Controlling interfacial defects and energy level aligment at CZTS/TiO2 heterojunction for photovoltaic applications

Abstract

The quaternary semiconductor kesterite-structured Cu2ZnSnS4 (CZTS) has been widely recognized as a potential light-absorbing material for thin-film solar cells. With all non-toxic and earth-abundant constituent elements, CZTS exhibits direct bandgap (Eg) with optimal value (1.4 eV – 1.5 eV) and high absorption coefficient (>104 cm-1). In the photovoltaic (PV) community, two critical factors are recognized to substantially impact the power conversion efficiency (PCE) of solar cells; phase purity of absorber film and the quality of heterointerface formed at p-n junction, which controls the generation and extraction of carriers in PV devices. With recent development, the maximum reported PCE of CZTS based solar cells is 11.4% to date. For single-junction CZTS solar cells, the theoretically predicted maximum efficiency by Shockley and Queisser is as high as 32.4%. The prime source for this huge performance disparity between theoretically predicted and experimentally achieved efficiency is the high open-circuit voltage (VOC) deficit. The interface-dominated recombination due to interface trap states, unfavorable band alignment and bulk-dominated non-radiative recombination owing to intrinsic point defects, extended band tail states and secondary phases are paramount for low VOC. The above background sets the motivation for the present thesis work to control the CZTS absorber defects, interfacial defects and energy level alignment at the absorber/buffer interface using a new wide bandgap metal oxide buffer layer. In the present thesis work, amorphous-TiO2 is proposed as a potential substitute to the conventional CdS buffer layer. Metal oxide buffer layer effectively inhibits the shunting path between absorber film and the top electrode and act as an excellent hole blocking layer which suppresses the back carrier recombination at the interface. Apart from these benefits, the experimentally calculated conduction band offset (CBO) is found to be positive at CZTS/TiO2 interface, which is required for low interface recombination. Kelvin probe force microscopy (KPFM) probe the interfaces at nanoscale and analysed the variation in surface potential at the interface. The KPFM analysis reveals that the p-n junction formed at CZTS/TiO2 interface has a wider depletion region and higher built-in potential, which is required for excellent charge separation conditions. We have critically chosen the transient photoresponse at the absorber/buffer interface only, as fabrication of solar cell would add other interfaces in the device. Therefore, to understand the charge separation mechanism, the transient photoresponse at absorber/buffer interface is investigated. The faster decay response of CZTS/TiO2 based device eventually highlights effective separation and reduced trapping of charge carriers at interface. We have compared the interface properties like interface chemistry, energy level alignment, electric field, depletion region width etc. for both CZTS/CdS and CZTS/TiO2 heterojunction and illustrate that TiO2 has potential to successfully substitute the toxic CdS buffer layer in earth-abundant CZTS solar cells. Further, we deployed a new chemical passivation approach in CZTS to suppress the tail states associated to charged defects and uncompensated bonds. The high concentration of charged intrinsic defects and dangling bonds generate defect levels within the forbidden gap and induce spatial potential fluctuations and band tailing. The band tailing in CZTS is reduced by using a Lewis base molecule which neutralizes charged defects and dangling bonds not only on the CZTS surface but also within the CZTS absorber. Absorption spectra performed on CZTS films disclose that after treatment CZTS film band gap increases with a reduction in Urbach energy by 0.29 eV. Blue-shifted PL peaks and enhanced band-to-band transitions (~ 6 times) over band-to-tail transitions in PL spectra represent the significant suppression of non-radiative recombination. In addition to optical analysis, electrical characterization like capacitance-voltage (C-V) characteristics and transient photoresponse are conducted on CZTS/TiO2 interface. Reduced interfacial trap states at treated CZTS surface tend to form a quality junction with TiO2 buffer layer. Increase in photo-to-dark current ratio and reduction in response time over non-treated CZTS based device advocates above outcomes. This work provides an unexplored direction for CZTS absorber which can be further explored to modulate its optical and electrical properties. Finally, we have developed a facile approach which can simultaneously passivate the CZTS/TiO2 interface and CZTS GBs. A systematic investigation is carried out to understand the influence of oxygen vacancies settled at the CZTS/TiO2 interface on the charge transfer rate at heterostructures. Modulation of OV shifted the CBO from unfavorable to favorable band alignment and increased the PL quenching at CZTS/TiO2 heterojunction. In order to probe the generated photovoltage at CZTS/TiO2 interface and CZTS GBs, KPFM is conducted in surface and junction mode under dark and light conditions. The acquired photovoltage map exhibit a higher value at the GBs, which reveals an increment in downward band bending after oxygen diffusion inside the bulk of CZTS. The improvement in VOC value from 157 mV to 335 mV after oxygen diffusion further supports our outcomes. Furthermore, comparatively lower trap concentration (evaluated from C-V measurements) was observed in O2-1.0 sample due to passivation effect induced by oxygen ambient. Thus, our findings illustrate a simple approach to engineer the CZTS absorber surface and bulk GBs, leading to higher photovoltage generation at CZTS/TiO2 heterojunction.