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The use of organic hole transport layer (HTL) Spiro-OMeTAD in various solar cells imposes serious stability and cost problems, and thus calls for inorganic substitute materials. In this work, a novel inorganic MnS film prepared by thermal evaporation has been demonstrated to serve as a decent HTL in high-performance Sb2(S,Se)3 solar cells, providing a cost-effective all-inorganic solution. A low-temperature air-annealing process for the evaporated MnS layer was found to result in a significant positive effect on the power conversion efficiency (PCE) of Sb2(S,Se)3 solar cells, due to its better-matched energy band alignment after partial oxidation. Impressively, the device with the optimized MnS HTL has achieved an excellent PCE of about 9.24%, which is the highest among all-inorganic Sb2(S,Se)3 solar cells. Our result has revealed that MnS is a feasible substitute for organic HTL in Sb-based solar cells to achieve high PCE, low cost, and high stability.

MoS2 thin film is deposited on ITO/CdS substrate by in-situ hydrothermal method, which is expected to be a potential absorber material for planar heterojunction solar cell due to its high absorber coefficient and carrier mobility. The CdS buffer layer provides a robust template for the quasi-epitaxial growth of MoS2 with (0 0 2) preferred orientation by enjoying the analogical hexagonal structures and the shared sulfur atoms, resulting in a benign interface of CdS/MoS2. Then a device with ITO/TiO2/CdS/MoS2/Carbon-Ag structure is assembled after coating carbon-Ag electrode. The effects of the growth characteristics, morphology changes and optical properties of MoS2 on the device performance are studied systematically. Finally, a primary efficiency of 0.12% is achieved using a P-type MoS2 absorber with a thickness of 305 nm, exhibiting an outstanding stability. The one-dimensional solar cell capacitance simulator (SCAPS) is further used to reveal the bottleneck of device performance. After optimizing the doping density (1 × 1015 cm−3) and defect density (

Hydrothermal deposition is emerging as a highly potential route for antimony-based solar cells, in which the Sb2(S,Se)3 is typically in situ grown on a common toxic CdS buffer layer. The narrow band gap of CdS causes a considerable absorption in the short-wavelength region and then lowers the current density of the device. Herein, TiO2 is first evaluated as an alternative Cd-free buffer layer for hydrothermally derived Sb2S3 solar cells. But it suffers from a severely inhomogeneous Sb2S3 coverage, which is effectively eliminated by inserting a Zn(O,S) layer. The surface atom of sulfur in Zn(O,S) uniquely provides a chemical bridge to enable the quasi-epitaxial deposition of Sb2S3 thin film, confirming by both morphology and binding energy analysis using DFT. Then the results of the first-principles calculations also show that Zn(O,S)/Sb2S3 has a more stable structure than TiO2/Sb2S3. The resultant perfect Zn(O,S)/Sb2S3 junction, with a suitable band alignment and excellent interface contact, delivers a remarkably enhanced JSC and VOC for Sb2S3 solar cells. The device efficiency with the TiO2/Zn(O,S) buffer layer boosts from 0.54% to 3.70% compared with the counterpart of TiO2, which has a champion efficiency of Cd-free Sb2S3 solar cells with a structure of ITO/TiO2/Zn(O,S)/Sb2S3/Carbon/Ag by in situ hydrothermal deposition. This work provides a guideline for the hydrothermal deposition of antimony-based films upon a nontoxic buffer layer.

