Integrated Materials and Device Engineering Towards High-Performance Perovskite Solar Cells

面向高性能鈣鈦礦太陽能電池的材料及器件工程

Student thesis: Doctoral Thesis

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Award date15 Oct 2020

Abstract

Organic-inorganic perovskite solar cells (PVSCs), as an emerging photovoltaic technology, have been vigorously developed in the past few years. The impressive optoelectronic properties of perovskites, such as high absorption coefficients, low exciton-binding energies and tunable bandgaps, enabling them as a promising candidate for large-scale photovoltaic applications. Recently, a very high certified efficiency of 25.5% has been reported, which is on par with those of the inorganic photovoltaics, including polycrystalline silicon (p-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe).

Despite these great efforts, the efficiencies of PVSCs are still lower than their theoretically achievable values (calculated from the Shockley-Queisser model) because of the defects formed in polycrystalline perovskite films. These defects are usually formed during the growth process of perovskite crystals, which can be broadly categorized as point defects (including vacancies, interstitial, anti-site substitution) and higher dimensional defects (edge dislocation, grain boundary, precipitate). Meanwhile, the heterojunctions between perovskites and charge-transporting layers (CTLs) are another primary source of defects, due to the dangling bonds on the surface of perovskites and CTLs. Fortunately, when a defect state lies shallowly within the bandgap (Eg) of the perovskites, the trapped carriers can be relatively easily de-trapped and excited back to the conduction/valence band by small activation energy. However, the deep trap states will significantly quench the carrier emission via the annihilation/recombination processes that occurred between the opposite carriers, resulting in substantial energy losses in the devices. Moreover, the deep trap states can interact with the environmental stimuli thus accelerate the degradation process of the device.

The purpose of my work conducted in this thesis is aiming to realize high-efficiency PVSCs with enhanced device stability. To fundamentally address the above issues, the quality of perovskite film is the most critical point which determines the intrinsic properties of the perovskite layer. Different strategies have been employed to obtain the high-quality perovskites, such as the exploration of alternative A-site organic cations and X-site halide anions (general formula of perovskite: ABX3) and the use of additives for the perovskite layer. As expected, crystal growth of perovskite could be controlled therefore obtain high-quality perovskite film with reduced density of trap states. Moreover, the non-radiative recombination losses at both hole-transporting layer (HTL)/perovskite and perovskite/electron-transporting layer (ETL) interfaces are also effectively suppressed by introducing a large alkylammonium interlayer (LAI) between perovskite and HTL.

Specifically, X-site composition engineering was studied initially to tune the properties of the perovskite layer. The impact of pseudo-halide, i.e., tetrafluoroborate (BF4- ) on the crystal structure and optoelectrical properties of perovskites were systematically investigated. It was found that the BF4- can be incorporated into the perovskite lattice, causing the lattice relaxation, prolonged TRPL lifetime, larger recombination resistance, and improved perovskite quality with low defect density. These impressive properties yielded a high power conversion efficiency (PCE) of 20.16% for the resultant PVSC, which is the highest value for the reported PVSCs based on BF4- substituted perovskite. This work provides insight for further exploration of anion substitutions for perovskites to improve the PVSCs performance.

Then, the A-site composition engineering for perovskites was demonstrated to manipulate the growth of perovskites. The large guanidinium (Gua) cation with zerodipole was incorporated into mixed cation perovskites, enabling the perovskite film to have a significantly longer carrier lifetime, sharper Urbach tail and lower trap density of states (tDOS). As a result, the 10% molar ratio of Gua based PVSC achieved a champion PCE of 21.12%. More impressively, an substantially enhanced Voc of 1.19 V could be realized with a Voc,loss of only 420 mV. These results indicate that Gua can act as a promising A-site cation to modulate the perovskite crystal structure, which should be highly applicable for other optoelectronic devices based on perovskites.

The third work is related to the study of passivation effect of additives on the defects. A series of novel functionalized graphitic carbon nitride (F-C3N4) with the different functional groups is designed and used as additives for perovskites precursor solution, yielding an obviously enhanced PCE from 17.85% (control PVSC) to 20.08% (PVSC with NO3-C3N4 as additive) for inverted PVSC. These findings present an effective strategy to design the novel passivation materials to further enhance the performance of PVSCs.

The last part of my work is focused on the development of an effective strategy to suppress the significant recombination and charge-extraction losses at the dual interfaces for inverted PVSCs. A large alkylammonium interlayer (LAI) was predeposited on the HTL to simultaneously manipulate the perovskite growth and passivate the interfacial defects. Compared with the previously reported surface passivation strategies for a single interface, the use of LAIs can suppress the nonradiative energy losses at both top and bottom interfaces of perovskite. As expected, the reduced surface recombination velocity (SRV) and mitigated phase segregation enable a remarkable improvement in photovoltage from 1.12 to 1.21 V for the inverted PVSCs with the bandgap (Eg) of 1.59 eV, leading to a champion PCE of over 22%, which is among the highest values for the reported PVSCs with inverted structure.