Planar structures for halide perovskite solar cells have recently garnered attention, due to their simple and low-temperature device fabrication processing. Unfortunately, planar structures typically show I–V  hysteresis and lower stable device efficiency compared with mesoporous structures, especially for TiO2-based n-i-p devices. SnO2, which has a deeper conduction band and higher electron mobility compared with traditional TiO2, could enhance charge transfer from perovskite to electron transport layers, and reduce charge accumulation at the interface. Here we report low-temperature solution-processed SnO2 nanoparticles as an efficient electron transport layer for perovskite solar cells. Our SnO2-based devices are almost free of hysteresis, which we propose is due to the enhancement of electron extraction. By introducing a PbI2 passivation phase in the perovskite layer, we obtain a 19.9 ± 0.6% certified efficiency. The devices can be easily processed under low temperature (150 ∘C), offering an efficient method for the large-scale production of perovskite solar cells. Planar structured perovskite solar cells often show hysteresis and lower efficiency than mesoporous ones. Jiang et al. show that using a SnO2 electron transport layer improves the performance of planar devices, reporting a certified efficiency of 19.9%, and enables a lower processing temperature.

Enhanced electron extraction using SnO 2 for high-efficiency planar-structure HC(NH 2 ) 2 PbI 3 -based perovskite solar cells

Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–xCsxPbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 p...

Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys

In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1–3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4–6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers. An atomically coherent interlayer between the electron-transporting and perovskite layers in perovskite solar cells enhances charge extraction and transport from the perovskite, enabling high power conversion efficiency.

Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes.

Organic and inorganic hybrid perovskites (e.g., CH(3)NH(3)PbI(3)), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications

A new chapter in the long and distinguished history of perovskites is being written with the breakthrough success of metal halide perovskites (MHPs) as solution-processed photovoltaic (PV) absorbers. The current surge in MHP research has largely arisen out of their rapid progress in PV devices; however, these materials are potentially suitable for a diverse array of optoelectronic applications. Like oxide perovskites, MHPs have ABX3 stoichiometry, where A and B are cations and X is a halide anion. Here, the underlying physical and photophysical properties of inorganic (A = inorganic) and hybrid organic–inorganic (A = organic) MHPs are reviewed with an eye toward their potential application in emerging optoelectronic technologies. Significant attention is given to the prototypical compound methylammonium lead iodide (CH3NH3PbI3) due to the preponderance of experimental and theoretical studies surrounding this material. We also discuss other salient MHP systems, including 2-dimensional compounds, where rele...

Intriguing Optoelectronic Properties of Metal Halide Perovskites

Lead halide perovskite solar cells with the high efficiencies typically use high-temperature processed TiO2 as the electron transporting layers (ETLs). Here, we demonstrate that low-temperature solution-processed nanocrystalline SnO2 can be an excellent alternative ETL material for efficient perovskite solar cells. Our best-performing planar cell using such a SnO2 ETL has achieved an average efficiency of 16.02%, obtained from efficiencies measured from both reverse and forward voltage scans. The outstanding performance of SnO2 ETLs is attributed to the excellent properties of nanocrystalline SnO2 films, such as good antireflection, suitable band edge positions, and high electron mobility. The simple low-temperature process is compatible with the roll-to-roll manufacturing of low-cost perovskite solar cells on flexible substrates.

Low-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells

Thin-film photovoltaics based on organic–inorganic hybrid perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. Over the past several years, we have witnessed a great and unexpected progress in organic–inorganic perovskite solar cells (PSCs). The power conversion efficiency (PCE) increased from 3.8% to 20.1% and exceeded the highest efficiency of conventional dye-sensitized solar cells. Here, the focus is specifically on the recent developments of the electron transport layer (ETL) in PSCs, which is an important part for high performing PSCs. This review briefly discusses the development of the structure of PSCs, and we attempt to give a systematic introduction about the optimization of ETL and its related interfaces for efficient PSCs. Moreover, the introduction of appropriate interfacial materials is another important issue to improve PSC performance by optimizing the interfacial electronic properties between the perovskite layer and the charge-collecting electrode. Besides, some related issues such as device stability and hysteresis behavior are also discussed here.

Recent progress in electron transport layers for efficient perovskite solar cells

Review on the Application of SnO2 in Perovskite Solar Cells

Lead halide perovskite solar cells use hole-blocking layers to allow a separate collection of positive and negative charge carriers and to achieve high-operation voltages. Here, the authors demonstrate efficient lead halide perovskite solar cells that avoid using this extra layer.

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Efficient hole-blocking layer-free planar halide perovskite thin-film solar cells.

We report a simple and effective interface engineering method for achieving highly efficient planar perovskite solar cells (PSCs) employing SnO2 electron selective layers (ESLs). Herein, a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) was used to modify the SnO2 ESL/perovskite layer interface. This APTES SAM demonstrates multiple functions: (1) it can increase the surface energy and enhance the affinity of the SnO2 ESL, which induce the formation of high quality perovskite films with a better morphology and enhanced crystallinity. (2) Its terminal functional groups form dipoles on the SnO2 surface, leading to a decreased work function of SnO2 and enlarged built-in potential of SnO2/perovskite heterojunctions. (3) The terminal groups can passivate the trap states at the perovskite surface via hydrogen bonding. (4) The thin insulating layer at the interface can hinder electron back transfer and reduce the recombination process at the interface effectively. With these desirable properties, the best-performing cell employing a APTES SAM modified-SnO2 ESL achieved a PCE over 18% and a steady-state efficiency of 17.54%. Impressively, to the best of our knowledge, the obtained VOC of 1.16 V is the highest value reported for the CH3NH3PbI3 (MAPbI3) system. Our results suggest that the ESL/perovskite interface engineering with a APTES SAM is a promising method for fabricating efficient and hysteresis-less PSCs.

Guang Yang

Papers

Low-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells

Recent progress in electron transport layers for efficient perovskite solar cells

Review on the Application of SnO2 in Perovskite Solar Cells

Efficient hole-blocking layer-free planar halide perovskite thin-film solar cells.

Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement