Renewable Energy Global Innovations features: A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells

Significance 

Solution-processable organic solar cells are an emerging technology that is capable of providing a cheap route for solar energy conversion. Researchers continue to set new records in power conversion efficiencies every year, yet the organic solar cell technology has remained more of an academic interest. In order to extend the existing lab-scale methodologies to large-scale commercialization, there are a number of inadequacies that will have to be addressed.

Transitioning to high-performance practical materials from currentmaterials with complex and multi-step preparations remains a major issue. This has made the materials expensive and more or less limited to academic settings. To address this issue, low cost and scalable N-annulated perylene diimide building blocks have been incorporated into a wide range of final materials with overall power conversion efficiencies between 2-8% when implemented as a non-fullerene acceptor. While these materials are simple to access and scale-up, their high efficiencies have relied on the use of polymeric donor materials, which are often quite expensive.

In view of the above limitations, solar energy scientists have now shifted their focus to finding simple and scalable donor materials that can sufficiently complement these acceptor materials. Researchers led by Professor Gregory Welch at the University of Calgary streamlined the screening of low cost and scalable donor materials using an N-annulated perylene diimide derivative. The authors established a simple air-processed and air-tested organic photovoltaic device preparation method in order to realize their objective. Their research work is published in Journal of Materials Chemistry A.

The authors took advantage of the diagnostic morphological handle inherent in N-annulated perylene diimide derivative and devised an approach for screening compatible donor materials. Implementing this efficient approach, the authors were able to screen a series of simple donor polymers constructed from low-cost building blocks and settled on PDTT-BOBT as good competitor to the now standard, high performance, yet expensive polymer, PTB7-Th.

The authors observed that optimizing the active layer blend of PDTT-BOBT:PDI-DPP-PDI led to an increase in the performance upon post-deposition chloroform vapor annealing. The best cell power conversion efficiency improved from 1.9 to 4.5% compared to 1.7 to 4.6% for the PTB7-Th. These high efficiencies made the authors recognize PDTT-BOBT as an alternative to PTB7-Th and supported its credibility for screening new acceptor materials.

While performance was impressive, negligible light absorption of PDTT-BOBT beyond 700nm as well as poor photochemical stability in air appear to be the major drawbacks to the polymer design. This polymer has high ionization potential and a high open circuit voltage. It would be therefore prudent to red-shift the onset of absorption with less alteration on the ionization potential. For this reason, any future modifications to the polymer design must be centrally focused on modifying the acceptor component.

Enhancing light stability of this polymer would definitely necessitate substituting the alkoxy side chains on the benzothiadiazole moiety with stable solubilizing substituents, without necessarily minimizing polymer solubility or affecting its self-assembly tendencies.

Addressing these challenges will call for the preparation of a number of new polymeric materials. Seth McAfee and colleagues in this study therefore proposed a simple approach for easy screening of these derivatives to come up with superior polymer designs.

About the author

Seth McAfee is a PhD candidate in Chemistry at the University of Calgary (Canada) working under the supervision of Dr. Gregory Welch.

Seth’s research in the Welch Research Group is focused on practical organic materials development for use in electronic devices, specifically organic solar cells. Motivated to access more sustainable and cost-effective active layer materials, Seth has been designing his organic pi-conjugated compounds to make use of organic dyes, known for their ease of commercial accessibility and excellent light harvesting capabilities.

Current efforts are focused on exploiting a material composed of perylene diimide (structural derivative of Pigment Red 190) and diketopyrrolopyrrole (structural derivative of Pigment Red 254). This compound is easily synthesized in high yields on multi-gram scale and has been able to achieve impressive device efficiencies acting as the electron-accepting material within the bulk heterojunction of solution-processable organic solar cells. A key feature of this material is the solvent vapour annealing induced solid-state re-organization of the compound. This was found to dramatically improve organic solar cell device efficiencies with an array of different electron-donating materials and highlights the versatile compatibility of the compound.

Contact: seth.mcafee@ucalgary.ca

Reference

Seth M. McAfee, Abby-Jo Payne, Sergey V. Dayneko, Gururaj P. Kini, Chang Eun Song, Jong-Cheol Lee and Gregory C. Welch. A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells. Journal of Materials Chemistry A, 2017, 5, 16907.

 

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Renewable Energy Global Innovations features: Monolithic tandem solar cells comprising electrodeposited CuInSe2 and perovskite solar cells with a nanoparticulate ZnO buffer layer

Significance Statement

High production cost and low efficiency are the key roadblocks to realizing advances in solar technology. At present, solar photovoltaics can only account for less than 2% of today’s global electricity. More so, a complex inhibition is encountered when attempts to increase the power conversion efficiency of a single-junction solar cell are made. As a result, inquest into the formation of multiple-junctions in a cell is receiving considerable attention. In recent advancements, organometal halide perovskite-based solar cells have emerged as promising top cell candidates due to their suitable electrical and optical properties. To achieve highly efficient tandem devices, the realization of a high efficiency semi-transparent perovskite solar cell is still an essential challenge. Moreover, the development of a suitable deposition process that does not deteriorate the low-cost advantage and maintains the high performance of the perovskite solar cells will be of great importance.

A team of researchers led by Doh-Kwon Lee at the Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology designed semi-transparent inverted planar perovskite solar cells for monolithic tandem construction with an electrodeposited copper-indium-selenide solar cell. Their aspirations were to attempt and realize a high-efficiency monolithic thin film tandem device. In order to achieve this, they hoped to eliminate the severe degradation of the perovskite/PCBM layers during the sputtering process of the transparent conducting oxide, using a thin zinc oxide-nanoparticle introduced onto the electron-extracting PCBM layer by a simple solution process without a post-annealing step. Their research work is now published in the journal, Journal of Materials Chemistry A.

The procedure undertaken involved selecting a sputter aluminum-doped zinc oxide layer as the top transparent conducting oxide while employing a thin zinc oxide-nanoparticles layer prepared by a simple solution process as a buffer layer to protect the underlying PCBM and/or perovskite layers from possible damage during the sputter deposition of the aluminum-doped zinc oxide electrode. To demonstrate the feasibility of all-solution processed, monolithic two-terminal tandems, an electrodeposited copper-indium-selenide thin-film solar cell with a power conversion efficiency of ca. 10% was used as a bottom cell. An intrinsic zinc oxide/ aluminum-doped zinc oxide double layer was applied as a transparent recombination layer at the junction between the sub-cells by tuning its thickness. As a result, monolithically integrated, two-terminal copper-indium-selenide/perovskite tandem solar cells with a higher power conversion efficiency than the constituent single-junction devices were realized.

The research team observed that the thin zinc oxide-nanoparticles layer with an optimal thickness facilitates the electron transfer from PCBM to the silver back contact in opaque devices while also helping to protect the underlying layers from plasma-induced damage in semi-transparent devices. As a result, semi-transparent perovskite solar cell devices with an inverted architecture having a sputtered aluminum-doped zinc oxide top electrode are realized with power conversion efficiencies over 10%.

