Core achievements
【summary】
- Mechanism: Increasing the lattice perturbation of perovskite can inhibit the migration of perovskite halogen ions and light-induced phase separation.
- Materials: Based on this mechanism, a light-stable 2.0eV Rb/Cs all-inorganic perovskite material was developed.
- Device: The all-inorganic perovskite material is used in the top cell of the three-junction photovoltaic, so that the perovskite-based three-junction photovoltaic efficiency is improved from the original 20% to 24%, and the working stability is improved from the original less than one hour to more than 400 hours, which achieves the first reported certification efficiency of perovskite-based three-junction photovoltaic (23.3%).
【achievements】
Multi-junction photovoltaic can reduce the thermal relaxation loss and absorption loss and break through the efficiency limit of single-junction photovoltaic. Perovskite is an ideal multi-junction photovoltaic light-absorbing material due to its good photoelectric properties, adjustable band gap and easy preparation. However, the wide band gap (>1.65eV) I/Br mixed perovskite light-absorbing materials face the problem of light-induced phase separation (LIPS), which seriously limits the stability and efficiency of multi-junction photovoltaic devices. Perovskite-based triple-junction photovoltaics (including all-perovskite, perovskite/perovskite/Si, perovskite/perovskite/CuInGaSe, etc.) have better theoretical efficiency than well-developed single-junction and two-junction photovoltaics. However, the ~2.0eV perovskite applied to their top cells is facing a serious light-induced phase segregation problem, which makes the efficiency of perovskite-based triple-junction photovoltaics only 20%, and the working stability is less than 1 hour.
We have noticed that the previously reported methods for suppressing the light-induced phase separation of I/Br mixed perovskite materials mainly include adding small A-site cations and increasing the concentration of large X-site iodine anions. Both methods lead to the increase of perovskite lattice perturbation (lattice distortion). Therefore, we propose a conjecture: increasing the I/Br mixed perovskite lattice perturbation can effectively suppress the light-induced phase segregation of perovskite.
Based on this conjecture, we compared the currently reported perovskite material systems. Under similar band gaps, cesium-based inorganic perovskites have smaller A-site cation radii and more larger I anions than the more widely studied organic-inorganic hybrid perovskites, with greater lattice disruption and better light stability. However, Cs-based inorganic perovskites with a band gap of 2.0eV (such as CsPbI1.4Br1.6) still face the challenge of light-induced phase segregation.
We found that Rb+, which is smaller than Cs+, can be doped into the I/Br mixed inorganic perovskite lattice, and the maximum content of Rb doping is positively correlated with the content of Br. The lattice doping of Rb broadens the band gap of inorganic perovskites, which means that Rb/Cs perovskites need more I to achieve a band gap of 2.0eV relative to Cs-based perovskites (such as ~2.0eV Rb0.15Cs0.85PbI1.75Br1.25, relative to ~2.0eV CsPbI1.4Br1.6). Compared with Cs-based inorganic perovskites, 2.0eV Rb/Cs perovskites with larger lattice perturbation exhibit suppressed light-induced phase separation. Our further study confirms that increasing the lattice perturbation of I/Br mixed perovskites reduces the average interatomic distance between A-site cations (such as Cs and Rb) and I, thereby increasing the energy barrier of halogen ion migration, thereby inhibiting halogen ion migration and light-induced phase segregation. Based on ~2.0eV Rb/Cs inorganic perovskite, we achieved a laboratory efficiency of 24.3% for all-perovskite triple-junction solar cells, and we also achieved the first reported certified perovskite-based triple-junction solar cell efficiency (23.3% quasi-steady-state efficiency). The device maintains 80% of the initial efficiency after 420 hours of continuous operation. This work was recently published in Nature under the title ‘Suppressed phase segregation for triple-junction perovskite solar cells’.
【graphic introduction】
