Spectroscopic and electrical signatures of acceptor states in solution processed Cu₂ZnSn(S,Se)₄ solar cells

Abstract

The nature and dynamics of acceptor states in solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) thin films are investigated by variable temperature photoluminescence (PL) and electrical impedance spectroscopy. Highly pure I-4 phase CZTSSe with the composition Cu1.6ZnSn0.9(S0.23Se0.77)4 is synthesized by sequentially spin coating of dimethyl-formamide/isopropanol solutions containing metal salts and thiourea onto Mo coated glass, followed by annealing in an Se atmosphere at 540 C. As annealed films are highly compact with a thickness of 1.3 micron and grain sizes above 800 nm, with a band gap of 1.18 eV. Photovoltaic devices of 0.25 cm2 with the architecture glass/Mo/CZTSSe/CdS/i-ZnO/Al:ZnO demonstrate a power conversion efficiency reaching up to 5.7% in the absence of an antireflective coating. Under AM 1.5G illumination at 296 K, the best device shows a 396 mV open-circuit voltage (VOC), 27.8 mA cm-2 short-circuit current (JSC) and 52% fill factor (FF). The overall dispersion of these parameters is under 15% for a total of 20 devices. In the near IR region, PL spectra are dominated by two broad and asymmetrical bands at 1.14 eV (PL1) and 0.95 eV (PL2) with characteristic power and temperature dependences. Analysis of the device electrical impedance spectra also reveals two electron acceptor states with the same activation energy as those observed by PL. This allows assigning PL1 as a radiative recombination at localized copper vacancies (VCu), while PL2 is associated with CuZn antisites, broadened by potential fluctuations (band tails). The impact of these states on device performance as well as other parameters, such as barrier collection heights introduced by partial selenization of the back contact, are discussed

Mapping Shunting Paths at the Surface of Cu2ZnSn(S,Se)4 Films via Energy-Filtered Photoemission Microscopy

“…Figure 1 A shows a characteristic J-V curve of a CZTSSe device (0.25 cm 2 ) under AM1.5G illumination at room temperature. The CZTSSe film was obtained by annealing of a single molecular precursor spin-coated onto Mo-coated glass in the presence of Se, based on a protocol reported elsewhere ( Tiwari et al., 2016 , Tiwari et al., 2017b ). 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.…”

“…Clustering of these point defects into domains can lead to potential energy fluctuations, which manifest themselves as band tails. Broadening and complex temperature dependence of photoluminescence responses provide the clearest manifestation of band tails in these materials ( Tiwari et al., 2017b ). Elemental disorder has also been investigated by X-ray and neutron diffraction ( Schorr, 2011 , Tobbens et al., 2016 ), as well as solid-state nuclear magnetic resonance ( Paris et al., 2015 ).…”

“…In this work, we employ for the first time sub-micron-resolution energy-filtered photoemission microscopy (EF-PEEM) to examine the complex and poorly understood surface electronic structure of CZTSSe thin films. Our investigation focuses on thin films generated by annealing of molecular precursors, which are characterized by a high degree of crystallinity and phase purity and power conversion efficiencies of 5.7% under AM 1.5G illumination ( Tiwari et al., 2017b ). Photoemission maps show a distribution in the onset energy of secondary electron emission, which we rationalized in terms of local effective work functions (LEWF).…”

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

“…10 Voltage losses in CZTSSe are often linked to structural disorder which manifests itself as band tails and deep trap states. 11,12 Cu-Zn antisite is the main type of defect contributing to band tails, while Sn disorder generates energy levels deep in the band gap. [13][14][15] In addition, secondary phases including binary (chalcogenides), ternary (Cu 2 SnS 3 ) and quaternary (I-42m phase, i.e.…”

A luminescence study of Cu2ZnSnSe4/Mo/glass films and solar cells with near stoichiometric copper content

“…These complexes are considered to be beneficial for the device performance [6,10]. The presence of V Cu and Zn Cu in copper deficient and zinc rich Cu 2 ZnSn(SSe) 4 has also been supported by experimental studies [13].…”