Chen Weijie

The ferroelectric P(VDF‐TrFE) polymer dopant results in promoting the built‐in electric field in the absorber perovskite layer, which leads to both increasing the minority carrier diffusion length and reducing the nonradiative recombination loss. Compared to typical CH₃NH₃PbI₃‐based perovskite solar cells (PSCs), optimized FE‐PSCs present higher V _OCs up to 1.17 V and power conversion efficiency up to 18%.

Abstract

A novel ferroelectric coupling photovoltaic effect is reported to enhance the open‐circuit voltage (V _OC) and the efficiency of CH₃NH₃PbI₃ perovskite solar cells. A theoretical analysis demonstrates that this ferroelectric coupling effect can effectively promote charge extraction as well as suppress combination loss for an increased minority carrier lifetime. In this study, a ferroelectric polymer P(VDF‐TrFE) is introduced to the absorber layer in solar cells with a proper cocrystalline process. Piezoresponse force microscopy (PFM) is used to confirm that the P(VDF‐TrFE):CH₃NH₃PbI₃ mixed thin films possess ferroelectricity, while the pure CH₃NH₃PbI₃ films have no obvious PFM response. Additionally, with the applied external bias voltages on the ferroelectric films, the devices begin to show tunable photovoltaic performance, as expected for the polarization in the poling process. Furthermore, it is shown that through the ferroelectric coupled effect, the efficiency of the CH₃NH₃PbI₃‐based perovskite photovoltaic devices is enhanced by about 30%, from 13.4% to 17.3%. And the open‐circuit voltages (V _OC) reach 1.17 from 1.08 V, which is reported to be among the highest V _OCs for CH₃NH₃PbI₃‐based devices. It should be noted in particular that the thickness of the layer is less than 160 nm, which can be regarded as semi‐transparent.

This work provides a new strategy (by alternating the core) for fine‐tuning the energy levels of nonconjugated zwitterionic molecules and by which a series of stable and one‐step synthesized electron transport layers are obtained for achieving higher performance of polymer solar cells and reducing the cost of industrial manufacture.

Developing stable and cheap organic cathode interlayers (OCIs; to replace metal cathode and expensive OCIs for enhancing the device stability and reducing the manufacturing cost) is an important topic for commercial applications of polymer solar cells (PSCs). Herein, four one‐step synthesized organic electron transport layers (G‐Series electron‐transport layers [ETLs]) with a novel star‐shaped molecular structure consisting of a series of different heteroatom atoms as cores and sulfobetaine ions as a terminal substituent are explored. The energy levels can be finely tuned by applying different heteroatom atoms as cores. With the conventional device structure with poly[[2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene][3‐fluoro‐2[(2‐ethylhexyl) carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) as a donor and [6,6]‐phenyl‐C₇₁‐butyric‐acid‐methyl‐ester (PC₇₀BM) as an acceptor, the G‐C2‐based devices exhibit a power conversion efficiency (PCE) of 8.90% with Al as the top electrode, much higher than that of the corresponding Ca/Al‐based device (7.43%). Furthermore, G‐Series‐based solar cells are also more stable than the reference device based on Ca. In addition, these easy‐to‐get ETLs can be widely suitable for other PSCs based on different active layer systems. This work not only shows a new strategy for fine‐tuning energy levels of nonconjugated zwitterionic molecules but also provides simple and stable ETLs for low‐cost and high‐performance PSCs.

Time‐resolved grazing‐incidence X‐ray scattering reveals cascade effects of PbS nanocrystals seeded in perovskite thin films. During annealing, PbS nanocrystals accelerate the nucleation of a highly oriented intermediate phase L ₁ through capped organic perovskite precursors, and subsequently catalyze the L ₁‐to‐perovskite conversion via lattice anchoring alignment with the cubic lattice of the perovskite, leading to improved crystal grain connectivity, texture, and thermal stability.

