Harmonizing the bilateral bonds of the interfacial molecule boosts the power conversion efficiency of n-i-p perovskite solar cells to 26.5%: A new research perspective on the buried interface
Perovskite solar cells (PSCs), as a kind of promising photovoltaic technology, have the advantages of high efficiency, low cost, and easy manufacturing, which are expected to provide a feasible technical choice for the realization of the carbon peaking and carbon neutrality goals. The buried interface between the substrate and the perovskite layer is one of the major defect-rich areas in PSCs. The optimization and regulation of the buried interface is crucial to further improve the performance of PSCs, which have attracted more and more research attention in recent years. As early as the beginning of 2019, our research group, i.e. Professor Rui Zhu’s group at Peking University, has already started to focus on the buried interface of perovskite films and to unveil the mystery of the buried interface of PSCs (Advanced Materials 2021, 33, 206435).
Usually, the researchers choose to introduce the interfacial molecules at the buried interface of the PSCs, which are validated to be able to effectively regulate the buried interface and improve the photoelectric conversion performance of PSCs. Then as the assistant mentor of Mr. Qiuyang Li, I selected bis(2-aminoethyl) ether (BAE) containing two –NH2 groups at both ends of the molecule, which seems to be promising as a kind of interfacial molecule, and gave it to him one day in the middle of 2022. Because from the end of 2016 to now, I have almost devoted myself to the study of regular-structure n-i-p PSCs, we chose to start from the n-i-p PSCs with SnOx as the electron transport layer (ETL). I soon got the feedback from Qiuyang that BAE works with SnOx-based n-i-p PSCs and it can effectively enhance the open-circuit voltage (VOC) and the power conversion efficiency (PCE) of PSCs. This verifies my speculation that BAE can serve as the interfacial molecule.
Considering the universality of the strategy, we then applied the BAE molecule at the buried interface of mesoscopic n-i-p PSCs with mesoporous-structure TiOx as ETL. However, there appeared a significant drop in PCE after the introduction of BAE with a serious decrease in VOC and fill factor. Even more strangely, when observing the PSC entities, we found that the backside of the BAE-treated TiOx-based device exhibited aberrant white streaks along the radiating pattern of TiOx rather than uniform dark reddish brown like both untreated TiOx-based and SnOx-based devices. Some previous literature reports that this may be caused by a large number of dendritic crystals at the bottom of the perovskite film, which is also responsible for the deterioration of device performance. The cross-sectional scanning electron microscopy (SEM) images of the complete TiOx-based PSC devices confirm that there are some aggregation-like clusters or voids at the buried interface due to the introduction of BAE.

We thus have raised a quite interesting and prevalent but unsolved scientific problem which often happens in the daily research of the field of perovskite optoelectronic devices “Why do there appear variations or contrary results of applying the same modifier molecule on different charge transport layer substrates of the same semiconducting type such as n-type TiOx and SnOx?”. Then we discussed these results and thoughts with Professor Rui Zhu, a wise scientist and a supportive supervisor who always gives us enough freedom to do the research that we are really interested in. We have realized that the underlying mechanism of this phenomenon as well as the corresponding modulation strategies to solve such a universal problem are worth being studied, while it has always been ignored and there are few studies on it.
We have noted that in most cases, in order to exert the modulation effects, the interfacial molecules need to be able to strongly interact/react with perovskite components. However, excessively strong interaction/reaction could lead to a series of problems, that is, the reactive interfacial molecules can uncontrollably insert into the perovskite bulk layer during film formation, leading to a decline in device performance, or inducing progressive ligand intercalation during device working, resulting in device degradation. We have also noted that the bilateral bonds of a BAE molecule, that is, the bond of BAE with buried ETL and the bond of BAE with perovskite, are competitive. The too-strong BAE-perovskite bond implies the weak BAE-buried-ETL bond. We thus performed the density functional theory (DFT) calculations and the results show that the binding energy (Eb) of BAE molecules on the TiOx surface is almost equal to Eb of BAE on the perovskite surface while more positive than Eb of BAE on sulfhydryl- and chlorine-rich SnOx surface. Combined with the experimental results, we speculate that the bonding of the interfacial molecule BAE with TiOx is not enough to resist the sequential solvent damage, and a weaker or appropriate bond with the perovskite to avoid the insertion into perovskite bulk is necessary.

This inspires us that we can weaken the BAE-perovskite bond through enhancing the BAE- TiOx bond. We therefore doped TiOx with Li2CO3 to modulate its electronic structure, in order to enhance the chemical bonding between BAE and TiOx as evidenced by the more negative Eb from DFT calculations. This accordingly weakens the BAE-perovskite bonding, resulting in the harmonization of the bilateral bond strength of the interfacial molecule BAE. Upon such modification, the PSC device appears again with a dark reddish brown backside and no obvious streaks or clusters, which is much different from that of the BAE-treated one but similar to that of the control one. Our harmonization strategy, as expected, led to the uniform assembly of BAE molecules and fewer BAE molecules entering perovskite film, indicating better surface affinity between BAE molecules and Li-doped TiOx. The resultant more robust buried interface with lower interfacial defect density significantly improves the photovoltaic performance of PSCs, as evidenced by the results of characterizations and photovoltaic test. Our strategy has ultimately delivered a breakthrough in PCE of n-i-p PSCs based on TiOx up to 26.52%, with a certified value of 26.31% from National Institute of Metrology (NIM) in China. This is the highest efficiency for mesoporous TiOx-based PSCs, which is comparable to that of other types of PSCs. It also works with SnOx with an improved PCE. In addition, our bilateral bond harmonization strategy can stabilize α-phase formamidinium lead iodide and can enhance the stability of corresponding PSCs upon humidity, thermal, or light stress. Our work may also guide the future modulation of other buried interfaces such as doped tin oxide electrode/self-assembled monolayer (SAM)/perovskite or NiOx/SAM/perovskite in p-i-n PSCs.

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