Alkali-metal-enhanced grain growth in Cu₂ZnSnS₄ thin films

Abstract

The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu-Zn-Sn alloy films co-sputtered on SLG, Pyrex, and quartz.CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100's of nm to greater than 1 mm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10 À6 moles of Na or 10 À7 moles of K resulted in a dramatic increase in grain size, from 100's of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth. Broader contextState-of-the-art thin-lm solar cells use cadmium telluride or copper indium gallium selenide as the light absorbing material. However, the production of these solar cells may be limited to less than 1 terawatt by the scarcity of tellurium and indium, and possibly by the toxicity of cadmium. These sustainability concerns provide a strong motivation to search for new photovoltaic materials comprised of nontoxic and abundant elements. The rapid rise in power conversion efficiencies of solar cells based on copper zinc tin sulde and selenide (CZTS and CZTSe) attests to their potential as low-cost, earth-abundant solar absorber materials. One of the properties that affects the CZTS solar cell performance is the lm's microstructure. Ideally, a monolayer of single crystal grains with sizes on the order of the lm thickness (1-3 micrometers) is required. The present study shows the remarkable effects of the presence of very small amounts of alkali metal atom impurities in enhancing grain growth in CZTS thin lms. The article also presents an approach for delivering preci...

Efficiency Enhancement of Cu2ZnSn(S,Se)4 Solar Cells via Alkali Metals Doping

“…[ 8 ] Because of the high concentration of Ca within soda lime glass, its effect on the CZTS devices have also been investigated. [ 9 ] However, no obvious infl uences on CZTS device performance were observed, which was attributed to the +2 oxidation state of Ca. This indicates that the ability of Na and K dopants to improve the effi ciency of copper based solar cells wileyonlinelibrary.com doped fi lms are continuous and grain sizes are larger than those in undoped CZTSSe fi lm.…”

“…[ 9 ] Therefore, all alkali metals with the +1 oxidation state are expected to have positive effect on CZTSSe solar cells. However, no systematic studies have been reported on the effects of alkali metals dopants so far.…”

A Bottom‐Up Approach toward All‐Solution‐Processed High‐Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting

“…Few studies have analyzed the effects of alkali metals from Group II on chalcopyrite or kesterite samples, [ 55 ] compared to the abundant literature on sodium, or more recently, on potassium. [ 7,56 ] We hypothesize that the magnesium operates as a surfactant for the chalcopyrites, as reported for the sodium, altering the interface energies and promoting the surface and bulk mobility of atoms to promote the grain growth.…”

Sodium doping effects on the crystalline and electrical properties of Cu2ZnSnSe4 thin films

“…Co-evaporating CZTS to substrates containing Na led to higher Hole concentrations, increased carrier mobility, and improved solar cell efficiencies (Li et al, 2013). By comparing the Na, K, Mg and Al including in the SLG, the Na diffusion from SLG was confirmed to be the key factor for high efficiency CZTS solar cell (Johnson et al,2014).…”