Abstract

Recently, a new seeding growth approach for perovskite thin films is reported to significantly enhance the device performance of perovskite solar cells. This work unveils the intermediate structures and the corresponding growth kinetics during conversion to perovskite crystal thin films assisted by seeding PbS nanocrystals (NCs), using time‐resolved grazing‐incidence X‐ray scattering. Through analyses of time‐resolved crystal formation kinetics obtained from synchrotron X‐rays with a fast subsecond probing time resolution, an important “catalytic” role of the seed‐like PbS NCs is clearly elucidated. The perovskite precursor‐capped PbS NCs are found to not only accelerate the nucleation of a highly oriented intermediate phase, but also catalyze the conversion of the intermediate phase into perovskite crystals with a reduced activation energy E _a = 47 (±5) kJ mol⁻¹, compared to 145 (±38) kJ mol⁻¹ for the pristine perovskite thin film. The reduced E _a is attributed to a designated crystal lattice alignment of the perovskite nanocrystals with perovskite cubic crystals; the pivotal heterointerface alignment of the perovskite crystals coordinated by the Pb NCs leads to an improved film surface morphology with less pinholes and enhanced crystal texture and thermal stability. These together contribute to the significantly improved photovoltaic performance of the corresponding devices.

Molecular doping is demonstrated as a powerful strategy for enhancing electrical conductivity in arrays of electronically coupled CsPbI₃ nanocrystals. The high surface‐area‐to‐volume ratio enables physisorbed organic redox molecules to inject charge carriers into a nanocrystal array. p‐Type doping improves both ground‐state conductivity and photoconductivity, enabling pronounced performance enhancements to both transistors and phototransistors.

Abstract

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead‐halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface‐area‐to‐volume ratio, are particularly well‐suited for doping via ground‐state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p‐type with increasing conductivity achieved by incorporating the electron‐accepting dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) and decreasing conductivity for the electron‐donating dopant benzyl viologen. Time‐resolved spectroscopic measurements reveal the time scales of Auger‐mediated recombination in the presence of excess electrons or holes. Microwave conductance and field‐effect transistor measurements demonstrate that both the local and long‐range hole mobility are improved by F₄TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p‐type arrays leads to a pronounced enhancement in phototransistors.

A new type of ACI perovskite is prepared through the alternating ordering of BEA²⁺ and MA⁺ cations in the interlayer space (B‐ACI). The high exciton extraction efficiency and a narrow distribution of quantum well widths of B‐ACI perovskite enable a device with a record efficiency of 17.39%. Furthermore, the devices show stronger resistance to humidity, heating, and light soaks than previous equivalents.

Abstract

Low‐dimensional Ruddlesden–Popper (LDRP) perovskites are a current theme in solar energy research as researchers attempt to fabricate stable photovoltaic devices from them. However, poor exciton dissociation and insufficiently fast charge transfer slows the charge extraction in these devices, resulting in inferior performance. 1,4‐Butanediamine (BEA)‐based low‐dimensional perovskites are designed to improve the carrier extraction efficiency in such devices. Structural characterization using single‐crystal X‐ray diffraction reveals that these layered perovskites are formed by the alternating ordering of diammonium (BEA²⁺) and monoammonium (MA⁺) cations in the interlayer space (B‐ACI) with the formula (BEA)_0.5MA _n PbnI_3n+1. Compared to the typical LDRP counterparts, these B‐ACI perovskites deliver a wider light absorption window and lower exciton binding energies with a more stable layered perovskite structure. Additionally, ultrafast transient absorption indicates that B‐ACI perovskites exhibit a narrow distribution of quantum well widths, leading to a barrier‐free and balanced carrier transport pathway with enhanced carrier diffusion (electron and hole) length over 350 nm. A perovskite solar cell incorporating BEA ligands achieves record efficiencies of 14.86% for (BEA)_0.5MA₃Pb₃I₁₀ and 17.39% for (BEA)_0.5Cs_0.15(FA_0.83MA_0.17)_2.85Pb₃(I_0.83Br_0.17)₁₀ without hysteresis. Furthermore, the triple cations B‐ACI devices can retain over 90% of their initial power conversion efficiency when stored under ambient atmospheric conditions for 2400 h and show no significant degradation under constant illumination for over 500 h.

Three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold were synthesized. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps.

Abstract

The synthesis of three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold are reported. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps. The best performing material, HTM3, demonstrated 18.62 % PCE in PSCs, rivaling spiro‐OMeTAD in efficiency, and showing markedly superior long‐term stability in non‐encapsulated devices. In dopant‐free PSCs, HTM3 outperformed spiro‐OMeTAD by a factror of 1.6. The high glass‐transition temperature (T _g=176 °C) of HTM3 also suggests promising perspectives in device applications.

