Conduction-Band-Offset Rule Governing J-V Distortion in CdS/CI(G)S Solar Cells

A type-I (“spike”) conduction-band offset (CBO) greater than a few tenths of an eV at the n/p interface of a solar cell can lead to significant distortion of the current-voltage (J-V) curve. Such distortion has been observed in CdS/CIS cells, low-gallium CdS/CIGS cells, and CIGS cells with alternative windows that increase the CBO. The basic feature is reduced current collection in forward bias. The distortion is mitigated by photoconductivity in the CdS or other window layer, and it is therefore more severe if the illumination contains no photons with energies greater than the band gap of the window layer. The device-physics analysis of such distortion is numerical simulation incorporating a three-layer [TCO/CdS/CI(G)S] approximation for the solar cell. The parameters that influence the barrier height, and hence the distortion, are the magnitude of the CBO, the doping of the pand nlayers, the defect density of the CdS, and the thicknesses of the CdS and TCO layers. The key value, however, is the energy difference between the quasi-Fermi level for electrons and the conduction band at the CdS/CIS interface. Thermionic emission across the interface will limit the current collection, if the difference exceeds approximately 0.48 eV at 300 K and one-sun illumination. This constraint is consistent with experiment, and strategies to satisfy the 0.48-eV rule when designing solar cells are enumerated. INTRODUCTION Thin-film solar cells are promising for energy conversion due to their low cost and good conversion efficiency. Cells with CIGS absorbers have achieved efficiencies of 19.5% [1]. CIGS cells generally use CdS buffer layers, but due to the reduced current collection in the low wavelength region, other alternatives have been proposed. To date, no alternative buffer has achieved efficiency as high as the CdS buffer, but some have come fairly close [2]. To make the search for alternative buffer more successful, it is important to investigate the role of CdS in the cell. One of the features of the CdS/CIS cell can be a deviation from a standard current–voltage curve when the cell is illuminated only with low energy photons. This distortion has been referred to as the “red kink”. The magnitude of the distortion depends on several parameters, and its dependence on the CdS thickness and on the CdS/CIGS conduction-band offset (CBO) has been investigated experimentally and numerically [3, 4]. The height of the “spike” compared to the conduction band energy in the absorber was proposed as a parameter that determines whether there is a distortion. This work summarizes the dependence of the distortion on additional parameters, provides a physical explanation for the distortion, and introduces a new key parameter that distinguishes well-behaved devices from devices with the J-V distortion. Mater. Res. Soc. Symp. Proc. Vol. 865 © 2005 Materials Research Society F5.32.1