By Katherine Hooper 22 June 2015
Printed Photovoltaics: From Materials to Manufacture took place on 21-22 April against the stunning backdrop of Swansea. This two day workshop was the first conference organised by the Sêr Solar team and included an impressive variety of invited speakers (both academic and industrial), as well as poster sessions and early career researcher presentations. The theme highlighted the importance of scale up for emerging solution processable photovoltaics and aimed to address the issues and challenges associated with it.
The conference opened with Michael Grätzel (EPFL) who gave a whirlwind overview of the background and latest research for dye-sensitised solar cells (DSCs) and perovskite solar cells (PSCs). He also presented data for a 21.5% tandem device based on a DSC top cell and a perovskite bottom cell. The discussion following his talk focused on the question of if a scaffold in PSCs is necessary, which architecture would be most stable, and fundamentally which has the potential for the highest efficiency. Of course no consensus was reached and it remains a hot debate within the perovskite community.
The rest of the talks focused on fundamental material behaviour, stability, scaling, and the challenges of implementing photovoltaics into the market. Henry Snaith (University of Oxford) presented photoluminescence studies of perovskites and associated the decay rates to either excitionic or free carrier recombination. He also reiterated the importance of homogeneity in perovskite films and how crucially processing influences this. With regards to stability Henry mentioned the higher thermal stability of formamidinium based perovskites, encapsulation methods, and full sun light soaking tests. Saif Haque (Imperial College London) showed some light and oxygen degradation studies of perovskite based on different architectures (TiO2 scaffold, Al2O3 scaffold and planar heterojunction). The aim was to investigate if the degradation pathway is affected by the different device structures which each have different interfacial charge transfer processes. Brian O’Regan (Swansea University) stressed the importance of stability studies and adhering to certain standards. Throwaway statements about the stability of devices with no detailed evidence or methods in a paper are misleading and it’s important to gain deep understanding as to why degradation occurs so we can improve stability. He discussed the stability of perovskite, dye-sensitised and organic solar cells with accelerated light degradation data obtained by soaking at intensities equivalent to 40 suns, citing this as a faster and cheaper way to acquire long term stability information.
Aldo Di Carlo (University of Rome) began the presentations on scaling with his group’s work on PSCs. He showed a module fabricated using a two step deposition of perovskite on mesoporous TiO2 with P3HT as the hole transport material (to avoid using the extremely expensive spiro-OMeTAD). To optimise this involved a lot of work on the patterning of the device layers. For example the compact TiO2 hole blocking layer is very resistive compared to FTO and it is important that the gold electrode makes direct contact with the FTO to avoid significant charge losses. He also showed modules of 10 cm2 and 100 cm2 in area with efficiencies of 13% and 9% respectively. Hans-Joachim Egelhaaf (ZAE Bayern) presented work on organic solar cell modules with similar emphasis on the importance of module design, low resistance interconnection, thickness optimisation, and film patterning. He also brought up a point of significant importance but is often overlooked - “Development of friendly solvent formulations is critical; often the coating line output isn’t limited by the speed of the process but by solvent vapour exposure legal limits.”
Trystan Watson (Swansea University) showcased the perovskite scaling work from SPECIFIC which addressed the choice of deposition techniques, process simplification, heating bottlenecks, and the opaque evaporated cathode. In order to scale up it is extremely important to keep processes as simple as possible to limit yield losses and increase the tolerance window for each step. He showed methods for significantly reducing the time and energy required for the crystallisation of the perovskite layer and work on a transparent adhesive cathode that even enabled devices to be built on metallic substrates.
Mark Spratt (G24 Power) gave an engaging perspective on the challenges for printed photovoltaics from the experiences of a start up company. The most important considerations are the overall yield (with losses accumulated from each individual manufacturing step), reducing costs (in accordance with how low they will need to be to compete in the predicted future market), and technology/product development. At the end of the conference Jenny Nelson (Imperial College London) gave a talk on the role of solar in the non carbon energy sector. Cost and life cycle analysis is absolutely crucial for our research to identify the areas we need to develop so that the technology can be viable in the real world. The need for storage of generated energy is another key component that must be addressed. She rounded things up with the question, can printed PV contribute? Increasing the module lifetime is critical, 10 years is no good to compete for building integration. We also need to eliminate the bottlenecks of cost and embedded energy. These areas are of significant research interest if the scaling of emerging PV is possible.
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