Temperature and bias dependence of hydrogenated amorphous silicon – crystalline silicon heterojunction capacitance: the link to band bending and band offsets

Abstract

The temperature dependence of the capacitance–voltage data (C–V–T) of very high efficiency silicon heterojunction solar cells in a wide temperature range, up to 400 K, is analyzed. We show that the temperature dependence of the capacitance exhibits an anomalously large increase with temperature that cannot be explained under the usual depletion approximation. Using the complete analytical calculation of the capacitance, where the contribution of both types of carriers is taken into account, this large increase of capacitance with temperature of p-type hydrogenated amorphous silicon – n-type crystalline silicon ((p) a-Si:H – (n) c-Si) heterojunctions observed experimentally is reproduced. This increase of the capacitance is due to a strong inversion layer at the c-Si surface, which is promoted as the temperature increases. Further we show that the temperature dependence of the 1/C2 versus applied reverse voltage (Va) plot is as well strongly affected by the strong inversion layer at the c-Si surface. Consequently, the intercept of the linear extrapolation of 1/C2 versus Va with the voltage axis (Vint) differs significantly from the total diffusion potential predicted by depletion capacitance theory. These underestimated values of the total diffusion potential can consequently lead to erroneous estimation of the band offsets. The temperature dependence of Vint is considerably enhanced for the case of the full analytical calculation when compared with the depletion approximation approach. These data, obtained directly on the final solar cell device, thus confirm the existence of a surface strong inversion layer that was previously revealed by measurements performed by other techniques on dedicated or precursor devices, allowing one to get information on the band diagram and the heterointerface.