We present the special device properties of CuZnSn(SSe) (0≤y≤1) (CZTSSe) based solar cells under low illumination conditions, reporting an unexpected difference in behavior of the quaternary-like compounds (close to S and Se pure compositions), compared with those observed in the CZTSSe solid solutions with comparable S and Se contents.
We study the optoelectronic properties of CZTSSe devices with efficiencies ranging from 3% to 6%, using illuminated I-V curves under different irradiance intensities (from 20 to 1000 W/m). Furthermore, external quantum efficiency curves (EQE) were measured under different white bias light intensities, under different monochromatic bias lights wavelengths at approximately constant intensity (455 nm, 505 nm, 660 nm and 850 nm), as well as under different bias voltages. Complementary conductive atomic force microscopy (c-AFM) studies under different voltages and extra light illumination were also performed with the aim to analyze the role of grain boundaries and bulk properties in these effects.
A remarkable increase of the corrected conversion efficiency from 5.6% and 3.3% (measured at 1000 W/m), to 11.5% and 8.4% (measured at 20 W/m and corrected to 1000 W/m), for CZTSe and CZTS based devices respectively, were obtained. This gives a strong interest to these materials for the development of in-door photovoltaic applications related to the need for power supply in small autonomous systems, obtaining efficiency values that are comparable to those achieved in devices already at an industrial stage, and opens new interesting application fields for emerging kesterite based technologies. The main changes observed in the devices at low illuminations intensity are associated to a strong increase of the shunt resistance. This could be related to the grain boundaries behavior under different light intensities.
Conversely, CZTSSe solid solutions behave very different, and the corrected efficiency and the others optoelectronic parameters are almost constant with the illumination. Trying to understand this contrasting behavior, we analyze the EQE curves as a function of bias light intensity and wavelength, and bias voltage. The results confirm the previously observed trends, whereas the EQE for the CZTSSe based devices are almost unaffected for these parameters, those corresponding to pure and nearly-pure CZTSe and CZTS ones are strongly affected.
In particular for this last device, EQE decrease can especially been observed when using long wavelength bias lights, whereas short wavelength bias light which is mainly absorbed in the buffer/absorber junction has less effect. This suggests that the light sensitivity is mainly due to the absorber bulk properties. Complementary c-AFM measurements under equivalent illumination conditions support the observed different roles of the grain boundaries. Potential origin of this behavior and the differences between CZTSe and CZTS, with CZTSSe based devices will be discussed.