Suppression of the interface-dependent nonradiative recombination by using 2-methylbenzimidazole as interlayer for highly efficient and stable perovskite solar cells

Highlights

• A small molecule, 2-methylbenzimidazole (MBIm), is used to modify the SnO₂ ETL/perovskite interface.

• MBIm interlayer shifts the work function of SnO₂ resulting in more effective electron extraction and an increase in the built-in potential.

• Lone pair electrons in MBIm passivate uncoordinated ions in perovskite by forming Lewis adducts, minimizing nonradiative recombination.

• SnO₂ passivation induces the formation of compact perovskite film with improved crystallinity, allowing reduction in the moisture penetration.

• Not only prolonged stability but also breakthrough efficiency of 21.6% is achieved with a dramatic increase in V_OC (~90 mV).

Abstract

Despite the tremendous progress in the efficiency of perovsite solar cells (PSCs), nonradiative recombination losses, mainly associated with the interfacial defects, still remain as a challenge that hinders their commercialization. In this study, we report a facile interface engineering strategy for highly-efficient planar PSCs by employing in a series concentration of 2-methylbenzimidazole (MBIm) between SnO₂ electron transporting layer (ETL) and photoactive perovskite layer. The preliminary results demonstrate that MBIm molecules reduce the band‐offset and enlarge the built-in potential (V_bi) between perovskite and SnO₂, resulting in a lower photovoltage loss. Besides, MBIm provides an efficient passivation by donating the lone pair electrons to the uncoordinated Pb²⁺ ions in perovskite structure through the formation of Lewis adducts, thereby minimizing nonradiative recombination in the ensuing devices. As a result, a remarkable increase in the efficiency from 19.5% (pristine cell) to 21.6% (3 mM-MBIm modified cell) was achieved with a dramatic increase in V_OC (~90 mV). Meanwhile, an admissible improvement in long-term stability was obtained by retaining ~85 and 90% of initial performance under high humidity and continuous light soaking conditions, respectively. The prolonged stability is ascribed to the formation of compact and high-quality perovskite layer deposited on the modified surfaces. We believe that this study offers an efficient strategy by minimizing the nonradiative recombination losses through ETL/perovskite interface for high-efficiency and stable perovskite cells.

Introduction

Since the energy consumption is growing as a function of population and economic development, green technologies are improving far faster to meet the world's rising energy demand beyond fossil fuels [1]. Among various clean energy technologies, photovoltaics (PVs), which directly converts the solar energy into electrical energy, can strongly promote the transition from fossil sources to alternative renewable energy sources. Organic−inorganic metal halide perovskite solar cells (PSCs) are the most emerging area of research among different PV technologies owing to their salient optoelectronic properties and facile processing routes [[2], [3], [4], [5], [6], [7], [8]]. Following the first demonstration by Miyasaka et al. [9], a striking progress has been registered on the cell performance and thanks to this, efficiencies have gone from single digits to a certified value of 25.2% in a short period of time [10]. Notwithstanding, substantial photovoltage losses due to nonradiative recombination in such devices, mainly originated from interfacial defects, prohibit them from reaching the radiative limit defined by the Shockley–Queisser (SQ) theory [[11], [12], [13], [14]]. Thereof, highly-efficient cells with low open-circuit voltage (V_OC) deficit can be fabricated only by minimizing the concentration of defects associated with recombination processes in the active layer and by eliminating the defect-states at the corresponding interfaces [15].

As far as we know, the interfaces between perovskite and charge transporting layers, especially electron transporting layer (ETL) ̵̶̶ as revealed by the recent impedance studies [[16], [17], [18], [19]] ̵̶̶ play an indispensable role in reducing the nonradiative recombination losses by transferring the photo-generated charges effectively [20]. The utilization of an engineered interface between the ETL and perovskite is one of the most effective ways to eliminate these parasitic charge carrier recombinations. Such an engineered interface can also acquire a high-quality electron conductor with reduced band‐offset. Additionally, it plays a subsidiary role in inducing the formation of compact perovskite layer for improving the efficiency and stability of ensuing perovskite cells [21]. Therefore, the interfaces and the interfacial materials need to be carefully engineered to avoid all the aforementioned issues.

Thus far, tremendous efforts have been devoted to suppress such defects and perform molecular modification at ETL/perovskite interface by introducing organic or inorganic interlayer materials [17,22,23]. Compared to inorganic materials, organic materials, especially small molecules, possess the advantages of well‐defined structure, multifunctional anchoring groups, good surface coverage, and perfect batch‐to‐batch reproducibility [24]. In one of the pioneering works in this field, Liu et al. introduced organic silane layer between TiO₂ ETL and CH₃NH₃PbI₃ perovskite [25]. With this method, they successfully obtained an ETL with lower work function as well as compact perovskite film with improved crystallinity and less defects. The resulting devices indicated a noticeable improvement in the efficiency from 9.6 to 12.7%. In another interesting study, a zwitterionic compound, 3-(1-pyridinio)-1-propanesulfonate, was introduced to modify the SnO₂ ETL/perovskite interface [26]. This modification led to a shift in the work function of ETL resulting in more effective electron extraction and an enhancement in built-in potential (V_bi). Moreover, an improvement in the stability of devices was reported by the help of positively charged atoms in the zwitterion, which passivated Pb–I antisite defects. In a similar direction, the density of trap states was reduced by introducing a thin layer of 3,3-diphenylpropylamine (DPPA) as a passivation material at the interface between perovskite and ZnO ETL [27]. The oxygen deficiency based defects on the ZnO surface were passivated by the formation of Zn–N bond, indicating improved charge collection and reduced interfacial (parasitic) charge recombination. Also, perovskite film on modified ETL showed an improved crystallinity with compact grains and reduced trap density. As a result, a significant enhancement in the performance associated with the remarkable increase in V_OC was reported.

Inspired by these works, we employed small organic molecule 2-methylbenzimidazole (denoted as MBIm) for the first time to modify the mixed cation/halide based perovskite and ETL interface. Not only facile deposition method, but also low cost and commercial availability of MBIm make it as a feasible candidate for large-scale industrial applications. In this study, low-temperature processed SnO₂ is preferred as ETL owing to its wide optical bandgap (3.6–4.0 eV), high electron mobility over 100 cm²V⁻¹s⁻¹, high conductivity, deep conduction band, and excellent chemical/UV stability. The influence of MBIm molecule on the photovoltaic performance and long-term stability of the PSCs was elucidated by modulating the passivating layer concentration (1 mM–10 mM). As an imidazole derivative, MBIm is a heterocyclic building block [28]. As reported by Hagfeldt et al., heterocyclic building blocks efficiently attach to the metal oxide-based ETL surface, and shift the conduction-band edge of ETL toward higher energy levels, and thus, retard the interfacial electron recombination by increasing the electron lifetime [29,30]. Thanks to the molecular structure of MBIm, it can interact with the Lewis acid defects in perovskite thorough N atom, whereas methyl chain in the structure presents hydrophobicity. These functional groups can provide considerable potential for more effective passivation effect in device applications.

In addition to significantly improved crystal quality and open grain boundary-free surface morphology of perovskite, the obtained results have been intensively shown that interfacial treating reduces the density of trap states along the interface and enables faster electron transport by minimizing the energetic disorder. Consequently, an admissible efficiency of 21.6% was achieved with a notable increase in V_OC of ~90 mV compared with those yielded by the MBIm-free cells (19.5%). The possible reasons of the performance improvement were investigated using various techniques such as space-charge limited current (SCLC), Mott-Schottky (M − S), photoluminescence (PL), and electronic impedance spectroscopy (EIS) measurements.