Mahui

A formamidinium (FA)‐based quasi‐2D Ruddlesden–Popper (RP) perovskite, namely, (ThMA)₂(FA) _{n −1}Pb _n I_{3 n +1} (nominal n = 5), is successfully demonstrated with high photovoltaic performance by using an organic‐salt‐assisted crystal growth method. The optimized device exhibits a champion efficiency of 19.06%, which is a record for quasi‐2D RP perovskite solar cells with nominal n‐value lower than 6.

Abstract

Quasi‐2D Ruddlesden–Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi‐2D RP PSCs using 2‐thiophenemethylammonium as spacer with nominal n‐value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi‐2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)‐based quasi‐2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4‐(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG‐processed quasi‐2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI₃‐based

3D PSCs.

Judicious incorporation of ambiopolar black phosphorene with tailored thickness to concurrently impart electron and hole extractions in perovskite solar cells is reported by Jinsong Huang, Zhiqun Lin, and co‐workers in article number https://doi.org/10.1002/adma.2020009992000999. This work underpins the potential implementation of black phosphorene as a dual‐functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, and light‐emitting diodes.

Nature Materials, Published online: 13 July 2020; doi:10.1038/s41563-020-0720-x

Voltage matching and rational design of redox couples enable high solar-to-output electricity efficiency and extended operational lifetime in a redox flow battery integrated with a perovskite/silicon tandem solar cell.

Herein, perovskite composite nanofibers are incorporated into a solar yarn through an electrospinning process, to work as the photoactive layer of the device. Electrospinning is conducted at high relative humidity (75%) and voltage (17 kV) to enhance perovskite crystal growth on polyvinylpyrrolidone (PVP) nanofibers. The resulting yarn demonstrates a 15.7% efficiency, with a good absorption peak, flexibility, and increased active lifetime.

A flexible perovskite solar yarn with an impressive active lifetime (>216 h) and an exceptional photon conversion efficiency is prepared under ordinary conditions. The champion device demonstrates an average linear mass density of 0.89 mg cm⁻¹ and can be bent over a loop diameter of 2.5 mm, with a negligible efficiency loss. Photoactive nanofibers composed of a polyvinylpyrrolidone (PVP) central strain and a perovskite phase on the surface (with average grain size of 275 ± 14.3 nm), are prepared by electrospinning, at 18 kV, relative humidity of 75%, and a temperature of 25 °C. This bilayered configuration promises superior mechanical strength and flexibility, together with an excellent photovoltaic character, compared with their dip coated counterparts. Photoactive perovskite nanofibers are incorporated into a plied‐solar yarn, with an organic hole‐conductive layer, poly(3‐hexylthiophene‐2,5‐diyl)‐coated on silver yarn electrode, and a composite electron conductive layer, phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM)‐SnO₂ coated on a carbon yarn. An individual double‐twisted solar yarns yields 15.7% champion power conversion efficiency, while a 30.5 mm × 30.5 mm active area of plain‐woven fabric generates a maximum power density of 1.26 mW cm⁻² under one sun (1000 W m⁻²) solar illumination.

Hole‐Transporting Materials

In article number 2000109, Zong‐Xiang Xu and co‐workers determine that the highest power conversion efficiency of perovskite solar cells based on a poly(3‐hexylthiophene)/gold nanorod (P3HT/AuNR) composite hole‐transporting material reaches up to 16.88%, which is an increase of 26% from that of a pristine P3HT‐based device (13.40%). The enhanced performance is attributed to the higher carrier mobility and increased light utilization efficiency induced by the addition of AuNRs with localized surface plasmon resonance effect in the polymer matrix.

Passivation Agents

In article number 2000082, Fei Zhang, Kai Zhu, and co‐workers design a more efficient and stable perovskite solar cell by partially replacing phenylethylammonium (PEA⁺) with pentauorophenethylammonium (F5PEA⁺) as the 2D perovskite passivation agent by forming a strong noncovalent interaction between the two bulky cations, which enhances charge transport, and presents the highest performance for reported wide‐bandgap perovskite solar cells.

Perovskite Solar Cells

In article number 2000093, Ming‐Chung Wu, Kun‐Mu Lee, and co‐workers use polyethylene glycol (PEG) as an additive for the perovskite active layer in lead‐reduced perovskite solar cells. The PEG can effectively control the surface morphology of the perovskite film and improve the charge carrier transport. For 10% lead‐reduced perovskite solar cells, a champion power conversion effi ciency of 16.1% is obtained without signifi cant hysteresis.

Perovskite Solar Cells

In article number 2000001, Jingjing Chang and co‐workers demonstrate a novel approach where a short‐period deep‐ultraviolet (DUV) photoactivation process is employed to modify SnO₂ electron transport layers to achieve all‐inorganic perovskite solar cells with high efficiency exceeding 15% with good stability. The DUV treatment induces better energy level alignment, more ordered crystal growth, and reduces interface stress related defects.

High‐performance all‐inorganic perovskite solar cells with efficiency exceeding 15% are achieved via short‐period deep‐ultraviolet (DUV) photoactivation process. The DUV treatment can efficiently decrease the work function, resulting in better band alignment. The unencapsulated device exhibits enhanced operational stability under continuous simulated sunlight illumination, thermal stability, and outstanding air stability after 30 days of storage under N₂ condition.

All‐inorganic perovskite CsPbI₂Br have been regarded as a promising candidate to tackle the thermal instability issue of organic–inorganic perovskite solar cells. However, the serious interfacial charge recombination and large voltage potential loss in cells circumscribe their performance and commercialization. Herein, a facile approach is demonstrated in which the SnO₂ electron transport layer is modified with short‐period deep‐ultraviolet (DUV) photoactivation process to decrease the work function and achieve better energy alignment with the conduction band of perovskites. Such modification triggers efficient charge transfer and reduces the charge recombination. Moreover, first‐principles calculation further demonstrates that DUV‐treated SnO₂ can strengthen the interface interaction, reduce the interface stress caused by lattice mismatch, induce more ordered perovskite structure, enlarge transfer charge from 0.71 to 2.33 e, gain larger built‐in field (from 0.74 to 2.09 eV), lower work function, and smaller conduction band offset. Thus, all‐inorganic CsPbI₂Br solar cells based on DUV‐treated SnO₂ exhibit a significant enhancement in power conversion efficiency, and the champion cell achieves an elevated efficiency of 15.1% with a superior V _oc of 1.22 V and better stability.

The nonionic polymer with multiple amino groups is introduced to passivate both metal‐ and halide‐induced defects of all‐inorganic CsPbI₂Br perovskite by coordination and hydrogen bonds, simultaneously. Consequently, a well‐controlled grain size, reduced defects, and reinforced phase structure of CsPbI₂Br film are achieved, which boosts the efficiency of perovskite solar cells up to 15.48% with excellent humidity stability.

The all‐inorganic CsPbI₂Br perovskite with superior thermal durability faces challenges of low‐phase stability and high moisture sensitivity. Herein, a nonionic additive of polyethyleneimine (PEI) with multiple amino groups is introduced to form hydrogen bond with I⁻/Br⁻ ions and coordinate with Pb²⁺/Cs⁺ ions simultaneously. The strong interplay between PEI and CsPbI₂Br achieves a well‐controlled grain size, reduced defects, and reinforced phase structure of CsPbI₂Br film, which boosts the power conversion efficiency (PCE) of perovskite solar cells to 15.48%. The hydrophobic long alkyl chain of PEI greatly improves the humidity resistance, retaining 81.9% of initial PCE of zjr unsealed device under 20 ± 5% relative humidity (RH) for 500 h. Remarkably, a PCE of 13.37% is achieved by the device based on CsPbI₂Br–PEI film processed under ambient condition (≈22% RH, ≈25 °C).

