ZiQi Sun

Outstanding material properties of organic-inorganic hybrid perovskites have triggered a new insight into the next-generation solar cells. Beyond solar cells, a wide range of controllable properties of hybrid perovskites, particularly depending on crystal growth conditions, enables versatile high-performance optoelectronic devices such as light-emitting diodes, photodetectors, and lasers. This article highlights recent progress in the crystallization strategies of organic–inorganic hybrid perovskites for use as effective light harvesters or light emitters. Fundamental background on perovskite crystalline structures and relevant optoelectronic properties such as optical band-gap, electron-hole behavior, and energy band alignment are given. A detailed overview of the effective crystallization methods for perovskites, including thermal treatment, additives, solvent mediator, laser irradiation, nanostructure, and crystal dimensionalityis reported offering a comprehensive correlation among perovskite processing conditions, crystalline morphology, and relevant device performance. Finally, future research directions to overcome current practical bottlenecks and move towards reliable high performance perovskite optoelectronic applications are proposed.

Organic–inorganic hybrid perovskite is a promising material for next-generation optoelectronic devices. A wide range of optoelectronic properties of perovskite are controllable for target applications using effective crystal growth. Recent progress in perovskite optoelectronics is highlighted, focusing particularly on solar cells and light-emitting diodes, as well as crystallization strategies for light harvesting and light emitting.

Organic–inorganic halide perovskite (OIHP) solar cells with efficiency over 18% power conversion efficiency (PCE) have been widely achieved with lab scale spin-coating method which is however not scalable for the fabrication of large area solar panels. The PCEs of OIHP solar cells made by scalable deposition methods, such as doctor-blading or slot-die coating, have been lagging far behind than spin-coated devices. In this study the authors report composition engineering in doctor-bladed OIHP solar cells with p–i–n planar heterojunction structure to enhance their efficiency. Phase purer OIHP thin films are obtained by incorporating a small amount of cesium (Cs⁺) and bromine (Br⁻) ions into perovskite precursor solution, which also reduces the required film formation temperature. Pinhole free OIHP thin films with micrometer-sized grains have been obtained assisted by a secondary grain growth with added methylammonium chloride into the precursor solution. The OIHP solar cells using these bladed thin films achieved PCEs over 19.0%, with the best stabilized PCE reaching 19.3%. This represents a significant step toward scalable manufacture of OIHP solar cells.

By tuning the composition of precursor solution high phase purity, perovskite thin films are obtained at a temperature of 120 °C via doctor-blading, and over 19% power conversion efficiencies are achieved in inverted p–i–n structured perovskite solar cells.

Remarkable power conversion efficiencies (PCE) of metal–halide perovskite solar cells (PSCs) are overshadowed by concerns about their ultimate stability, which is arguably the prime obstacle to commercialization of this promising technology. Herein, the problem is addressed by introducing ethane-1,2-diammonium (⁺NH₃(CH₂)₂NH₃⁺, EDA²⁺) cations into the methyl ammonium (CH₃NH₃⁺, MA⁺) lead iodide perovskite, which enables, inter alia, systematic tuning of the morphology, electronic structure, light absorption, and photoluminescence properties of the perovskite films. Incorporation of <5 mol% EDA²⁺ induces strain in the perovskite crystal structure with no new phase formed. With 0.8 mol% EDA²⁺, PCE of the MAPbI₃-based PSCs (aperture of 0.16 cm²) improves from 16.7% ± 0.6% to 17.9% ± 0.4% under 1 sun irradiation, and fabrication of larger area devices (aperture 1.04 cm²) with a certified PCE of 15.2% ± 0.5% is demonstrated. Most importantly, EDA²⁺/MA⁺-based solar cells retain 75% of the initial performance after 72 h of continuous operation at 50% relative humidity and 50 °C under 1 sun illumination, whereas the MAPbI₃ devices degrade by approximately 90% within only 15 h. This substantial improvement in stability is attributed to the steric and coulombic interactions of embedded EDA²⁺ in the perovskite structure.

Mixed organic cation lead–halide perovskite solar cells demonstrate remarkably improved stability while maintaining high efficiency. Incorporation of low concentrations of ethylenediammonium into CH₃NH₃PbI₃ perovskite enables fabrication of planar solar cells with up to 18.6% power conversion efficiency that retain 75% of their performance after 72 h of continuous operation under 1 sun irradiation at 50 °C and 50% relative humidity.

The fabrication of high-quality cesium (Cs)/formamidinium (FA) double-cation perovskite films through a two-step interdiffusion method is reported. Cs_xFA_1-xPbI_3-y(1-x)Br_y(1-x) films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs⁺ and Br⁻ on the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm²) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs_0.1FA_0.9PbI_2.865Br_0.135) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).

A modified two-step interdiffusion method is developed to fabricate high-quality cesium/formamidinium double cation perovskites with various compositions that have superior intrinsic thermal stability than those with methylammonium cation. Perovskite solar cells and modules based on Cs_0.1FA_0.9PbI_2.865Br_0.135 show the highest power conversion efficiency of 19.3% and 16.4%, respectively, in a planar structure.