Within a semi-transparent perovskite solar cells in the p–i–n architecture, in which a solution processed zinc oxide-nanoparticles layer was introduced as an effective charge selective layer between the PCBM and transparent conducting oxide layers, has been demonstrated. It has been seen that due to the sputter-buffering ability of the zinc oxide-nanoparticles layer, an aluminum-doped zinc oxide top transparent conducting oxide layer could be deposited without causing severe plasma damage to the underlying perovskite/PCBM layers. The technique presented in their paper is important in that it allows current matching between the sub-cells, optimization of the recombination layer for good transmission of low-energy photons and low interfacial resistance and the development of a fabrication strategy that is viable for industrial use. All in all, the results obtained shed light on the possibility of all-solution-processed, highly efficient tandem solar cells.

PCBM – phenyl-C61-butyric acid methyl ester

Reference

Yoon Hee Jang, Jang Mi Lee, Jung Woo Seo, Inho Kim, Doh-Kwon Lee. Monolithic tandem solar cells comprising electrodeposited CuInSe₂ and perovskite solar cells with a nanoparticulate ZnO buffer layer. J. Mater. Chem. A, 2017, 5, 19439–19446

 

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Renewable Energy Global Innovations features: Relevant efficiency enhancement of emerging Cu2MnSnS4 thin film solar cells by low temperature annealing

Significance Statement

In the recent years, thin film solar cell manufacturers have suffered from the abrupt decrease of silicon module price. Irrespective of the current efficiency of Cu(In, Ga) Se2 thin film solar cells being very near to the already established silicon-based photovoltaic technology, the low availability of gallium and indium in the Earth’s crust will result in their high cost, and this will definitely limit their future role in terawatt range photovoltaic applications. Therefore, researchers have shifted their attention to low cost materials based on earth abundant elements.

Fortunately, there is an attractive alternative for the preparation of indium and gallium free terawatt-scale chalcogenides. These alternatives depend on I₂-II-IV-VI₄ species, which include copper zin tin sulfide, copper zinc tin selenide and the related sulphur-selenium alloy. Copper manganese tin sulfide, a p-type semiconductor based fully on earth abundant as well as low cost elements, is another member of this group of materials . In view of the fact that manganese is cheaper than zinc, copper manganese tin sulfide can provide W_p cost lower as compared to copper zinc tin sulfide.

Copper manganese tin sulfide that crystalizes into a stannite structure exhibits direct band gap and high absorption coefficient all of which are necessary for photovoltaic applications. Several studies on copper manganese tin sulfide have been mainly based on single crystals and nanocrystals. However, recent studies have been reported on copper manganese tin sulfide thin films for photovoltaic applications.

Alessia Le Donne, Maurizio Acciarri and Simona Binetti at University of Milano-Bicocca in collaboration with Stefano Marchionna and Federico Cernuschi at RSE SpA grew copper manganese tin sulfide thin films through a two-step vacuum process. They grew metal precursor stacks through thermal evaporation and then heat treated them in elemental sulfur vapors. Their research work is published in Solar Energy.

The authors settled for Cu-poor/Mn-rich copper manganese tin sulfide films with Mn/Sn ratio of 1 in a bid to avoid the development of insulating and highly conductive secondary phases. The researchers tested the proposed copper manganese tin sulfide thin films by photoluminescence, Raman, Scanning Electron Microscopy and Energy Dispersive Spectroscopy.

The research team were able to obtain Cu-poor/Mn-rich copper manganese tin sulfide specimens with an acceptable homogeneity of the metal compositional ratios through a stringent control of the manganese evaporation rate. Solar cells manufactured from the films indicated good performance as opposed to a previous study. In view of the advantages of low temperature post-deposition annealing in inert and air atmosphere reported in the literature, the authors investigated the impact of thermal treatments they did between 200 and 275 °C on the copper manganese tin sulfide solar cell efficiency. The analysis encompassed both modification of material attributes and electrical performance.

The best annealing at 225 °C in air for about 40 minutes allowed for significant enhancement of their performance, open circuit voltage 354 mV, short circuit current density 5.8 mA/cm2, 40% fill factor and efficiency of 0.83%. This therefore increased the efficiency of this promising material.

About The Author

Alessia Le Donne got a M.S. degree in Materials Science from the University of Milano-Bicocca in 2001 and in 2004 a Ph.D. in Materials Science from the same Institution. Since 2005 she got several postdoctoral fellowships at the University of Milano-Bicocca and a research fellowship at CNISM (National Interuniversity Consortium for the Physical Sciences of Matter).

She co-authored 63 peer-reviewed papers, 1 book chapter and more than 80 contributions at national or international scientific conferences. Since 2001 she has been involved in several European and national Projects. She regularly serves as peer-reviewer for high impact factor international scientific journals. She is associated editor of the international journals ‘Reviews in Advanced Sciences and Engineering’ and ‘Materials Focus’ and member of the editorial board of ‘Conference Papers in Energy’ and ‘Indian Journal of Materials Science’.

About The Author

Stefano Marchionna got a M.S. degree with honours in Materials Science from the University of Milano-Bicocca in 2003 and a Ph.D. in Materials Science from the same University in 2006. He co-authored 20 peer-reviewed papers and more than 30 communications at national or international scientific conferences. In 2007, he was process engineer at NED Silicon Company (Italy), working on the development of an innovative production line for solar grade silicon. From 2008 to 2013, he was process engineer at Voltasolar Company (Italy), working on the development of low-cost thin films solar cells based on Cu(In,Ga)Se₂ (CIGS). Presently, his research activity at RSE SpA (Italy) is focused on the development of new and alternative materials based on Earth abundant elements both for photovoltaic and energy storage applications.

About The Author

Maurizio Acciarri is Associate Professor in Physics at the Department of Materials Science of the University of Milano-Bicocca. His research activity is mainly addressed to the study of electrical properties of semiconductors for photovoltaic applications. His research in the field of thin films for photovoltaic applications led to an international patent and to the technological transfer of the related Cu(In, Ga)Se₂ growth process to a pilot line. He is co-author of 4 patents. From 2017 he is Director of the Management Committee of the Microscopy Platform of the University of Milano-Bicocca.

Since 2011 he is member of the scientific committee of the Milano-Bicocca Solar Energy Research Center (MIBSOLAR) and since 2013 he is co-director of the center. Since 2014 he is member of the scientific committee and teacher of the Green Energy Management Summer School. Since 2017 he is member of the scientific committee and teacher for the PhD school in Sustainable Human Development. Since 2017 he is member of the editorial board of the international journal ‛Solar Energy’.

About The Author

Federico Cernuschi is the head of the Materials for Energy Research Group at RSE SpA. After completing his studies in physics at the University of Milan, since 1990 he worked on the development and application of advanced non-destructive techniques for the integrity assessment of power plant components and for the physical, thermophysical characterization and wear resistance of coatings and materials for energy applications. He has been responsible for several EU funded research projects. He has published more than 60 papers in international scientific journals. He sits on national and international standards committees focusing on wear and NDE&T and advanced ceramics.

About The Author

Simona Binetti is Associated Professor of Physical Chemistry at University of Milano-Bicocca,  vice director of Milano-Bicocca Solar Energy Research Center (MIBSOLAR), representing UNIMIB in the Joint Program on Photovoltaics of European Energy Research Alliance. Qualified Full Professor in Physical Chemistry. Graduated in Physics, Master in Materials Science and PhD in Chemistry.

Recognized expert in effect of defects on optoelectronic properties of silicon based semiconductors. Involved in 10 European Projects, 9 national about PV, some of them as leader, collaborating in research for private owned companies. She is currently leading 3 projects. Co-author of 120 peer-reviewed publications, 4 book chapters, 4 patents.