Post‐treating the mesoporous TiO₂/ZrO₂/carbon triple layer by alkali metal sulfonate compounds enables a significantly enhanced photovoltage for hole‐conductor‐free printable mesoscopic perovskite solar cells. The devices demonstrate high operational stability, retaining 91.7% of their initial efficiency after 1000 h continuous operation at the maximum power point under 1 sun illumination.

Triple‐mesoscopic perovskite solar cells (PSCs) based on TiO₂/ZrO₂/carbon architecture have attracted much attention due to their excellent long‐term stability and screen‐printing technique‐based fabrication process. However, the relatively low open‐circuit voltage (V _OC) limits the further improvement of power conversion efficiency (PCE) for triple‐mesoscopic PSCs. Herein, 2‐phenyl‐5‐benzimidazole sulfonate‐Na to post‐treat the triple‐mesoscopic structured scaffold is introduced. The conduction band of the mesoporous TiO₂ layer (electron transport layer [ETL]) is significantly shifted up from −4.22 to −4.11 eV, which favors the electron transfer from the perovskite absorber to the ETL. At the same time, the recombination at the interface of ETL/perovskite is effectively suppressed. Correspondingly, the V _OC and fill factor (FF) of the devices are enhanced without sacrificing the photocurrent density (J _SC). With optimal post‐treatment conditions, the champion device delivers a V _OC of 1.02 V and an FF of 0.70 with J _SC of 23.06 mA cm⁻², showing an overall PCE of 16.51%. After 1000 h continuous operation at the maximum power point under AM1.5G 1 sun illumination, the devices can maintain 91.7% of the initial efficiency. This simple procedure and significant photovoltage enhancement render this method promising for fabricating efficient PSCs based on mesoporous charge transport layers.

Three novel donor–π‐bridge–acceptor (D–π–A)‐type small organic molecules are designed and synthesized as dopant‐free hole transport materials for perovskite solar cells. Combination of triazatruxene donor, terthiophene π‐bridge, and dicyanovinylene N‐ethyl rhodanine electron‐accepting unit as CI‐B3 creates well‐ordered edge‐on aggregated π–π stacking. Solar cell performance and long‐term stability are significantly improved.

Three donor–π‐bridge–acceptor (D–π–A)‐type organic small molecules coded CI‐B1, CI‐B2, and CI‐B3 are designed, synthesized, and used as dopant‐free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron‐donating triazatruxene central core (D), terthiophene conjugated arms (π), and three different strong electron‐accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end‐capping, three different organizations of self‐assembly with π–π stacking are observed in the solid thin films. Dopant‐free CI‐B1, CI‐B2, CI‐B3, and spiro‐OMeTAD with dopants are used with triple cation perovskite composition Cs_0.1(MA_0.15FA_0.85)_0.9Pb(I_0.85Br_0.15)₃ (MA: CH₃NH₃ ⁺, FA: NHCHNH₃ ⁺) in n‐i‐p architecture. The cells prepared with CI‐B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state‐of‐art of spiro‐OMeTAD with dopants (18.02%), but also demonstrates improved long‐term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant‐free nature, high durability, and edge‐on molecular orientation of CI‐B3 show its great promise as a HTM candidate for efficient and stable PSCs.

A synthetic polyhalide ligand (2‐picolyl)amine triiodide as a molecular glue is used to passivate halide vacancies at grain boundaries directionally and throughout grain bulk of perovskites. The inverted perovskite solar cells after passivation are allowed to be more efficient, and are profoundly stabilized in both ambient air and light‐soaking circumstances.

