Mahui

A polymer solar cell (PSC) with a large active area of 216 cm² and high power conversion efficiency of 7.7% is presented, involving a nonfullerene acceptor and the solution‐processable ZrOx interfacial layer made by blade coating. This represents the highest reported efficiency for PSCs with an active area more than 10 cm². More encouragingly, the large‐area PSC shows good long‐term thermal stability as well.

A polymer solar cell involving a nonfullerene acceptor is made by blade coating. In the ternary bulk‐heterojunction layer, the donor is poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b’]dithiophene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐ 5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c’]dithiophene‐4,8‐dione)] (PBDB‐T) and the acceptor is a mixture of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2’,3’‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]dithiophene) (ITIC) and [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM). The device structure is an indium tin oxide (ITO)‐coated glass substrate/PEDOT:PSS/ternary active layer/interfacial layer/Al. For a small active area of 0.04 cm², the best power conversion efficiency is 9.8% with the LiF interfacial layer. For a large active area of 216 cm², the best efficiency is 7.7% with the ZrOx interfacial layer. After annealing at 85 °C for 30 days, the large‐area device keeps 75% of the initial efficiency. The efficiency of 4.9% is achieved for a large‐area semi‐transparent device.

The power conversion efficiency of poly(3‐hexylthiophene) P3HT:O‐IDTBR bulk heterojunction solar cells peaks at intermediate (34 kDa) polymer molecular weights (MWs). Combined transient absorption and time‐delayed collection field experiments demonstrate that charges are generated more efficiently at intermediate P3HT MWs compared with high and low MWs.

Large‐scale production of organic solar modules requires low‐cost and reliable materials with reproducible batch‐to‐batch properties. In case of polymers, their (photo)physical properties depend strongly on the polymers’ molecular weight (MW). Herein, the impact of the MW of the donor polymer poly(3‐hexylthiophene) (P3HT) on the photophysics is studied in blends with a recently developed rhodanine‐endcapped indacenodithiophene nonfullerene acceptor (IDTBR), a bulk heterojunction (BHJ) system that potentially fulfills the aforementioned criteria for large‐scale production. It is found that the power conversion efficiency (PCE) increases when the weight‐average MW is increased from 17 kDa (PCE: 4.0%) to 34 kDa (PCE: 6.6%), whereas a further increase in MW leads to a reduced PCE of 4.4%. It is demonstrated that the charge generation efficiency, as estimated from time‐delayed collection field experiments, varies with the P3HT MW and is the reason for the differences in photocurrent and device performance. These findings provide insight into the fundamental photophysical reasons of the MW dependence of the PCE, which is taken into account when using polymer‐based nonfullerene acceptor blends in solar cell devices and modules.

Pb(SCN)₂ functions at the grain boundaries and pinholes to in situ polish the perovskite film surface. A 425 nm‐thick CsPbI₂Br film with high crystalline, smooth, and uniform surface morphology is obtained, with an efficiency of 10.44% for a low cost and stable carbon‐based perovskite solar cell processed under low‐temperature (150 °C).

Improvement in stability and an economical processing technique are the main aspects of the commercialization of perovskite solar cells (PSCs). In this study, a 425 nm‐thick CsPbI₂Br film with a high crystalline, smooth, and uniform surface morphology is obtained by Pb(SCN)₂ passivating the grain boundaries under low temperature (150 °C). The results of a series of electrochemical analyses, including space‐charge‐limited‐current (SCLC), open‐circuit voltage decay (OCVD), electrical impedance spectroscopy (EIS), intensity‐modulated photocurrent spectroscopy (IMPS), and intensity‐modulated photovoltage spectroscopy (IMVS), demonstrate that the trap density of the CsPbI₂Br film is greatly reduced with Pb(SCN)₂, which effectively inhibits the interface recombination and promotes charge transport in CsPbI₂Br PSC. Efficiencies of 12.22% and 10.44% are achieved for low‐temperature‐processed CsPbI₂Br planar‐architecture PSCs with ITO/SnO₂/CsPbI₂Br/ poly(3‐hexylthiophene) (P3HT)/Ag and ITO/SnO₂/CsPbI₂Br/carbon, respectively. This low‐cost, high‐efficiency carbon‐based inorganic PSC shows potential industrial application, especially for tandem solar cells.

The optical, morphological, and photovoltaic changes as a result of the halide exchange process at the solid‐state phase of perovskite‐based solar cells are studied.

