Recent advancement in inorganic-organic electron transport layers in perovskite solar cell: current status and future outlook

Highlights

• Role of organic-inorganic ETLs in PSCs are discussed briefly.

• Modifications adopted in synthesis method, doping, and low-temperature process techniques for different PSCs are summarized.

• Inorganic-organic ETLs with various structures of PSCs, mainly focusing on crystallization of thin-films, are also reviewed.

Abstract

Organic-inorganic lead halide perovskite solar cells have captured significant attention in recent years due to low processing costs and unprecedented development in power conversion efficiency (PCE). It has appeared from 2009 with PCE of 3.8% to being claimed more than 25.2% PCE in a very short span of time, showing their future prospective toward the fabrication of less expensive and stable solar cells. The incredible advancement in this technology encourages at one end, whereas several hurdles restricting its complete utilization for commercial purposes at another end. Although the selection of perovskite structure is limited with planar and mesoporous electron transport layers (ETLs), but identification of appropriate ETLs necessitates excellent effort to improve the surface morphology of absorber and obtain enhanced PCE with higher stability. In the present review, we have investigated various inorganic-organic ETLs with different device configurations of PSCs, primarily focusing on crystallization and morphology control techniques of ETL thin films. Numerous strategies such as surface functionalization, doping, and addition of interfacial layer are adopted for ETLs, and their effect on device efficiency, performance, and hysteresis is also discussed in detail. Additionally, designs of PSCs with different device configurations are discussed as well, providing future guidelines for significant progress in PSCs structure with different ETLs.

Introduction

The continuous growth of the industrial sector worldwide leads to demanding a tremendous amount of power for their operation [[1], [2], [3]]. In 21^st century, the energy supply is a major worldwide challenge. To fulfill the energy demand, the transition from fossil fuel to renewable energy is essential to the contribution of future energy production and promotion [[4], [5], [6]]. Moreover, it has been predicted that the global energy demand becomes double by 2050. About 85% of the world's energy need is entirely filled by traditional fossil fuel that offers to answer the increasing concern of greenhouse gases and global warming, which can also be linked to climate change [7,8]. Therefore, the search for renewable and sustainable energy sources is the primary concern for humankind in the coming years as per our energy requirements. Solar cells, being pollution-free, environmentally friendly, and eternal, are some of the most promising technologies to inhibit the excessive discharge of greenhouse gases in the atmosphere to solve the serious issue of global warming [[9], [10], [11], [12]]. Sunlight is one of the most abundant renewable sources of energy that can satisfactorily fulfill the energy demand of the entire world. But, it is possible if the technologies are readily available for harvesting and supplying it from the sun without any detrimental impact on the environment [[13], [14], [15], [16], [17]].

In recent years, perovskite-based solar cells have been revolutionized device performance. They have rapidly surpassed the efficiencies of many emerging and commercial photovoltaic technologies, such as organic, amorphous, and dye-sensitized silicon solar cells (DSSC) [7,[18], [19], [20]]. The perovskite is an ideal material for solar cells because of its appropriate direct bandgap, apparent tolerance defect, excellent carrier transport, and high absorption coefficient [[21], [22], [23], [24]]. The perovskite-based solar devices have been widely investigated as the most promising technology and attracted great attention of photovoltaic researchers community, working on different solar cell technologies such as organic photovoltaics and dye-sensitized solar cells because of their incredible power conversion efficiency (PCE) from 3.8% to 25.2% from 2009 to till date [[25], [26], [27], [28], [29], [30]].

The general configuration of PSCs typically consists of an absorbing layer which is inserted between ETL, usually an n-type semiconductor, and HTL, generally, a p-type semiconductor, as well as fluorine-doped tin oxide (FTO) and metal electrodes are served as substrate material and back electrode, respectively [[31], [32], [33], [34]]. However, wide variation is possible in the configuration of the perovskite structure. The typical working principle is described as in which electron and holes generated at light-harvesting material are extracted and transported by ETL and HTL, respectively, as shown in Fig. 1(a). The working principle and charge transport mechanism are described in Fig. 1(b, c), in which routes 1 to 4 represent the electron transfer, hole transfer, electron transportation and collection, charge recombination at ETL/perovskite interface, and charge recombination between electrons and holes of ETL and HTL, respectively [35]. Further, these charge carriers are collected by electrode layers and formed PSCs.

Moreover, ETLs are also defined as electron transportation layers, electron collection, or extraction layers whose conduction band minimum (CBM) should be lower than that of perovskite absorber [[36], [37], [38]]. The PSC research started with TiO₂ as ETL in 2009, where a solar cell was designed with Pt-coated FTO glass substrate and CH₃NH₃PbX₃-TiO₂ photo-electrode, exhibited PCE of 3.81% and 3.13% for CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ absorber, respectively [25]. Kim et al. have designed PSCs with submicro-thin mesoporous TiO₂ as ETL and delivered a maximum PCE of 9.7% [39]. However, the requirement of high sintering temperature for the growth of TiO₂ and often hysteresis effect has also been observed in the J-V curve of TiO₂-based devices, given the birth to another potential ETL candidate such as ZnO and SnO₂ owing to their compatibility for low-temperature deposition [[40], [41], [42]]. Zhang et al. have stated that reduced device efficiency has been observed for low-temperature processed devices mainly due to the recombination at deficient interfaces or structural and chemical defects [43]. Research in PSCs specifically focuses on low-temperature processing, which views different types of other ETLs such as CdS, Nb₂O₅, Zn₂SO₄, Fe₂O₃, In₂S₃, IGZO, and In₂O₃ have also been widely investigated and applied in PSCs [[44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]]. In fact, organic ETLs (fullerene, fullerene derivative, and non-fullerene acceptor) have also shown their presence in PSCs and reduced the recombination losses and photocurrent hysteresis in PSC devices [[55], [56], [57], [58]]. Furthermore, different strategies such as preparation of multilayer ETLs, optimization of thickness and doping concentration, development of nanostructured nanorods, nanocones, as well as interfacial and surface engineering approaches were adopted to develop the stable and highly efficient PSCs [[59], [60], [61], [62], [63]].

There are many review articles already published on organic and inorganic perovskite solar cells, but our motivation is not to follow the same trend. We have identified all commonly used ETLs (inorganic and organic) separately and provide a brief discussion about each ETL's physical and optoelectronic properties along with the variation in device parameters of PSCs. In this study, a comprehensive and updated review on inorganic-organic ETLs with different device architectures is presented. We have examined the commonly used ETLs and mainly focused on several syntheses and deposition techniques and typical strategies adopted to optimize the thickness and doping of ETLs and their effect on device performance of PSCs. Different perovskite structures along with several device configurations of PSCs are also discussed in brief.