The highest conversion efficiencies for Cu(In,Ga)Se2 (CIGSe) thin-film solar cells are currently close to 23%. In spite of extraordinary photovoltaic performance, the open-circuit voltage (Voc) and the short-circuit current density (jsc) do not reach levels of the theoretical, radiative Shockley-Queisser limit. The present work reports about potential limitations caused by enhanced nonradiative recombination at grain boundaries (GBs) and by electrostatic potential fluctuations. A CIGSe solar cell with a conversion efficiency of almost 21% (without anti-reflection coating) and a stacking sequence ZnO:Al/(Zn,Mg)O/CdS/CIGSe/Mo/glass, of which the coevaporated CIGSe absorber underwent a RbF postdeposition treatment (PDT), was characterized. In order to get an insight into the microstructural, compositional, electrical and optoelectronic properties, electron backscatter diffraction, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on a cross-sectional specimen in a scanning electron microscope. In addition, quantitative photoluminescence (PL) hyperspectral imaging and capacitance-voltage (C-V) analyses were carried out as well as two-dimensional device simulations, in order to corroborate the microscopy results.
From EBIC results, the width of the space-charge region (wSCR) and the minority-carrier diffusion length in the quasi-neutral region (LD) can be correlated to results obtained by absolute PL and C-V measurements. EDX elemental distribution analysis did not show any substantial, spatial variations in composition, except for the Ga/In gradient perpendicular to the substrate, which can be also correlated to the detected spectral shift in the CL peak in the same perpendicular direction. However, the impact of electrostatic potential, lifetime, as well as band-gap fluctuations present in neighboring regions in the absorber layer on the device performance of CIGSe solar cells can be neglected. This result was confirmed by two-dimensional device simulations.
In contrast, recombination velocities at GBs were determined to be on the order of 103 cm/s from CL profiles extracted across GBs, which is the same order of magnitude that was determined also for GBs in various Na-containing CIGSe thin films without any PDT. Two-dimensional device simulations show that indeed, such recombination velocities could be one origin for the limited Voc and jsc in CIGSe solar cells.
Voc and jsc in CIGSe solar cells.