Black phosphorus induced photo-doping for high-performance organic-silicon heterojunction photovoltaics

In conventional crystalline silicon (Si) homojunction solar cells, a strategy of doping by transporting phosphorus or boron impurities into Si is commonly used to build Ohmic contacts at rear electrodes. However, this technique involves an energy intensive, high temperature (similar to 800 degrees C) process and toxic doping materials. Black phosphorus (BP) is a two-dimensional, narrow bandgap semiconductor with high carrier mobility that exhibits broad light harvesting properties. Here, we place BP: zinc oxide (ZnO) composite films between Si and aluminum (Al) to improve their contact. Once the BP harvests photons with energies below 1.1 eV from the crystalline Si, the ZnO carrier concentration increases dramatically due to charge injection. This photo-induced doping results in a high carrier concentration in the ZnO film, mimicking the modulated doping technique used in semiconductor heterojunctions. We show that photo-induced carriers dramatically increase the conductivities of the BP-modified ZnO films, thus reducing the contact resistance between Si and Al. A photovoltaic power conversion efficiency of 15.2% is achieved in organic-Si heterojunction solar cells that use a ZnO:BP layer. These findings demonstrate an effective way of improving Si/metal contact via a simple, low temperature process.