Reference

A. Le Donne, S. Marchionna, M. Acciarri, F. Cernuschi, S. Binetti. Relevant efficiency enhancement of emerging Cu₂MnSnS₄ thin film solar cells by low temperature annealing. Solar Energy, volume 149 (2017), pages 125–131.

 

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Renewable Energy Global Innovations features: Copper nanowire/multi-walled carbon nanotube composites as all-nanowire flexible electrode for fast-charging/discharging lithium-ion battery

Significance Statement

With the rapid evolution of autonomous vehicles, electric vehicles are anticipated to continue to grow rapidly. The electric vehicles offer many benefits such as zero emissions, less noise and vibrations, and are operated by simple electric motors with energy conversion in the range of 80-90%. Electric vehicles also have superior energy resilience since they can be charged using a number of energy sources such as renewable energy, conventional power-plant energy, and regenerative braking energy.

Unfortunately, the electric vehicles suffer some limitations such as cost, safety, mileage, limited lifespan, long charging time, and lack of grid for charging. These problems have led to the development of power systems with Lithium-ion batteries. In a bid to fix cost and mileage issues, the energy density of lithium-ion batteries must be increased, and the only way to improve the energy density would be to come up with new active materials with high theoretical capacity for anodes and cathodes.

Although high capacity materials can be applied to the Li-ion batteries, Li-ion batteries would still take long time to charge owing to their low power densities. Unfortunately, even the recently developed fast chargers with pulse power cannot overcome energy-density fading in the course of high-current charge/discharge reference to the limitation in the energy-conversion reaction of Li-ion batteries.

Researchers led by Professor Youn Sang Kim at Seoul National University, Republic of Korea, proposed a novel all-nanowire electrode structure for fast-charging-discharging Li-ion batteries implementing copper nanowires and multi-walled carbon nanotubes without binders or even conductive agents. Theoretically, the multi-walled carbon nanotubes as the representative one-dimensional carbon-based nanostructure provided fast channels for the effective transport of both electrons as well as ions for Li-ion batteries owing to their unique features that had high aspect ratio as well as large surface area. However, the large voltage range between charging and discharging is normally limited the multi-walled carbon nanotubes to be used for active materials in full cells, due to their morphology and resistivity. The authors firstly overcame this limitation of multi-walled carbon nanotubes, and their work is published in peer-reviewed journal, Nano Research.

The authors fabricated a lightweight 3-dimensional composite anode for a fast charging-discharging Li-ion battery implementing two of 1-dimensional nanomaterials, which were copper nanowires and multi-walled carbon nanotubes. Reference to superior electrical conductivity, large surface areas, and high aspect ratio of these materials, the copper nanowire-multi walled carbon nanotubes composite with 3-dimensional structure provided several advantages concerning transport channels of ions and electrons.

The copper nanowires applied as the current collector and multi-walled carbon nanotubes applied as the active materials provided a number of benefits for enhancing the Li-ion battery performances. These included efficient ion diffusion, thick electrode formation, fast electron transport, and flexible cell design. As an advanced binder-free anode, the proposed composite film with tunable thickness indicated a significant low sheet resistance and internal cell resistance. The copper nanowires network with 3-dimensional structure functioned as a rigid framework connected to the multi-walled carbon nanotubes. They prevented the shrinkage and expansion of the multi-walled carbon nanotubes owing to swelling and aggregation, and minimized the effects of volume change of the carbon nanotubes during the charging-discharging process.

Both the full and half-cells of the Li-ion batteries with 3D-composite film anode indicated high specific capacities and Coulombic efficiencies even at high currents. The authors were able to overcome, for the first time, the limitations of carbon nanotubes as anode materials for fast charging and discharging Li-ion batteries by implementing copper nanowires, and the proposed anode can be used in flexible Li-ion batteries. This new development could result in the development of ultrafast chargeable Li-ion batteries for electric vehicles.

About The Author

Zhenxing Yin completed his Bachelor’s studies at Changchun University of Technologies (China) in 2012. Then, he received his Master’s degree at Seoul National University (Republic of Korea) in 2014, and is currently a Ph.D. candidate at Graduate School of Convergence Science and Technology, Seoul National University. His research interests mainly focus on copper nanowire synthesis and applications.

About The Author

Prof. Jeeyoung Yoo is the research professor in Graduate School of Convergence Science and Technology, Seoul National University at Seoul, Korea. A graduate of Chung-Ang University, she holds a Bachelor of Chemical Engineering, Master of Chemical Engineering, and PhD in Chemical Engineering, specializing in electrochemical engineering. And she is an expert in energy storage materials and device-related research and development.

About The Author

Prof. Youn Sang Kim is the Professor in Graduate School of Convergence Science and Technology, Seoul National University at Seoul, Korea. He received Ph.D. in the Department of Chemical Engineering from Seoul National University at Seoul, Korea in 2002 and then worked for two years as a postdoctoral fellow in Massachusetts Institute of Technology, USA. His current research interests are concentrated on interface engineering for novel devices such as energy harvesting devices, oxide or hybrid TFTs, oxide diodes and printing electronics.

Reference

Zhenxing Yin, Sanghun Cho, Duck-Jae You, Yong-keon Ahn, Jeeyoung Yoo, and Youn Sang Kim. Copper nanowire/multi-walled carbon nanotube composites as all-nanowire flexible electrode for fast-charging/discharging lithium-ion battery. Nano Res. 2017, DOI: 10.1007/s12274-017-1686-0.

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Renewable Energy Global Innovations features: CH3NH3PbI3 Converted from Reactive Magnetron Sputtered PbO for Large Area Perovskite Solar Cells

Significance Statement

Exponential growth in both interest and attention paid to the organic-inorganic metal halide perovskites materials have spiked undeniable concern of late. The outstanding properties possessed by these materials carry all the credit. These properties, including: long exciton diffusion length, strong absorption coefficients, low cost, ease of Synthesis and environmental-friendliness have led to great advancement in perovskite solar cells such as improving the power conversion efficiency from around ten percent in the early years of this decade to about twenty percent at present. Recent studies have shown that the quality and morphology of the perovskite films are crucial to its photoelectric properties and that they directly influence the performance of the resultant perovskite solar cells. Even though several deposition techniques have been proposed for synthesis of the perovskite light-absorption layers, great difficulties are still being encountered in the bid to fabricate perovskite films with both satisfactory coverage and uniformity over a large area.

Researchers led by Professor Meicheng Li at the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources in North China Electric Power University developed a novel process route based on direct current reactive magnetron sputtering in the preparation of the CH₃NH₃PbI₃ film. They aimed at presenting a complete approach for the fabrication of large area perovskites solar cells with the advantages of easy control, economical and requiring less use of toxic reagents but with diverse potential applications. Their research work is now published in Solar Energy Materials & Solar Cells.

The research team began by fabricating the lead oxide film on an FTO-glass substrate coated with a nanocrystalline rutile titania by using a pure metallic lead target in an argon-oxygen mixture. They then converted the prepared lead oxide film to CH₃NH₃PbI₃ through the sequential reactions setup in isopropanol solution of CH₃NH₃I. Eventually, the research team fabricated solar cells of a complex structure that employed nanocrystalline rutile titania as the contact layer of the photovoltaic devices.

