Assessing the Impact of Na and Sb Doping in Solution Processed Cu2ZnSnS4 Thin-Film Solar Cells

Abstract

Thin-film inorganic solar cells based on Cu2ZnSn(SxSe1-x)4 absorbers remain as one of the most promising PV technologies featuring Earth abundant and low toxic elements. These devices conventionally adopt the so-called substrate configuration which characterises commercial CIGS technologies. Interestingly, similar record cell efficiencies have been achieved using solution processing (12.6%)1 and sputtering based methods (13.0%).2 It is widely recognised that cell voltage is the key limiting factor in these devices, yet the physical origin of voltage losses remains to be elucidated.The composition complexity of these materials can lead to a variety of point defects which have been extensively investigated from the experimental and computational point of view. For instance, the loss of voltage has been linked to Cu/Zn antisites which can give rise to tails in the band edge energies (so-called Urbach tails).3 Furthermore, we have also observed Sn disorder in the bulk3 and at the surface of these compounds,4 which are responsible for deep states highly detrimental to cell performance.Numerous studies have reported improvement in Cu2ZnSn(SxSe1-x)4 cells performance upon doping and/or alloying with a variety of alkali, group IV and V elements.5 The introduction of dopants can have multiple effects such as supressing elemental disorder, doping, passivation of recombination sites, changes in grain size and so forth. The material complexity makes rather challenging the establishment of meaningful correlations between composition and device properties. In this contribution, we uncover the effects of Na and Sb doping in the bulk and interfacial electronic properties of solution-processed Cu2ZnSnS4 (CZTS) thin films, which manifest itself by an increase in power conversion efficiency from 3.2 to 5.7%.6 I will first show how the surface electronic landscape of CZTS thin films evolves upon doing, employing high-resolution energy-filtered photoemission of electron microscopy (EF-PEEM). This technique allows mapping fluctuations in the local effective work function (LEWF) with sub-micron resolution.4,7 We uncover unprecedented correlations between surface electronic properties with key device parameters such as the activation energy for the predominant recombination pathway and Urbach tails. We will also discuss new data correlating variable temperature photoluminescence and admittance spectroscopy, demonstrating the fundamental role of Sb in diminishing Sn disorder.