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Understanding the intrinsic phase stability and inherent band gap of formamidinium lead triiodide (FAPbI

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.

Summary In small-area perovskite solar cells, efficiencies of >25% have been achieved using an antisolvent dripping technique; however, it is not applicable for coatings on an industrial scale. Currently, large-area devices based on scalable perovskite coatings still show a very large gap with small-area devices. This means that a uniform perovskite coating and defect control on large-area devices are not as secure as they are on small devices. Here, we report the deposition of dense and uniform perovskite films using an air-knife-assisted bar-coating employing a perovskite [(FAPbI3)0.95(MAPbBr3)0.05] precursor solution dissolved in 2-methoxyethanol. A pinhole-free and homogeneous surface morphology was achieved by adding n-cyclohexyl-2-pyrrolidone to ensure a balance between rapid nucleation and slowed crystal growth in the precursor solution. By applying surface passivation with acetylcholine bromide and laser etching to thin layers, mini-modules with an aperture area of 31 cm2 were fabricated, attaining an efficiency of >20% (17.53% in the efficiency certified by a quasi-steady-state protocol).

Formamidinium lead triiodide (FAPbI3) perovskite solar cells (PSCs) are mainly fabricated by sequentially coating lead iodide and formamidinium iodide, or by coating a solution in which all components are dissolved in one solvent (one-pot process). The PSCs produced by both processes exhibited similar efficiencies; however, their long-term stabilities were notably different. We concluded that the major reason for this behaviour is the stabilization of the α-FAPbI3 phase by isopropylammonium cations produced by the chemical reaction between isopropyl alcohol, used as solvent, and methylammonium chloride, added during the process. On this basis, we fabricated PSCs by adding isopropylammonium chloride to the perovskite precursor solution for the one-pot process and achieved a certified power conversion efficiency of 23.9%. Long-term operational current density–voltage measurements (one sweep every 84 min under 1-Sun irradiation in nitrogen atmosphere) showed that the as-fabricated device with an initial efficiency of approximately 20% recorded an efficiency of about 23% after 1,000 h that gradually degraded to about 22% after an additional 1,000 h. The operational stability of formamidinium lead triiodide solar cells varies with the fabrication method of the perovskite layer. Now Park et al. find that isopropylammonium stabilizes the perovskite structure and leads to solar cells with 2,000-h stability under constant illumination.

Summary Cesium lead triiodide (CsPbI3) presents a desirable band gap, does not require the use of mixed halides for Si tandem solar cells, and possesses relatively high thermal stability owing to its inorganic components. However, the power conversion efficiency (PCE) of CsPbI3 is lower than that of organic cation-based halide perovskites with identical band gaps. The main factors that govern the PCE of CsPbI3 are the surface morphology and defect passivation of its thin films on substrates. In this study, we used the sequential dripping of a methylammonium chloride (MACl) solution (SDMS) to obtain highly uniform and pinhole-minimized thin films by controlling the intermediate stages of the crystallization process, followed by surface passivation using octylammonium iodides in ambient air. SDMS accelerated the crystallization process of the CsPbI3 perovskite layer, resulting in the formation of a uniform and dense surface with few pinholes. Consequently, we fabricated CsPbI3 solar cells with excellent PCE (20.37%).

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

The A cation in ABX 3 organic-inorganic lead halide perovskites (OLHPs) was conventionally believed to hardly affect their optoelectronic properties. However, more recent developments have unraveled the critical role of the A cation in the regulation of the physicochemical and optoelectronic properties of OLHPs. We review the important breakthroughs enabled by the versatility of the A cation and highlight potential opportunities and unanswered questions related to the A cation in OLHPs.

Halide perovskites have attracted tremendous attention from many researchers recently, particularly for their excellent optoelectronic properties in applications such as photovoltaic solar cells and light-emitting diodes. In recent years, the application of halide perovskites has rapidly extended into nanoelectronics, such as thermoelectric, memory, and artificial synapse applications. Halide perovskites can be synthesized easily, even at relatively low temperatures, and organic and inorganic ions can even coexist in one crystal structure. Moreover, the structural flexibility is excellent, where two- and three-dimensional crystals can be linked together. The combination of various types of halide ions not only controls the physical properties of the halide perovskite, but also facilitates control of the bandgap by varying the size of nanoparticles when they exhibit quantum effects. Halide perovskites thus provide an excellent platform for optoelectronics with interesting optical, electrical, and magnetic properties. The articles in this issue introduce the wide range of basic properties and potential applications of halide perovskites.