The fundamental instability of hybrid perovskite solar cells originates from the considerable halide vacancies. Furthermore, the local roles of halide vacancies between grain boundaries and grain bulk generally conflict, thus inhibiting complete passivation. To overcome this obstacle, a rational polyhalide ligand, di‐(2‐picolyl)amine triiodide, is designed as a molecular “glue” to achieve comprehensive passivation. Unlike a monohalide ligand, this ligand has multiple iodide ions and a quasiplanar tridentate chelation capability, contributing to directional passivation along the grain boundaries and overall passivation throughout the grain bulk. Using this molecular glue passivation, the best inverted solar cell yields an efficiency of 20.02%. Moreover, the relative stability of this cell in ambient air (≈40% humidity, 800 h aging) and under light‐soaking conditions (500 h aging) is profoundly enhanced by 33.33% and 22.26%, respectively. Herein, important insights into the design of passivating molecules to achieve low‐defect perovskites toward the development of multifunctional devices are provided.

A simple and effective method of fine‐tuning the energy level of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) is demonstrated. The as‐prepared hole‐transporting material aligns well with the highest occupied molecular orbital level of the electron donor in nonfullerene‐based organic photovoltaic (OPV) devices in inverted architecture, reaching a power conversion efficiency of 10%, which would benefit the future commercialization of highly efficient OPV devices.

Solution‐processable hole‐transporting materials are demonstrated to improve the performance of nonfullerene‐based organic photovoltaic devices in an inverted structure. A vanadium oxide (VO _X ) precursor, used as a sol–gel, is mixed with commercial poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to form a well‐dispersed VO _X :PEDOT:PSS solution. The work function and molecular distribution of the VO _X :PEDOT:PSS thin film are examined by ultraviolet photoelectron spectroscopy (UPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), respectively. Unlike conventional PEDOT:PSS, VO _X :PEDOT:PSS not only is compatible with highly hydrophobic photoactive layers but also aligns well with the highest occupied molecular orbital (HOMO) level of the polymer donor, reaching a power conversion efficiency of 10% (≈100% boost) and achieving an excellent device stability.

A new series of dopant‐free hole‐transporting materials (HTMs) YZT1–YZT4 featuring porphyrin backbone is achieved in which the best power conversion efficiency (PCE) of YZT4 is 14.95% for doped and that of YZT1 is 13.10% for dopant‐free perovskite solar cells (PSCs) based on TiO₂ semiconductors.

A new series of structurally simple and easily accessible hole‐transporting materials (HTMs) YZT1–YZT4 using porphyrin backbone is devised for high‐performance perovskite solar cells (PSCs) with and without the aid of doping. The YZT‐series HTMs have either push–push or push–pull type planar linear molecular geometry with substitution of linear or branched alkylamine. UV–vis absorption, photoluminance (PL) quenching experiments, and theoretical studies all suggest a different pattern of molecular packing induced by molecular geometry and/or substituted chains. Nonetheless, both types of porphyrin HTMs perform well in TiO₂‐based PSCs when doped YZT4 with a power conversion efficiency (PCE) of 14.95% and undoped YZT1 with a PCE of 13.10%. The results clearly reveal the potential for porphyrin‐based HTMs for use in dopant‐free PSCs.

The article studies SnO₂'s role in the stability of air‐processed planar perovskite solar cells. UV treatment of sub‐cells (500 h N₂ environment) speeds up the depletion of perovskite films, leading to excess PbI₂ formation at the perovskite surfaces. This inadvertently leads to full device stabilization through passivation as seen in maximum power point (MPP) measurements of perovskite solar cells incorporating UV‐treated sub‐cells.

SnO₂ is nowadays the widely preferred material as an electron transport layer (ETL) in most n‐i‐p planar perovskite solar cells (PSCs) due to its facility for ambient, low temperature processing, and ultraviolet (UV) stability. Most reports published so far study device stability on full cells. Herein, the role of slot‐die‐coated SnO₂ on air‐processed planar PSCs by analyzing sub‐cells (indium tin oxide [ITO]/SnO₂/perovskite) under UV exposure is investigated. Results from UV–vis spectroscopy, depth profiling using X‐ray diffraction measurement in grazing incidence mode (GIXRD), X‐ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy measurements show that UV treatment of ITO/SnO₂/perovskite leads to a reduced electron transfer to the SnO₂ layer and a gradual increase in the amount of PbI₂ toward the perovskite surfaces. Subsequently, hole transport layer (HTL) and electrodes are applied on SnO₂/perovskite interfaces (UV‐treated and non‐UV‐treated) and complete devices are fabricated. Device performance is compared and analyzed through J –V curves and maximum power point (MPP) tracking. Results show that devices built on a UV‐treated SnO₂/perovskite interface show better stability attributed to the presence of excess PbI₂ resulting in a passivation effect. Challenges in uniform film formation of slot‐die‐coated SnO₂ and potential solutions using a polymeric additive are also highlighted.

