The article presents a strategic review of secondary phases, defects and defect-complexes in kesterite CZTSSe solar cells responsible for performance gap compared to CIGS solar cells.

CZTSSe solar cells are considered to be potential and cost-effective alternative solution to matured photovoltaic technology to meet future energy demands. However, the current performance of CZTSSe solar cells is...

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Efficiency of earth abundant and pure sulfide kesterite Cu-Zn-Sn-S (CZTS) solar cell has been stagnant around 9.4% for years, while its counterpart Cu-In-Ga-Se (CIGS) reports an efficiency of more than 22%. Low open circuit voltage (VOC) is the major challenging factor for low efficiency due to severe nonradiative interface recombinations. The existence of higher defect states at the conventional CZTS-CdS interface due to undesirable energy level alignment and lattice misfit promotes trap-assisted recombinations and results in low VOC. In this work, amorphous TiO2 (Eg=3.8eV) is proposed as a promising substitute to the conventional and low bandgap CdS (Eg=2.4eV) layer. The surface and interface of the CZTS-TiO2 layer were investigated using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The result reveals favorable “spike”-like conformations at the CZTS-TiO2 interface with a conduction band offset value of 0.17 eV. The nanoscale probing of the interface by Kelvin probe force microscopy across CZTS-TiO2 layers shows a higher potential barrier for interface recombination at CZTS-TiO2 in contrast to the conventional CZTS-CdS interface. Finally, the fast decay response and lower persistent photoconductivity of photogenerated carriers for CZTS-TiO2 heterojunction based photodetectors further validate our results. The energy level alignment and nanoscale interface studies signify TiO2 as a promising alternate buffer layer for earth abundant CZTS solar cells.

Earth abundant CZTS (Cu2ZnSnS4) absorber layers are promising for the development of cost-effective and large area photovoltaics; however, interfacial nonradiative recombination is a major obstruction to the pathways toward high performing CZTS devices. Elimination of interfacial recombination losses via interface engineering is paramount to obtain efficient CZTS solar cells. Herein, we report a systematic investigation of the influence of oxygen vacancies (OV) settled at the CZTS/TiO2 interface on the charge transfer rate in heterostructures. Modulation of OV by varying oxygen flow rate during TiO2 deposition was confirmed by x-ray photoelectron spectroscopy. Lower OV concentration shifted the conduction band offset from negative to positive at the CZTS/TiO2 heterojunction, which is essential for efficient charge transportation through the interface. Photoluminescence quenching of the CZTS/TiO2 heterojunction also showed a strong correlation between charge dynamics and OV at the interface. Finally, we found the fast decay response of photogenerated charge carriers for the CZTS/TiO2 device with lower OV strongly favors the suppression of carrier trapping at the interface. This work provides a critical insight into interface engineering in CZTS solar cells through regulating interfacial OV, particularly when an oxide electron transport layer is applied.

This research presents a solar-blind (UVC) office paper-based photodetector that is self-biased, superflexible, nonwettable, high-performance, and high-voltage stable. In contrast to the traditional design of photodetectors involving sophisticated techniques, this study reports unique and facile hand-sketched bottom asymmetric graphite and silver electrodes. The asymmetric bottom electrodes offer minimum reflectivity that limits the dark current to 1.2 pA and boosts the device performance even at zero bias; this results in a significantly increased On/Off ratio of more than 10 4 at 254 nm. The photodetector exhibits high responsivity (3.1 mA W −1 ), and fast response (0.459 s/ 0.386 s), even at zero bias. For good formability deformations (30 cm −1 ) as well as for wide turning angles (±180°), a high strenuous durability test demonstrated impressive photoswitching characteristics. Highly stable bottom electrodes contributed to the exceptionally stable operation (>720 h) and much enhanced performance along with tremendous capability of bearing high voltages up to 100 V. The performance of the reported photodetector has considerably exceeded previously published photodetectors even on costly nonflexible substrates. Moreover, the paper-based device was coated with UV transparent enamel paint to make it completely protected from wettability.

The performance of earth abundant Cu 2 ZnSnS 4 (CZTS) material is limited by high deficit of open circuit voltage (V OC ) which is mainly due to the easy formation of Cu Zn antisite defects. Suppression of Cu Zn defects is thus inevitably required for further developments in CZTS based solar cells. We studied systematic increase of Ag doping in CZTS thin film and investigated the nanoscale electrical properties using Kelvin probe force microscopy and current sensing atomic force microscopy (CAFM) to probe Cu Zn defects. Crystallographic analysis indicated the successful partial substitution of Cu + ions by large size Ag + ions. The considerable decrease in grain boundary potential from 66.50±5.44 mV to 13.50±2.61 mV with Ag doping, suggesting the substantial decrease in Cu Zn defects. Consequently, CAFM measurement confirms the remarkable increment in minority carrier current with Ag doping and their local mobility in CZTS layer. Finally, the lower persistent photoconductivity and fast decay response of photogenerated carriers for Ag doped CZTS photodetector further validate our results. This study provides a fresh approach of controlling deleterious Cu Zn defects in CZTS by tuning Ag content that may guide researchers to develop next generation high-performance CZTS based solar cells.