Additive improves perovskite solar cell stability, efficiency

Perovskite solar cells (PVSCs) are a potential alternative to traditional silicon-based solar cells because of their high power-conversion efficiency and low cost. However, achieving long-term stability has presented a challenge to their development. A research team from City University of Hong Kong (CityU) has developed an innovative multifunctional and non-volatile additive which can improve the efficiency and stability of perovskite solar cells by modulating perovskite film growth. This simple and effective strategy has potential for facilitating the commercialisation of PVSCs.

Professor Alex Jen Kwan-yue, Lee Shau Kee Chair Professor of Materials Science, who led the study, said that this type of multifunctional additive can be used to make different perovskite compositions for fabricating efficient and stable perovskite solar cells. “The high-quality perovskite films will enable the upscaling of large-area solar panels,” Jen said.

PVSCs have attracted attention due to their impressive solar power conversion efficiency (PCE). Since perovskites can be deposited from solutions onto the fabrication surfaces, PVSCs have the potential to be applied in building-integrated photovoltaics (BIPV), wearable devices and solar farm applications. However, the efficiency and stability are still affected by the severe energy loss associated with defects embedded at the interfaces and grain boundaries of the perovskites. Therefore, the intrinsic quality of perovskite film plays a critical role in determining the achievable efficiency and stability of PVSCs.

Although many previous research studies have focused on improving the film morphology and quality with volatile additives, these additives tend to escape from the film after annealing, creating a void at the perovskite-substrate interface. To tackle these issues, the CityU researchers developed a strategy of modulating the perovskite film growth to enhance the film quality. They found that by adding a multifunctional molecule (4-guanidinobenzoic acid hydrochloride, (GBAC)) to the perovskite precursor, a hydrogen-bond-bridged intermediate phase is formed and modulates the crystallisation to achieve high-quality perovskite films with large perovskite crystal gains and coherent grain growth from the bottom to the surface of the film. This molecule can also serve as an effective defect passivation linker (a method to reduce the defect density of perovskite film) in the annealed perovskite film due to its non-volatility, resulting in significantly reduced non-radiative recombination loss and improved film quality.

The research showed that the defect density of perovskite films can be reduced after introducing GBAC. The power conversion efficiency of inverted (p-i-n) perovskite solar cells based on the modified perovskites was boosted to 24.8% (24.5% certified by the Japan Electrical Safety & Environment Technology Laboratories). The overall energy loss of the device was reduced to 0.36 eV, representing one of the lowest energy losses among the PVSC devices with high power-conversion efficiency. Additionally, the unencapsulated devices exhibit improved thermal stability beyond 1000 hours under continuous heating at 65 ± 5°C in a nitrogen-filled glovebox while maintaining 98% of the original efficiency.

The researchers demonstrated the applicability of this strategy for different perovskite compositions and large-area devices. A larger area device (1 cm²) in the experiment delivered a high PCE of 22.7% with this strategy, indicating potential for fabricating scalable, efficient PVSCs. “This work provides a clear path to achieving optimised perovskite film quality to facilitate the development of highly efficient and stable perovskite solar cells and their upscaling for practical applications,” Jen said.

The researchers aim to extend the molecular structures and optimise the device structure through compositional and interfacial engineering. They will also focus on the fabrication of large-area devices. The findings were published in the journal Nature Photonics.

Image caption: Photo of a 1 cm² perovskite solar cell with additive. Image credit: City University of Hong Kong.