Antimony chalcogenide Sb2(S, Se)3 is attracting a lot of attention as photovoltaic absorber owing to its rewarding photoelectric properties, low toxicity, and earth abundance. However, its device efficiency is still limited by the absorber material quality and device interface recombination. In this work, a fluorine-doped tin oxide (FTO) substrate with ultra-thin SnO2 layer and a low-cost stabilized carbon paste are introduced as a front and back contact layer respectively in Sb2(S, Se)3 based planar solar cells. Over 5.2 % efficiency is demonstrated in the structure of FTO/SnO2/CdS/Sb2(S, Se)3/Carbon/Ag, where the Sb2(S, Se)3 is prepared by hydrothermal technique. The complementary device physics characterizations reveal that the interfacial recombination between TCO and CdS is significantly suppressed by the introduction of ultra-thin SnO2 layer, which is profited from the leakage protection and bandgap offset engineering by its high resistivity and suitable conduction band minimum. Meanwhile, the successful adoption of the low-cost stabilized carbon as a back contact here shows an enormous potential to replace the conventional organic hole transport materials and noble metal. We hope this work can provide positive guidance to optimize Sb2(S, Se)3 based planar solar cells in the future.

In this work, a three-step (sulfurization-selenization-sulfurization) annealing process was designed to optimize the surface constitution of Cu2ZnSn(S, Se)4 (CZTSSe) thin films, which was prepared by oxide nanoparticles-based approach. The devices with the Mo/CZTSSe/CdS/ZnMgO/ZnO:Al/Al structure were fabricated and their performances were studied. The additional post-sulfurization with low toxicity sulfur powder has negligible impact on the structure, morphology and composition of CZTSSe bulk, however, it improves open circuit voltage of device significantly. The open circuit voltage can be increased significantly from 408 mV (without surface sulfurization) to 497 mV (with surface sulfurization). This is benefited from the increase of surficial sulfur content and the broadening of the surface band-gap of the CZTSSe thin film. The result is confirmed by X-ray photoelectron spectroscopy analysis. Such a crucial three-step annealing process promotes the power conversion efficiency from 4.71% (2-step) to 6.37% (3-step), which is the champion efficiency of oxide precursor derived CZTSSe solar cell.

Germanium selenide (GeSe), a IV-VI chalcogenide containing earth-abundant and low-toxic elements, has attracted increasing interest as a potential absorber material for photovoltaics due to its excellent optical and electrical properties. In this work, a simple and effective magnetron sputtering method was proposed to deposite GeSe precursor thin films using GeSe alloy target. Subsequently, the as-deposited samples were post-annealed in vacuum atmosphere. The effects of different annealing temperatures on crystallinity, morphology and phase transformation of the as-prepared films were systematically investigated. It has been revealed that phase-pure and uniform GeSe thin films were obtained through the self-decomposition of slight GeSe2 impurity at 400 °C. Finally, a prototypical FTO/CdS/GeSe/C-Ag solar cell with 220 mV open circuit voltage and 0.05% power conversion efficiency was fabricated, which suggests the potential of sputtering processed GeSe thin film for low cost solar cell.

Ternary Cu2SnS3 (CTS) thin films have attracted lots of attention due to its low cost, nontoxic element for solar cell. However, Tin losses always happen during sulfurization, which will affect the phase purity and morphology of CTS thin films and relevant solar cell. In this work, a low cost oxides route was used to prepare CTS thin films. The influences of sulfurizaton temperature on the crystal structure, morphology and composition were studied carefully. From the XRD, SEM and EDX results, it was found that the stable oxides were conducive to alleviate the Sn loss even at the higher sulfurization temperature (580 °C). Meanwhile, the crystal structure evolution along annealing temperature was also observed from mixed phase (Monoclinic and Tetragonal) to single phase Monoclinic. The Monoclinic CTS thin films sulfurized at 580 °C were used to fabricate CTS solar cell with Mo/CTS/CdS/ZnO/AZO/Ag structure, exhibiting efficiency of 0.16%. The comparable Voc of 300 mV was achieved by benefiting from the precise composition control. Besides, the photoelectric properties of the device were carefully studied as well. This study provides a low-cost and facile strategy for the fabrication of Sn-loss free CTS solar cells.