For the practical application of perovskite solar cells (PSC), it is desirable to have high efficiency, long-term stability, and low manufacturing cost. Therefore, it is required to develop inexpensive and well-performing hole-transporting materials (HTMs). In this study, we synthesized SFXDAnCBZ, which is a new carbazole-based spiro[fluorene-9,9'-xanthene] (SFX) derivative, where the central core and end-cap units consist of SFX and N3,N6-bis(di-4-anisylamino)-9H-carbazole (DAnCBZ), respectively, as an efficient and low-cost HTM for PSCs. Photoluminescence quenching at the SFXDAnCBZ/perovskite interface was more effective than at the perovskite/Spiro-OMeTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxy-phenyl-amine) 9,9'spiro-bifluorene) interface. We fabricated a PSC with a power conversion efficiency (PCE) of 20.87% under 1 sun illumination (100 mW cm-2) using SFXDAnCBZ as an HTM. This value is comparable to that measured for the benchmark Spiro-OMeTAD. Thus, this result confirms that SFX core-based materials can be a new kind of HTMs for high-efficiency and low-cost PSCs.

For the fabrication of perovskite solar cells (PSCs) using a solution process, it is essential to understand the characteristics of the perovskite precursor solution to achieve high performance and reproducibility. The colloids (iodoplumbates) in the perovskite precursors under various conditions were investigated by UV-visible absorption, dynamic light scattering, photoluminescence, and total internal reflection fluorescence microscopy techniques. Their local structure was examined by in situ X-ray absorption fine structure studies. Perovskite thin films on a substrate with precursor solutions were characterized by transmission electron microscopy, X-ray diffraction analysis, space-charge-limited current, and Kelvin probe force microscopy. The colloidal properties of the perovskite precursor solutions were found to be directly correlated with the defect concentration and crystallinity of the perovskite film. This work provides guidelines for controlling perovskite films by varying the precursor solution, making it possible to use colloid-engineered lead halide perovskite layers to fabricate efficient PSCs.

Perovskite materials have demonstrated remarkable optoelectronic properties, which have placed them at the crux of the photovoltaic technology. In this era of striving for clean and economical methods of energy production, lead-based perovskites have become the key materials in photovoltaics owing to their facile solution processability and phenomenal performance. Unfortunately, lead toxicity poses a major hurdle to their scalability and widespread commercialization. Hence, to fulfill the indisputable need for energy, lead-free perovskites are considered to be a boon to the future of photovoltaic technology. In this perspective, we provide a comprehensive assessment of lead-free perovskite solar cells enabled by hetero-valent substitutes. These comprise A3B2X9-structured perovskites, halide double perovskites, and mixed metal halide–chalcogenide perovskites. Importantly, we emphasize the effects of cationic–anionic sites, metal substitutions, and solvents on the chemical and structural properties of A3B2X9-structured perovskites. Moreover, we also focus on antimony(V) perovskite-like materials and antimony-based perovskite nanocrystals (NCs) in the fabrication of devices. Two types of halide double perovskites are described, including A2B(I)B(III)X6 and A2B(IV)X6 double perovskites. Subsequently, chalcogenide perovskite solar cells are also discussed. Overall, the primary purpose of this perspective is to explicitly describe lead-free perovskite solar cells enabled by hetero-valent substitutions more broadly as a category of next-generation optoelectronic materials. Finally, we propose that mixed halide–chalcogenide perovskites offer a promising pathway towards achieving highly efficient and stable perovskite solar cells.

