Effects of grain boundaries in polycrystalline silicon thin-film solar cells based on the two-dimensional model

The two-dimensional calculation for polycrystalline Si thin-film solar cells was performed. Two models, “stripe structure” and “columnar structure”, were applied for the solar cells composed of grains. For the stripe structure of 20 μm active layer, to keep the efficiency distribution within 5% for individual unit cells, the stripe width requires more than 500 μm for a minority-carrier lifetime of 1×10−5 s and recombination velocity at the grain boundary of 1×104 cm/s. For the columnar structure of 10 μm active layer, to keep the efficiency independent of grain size, the recombination velocity should be kept less than 1×103 cm/s. If imperfect passivation of a grain boundary is given, the way of decreasing carrier concentration to 1014 cm−3 in an active layer may realize insusceptible output. An appropriate device modeling is needed in the two-dimensional calculation for polycrystalline Si thin films with an electron diffusion length close to or more than grain size and with a poorly passivated grain boundary. The calculated efficiency using bad model will include an error of about 1% as overestimation.