In article no. 1900032, Lei Ying, Feng Liu, Fei Huang, Thomas P. Russell, and co‐workers study the multiple crystallization kinetics during bulk‐heterojunction film drying for all‐polymer solar cells by using in situ grazing incidence wide‐angle X‐ray scattering. Printing with 1,8‐diiodooctance is helpful for the formation of a multi‐length‐scale phase separation, and ultimately improves the solar cell performance.

In article no. 1900094, Doo Kyung Moon and co‐workers newly synthesize a chlorinated thiophenebased donor polymer, P(Cl), which has a relatively low synthetic complexity. P(Cl) shows an excellent power conversion efficiency and high atmospheric stability in non‐fullerene polymer solar cells.

In article no. 1900063, Kai Wang and co‐workers report the spin‐dependent electron‐hole recombination and dissociation in nonfullerene acceptor ITIC‐based binary and ternary organic bulk heterojunction systems. ITIC itself exhibits a negative magneto‐photocurrent due to the exciton‐charge reaction. The effect becomes critically important for electron‐hole dissociation in both systems at large fields and longer photo‐excitation wavelengths.

In article no. 1900091, Lai Feng and co‐workers demonstrate that solution‐processed two‐dimensional Nb₂O₅(001) nanosheets can be combined with PC₆₁BM and employed as a double‐layered electron transport layer for inverted inorganic CsPbI₂Br perovskite solar cells with high performance and excellent stability.

A hole‐transporting material (HTM), termed V1160, based on four TPD‐type fragments connected by a Tröger's base structural core, is synthesized, characterized, and applied as an HTM in perovskite solar cells. Demonstrating an over 18% power conversion efficiency, the fully amorphous nature of V1160, suggesting further studies in TPD‐based materials, is warranted.

One of the obstacles to the commercialization of perovskite solar cells (PSCs) is the high price and morphological instability of the most common hole‐transporting material (HTM) Spiro‐OMeTAD. Herein, a novel HTM, termed V1160, based on four N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD)‐type fragments, fused by a Tröger's base core, is synthesized and successfully applied in PSCs. Investigation of the optical, thermal, and photoelectrical properties shows that V1160 is a suitable candidate for application as an HTM in PSCs. A promising power conversion efficiency (PCE) of over 18% is demonstrated, which is only slightly lower than that of Spiro‐OMeTAD. Moreover, V1160‐based devices exhibit improved performances in dopant‐free configurations and superior stability. Favorable morphological properties in combination with a simple synthesis make V1160 and related materials promising for HTM applications.

An amorphous passivation layer using phenethylammonium iodide for amply covering the surface and grain boundaries of the CH₃NH₃PbI₃ film, results in the reduction of trap density and suppression of nonradiative recombination.

Organic–inorganic lead halide perovskite solar cells have realized a rapid increase of power conversion efficiency (PCE) in the past few years. However, their performance still suffers trap‐assisted decline due to defects at the surface and grain boundaries of the perovskite film. Herein, a phenethylammonium iodide‐lead iodide (PEAI‐PbI₂) passivation layer is formed on the CH₃NH₃PbI₃ perovskite film. The characterization results indicate that the PEAI covering layer leads to the reduction of surface defects and suppression of nonradiative recombination. By manipulating this surface passivation method, a remarkably improved V _OC of 1.16 V and an enhanced PCE of 20.8% are achieved.

Surface‐modified NiO _x nanoparticles (NPs) as hole transport materials in n‐i‐p‐structured perovskite solar cells are studied. The modified NiO _x NPs disperse well in chlorobenzene, and their film forms smooth and pinhole‐free layers, which show good electrical conductivity and improved extraction properties. The power conversion efficiency is improved from 5.5% to 13.1%.

Modified NiO _x nanoparticles (NPs) developed via surface engineering are applied to a hole transport layer (HTL) in n‐i‐p‐structured perovskite solar cells (PSCs). Hexanoic acid (HA) as a surfactant improves the dispersibility of NiO _x NPs in chlorobenzene (CB). The conductivity of the NiO _x ‐HA film is 1.20×10−5S cm−1, which is superior to that of 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) with dopants. The NiO _x ‐HA film shows better hole extraction properties compared with the pristine NiO _x film. The NiO _x ‐HA NPs form closely packed and pinhole‐free films, leading to improved device performance with a power conversion efficiency from 5.5% to 13.1%.

