The latest progress and future perspectives of thin film photovoltaic kesterite technology are reviewed herein. Kesterite is currently the most promising emerging fully inorganic thin film photovoltaic technology based on critical raw-material-free and sustainable solutions. The positioning of kesterites in the frame of the emerging inorganic solar cells is first addressed, and the recent history of this family of materials briefly described. A review of the fast progress achieved earlier this decade is presented, toward the relative slowdown in the recent years partly explained by the large opencircuit voltage (V OC ) deficit recurrently observed even in the best solar cell devices in the literature. Then, through a comparison with the close cousin Cu(In,Ga)Se 2 technology, doping and alloying strategies are proposed as critical for enhancing the conversion efficiency of kesterite. In the second section herein, intrinsic and extrinsic doping, as well as alloying strategies are reviewed, presenting the most relevant and recent results, and proposing possible pathways for future implementation. In the last section, a review on technological applications of kesterite is presented, going beyond conventional photovoltaic devices, and demonstrating their suitability as potential candidates in advanced tandem concepts, photocatalysis, thermoelectric, gas sensing, etc.Kesterite Photovoltaics He has supervised ten Ph.D. theses in the photovoltaic field. He is currently the coordinator of the research and innovation H2020 project STARCELL (www.starcell.eu), and the RISE (Marie Curie) project INFINITE-CELL (www.infinite-cell.eu).the V OC deficit for bandgaps close to 1.0 or 1.5 eV are comparable, and one can consider that kesterite is reasonably close to the state of the art of chalcopyrite materials in that range. But, while for kesterite the V OC deficit increases almost monotonically with the bandgap, it is markedly reduced for intermediate

A fundamental analysis of the impact of Ge on the synthesis of Cu2ZnSnSe4:Ge by a sequential process is presented, reporting the consequences on the absorber morphology and solar cell devices performance.

Improvement of the efficiency of Cu(2)ZnSnS(4) (CZTS)-based solar cells requires the development of specific procedures to remove or avoid the formation of detrimental secondary phases. The presence of these phases is favored by the Zn-rich and Cu-poor conditions that are required to obtain device-grade layers. We have developed a selective chemical etching process based on the use of hydrochloric acid solutions to remove Zn-rich secondary phases from the CZTS film surface, which are partly responsible for the deterioration of the series resistance of the cells and, as a consequence, the conversion efficiency. Using this approach, we have obtained CZTS-based devices with 5.2% efficiency, which is nearly twice that of the devices we have prepared without this etching process.

This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

This work presents a complete analysis of all Raman active modes of Cu 2 ZnSnS 4 measuring with six different excitation wavelengths from near infrared to ultraviolet. Simultaneous fitting of spectra allowed identification of 18 peaks from device grade layers with composition close to stoichiometry that are attributed to the 27 optical modes theoretically expected for this crystalline structure, including detection of 5 peaks not observed previously, but theoretically predicted. Resonance effects are assumed to explain the observed increase in intensity of weak modes for near infrared and ultraviolet excitations. These results are particularly relevant for experimental discrimination of Raman modes related to secondary phases. V C 2014 AIP Publishing LLC.

The control and removal of secondary phases is one of the major challenges for the development of Cu 2 ZnSn(S,Se) 4 (CZTSSe)-based solar cells. Although etching processes have been developed for Cu(S,Se), Zn(S,Se), and CuSn(S,Se) secondary phases, so far very little attention has been given to the role of Sn(S,Se). In this paper, we report a chemical route using a yellow (NH 4 ) 2 S solution to effectively remove Sn(S,Se). We found that Sn(S,Se) can form on the surface either because of stoichiometric deviation or by condensation. After etching, the efficiency of devices typically increases between 20 and 65% relative to the before etch efficiencies. We achieved a maximum 5.9% efficiency in Se-rich CZTSSe-based devices. It is confirmed that this feature is related not only to the removal of Sn(S,Se) but also to the unexpected passivation of the surface. We propose a phenomenological model for this passivation, which may open new perspectives for the development of CZTSSe-based solar cells.

Secondary phases like Cu2SnS3 are major obstacles for kesterite thin film solar cell applications. We prepare Cu2SnS3 using identical annealing conditions as used for the kesterite films. By x-ray diffraction, the crystal structure of Cu2SnS3 was identified as monoclinic. Polarization-dependent Raman investigations allowed the identification of the dominant peaks at 290 cm−1 and 352 cm−1 with the main A′ symmetry vibrational modes from the monoclinic Cu2SnS3 phase. Furthermore, micro-resolved Raman investigations revealed local variations in the spectra that are attributed to a secondary phase (possibly Cu2Sn3S7). This exemplifies the abilities of micro-resolved Raman measurements in the detection of secondary phases.

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