Role of bulk- and interface-defects in spectrum-dependent energy harvesting of CZTSSe-based indoor photovoltaic devices

In this work, we theoretically investigate spectrum dependent energy harvesting of a thin-film indoor photovoltaic (PV) device, taking into account the role of defects. By numerically solving Poisson’s equation and the continuity equation under optical generation-recombination conditions, performance characteristics of a Cu2ZnSn(S,Se)4-based thin-film PV device have been evaluated under spectrally varying white light emitting diodes (LEDs). Without any loss of generality, the results of the experimentally validated theoretical model suggest that a thin-film PV device becomes significantly tolerant to both bulk and interface defects when a fraction of blue emission in the white LED spectra remains relatively low. For a white LED having cool white emission characteristics, the efficiency of a CZTSSe-based PV device can equal the efficiency of the CZTSSe solar cell, the experimentally reported champion, while having about two orders of magnitude higher interface defect density, as well as about twenty times higher bulk defect density. In addition, for all practical densities of both types of defects, the efficiency of the indoor PV device remains at least 20% higher than the efficiency obtained under AM1.5 solar irradiation. The underlying reasons behind such observations have been traced back to the wavelength dependent carrier generation recombination dynamics of the thin-film device stack. The results of this work in effect provide guidelines for designing low-cost yet energy-efficient indoor photovoltaic devices with defect-rich thin film material systems.