A new azeotropic promoted approach is proposed to successfully synthesize In doped CuCrO₂ under low temperatures in a short time. This In doped CuCrO₂ HTL has thermal stability up to 200 °C, and exhibits improved optical transmission and carrier mobility, which is beneficial for achieving high performance perovskite solar cells.

Abstract

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.

Solution‐processed double‐walled carbon nanotubes function as transparent electrodes in inverted‐type planar heterojunction perovskite solar cells. Double‐walled carbon nanotubes exhibit high optical conductivity and solubility. Good energy level alignment and morphology of the electrodes leads to an operating power conversion efficiency of 17.2%, which is the highest among the carbon nanotube electrode‐based perovskite solar cells.

Abstract

Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.

Biodegradable and biocompatible transparent conductive electrodes are fabricated from bamboo for flexible perovskite solar cells. After extensive mechanical tests, including bending and crumpling tests, they still exhibit excellent electrical performance and negligible decay. The bamboo‐based bioelectrode perovskite solar cell shows a record power conversion efficiency of 11.68%, maintaining over 70% of initial power conversion efficiency after the bending tests.

Abstract

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light‐weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo‐based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass‐based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Interfacial hot carrier extraction in MAPbI₃ perovskite films is explored by femtosecond transient absorption spectroscopy. Compared to hot electrons, the extraction of hot holes is more efficient at the interface of MAPbI₃.

Abstract

Charge‐carriers photoexcited above a semiconductor's bandgap rapidly thermalize to the band‐edge. The cooling of these difficult to collect “hot” carriers caps the available photon energy that solar cells–including efficient perovskite solar cells–may utilize. Here, the dynamics and efficiency of hot carrier extraction from MAPbI₃ (MA = methylammonium) perovskite by spiro‐OMeTAD (a hole‐transporting layer) and TiO₂ (an electron‐transporting layer) are investigated and explained using both ultrafast electronic spectroscopy and theoretical modeling. Time‐resolved spectroscopy reveals a quasi‐equilibrium distribution of hot carriers forming upon excess‐energy excitation of the perovskite–a distribution largely unaffected by the presence of TiO₂. In contrast, the quasi‐equilibrium distribution of hot carriers is virtually nonexistent when spiro‐OMeTAD is present, which is indicative of efficient hot hole extraction at the interface of MAPbI₃. Density functional theory calculations predict that deep energy‐levels of MAPbI₃ exhibit electronically delocalized character, with significant overlap with the localized valence band charge of the spiro‐OMeTAD molecules lying on the surface of MAPbI₃. Consequently, hot holes are easily extracted from the deep energy‐levels of MAPbI₃ by spiro‐OMeTAD. These findings uncover the origins of efficient hot hole extraction in perovskites and offer a practical blueprint for optimizing solar cell interlayers to enable hot carrier utilization.

The incorporation of π‐extended phosphoniumfluorene electrolytes as hole‐blocking layers in planar perovskite solar cells results in a significant enhancement in both the fill factor and the open‐circuit voltage of the devices. The latter can be enhanced by up to 120 mV as compared to the commonly used bathocuproine hole blocking layer.

Abstract

Four π‐extended phosphoniumfluorene electrolytes (π‐PFEs) are introduced as hole‐blocking layers (HBL) in inverted architecture planar perovskite solar cells with the structure of ITO/PEDOT:PSS/MAPbI₃/PCBM/HBL/Ag. The deep‐lying highest occupied molecular orbital energy level of the π‐PFEs effectively blocks holes, decreasing contact recombination. It is demonstrated that the incorporation of π‐PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in significant enhancement of the built‐in potential of the device. This enhancement results in an increase in the open‐circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. This work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress nonradiative recombination in perovskite devices and compensates for a nonideal energetic alignment at the hole‐transport layer/perovskite interface.

Interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro‐OMeTAD, which contributes to more efficient charge extraction and less accumulation of charge carriers.

Abstract

Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C‐9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro‐OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6%) is achieved by using the new HSMs in suitable perovskite solar cells. Time‐resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro‐OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS‐integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro‐OMeTAD. Importantly, the low‐cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.

