Bifacial silicon heterojunction solar cell with deposited back surface field

Silicon heterojunction cells (SHJ) using crystalline silicon wafers and deposited heterojunction thin film emitters are interesting from an economical, technological, and scientific point of view. Modules using such cells (so called HIT cells) are commercially produced by Sanyo Electric Co., using single crystal wafers. Recently reported alternatives still comprise a high-temperature diffused back surface field (BSF). In order to provide a low cost alternative, our goal is to develop SHJ using multicrystalline silicon wafers with both a deposited emitter and a deposited BSF. The present approach truly allows the development of a cheap, low temperature, all-deposited alternative for the HIT cell. We made a bifacial silicon heterojunction solar cell, with an emitter consisting of a 7 nm intrinsic a-Si:H layer and a 15 nm n-type mu c-Si:H layer. The back surface field is formed by a 30 nm p(++) mu c-Si:H layer. In order to achieve a functional deposited BSF the thin p(++)-layer has to have higher effective dopant concentration than the substrate. We used a 375 mu m thick FZ wafer with a resistance of 1 Omega cm with an activation energy of 0.2 eV. The mu c-Si:H p(++)-layer has an activation energy of E-a = 0.082 eV. To detect the operation of the BSF the cell was made bifacial. The cell has an efficiency of 14.87 %, V-oc, = 571.4 mV and J(sc) = 33.3 mA/cm(2), R-s = 1.2 Omega cm(2) and R-p - 1.8 k Omega cm(2). The cell, illuminated from the back, shows a V-oc of 0.1 V and J(sc) of 7 in A/cm(2). Reference bifacial cells without BSF show no cell behavior illuminated from the rear side. External quantum efficiency (EQE) measurements with illumination at the rear side of the bifacial heterojunction cell with BSF show a quantum efficiency value of 0.17 in the range 500-800 nm, while the reference cell without BSF shows zero quantum efficiency in this range. These results show evidence for the feasibility of a truly functioning deposited BSF combined with a SHJ with deposited emitter.