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KSC Seminar: Passivating Contacts for Silicon and Perovskite Solar Cells

Speaker: Prof. Stefaan De Wolf

KAUST Solar Center

Abstract: 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 W_OC = E_g/q – V_OC (E_q is the bandgap, q the elementary charge and V_OC is the open-circuit voltage). Quite generally, a high W_OC also results in a poor FF.

To fabricate functional contacts, two strategies are possible: Classically, for silicon solar cells, heavy surface doping is applied, which narrows the Schottky barrier, enabling majority-carrier tunneling to the contacting material, but resulting in undesired high-doping phenomena such as Auger recombination and free-carrier absorption. Alternatively, passivating-contact technology aims at minimizing the interface-gap state density. For silicon, using density-functional theory calculations, we find that a good passivating contact needs chemical passivation and a displaced outer electrode, else metal-induced gap states are present [2]. A prime example of passivation-contact technology are silicon heterojunction solar cells, 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 MoO_x 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 TiO_x and the metal nitrides TaN_x and TiN_x [3,4]

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 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 V_OCs, FFs, as well as to quench hysteresis in their current-voltage characteristics [1]. 

Biography: Stefaan De Wolf has been working on silicon solar cells and materials at IMEC in Belgium and AIST in Japan. In 2008, he joined PV-lab at EPFL in Switzerland, as a team leader for its activities on high-efficiency silicon solar cells. Since September 2016, he is an Associate Professor at KAUST in Saudi Arabia working on high-efficiency photovoltaics for hot climates.