By Prof. Stefaan De Wolf, KAUST Solar Center, King Abdullah University of Science and Technology, Saudi Arabia.
In silicon and perovskite solar cells, the presence of surface states at their electrical contacts is increasingly recognized as a key factor limiting device performance. In this presentation, first I discuss how such surface states result in Fermi-level pinning (FLP) and surface recombination. FLP impairs the ability of contact materials through their work function to induce an anticipated band bending in the photovoltaic absorber underneath, affecting charge collection. On the other hand, surface recombination inflates the open-circuit deficit WOC = Eg/q – VOC (Eq is the bandgap, q the elementary charge and VOC is the open-circuit voltage). Quite generally, for all photovoltaic technologies, a high WOC also results in a poor FF, as illustrated in the figure.
Passivating-contact technology aims at minimizing the interface-gap state density; a prime example is silicon heterojunction solar cell technology, using very thin amorphous silicon films for contact formation and passivation. Such devices have demonstrated record-high voltages and power-conversion efficiencies. Nevertheless, the used very thin amorphous silicon films may result in some parasitic absorption losses. Therefore, an important area of research in recent years has been the development of strategies to improve current generation in these devices. Essentially, three options are available: the use of more transparent contacting materials, placing all contacts at the back of the device, or develop tandem technologies to harvest more effectively the blue part of the solar spectrum. A prominent example of the first strategy is the use of metal oxides to replace the (doped) amorphous silicon films, such MoOx replacing p-type amorphous silicon. In the context of such ‘doping-free’ contact materials, recently several attractive materials have been proposed for electron collection as well such as TiOx and the metal nitrides TaNx and TiNx [2,3]
Regarding silicon-based tandems, the combination with a perovskite top cell has been identified to be particularly promising to enable ultra-high efficiency photovoltaics at an affordable cost. I will finish my presentation with a discussion how passivating contacts are also of increasing interest in perovskite solar cells to enable higher VOCs, FFs, as well as to quench hysteresis in their current-voltage characteristics [1,4].
Stefaan De Wolf received his Ph.D. degree in 2005 from the Katholieke Universiteit Leuven in Belgium, during which time he was also affiliated with imec in Belgium, working on crystalline silicon solar cells. From 2005 to 2008, he was with the National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, investigating silicon heterojunction structures and devices. In 2008, he joined the Photovoltaics and Thin-Film Electronics Laboratory, Ecole Polytechnique Federale de Lausanne (EPFL), Neuchatel, Switzerland, as a team leader for its activities on high-efficiency silicon solar cells. Since September 2016 he is an associate professor at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, investigating high-efficiency silicon and perovskite solar cells.
 E. Aydin, M. De Bastiani, and S. De Wolf, Defect and Contact Passivation for Perovskite Solar Cells, Adv. Mater. 31, 1900428 (2019).
 X. Yang, E. Aydin, H. Xu, J. Kang, M.N. Hedhili, W. Liu, Y. Wan, C. Samundsett, A. Cuevas, and S. De Wolf, Tantalum Nitride Electron-Selective Contact for Silicon Solar Cells, Adv. Energy Mater. 8, 1800608 (2018).
 X. Yang, W. Liu, M. De Bastiani, H. Xu, J. Kang, E. Aydin, T. Allen, L. Xu, H. Dang, E. Alhabshi, K. Kotsovos, A. Alsaggaf, I. Gereige, Y. Wan, J. Peng, C. Samundsett, A. Cuevas, and S. De Wolf, Dual-functional, electron-conductive, hole-blocking titanium nitride contact for efficient silicon solar cells, Joule 3, 1314 (2019).
 J. Peng, J.I. Khan, W. Liu, E. Ugur, T. Duong, Y. Wu, H. Shen, K. Wang, H. Dang, E. Aydin, X. Yang, Y. Wan, K.J. Weber, K.R. Catchpole, F. Laquai, S. De Wolf, and T.P. White, A Universal Double-Side Passivation for High Open-Circuit Voltage in Perovskite Solar Cells: Role of Carbonyl groups in Poly(methyl methacrylate), Adv. Energy Mater. 8, 1801208 (2018).