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Nature Energy, Published online: 20 January 2020; doi:10.1038/s41560-019-0538-4

Nature Energy, Published online: 20 January 2020; doi:10.1038/s41560-019-0535-7

A stable intermediate‐state film is obtained by using teramethylene sulfoxide (TMSO), originating from the formation of stronger coordination bond between TMSO and all perovskite precursors, which extends the annealing window and promotes the formation of a high‐quality film with larger grains and textured surface. 21.14% efficiency is achieved attributable to the improvement of the long‐wavelength response and fill factor.

Abstract

As the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is increased to as high over 25%, it becomes pre‐eminent to study a scalable process with wide processing window to fabricate large‐area uniform perovskite films with good light‐trapping performance. A stable and uniform intermediate‐state complex film is obtained by using tetramethylene sulfoxide (TMSO), which extends the annealing window to as long as 20 min, promotes the formation of a high‐quality perovskite film with larger grains (over 400 nm) and spontaneously forms the surface texture to result in an improved fill factor and open‐circuit voltage (V _oc). Moreover, the superior surface texture significantly increases the long‐wavelength response, leading to an improved short‐circuit current density (J _sc). As a result, the maximum PCE of 21.14% is achieved based on a simple planar cell structure without any interface passivation. Moreover, a large area module with active area of 6.75 cm² is assembled using the optimized TMSO process, showing efficiency as high as 16.57%. The study paves the way to the rational design of highly efficient PSCs for potential scaled‐up production.

Low‐temperature black‐phase CsPbI₃ evolution processes are designed using chemical bond engineering for the fabrication of efficient and ambient‐air‐stable solar cells. After optimization, the low temperature (160 °C)‐annealed 3% polyvinylpyrrolidone device shows the highest efficiency of 10.0% and sustains ≈80% of its initial power conversion efficiency after 5 months of storage in ambient‐air conditions.

Cesium‐based fully inorganic black‐phase (BP) lead halide perovskites (such as α‐, β‐, and γ‐CsPbI₃) with excellent thermal stability and a decently high photovoltaic performance have attracted increasing attention. However, a below 200 °C fabrication process of the desirable BP CsPbI₃ has rarely been reported. Herein, the detailed crystal structure evolution of ambient‐air‐stable BP CsPbI₃ prepared under low temperature conditions is investigated by exploiting the strong coordination bonding between CO in polyvinylpyrrolidone (PVP) and Pb in CsPbI₃ and inflection effect of PVP under annealing. It is found that ambient‐air‐stable BP CsPbI₃ films are formed and the energy barrier for the long‐term stable BP CsPbI₃ formation is significantly reduced (the required annealing temperature is only 80 °C). After optimization, the highest power conversion efficiencies (PCEs) of ≈4.0% and 10.0% are recorded for the 3% PVP‐added devices with light absorbers annealed at 80 and 160 °C, respectively. More importantly, the 3% PVP device annealed at 160 °C maintains ≈80% of its original PCE after 5 months storage under ambient‐air conditions.

Flexible perovskite solar cells have attracted plenty of attention in both educational and business communities. Herein, the research background is summarized and technological advancement with regard to flexible substrates is evaluated. In addition, different stability tests with and without encapsulation are briefly examined. Last but not least, the upscaling issues and the material costs are also vividly discussed.

In recent years, the era of perovskites has experienced splendid development. Among perovskites, flexible perovskite solar cells (FPSCs) have received increasing attention due to their high efficiency, light weight, low cost, excellent flexibility, and low‐temperature solution processing ability. In the last decade, the power conversion efficiency of FPSCs has increased significantly from 2.62% to more than 20%. Herein, a succinct overview of the current endeavor to achieve low‐temperature FPSCs is provided. The recent developments, including flexible substrates, transparent conductive electrodes, perovskite absorbers, and device manufacturing methods, are vividly discussed. The strategies for enhancing the stability and flexibility of FPSCs are presented in terms of electrode materials, device encapsulation, and structural effects. Finally, the most encouraging and potential studies for the future of flexible PSCs are revealed.