Conjugated polyelectrolytes (CPEs) are studied as interlayers in perovskite‐based solar cells. By modulating the ionic density in CPEs, wetting, perovskite crystal growth, and interfacial defect passivation are optimized, achieving 18.38% efficiency for a large‐area (1 cm²) device with negligible hysteresis and stable power output.

Abstract

A series of anionic conjugated polyelectrolytes (CPEs) is synthesized based on poly(fluorene‐co‐phenylene) by varying the side‐chain ionic density from two to six per repeat units (MPS2‐TMA, MPS4‐TMA, and MPS6‐TMA). The effect of MPS2, 4, 6‐TMA as interlayers on top of a hole‐extraction layer of poly(bis(4‐phenyl)‐2,4,6‐trimethylphenylamine (PTAA) is investigated in inverted perovskite solar cells (PeSCs). Owing to the improved wettability of perovskites on hydrophobic PTAA with the CPEs, the PeSCs with CPE interlayers demonstrate a significantly enhanced device performance, with negligible device‐to‐device dependence relative to the reference PeSC without CPEs. By increasing the ionic density in the MPS‐TMA interlayers, the wetting, interfacial defect passivation, and crystal growth of the perovskites are significantly improved without increasing the series resistance of the PeSCs. In particular, the open‐circuit voltage increases from 1.06 V for the PeSC with MPS2‐TMA to 1.11 V for the PeSC with MPS6‐TMA. The trap densities of the PeSCs with MPS2,4,6‐TMA are further analyzed using frequency‐dependent capacitance measurements. Finally, a large‐area (1 cm²) PeSC is successfully fabricated with MPS6‐TMA, showing a power conversion efficiency of 18.38% with negligible hysteresis and a stable power output under light soaking for 60 s.

Phenylhydrazine hydrochloride is introduced into FASnI₃‐based perovskite solar cells (where FA = NH₂CHNH₂ ⁺) in order to reduce the existing Sn⁴⁺ and prevent the further degradation of the FASnI₃. Consequently, the champion device shows a high power conversion efficiency up to 11.4%, a long‐term storage stability over 2300 h, and an efficiency recovery capability after being exposed to air.

Abstract

The development of tin (Sn)‐based perovskite solar cells (PSCs) is hindered by their lower power conversion efficiency and poorer stability compared to the lead‐based ones, which arise from the easy oxidation of Sn²⁺ to Sn⁴⁺. Herein, phenylhydrazine hydrochloride (PHCl) is introduced into FASnI₃ (FA = NH₂CH  NH₂ ⁺) perovskite films to reduce the existing Sn⁴⁺ and prevent the further degradation of FASnI₃, since PHCl has a reductive hydrazino group and a hydrophobic phenyl group. Consequently, the device achieves a record power conversion efficiency of 11.4% for lead‐free PSCs. Besides, the unencapsulated device displays almost no efficiency reduction in a glove box over 110 days and shows efficiency recovery after being exposed to air, due to a proposed self‐repairing trap state passivation process.

Di‐n‐propylamine solution in methyl acetate is successfully demonstrated as an efficient solid‐state treatment for CsPbI₃ perovskite quantum dot (PQD) solar cells, and a record power conversion efficiency of ≈15% and high reproducibility are achieved for CsPbI₃ PQD solar cells.

