Ultrathin SnO₂ Buffer Layer Aids in Interface and Band Engineering for Sb₂(S,Se)₃ Solar Cells with over 8% Efficiency

Abstract

The environmentally friendly antimony selenosulfide (Sb2(S,Se)3) semiconductor emerges as a promising light harvester for thin-film photovoltaics owing to its excellent material and optoelectronic properties. The alloyed Sb2(S,Se)3 is endowed with the complementary benefits of Sb2S3 and Sb2Se3, such as a tunable band gap within the range of 1.10–1.70 eV. In Sb2(S,Se)3 solar cells, the n-type semiconductor CdS is extensively used as an electron transport layer (ETL), which plays a role in extracting photogenerated electrons from absorbers and transporting them to conducting substrates. However, the unsatisfactory ETL/absorber interface contact often involves severe interface recombination. Herein, we report that an ultrathin SnO2 buffer layer of ∼10 nm applied on the high-roughness fluorine-doped tin oxide (FTO) substrate aids in effective interface and band engineering for superstrate CdS/Sb2(S,Se)3 solar cells. Careful characterizations confirm that the ultrathin SnO2 buffer layer plays a positive role in inhibiting the shunt current leakage at the ETL/absorber interface and manipulating the cascade energy band structure for more effective interface passivation and efficient electron extraction. Consequently, the resultant SnO2/CdS ETL-based Sb2(S,Se)3 solar cells exhibited a remarkable device efficiency of 8.67%, coupled with a considerable open-circuit voltage of 0.72 V. Our finding demonstrates a facile approach to engineer the interface contact and band offset to accelerate electron extraction, transport, and collection efficiencies.

The Design of SnO2‐Dominated Electron Transport Layer for High‐Efficiency Sb2(S,Se)3 Solar Cells

“…In our previous work, we constructed a single‐layer SnO 2 ‐based device of FTO/SnO 2 /Sb 2 (S,Se) 3 /spiro‐OMeTAD/Au, and found that the corresponding device exhibited a quite low efficiency of 0.92% that lagged far behind the current efficiency record of Sb 2 (S,Se) 3 solar cells. [ 21 ] This fact is closely associated with the poor quality of Sb 2 (S,Se) 3 films deposited onto the SnO 2 layer via the hydrothermal method. In contrast, the CdS substrates allow the fabrication of compact and uniform Sb 2 (S,Se) 3 absorber layer, which might be benefitted from the low lattice‐mismatch between CdS and Sb 2 (S,Se) 3 .…”

“…A too‐thin layer would result in increased leakage current while a too‐thick layer would reduce the transmittance and increase the charge transfer resistance; both cases will affect the heterojunction quality, hindering the extraction and transport of electrons and thereby reducing the device performance. [ 21,30 ] In this work, we fabricated the SnO 2 ETL by spin‐coating commercial SnO 2 hydrosol (SH) precursor diluted by deionized water (DI), and the film thickness can be tuned by changing the spinning speed, the precursor concentration, and the number of spin‐coating. First, we optimized the volume ratio of SH/DW to obtain a reasonable thickness of ETL for high‐performance Sb 2 (S,Se) 3 solar cells.…”

“…Figure 5b illustrates the energy level diagram of functional layers, and the corresponding energy level values are referred from the literature. [ 21 ] The energy difference promotes the transfer of photogenerated electrons and holes from the Sb 2 (S,Se) 3 absorber to the ETL and the hole transport layer (HTL), respectively. The ETL plays a critical role in extracting electrons from the absorber as well as blocking holes.…”

“…In addition, the morphology and physical properties also exert some influence on the quality of subsequently deposited absorbers. [ 21,22 ] Therefore, an ideal ETL should have the following features: 1) reasonable band structure between the ETL and the absorber; 2) high electron mobility to ensure efficient electron transport; 3) low defect state density to reduce the electron–hole recombination; 4) high light transmittance; 5) free of toxic elements. [ 20,22 ] Nowadays, a number of electron transport materials have been developed for solar cell applications, such as metal oxides (TiO 2 , SnO 2 , ZnO, (Zn,Mg)O, etc.…”

“…In our previous work, we found that the hydrothermal deposition of Sb 2 (S,Se) 3 films on SnO 2 ETLs suffer from poor quality, resulting in relatively low device efficiency. [ 21 ] In this work, an ultrathin CdS modification layer was introduced between the SnO 2 film and the absorber to obtain high‐quality Sb 2 (S,Se) 3 films; coupled with the optimization of the fabrication process for depositing SnO 2 , we achieved high‐performance SnO 2 ‐dominated environmental‐friendly Sb 2 (S,Se) 3 solar cells.…”