Effect of defects and oblique incidence on the performance of earth-abundant CZTSSe material based indoor photovoltaic devices

Abstract

In this thesis, 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 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 the fraction of blue emission in the white LED spectra remains relatively low. The efficiency of the highest efficiency solar cell is 12.6% whereas it is 15.5% under a warm white LED spectrum. For a white LED having warm white emission characteristics, the efficiency of a CZTSSe-based PV device can equal the efficiency of the experimentally reported champion CZTSSe solar cell while having about two orders of magnitude higher interface defect density of ~1×1011 cm-2, as well as about twenty times higher bulk defect density of ~4×1016 cm-3. Also, 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 AM1.5G 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. Next, we analyzed the performance of practical defect rich PV devices under oblique incidence for different polarizations of solar and indoor light source employing finite-difference time domain (FDTD) analysis technique, two- dimensional (2-D) Poisson’s equation and continuity equations. The results of this work demonstrate that efficiency of indoor PV device is always higher than solar PV device efficiency for all incident angle and polarization. For S polarized and unpolarized light, efficiency decreases as the incident angle increases; however, for P polarized light, initially efficiency increases with incident angle, reached its maximum (~16%) at the Brewster’s angle which is 680 for CdS, then starts to decrease rapidly. These phenomena can be explained in the context of Fresnel’s reflection coefficient for a particular polarization. Thus, the results of this thesis provide guidelines for designing low-cost yet energy- efficient indoor photovoltaic devices with defect-rich thin film material systems keeping the practical operating illumination conditions with having different angles of incidence and degrees of polarization in consideration.