CZTSSe solar cells are considered to be potential and cost-effective alternative solutions to mature photovoltaic technology for meeting future energy demands. However, the current performance of CZTSSe solar cells is limited due to their high voltage deficit. The comparison of figure-of-merits of the two technologies show that CZTSSe has 62% lower open circuit voltage compared to that of 35% for CIGS, with respect to their associated band gaps. Efficient charge separation and extraction at the absorber–buffer (p–n) junction is paramount for mitigating the voltage loss and achieve high photo-conversion efficiency. The rapid progress for achieving high deliverables in CZTSSe is impeded by interface recombination, which is a consequence of poor-quality p–n junction. The inherent association of CZTSSe with secondary phases and defects due to narrow phase stability plays an unfavorable role in producing a good quality interface. The high density of interface defects, unfavorable band alignment, and structural inhomogeneities across the interface are some of the leading causes that nurture interface dominant recombination pathways. These interface-related concerns have drawn the scientific community towards interface engineering and modification of the interface to bring closer performance parity between CZTSSe and matured CIGS solar cell technology. Several approaches have attempted to develop favorable interface features that facilitate improved device performance. This work addresses the critical aspects of interface engineering of the absorber–buffer heterojunction in CZTSSe solar cells and the importance of tools that are essential to identify and eradicate the root causes of low efficiency.

Progress and prospects of CZTSSe/CdS interface engineering to combat high open-circuit voltage deficit of kesterite photovoltaics: a critical review

Herein, we report a spin coating route for the deposition of multifunctional quaternary copper chalcogenide Cu2–II–Sn–S4 (II = Fe, Co and Ni) thin films on ITO glass substrates. The surface morphology of the thin films is a uniform porous like structure with an average film thickness of 1.5 μm. The formation of single phase Cu2FeSnS4 (CFTS), Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) thin films was confirmed by Raman spectroscopy, which reveals two principal Raman peaks of the A1 phonon mode. Due to the presence of the d6, d7 and d8 electronic configurations within Fe2+, Co2+ and Ni2+ ions, respectively, all the materials show a spin magnetic moment per ion of 3.33 μB, 2.45 μB and 1.31 μB for CFTS, CCTS and CNTS, respectively, as described by the electronic band structure calculation (WIEN2k code). All the thin films have estimated band gap values of 1.87 eV, 1.57 eV and 1.74 eV, respectively, with enhanced photocurrent properties and electrocatalytic activity. Significantly improved charge transfer properties of the thin films were observed from electrochemical impedance spectroscopy analysis under dark and visible light illumination conditions. These thin films are promising for use in photoelectrochemical and solar cell applications.

Photo-electrochemical properties and electronic band structure of kesterite copper chalcogenide Cu2–II–Sn–S4 (II = Fe, Co, Ni) thin films

Cation disorder which arises from the size and chemical environment similarity of Cu and Zn is the limiting factor in Cu2ZnSnSxSe4−x (CZTSSe) performance. Cation substitution is one effective way to solve this issue, however, the most commonly reported substitutes, Ag and Cd, are not ideal as they detract from the earth-abundant and non-toxic motivation of CZTSSe. Mn is a promising candidate in comparison with other candidates (e.g. Fe, Ni or Co), because of its oxidation state stability and larger ionic size mismatch with Cu. In this study, Cu2MnxZn1−xSn(S,Se)4 (CMZTSSe) thin film solar cells were prepared by chemical spray pyrolysis and a subsequent selenization process. We study the influence of Mn substitution on the morphological, structural, optical, electrical and device properties. A distinct phase transformation from CZTSSe kesterite to C(M,Z)TSSe stannite is observed at 20% Mn substitution. A high amount of Mn substitution (x ≥ 0.6) is shown to increase the carrier density significantly which introduces more defects and non-radiative carrier recombination as shown by quenched photoluminescence intensity. Consequently, reduction in device performance is observed for these samples. The highest power conversion efficiency is achieved at x ≈ 0.05 with η = 7.59%, Voc = 0.43 V, Jsc = 28.9 mA cm−2 and FF = 61.03%. The improved open circuit voltage (Voc) and fill factor (FF) are attributed to the improved shunt resistance and carrier transport due to low defect density especially at the CdS/CMZTSSe interface. Finally, based on our electrical characterization, a few suggestions to improve the efficiency are proposed.

