The silicon heterojunction (SHJ) concept is definitely the most successful solution to achieve high-efficiency c-Si solar cells at low temperature processing. Namely, Panasonic has demonstrated the possibility to achieve record efficiencies of 25.6% on very thin n-type c-Si substrates. Attending to the band offsets at the a-Si:H/c-Si interface, n-type wafers would be fundamentally more suited to SHJ devices. Nevertheless, conversion efficiencies above 21% have been also demonstrated on p-type wafers. This result makes research on p-type SHJ solar cells also very interesting, since these substrates are more extended in the industry for the fabrication of conventional (diffused-emitter) solar cells. In this work, we will focus on doping-free alternatives for the back contact of SHJ solar cells. This rear contact consists typically in a heterojunction structure with the same doping-type of the base. This structure can provide a high quality passivated back contact, but implies the use of both doping gas precursors. For that reason, duplicated deposition chambers or time consuming conditioning treatments are required to avoid an eventual cross-contamination of the deposited layers. In this regard, an advantage of p-type substrates is the possibility to use a pattern of locally-diffused aluminum point contacts at the rear side in combination with an adequate passivation layer. In our group, we have found that an excellent surface passivation can be obtained with intrinsic a-SiC:H layers. This alternative is technologically relevant, as it allows eliminating one of the doping gases from the fabrication process. The pattern of locally-diffused aluminum point contacts can be obtained by applying the laser-firing technique. A further simplification of the fabrication process could be obtained by eliminating the laser-firing step. Looking for a doping-free alternative, we are studying the use of hole selective contacts based on transition metal oxides such as molybdenum oxide or tungsten oxide. These materials have already demonstrated excellent hole injection and extraction properties in organic semiconductor devices. First hybrid devices using molybdenum oxide as the emitter of n-type SHJ solar cells were reported very recently. In this work, we show that these materials can be also incorporated on the rear side of p-type wafers to obtain full-area passivated back contacts at low processing temperatures. Preliminary devices have already reached conversion efficiencies of about 15%.
Dr. Cristobal Voz obtained his Ph.D. degree in Physics from the University of Barcelona in 2001 in the area of fabrication and physical modeling of thin-film microcrystalline silicon solar cells. Later on, he joined the Polytechnic University of Catalonia (UPC), where he he is working as a Associate Professor since 2003 at the Department of Electronic Engineering. His main research interests are the physics and technology of semiconductor devices, with special focus on high-efficiency silicon heterojunction solar cells.