Two important factors in solar cells are efficiency and long-term stability. The commercialization of halide perovskite solar cells (PSCs) containing organic cations may be limited due to their low stability, in spite of their high efficiency. The molecular features of the organic cations adopted in perovksites belong to a subset of ammonium cations, such as methylammonium (MA), which are known to display poor humidity stability. Based on the molecular aspects of organic cations, the humidity stability of perovskites can be affected by their structural stability, the hydrophobicity, chemical environment near the heteroatom, and stereochemistry. Organic cations with large ionic radii produce two-dimensional (2D) structures with significantly improved stability. 2D perovskites show high humidity stability due to their increased chemical stability and hydrophobicity, but have the drawback of slow charge transport due to the insulating properties of the organic cations. Accordingly, many efforts have been devoted to form conductive inorganic layers in 2D perovskites in addition to the light-active layer of PSCs in several ways. In this perspective, we have analyzed the possible degradation initiators formed under humid conditions based on a comprehensive review of the literature, followed by practical experimental results using ammonium-driven PSCs, focusing on their humidity stability and device performance. As a strategy to enhance the wet-fastness of perovskites, we propose new under-explored sulfonium cations (R3S+), showing characteristic stereochemistry and significantly increased humidity stability of perovskites, which differ from conventional protic ammonium cations.

Maintaining the bandgap The bandgap of the black α-phase of formamidinium-based lead triiodide (FAPbI 3 ) is near optimal for creating high-efficiency perovskite solar cells. However, this phase is unstable, and the additives normally used to stabilize this phase at ambient temperature—such as methylammonium, caesium, and bromine—widen its bandgap. Min et al. show that doping of the α-FAPbI 3 phase with methylenediammonium dichloride enabled power conversion efficiencies of 23.7%, which were maintained after 600 hours of operation. Unencapsulated devices had high thermal stability and retained >90% efficiency even after annealing for 20 hours at 150°C in air. Science , this issue p. 749

Exceptional power conversion efficiency (PCE) of 25.7% in perovskite solar cells (PSCs) has been achieved, which is comparable with their traditional rivals (Si-based solar cells). However, commercialization-worthy efficiency and long-term stability remain a challenge. In this regard, there are increasing studies focusing on the interface engineering in PSC devices to overcome their poor technical readiness. Herein, the roles of electrode materials and interfaces in PSCs are discussed in terms of their PCEs and perovskite stability. All the current knowledge on the factors responsible for the rapid intrinsic and external degradation of PSCs is presented. Then, the roles of carbonaceous materials as substitutes for noble metals are focused on, along with the recent research progress in carbon-based PSCs. Furthermore, a sub-category of PSCs, that is, flexible PSCs, is considered as a type of exceptional power source due to their high power-to-weight ratios and figures of merit for next-generation wearable electronics. Last, the future perspectives and directions for research in PSCs are discussed, with an emphasis on their commercialization.

Organic-inorganic hybrid perovskite is the most promising active layer for new generation of solar cells. Despite of highly efficient perovskite active layer conventionally fabricated by spin coating methods, the need for using toxic solvents like dimethylformamide (DMF) required for dissolving low soluble metal precursors as well as the difficulties for upscaling the process have restricted their practical development. To deal with these shortcomings, in this work, lead sulphide as the lead metal precursor was produced by aqueous chemical bath deposition. PbS films were subsequently chemically converted to PbI2 and finally to mixed-cation mixed halide perovskite films. The microstructural, optical and solar cell performance of mixed cation mixed halide perovskite films were exploited. Results show that controlling the morphology of PbI2 platelets achieved from PbS precursor films enabled efficient conversion to perovskite. Using this processing technique, smooth and pin hole-free perovskite films having columnar grains of about 800 nm and a bandgap of 1.55 eV were produced. The solar cell performance consisting of such perovskite layers gave rise to a notable power conversion efficiency of 11.35% under standard solar conditions. The proposed processing technique is a very promising environmentally friendly method for the production of large-scale high efficient perovskite solar cells.

Next-generation solar cells, such as dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs), are fabricated in a configuration where light absorbers are located between the electron transporting layer (ETL) and the hole transporting layer (HTM). Although the most efficient DSSCs and PSCs have been fabricated using TiO2 as the ETL, TiO2 exhibits inherently low electron mobility with difficulty controlling the energy levels (i.e., conduction and valence bands) as it possesses a single phase of two components. Here, we report the synthesis of Sr-substituted BaSnO3 (BSSO) by a low-temperature solution process as a new alternative to TiO2 for both PSCs and DSSCs. The energy-level tailoring by Sr incorporation into BaSnO3 minimizes the open-circuit voltage (VOC) loss at the interfaces of ETL/perovskite and ETL/electrolyte in the PSCs and DSSCs, thereby leading to an improved VOC from 0.65 to 0.72 V in DSSC and 1.07 to 1.13 V in PSCs. Additionally, the BSSO ETL-based PSC shows improved photostability compared to the TiO2 analog. Our results show that energy-level tuned BSSO can be applied as a universal ETL for improving efficiency in both PSCs and DSSCs.