The significant improvement of photovoltaic performances by synergetic effects of GeI₂ and methylammonium chloride (MACl) is described. The improved solubility of GeI₂ with the help of MACl in the precursor leads to high performance perovskite solar cells.

Abstract

Interfacial engineering, grain boundary, and surface passivation in organic–inorganic hybrid perovskite solar cells (HyPSCs) are effective in achieving high performance and enhanced durability. Organic additives and inorganic doping are generally used to chemically modify the surface contacting charge transport layers, and/or grain boundaries so as to reduce the defect density. Here, a simple but tricky one‐step method to dope organic–inorganic hybrid perovskite with Ge for the first time is reported. Unlike Ge doping to all‐inorganic perovskites, application of GeI₂ in organic–inorganic perovskite precursors is challenging due to the extremely poor solubility of GeI₂ in hybrid perovskite ink, leading to failure in the formation of uniform films. However, it is found that addition of methylammonium chloride (MACl) into the precursor remarkably increases the solubility of GeI₂. This MACl‐assisted Ge doping of hybrid perovskites produces high‐quality crystalline film with its surface passivated with nonvolatile GeI₂ (GeO₂) and the volatile MACl additive also improves the uniformity of GeO₂ distribution in the perovskite films. The resulting Ge‐doped mixed cation and mixed halide perovskite films with composition FA_0.83MA_0.17Ge_0.03Pb_0.97(I_0.9Br_0.1)₃ show superior photoluminescence lifetime, power conversion efficiency above 22%, and greater stability toward illumination and humidity, outperforming photovoltaic properties of HyPSCs prepared without the Ge doping.

In article number https://doi.org/10.1002/aenm.2019015241901524, Angel Barranco, Juan R. Sanchez‐Valencia, and co‐workers develop a simple approach to fabricate dopant‐free Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine)9,9′‐spirobifluorene] layers by vacuum sublimation. Temperature control of the samples during evaporation induces crystalline and microstructural changes, as illustrated in the lateral side of the cover. The implementation in perovskite solar cells demonstrates unprecedented superior stability with respect to solution‐processed devices.

Retaining the α‐phase of all‐inorganic perovskites (AI‐PSCs) at ambient condition is a difficult task. In article number https://doi.org/10.1002/aenm.2019027081902708, Sawanta S. Mali, Jyoti V. Patil, and Chang Kook Hong design air‐processed MnCl₂:CsPbI₂Br perovskite devices in combination with MgZnO and P3HT electron and hole transporting layers, respectively. This strategy enables the fabrication of highly‐efficient AI‐PSCs with 15.50% power conversion efficiency with excellent stability under 85 °C thermal stress.

Transient techniques are applied to probe the charge recombination and trap distribution in efficient perovskite solar cells (PSCs). The observations reveal that the possible water and NO₃ ⁻ residues in the perovskite lattice play a role in benign trap passivation, which elicits the superior photovoltaic properties of PSCs fabricated using the Pb(NO₃)₂/water protocol.

Abstract

Studies on the photoelectronic properties of perovskite solar cells (PSCs) made from non‐PbI₂ precursors are seldom reported. In this study, a series of transient techniques are applied to investigate the charge recombination and trap distribution in an efficient PSC fabricated using a low‐toxicity Pb(NO₃)₂/water protocol. A device with identical conversion efficiency fabricated using a conventional PbI₂/dimethylformamide protocol is also studied for comparison. Transient photovoltage and time‐resolved photoluminescence analysis reveal that the Pb(NO₃)₂/water‐based device exhibits a long lifetime in both bimolecular and trap‐assisted recombination. However, differential capacitance and differential charging analysis indicate that there are more charges stored in the Pb(NO₃)₂/water‐based perovskite layer, which stretches the energy tail from band edge to midband and should provoke serious trap‐assisted recombination. The exceptional long electron lifetime in the Pb(NO₃)₂/water‐based device is explained by a benign defect inactivation, which originates from water and NO₃ ⁻ residues from the aqueous precursor solution and is involved in the formation of perovskite crystal. Consequently, despite the perovskite film made from Pb(NO₃)₂/water protocol possessing high trap density, its photovoltaic device still exhibits a long electron lifetime and superior photovoltaic properties.