The authors were able to observe that the as-prepared perovskite film exhibited a surface morphology of high uniformity and excellent coverage over a large scale. Also the crystal grains were seen to reach the size of up to 600 nm, which is beneficial to extract photo-generated electrons more effectively and prepare the perovskites solar cells at low temperature.

The new approach employed in their study is technically spin-coating-free for the formation of large area CH₃NH₃PbI₃ film and exhibits advantages ranging from easy process control, economical all the way to less use of toxic reagents. Of crucial importance, it is expected that this novel technique will be applied for the synthesis of perovskites solar cells or other thin-film devices and thus entails potential applications and practical significance.

The schematic illustration of CH3NH3PbI3 formation (on NRT-coated FTO glass substrate) through the sputtered PbO.

The top-view SEM of CH3NH3PbI3 converted from the sputtered PbO, where the insertions are the corresponding one with high magnification.

About The Author

Zhirong Zhang is a Ph.D candidate, who studied at the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources in North China Electric Power University, under the supervision of Prof. Meicheng Li. He received his B.S. degree majored in Radio Physics, from Lanzhou University, in 2008. His research interests include development of thin film solar cells and the design & application of photovoltaic system. He has been working on perovskite solar cells since the year of 2013.

About The Author

Prof. Meicheng Li is the Director of New Energy Materials and PV Technology Center, and the Vice Dean of the School of Renewable Energy, North China Electric Power University. He obtained his PhD at Harbin Institute of Technology in 2001. He worked in University of Cambridge as Research Fellow from 2004 to 2006. He won the Excellent Talents in the New Century by the ministry of education in 2006. His current research topic is the New Energy Materials and Devices, such as perovskite solar cells, lithium ion battery system. Till now, he contributed more than 200 journal articles and performed the review services for about 80 SCI journals. He got almost more than 10 items of awards for the science and technology success. He served more than 20 academic conferences as the chair, track co-chair or session chair. He is an executive fellow of the China Energy Society, fellow of Chinese Society for Optical Engineering.

Website , Research Gate.

Reference

Zhirong Zhang, Meicheng Li, Wenjian Liu, Xiaopeng Yue, Peng Cui, Dong Wei. CH₃NH₃PbI₃ converted from reactive magnetron sputtered lead oxide (PbO) for large area perovskite solar cells. Solar Energy Materials & Solar Cells, volume 163 (2017) pages 250–254.

 

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Renewable Energy Global Innovations features: Solution processed Cu2CdSnS4 as a low-cost inorganic hole transport material for polymer solar cells

Significance Statement

Polymer solar cells have a wide range of advantages, and their bulk heterojunction formations, especially the solution-processed type, contribute to improved power conversion efficiency. Traditionally, PEDOT:PSS is used as a hole transport layer in an polymer solar cells (PSCs) owing to its potential advantages such as ease in processability, smooth surface topography, and matching work function with HOMO of many donor-type organic semiconductors. But the acidity and hygroscopic nature of the PEDOT:PSS makes it inefficient electron-blocking performance, and thus negatively contribute to the performance and stability of PSCs under ambient conditions. The transparent metal oxides (TMOs) are being used to replace PEDOT:PSS requires expensive and complicated vacuum based processes, the toxicity and scarcity of some materials such as NiOx and VOx makes it immaterial for PSCs application. So it is essential to focus on materials that are based on earth abundant elements, low-cost, nontoxic and environmentally stable HTMs for highly efficient OSCs.

Professor Sudip Batabyal and colleagues at Amrita University in India demonstrated the capability of applying solution-processed copper cadmium tin sulfide nanoparticles as low-cost inorganic hole transporting materials in polymer solar cells, and studied the effect of particles structure on the functioning of the cells. Their research is published in Solar Energy Materials and Solar Cells

The authors prepared 4 different samples by spin coating a cernyite solution on an indium tin oxide substrate and varied the number of layers in the samples from 1 to 4. A reference cell was fabricated using poly(3,4-ethylenedioxythiophene): polystyrenesulfonate as the hole transporting layer.

Structural features of the particles showed that they had a size of about 7-12nm, and a tetragonal structure. The band gap values for the as-deposited thin films were 1.65eV for 1 layer, 1.60eV for 2 layers, and 1.35eV for both 3 and 4 layers. This shows that the band gap values decrease with an increase in thickness for the as deposited thin films, which confirms that particles agglomerate and their sizes increase with an increase in layer thickness. Further, the absorption spectrum of the as-deposited thin films was observed to cover the entire visible spectrum and increases with an increase in thickness, which shows the nanoparticles have an additional contribution in photocurrent.

The authors calculated the surface roughness values for the samples which were 23.54 nm for 1 layer, 13.11 nm for 2 layers, 11.07 nm for 3 layers, and 21.00 nm for 4 layers. The as-deposited thin film having 3 layers had the minimum surface roughness which implies a more uniform coating over its surface.Although increasing the number of layers improves the compactness in the thin films, it reaches a point where particles aggregate which in effect deters transportation of charge carriers and thus reduces the device’s overall performance. Therefore, the 3 layers were observed to be the optimum number for use of the nanoparticles as a hole transporting layer.

The research team also noted that the solar cell power conversion efficiency improves with an increase in thickness of the as developed thin film and then it decreases significantly. This power conversion efficiency is comparable to that of the poly(3,4-ethylenedioxythiophene): polystyrenesulfonate hole transporting layer. Furthermore, the sample having 3 layers of the nanoparticles had the highest power conversion efficiency as compared with the other samples.This is as a result of the uniform thickness of the film and its compactness, which generates an optimum interface as well as improves the dynamics of the charge transfer.

Comments from Authors:

Metal sulfide nanomaterials have drawn drastic attention because of their exotic electronic properties and high specific surface areas that are potentially useful in photovoltaic applications. For the first time, the compound chalcogenides were used as a hole transport layer in a polymer solar cells. In addition to the transporting property of the p-type buffer layer it is also expected in generation of excitons resulting in increased photo generated currents. The inorganic materials are known for its stability than the organic materials and in future it is interested to work in this direction.

About The Author

Dr. Sudip Kumar Batabyal is the Senior Research Scientist in ACIRI. Prior to joining ACIRI in 2015, Sudip has over 8 years of research experience in nanomaterials fabrication and application in renewable energy sector. His areas of expertise and research interests include semiconducting nanomaterials for energy harvesting and storage, perovskite materials, printed electronics, and energy storage. Sudip received his M.Sc in Physics from Vinoba Bhave University and Ph.D from Indian Association for the Cultivation of Science ( Jadavpur University). Sudip worked in National University of Singapore and Energy Research Institute @ Nanyang Technological University (ERI@N) on the project of solution processed Cu2InGa(S/Se)2 (CIGS) and Cu2SnZn(S/Se)4 solar cell. He developed the CIGS and CZTS absorber layer deposition on Mo substrate by spray pyrolysis method. He successfully fabricated the solution processed CIGS device with more than 10% efficiency. Sudip developed some CNT based perovskite device with more than 10% efficiency. He developed some metal chalcogenide based holetransporting materials for OPV.

 In his research career, he has focused on a wide variety of novel materials (metal chalcogenide, metal oxide, organic semiconductors, carbon nanotubes and graphene) synthesised by a range of fabrication procedures. His main emphasis was on the electronic and optical properties of these materials and direct application of these nanostructures in practical devices. His primary research interests are photovoltaics, photoelectrochemical systems and energy storage. His research work has been published (90 publications) in many high impact factor journals such as Nature Communication, Advanced Materials, ACS Nano and Advanced Energy Materials etc.