Pinhole-free, dense perovskite films with large grains, which can be achieved by control of the nucleation and crystal growth, are essential for high-performance perovskite solar cells. These perovskite-solar-cell systems are discussed by Fuzhi Huang, Yi-Bing Cheng, and co-workers in article number 1601715.

Solution-processed perovskite (PSC) solar cells have achieved extremely high power conversion efficiencies (PCEs) over 20%, but practical application of this photovoltaic technology requires further advancements on both long-term stability and large-area device demonstration. Here, an additive-engineering strategy is developed to realize a facile and convenient fabrication method of large-area uniform perovskite films composed of large crystal size and low density of defects. The high crystalline quality of the perovskite is found to simultaneously enhance the PCE and the durability of PSCs. By using the simple and widely used methylammonium lead iodide (MAPbI₃), a certified PCE of 19.19% is achieved for devices with an aperture area of 1.025 cm², and the high-performing devices can sustain over 80% of the initial PCE after 500 h of thermal aging at 85 °C, which are among the best results of MAPbI₃-based PSCs so far.

By enhancing the crystalline quality of the simple and widely used MAPbI₃ perovskite through additive engineering, unprecedented photovoltaic performance and thermal stability are achieved. A certified efficiency of 19.19% is obtained for devices with active area over 1 cm², and the high-performing devices show unprecedented durability, maintaining >80% of the initial efficiency after 500 h of thermal aging at 85 °C.

Solar cells with organic-inorganic lead halide perovskites have achieved great success and their power conversion efficiency (PCE) has reached to 22.1%. To address the toxicology of lead element and some stability issues associated with the organic-inorganic lead halide perovskites, inorganic lead-free perovskites have gained more attentions from the photovoltaic research community. Herein, a series of chalcogenide perovskites are proposed as optical absorber materials for thin-film solar cells. SrSnSe₃ and SrSnS₃ are predicted to be direct bandgap semiconductors with the bandgap value being within the optimal range of 0.9–1.6 eV. Both SrSnSe₃ and SrSnS₃ not only exhibit good optical absorption properties and carrier mobility, but also possess flexible bandgaps that can be continuously tuned within the grange of 0.9–1.6 eV via the element-mixing strategy, thereby render both perovskites as promising candidates for photovoltaic applications.

SrSnSe₃ and SrSnS₃ are predicted to be direct gap semiconductors with bandgap value being within the optimal range of 0.9–1.6 eV, to exhibit good optical absorption properties and high carrier mobility, and to enable flexible bandgaps continuously tuned within the range of 0.9–1.6 eV via the elemental mixing strategy, thereby render both materials as promising candidates for photovoltaic applications.

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high-temperature preparation of metal oxides as a charge transport layer in most state-of-the-art PSCs has been becoming a big stumbling block for future low-temperature and large-scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low-temperature-processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all-solution-processed PSCs, we propose a novel n-type sulfur-containing small molecule hexaazatrinaphtho[2,3-c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10⁻² cm² V⁻¹ s⁻¹ as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via SI or SPb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n-type organic small molecules for high-performance PSCs.

A new sulfur-containing n-type organic small molecule hexaazatrinaphtho[2,3-c][1,2,5]thiadiazole (HATNT) is proposed for perovskite solar cells. In comparison with traditional PC₆₁BM, benefitting from much more significant suppression of charge recombination at the MAPbI₃/HATNT interface and strong interfacial interaction between the MAPbI₃ and HATNT via SI or SPb bonding, solution-processed high-performance HATNT-based perovskite solar cells are demonstrated with an optimized efficiency up to 18.1%.

The meteoric rise of perovskite single-junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single-junction efficiencies. Unconstrained by the Shockley–Quiesser single-junction limit, perovskite tandems suggest a real possibility of true third-generation thin-film photovoltaics; monolithic all-perovskite tandems have reached 18% efficiency and will likely pass perovskite single-junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long-range charge-transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap-tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple-junction perovskite cells are presented, outlining roadmaps for future focused research.

The remarkable progress of perovskite tandem solar cells, now above 25% efficiency for a silicon–perovskite four-terminal tandem and 18% for monolithic all-perovskite tandems, is reviewed. In detail, the candidate materials, contact layers, and device challenges are examined, outlining a roadmap toward a future of true third-generation thin-film photovoltaics comprising high-efficiency at low cost.

