Event time
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 [1].
Passivating-contact technology aims at minimizing the interface-gap state density; a prime example 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 [2]. 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 as 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 OC s, FFs, as well as to quench hysteresis in their current-voltage characteristics [1,5].
Vergaderzaal 1.2 James Maxwell, iGent Campus Ardoyen
Passivating contacts for silicon and perovskite solar cells
External speaker
Prof. Stefaan De Wolf
Event type
Prof. Stefaan De Wolf