Ligang Yuan

A novel organic compound 1‐(ammonium acetyl) pyrene is successfully introduced for preparing the 2D/3D heterostructured MAPbI₃ perovskite. Because of the functional organic pyrene group with high humidity resistance and strong absorption in the ultraviolet region, the 2D/3D perovskite showed notable stability, comparable photovoltaic performance in humid air atmosphere and ultraviolet irradiation.

Abstract

Despite the eminent performance of the organometallic halide perovskite solar cells (PSCs), the poor stability for humidity and ultraviolet irradiation is still major problem for the commercialization of PSCs. Herein, a novel functional organic compound 1‐(ammonium acetyl)pyrene is successfully introduced for preparing the 2D/3D heterostructured MAPbI₃ perovskite. Because of the functional organic pyrene group with high humidity resistance and strong absorption in the ultraviolet region, the 2D/3D perovskite film shows notable stability with no degradation in ≈60% relative humidity after even six months and exhibits a high ultraviolet irradiation stability which keeps nearly no degradation after 1 h in the UV Ozone treatment. Planar PSCs are fabricated in the ≈60% relative humidity air outside glovebox. The champion efficiency of (PEY₂PbI₄)_0.02MAPbI₃ perovskite solar cells is 14.7% with nearly no hysteresis which is equal performance of 3D MAPbI₃ devices (15.0%). This work presents a new direction for enhancing the solar cells' performance and stability by incorporating a functional organic aromatic compound into the perovskite layer.

The device performance of all‐polymer solar cells is systematically optimized by modulating the crystallinity of the donor and acceptor polymers via controlling the regularity of their main chains. Experimental and coarse‐grained modeling results reveal that reducing the backbone regularity of either donor or acceptor results in poorer device performance due to a reduced crystallinity and contracted domain size in blends.

All‐polymer solar cells (APSCs), composed of semiconducting donor and acceptor polymers, have attracted considerable attention due to their unique advantages compared to polymer‐fullerene‐based devices in terms of enhanced light absorption and morphological stability. To improve the performance of APSCs, the morphology of the active layer must be optimized. By employing a random copolymerization strategy to control the regularity of the backbone of the donor polymers (PTAZ‐TPDx) and acceptor polymers (PNDI‐Tx) the morphology can be systematically optimized by tuning the polymer packing and crystallinity. To minimize effects of molecular weight, both donor and acceptor polymers have number‐average molecular weights in narrow ranges. Experimental and coarse‐grained modeling results disclose that systematic backbone engineering greatly affects the polymer crystallinity and ultimately the phase separation and morphology of the all‐polymer blends. Decreasing the backbone regularity of either the donor or the acceptor polymer reduces the local crystallinity of the individual phase in blend films, affording reduced short‐circuit current densities and fill factors. This two‐dimensional crystallinity optimization strategy locates a PCE maximum at highest crystallinity for both donor and acceptor polymers. Overall, this study demonstrates that proper control of both donor and acceptor polymer crystallinity simultaneously is essential to optimize APSC performance.

Dopant‐free hole transporting materials for perovskite solar cells are reviewed. The efficiency growth of perovskite solar cells (PSCs) based on dopant‐free hole transporting materials (HTMs) is highlighted. Hydrophilic chemical dopants can accelerate the degradation of the perovskite layer. Dopant‐free HTMs play a significant role in both efficiency and stability of PSCs.

This article reviews various dopant‐free hole transporting materials (HTMs) used in perovskite solar cells (PSCs) in three main categories including inorganic, polymeric, and small molecule HTMs. PSCs have undergone rapid progress, achieving power conversion efficiencies (PCEs) above 22%. With their low production cost and high efficiencies, PSCs are considered promising next‐generation solar cell technology. In all developed architectures for PSCs, including planar and mesoscopic with conventional and inverted structures, HTMs play a significant role in determining the photovoltaic performance of PSCs. Using p‐type dopants, however, is considered a common strategy to increase the hole conductivity of HTM, which is usually compensated by a more complicated fabrication procedure, higher production costs, and lower stability of PSC. Although several reviews on HTMs have been published, progress on dopant free HTMs needs to be reviewed and analyzed. Here, a review covering most of the published reports on dopant‐free HTMs is presented, and the device structure and fabrication method, HTM layer deposition techniques, and the efficiency and the stability of PSCs are addressed during discussions in each main category. Finally, an outlook on stability and PCE growth in PSCs based on dopant‐free HTMs is presented.