Reference

Suresh Kumar, KallolMohanta, Sudip K. Batabyal. Solution processed Cu₂CdSnS₄ as a low-cost inorganic hole transport material for polymer solar cells. Solar Energy Materials and Solar Cells, 161 (2017) 157-161.

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Renewable Energy Global Innovations features: Copper yttrium selenide: A potential photovoltaic absorption material for solar cells

Significance Statement

The unique structural and photoelectric characteristics of copper based ternary and quaternary semiconductor compounds favors their wide applicability as absorption materials in thin film solar cells. Most research on this type of materials has focused mainly on group IIIA compounds of the periodic table. The polycrystalline compound Cu(In,Ga)Se₂ has yielded the highest conversion efficiency to date. However, indium and gallium are expensive due to their scarcity, thereby increasing the cost of production of solar cells and limiting their wide-scale applicability. Researchers have thus been coerced to seek for alternatives where the advantages of doping semiconductors such as copper yttrium selenide with rare earth elements has attracted their attention and presented an alternative. Yttrium is cheaper, more abundant and less toxic than both indium and gallium. So far, no report on the optoelectronic properties of the Copper yttrium selenide as an absorption material for solar cells has been presented.

 A team of researchers led by Professor Ruixin Ma at the University of Science and Technology in China proposed a study on the optoelectronic properties of copper yttrium selenide. They successfully synthesized the Copper yttrium selenide using self-propagating high temperature synthesis technique and measure its optoelectronic properties. Their work is now published in Materials and Design.

First, the research team synthesized the novel photovoltaic material: copper yttrium selenide, using a self-propagating high temperature synthesis method. The process had three stages and the optimum temperature of the copper yttrium selenide synthesis was 1016.2°C. The authors of this paper then characterized the crystalline morphology of the copper yttrium selenide using X-ray diffraction and field emission scanning electron microscopy. They then estimated the band gap of the copper yttrium selenide material based on the ultraviolet-visible spectroscopy spectrum of the material. A thin film of the copper yttrium selenide was prepared and used to determine the current-voltage properties.

The team noticed that the newly synthesized copper yttrium selenide exhibited outstanding absorption property in the visible light region. They also recorded the band gap to be 1.53 eV which is close to the optimal value for use in the solar cells. Most important of all, the copper yttrium selenide film used exhibited an exceptional photo-electron responsive behavior of a I_light/ I_dark  ratio of 2.81. This showed that the copper yttrium selenide was very suitable for use as an absorption material of thin film solar cells.

The work described herein presents and illustrates the novel copper yttrium selenide as a promising candidate for use as an absorption material in low cost and mass production fabrication of thin film solar cells. The success of copper yttrium selenide lengthens the list of available species of materials for use in low cost solar cells. 

Reference

Shina Li, Ruixin Ma, Xiaoyong Zhang, Xiang Li, Weishuang Zhao, Hongmin Zhu. Copper yttrium selenide: A potential photovoltaic absorption material for solar cellsMaterials and Design, volume 118 (2017) pages 163–167.

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Renewable Energy Global Innovations features: Electrochemical and photocurrent characterization of polymer solar cells with improved performance after graphene oxide addition to PEDOT:PSS hole transporting layer

Significance Statement

Extensive knowledge exists regarding the predominant role that in bulk heterojunction solar cells donor and acceptor interfaces play in charge carrier formation and separation. Interface improvements by addition of surfactants and by active layer annealing has already been attempted. Interfacial layers have also been optimized so as to avoid charge recombination at the collecting electrodes. These interfacial layers however must qualify as efficient carriers, possess a reduced resistance and be characterized by low light absorption capacities. Consequently, graphene oxide in polymer solar cells possess such qualities that defend their application, contrary to the popularly used tin-doped indium oxide and aluminum electrodes, so as to improve the photovoltaic performance and stability of the devices. Herein, chemically fabricated graphene oxide is added so as to modify the performance and electrochemical properties of bulk heterojunction solar cells of varying architecture.

In a recent research collaboration between Polish and Mexican researchers, Agnieszka Iwan, Felipe Caballero-Briones and colleagues investigated the photocurrent and electrochemical characterization of polymer solar cells with improved performance, after addition of graphene oxide to the PEDOT:PSS hole transporting layer. They focused on applying chemically synthesized graphene oxide in polymer solar cells of varying architectures while altering the placement and amount of the graphene oxide in the polymer solar cells. Their aim was to establish a knowledge base that enlightens on graphene oxide addition in different layers of the same device. Their research work is now published in Solar Energy.

First, the research team obtained the graphene oxide by modified Hummers method and fully characterized by Raman spectroscopy, Fourier Transform Infrared Spectroscopy, X-ray diffraction as well as with cyclic voltammetry. They then constructed bulk heterojunction polymer solar cells with P3HT:PC61BM or PTB7:PC71BM active layers and PEDOT:PSS as hole transport layers. The constructed layers were then subject to investigation relative to: the concentration of graphene oxide in hole transport layer, the acidity of the graphene oxide, the type of polymer used in the active layer, the annealing temperatures of the active layer and the place where the graphene oxide is incorporated in the devices.

The authors observed that the best performance for the polymer solar cells was obtained for the devices with the ITO/PEDOT: PSS:GO/PTB7:PC71BM/Al architecture and at the point where the volume ratio of the graphene oxide to PEDOT:PSS was 1:1. Under these conditions, the researchers noted that higher power conversion efficiency was obtained. They also observed a better active layer performance of the polymer solar cells with the graphene oxide annealed at 130⁰ C.

Herein, the positive effects of incorporation of graphene oxide in bulk heterojunction polymer solar cells, as additive to the hole transport layer PEDOT:PSS with the volume ratio 1:1 are demonstrated. Improved performance of the polymer solar cells is notably achieved in both photocurrent and electrochemical characterization. In totality, the improvement of the polymer solar cells performance upon graphene oxide addition can therefore be comprehended in terms of hole movement and better HOMO-LUMO matching within the structure.

About The Author

Dr. Agnieszka Iwan, assoc. prof. has completed her Ph.D. from Technical University in Silesia (Poland) and postdoctoral studies from Centre National De La Recherche Scientifique in Grenoble (France). She received Ph.D., D.Sc. in Technical University in Wroclaw (Poland). She formerly worked at the Centre of Polymer and Carbon Materials, PAS (Zabrze, Poland) and next at the Electrotechnical Institute (Wroclaw, Poland) as head of the New Technologies Lab., in October 2016 moved to Military Institute of Engineer Technology (Wroclaw, Poland) and has professor position in Institute.

Her research focuses on the organic/polymer/perovskite solar and fuel cells, flexible electronics, nanomaterials such as graphene, TiO₂ or ZnO, liquid crystals and acid-base interactions.

She is author and co-author of more than 260 articles, including 8 book chapters, 3 books and more than 135 presentations in scientific conferences.

About The Author

Dr. Felipe Caballero-Briones, Full Professor, has completed his PhD at the University of Barcelona (Spain) in 2009 and did a postdoctoral stay at Institute of Engineering of Catalonia (IBEC) and Department of Chemical Physics-UB in 2010-2011. From 1999 to 2009 was appointed as associate professor and from 2010 became full professor at the Center for Applied Science and Advanced Technology (CICATA Unidad Altamira) of the Instituto Politecnico Nacional (Mexico) where he is Leader of the Materials and Technologies for Energy, Health and Environment Group (GESMAT).

