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Towards Stable and Highly Efficient Tin-based Perovskite Solar Cells

Periodic Reporting for period 1 - TinPSC (Towards Stable and Highly Efficient Tin-based Perovskite Solar Cells)

Reporting period: 2018-08-01 to 2020-07-31

Solar cells are considered as one of the most promising renewable energy resources that can meet growing energy demand and mitigate greenhouse gas emissions. Impressively, solution-processed lead halide perovskites have attracted a great deal of attention in photovoltaic applications with an incredible device efficiency improvement from 3.8% to 25.2% during the past ten years. Unfortunately, these perovskite devices suffer from the toxicity of Pb and poor stability against moisture and heat, which are key challenges hindering their practical applications.

Issue 1: Toxicity of Lead-based perovskites. It is prospective to replace Pb with less toxic Sn, since both Sn and Pb elements belong to the IVA group and they have similar ionic radii due to relativistic effects (Sn2+ 1.35Å and Pb2+ 1.49Å). However, the poor stability limits their progress and high efficiency of solar cells. Meanwhile, we found another more promising material-double perovskites with the formula of A2M+M3+X6, which can be formed by substituting divalent Pb2+ cations with a combination of non-toxic monovalent M+ and trivalent M3+ cations, are promising as lead-free photovoltaic materials. They possess a three-dimensional crystal structure similar to lead-based perovskites, and good stability against moisture and heat. Currently, the large bandgap limits its photovoltaic application with high efficiency. Therefore, in this project, we focus on both Sn-based perovskites and the benchmark double perovskite, Cs2AgBiBr6.

Issue 2: Unstable organic hole transport materials (HTMs). Transport layers are crucial for achieving high-efficiency optoelectronic devices by promoting efficient selective carriers collection or injection. Organic semiconductors are attractive as transport materials with advantages of amorphous, light, flexible, good solubility in organic solvents, and easy to the solution process. Due to the low intrinsic carrier concentration of most organic transport materials, dopants are conventionally required to improve their carrier mobility and facilitate the carrier transfer. For example, LiTFSI and tBP are typical dopants for spiro-OMeTAD HTMs in high-efficiency solar cells. However, they still present a few challenges: 1) the need of a long-post oxidization process (around 10-24 hours) increases the production cycle of devices, 2) the poor stability of dopants leads to the poor long-term stability of devices.

The overall objectives are 1) to improve the stability of Sn-based perovskites, 2) enhance the absorption properties of Cs2AgBiBr6, and 3) to develop stable HTMs for devices with high efficiency.
1) Improve the stability of Sn-based perovskites
We have improved the stability of Sn-based perovskites by developing benzylamine iodide (BAI) as an organic ligand with the formation of 2D-3D Sn-based perovskites, BA0.1FA0.5MA0.4SnI3. Besides, we also develop a new sacrificial agent, ammonium hypophosphite. It could prevent the oxidization of Sn2+, as evidenced by the reduction of Sn4+ by ammonium hypophosphite. Now, an average power conversion efficiency of 5.5% has been achieved with a good reproducibility with little variation (Manuscript is in preparation).

2) Reduce the bandgap of Cs2AgBiBr6 from 1.98 eV to 1.72 eV.
We have decreased the bandgap by ~0.26 eV by a novel crystal-engineering strategy, reaching the smallest reported bandgap of 1.72 eV for Cs2AgBiBr6 at ambient conditions. The bandgap narrowing is confirmed by both absorption and photoluminescence measurements. The first-principles calculations indicate that enhanced Ag-Bi disorder has a large impact on the band structure and decreases the bandgap, providing a possible explanation of the observed bandgap narrowing effect. (Accepted by Angew. Chem. Int. Ed.)

3) Increase the absorption of Cs2AgBiBr6 to the near-infrared range.
We have further significantly broadened the absorption band edge of Cs2AgBiBr6 double perovskite to the near-infrared range by developing doped double perovskites-Cs2(Ag:Cu)BiBr6. The X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (ssNMR) measurements confirm the partial replacement of Ag ions by Cu ions in the crystal lattice. More interestingly, the near-infrared absorption can generate band carriers upon excitation, indicated by the photoconductive measurement. (Accepted by Advanced Functional Materials)

4) Develop a stable and efficient hole transport layer (HTL).
We also have developed a new recipe for the hole transport layer. The corresponding solar cell devices demonstrate improved stability in terms of moisture, heat, and light than the traditional one. Meanwhile, the device efficiency shows an increase compared to those based on the traditional recipe. Moreover, the new recipe can bypass the long post-oxidization process, and thus reduce the production cycle of devices. (Patent application ongoing and the manuscript in preparation)
1) Progress beyond the state of the art: i) we have reached the smallest reported band gap of 1.72 eV and 1.44 eV for pristine Cs2AgBiBr6 and doped Cs2AgBiBr6 under ambient conditions, respectively. ii) we have revealed the mystery of the traditional hole transport layer with improved stability, increased efficiency, as well as bypassing the post-oxidization process.

2) Expected results until the end of the project: i) Achieve high-efficiency solar cell devices based on developed pristine and doped Cs2AgBiBr6. ii) Achieve highly stable Cs2AgBiBr6 based solar cell devices based on developed new HTL recipe.

3) Potential impacts: Our achievements are of great importance in terms of academic and industrial communities, as well as society. Firstly, the developed double perovskite with narrow bandgaps would not only provide new insights for achieving lead-free double perovskites with suitable band gaps but also stimulate the exploration of halide double perovskites for other optoelectronic devices. Secondly, our results on HTL is the general rule for all organic-based transport layers. Thus, we believe our findings will open a new era for organic semiconductor family, and lead to further gains in the efficiency and stability of optoelectronic devices eventually. Lastly, based on the indication of the technology readiness levels (TRLs), we think the TRL for the commercialization of our new HTL recipe is around 5-7. We plan to commercialize the product after testing by different research groups. Undoubtedly, the commercialisation will promote economic growth and job supplies.
Papers and Patent