Abstract
SummaryThe performance of Cu2ZnSn(S,Se)4 thin-film solar cells, commonly referred to as kesterite or CZTSSe, is limited by open-circuit voltage (VOC) values less than 60% of the maximum theoretical limit. In the present study, we employ energy-filtered photoemission microscopy to visualize nanoscale shunting paths in solution-processed CZTSSe films, which limit the VOC of cells to approximately 400 mV. These studies unveil areas of local effective work function (LEWF) narrowly distributed around 4.9 eV, whereas other portions show hotspots with LEWF as low as 4.2 eV. Localized valence band spectra and density functional theory calculations allow rationalizing the LEWF maps in terms of the CZTSSe effective work function broadened by potential energy fluctuations and nanoscale Sn(S,Se) phases.
Highlights
Thin-film photovoltaic (PV) solar cells comprise approximately 10% of the PV installed capacity worldwide (Fraunhofer ISE, 2017; Haegel et al, 2017)
Density functional theory (DFT) analysis predicts that Sn disorder may generate states deeper in the band gap, which may act as recombination centers (Chen et al, 2009)
The film is etched in KCN before chemical bath deposition of CdS and deposition of i-ZnO and Al-doped ZnO via radio frequency (RF) sputtering
Summary
Thin-film photovoltaic (PV) solar cells comprise approximately 10% of the PV installed capacity worldwide (Fraunhofer ISE, 2017; Haegel et al, 2017). Systematic approaches to reduce bulk composition disorder via controlled annealing and the introduction of additives (e.g., alkali metal, Ag, Cd, Ge, and Sb) have led to improvement in device efficiency, yet VOC values remain in the 500–600 mV range (Johnson et al, 2014; Kumar et al, 2015; Su et al, 2015; Tiwari et al, 2016; Qi et al, 2017; Giraldo et al, 2018; Sai Gautam et al, 2018) These observations have focused our attention away from bulk and into interfacial defects as the key factor determining voltage losses, which is emphasized by the fact that little is known about the surface structure of these complex materials
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