Electrical characterization of all-layers-sprayed solar cell based on ZnO nanorods and extremely thin CIS absorber

Abstract

Nanostructured ZnO/In2S3/CuInS2 superstrate solar cell with all component layers deposited by a low cost chemical spray pyrolysis (CSP) method is characterized. The characteristics of a cell based on a nano-columnar ZnO window layer and a thin film reference prepared by spray are compared. The aim is to determine the dominating non-ideality and the recombination mechanism of these cells in both dark and illuminated conditions and relate the findings to the imperfections in the cell materials. We performed J–V–T (current–voltage–temperature) measurements in dark and under 0.5–100mW/cm2 illumination intensities, and admittance spectroscopy, in the temperature range of 100–360K. We further measured EQE (external quantum efficiency) at room temperature. The solar cell outputs at AM1.5 of the flat reference cell are: Voc=497mV, Jsc=6.4mA/cm2, FF=62%, η=2%. The use of a nanostructured instead of a flat window layer resulted in short-circuit current density Jsc=12.2mA/cm2 and efficiency of η=3% at the expense of slightly reduced open-circuit voltage Voc=430mV and fill factor FF=58%. Interestingly, the nanostructured cell performs worse than the flat reference at low illumination intensity, as also indicated by illumination dependent shunt conductance. The diode ideality factor of both cells has a significant temperature and illumination dependence. The absorber bandgaps deduced from EQE are 1.5eV for the flat cell and 1.3eV for the structured cell. The nanostructured cell shows an increase of light scattering ability accompanied by a less effective charge carrier separation, compared to those of the flat reference cell. The extrapolation of Voc(T) to 0K yields 740–830meV and 950–990meV at varied illumination intensity, for the structured and the flat reference cell respectively, pointing to a non-midgap defect recombination. C–f–T (capacitance – ac frequency – temperature) analysis indicates that the structured cell has a higher concentration of defects and an additional band of defects. This further explains the limited performance enhancement of the structured cell over the flat cell. The illumination dependent cell parameters reveal that copper diffusion from absorber to buffer layer is likely. To eliminate problems associated with Cu diffusion, binary compounds like Sb2S3 or SnS for use as absorber material are considered to further develop the low-cost superstrate type solar cell deposited by the CSP method.