Design of impressed current cathodic protection (ICCP)-anode locations to minimize underwater electric field is important to increase the survivability of naval ships. However, the evaluation of all ICCP-anode location scenarios is time-consuming process even though it is conducted by numerical simulation, especially boundary element method (BEM). To solve this problem, the randomly selected cases of ICCP-anode location in 3, 4 and 5-pair, which were produced by the beta distribution model, were simulated using BEM simulations. Then, the optimized ICCP-anode location scenario from the cases was verified by statistical methods (multiple linear regression and artificial neural network). Predictions of ICCP-anode location and underwater electric field were more correlated with the artificial neural network than the multiple linear regression analysis. Also, the selected ICCP-anode locations were verified by the artificial neural network considering data interactions. Thus, the application of an artificial neural network can be more useful for designing ICCP-anode location that minimizes underwater electric fields.

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 devices

To boost the photoelectrochemical water oxidation performance of a hematite photoanode, high temperature annealing has been widely applied to enhance crystallinity and remove the physical interface between the hematite and the fluorine doped thin oxide (FTO) substrate. However, the high temperature also results in unintentional Sn-doping due to thermal diffusion from the bottom FTO substrate. Therefore, when using additional dopants and the subsequent high temperature annealing process to enhance performance, the procedure should more precisely be considered co-doping of the hematite photoanode. However, at present, the interaction between the unintentional Sn and intentional dopant is poorly understood. Here, using germanium (Ge), which has been proven a promising dopant in previously reported simulations, we investigated how Sn diffusion affects overall PEC performance in Sn:Ge co-doped systems. After revealing the negative interaction of Sn and Ge dopants, we developed a facile Ge-doping method which suppresses Sn diffusion from the FTO substrate, significantly improving hematite performance. The Sn:Ge-hematite photoanode showed a photocurrent density of 4.6 mA cm− 2 at 1.23 VRHE with an excellent low turn-on voltage. After combining with a perovskite solar cell, our tandem system achieved outstanding 4.8% solar-to-hydrogen conversion efficiency (3.9 mA cm− 2 in an unassisted water splitting system). Our work provides important insights on a promising diagnostic tool for future co-doping system design.

Relieving unwanted strain Although the α-phase of formamidinium lead iodide (FAPbI 3 ) has a suitable bandgap for use in solar cells, it must be stabilized with additional cations. These compositions can adversely affect the bandgap and produce lattice strain that creates trap sites for charge carriers. Kim et al. found that substituting small, equimolar amounts of cesium and methylenediammonium cations for formamidinium reduced the lattice strain and trap densities. The enhancement in open-circuit voltage led to a certified power conversion efficiency of 24.4%, and encapsulated devices retained 90% of their initial efficiency after 400 hours of maximal power point operating conditions. Science , this issue p. 108

The quaternary chalcogeno-iodides Pb2SbS2I3, comprising group IV and V elements, has attracted significant attention because of its unique semiconducting and ferroelectric properties. However, it has not yet been applied in solar cells. Herein, we report the first fabrication of nanostructured solar cells using Pb2SbS2I3 as a light harvester, prepared through a reaction between antimony sulfide, deposited by chemical bath deposition on mesoporous (mp)-TiO2, and lead iodide under an Ar atmosphere at 300 °C. A power conversion efficiency (PCE) of 3.12% under the standard illumination conditions of 100 mW/cm2 was achieved for the Pb2SbS2I3 layer sandwiched between mp-TiO2 and an organic hole conductor. Pb2SbS2I3 cells without encapsulation show good humidity stability over 30 days, retaining about 90% of the initial performance.

