Revealing the Role of Potassium Treatment in CZTSSe Thin Film Solar Cells

Potassium (K) post-treatment on CIGSSe has been shown to yield the highest efficiency reported to date. However, very little is known on the effect of K doping in CZTSSe and the mechanism behind the efficiency improvement. Here we reveal the mechanism by which K enhances the charge separation in CZTSSe. We show that K accumulates at the CdS/CZTSSe, passivating the recombination at the front interface and improving carrier collection. K is also found to accumulate at the CZTSSe/Mo interface and facilitates the diffusion of Cd into the absorber which affects the morphology and grain growth of CZTSSe. As revealed by the <i>C</i>–<i>V</i>, external quantum efficiency, and color <i>J</i>–<i>V</i> test, K doping significantly increases the carrier density, improves carrier collection, and passivates the front interface and grain boundaries, leading to the enhancement of <i>V</i>_oc and <i>J</i>_sc. The average power conversion efficiency has been promoted from 5% to above 7%, and the best 7.78% efficiency has been achieved for the 1.5 mol % K-doped CZTSSe device. This work offers some new insights into the K doping effects on CZTSSe via solution-based approach and demonstrates the potential of facile control of K doping for further improvement of CZTSSe thin film solar cells

Solution-Processed Trigonal Cu₂BaSnS₄ Thin-Film Solar Cells

Recently, Cu₂BaSnS₄ (CBTS) thin film has emerged as a promising candidate for single- or multiple-junction photovoltaics (PVs) due to its excellent optical and electrical properties, and earth-abundant, nontoxic constituents. In this study, a molecular solution-based nonvacuum process has been employed to prepare CBTS thin films for solar cells. The obtained CBTS films show trigonal structure with band gap of 2.01 eV and p-type conductivity. The solar cell device with configuration of Mo-coated glass/CBTS/CdS/i-ZnO/ITO has achieved a power conversion efficiency (PCE) of 1.72% for CBTS with a Ba/Sn atomic ratio of 1.30. An abrupt band gap cutoff in external quantum efficiency (EQE) data coupled with the very small offset (only 10 meV) in band gap between EQE and photoluminescence (PL) measurements reveals that band tailing is not the limiting factor in CBTS

Kesterite Cu₂ZnSn(S,Se)₄ Solar Cells with beyond 8% Efficiency by a Sol–Gel and Selenization Process

A facile sol–gel and selenization process has been demonstrated to fabricate high-quality single-phase earth abundant kesterite Cu₂ZnSn­(S,Se)₄ (CZTSSe) photovoltaic absorbers. The structure and band gap of the fabricated CZTSSe can be readily tuned by varying the [S]/([S] + [Se]) ratios via selenization condition control. The effects of [S]/([S] + [Se]) ratio on device performance have been presented. The best device shows 8.25% total area efficiency without antireflection coating. Low fill factor is the main limitation for the current device efficiency compared to record efficiency device due to high series resistance and interface recombination. By improving film uniformity, eliminating voids, and reducing the Mo­(S,Se)₂ interfacial layer, a further boost of the device efficiency is expected, enabling the proposed process for fabricating one of the most promising candidates for kesterite solar cells

Enhancement of Open-Circuit Voltage of Solution-Processed Cu₂ZnSnS₄ Solar Cells with 7.2% Efficiency by Incorporation of Silver

Recently, considerable attention in the development of Cu₂ZnSnS₄ (CZTS)-based thin-film solar cells has been given to the reduction of antisite defects via cation substitution. In this Letter, we report the substitution of copper atoms by silver, incorporated into the crystal lattice through a solution processable method. We observe an increase in open-circuit voltage (<i>V</i>_OC) by 50 mV and an accompanying rise in device efficiency from 4.9% to 7.2%. The incorporation of Ag is found to improve the grain size, enhance the depletion width of the pn-junction, and reduce the concentration of antisite defect states. This work demonstrates the promising role of Ag in reducing the <i>V</i>_OC deficit of Cu-kesterite thin-film solar cells

Origin of Photocarrier Losses in Iron Pyrite (FeS₂) Nanocubes

Iron pyrite has received significant attention due to its high optical absorption. However, the loss of open circuit voltage (<i>V</i>_oc) prevents its further application in photovoltaics. Herein, we have studied the photophysics of pyrite by ultrafast laser spectroscopy to understand fundamental limitation of low <i>V</i>_oc by quantifying photocarrier losses in high quality, stoichiometric, and phase pure {100} faceted pyrite nanocubes. We found that fast carrier localization of photoexcited carriers to indirect band edge and shallow trap states is responsible for major carrier loss. Slow relaxation component reflects high density of defects within the band gap which is consistent with the observed Mott-variable range hopping (VRH) conduction from transport measurements. Magnetic measurements strikingly show the magnetic ordering associated with phase inhomogeneity, such as FeS_2−δ (0 ≤ δ ≤ 1). This implies that improvement of iron pyrite solar cell performance lies in mitigating the intrinsic defects (such as sulfur vacancies) by blocking the fast carrier localization process. Photocarrier generation and relaxation model is presented by comprehensive analysis. Our results provide insight into possible defects that induce midgap states and facilitate rapid carrier relaxation before collection