The inhibition and removal of the SnS2 impurity phase in the kesterite Cu2ZnSnS4 (CZTS) layer is a major challenge for the improvement of CZTS solar cells; this impurity phase can critically damage device performance by forming a diode and a barrier for carrier collection. However, the formation and growth mechanism of SnS2 is incomplete and the common etching treatment with (NH4)2S is problematic because of its toxic characteristics. In this study, the formation and growth of large-size SnS2 sheets were observed to occur on the surface of the CZTS thin film via condensation during the cooling process. Sequently, a novel and benign peeling process was carried out using a sticky tape to effectively remove the SnS2 formed on the surface, resulting in a uniform CZTS thin film. A maximum efficiency of 4.3% of the oxides-derived CZTS device was obtained after this mechanical exfoliation, which is nearly eleven times that of the device without stripping. It was confirmed that this surface treatment is an effective and green way to remove the SnS2 impurity, which largely improved the performance of the CZTS device and may guide the preparation of pure phase CZTS thin films.

The earth-abundant material CuSbS2 (CAS) exhibits excellent optical and electrical properties as a potential absorber material for thin-film solar cells. However, the preparation of phase-pure CAS is still challenging due to the multi-phase characters of Cu-Sb-S system and antimony (Sb) loss during the annealing. In this work, an oxides nanoparticles-derived route is first investigated to synthesize CAS thin film. The Sb loss can be alleviated slightly by increasing sulfurization pressure. To control the film composition and phase purity, the effect of Cu/Sb ratios on the phase evolution is carefully investigated, and phase formation mechanism is then well established. Finally, the phase-pure CAS thin film with large grains is prepared with the sufficient Sb provided as a sacrificial material. An obvious photoelectric response of CAS thin film and its related solar cell are observed, suggesting a potential strategy for the fabrication of CAS solar cells from oxides nanoparticles ink.

GeSe is considered as a potential absorber material for thin film solar cells owing to its ideal band gap, strong light absorption, remarkable air durability, Earth-abundance and non-toxic constituents. However, the high vapor pressure of GeSe at a temperature below its melting point makes it difficult to synthesize a high-quality GeSe film. To alleviate this limitation, in this work, a thermal evaporation combining a novel sandwiching post-annealing method was introduced to deposit high quality GeSe thin films with (100)-orientation. The self-assembling mechanism of the highly oriented GeSe film was carefully investigated by the systematic experiments and confirmed by the lowest total energy of the (100) crystal plane. Finally, the fully-inorganic, low-cost and non-toxic planar device with the superstrate configuration of FTO/TiO2/GeSe/carbon/Ag was also successfully fabricated. Notably, as a result, an impressive open circuit voltage (VOC) of 340 mV (maximum: 456 mV) was achieved, which is the highest VOC of GeSe solar cells reported so far. Furthermore, through current-voltage, capacitance-voltage profiling and drive level capacitance profiling measurements, it was demonstrated that the limiting factors of the GeSe solar cell performance were the narrow depletion width (138 nm) and the drastic recombination at the TiO2/GeSe interface.

Molecular solution with the metal-thiourea complex is a facile and effective route for Cu2SnS3 (CTS) thin film deposition. However, its stability is still challenging due to flexible metal salts precursors. Here we report a strategy to enhance the stability of the Cu–Sn–S molecular solution that use bivalent Sn to suppress the oxidation of the S2- of thiourea via the redox reaction. The self-stabilizing mechanism has been carefully revealed by the systematic experiments and confirmed by the spontaneous reaction process of metal-thiourea system. It demonstrates that Sn with a low valence state weakens the transfer of lone pair electrons from thiourea to Cu2+ and Sn4+ ions, alleviating the sulfur precipitates from thiourea oxidation because of the easily reducible character of Sn2+ ion. The solution with Sn2+ antioxidant additive can be stored for a long time under air ambient, which is then spin-coated and sulfurized with Sn powder compensation. The uniform CTS thin film with composition control is finally achieved and delivers a device efficiency of 1.15%. This work provides a guide on stabilizing the solution precursor for chalcogenide thin film deposition.