Sensitivity of the dark spectral response of thin film silicon based tandem solar cells on the defective regions in the intrinsic layers

The spectral response (SR) measured without bias light (dark SR) normally matches at each wavelength the lowest of the two sub cell responses under bias light (light SR), showing nearly a triangular shape. However, in some tandem structures a departure of this typical triangular shape has been observed, indicating the presence of an apparent current leakage. The dark SR shows some additional response coming from one of the sub cells and the open circuit voltage (Voc) of the tandem cell is observed to decrease. In this paper we investigate with computer modeling the causes behind the appearance of dark SR curves departing from the characteristic triangular shape by studying various amorphous and microcrystalline silicon based tandem structures. Our simulations indicate that the presence of thin defective regions perpendicular to the charge transport direction inside the intrinsic layers, especially near the p/i interface, can give rise to anomalous dark SR and to the deterioration of the tandem cell performance. Moreover, the dark SR is more sensitive to the presence of defects in the front region of the intrinsic layer of the bottom sub cell.