Recent advances in probing polar domains in methylammonium lead iodide thin films and their implications for perovskite solar cells are reviewed. The fundamental crystal properties and the formation of domains with predominant in‐plane polarization, as monitored with piezoresponse force microscopy, provide evidence of the semiconducting ferroelectric nature of methylammonium lead iodide thin films.

Abstract

Whether or not methylammonium lead iodide (MAPbI₃) is a ferroelectric semiconductor has caused controversy in the literature, fueled by many misunderstandings and imprecise definitions. Correlating recent literature reports and generic crystal properties with the authors' experimental evidence, the authors show that MAPbI₃ thin‐films are indeed semiconducting ferroelectrics and exhibit spontaneous polarization upon transition from the cubic high‐temperature phase to the tetragonal phase at room temperature. The polarization is predominantly oriented in‐plane and is organized in characteristic domains as probed with piezoresponse force microscopy. Drift‐diffusion simulations based on experimental patterns of polarized domains indicate a reduction of the Shockley–Read–Hall recombination of charge carriers within the perovskite grains due to the ferroelectric built‐in field and allow reproduction of the electrical solar cell properties.

Electron and soft X‐ray spectroscopies are powerful techniques to study the chemical and electronic structure of surfaces and interfaces. The use of these techniques to study solar devices and to unravel some of the most pertinent aspects of recent cutting‐edge developments (and world‐record efficiency improvements) in chalcopyrite thin‐film solar cells is discussed.

Abstract

Thin‐film solar cells have great potential to overtake the currently dominant silicon‐based solar cell technologies in a strongly growing market. Such thin‐film devices consist of a multilayer structure, for which charge‐carrier transport across interfaces plays a crucial role in minimizing the associated recombination losses and achieving high solar conversion efficiencies. Further development can strongly profit from a high‐level characterization that gives a local, electronic, and chemical picture of the interface properties, which allows for an insight‐driven optimization. Herein, the authors' recent progress of applying a “toolbox” of high‐level laboratory‐ and synchrotron‐based electron and soft X‐ray spectroscopies to characterize the chemical and electronic properties of such applied interfaces is provided. With this toolbox in hand, the activities are paired with those of experts in thin‐film solar cell preparation at the cutting edge of current developments to obtain a deeper understanding of the recent improvements in the field, e.g., by studying the influence of so‐called “post‐deposition treatments”, as well as characterizing the properties of interfaces with alternative buffer layer materials that give superior efficiencies on large, module‐sized areas.

Same‐spot Raman photoluminescence with two lasers in a diamond anvil cell under hydrostatic pressure reveals that CsPbBr₃ nanocrystals, mostly located on the edges of CsPb₂Br₅ 2D platelets, are responsible for CsPb₂Br₅'s green emission. This sensitive noninvasive technique combining static and dynamic probes establishes a one‐to‐one property–structure relationship and distinguishes light emission from point defects versus nanoinclusions.

Abstract

Since the first report of the green emission of 2D all‐inorganic CsPb₂Br₅, its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb₂Br₅, a two‐step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same‐spot Raman‐PL as a static property–structure probe reveals that CsPbBr₃ nanocrystals are contributing to the green emission of CsPb₂Br₅; pressure‐dependent Raman‐PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb₂Br₅ is 0.3–0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full‐fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr_{3− x} X _x (X = Cl or I) is responsible for the strong visible PL in CsPb₂Br_{5− x} X _x . This experimental approach is applicable to all PL‐active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all‐inorganic lead halides.

A rutile TiO₂ electron transport layer (ETL) was prepared. The thickness and crystallinity can be controlled by deposition time and sintering temperature. Rutile TiO₂ has higher conductivity than anatase for faster electron transfer, better interface contact with the perovskite layer, and a lower trap density. These facilitate the charge extraction and collection and reducing carrier recombination.

Abstract

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase‐dependent charge collection is investigated by using rutile and anatase TiO₂ electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO₂ ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH₃NH₃PbI₃ (MAPbI₃) layer. Moreover, this may be also attributed to the fact that rutile TiO₂ has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO₂ ETL, MAPbI₃ PSCs with rutile TiO₂ ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (V _OC) of 1.17 V.