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring, Published online: 24 September 2018; doi:10.1038/s41467-018-06425-5

A new group of aniline‐based enamine hole‐transporting materials is synthesized, characterized, and tested in perovskite solar cells, yielding a champion power conversion efficiency over 20%. The investigated materials are obtained via one‐step synthesis procedure, without the use of a transition metal catalyst, from a very common and inexpensive precursor—aniline.

Abstract

Metal‐halide perovskites offer great potential to realize low‐cost and flexible next‐generation solar cells. Low‐temperature‐processed organic hole‐transporting layers play an important role in advancing device efficiencies and stabilities. Inexpensive and stable hole‐transporting materials (HTMs) are highly desirable toward the scaling up of perovskite solar cells (PSCs). Here, a new group of aniline‐based enamine HTMs obtained via a one‐step synthesis procedure is reported, without using a transition metal catalyst, from very common and inexpensive aniline precursors. This results in a material cost reduction to less than 1/5 of that for the archetypal spiro‐OMeTAD. PSCs using an enamine V1091 HTM exhibit a champion power conversion efficiency of over 20%. Importantly, the unsealed devices with V1091 retain 96% of their original efficiency after storage in ambient air, with a relative humidity of 45% for over 800 h, while the devices fabricated using spiro‐OMeTAD dropped down to 42% of their original efficiency after aging. Additionally, these materials can be processed via both solution and vacuum processes, which is believed to open up new possibilities for interlayers used in large‐area all perovskite tandem cells, as well as many other optoelectronic device applications.

A low‐temperature and substrate‐independent growth method is demonstrated to grow millimeter‐level inorganic perovskite monocrystalline thin films. These films present good optical and electrical properties comparable to bulk ones. What is more, they exhibit excellent long‐term stability toward humidity and thermal treatment. The as‐grown CsPbBr₃ monocrystalline films are fabricated into photodetectors with high photodetecting performance.

Abstract

Stability is a key problem that hinders the practical application of lead halide perovskite. Therefore, all‐inorganic perovskite CsPbX₃ monocrystalline films are urgently needed to fabricate photoelectric devices. Herein, a low‐temperature and substrate‐independent growth method is demonstrated to grow millimeter‐level inorganic perovskite monocrystalline thin films. These films present good optical and electrical properties comparable to bulk ones. What is more, they exhibit excellent long‐term stability toward humidity and thermal treatment. The as‐grown CsPbBr₃ monocrystalline films are then fabricated into photodetectors with high photodetection performance. These results demonstrate that the CsPbBr₃ monocrystalline films have potential in fabricating high‐performance optoelectronic devices.

Epitaxial BiFeO₃–SrTiO₃ thin films are investigated in order to understand the correlation between electronic structure and carrier dynamics in perovskite solid solution photoelectrodes. As well as shifting the conduction band edge up, an exponential tail of trap states emerges near the conduction band edge which suppresses the fast recombination of electrons and holes and strongly enhances the photocurrent density.

Abstract

Owing to the versatility in their chemical and physical properties, transition metal perovskite oxides have emerged as a new category of highly efficient photocatalysts for photoelectrochemical (PEC) water splitting. Here, to understand the underlying mechanism for the enhanced PEC water splitting in mixed perovskites, ideal epitaxial thin films of the BiFeO₃–SrTiO₃ system are explored. The electronic structure and carrier dynamics are determined from both experiment and density‐functional theory calculations. The intrinsic phenomena are measured in this ideal system, contrasting to commonly studied polycrystalline solid solutions where extrinsic structural features obscure the intrinsic phenomena. It is determined that when SrTiO₃ is added to BiFeO₃ the conduction band minimum position is raised and an exponential tail of trap states from hybridized Ti 3d and Fe 3d orbitals emerges near the conduction band edge. The presence of these trap states strongly suppresses the fast electron–hole recombination and improves the photocurrent density in the visible‐light region, up to 16× at 0 V _RHE compared to the pure end member compositions. This work provides a new design approach for optimizing the PEC performance in mixed perovksite oxides.

Fully‐solution‐processed regular architecture perovskite solar cells utilizing a low‐temperature‐printed carbon electrode are demonstrated. The carbon electrode is printed from toluene ink with the active layer architecture ITO/self‐assembled monolayer/MAPI/hole transport materials/Ta‐WO _x /carbon, which shows dramati‐cally increased stability and reasonable performance.

