penghuang

Pure sulfide Cu₂ZnSnS₄ thin films were fabricated on Mo-coated glass substrates by facile spray deposition of aqueous precursor solutions containing Cu(NO₃)₂, Zn(NO₃)₂, Sn(CH₃SO₃)₂, and thiourea followed by annealing at 600 °C. When a precursor solution containing a stoichiometric composition of Cu, Zn, and Sn was used, the resulting Cu₂ZnSnS₄ thin film contained a Cu_2−xS impurity phase owing to the evaporation of Sn components during the annealing process. The Cu_2−xS impurity in the Cu₂ZnSnS₄ thin film was removed by reducing the concentration of Cu in the precursor solution. This resulted in an improvement of the structural features (i.e., grain sizes and compactness) as well as the electric properties such as acceptor densities, the nature of the acceptor defects, and carrier lifetimes. A solar cell based on the Cu₂ZnSnS₄ film with an empirically optimal composition showed conversion efficiency of 8.1 %. The value achieved was one of the best efficiencies of Cu₂ZnSnS₄-based cells derived from a non-vacuum process.

Efficient and cheap: The metallic compositions of the precursor solutions were found to have a significant effect on the grain sizes, morphologies, acceptor densities, the nature of the acceptor defects, and carrier density of Cu₂ZnSnS₄ films. A device with high conversion efficiency of 8.1 % was obtained from the film with an optimal composition.

We examined different encapsulation strategies for perovskite solar cells by testing the device stability under continuous illumination, elevated temperature (85 °C) and ambient humidity of 65 %. The effects of the use of different epoxies, protective layers and the presence of desiccant were investigated. The best stability (retention of ∼80 % of initial efficiency on average after 48 h) was obtained for devices protected by a SiO₂ film and encapsulated with a UV-curable epoxy including a desiccant sheet. However, the stability of ZnO-based cells encapsulated by the same method was found to be inferior to that of TiO₂-based cells. Finally, outdoor performance tests were performed for TiO₂-based cells (30–90 % ambient humidity). All the stability tests were performed following the established international summit on organic photovoltaic stability (ISOS) protocols for organic solar cell testing (ISOS-L2 and ISOS-O1).

Seal in the stability: Different encapsulation techniques are examined for perovskite solar cells. The cells encapsulated with a SiO₂ thin film and cover glass packaging incorporating desiccant and UV-curable epoxy exhibit good performance in accelerated lab testing under illumination and at 85 °C, as well as in outdoor testing in Hong Kong.

The electron-selective contact layer (ESL) in organometal halide-based perovskite solar cells (PSCs) determines not only the power conversion efficiency (PCE) but also the thermostability of PSCs. To improve the thermostability of ZnO-based PSCs, we developed Mg-doped ZnO [Zn_1−xMg_xO (ZMO)] as a high optical transmittance ESL for the methylammonium lead trihalide perovskite absorber [CH₃NH₃PbI₃]. We further investigated the optical and electrical properties of the ESL films with Mg contents of 0–30 mol % and the corresponding devices. We achieved a maximum PCE of 16.5 % with improved thermal stability of CH₃NH₃PbI₃ on ESL with the optimal ZMO (0.4 m) containing 10 mol % Mg. Moreover, this optimized ZMO PSC exhibited significantly improved durability and photostability owing to the improved chemical/photochemical stability of the wider optical bandgap ZMO.

Mg+ZnO=improved stability: To improve the power conversion efficiency and the thermostability in ZnO/CH₃NH₃PbI₃-based perovskite solar cells (PSCs), magnesium-doped ZnO is developed as the electron-selective contact layer. The optimized ZMO PSCs exhibit significantly improved durability and photo-stability.