Perovskite solar cells (PSCs) require both high efficiency and good long-term stability if they are to be commercialized. It is crucial to finely optimize the energy level matching between the perovskites and hole-transporting materials to achieve better performance. Here, we synthesize a fluorene-terminated hole-transporting material with a fine-tuned energy level and a high glass transition temperature to ensure highly efficient and thermally stable PSCs. We use this material to fabricate photovoltaic devices with 23.2% efficiency (under reverse scanning) with a steady-state efficiency of 22.85% for small-area (~0.094 cm2) cells and 21.7% efficiency (under reverse scanning) for large-area (~1 cm2) cells. We also achieve certified efficiencies of 22.6% (small-area cells, ~0.094 cm2) and 20.9% (large-area, ~1 cm2). The resultant device shows better thermal stability than the device with spiro-OMeTAD, maintaining almost 95% of its initial performance for more than 500 h after thermal annealing at 60 °C. Interfacial losses between device layers play a key role in determining characteristics of solar cells. Jeon et al. address this in perovskite solar cells by synthesizing a hole-transporting layer that is better matched to the surrounding layers, and show high-efficiency and high-stability devices.

Growing interest in hybrid organic–inorganic lead halide perovskites has led to the development of various perovskite nanowires (NWs), which have potential use in a wide range of applications, including lasers, photodetectors, and light-emitting diodes (LEDs). However, existing nanofabrication approaches lack the ability to control the number, location, orientation, and properties of perovskite NWs. Their growth mechanism also remains elusive. Here, we demonstrate a micro/nanofluidic fabrication technique (MNFFT) enabling both precise control and in situ monitoring of the growth of perovskite NWs. The initial nucleation point and subsequent growth path of a methylammonium lead iodide–dimethylformamide (MAPbI3·DMF) NW array can be guided by a nanochannel. In situ UV–vis absorption spectra are measured in real time, permitting the study of the growth mechanism of the DMF-mediated crystallization of MAPbI3. As an example of an application of the MNFFT, we demonstrate a highly sensitive MAPbI3-NW-based photod...

The use of divalent chalcogenides and monovalent halides as anions in a perovskite structure allows the introduction of 3+ and 4+ charged cations in the place of the 2+ metal cations. Herein we report for the first time on the fabrication of solar cells exploiting methylammonium antimony sulfur diiodide (MASbSI2) perovskite structures, as light harvesters. The MASbSI2 was prepared by annealing under mild temperature conditions, via a sequential reaction between antimony trisulfide (Sb2S3), which is deposited by the chemical bath deposition (CBD) method, antimony triiodide (SbI3), and methylammonium iodide (MAI) onto a mesoporous TiO2 electrode, and then annealed at 150 °C in an argon atmosphere. The solar cells fabricated using MASbSI2 exhibited power conversion efficiencies (PCE) of 3.08%, under the standard illumination conditions of 100 mW/cm2.

Methylammonium lead iodide perovskites have limited practical applications due to a lack of stability under operating conditions. Environmental stability under heating has not yet been achieved, although recent studies modifying perovskites with long organic cations have reported the progress of stabilisation under humid conditions. In this work, we report the structural evolution of long alkylammonium-modified MAPbI3 and its functions for highly efficient and stable solar cells. As an encapsulating agent, octylammonium (OA) cation produced individual MAPbI3 grains in full armour without the formation of layered structures in contrast to butylammonium (BA) and phenethylammonium (PEA) cations. Our OA-armoured MAPbI3 achieved a stabilised power conversion efficiency of 20.1% without a deterioration of charge transport properties due to highly preferential orientation suppressing non-radiative recombination. The structural features also led to a much improved thermal stability at 85 °C in ambient atmosphere retaining 80% of the initial efficiency after 760 h without any encapsulation, as well as water tolerance. This work addresses widespread concerns associated with the photovoltaic efficiency and stability of MAPbI3 by exploring the inter-relationship between structural features and their functions in surfactant-modified perovskites.

In Sn-based halide perovskite solar cells (PSCs), the oxidation of Sn2+ to Sn4+ under ambient air leads to unwanted p-type doping in the perovskite film, which is a main reason for increased background carrier density and low efficiency. Here, we find that the introduction of bromide into formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) lattice significantly lowers the carrier density of perovskite absorber, which is thought to be a result of reduction of Sn vacancies. It reduces the leakage current of devices, increases recombination lifetime, and finally improves open-circuit voltage and fill factor of the resulting devices employing mesoporous TiO2 as an electron transport layer. Consequently, a high power conversion efficiency (PCE) of 5.5% is achieved with an average PCE of 5%, and after encapsulation the devices are highly stable over 1000 h under continuous one sun illumination including the ultraviolet region. This study suggests a simple approach for improving stability and efficiency in FASnI3-ba...