His research is directed to design and develop graphene-based and semiconducting materials and oxides for photovoltaics, microbial and polymeric fuel cells, supercapacitors, thermoelectrics, and photocatalyts. Other current research interests are graphene-based materials for water remediation, cancer treatment and desalination.

Dr. Caballero-Briones advised or is advising 5 PhD, 14 MSc and 13 BSc thesis and has authored or coauthored 55 articles and more than 200 presentations in scientific conferences; he has 601 cites in Google Scholar (H index 16).

Reference

Agnieszka Iwan, Felipe Caballero-Briones, Michal Filapek, Bartosz Boharewicz, Igor Tazbir, Agnieszka Hreniak, Jesus Guerrero- Contreras. Electrochemical and photocurrent characterization of polymer solar cells with improved performance after graphene oxide addition to the tin-doped indium oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) hole transporting layer. Solar Energy volume 146 (2017) page 230–242.

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Renewable Energy Global Innovations features: Comparisons among Bat algorithms with various objective functions on grouping photovoltaic power patterns

Significance Statement

Power generation from photovoltaic systems has been the focus of many research studies. The main aim has been to tackle environmental and financial issues of typical power resources. Unstable fossil fuel prices and a considerable portion of environmental pollutions and greenhouse emissions are major concerns when it comes to industrialized countries. Ability to produce electricity for a long time with minimal maintenance and reduction in capital costs are factors to be considered while integrating photovoltaic systems into the electrical grid.

Unfortunately, output power of photovoltaic system is dependent on ambient temperature and irradiation level. In addition, fluctuations in the output power could be experienced owing to shadowing or power quality interference. Therefore, it is important to study the effect of these output power fluctuations before photovoltaic systems installation. To achieve this, simulation implementing historical data and extensive analysis should be done.

Handling this data is computationally expensive and time consuming. Therefore, developing solutions that can ease the burden of extensive studies and simulations relating to integrating photovoltaic systems into the electrical grid is of outmost importance. Clustering methods can be used to group photovoltaic power patterns with similar properties. Thus, a representative power pattern for every group can be integrated in the simulations.

Amr Munshi and Yasser Mohamed from the University of Alberta presented the outcomes of an in-depth analysis of Bat clustering algorithms based on a number of objective functions in a bid to establish the grouping mechanism of photovoltaic power patterns. Their main objective was to enhance the clustering formation of the former clustering algorithm, Bat J. Their research work is published in Solar Energy.

The authors performed and in-depth analysis of the performance of Bat clustering algorithms dictated by a number of integrated objective functions to validate the clustering of photovoltaic power pattern process. They then compared the performance of the K-means and Bat J clustering algorithms with the new Bat clustering on the clustering process of photovoltaic power patterns data.

The researchers also illustrated the original Bat clustering algorithm methods to undertake photovoltaic power patterns grouping. They adopted the principle component analysis in a bid to reduce the dimensionality of the photovoltaic power patterns data.

Bat clustering algorithms were comparable or surpassed K-means in the validity index, compactness and separation values. The within-cluster-sum-of-squares validity index values of Bat were observed to have improved as opposed to K-means by approximately 14.10% and 14.71% over the knee-points for the first and second datasets, respectively. The authors observed that Bat within-cluster-sum-of-squares posted the best outcomes and was capable of enhancing Bat J algorithm that exhibited the best cluster data.

Nevertheless, this corresponded to more complexity since the number of parameters ought to have been priori calibrated. The preferable combination presenting the optimum number of clusters was observed to be Bat within-cluster-sum-of-squares clustering and within-cluster-sum-of-squares validity index. They presented considerably high separated and compact clusters.

Lower within-cluster-sum-of-squares values at a selected partition presented the most preferable combination of separation and compactness. Therefore, Munshi and Mohamed study on the Bat within-cluster-sum-of-squares could offer well-defined photovoltaic power pattern clusters as well as cluster representatives that can be used in photovoltaic output power analyses.

About The Author

Amr A. Munshi received the B.Sc. degree in computer engineering from Umm Al-Qura University, Makkah, Saudi Arabia, in 2008, and the M.Sc. degree in computer engineering from the University of Alberta, Edmonton, AB, Canada, in 2014, where he is currently pursuing the Ph.D. degree in computer engineering. His research interests include machine learning, data mining and big data analytics. Mr. Munshi is a Member of the Golden Key International Honor Society. He is currently an Editor of the Alberta Academic Review Journal.

Reference

Amr A. Munshi and Yasser A.-R.I. Mohamed. Comparisons among Bat algorithms with various objective functions on grouping photovoltaic power patterns. Solar Energy, volume 144 (2017), pages 254–266.

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Renewable Energy Global Innovations features: Thinness- and Shape-Controlled Growth for Ultrathin Single-Crystalline Perovskite Wafers for Mass Production of Superior Photoelectronic Devices

Significance Statement

The commercialization of organic-inorganic hybrid perovskite light-absorber material has been hampered by factors such as environmental stability, substandard interface and defects. Focus has shifted to the single-crystalline perovskite which is believed to be defect free, has better stability, longer carrier lifetime and diffusion length, wider optical absorption spectrum, and lower trap-state-density.

Professor Shengzhong (Frank) Liu and colleagues have successfully used a dynamic flow microreactor system to grow geometry-controlled ultrathin single crystalline perovskite wafers of different ranges of thicknesses. Their work is published in Advanced Materials, 2016, 28, 9204-9209; Adv. Opt. Mater. 2016, 4 (11), 1829-1837; Sci. China Chem. 2017, DOI:10.1007/s11426-017-9081-3.

The authors employed a dynamic-flow reaction system. They used 2 spacers to separate and align 2 thin glass slides so as to limit the crystal growth to a slit channel. A peristaltic pump was used to achieve dynamic flow of the precursor solution.

The research team fabricated single crystalline wafers of approximately 150, 330, 670, and 1440 mm in thickness, which showed that the crystal growth was confined within the microreactor.

The authors observed no obvious grain boundaries and cracks from the scanning electron microscopy examination, indicating that the wafer is of  high single-crystalline quality throughout. The mapping analysis and line scan results of the scanning electron microscopy energy-dispersive x-ray spectroscopy show an even distribution as well as consistency in atomic ratio of carbon, nitrogen, lead, and iodine constituents of the produced wafer.

When the single crystalline perovskite wafer is compared with the microcrystalline thin films in UV-Vis-NIR spectrophotometry, the authors observed that the former displayed a significant red-shifted light absorption edge at 900 nm as compared with 800 nmfor the latter, a significant advantage for PV and optoelectronic applications.

From the thermogravimetric analysis, the single crystalline wafer is similar to bulk single crystals in thermal decomposition,  exhibiting stability at higher temperatures over the microcrystalline films.

The authors designed a hole only device to analyze the trap density of the single crystalline wafer by testing, at different biases, the evolution of space-charge-limited current. The trap density of both the single crystalline wafer and the large single crystals was found to be similar. The Hall effect is also similar for both the single crystalline wafer and the single large crystals.

