The electrical performance of thinn +-window layers ina-Si: H solar cells

The electrical properties ofn ⁺-window layers inp-i-n a-Si:H solar cells were characterised as a function ofn ⁺-layer thickness, , by measuring firstly the activation energyE _a of the dark conductivity and secondly the built-in potentialV _bi of the cells.E _a was found to increase with decreasing attaining values as high as 0.8 eV for 5nm; bulk values, e.g.E _a . 2eV in the amorphous andE _a<0.01 eV in the microcrystalline case, were only observed for >20nm and for >200nm, respectively. In contrast,V _bi did not depend on at all and was further found to be consistent with expectations based on the Fermi level positions in bulkn ⁺ andp ⁺-material. As a consequenceE _a in very thin films can no longer be considered as a measure of (E _C –E _F), the distance of the Fermi level from the conduction band edge. The apparent inconsistency inherent to theE _a and theV _bi results can be resolved by assuming that the deposition of then ⁺-material proceeds via the growth and coalescence of small islands.