Abstract

Thin‐film solar cells based on hybrid organo‐halide lead perovskites achieve over 22% power conversion efficiency (PCE). A photovoltaic technology at such high performance is no longer limited by efficiency. Instead, lifetime and reliability become the decisive criteria for commercialization. This requires a standardized and scalable architecture which does fulfill all requirements for larger area solution processing. One of the most highly demanded technologies is a low temperature and printable conductive ink to substitute evaporated metal electrodes for the top contact. Importantly, that electrode technology must have higher environmental stability than, for instance, an evaporated silver (Ag) electrode. Herein, planar and entirely low‐temperature‐processed perovskite devices with a printed carbon top electrode are demonstrated. The carbon electrode shows superior photostability compared to reference devices with an evaporated Ag top electrode. As hole transport material, poly (3′hexyl thiophene) (P3HT) and copper(I) thiocyanate (CuSCN), two cost‐effective and commercially available p‐type semiconductors are identified to effectively replace the costlier 2,2′,7,7′‐Tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐MeOTAD). While methylammonium lead iodide (MAPbI₃)‐based perovskite solar cells (PSCs) with an evaporated Ag electrode degrade within 100 h under simulated sunlight (AM 1.5), fully solution‐processed PSCs with printed carbon electrodes preserve more than 80% of their initial PCE after 1000 h of constant illumination.

Lanthanide ions are doped into CsPbBr₃ films to modulate crystal lattice for high‐performance all‐inorganic perovskite solar cells. Arising from the improved grain size and carrier lifetime, the solar cell achieves a champion power conversion efficiency of 10.14%, an ultrahigh V _oc of 1.594 V, and excellent stability.

Abstract

All‐inorganic cesium lead bromide (CsPbBr₃) perovskite solar cells have attracted enormous attention owing to their outstanding stability in comparison with organic–inorganic hybrid devices. The greatest weakness for inorganic CsPbBr₃ solar cells is their lower power conversion efficiencies, mainly arising from inferior light‐absorbance range and serious charge recombination at interfaces or within perovskite films. To address this issue, the lattice doping of lanthanide ions (Ln³⁺ = La³⁺, Ce³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Ho³⁺, Er³⁺, Yb³⁺, and Lu³⁺) into CsPbBr₃ films for all‐inorganic solar cells free of hole‐transporting materials and precious metal electrodes is presented. Arising from the enlarged grain size and prolonged carrier lifetimes upon incorporating Ln³⁺ ions into perovskite lattice, the performances of these inorganic CsPbBr₃ solar cell devices are significantly enhanced, achieving a champion efficiency as high as 10.14% and an ultrahigh open‐circuit voltage of 1.594 V under one sun illumination. Meanwhile, the nearly unchanged efficiency upon persistent attack by 80% RH in air atmosphere over 110 d and enhanced thermal stability at 80 °C over 60 d provide new opportunities of promoting commercialization of all‐inorganic CsPbBr₃ perovskite solar cells.

The effect of grain cluster size on back‐contact perovskite solar cells is investigated. It is found that the photovoltaic performance correlates positively with the perovskite grain cluster size. This is attributed to the reduced charge recombination and more efficient charge injection accompany perovskite films with larger grains.

Abstract

Incorporating interdigitated back‐contact electrodes into organic–inorganic halide perovskite solar cells overcomes the optical losses and low architectural defect tolerance present in conventional “sandwich” cell configurations. However, other factors limit device performance in back‐contact architectures, such as the short charge‐carrier diffusion length within the perovskite film relative to the electrode spacing. As charge‐carrier diffusion length is crystal‐size related, in order to understand the effect of perovskite morphology on the performance of back‐contact perovskite solar cells (bc‐PSCs), perovskite films with four different grain cluster sizes, i.e., large, medium, small, and extra small, are fabricated via a solvent annealing approach. Crystallization of the perovskite is found to be closely related to the surface chemistry and topography of the substrate. The bc‐PSC photovoltaic performance correlates positively with the perovskite grain cluster size. Through a detailed analysis of transient photovoltage decay measurements, time‐resolved photoluminescence, and space charge‐limited current measurements, the effect of defect densities associated with grain cluster boundaries is elucidated.

