New Approaches toward Stable Perovskite Solar Cells

製備稳定鈣鈦礦太陽能電池的新方法

Student thesis: Doctoral Thesis

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Award date14 Jan 2021

Abstract

Solar energy harvesting has been considered as one of the best choices to solve the energy crisis and air pollution. One of the most common technologies to utilize solar energy is photovoltaic solar cells. In recent years, a revolution has been witnessed in the photovoltaics field due to the rapid evolution of the emerging perovskite solar cells (PSC). To date, a certified power conversion efficiency (PCE) of 25.2%, which is comparable to commercial crystalline silicon solar cells, has been achieved. However, the PSC instability against water/moisture remains a key obstacle toward practical application.

In this thesis, the water effect on the performance of PSC has been studied. It is demonstrated that two forms of water (hydrate water and bulk water) exist in the hybrid perovskite film. Hydrate water induces a recrystallization process, resulting in perovskite film with enhanced crystallinity, uniformity, and reduced impurities. While bulk water leads to decomposition of perovskite. Further, by controlling the annealing temperature, it is found that 60 ℃ is the most suitable temperature for the formation of hydrated perovskite. With further annealing, the resulting perovskite film reveals further enhanced crystallinity. In addition, the effect of water on different types of perovskites is studied, revealing that the formation of hydrated perovskite is mainly determined by the cation of the perovskite itself.

After reaching a good understanding of the water effect on perovskites, compositional engineering was conducted to design perovskite with enhanced stability. Large guanidinium (GA) cation was incorporated in traditional perovskite to fabricate a novel quadruple cation based CsFAMA1-xGAx perovskite. The introduction of GA induces a phase separation of 3D CsFAMA1-xGAx, 2D FAGAPbI4, and 1D δ-FAPbI3. By tuning the content of GA, a δ-FAPbI3/CsFAMA1-xGAx (1D/3D) perovskite with superior optoelectronic properties and much improved stability is demonstrated.

Subsequently, mixed halide all-inorganic CsPbI3-xBrx perovskite, where x represents the content of bromine in the precursor, was systematically studied. To solve the notorious phase transition of CsPbI2Br perovskite, an air stable rubidium (Rb) incorporated Cs(1-x)RbxPbI2Br perovskite with guanidinium bromide (GABr) post-treatment is demonstrated. The resulting films reveal a 2D/3D heterostructure with high crystallinity, excellent surface morphology, and significantly enhanced ambient stability. After that, all-inorganic CsPbIBr2 perovskite was investigated due to its excellent environmental stability. To reduce the required high annealing temperature (>250 ℃), a novel seed-assisted growth (SAG) method was conceived by introducing methylammonium halides (MAX, X=I, Br, Cl) during the fabrication for low temperature processed CsPbIBr2 solar cells. It is found that MABr treated perovskite (Pvsk-Br) film annealed at 150 ℃ shows high crystallinity with micrometer-sized grains and superior charge dynamic properties. The resulting devices show an average PCE of over 10%. Finally, an organic dye, fluorescein isothiocyanate (FITC), was adopted as passivator to reduce the open-circuit voltage loss (Voc, loss) of all-inorganic PSC. The results suggest that the carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interaction with the under-coordinated Pb2+ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, the resulting passivated device based on the optimal halide compositional inorganic CsPbI1.5Br1.5 perovskite yields a PCE of 14.05%, which presents the highest reported efficiency for inorganic CsPbI1.5Br1.5 solar cells thus far.

Overall, this thesis work contributes to providing a comprehensive understanding of the water effect on hybrid perovskite solar cells. Besides, new strategies related to cation exchange demonstrate effectiveness in fabricating stable perovskite photovoltaic cells. The results are promising and of great significance in designing and fabricating robust perovskite materials for future practical application.

    Research areas

  • stability, Perovskite Solar Cells, all-inorganic