| Author: | Wang, Tianyue |
| Title: | The study of tin-based perovskite solar cells with high efficiency and stability |
| Advisors: | Yan, Feng (AP) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Perovskite solar cells Solar cells Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xx, 132 pages : color illustrations |
| Language: | English |
| Abstract: | Organic-inorganic hybrid perovskite materials have aroused wide research interests due to their superior optical and electrical properties. One successful application of such materials is solar cell, which has recently achieved a power conversion efficiency (PCE) of 25.5%. However, the involvement of toxic lead in common perovskite solar cells (PSCs) may constitute an obstacle for their further commercialization. Less toxic tin-based perovskites are regarded as the most promising alternative among various kinds of lead-free perovskite compounds. It has been found that tin-based PSCs suffer from lower efficiency and stability than their lead analogues because Sn2+ in tin-based perovskites can be easily oxidized to Sn4+ and induce Sn vacancy defects that lead to severe nonradiative charge recombination in the films. In addition, the rapid crystallization of tin-based perovskites during solution processes can result in poor film morphology and high defect concentration. Moreover, special requirements are needed in charge transport layers of tin-based PSCs to get better energy alignment and interface property. All of the issues make it challenging to achieve high performance tin based PSCs. In the thesis, antioxidant gallic acid (GA) additive together with excess SnCl2 were first introduced into FASnI3 perovskite to improve its stability and the device performance. This coadditive engineering approach enables capping of the perovskite grains with an amorphous SnCl2-GA complex and leads to suppressed Sn2+ oxidation and defect densities in the perovskite films. Moreover, the bandgap of SnCl2-GA complex is decreased and its conduction band is shifted downward compared to SnCl2, which facilitates electron transfer from perovskite grains through the complex layer to electron transport layer and reduces recombination loss. By systematically adjusting the amount of coadditives, PSCs based on the inverted structure can attain a PCE of 9.03%. The FASnI3-based PSCs also exhibit striking long-term stability, which show no degradation in PCE after being stored in N2 filled glovebox for more than 1500h. And the unencapsultaed devices can maintain ~80% of the initial efficiency over 1000 h storage upon air exposure (~20% RH). Second, we have developed a special technique by utilizing liquid-exfoliated two-dimensional (2D) transition-metal dichalcogenides as the interlayer for synergistic modulation of perovskite growth and the interfaces in PSCs. Liquid exfoliated MoS2, WS2 and WSe2 flakes (generally referred as MX2) were introduced between hole transport layer (NiOx) and FASnI3 perovskite. The smooth and defect-free face of MX2 can promote van der Waals epitaxial growth of perovskite film with enlarged sizes. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) provide evidence that perovskite films on MX2 exhibit highly oriented packing of (100) plan in the out-of-plane direction. In comparison with MoS2 and WS2, WSe2 possesses shallower VBM and is judiciously selected as an efficient charge transport interlayer due to its high hole mobility and good energy alignment with the VBM of HTL and perovskite film. The resultant WSe2 incorporated PSCs exhibit increased photovoltaic parameters in Jsc, Voc and FF, which can be attributed to the synergy of enhanced charge transport, less interfacial recombination loss and stronger light absorption. Consequently, a PCE of 10.47% is obtained for the champion device. Third, perovskite dimensionality manipulation and replacement of PCBM with novel electron transport layer (ETL) were performed simultaneously to reduce the trap density in the perovskite film and optimize band energy alignment. Bulky organic cation phenethylammonium (PEA) was incorporated into FASnI3, which effectively generates low-dimensional (LD) perovskite phase in the 3D perovskite. PEA was found to induce superior crystallinity and oriented growth of FASnI3 on NiOx substrates with preferential facet of (100). The high quality LD/3D perovskite with well packing of crystal planes exhibits higher structure robustness than FASnI3 perovskite, resulting in reduced trap density within the bulk films. Through further using indene-C60 bisadduct (ICBA) with shallower Lowest Unoccupied Molecular Orbital (LUMO) level as the novel ETL, the band energy off-set between LD/3D perovskite and ETL is minimized, leading to enhanced charge extraction and reduced non-radiative recombination at the interface. By adopting these strategies, the Voc of the device is increased to more than 0.9V, which is among the highest Voc reported so far. This work highlights the significance of perovskite compositional engineering and device structure modulation in enhancing the Voc and efficiency of tin-based PSCs. |
| Rights: | All rights reserved |
| Access: | open access |
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