Unique J‐architecture from a new fused ring–based nonfullerene acceptor is demonstrated. The well correlations between the single crystal data and the graze‐incidence X‐ray diffraction (GIXRD) data give a clear picture of the acceptor molecule packing in the donor:acceptor blend films and the assignments of the well often used GIXRD signals. A power conversion efficiency of 10.5% is obtained.

Abstract

An interesting and important question emerges with the rapid advances of the highly efficient fused‐ring nonfullerene acceptors; that is, how the acceptor molecules form aggregates in its blended film with a donor polymer/small molecule so as to offer highly efficient exciton diffusion and electron transport? To answer this question, a new acceptor molecule, 3,9‐bis(5‐methylene‐4‐one‐6‐(1,1‐dicyanomethylene)‐cyclopenta[c]thiophen‐2,8‐dimethyl)‐5,5,11,11‐tetrakis(4‐n‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITCT‐DM), is designed and synthesized herein and its unique interpenetrating J‐architecture is presented in which the acceptor molecules form compacted and displaced ππ‐stacks with the distances of 3.1−4.2 Ǻ. Again the crystal structure data are correlated with the grazing‐incidence X‐ray diffraction (GIXRD) data of the pure acceptor and its polymer:acceptor blended films, which gives a clearer picture about the origins of the acceptor's GIXRD signals in both the pure and its blended films. Again, these results unveil the key roles of the uses of 1,8‐diiodooctane (DIO) and thermal annealing treatment in optimizing the acceptor phase morphologies in the donor:acceptor blended film, and the combination of the thermal annealing and DIO treatment leads to obtain higher crystallinity for both the donor and acceptor phases, more compacted packing, and finer morphologies. A power conversion efficiency of 10.5% is obtained.

A novel concept for the formation of the hole selective layer in efficient perovskite solar cells is presented. Carbazole‐based material is synthesized and used for the formation of a self‐assembled monolayer on top of the indium tin oxide transparent conductive substrate. Power conversion efficiency as high as 17.8% is achieved.

Abstract

The unprecedented emergence of perovskite‐based solar cells (PSCs) has been accompanied by an intensive search of suitable materials for charge‐selective contacts. For the first time a hole‐transporting self‐assembled monolayer (SAM) as the dopant‐free hole‐selective contact in p–i–n PSCs is used and a power conversion efficiency of up to 17.8% with average fill factor close to 80% and undetectable parasitic absorption is demonstrated. SAM formation is achieved by simply immersing the substrate into a solution of a novel molecule V1036 that binds to the indium tin oxide surface due to its phosphonic anchoring group. The SAM and its modifications are further characterized by Fourier‐transform infrared and vibrational sum‐frequency generation spectroscopy. In addition, photoelectron spectroscopy in air is used for measuring the ionization potential of the studied SAMs. This novel approach is also suitable for achieving a conformal coverage of large‐area and/or textured substrates with minimal material consumption and can potentially be extended to serve as a model system for substrate‐based perovskite nucleation and passivation control. Further gains in efficiency can be expected upon SAM optimization by means of molecular and compositional engineering.

A sequential‐vapor‐deposition method is exploited to successfully fabricate Cs₂AgBiBr₆ double perovskite films that are desirable for lead‐free solar cell applications. The films exhibit high quality in terms of large compact grains, high uniformity, and long‐term stability. The planar structure solar cells based on the vapor deposited films show a power conversion efficiency of 1.37%.

Lead‐free double perovskites have been demonstrated as promising alternatives to solve the toxicity and stability issues in conventional lead trihalide perovskites. However, different solubility of components in the precursors hinders fabrication of double perovskite films with commonly used solution procedures. Here, for the first time, the authors successfully prepared double perovskite Cs₂AgBiBr₆ thin films throughout a sequential‐vapor‐deposition procedure. The obtained thin films with pure double perovskite phase show large grain sizes, uniform, and smooth surface properties. In addition, the high‐quality vapor‐deposited Cs₂AgBiBr₆ films exhibit a photoluminescence (PL) lifetime of 117 ns, indicative of significant potential in photovoltaic applications. The resulting solar cells with planar device structure show an optimized power conversion efficiency of 1.37%, which can be maintained at 90% after 240 h of storage under ambient condition. Our results demonstrate the feasibility of employing vapor deposition technique to fabricate high‐quality double perovskite thin films, which paves the way for further development of various optoelectronic devices based on these promising lead‐free semiconductors.