Abstract

Lead‐halide perovskite quantum dots (PQDs) or more broadly, nanocrystals possess advantageous features for solution‐processed photovoltaic devices. The nanocrystal surface ligands play a crucial role in the transport of photogenerated carriers and ultimately affect the overall performance of PQD solar cells. Significantly improved CsPbI₃ PQD synthetic yield and solar‐cell performance through surface ligand management are demonstrated. The treatment of a secondary amine, di‐n‐propylamine (DPA), provides a mild and efficient approach to control the surface ligand density of PQDs, which has an apparently different working mechanism compared to previously reported surface treatments. Using an optimal DPA concentration, the treatment can simultaneously remove both long‐chain insulating surface ligands of oleic acid and oleylamine, even for unpurified PQDs with high ligand density. As a result, the electrical coupling between PQDs is enhanced, leading to improved charge transport, reduced carrier recombination, and a high power conversion efficiency approaching 15% for CsPbI₃‐PQD‐based solar cells. In addition, the production yield of CsPbI₃ PQDs can be increased by a factor of 8. These results highlight the importance of developing new ligand‐management strategies, specifically for emerging PQDs to achieve scalable and high‐performance perovskite‐based optoelectronic devices.

The mechanism of 2D halide perovskite film formation is resolved using in situ grazing‐incidence wide‐angle scattering (GIWAXS). The film begins as a sol–gel precursor before first forming a 3D MAPbI₃‐like phase at the air/liquid interface. This acts as a template for the highly textured 2D phase with the layers perpendicular to the substrate, which grows closer to the substrate.

Abstract

2D hybrid halide perovskites with the formula (A′)₂(A) _{n ‐1}Pb _n I_{3 n +1} have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to monitor film formation during spin‐coating. This elucidates the general film formation mechanism of 2D halide perovskites during one‐step spin‐coating. There are three stages of film formation: sol–gel, oriented 3D, and 2D. Three precursor phases form during the sol–gel stage and transform to perovskite, first giving a highly oriented 3D‐like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.

A gradient grain‐sized (GGS) CsPbI₃ bilayer is developed to stabilize the α phase via a single‐step film‐deposition process. The perovskite solar cell based on the GGS CsPbI₃ bilayer shows an efficiency of 15.5% and operates stably for 1000 h under ambient conditions.

Abstract

The emerging inorganic CsPbI₃ perovskites are promising wide‐bandgap materials for application in tandem solar cells, but they tend to transit from a black α phase to a yellow δ phase in ambient conditions. Herein, a gradient grain‐sized (GGS) CsPbI₃ bilayer is developed to stabilize the α phase via a single‐step film deposition process. The spontaneously upward migration of (adamantan‐1‐yl)methanammonium (ADMA) based on the hot‐casting technique causes self‐assembly of the hierarchical morphology for the perovskite layers. Due to the strong steric effect of the surficial ADMA cation, a self‐assembly tiny grain‐sized CsPbI₃ layer is in situ formed at the surface site, which exhibits notably enhanced phase stability by its high surface energy. Meanwhile, a large grain‐sized CsPbI₃ layer is obtained at the bottom site with high charge mobility and low trap density of states, which benefits from the regulated growth rates by the interaction between ADMA and perovskites. The perovskite solar cell (PSC) based on the GGS CsPbI₃ bilayer shows an efficiency of 15.5% and operates stably for 1000 h under ambient conditions. This work confirms that redistributing the surface energy of perovskite films is a facile strategy to stabilize metastable PSCs without the cost of efficiency loss.

The recent advances achieved in building‐integrated photovoltaics based on perovskite materials for four areas of applications, namely semitransparent windows, colorful wall facades, electrochromic windows, and thermochromic windows, are presented. Critical roadmaps on future developments of this cutting‐edge research field are provided.

