A study to control secondary phases and defects in Kesterite Cu2ZnSnS4 absorber layer for photovoltaic applications

Abstract

Chalcogenide based quaternary compound: Cu2ZnSnS4 (CZTS), a Kesterite material, has been identified as a potential absorber for thin film solar cells. Salient features of CZTS such as, high absorption coefficient (>104 cm-1), suitable band gap (~ 1.5 eV), earth abundant and eco-friendly elements make it viable candidate for cost-effective photovoltaic (PV) technology. Highest power conversion efficiency (PCE) reported for CZTS is 11.4% till date, which is much lower than its theoretically predicted efficiency. Unfortunately, CZTS suffers from narrow phase stability which leads to the formation of secondary phases, defects and defect complexes. The co-existence of different secondary phases and defects within CZTS are the root causes of lower PCE recorded in CZTS solar cells. Moreover, a correlation between Cu poor and Zn rich composition and high PCE is reported in CZTS solar cells. However, Cu poor and Zn rich composition (off-stoichiometry) is difficult to control, which eventually results in variety of secondary phases along with CZTS phase. In addition, even small changes in the stoichiometry cause variety of defects, specifically defects associated to Cu-Zn disorder due to their low formation energy. The above background sets the motivation for the present thesis work to control secondary phases and defects in CZTS by using controlled growth approach, doping and strain engineering in CZTS absorbers for enhanced device performance. The off-stoichiometry of CZTS was uniquely controlled by fine tuning the Zn/Sn composition ratio. Zn/Sn ratio influence the formation of detrimental secondary phases such as Cu2-xS which is highly metallic phase and leads to shunting the device. We focused to tune Zn/Sn composition to suppress deleterious Cu2-xS phase during growth of CZTS itself, which eventually helped to avoid additional step of chemical etching to remove such secondary phases from CZTS surface. We successfully achieved threefold enhancement in PCE and recorded PCE of 6.11% by controlling Cu2-xS phase. Increase in efficiency is mainly attributed to the enhanced shunt resistance due to suppression of Cu2-xS phase with Zn/Sn ratio of 1.12. In addition, impact of Zn/Sn ratio on growth kinetics was investigated and mechanism has been proposed for the same. Further, we deployed the doping approach in CZTS to suppress different defects associated to Cu and Zn lattice sites. Cu-Zn antisite defects in CZTS were reduced by doping Ag in CZTS matrix. Easy formation of Cu-Zn antisite defects was suppressed by partial substitution of Cu+ ions by large size Ag+ ions. Ag content in CZTS was simultaneously optimized to improve nanoscale electrical properties of CZTS. The nanoscale characterization data from kelvin probe force microscopy (KPFM) and conducting atomic force microscopy (CAFM) is correlated with the transient photoresponse of photodetectors (PDs) fabricated with pristine CZTS and CAZTS (Ag doped CZTS) films, to facilitate the understanding of suppression of CuZn trap centers with increased Ag content. In addition, we have considered potential substitution of Cr in CZTS matrix owing to its existence in wide range of oxidation states including +1, +2 and +4. Partial substitution of Zn with Cr has been identified using X-ray photoelectron spectroscopy. Using KPFM and CAFM, we verify the surface potential variation and nanoscale electrical conductivity in pristine and Cr-CZTS (Cr doped CZTS) films. Substantial increase in the grain boundary (GB) current was determined for Cr-CZTS films. This work widens the opportunity of exploring potential cationic substitution in CZTS for developing high efficiency CZTS solar cells. Finally, residual strain in CZTS absorber has been modulated by using facile approach of high temperature seed layer (SL)-assisted growth. Cu-Zn disorder in CZTS easily arises due to its low formation energy, and is sensitive to small changes in the structural inhomogeneities. Huge disparity in lattice structure and coefficient of thermal expansion (CTE) among CZTS and Mo causes strained growth of CZTS over Mo (molybdenum) coated soda lime glass (SLG). Introducing SL before growing bulk CZTS on Mo film has shown beneficial traits on regulating residual strain in the film. SL-assisted growth reduced the overall residual strain in CZTS films, thereby improving the Cu-Zn ordering and reducing band tailing. The mobility of majority charge carriers (holes) has shown remarkable enhancement from 2.3 cm2 V/s (without SL) to 13.5 cm2 V/s (with SL). Additionally, depth dependent investigations of residual strain with two different thicknesses of SL i.e 50 nm (Thin SL) and 250 nm (Thick SL), and for No SL samples were carried out using grazing incidence X-ray diffraction measurements. Graded distribution of strain has been realized in SL CZTS films in contrast to uniformly strained No SL CZTS films. Reduction in band tailing and improved Cu-Zn ordering in SL samples is governed by simulation and theoretical fitting of the absorption data recorded from UV-Visible spectroscopy. Substantial quenching of the photoluminescence (PL) signal in SL samples is attributed to the changes in the electronic structure due to graded strain. The graded strain would induce formation of multiple homojunction which sets up a built-in electric field array which helps in charge transfer, thereby reducing the PL signal. The high strain gradient in SL CZTS films plays a crucial role and is majorly responsible for the faster carrier transition in Thin SL CZTS PDs.