UCLA

Organic-inorganic metal halide perovskites (MHPs) for photovoltaic applications have emerged rapid progress since their first successful demonstration since a decade ago. The record power conversion efficiency (PCE) of lab-sized (typically <1 cm2) perovskite solar cells (PSCs) have risen from 14.1% to 25.5%, and small modules have reached 17.9% PCE in early 2021. With representative compositions including methylammonium lead triiodide (MAPbI3), formamidinium lead triiodide (FAPbI3), cesium lead triiodide (CsPbI3), or hybrids of these cations with mixed halide compositions (e.g. FAxCs1-xPbI3, FAxMA1-xPbI3-yBry), perovskite materials generally exhibit near infrared bandgap, ideal for single-junction solar cells. However, as the band edges formed by the hybridization of the electronic orbitals from the B-site lead and the X-site halide, the optical bandgap of perovskites could be precisely tuned by controlling the halide I/Br ratio, making configuring tandem photovoltaics with halide-perovskite possible which is also the core of this dissertation. Starting from Chapter 1, I will give a brief introduction on MHPs and tandem photovoltaics. The critical role of defect physics plays in halide perovskite solar cell’s performance and stability will also be discussed in this chapter, and I will introduce my early work at UCLA in delivering a certified 22.0% efficiency monolithic perovskite-CIGS tandem which is the origin of most of my ideas proceed to this thesis. After that, I will introduce strategies we developed that achieved highly efficient 4-terminal perovskite-Cu(In,Ga)(Se,S)2 and perovskite-silicon tandem solar cells in Chapter 2. Numerical simulation and analysis unraveled the efficiency losses in the perovskite sub-cell were still the bottlenecking factor in limiting the performance of perovskite-based tandem photovoltaics. An electronic defects passivation strategy at perovskite surface were developed utilizing a synergetic effect of post-treated surface fluoride and phenethylammonium iodide (PEAI) that mitigated surface traps. In Chapter 3, a more in-depth investigation of the surface reconstruction of perovskite during post-treatments was carried out at the microscale level based on a series of surface-sensitive or depth-resolved characterizations. A change of the perovskite surface could be induced by the commonly used post-treatment solvent, isopropyl alcohol (IPA), which directly affect the surface electronic affinity, surface termination, and surface defect feature. However, it was also found the IPA wash in fact assisted the ligand adsorption (e.g. PEA+) to the perovskite surface and thus enhanced their defect passivation effect. In Chapter 4, a set of in-situ or carefully designed ex-situ experiments were developed to distinguish the different formation dynamics and defect physics between the wide bandgap mixed-halide perovskite for tandem applications and the low bandgap tri-iodide perovskite for single junction solar cells, as the performance losses in the mixed-halide perovskite were significantly larger than their tri-iodide counterpart. The results show that the inclusion of bromide introduced a halide homogenization process during the perovskite growth stage from an initial bromide-rich phase towards the final target stoichiometry. We further elucidated a physical model that correlates the role of bromide with the formation dynamics, defect physics, and eventual optoelectronic properties of the film. After recognizing the critical role of surface properties and surface defects in halide perovskite, Chapter 5 will introduce a theoretical investigation of the formation and energy levels of perovskite surface defects. FAPbI3, which is the composition that delivered the record photovoltaic performance among the perovskite family, was chosen for this study. All surface point defects were considered under two most commonly seems terminations, namely PbI2 termination and FA-I termination. Insights of combining theoretical results and experimental outcomes were also discussed. In Chapter 6, I will introduce organic small molecules named Y1 and Y2 that also showed tremendous potential in tandem application. They are non-fullerene acceptors (NFAs) with near infrared absorption and could serve as an ideal absorber as rear cell. The optoelectronic properties of Y1 and Y2 were delicately tuned by the introduction of unconventional electron-deficient-core-based fused structure, and their single junction devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm−2, resulting in record-breaking power conversion efficiencies of over 13.4% (certified 12.6%).