Renewable Energy Global Innovations features: Fabrication of air-stable, large-area, PCDTBT:PC70BM polymer solar cell modules using a custom built slot-die coater

Significance Statement

The quest for understanding the operation of polymer solar cells has made it possible for the development of conjugated polymers. This has led to the fabrication of devices with over 10% power conversion efficiency. Longevity stability problems, common for typical polymer solar cells where the cathode is susceptible to water and oxygen degradation, has been curtailed through device structure inversion. Above all, low band gap polymers including PCDTBT have exhibited potential photochemical stability in laboratory and real world operational conditions, posting 7-15 years useful life.

However, production of large-scale polymer solar cells using slot-die coating, inkjet printing, rotary screen-printing, and spray coating methods in the laboratory is less practical considering the high cost of purchase and repeatable operation of the equipment needed. Therefore, most researchers rely on the spin-coating method, which only produces polymer solar cell devices of up to 1cm², and is unsuitable for large-scale production in view of poor material utilization and issues with scaling up. The use of spin coating also creates discrepancies in industrial and research laboratory results owing to significant non-identical effects in the deposition process on device performance.

To address this, researchers led by Professor S. Ravi P. Silva at the University of Surrey described in their work, the adaptation of an entry level paint applicator to function as a functional slot-die coater. This allowed for the attachment of various coating heads for flexible coating. They described the design of slot-die head and its supporting elements. The slot-die coater was then used to produce polymer solar cell modules with over 3% power conversion efficiency determined over 35cm² photoactive area. Their work is published in Solar Energy Materials & Solar Cells.

The authors fabricated polymer solar cell devices with a structure of glass -ITO/ZnO/PCDTBT: PC70BM/MoO3/Al. The electron transport layer solution was then slot-die coated onto the glass indium tin-coated glass substrates. The coating process of the PCDTBT: PC₇₀BM layer was carefully undertaken at ambient conditions on top of the zinc oxide layers.

For modules fabrication, the viscosity of the active layer and subsequently the resulting layer thickness was changed by altering the processing temperature and applying three varying donor acceptor concentrations. The authors made two modules from the first two concentrations and one module from the latter.

The modified sheet-to-sheet slot-die coater was used to produce polymer solar cell modules with an active area above 35cm² and over 3% power conversion efficiency. Optimization of the processing parameters led to homogeneous layers that were characterized by light beam induced current, micro Raman mapping, and micro photoluminescence of the modules. The authors also investigated the behavior of the module at various annealing temperatures as well as its stability during operation, and provide supplementary information on the fabrication of the modified slot die head.

The outcomes of their study provide a route for fabrication of large-scale slot-die coated modules with viable polymer solar cells. This infers that large-scale polymer solar cells can be made with simple coating equipment, therefore, avoiding the high cost of purchase and operation.

The reported work was completed partly in collaboration with European partners through a European Union 7^th Framework Programme (FP7) project entitled Smartonics, and partly in collaboration with the United Kingdom’s national measurement institute, The National Physical Laboratory (NPL). The collaboration with NPL has recently been further supported by a grant from the European Union Horizon 2020 Framework Programme to which will create an effective Open Innovation Environment (OIE) for printed electronics, entitled CORNET, combining world-class expert academic, research and industrial entities from 6 countries, as part of a national metrology effort to standardize printable electronics in an organic electronics market which is predicted to grow to $48B in 2019 (IDTechEx).

Prof Silva indicated that: “Consortia such as the FP7 Smartonics programme have contributed much in setting up a framework to establish printable electronics within the European Union. We hope research such as this, and the recently funded CORNET project, will help firmly establish plastic electronics within the UK and Europe, and provide a resource for researchers worldwide.”

Images produced by Dr Dimitar Kutsarov (ATI, University of Surrey)

About The Author

Professor Ravi Silva is the Director of the University of Surrey’s multi-disciplinary Advanced Technology Institute (ATI, http://ift.tt/2xtEbOq) incorporating over 150 researchers. The ATI is one of University’s world-leading research centres, bringing together researchers with an international outlook in Quantum Information, Nanotechnology, Energy and Advanced Materials.

His research interest encompass a wide range of activities, with nanotechnology and renewables being two underlying themes which thread through a plethora of fields. Within nanotechnology, Ravi is active in carbon nanomaterials (including carbon nanotubes, graphene, diamond like carbon and carbon fibre reinforced plastics), transistor designs & simulations, source gate transistors, nano-biotechnology, large area electronics, and electronic and photonic devices. While within renewables, fields include solar cells, organic photovoltaics, organic light emitting diodes, energy materials for thermo- and tribo-electrics, water technology. In particular, he has an established interest in the development of next generation large area photovoltaics and the production of carbon nanomaterials for advanced manufacturable technologies, as well as the technology required to develop both fields.

The ATI has in particular been recognised for its pioneering work in these fields. For example, Prof. Silva has defined a new generation of materials that is enabling the development of next generation (4G) solar cells. Engineered using nanotechnology, the 4^th generation (4G) solar cell materials are a hybrid of organic and inorganic materials. These devices maximise the harvesting of solar radiation, offering a more efficient, cost-effective solution than existing solar cells. Additionally, researchers at the ATI have used a graphene production technique known as nanotexturing, which involves growing graphene around a textured metallic surface, to create ultra-thin graphene sheets designed to more effectively capture light. Just one atom thick, graphene is very strong but traditionally inefficient at light absorption. To combat this, the team used the nano-patterning to localise light into the narrow spaces between the textured surface structures, enhancing the amount of light absorbed by the material by about 90%, analogous to moths’ eyes which have microscopic patterning that allows them to see in the dimmest conditions.

Ravi passionately believes in developing enabling technologies relevant to major societal challenges, and over 20 years has contributed to the UK knowledge economy in the materials and manufacturing sectors by training more than 50 PhD and 65 postdoctoral staff.

Professor Silva’s work was recently recognised by the award of the IET’s JJ Thomson medal (2014) for major and distinguished contributions in electronics, citing his work on large area [carbon] nanotube-organic solar cells, and the Institute of Materials, Minerals and Mining’s Platinum Medal (2015) for the advancement and promotion of carbon nanomaterials for technology applications.

Reference

Dimitar I. Kutsarov, Edward New, Francesco Bausi, Alina Zoladek-Lemanczyk, Fernando, A. Castro, S. Ravi P. Silva. Fabrication of air-stable, large-area, PCDTBT:PC70BM polymer solar cell modules using a custom built slot-die coater. Solar Energy Materials & Solar Cells, volume 161 (2017), pages 388–396.

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