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

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThin-film photovoltaic (PV) solar cells comprise approximately 10% of the PV installed capacity worldwide (, ). Considering the exponential increase in the PV market over the last 10 years, it is crucial to develop new scalable technologies based on low-cost earth-abundant materials (, ). Cu2ZnSn(S,Se)4 (CZTSSe) largely fulfills the key requirements in terms of optical properties, electronic structure, Earth abundance, and stability, although device power conversion efficiency (η) has been limited by low open-circuit voltage (VOC) and fill factor (FF) (, , , ). The most efficient cells certified to date, with a band gap of 1.1 eV, have shown VOC values of 513 mV (η = 12.6%) (), 521 mV (η = 12.3%) (), and 670 mV (11.9%) (). These cells have been fabricated by physical vapor deposition () as well as solution-based methods (), suggesting that key power conversion losses are linked to intrinsic material properties rather than the preparation method.Low VOC values have been linked to non-optimal band alignment at CZTSSe/CdS boundary as well as elemental disorder in CZTSSe, mainly point defects such as CuZn antisites (, ). 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 (href="#bib51" rid="bib51" class=" bibr popnode">Tiwari et al., 2017b). Elemental disorder has also been investigated by X-ray and neutron diffraction (href="#bib42" rid="bib42" class=" bibr popnode">Schorr, 2011, href="#bib52" rid="bib52" class=" bibr popnode">Tobbens et al., 2016), as well as solid-state nuclear magnetic resonance (href="#bib34" rid="bib34" class=" bibr popnode">Paris et al., 2015). On the other hand, density functional theory (DFT) analysis also predicts that Sn disorder may generate states deeper in the band gap, which may act as recombination centers (href="#bib10" rid="bib10" class=" bibr popnode">Chen et al., 2009). Recently, we have shown the presence of Sn antisite domain boundaries in CZTS nanostructures employing atomic-resolution transmission electron microscopy (href="#bib25" rid="bib25" class=" bibr popnode">Kattan et al., 2016). Nanoscale domains of secondary phases have also been observed employing atom-probe tomography (href="#bib47" rid="bib47" class=" bibr popnode">Tajima et al., 2014) and high-resolution cathodoluminescence (href="#bib30" rid="bib30" class=" bibr popnode">Mendis et al., 2018). 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 (href="#bib22" rid="bib22" class=" bibr popnode">Johnson et al., 2014, href="#bib27" rid="bib27" class=" bibr popnode">Kumar et al., 2015, href="#bib46" rid="bib46" class=" bibr popnode">Su et al., 2015, href="#bib49" rid="bib49" class=" bibr popnode">Tiwari et al., 2016, href="#bib37" rid="bib37" class=" bibr popnode">Qi et al., 2017, href="#bib18" rid="bib18" class=" bibr popnode">Giraldo et al., 2018, href="#bib39" rid="bib39" class=" bibr popnode">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.Experimental evidences have shown the importance of understanding the structure of the buried junctions involving the absorber layer, namely, the Mo/CZTSSe and the CZTSSe/CdS interfaces. For instance, Haight and co-workers exfoliated CZTSSe cells from the Mo back-contact and mechanically reconnected them achieving a substantial increase in power conversion efficiency (href="#bib1" rid="bib1" class=" bibr popnode">Antunez et al., 2017a). This is due to the partial selenization of the Mo film during the thermal annealing step. However, investigating the CZTSSe/CdS boundary with the penetration depth offered by photoemission spectroscopy is significantly challenging (href="#bib4" rid="bib4" class=" bibr popnode">Bär et al., 2017).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 (href="#bib51" rid="bib51" class=" bibr popnode">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). Analysis of the valence band spectra, supported by DFT supercell calculations, provides a strong link between the most prominent LEWF features and the main CZTSSe phase. The photoemission landscape also unveils sub-micron domains of low LEWF, acting as shunting path for photogenerated electrons. Local valence band spectrum reveals that these photoemission hotspots correspond to Sn(S,Se) surface domains. We conclude that these surface domains, not detectable by conventional spectroscopic and diffraction techniques, can play a substantial role in the voltage losses in devices.