Exploring Methods to Achieve High-Efficiency Organometal Wide-Bandgap Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Awarding Institution
Award date9 Aug 2019


As a new promising photovoltaic technique organometal halide perovskite solar cells (PVSCs) are attracting tremendous attention for its widely-tunable bandgap (12-2.33 eV), great light absorption, high power conversion efficiency and low-cost fabrication method. Featured with the advantages, the record efficiency of PVSCs has been boosted from 3.8% to 24.2% in less than seven years.

Typically, PVSCs can be roughly divided into two categories according to their device structure, viz., single-junction PVSCs and two-junction (tandem) PVSCs. Although single-junction PVSCs have made great progress and occupied the recorded efficiency of PVSCs, the intrinsic theoretical efficiency limitation (33.7%) makes it difficult to get further development. On the other side, guaranteed by the broader light absorption range, two-junction perovskite/perovskite tandem device exhibits a much higher power conversion potential (46.1%) and regarded as the most promising commercialization technique. Although the advocated strategy has boosted perovskite to a new level, it is still not well studied and far from commercialization. Especially, for the top wide-bandgap cell, which plays pivotal roles in the tandem device, achieving high-performance (high efficiency and stability) devices is still challenging.

Achieving high-performance wide-bandgap PVSCs lies at the heart of the investigation of tandem devices, which is crucial to determine the tandem devices power conversion effects. Crystallization properties is critical important to understand the film formation mechanism. Solution engineering method plays an important role to achieve high quality perovskite films. To reveal the mechanism of the appearance of pin-holes in the MA-based wide-bandgap perovskites, perovskites with high Br composition (MA0.9Cs0.1Pb(I0.6Br0.4)3) is studied. In this study, an in-situ photoluminescence technique was used to investigate the perovskite crystallization during the thermal annealing process. It is found that the crystallization of a mixed halide perovskite with bromide (Br-) and iodine (I-) ions following the Ostward ripening crystal growth. Interestingly, it is found that the initial nucleation reaction is quickly completed in the first few seconds, however, leaving the small crystals with inhomogeneous composition. The different aggregation affinities of such inhomogeneous small crystals provoke the formation of pin-holes during the thermal annealing process. By engineering the precursor solution to control the nucleation rate, the chemical composition of the small crystals has become homogenous. Uniform pin-hole free high Br- composited wide-bandgap MA0.9Cs0.1Pb(I0.6Br0.4)3 perovskite films with bandgap energy of 1.8 eV have been realized. The corresponding photovoltaic devices have achieved an encouraging device efficiency of 13.3% with superb photostability.

Composition engineering is another crucial method that affects the film bandgap grain size and crystallinity, which will eventually affect film morphology. To tackle with the drawback of the MA-based wide-bandgap perovskites (different aggregation properties of the inhomogeneous initial small particles during the crystallization process), Pb(SCN)2 is introduced into MAPbI2Br perovskites. The merit of the Pb(SCN)2 additive for the formation of large grains can make the crystallization process overcome the drawback and achieve pin-hole free perovskites. With the further incorporation of Cs+ to improve the PbX2 (X=I, Br) solubility in the precursor solution, the aggregated PbI2 particles, induced by the Pb(SCN)2 additive, in the final perovskite film are also eliminated. The synergistic effect of Pb(SCN)2 additive and Cs+ guarantee a high quality uniform MA0.9Cs0.1PbI2Br(SCN)0.08 wide-bandgap (~1.77 eV) perovskite film. Since the appearance of pin-holes is associated with the organic cation MA+, FA+ is introduced into MAPb(I0.6Br0.4)3 as another composition engineering method by tuning the organic cation compositions. The big difference of the ionic radius across the A cation sites and X halide sites leads to the internal distortions of the crystals, which has a profound effect on the lattice parameter. Corroborated by the XRD, PL and SEM results, it is confirmed that the incorporation of FA+ is preferable to the formation of uniform high-quality wide-bandgap perovskites.

In summary, three types organometal wide-bandgap perovskites are rationally designed and well studied in the thesis, targeting at revealing the film formation mechanism and achieving high quality films. It is believed that the methods and results in this thesis not only provides new insight into the perovskite properties but also give new inspirations in the community.