Low-Dimensional Materials Enabled Highly Efficient and Stable Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Awarding Institution
Award date4 May 2023


Along with the rapid industrialization over the past centuries, incessant energy consumption and endless damage to the environment have aroused growing attention for seeking clean and renewable energy sources. To achieve net zero CO2 emissions by 2050, renewable power needs to expand significantly to meet the share of 60% by 2030. Renewables, including solar, wind, hydro, biofuels, and others, are at the center of the transition to a less carbon-intensive and more sustainable energy system. Solar PV is one of the most competitive technologies in the market today, but the average annual additions still need to be doubled in the next five years in the forecast. Compared to the conventional photovoltaics based on silicon, copper indium gallium selenide (CIGS), and cadmium telluride (CdTe), organic-inorganic hybrid perovskite solar cell (PVSC) has exhibited great potential for its low-costs, tunable bandgaps, good flexibility, semi-transparency, and light-weight properties. Although the power conversion efficiency (PCE) of PVSCs has been boosted to over 25%, further enhancement in performance and long-term stability are the main issues to meet the commercialization requirements.

Unlike the robust perovskite oxides, the frail Coulombic interaction and weak ionic bonding of organic-inorganic halide perovskites cause more vulnerable atomic reorganization and deviations in the derived film. The crystal lattice interference and atomic periodicity perturbation at perovskites alter the electronic behavior and band structure. Originating from this ionic soft nature of polycrystalline perovskite, a considerable number of charged defects are generated at interfaces, acting as trap centers and the triggers of film degradation. Such defects-induced nonradiative recombination loss drags the PCE of PVSCs behind the predicted values (calculated from the Shockley-Queisser model). Besides, to facilitate the commercialization of PVSCs, the long-term operational stability under humidity, heat, and light illumination needs to be significantly enhanced. The mismatched thermal expansion coefficients between the perovskite film and the substrate underneath induce a significant strain unevenly distributed over the perovskite film during thermal annealing. This residual strain plays a critical role in affecting the mechanical stability of PVSCs, especially for devices operating under nonconstant temperatures in practical day-night cycling conditions. In addition, the organic cations and halide anions of perovskites tend to migrate faster under light illumination to release the strain, further speeding up the degradation. My work in this thesis focuses on improving the efficiency and stability of PVSCs toward further commercialization. To address the issues mentioned above, low-dimensional (LD) materials are employed to modulate the crystallization kinetics, passivate the defects and alleviate the residual strain of perovskite film. Fundamental studies of the structure-property relationships are discussed to provide design rules for materials.

Firstly, quasi-2D PVSCs were studied to demonstrate good stability and performance of 2D layered perovskites. NH4SCN additive was employed and found can effectively tune the crystal orientation of quasi-2D perovskite films from random to vertical on the substrates. Also, the layer number distribution can be narrowed to be around n = 3 and n = 4 with NH4SCN addition. These combined effects can facilitate charge transport across the layers and improve photovoltaic performance. The derived quasi-2D PVSC demonstrated very good ambient stability, which retained 85% of its initial PCE under a relative humidity of 50 ± 5% for 900 h. And the PVSCs of vertically grown quasi-2D layered perovskites showed a champion PCE of 14.53%, which is among the highest values reported for 2D PVSCs prepared at room temperature. However, the efficiency of quasi-2D PVSCs is still far below that of the 3D-based perovskite, which requires more efforts toward commercialization.

Then, applying large space cations upon the surface of perovskite was introduced, which can initiate the phase transformation from 3D to LD perovskite for passivating the defects and protecting the bulk layer from decomposition. A novel bifunctional molecule, piperazinium iodide (PI), containing both R2NH and R2NH2+ group on the same six-membered ring with an in-plane dipole was developed, that is capable of functioning as an electron donor and an electron acceptor to react with different surface terminating groups. It was found that PI could form a Pb-N bond in the single crystal of (PI)2PbI2·2DMSO, which was grown as a model compound to investigate the chemical interactions between PI and PbI2. After introducing PI for surface treatment, the residual stress of the film surface could also be released. The tuning of surface termination with PI treatment also helps to achieve a more n-type characteristic film to facilitate charge transfer. The combined effects resulted in suppressed nonradiative recombination loss and a significantly reduced energy loss of 0.33 eV with very low nonradiative recombination-induced energy loss of 60.13 meV. The PCE of inverted (p-i-n) devices achieved 23.37% (with 22.75% certified) with an open-circuit voltage (Voc) of 1.17 V, which is the highest value for inverted PVSCs to date. This work reveals a very effective way of using rationally designed bifunctional molecules to simultaneously enhance the device's performance and stability.

The third work is related to the improvement of perovskite film quality through a multifunctional, nonvolatile additive (4-guanidinobenzoic acid hydrochloride, GBAC). A hydrogen bond-bridged intermediate phase (1D structured GBAC-PbI2-DMF) was formed and can be used to modulate the crystallization kinetics and serve as an effective nonvolatile passivation linker in the annealed film. The combined structural analysis of single crystals, in situ photoluminescence (PL) characterizations, and the density functional theory (DFT) calculations reveal that the kinetics of crystal growth is regulated by the additional energies required for breaking the hydrogen bonds and exchanging the cations between the intermediate phase and perovskite. Therefore, the obtained perovskite films exhibit significantly enhanced crystallinity with large-sized grains (up to 1 μm). Besides, the additives can tightly pack at the grain boundaries due to the π-π interaction of the benzene rings to effectively passivate the defects (such as uncoordinated lead cations and organic/inorganic vacancies ) through its carboxyl and guanidinium groups, improving the intrinsic stability and reducing the non-radiative recombination of perovskite films. The inverted PVSCs could achieve a champion PCE of 24.8% with a Voc of 1.19 V, short-circuit current density (Jsc) of 24.55 mA/cm2 and fill factor (FF) of 84.78%. A certified PCE of 24.5% was obtained from the Japan Electrical Safety & Environment Technology Laboratories (JET), which is among the highest value of inverted PVSCs reported so far. Due to the significantly improved film quality and minimized defects-induced non-radiative recombination, the total energy loss was diminished to 0.36 V, which is very close to that reported for GaAs photovoltaics. This strategy also demonstrates great versability as a similar photovoltage enhancement can be observed for wide-bandgap and large-area PVSCs, which paves the way for developing multijunction and scalable perovskite photovoltaics.

Finally, considering the residual strain induced during the growth of perovskite film significantly affects the efficiency and stability of PVSCs. SiO2-coated gold nanorods (GNR@SiO2) were introduced into perovskite film and took advantage of the plasmonic local heating effect for in situ strain relaxation. The GNR@SiO2-incorporated inverted PVSCs exhibit a champion PCE of over 23%, which is the highest PCE in plasmonic-incorporated PVSCs. Moreover, the intrinsic stability of the resulting PVSCs is greatly improved for the nonencapsulated device and retains 84% of its initial PCE after 2800 h aging under continuous illumination at 65 ± 5 ºC in an N2 glove box and nearly 90% after 1000 h repetitive 12 h light on-off cycles. This work provides an efficient yet easy-to-implement plasmonic heating strategy for simultaneously enhancing the efficiency and stability of PVSCs.