A new nonfullerene acceptor (TOBDT) with asymmetrical side chains is rationally designed and reported. Compared to the PM6:TOBDT binary devices, the as‐cast ternary devices based on PM6:IDIC:TOBDT obtain improved J _sc, V _oc, and fill factor values simultaneously, leading to a high power conversion efficiency of 14.0%, which is among the best reported for as‐cast fullerene‐free ternary devices.

Abstract

A new small‐molecule nonfullerene acceptor based on the benzo[1,2‐b:4,5‐b′]dithiophene (BDT) fused central core with asymmetrical alkoxy and thienyl side chains, namely TOBDT, is designed and synthesized. The alkoxy unit helps narrow the bandgap, and thienyl side chain helps enhance the intermolecular interaction. As a result, TOBDT is suitable to match the deep‐lying highest occupied molecular orbital (HOMO) of polymer donor PM6. Then, a strong crystalline acceptor IDIC is introduced as the third component to fabricate as‐cast nonfullerene ternary devices to achieve absorption and morphology control. Addition of IDIC not only mixes well with TOBDT but modulates the morphology of the blend film, which helps to balance the charge transport properties and reduce the photovoltage loss of ternary devices. All these contribute to synergetic improvement of J _sc, V _oc, and fill factor parameters, leading to a power conversion efficiency of 14.0% for the as‐cast fullerene‐free ternary device.

A Lewis acid tris(pentafluorophenyl)borane and nonhygroscopic lithium salt (LiClO₄) codoping strategy is introduced to tailor C₆₀ and fabricate highly efficient inorganic CsPbI₂Br perovskite solar cells with reduced hysteresis. Consequently, square‐centimeter inorganic CsPbI₂Br perovskite solar cells yield a record power conversion efficiency (PCE) of 14.44%. In addition, the first inorganic perovskite solar module with an efficiency exceeding 12% is reported, using a self‐developed quasi‐curved heating method.

Abstract

Although inorganic perovskite solar cells (PSCs) are promising in thermal stability, their large open‐circuit voltage (V _OC) deficit and difficulty in large‐area preparation still limit their development toward commercialization. The present work tailors C₆₀ via a codoping strategy to construct an efficient electron‐transporting layer (ETL), leading to a significant improvement in V _OC of the inverted inorganic CsPbI₂Br PSC. Specifically, tris(pentafluorophenyl)borane (TPFPB) is introduced as a dopant to lower the lowest unoccupied molecular orbital (LUMO) level of the C₆₀ layer by forming a Lewis acidic adduct. The enlarged free energy difference provides a favorable enhancement in electron injection and thereby reduces charge recombination. Subsequently, a nonhygroscopic lithium salt (LiClO₄) is added to increase electron mobility and conductivity of the film, leading to a reduction in the device hysteresis and facilitating the fabrication of a large‐area device. Finally, the as‐optimized inorganic CsPbI₂Br PSCs gain a champion power conversion efficiency (PCE) of 15.19%, with a stabilized power output (SPO) of 14.21% (0.09 cm²). More importantly, this work also demonstrates a record PCE of 14.44% for large‐area inorganic CsPbI₂Br PSCs (1.0 cm²) and reports the first inorganic perovskite solar module with the excellent efficiency exceeding 12% (10.92 cm²) by a self‐developed quasi‐curved heating method.

Integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells have shown great potential to further improve their performance by combining the advantages of perovskite solar cells and near‐infrared (NIR) BHJ organic solar cells. Combining with the maintained high V _OC, higher efficiencies are expected by fully optimizing the perovskite layers and NIR BHJ layers through device engineering and materials innovations.

Abstract

The recently emerged integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells (IPOSCs) without any recombination layers have generated wide attention. This type of device structure can take the advantages of tandem cells using both perovskite solar and near‐infrared (NIR) BHJ organic solar materials for wide‐range sunlight absorption and the simple fabrication of single junction cells, as the low bandgap BHJ layer can provide additional light harvesting in the NIR region and the high open‐circuit voltage can be maintained at the same time. This progress report highlights the recent developments in such IPOSCs and the possible challenges ahead. In addition, the recent development of perovskite solar cells and NIR organic solar cells is also covered to fully underline the importance and potential of IPOSCs.