Abstract

Perovskite‐based solar cells have attracted great attention due to their low cost and high photovoltaic (PV) performance. In addition to their success in the PV sector, there has been growing interest in employing perovskites in energy‐efficient smart windows and other building technologies owing to their large absorption coefficient and color tunability. The major challenge lies in integrating perovskite materials into windows and building facades and combining them with added functionalities while maintaining their remarkable power conversion efficiencies. Herein, advances that have been made in the application of perovskites to building‐integrated photovoltaics (BIPVs) in four areas are highlighted: semitransparent windows, colorful wall facades, electrochromic windows, and thermochromic windows. In addition, the opportunities and challenges of this cutting‐edge research area and important roadmaps for the future use of perovskites in BIPVs are discussed.

Phenylhydrazine hydrochloride is introduced into FASnI₃ (FA = NH₂CH = NH₂ ⁺)‐based perovskite solar cells in order to reduce the existing Sn⁴⁺ and prevent the further degradation of FASnI₃. Consequently, the champion device shows a high power conversion efficiency up to 11.4%, a long‐term storage stability over 2300 h, and an efficiency recovery capability after being exposed to air.

Abstract

The development of tin (Sn)‐based perovskite solar cells (PSCs) is hindered by their lower power conversion efficiency and poorer stability compared to the lead‐based ones, which arise from the easy oxidation of Sn²⁺ to Sn⁴⁺. Herein, phenylhydrazine hydrochloride (PHCl) is introduced into FASnI₃ (FA = NH₂CH = NH₂ ⁺) perovskite films to reduce the existing Sn⁴⁺ and prevent the further degradation of FASnI₃, since PHCl has a reductive hydrazino group and a hydrophobic phenyl group. Consequently, the device achieves a record power conversion efficiency of 11.4% for lead‐free PSCs. Besides, the unencapsulated device displays almost no efficiency reduction in a glove box over 110 days and shows efficiency recovery after being exposed to air, due to a proposed self‐repairing trap state passivation process.

A small molecule of 4,4′,4″,4′″‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N ,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) is applied to effectively p‐dope the FA _x MA_{1− x} PbI₃ (FA:HC(NH₂)₂; MA:CH₃NH₃) perovskite surface, with obvious conductivity and carrier concentration increase. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending at perovskite surface facilitates hole extraction to hole transport layer and expels electrons toward cathode, which reduces the surface charge recombination. The optimized devices demonstrate a stabilized efficiency of 22.9%.

Abstract

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N ,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) which can effectively p‐dope the surface of FA _x MA_{1− x} PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite films is reported. The intermolecular charge transfer property of PT‐TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT‐TPA. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.

The mechanism of 2D halide perovskite film formation is resolved using in situ grazing‐incidence wide‐angle scattering (GIWAXS). The film begins as a sol–gel precursor before first forming a 3D MAPbI₃‐like phase at the air/liquid interface. This acts as a template for the highly textured 2D phase with the layers perpendicular to the substrate, which grows closer to the substrate.

Abstract

2D hybrid halide perovskites with the formula (A′)₂(A) _{n ‐1}Pb _n I_{3 n +1} have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to monitor film formation during spin‐coating. This elucidates the general film formation mechanism of 2D halide perovskites during one‐step spin‐coating. There are three stages of film formation: sol–gel, oriented 3D, and 2D. Three precursor phases form during the sol–gel stage and transform to perovskite, first giving a highly oriented 3D‐like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Xuning Zhang, Nannan Yao, Rui Wang, Yanxun Li, Dongyang Zhang, Guangbao Wu, Jiyu Zhou, Xing Li, Hong Zhang, Jianqi Zhang, Zhixiang Wei, Chunfeng Zhang, Huiqiong Zhou, Fengling Zhang, Yuan Zhang

Publication date: October 2020

Source: Nano Energy, Volume 76

Author(s): Yueyue Gao, Zhitao Shen, Furui Tan, Gentian Yue, Rong Liu, Zhijie Wang, Shengchun Qu, Zhanguo Wang, Weifeng Zhang

An efficient and stable tin perovskite solar cell with a graded heterostructure which is composed of narrow‐bandgap and wide‐bandgap tin perovskites is reported. Such heterostructure facilitates charge extraction and suppresses the oxidation process of Sn²⁺ to Sn⁴⁺. Consequently, the device achieves a maximum power conversion efficiency of 11% with better operational stability.