A multi-step dimethylformamide (DMF)-free green synthesizing method based on (i) initial electrodeposition of lead precursor, i.e. lead sulfide (PbS) on mesoporous TiO2/fluorine-doped tin oxide (FTO) conductive glasses substrates, (ii) subsequent conversion of PbS to PbI2 and (iii) synthesis of methylammonium lead triiodide (CH3NH3PbI3) perovskite film and their microstructural, optical and solar cell performance are described. Different electrodeposition techniques including direct current and cyclic voltammetry deposition were investigated to produce PbS films. We find that the perovskite films produced based on PbS deposited by cyclic voltammetry exhibit compact layer consisting of cuboid grains with an average size of approximately 800 nm and a bandgap of 1.58 eV whose properties are comparable to those of perovskite films generally prepared by conventional methods like spin coating. It was observed that uniform perovskite layers deposited under different conditions as the absorber layer generate a power conversion efficiency (PCE) of up to 7.72% under the standard AM 1.5 condition in the first attempt by this fabrication approach. PCEs obtained under different electrodeposition conditions were improved by eliminating of pores between the cuboid perovskite crystallites. This approach neglects employing hazardous solvents from the routine perovskite solar cell fabrication method and has potential to enhance its PCE similar to the common strategies by spin-coating methods improved over last decade by further modification of the electrodeposition process of the metal precursors and other steps towards highly efficient green perovskite solar cells.

Organic inorganic metal halide perovskites (OIHPs) has emerged as promising photovoltaic materials the latest years. Many OIHPs, however, have complex material compositions with mixed cation and halide compositions, phase mixtures, as well as beneficial remains of PbI2 in the final solar cell materials where the complex material composition with dual conduction and valence band states and its effects on the performance remain unclear. Here, we report an approach to analyze the phase mixture, order-disorder phases and the emissive electronic states via a 4-state model of the photoluminescence yield. The approach is applied to scaffold layer perovskite materials with different mixed halide composition. The optical transitions and the full emission spectra are de-convoluted to quantify the band gaps and charge quenching yields in the OIHPs. An approach to extract the excited state coupling parameters within the 4-state model is also briefly given. The integration model is finally utilized in charge quenching yield analysis for the different materials and correlated with solar cell performance from MAPbI(3) and MAPbI(3-x)Cl(x) in mesoporous TiO2 layers where inclusion of Cl improves crystal formation and is compared to alternative approaches using optimized solvents and anti-solvent methods. A band gap grading effect was found to be present for the scaffold MAPbI(3) and increased for MAPbI(3-x)Cl(x), beneficial for decreased hole concentration at the back contact and thus reducing back contact recombination.

Healing defects with triiodide ions Deep-level defects in organic-inorganic perovskites decrease the performance of solar cells through unproductive recombination of charge carriers. Yang et al. show that introducing additional triiodide ions during the formation of layers of formamidinium lead iodide, which also contain small amounts of methylammonium lead bromide, suppresses the formation of deep-level defects. This process boosts the certified efficiency of 1-cm 2 solar cells to almost 20%. Science , this issue p. 1376

Transporter layers for greater stability Although perovskite solar cells (PSCs) can have power conversion efficiencies exceeding 20%, they can have limited stability under ultraviolet irradiation. This is in part because the mesoporous TiO 2 used as an electron-transporting layer can photocatalyze unwanted reactions in the perovskite layer. Shin et al. report a low-temperature colloidal method for depositing La-doped BaSnO 3 films as a replacement for TiO 2 to reduce such ultraviolet-induced damage. Solar cells retained over 90% of their initial performance after 1000 hours of full sun illumination. Science , this issue p. 167

A pyrazole based heterocyclic compound methyl 3, 5-dimethyl-3-(N′-methylsulfanylthio- carbonyl-hydrazino)-2,3-dihydro-pyrazole-1-carbodithioate (Hmdpc) and [Pb(mdpc) 2 ] have been synthesized and characterized to obtain nanosized visible light responsive PbS nanocrystals (NCs). The presence of four–SCH 3 groups in each Pb(II)-center in the SP controls the shape and size of the nanocrystals. A possible growth mechanism based on internal molecular transfer of thiol group for the preparation of spherical nanocrystals has been proposed. The synthesized PbS NCs were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The optical properties of the nanocrystals reveal the absorption in the entire visible region and can play significant role in showing photocatalytic activity. The PbS nanocrystals show prominent catalytic activity against degradation of Rose Bengal (RB) dyes in aqueous medium in presence of visible light.