Reducing the interfacial defect density of CZTSSe solar cells by Mn substitution

This article presents the photo-electrochemical and photovoltaic performances of solution processed Cu2XSnS4 (X = Fe, Co, Ni) thin films on ZnS coated vertically aligned ZnO nanorod (NR) arrays for the first time. The optical, structural and electrical properties of all the synthesized structures were investigated using various characterization techniques. The electrocatalytic activities of the superstrate structures were characterized by taking cyclic voltammetry (CV) measurements and performing electrochemical impedance spectroscopy (EIS) analyses. Furthermore, the photovoltaic performances of the devices with architecture ITO/ZnO-NR's/ZnS/Cu2XSnS4/Au were investigated under AM 1.5 illumination condition. The thin film solar cells with Cu2FeSnS4 (CFTS), Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) photoactive layers exhibited power conversion efficiency (PCE) values of 2.73%, 3.23% and 2.71% respectively. All the devices exhibit good electrochemical as well as photovoltaic performance under illuminations and have shown potential to fabricate nontoxic solution processed high efficient devices.

https://pubs.rsc.org/en/content/articlepdf/2018/RA/C8RA90072H

Solution-processed Cu2XSnS4 (X = Fe, Co, Ni) photo-electrochemical and thin film solar cells on vertically grown ZnO nanorod arrays

Kesterite (CZTSe) is a promising thin film photovoltaic absorber material due to its composition of more earth abundant materials compared to mature thin film photovoltaic technologies. Up to now, power conversion efficiencies are still lower and its main problem is the low open circuit voltage (Voc). Recently, a novel sintering approach using a nanometric Ge layer showed a large increase in device performance and especially in Voc. In this work, in-depth solar cell characterization as well as Raman and photoluminescence studies of devices employing different Ge doped CZTSe absorber layers is presented. The main focus is to reveal the beneficial effects of Ge doping and furthermore investigate the interaction of Ge and Na. For low Ge doping an increase in charge carrier concentration is observed, resulting in devices with a Voc of 460 mV, which corresponds to a Voc deficit (Eg/q − Voc) of 596 mV, a value comparable to current record devices. For high Ge amounts, admittance spectroscopy measurements identified the appearance of a deep defect which can explain the observed deterioration of solar cell performance. Additional Na provided during crystallization of high Ge doped devices can reduce the density of this deep defect and recover device performance. These results indicate that Na plays an important role in defect passivation and we propose a defect model based on the interaction of group IV elements and Na with Cu vacancies.

/pdf/revealing-the-beneficial-effects-of-ge-doping-on-cu2znsnse4-1edb8ji9w1.pdf

Revealing the beneficial effects of Ge doping on Cu2ZnSnSe4 thin film solar cells

Impedance spectroscopy (IS) and current–voltage characteristics measurements were applied to study properties of a Cu2ZnSnSe4 (CZTSe) thin film solar cell. IS measurements were done in the frequency range 20 Hz to 10 MHz. The measurement temperature was varied from 10 K to 325 K with a step ▵T = 5 K. Temperature dependence of V oc revealed an activation energy of 962 meV, which is in the vicinity of the band gap energy of CZTSe and hence the dominating recombination mechanism in this solar cell is bulk recombination. Different temperature ranges, where electrical properties change, were found. Interface states at grain boundaries with different properties were revealed to play an important role in impedance measurements. These states can be described by introducing a constant phase element in the equivalent circuit.

/pdf/temperature-dependent-electrical-characterization-of-thin-1oigq8d9sz.pdf

Temperature dependent electrical characterization of thin film Cu 2 ZnSnSe 4 solar cells

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Temperature dependent electrical characterization of thin film Cu 2 ZnSnSe 4 solar cells