Ionic liquids (ILs) have become an established option for the use as electrolytes in dye-sensitized solar cells. In the present study, the adsorption of a multitude of different ILs on a TiO₂ surface is studied systematically, focusing on the energetic modifications of the semiconductor. The cation was found to generally cause an energetic downward shift of the TiO₂ band levels by accepting electron density from the surface, and the anions were observed to function in the opposite direction, raising the energy levels by donating electron density. Both effects counterbalance each other, leaving the desired outcome dependent on the choice of the specific IL, i.e., the choice of the cation/anion combination. The correlation of the band levels with the properties of the IL was successfully achieved. The dipole moment of the adsorbed ionic liquid species showed little to no correlation with the semiconductor energetics, but the charge transfer calculated by radical Voronoi tessellation revealed a high correlation. The current findings contribute to a deeper understanding of the role of the electrolyte in dye-sensitized solar cells, and ILs in general, and help with choosing and tuning of the electrolyte solutions in existing applications.

The electrolyte effect! The adsorption of cations and anions from ionic liquids is decisive for the band level alignment in dye-sensitized solar cells. Electron deficiency typically caused by cations results in an energetic lowering of states, that is, valence and conduction bands. Consequently, electron donation from anions results in an energetic increase of states.

Bismuth- or antimony-based lead-free double perovskites represented by Cs₂AgBiBr₆ have recently been considered promising alternatives to the emerging lead-based perovskites for solar cell applications. These new perovskites belong to the Fm m space group and consist of two types of octahedra alternating in a rock-salt face-centered cubic structure. We show, by density functional theory calculations, that the stable chemical potential region for pure Cs₂AgBiBr₆ is narrow. Ag vacancies are a shallow accepters and can easily form, leading to intrinsic p-type conductivity. Bi vacancies and Ag_Bi antisites are deep acceptors and should be the dominant defects under the Br-rich growth conditions. Our results suggest that the growth of Cs₂AgBiBr₆ under Br-poor/Bi-rich conditions is preferred for suppressing the formation of the deep defects, which is beneficial for maximizing the photovoltaic performance.

Pb-free, defect-directed: Density functional theory calculations are performed to study the thermodynamic stability, electronic structure, and defect properties of Pb-free double perovskite Cs₂AgBiBr₆. It is found that Ag vacancies are shallow acceptors and can easily form, leading to intrinsic p-type conductivity, whereas Ag-on-Bi antisites are deep acceptors, whose formation can be suppressed under the Bi-rich/Br-poor growth conditions, which is necessary for photovoltaic and other optoelectronic applications of Cs₂AgBiBr₆.

The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I_1−xBr_x)₃ (MA=methylammonium) compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr₃ films exhibited no degradation, while MAPbI₃ and mixed halide MAPb(I_1−xBr_x)₃ films decomposed yielding crystallization of inorganic PbI₂ accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.

To be or not to be (efficient and stable): Photochemical stability studies of MAPb(I_1−xBr_x)₃ (MA=methylammonium) photovoltaic materials show that mixed halide compositions are less stable than pure halide ones. We postulate that these solid solutions contain internal stresses and structural defects, which might be responsible for their accelerated degradation.

A series of monobenzoporphyrins (WH1–WH4) bearing different conjugated spacer groups were designed and synthesized as sensitizers for dye-sensitized solar cells. Although a phenyl spacer only has a minimal impact on the absorption bands of the monobenzoporphyrin, an ethynylphenyl (WH3) or a vinyl (WH4) spacer redshifts and broadens the absorption bands of the dyes to result in much enhanced light-harvesting ability. Dye-sensitized solar cells based on these monobenzoporphyrin dyes displayed remarkable differences in power conversion efficiencies (PCEs). The monobenzoporphyrin bearing no spacer (WH1) resulted in a PCE of only 0.5 %; in contrast, the monobenzoporphyrin bearing vinyl spacers (WH4) achieved a PCE of 5.2 %. The high efficiency of the WH4 cell is attributed to the higher light-harvesting ability, the lesser extent of aggregation on the TiO₂ surface, and the more favorable electron-density distributions of the HOMO and LUMO for electron injection and collection. This work demonstrates the exceptional tunability of benzoporphyrins as sensitizers for dye-sensitized solar cells.

A spacer odyssey: Monobenzoporphyrins bearing conjugated spacer groups are synthesized as sensitizers for dye-sensitized solar cells. These monobenzoporphyrins demonstrate a significant spacer-group effect and give power conversion efficiencies in the range 0.5 to 5.2 %.