Thin film photovoltaic modules based on the CIGS absorber are already in the market, with record power conversion efficiencies of 16.5%. Both on module and on cell level, CIGS efficiency is close to crystalline Silicon (c-Si) technology. On cell level, record efficiencies of 22.6% for CIGS and 25.3% for c-Si were marked. Compared to c-Si, CIGS has many advantages, such as limited temperature sensitivity, better low-light performance and low shading sensitivity, which are attractive for various climatic conditions and for deployment in the built environment. Also, the cost of CIGS modules is on par with c-Si, but at much lower cumulative production volume, which gives margin for drastic cost-reduction along the CIGS learning curve. CIGS technology is also compatible with low-cost flexible substrates, such as polyimide. This can further increase the throughput and open opportunities in new market areas, such as the automotive industry. The highest efficiencies of flexible CIGS-modules based on polyimide substrates are currently at about 14%.
Sunplugged pursues CIGS, R2R development on polyimide substrate. The company’s current best efficiency is 11% on cell level, steadily increasing with the improvement of the absorber, which is processed with a unique hybrid sputtering/co-evaporation approach. Also, crucial for the cell efficiency is the heterojunction properties between the p-CIGS and the n-type “buffer” layer. Cadmium sulphide (CdS) is predominantly used as buffer, deposited in a chemical bath. The success of CdS is due to the specific interface chemistry that it induces, leading to large open circuit voltages and fill factors. On the other hand, CdS induces optical losses in the UV due to its low bandgap (affecting the extracted current) and Cd is classified as highly toxic.
In parallel, Sunplugged seeks to replace the CBD (chemical bath deposition) with sputter-deposition, thus keeping the whole cell fabrication in vacuum conditions. This will reduce the contamination of the absorber and spare the extra cost related to the management of Cd-contaminated CBD waste. At the same time, also the buffer thickness homogeneity is expected to improve with vacuum deposition.
Inspire will address the issues aforementioned by developing sputtered, Cd-free buffers, based on Zn(O,S), ZnMgO and TiO2, materials which match the energetics of the CIGS. Additionally, Zn(O,S) and ZnMgO offer energy level tunability through their composition, so that they can more accurately match different compositions of the CIGS absorber. Zn(O,S) and ZnMgO are widely reported, but the results scatter through various deposition techniques. Literature shows that these buffers have the potential to achieve efficiencies as high with CdS. On the other hand, there are no reports on sputtered TiO2 buffer, but a single report on ALD-TiO2, showing the high potential of this buffer.
The success of the project in developing a non-toxic, Cd-free, sputtered buffer layer through meticulous thin-film processing and heterojunction engineering, will enable Sunplugged to adapt their production line to a roll-to-roll, entirely vacuum-based production line of flexible CIGS solar cells, with a targeted efficiency above 12%.
Projekt manager: Dr Theodoros Dimopoulos