Lead‐free tin perovskite solar cells (TPSCs) have attracted widespread attention in recent years due to their low toxicity, suitable bandgap, and high carrier mobility. However, the photovoltage and efficiency of TPSCs are still much lower than those of the lead counterparts because of the high trap density and unfavorable band structure in tin perovskite films. To overcome these issues, efficient and stable TPSCs with a graded heterostructure of light‐absorbing layer are reported, in which the narrow‐bandgap tin perovskite dominates at the bulk, whereas the wide‐bandgap tin perovskite is distributed with a gradient from bulk to surface. This heterostructure can selectively extract the photogenerated charge carriers at the perovskite/electron transport layer interface, reduce the density of trap states, and impede the oxidation process of Sn²⁺ to Sn⁴⁺ in air. As a consequence, this graded heterostructure of tin perovskite layer contributes to an increase of 120 mV in the open‐circuit voltage and a maximum power conversion efficiency of 11% for TPSCs with longer operational stability.

A simple and effective method of fine‐tuning the energy level of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) is demonstrated. The as‐prepared hole‐transporting material aligns well with the highest occupied molecular orbital level of the electron donor in nonfullerene‐based organic photovoltaic (OPV) devices in inverted architecture, reaching a power conversion efficiency of 10%, which would benefit the future commercialization of highly efficient OPV devices.

Solution‐processable hole‐transporting materials are demonstrated to improve the performance of nonfullerene‐based organic photovoltaic devices in an inverted structure. A vanadium oxide (VO _X ) precursor, used as a sol–gel, is mixed with commercial poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to form a well‐dispersed VO _X :PEDOT:PSS solution. The work function and molecular distribution of the VO _X :PEDOT:PSS thin film are examined by ultraviolet photoelectron spectroscopy (UPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), respectively. Unlike conventional PEDOT:PSS, VO _X :PEDOT:PSS not only is compatible with highly hydrophobic photoactive layers but also aligns well with the highest occupied molecular orbital (HOMO) level of the polymer donor, reaching a power conversion efficiency of 10% (≈100% boost) and achieving an excellent device stability.

Conjugated polyelectrolytes (CPEs) are studied as interlayers in perovskite‐based solar cells. By modulating the ionic density in CPEs, wetting, perovskite crystal growth, and interfacial defect passivation are optimized, achieving 18.38% efficiency for a large‐area (1 cm²) device with negligible hysteresis and stable power output.

Abstract

A series of anionic conjugated polyelectrolytes (CPEs) is synthesized based on poly(fluorene‐co‐phenylene) by varying the side‐chain ionic density from two to six per repeat units (MPS2‐TMA, MPS4‐TMA, and MPS6‐TMA). The effect of MPS2, 4, 6‐TMA as interlayers on top of a hole‐extraction layer of poly(bis(4‐phenyl)‐2,4,6‐trimethylphenylamine (PTAA) is investigated in inverted perovskite solar cells (PeSCs). Owing to the improved wettability of perovskites on hydrophobic PTAA with the CPEs, the PeSCs with CPE interlayers demonstrate a significantly enhanced device performance, with negligible device‐to‐device dependence relative to the reference PeSC without CPEs. By increasing the ionic density in the MPS‐TMA interlayers, the wetting, interfacial defect passivation, and crystal growth of the perovskites are significantly improved without increasing the series resistance of the PeSCs. In particular, the open‐circuit voltage increases from 1.06 V for the PeSC with MPS2‐TMA to 1.11 V for the PeSC with MPS6‐TMA. The trap densities of the PeSCs with MPS2,4,6‐TMA are further analyzed using frequency‐dependent capacitance measurements. Finally, a large‐area (1 cm²) PeSC is successfully fabricated with MPS6‐TMA, showing a power conversion efficiency of 18.38% with negligible hysteresis and a stable power output under light soaking for 60 s.