2D perovskites have recently been shown to exhibit significantly improved environmental stability. Derived from their 3D analogues, 2D perovskites are formed by inserting bulky alkylammonium cations in-between the anionic layers. However, these insulating organic spacer cations also hinder charge transport. Herein, such a 2D perovskite, (iso-BA)₂(MA)₃Pb₄I₁₃, that contains short branched-chain spacer cations (iso-BA⁺) and shows a remarkable increase of optical absorption and crystallinity in comparison to the conventional linear one, n-BA⁺, is designed. After applying the hot-casting (HC) technique, all these properties are further improved. The HC (iso-BA)₂(MA)₃Pb₄I₁₃ sample exhibits the best ambient stability by maintaining its initial optical absorption after storage of 840 h in an environmental chamber at 20 °C with a relative humidity of 60% without encapsulation. More importantly, the out-of-plane crystal orientation of (iso-BA)₂(MA)₃Pb₄I₁₃ film is notably enhanced, which increases cross-plane charge mobility. As a result, the highest power conversion efficiencies (PCEs) measured from for current density versus voltage curves afford 8.82% and 10.63% for room-temperature and HC-processed 2D perovskites based planar solar cells, respectively. However, the corresponding steady-state PCEs are remarkably lower, which is presumably due to the significant hysteresis phenomena caused by low charge extraction efficiency at interfaces of C₆₀/2D perovskites.

2D organometal halide perovskites with high environmental stability are successfully obtained by introducing short branched-chain spacer cations. The resulting (iso-BA)₂(MA)₃Pb₄I₁₃ exhibits a remarkable increase of optical absorption, crystallinity, and in particular out-of-plane orientation in comparison to the conventional linear n-BA⁺. A high power conversion efficiency of 10.63% is hence obtained for such 2D perovskite solar cells.

All-inorganic perovskite light-emitting diodes (LEDs) reveal efficient luminescence with high color purity, but their modest brightness and poor stability are still critical drawbacks. Here, the luminescent efficiency and the stability of perovskite LEDs (PeLEDs) are boosted by antisolvent vapor treatment of CsPbBr₃ embedded in a dielectric polymer matrix of polyethylene oxide (PEO). A unique method is developed to obtain high quality CsPbBr₃ emitting layers with low defects by controlling their grain sizes. CsPbBr₃ in PEO matrix is post-treated with antisolvent of chloroform (CF), leading to microcrystals with a size of ≈5 µm along the in-plane direction with active emitting composite of 90%. A device based on CF post-treatment (CsPbBr₃-PEO-CF) film displays a brightness of up to 51890 cd m⁻² with an external quantum efficiency of 4.76%. CsPbBr₃-PEO-CF PeLED still maintains 82% of its initial efficiency after 80 h continuous operation in ambient air, which indicates relatively good device stability. This work highlights that film quality is not only key to promoting fluorescence in CsPbBr₃, but also to achieving higher performance PeLEDs.

Antisolvent vapor treatment of CsPbBr₃ films embedded in a dielectric polymer matrix film is proposed, resulting in microcrystal size of ≈5 µm with low defect density. A light-emitting diode based on composite CsPbBr₃ films with this antisolvent vapor treatment displays a brightness of 51890 cd m⁻² and an external quantum efficiency of 4.76%.

As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single- or dual-layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO₂/TiO₂ GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO₂ and TiO₂ networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO₂ network to the high electron mobility SnO₂ network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO₂ ETL constructed under the comparable conditions.

A gradient heterojunction electron transport layer (GHJ ETL), prepared by a facile low-temperature route, is utilized in perovskite solar cells (PSCs) for the first time. PSCs based on the potential GHJ ETL demonstrate an efficiency of 18.08% with less hysteresis effect, which is due to excellent management of charge transport and recombination.

Photodetectors with ultrafast response are explored using inorganic/organic hybrid perovskites. High responsivity and fast optoelectronic response are achieved due to the exceptional semiconducting properties of perovskite materials. However, most of the perovskite-based photodetectors exploited to date are centered on Pb-based perovskites, which only afford spectral response across the visible spectrum. This study demonstrates a high-performance near-IR (NIR) photodetector using a stable low-bandgap Sn-containing perovskite, (CH₃NH₃)_0.5(NH₂CHNH₂)_0.5Pb_0.5Sn_0.5I₃ (MA_0.5FA_0.5Pb_0.5Sn_0.5I₃), which is processed with an antioxidant additive, ascorbic acid (AA). The addition of AA effectively strengthens the stability of Sn-containing perovskite against oxygen, thereby significantly inhibiting the leakage current. Consequently, the derived photodetector shows high responsivity with a detectivity of over 10¹² Jones ranging from 800 to 970 nm. Such low-cost, solution processable NIR photodetectors with high performance show promising potential for future optoelectronic applications.

A high-performance NIR photodetector derived from a stable low optical bandgap (E _g) Sn-containing perovskite, MA_0.5FA_0.5Pb_0.5Sn_0.5I₃, is introduced. Ascorbic acid is used as an effective antioxidant additive to enhance the performance of the photodiode. Finally, a high detectivity of over 10¹² Jones between 800 and 970 nm with a high response rate is achieved.

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃ perovskite films deposited under different yet optimized conditions using X-ray scattering and cathodoluminescence spectroscopy, respectively. X-ray scattering shows that a CH₃NH₃PbI₃ film involving spin coating of CH₃NH₃I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH₃NH₃PbI₃ film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH₃NH₃PbBr₃, while the crystallinity decreases when the HBr solution is used in a single-step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH₂)₂PbBr₃ devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.

A comparative analysis of the local structural, morphological, and emission characteristics of different perovskite films rationally justifies the higher photovoltage yielded by the better performing devices.