In article number https://doi.org/10.1002/adma.2018058431805843, Yongsheng Chen and Yongsheng Liu review integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells (IPOSCs), which have recently emerged, and which could take the advantage of tandem cells using both perovskite and near‐infrared organic materials for wide‐range sunlight absorption. Combined with the reserved high open‐circuit voltage (V _OC), efficiencies close to or even exceeding the Shockley–Queisser limit of single‐junction cells are expected.

Nature Communications, Published online: 10 January 2020; doi:10.1038/s41467-019-13909-5

Moisture Stability

In article number 1900331, Baomin Xu, Chun Cheng, and co‐workers show that the cascade‐aligned energy levels and the defect‐passivation achieved by combining commercially accessible SnO₂ and home‐made TiO₂ nanoparticles effectively reduce energy loss and inhibit defects in the device. Consequently, the perovskite solar cell delivers a high power conversion efficiency (PCE) of 20.50%, which is superior to that of control devices based on SnO₂ with a PCE of 18.09%.

Through time‐resolved microwave conductivity measurements, the local intrinsic free‐carrier properties in three representative organic bulk heterojunction blends are comparatively investigated, and it is believed that the results shown here are important in understanding and improving the efficiency and stability of organic solar cells.

Polymer–polymer blends have been reported to exhibit exceptional thermal and ambient stability. However, power conversion efficiencies (PCEs) from devices using polymeric acceptors have been recorded to be significantly lower than those based on conjugated molecular acceptors. Herein, two organic nonfullerene bulk heterojunction (BHJ) blends ITIC:PBDB‐T and N2200:PBDB‐T, together with their fullerene counterpart, PCBM:PBDB‐T, are adopted to understand the effect of electron acceptors on device performance. Free charge carrier properties using time‐resolved microwave conductivity (TRMC) measurements are comprehensively investigated. The nonfullerene devices show an improved PCE of 10.06% and 6.65% in the ITIC‐ and N2200‐based cells, respectively. In comparison, the PCBM:PBDB‐T‐based devices yield a PCE of 5.88%. The optimal N2200:PBDB‐T produced the highest TRMC mobility, longest lifetime, and greatest free‐carrier diffusion length. It is found that such phenomena can be associated with the unfavorable morphology of the all‐polymer BHJ microstructure. In contrast, the solar cells using either the PCBM or ITIC acceptors display a more balanced donor and acceptor phase separation, leading to more efficient free‐carrier separation and transport in the operating device. By sacrificing efficiency for superior stability, it is shown that the improved structure in all‐polymer blend can deliver a more stable morphology under thermal stress.

An effective method is proposed to boost charge carrier concentration in poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) from 10¹⁸ to 10²² cm⁻³ by combining solvent p‐doping and DMF treatment. This highly conductive PEDOT:PSS enables a simple and efficient organic/n‐Si planar heterojunction solar cell with a power conversion efficiency exceeding 13% without using complex light‐trapping structures.

The key for fabricating efficient organic/n‐Si planar heterojunction solar cells is the organic semiconductor layer, which governs key steps in photocarrier harvesting such as charge carrier separation and extraction. Typical organic semiconductors, however, are inadequate to yield good device performance due to their low electrical conductivities and undesirable work functions. Herein, a method is proposed to boost charge carrier concentration in poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) from 10¹⁸ to 10²² cm⁻³ by combining solvent p‐doping and DMF treatment. This high carrier concentration induces significant band bending in the Si substrate and thus leads to an extended inversion zone near an organic/Si heterojunction. It is found that this type of heterojunction creates a broad built‐in electric field and effective interface passivation. The highly conductive PEDOT:PSS enables a simple and efficient organic/n‐Si planar heterojunction solar cell with a power conversion efficiency exceeding 13% without using complex light‐trapping structures. This power efficiency is comparable with the highest value reported to date. Herein, the possibility of making effective organic/n‐Si planar heterojunction solar cells simply by applying a layer of extremely conductive organic coating is demonstrated.