To simulate an optoelectronic device, the team designed 100 photodetectors on the perovskite wafer, which demonstrated the feasibility of integrated circuits being mass produced on these wafers. At different bias voltages and illumination, the authors observed that at a 2V bias the photocurrent in the wafer was approximately 700 µA, while this was limited to only 2 µA for the microcrystalline thin films which is approximately 350 times smaller. The single crystalline wafer detector showed significant response  at 880 nm while the microcrystalline thin film device shows no response at all, which confirmed that the former has a broader optical absorption than the latter.

Reference

Yucheng Liu, Yunxia Zhang, Zhou Yang, Dong Yang, Xiaodong Ren, Liuqing Pang, and Shengzhong(Frank) Liu. Thinness- and Shape-Controlled Growth for Ultrathin Single-Crystalline Perovskite Wafers for Mass Production of Superior Photoelectronic Devices. Advanced Materials, 2016, 28, 9204-9209.

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Renewable Energy Global Innovations features: 20-mm-Large Single-Crystalline Formamidinium-Perovskite Wafer for Mass Production of Integrated Photodetectors

Significance Statement

Defects such as surface imperfections and grain boundaries within microcrystalline thin films have been the cause of performance killer in perovskite-based devices. Large single-crystalline materials such as methylammonium iodide have been used since they are free from the defects and possess superior characteristics such as ultralow trap state density. Research advancements have shown that the formamidinium lead iodide performs even better as an optoelectronic material with solar cell efficiency greater than 22%. One of the impediments to the advancement of the formamidinium lead iodide is that only smaller single crystalline formamidinium perovskite have been produced and therefore larger wafers and crystals urgently need to be developed.

Shengzhong (Frank) Liu and coworkers applied an inverse temperature reactive crystallization strategy to develop large single crystalline formamidinium perovskite. Their work is reported in Advanced Optical Materials, 2016, 4 (11), 1829-1837;. Advanced Materials, 2016, 28, 9204-9209; Sci. China Chem. 2017, DOI:10.1007/s11426-017-9081-3.

The authors observed that most harvested crystals exhibited rhombic hexagonal dodecahedra shapes with black glossy surfaces. They also noted that a higher relative humidity accelerates color change of the crystal from black to yellow. Thin wafers were prepared using a slicing machine with the thinnest wafer obtained having a 100 µm thickness.

The x-ray diffraction test showed the single crystalline formamidinium perovskite wafer adopts a perovskite structure and that no residue of the reactant precursors is present. Further, from rocking curve measurements it was evident that the wafer exhibits excellent crystalline quality.

From the absorption spectrum of the formamidinium lead iodide there was a cut-off feature indicating that it is a direct bandgap semiconductor, with the absorption edge located at approximately 870 nm, and an optical bandgap of approximately 1.49 electron-volts. The thermogravimetric analysis of the perovskite shows that it is highly stable without any sign of thermal decomposition up to 300 °C as compared with the methylammonium lead iodide perovskite.

The authors established that the trap state density of the as-prepared perovskite was lower than many of the known inorganic semiconductors, and that it has a low carrier concentration with a greater carrier mobility. This affirms that the as-prepared perovskite is of high quality.

The research team also compared photodetectors made of the as-prepared perovskite wafer and a thin film microcrystalline perovskite. They observed that the photocurrent of as-prepared perovskite was about 90 times higher than that of the thin film perovskite, and that it exhibits a significant response in the near-infrared region as compared with the latter, which confirms that the former has a broader optical absorption. The external quantum efficiency, photoresponsivity and response time of the as-prepared perovskite wafer were found to be even greater in the as-prepared perovskite wafer.

Reference

Yucheng Liu, Jiankun Shun, Zhou Yang, Dong Yang, Xiaodong Ren, Hua Xu, Zupei Yang, Shengzhong(Frank) Liu. 20-mm-Large Single-Crystalline Formamidinium-Perovskite Wafer for Mass Production of Integrated Photodetectors. Advanced Optical Materials, 2016, 4, 1829-1837.

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Renewable Energy Global Innovations features: Investigation of donor-acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman micro-spectroscopy

Significance Statement

Low-band gap copolymers as well as their compounds with fullerenes have received a great deal of research attention owing to the fact that they can be used as active layers on organic photovoltaic gadgets. Photovoltaic systems with bulk heterojunctions as well as donor-acceptor blends appear to be the most promising devices.

Photovoltaic gadgets with bulk heterojunctions composed of blends of donor-acceptor copolymers as well as fullerene derivative, phenyl C61-butyric acid methyl ester have shown varying power conversion efficiencies dictated by side chain nature as well as the behavior of the donor units. Blend film quality is an important parameter for photovoltaic device performance. Raman spectroscopy appears as a useful tool for monitoring polymer order, and has enabled the study of chemical composition of blends normally used in bulk-heterojunction structures.

Researchers led by Věra Cimrová from the Academy of Sciences of the Czech Republic studied thin films with three low band gap donor-acceptor copolymers and their blends of varying ratios with the soluble fullerene derivative as heterojunction solar cell layers. They used UV-vis absorption spectroscopy as well as Raman microspectroscopy for the study. In particular, they used Raman measurements with varying excitation wavelengths to differentiate the low-wavelength photoluminescence of the two components in the blends. Their work is published in peer-reviewed journal, Organic Electronics.

 The authors prepared three donor-acceptor copolymers, CDTF, CDTDOP, and CDTDP, composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5[Symbol],5[Symbol]-diyl as an electron-acceptor structural unit and electron-donor structural units 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyloxy-1,4-phenylene and 2,5-didodecyl-1,4-phenylene, respectively. Recent research works of authors have shown that copolymers composed of electron- accepting thienothiadiazole-based components are promising for photovoltaic applications owing to their high electron affinity, low band gap and reversible redox attributes.They prepared polymer and polymers-blend thin films through spin coating onto fused silica substrates from 1,2-dichlorobenzene solutions.

The researchers observed that the maxima of the thin film spectra were red shifted as opposed to the solution spectra indicating strong intermolecular interactions in the solid state. The long-wavelength intra-chain charge transfer absorption of the CDTDOP was observed to be located at lower wavelengths compared to the other two copolymers owing to strong electron donor character of the 2,5-didodecyloxy-1,4-phenylene components as well as stronger aggregation that was also evident in its solution absorption.

The fullerene derivative absorbed in the UV spectral region, and for this reason, in the blend films, the UV absorption was observed to increase with the increase in the fullerene derivative. The absorption in the visible region decreased as compared with the absorption of the thin films. However, there were no shifts observed in the maxima positions of the CDTF and CDTDOP copolymers. This was an indication that blending the copolymers with the fullerene derivative did not hinder planarization of the main chain.

From the Raman spectroscopic studies of the thin films of the three copolymers and their blends with fullerene derivative, the authors observed different behavior in the CDTDP blends from that of CDTF and CDTDOP blends.

The authors found separate regions (in-homogeneities) in the blend films of the copolymers with fullerene. Photoluminescence of the two components determined in the Raman spectra indicated that the in-homogeneities were composed of more fullerene and a small copolymer amount as opposed to homogeneous blend.

The results of their study showed that Raman microscopic and optical absorption approaches are important in getting additional information about polymer molecule planarity as well as interactions in the polymer thin films.