We have designed and synthesized fluorinated indolo[3,2-b]indole (IDID) derivatives as crystalline hole-transporting materials (HTM) for perovskite solar cells. The fluorinated IDID backbone enables a tight molecular arrangement stacked by strong π–π interactions, leading to a higher hole mobility than that of the current HTM standard, p,p-spiro-OMeTAD, with a spherical shape and amorphous morphology. Moreover, the photoluminescence quenching in a perovskite/HTM film is more effective at the interface of the perovskite with IDIDF as compared to that of p,p-spiro-OMeTAD. As a consequence, the device fabricated using IDIDF shows superior photovoltaic properties compared to that using p,p-spiro-OMeTAD, exhibiting an optimal performance of 19%. Thus, this remarkable result demonstrates IDID core-based materials as a new class of HTMs for highly efficient perovskite solar cells.

We synthesized single-crystalline Sn-based oxides for use as electron-transporting layers (ETLs) in perovskite solar cells (PSCs). The control of the Zn-to-Sn cation ratio (Zn/Sn = 0–2) in a fixed concentration of hydrazine solution leads to the formation of various types of Sn-based oxides, i.e., spherical SnO2 and Zn2SnO4 nanoparticles (NPs), SnO2 nanorods, and Zn2SnO4 nanocubes. In particular, a ratio of Zn/Sn = 1 results in nanocomposites of single-crystalline SnO2 nanorods and Zn2SnO4 nanocubes. This is related to the concentration of free hydrazine unreacted with Zn and Sn ions in the reaction solution, because the resulting OH− concentration affects the growth rate of intermediate phases such as ZnSn(OH)6, Zn(OH)42− and Sn(OH)62−. Additionally, we propose plausible pathways for the formation of Sn-based oxides in hydrazine solution. The Sn-based oxides are applied as ETLs and annealed at a low temperature below 150 °C in PSCs. The PSCs fabricated by using the nanocomposite ETLs consisting of single-crystalline SnO2 nanorods and Zn2SnO4 nanocubes exhibit superior device performance to TiO2-based PSCs due to their excellent charge collection ability and optical properties, achieving a power conversion efficiency of ≥17%.

Significant research efforts are currently being devoted to improving both the crystalline quality and stability of lead halide perovskite absorbers to advance the commercial prospects of perovskite-based solar cells. Herein, we report a simple one-step dibutylhydroxytoluene (BHT) additive-based approach for simultaneously improving the crystallinity and resistance of perovskite films under adverse degradation conditions. We found that BHT, commonly known for its antioxidant properties, can considerably improve the performance of methylammonium lead iodide perovskite solar cells by modulating the chemical environment within the precursor medium to form intermediate complexes, and it can also suppress photooxidation, which results in perovskite degradation under environmental operating conditions. Consequently, a device exhibited a significant power conversion efficiency improvement to 18.1% with the BHT-additive-based perovskite absorber, exceeding the 17.1% efficiency achieved for the control device. The BHT additive also improved the perovskite stability by quenching intermediate reactions resulting in perovskite degradation to an undesirable lead iodide phase, as evidenced by detailed analysis of absorption spectra, grazing-incidence wide-angle X-ray scattering, X-ray photoelectron spectra, and photoluminescence measurements.

Because the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a number of ionized iodide (triiodide: I3-) ions during perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the long-term chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging of PSCs in the dark based on formamidinium lead triiodide by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in the PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference using grazing incidence wide-angle X-ray scattering, deep-level transient spectroscopy, scanning photoelectron microscopy, and high-resolution photoemission spectroscopy. The addition of I3- was found to reduce the amounts of hydroxide and Ox in the halide perovskites (HPs), affecting the migration of defects and the structural transformation of the HPs.

Sb

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S2-) into the host lattice (MAPb0.75Sn0.25Br3) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan-bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology.