Minimizing Energy Loss of Perovskite Solar Cells by Interface Engineering and Surface Modification

致力於減少鈣鈦礦太陽能電池能量損耗的界面調控和表面修飾的研究

Student thesis: Doctoral Thesis

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Award date14 Sep 2021

Abstract

Perovskite solar cells (PVSCs) have become a promising candidate for next generation commercialized photovoltaic technology due to their excellent performance. Recently, a certified power conversion efficiency (PCE) of 25.5% was realized for PVSCs, which is comparable to that of commercialized silicon solar cells. Despite their rapid development, there are still several limitations hindering the further large-scale production of PVSCs.

First, the PCE of PVSCs is still lower than the theoretical Shockley–Queisser limit with severe Eloss, which is mainly caused by defect- and interface-induced charge recombination. Despite of tremendous efforts dedicated to minimizing Eloss, there is considerable scope to develop facile but effective method to suppress the non-recombination loss from the aspect of crystal growth control, grain boundary and interfacial defect passivation, and band alignment optimization. Second, the stability of the device is another major concern and the stability of organic–inorganic hybrid species may be compromised by film degradation owing to the thermal instability of methylammonium (MA) ions. Developing MA-free perovskite film by replacing MA cations with cesium (Cs) and formamidinium (FA) cations offers the best approach for developing long-term stable PVSC devices. In addition, the benchmark hole-transport materials (HTMs) used in n-i-p PVSCs, such as spiro-OMeTAD and PTAA, usually need a complex doping process to ensure good hole-transport properties. However, the incorporated dopants and the relevant oxidation reactions will seriously deteriorate the long-term stability of PVSC devices. Therefore, it is of great urgency to suppress the energy loss of PVSCs by interface engineering and surface modification towards high performance PVSCs.

Given that perovskite growth is quite substrate-dependent, the underneath electron-transport layer (ETL) plays important roles in achieving a high-quality perovskite film for n-i-p PVSCs. Chapter 3 introduced the modification of the bottom-side ETL and incorporation of an ultra-thin PEIE layer between the SnO2 and perovskite layers. The higher-lying work-function of the ETL of SnO2/PEIE created a better energy alignment with the conduction band minimum of CsPbIBr2, while the favorable interactions between the amino groups on PEIE and CsPbIBr2 promoted the growth of a perovskite film with improved crystallinity and increased grain size. Accordingly, interfacial trap-assisted recombination has been greatly suppressed and photo-stability has been improved. As a result, remarkable Voc and PCE of SnO2/PEIE-based CsPbIBr2 PVSC was achieved with high values of 1.29 V and 11.2%. Moreover, PEIE modification resulted in enhanced photostability, SnO2/PEIE-based PVSC retained over 80% of its initial value after continuous one sun illumination for 500 h.

In Chapter 4, we paid efforts to improve the device stability via interface engineering on hole-transport layer (HTL) side. It is found that the intrinsic poor thermal stability of Spiro-OMeTAD and the ionic migration accelerated by the necessary dopants cause devastating effects on the long-term stability of PVSCs. Developing novel dopant-free HTMs to replace spiro-OMeTAD turned out to be effectively method to avoid dopant-induced instability issues, but the low intrinsic hole-mobility presents a challenge for these materials at the current stage. Therefore, we designed a novel spirofluorene–dithiolane-based small molecular HTM called SFDT-TDM, which was developed through facile and low-cost synthetic routes. The C-H…π interactions in the adjacent SFDT-TDM were beneficial for high hole-mobility, and the methylthio groups in SFDT-TDM served as a Lewis base to passivate the defects on the surface of perovskite films, leading to suppressed nonradiative recombination and enhanced charge extraction at the perovskite/HTL interface. Consequently, the CsxFA1-xPbI3-based PVSCs with SFDT-TDM as the HTM realized high PCEs of 21.7% and 20.3% for small (0.04 cm2) and large (1.0 cm2) area devices with negligible photocurrent hysteresis, respectively. Additionally, the fully inorganic CsPbI3-xBrx-based PVSCs with SFDT-TDM demonstrated an impressive PCE of 17.1% and excellent stability.

In addition, employing the p-i-n structure instead of n-i-p structure is also a strategy for promoting the long-term stability of PVSC devices since it can exempt the use of spiro-OMeTAD. But the main challenge for this kind of PVSCs lies in the serious Voc loss, especially for inorganic PVSCs. Chapter 5 highlighted the design of the device configuration to avoid dopant-reduced film degradation and the development of inorganic inverted p-i-n PVSCs. Considering that a large number of positively/negatively charged defects on the perovskite surface and at grain boundaries can induce large open-circuit voltage losses in PVSCs, a simple surface passivation strategy with Lewis base 6TIC-4F is presented. DFT calculations and XPS measurements illustrate that numerous nitrogen (N) atoms possessing lone pair electrons on 6TIC-4F could passivate the surface defects of CsPbIxBr3−x film via direct coordination with lead ions (Pb2+) through the formation of Lewis adducts. In addition, the employed 6TIC-4F also tended to trigger the nucleation of a perovskite precursor, leading to the formation of a larger grain size and denser film. Furthermore, 6TIC-4F enabled a better energy alignment across the perovskite/ETL interface to provide improved electron extraction efficiency. Consequently, the optimized device with the inverted structure of ITO/NiOx/CsPbIxBr3−x/ZnO/C60/Ag delivered a record PCE of 16.1% and a certificated value of 15.6%.

    Research areas

  • Perovskite solar cells, energy loss, interface engineering, surface modification