About The Author

Věra Cimrová received her M.Sc. in Biophysics and Chemical Physics (1984) at Faculty of Mathematics and Physics, Charles University in Prague, her Ph.D. in Physical Chemistry (1991) at the Institute of Macromolecular Chemistry (IMC), The Czechoslovak (now Czech) Academy of Sciences, and habilitation in Physics of Molecular and Biological Structures (2013) at Charles University in Prague. In the 1993-1995 and 1998 stayed abroad as a visiting researcher at the Max-Planck-Institute for Polymer Research, in Mainz, Germany. Since 1984 she is working at IMC. Currently she is a head researcher in the IMC (deputy head of Department of polymers for optoelectronics and photonics), and associate professor at Faculty of Mathematics and Physics Charles University in Prague.

She has published more than 100 original papers in impacted international journals and monographs, as well as numerous invited conference lectures and contributions. She was Editor of two Special Issues of Macromolecular Symposia, and also chairperson of 3 international conferences in the series of Prague Meetings on Macromolecules.

Her research interest includes photophysical, electrical, photoelectrical and electrochemical properties of organic materials, polymers and polymer blends, organic photovoltaics and electroluminescence, design and research of new polymers and polymer systems for photonics and electronics.

About The Author

Zuzana Morávková received her M.Sc. in Physics of condensed matter and materials in 2009 and her Ph.D. in 2013 in Polymer physics, both at the Faculty of Mathematics and Physics, Charles University in Prague She is now working as a research associate in the Institute of Macromolecular Chemistry of Czech Academy of Sciences, Department of Vibrational Spectroscopy. She has published 32 papers in impacted journals and 19 conference contributions.

Her research interests cover conducting polymers, related oligomers, and carbon materials, studied by vibrational spectroscopy, microspectroscopy, and spectroelectrochemistry, in various forms such as thin films, colloids, solutions, or composite materials. Within her one year her post-doctoral stay (2015–2016) at the Centre of Spectroelectrochemistry, Leibniz Institute of Solid State and Materials Research, Dresden, Germany, she worked on vibrational spectroelectrochemistry of conducting polymers.

About The Author

Veronika Pokorná received her M.Sc. in Technology of polymer synthesis and processing (1990) at Department of Polymers, University of Chemistry and Technology in Prague and her PhD. in Macromolecular Chemistry (1995) at the Institute of Macromolecular Chemistry, The Czech Academy of Sciences in Prague. She is now working as a research associate in the Institute of Macromolecular Chemistry (Department of polymers for optoelectronics and photonics). She has published 29 papers in reputed reviewed and impacted journals. Her research interest includes syntheses and characterization of polymers.

About The Author

Drahomír Výprachtický received his M.Sc. in Technology of macromolecular materials (1980) and his PhD. in Macromolecular chemistry (1986) at Department of Polymers, University of Chemistry and Technology in Prague. He is now working as head researcher in the Institute of Macromolecular Chemistry, The Czech Academy of Sciences, Prague (Department of polymers for optoelectronics and photonics).

He has published more than 70 papers in reputed impacted journals and more than 100 conference contributions. Within 1991-1997 he spent 5 years as research associate at Polytechnic University, Brooklyn, New York (now NYU).

His research interest includes syntheses and characterization of polymers with fluorescence labels, syntheses of polymers for organic photonics, syntheses of polymer ligands for lanthanides, syntheses of conjugated polymers or application of steady-state and time-resolved fluorescence spectroscopy and nonradiative energy transfer in polymer science.

Reference

Věra Cimrová, Zuzana Morávková, Veronika Pokorná, Drahomír Výprachtický. Investigation of donor-acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman micro-spectroscopy. Organic Electronics, volume 47 (2017), pages 194-199.

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Renewable Energy Global Innovations features: Alternating precursor layer deposition for highly stable perovskite films towards efficient solar cells using vacuum deposition

Significance Statement

Hybrid organic-inorganic perovskite semiconducting materials developed for field-effect transistors have received a significant research interest owing to their superior optical absorption, tolerances to defects, and charge-carrier diffusion. The power conversion efficiency of these perovskite solar cells have been recorded to rise over the past few years to about 20%. Notwithstanding, fabrication based on the planer architecture has become popular owing to its low fabrication temperature and compatibility with a wide range of substrates.

Unfortunately, incomplete and inhomogeneous coverage of the perovskite films have been identified as the main adversaries limiting device performance. Therefore, there has been extensive research aiming at improving the morphology of the perovskite films by modifying interface layers, precursor solution concentration, and by optimizing annealing time and temperature. Developing new methods for perovskite film deposition such as vacuum deposition and solution deposition have also been explored.

Vacuum processes for thermal co-deposition and sequential deposition of lead chloride and CH₃NH₃I have been focused most, as they are considered efficient for preparing films with excellent uniformity as well as high surface coverage. However, the diffusion of CH₃NH₃I into the vacuum chamber and poor uniformity of the solar cells have limited the number of successful reports implementing vacuum deposition methods.

Researchers led by Professor Shengzhong (Frank) Liu and Dr. Dong Yang from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, achieved a uniform layer-by-layer vacuum deposition by alternating lead chloride and CH₃NH₃I precursor layers. The method allowed the authors to relax the stringent deposition control and monitoring measures, and realized excellent uniformity in film morphology, smoothness, and surface coverage. They recorded a solar cell efficiency of 16.03%, the highest efficiency reported at the time of its publication in peer-reviewed journals, Journal of Materials Chemistry A and Advanced Materials. It is worthwhile to point out that the group has increased the efficiency to as high as 18.34%, remaining their lead in this category.

The authors prepared samples of lead chloride film with different thickness. This was in a bid to investigate the thickness effect of lead chloride layer. They then ensured sufficient amount of CH₃NH₃I and removed the excess amount through post-annealing.

They achieved uniform and full coverage perovskite film when the lead chloride film thickness was smaller than 100nm. When it was more than 100nm, voids appeared in the films because of unreacted lead chloride, and the density and size of the voids increased with the lead chloride film thickness. Therefore, they take turns to deposite 100 nm of lead chloride and then suitable thickness of CH₃NH₃I precursor layers to obtain high quality perovskite films. This proposed method offered very high solar cell efficiency with suppressed performance variation. The power conversion efficiency of these devices implementing the present development reached approximately 16.03%, and the power conversion efficiency of the large active area device reaches 13.87%.

The method proposed in this study comes with a number of advantages. First, it relaxes the complicated operations concerning deposition rates control and monitoring. It provides high quality perovskite films with smaller roughness, uniform morphology, full surface coverage, and crystalline phases of higher purity. The controlled deposition environment makes possible the manufacture of dense and pure perovskite films, which results in efficient moisture protection yielding excellent device stability.

The proposed method in the study provided an efficient approach for the fabrication of large area perovskite solar cells. It appears promising for application in the manufacture of large area perovskite solar cells.

Reference

Dong Yang, Zhou Yang, Wei Qin, Yuliang Zhang, Shengzhong (Frank) Liu and Can Li. Alternating precursor layer deposition for highly stable perovskite films towards efficient solar cells using vacuum deposition. J. Mater. Chem. A, 2015, 3, 9401–9405.

Yang; R. Yang; X. Ren; X. Zhu; Z. Yang; C. Li; S. F. Liu, Hysteresis-Suppressed High-Efficiency Flexible Perovskite Solar Cells Using Solid-State Ionic-Liquids for Effective Electron Transport. Adv Mater 2016, 28 (26), 5206-13.

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