The optical properties of halide perovskite are already tuned in the initial solution by choosing the preferred precursors. However, it is not obvious that the optical properties change following perovskite crystallization; moreover, the mechanism in this case is not clear. Herein, it is shown that the optical properties are not necessarily “fixed” following perovskite crystallization. The phenomenon using the halide exchange process in the solid phase of an already crystalized perovskite is demonstrated. The effects of formamidinium iodide (FAI) and formamidinium bromide (FABr) are investigated post‐treatment on the physical, optical, and PV properties of Cs_0.2FA_0.8Pb(I_0.75Br_0.25)₃ perovskite. Various FAI/FABr ratios in the post‐treatment solutions show that a halide exchange occurs in the solid‐state phase following the crystallization of the perovskite film. Energy‐dispersive spectroscopy line scan made on focused ion beam samples show that the bromide is present deep inside the film post‐treatment, which supports the fact that the reaction is not occurring only on the surface. Charge extraction and voltage decay measurements of the complete solar cells support the structural changes due to the halide exchange. This work enhances the knowledge of halide exchange in already crystalized perovskite.

A liquid crystal (LC) molecule (4′‐heptyl‐4‐biphenylcarbonitrile) is first used as a “binding agent” to connect grain boundaries of perovskite. The crystal orientation of perovskite grains is controlled and the electron transport process is accelerated after treating with LC; these are reflected by the significant improvement in power conversion efficiency and high fill factor. Remarkably, the LC greatly contributes to the humid‐stability of perovskite solar cells.

Hybrid perovskites have rapidly emerged as highly promising optoelectronic materials for perovskite solar cells (PSCs), whereas solution‐processed perovskite films usually contain a large amount of grain‐boundary network, which is unbeneficial for efficient film function, including charge transport and environmental stability. Herein, a liquid crystal (LC) molecule is first used as a “binding agent” to connect grains and fill grain boundaries of perovskite. The LC molecule (4′‐heptyl‐4‐biphenylcarbonitrile) interacts with PbI₂ to control the crystal orientation for fine and oriented perovskite grains, which accelerates electron transport and enhances environmental stability. Consequently, compared with the pristine devices, the power conversion efficiency of the LC‐based device increases from 17.14% to 20.19% with a high fill factor (over 80%). Remarkably, the LC‐based PSCs retain 92% of their initial efficiency at 25 °C, and a relative humidity of 70% after 500 h, whereas the control samples are almost degraded completely under the same conditions.

The addition of low‐cost polypropylene (PP) into the active layer of organic solar cells improves the crystallinity of donors with high crystallinity and has a slight negative effect on the crystallinity of donors with low crystallinity. PP can reduce the aggregation size of PC₇₁BM, which leads to optimized morphology of the active layer. These results lead to a high power conversion efficiency at a thick active layer.

In organic solar cells (OSCs), the sensitivity of device performance to active layer thickness is a limiting factor for the large‐scale manufacture and roll‐to‐roll production. To reduce the thickness‐dependent effects, a low‐cost insulating polypropylene (PP) material is incorporated into the system with a high crystallinity donor, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))]:[6,6]‐phenyl C71 butyric acid methyl ester (PBDB‐T:PC₇₁BM) and the system with a low crystallinity donor, poly[[4,8‐bis[(2ethylhexyl)oxy]‐benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno‐[3,4b]thiophenediyl] (PTB7):PC₇₁BM. The devices based on PBDB‐T:PC₇₁BM:PP (2 wt%) show a power conversion efficiency (PCE) of 7.6% at a thickness of 280 nm compared with the control device with a PCE of 7% at a thickness of 100 nm. The change is mainly due to the enhancement of the crystallinity of donors with high crystallinity. PTB7:PC₇₁BM with 4 wt% PP shows higher PCE than the control device for the same thickness (100–300 nm) and reduced PCE with increasing thickness. These are mainly due to the reduced PC₇₁BM aggregation. The optimization of crystallinity and morphology will further promote effective exciton dissociation, accelerated charge transport, and reduced charge recombination. It indicates that the low cost and simple method of adding PP is a promising option for roll‐to‐roll production in the future.

The development of lead‐free perovskites has attracted increasing attention. The design rules for lead‐free perovskite materials with diverse structures are presented. The structure–property relationships and optical‐, electric‐, and magnetic‐related applications of these lead‐free perovskites are summarized. Based on these structure–property relationships, strategies for multifunctional perovskite design are proposed.

Abstract

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high‐performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less‐toxic, elements while maintaining the superior properties. Here, the design rules for lead‐free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead‐free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic‐related properties. 3D perovskites, especially A₂B⁺B³⁺X₆‐type double perovskites, demonstrate very promising optoelectronic prospects, while low‐dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure–property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead‐free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead‐free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

Solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is observed. The in‐plane coupling between the perovskite and the MoS₂ crystal lattices leads to perovskite films with larger grain size, lower trap density, and preferential growth orientation. Consequently, the efficiency of fabricated perovskite solar cells is substantially improved by the MoS₂ flakes as interfacial layers.

Abstract

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution‐processed perovskite films. Here, solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is reported. Under transmission electron microscopy, in‐plane coupling between the perovskite and the MoS₂ crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS₂ surface. In perovskite solar cells, when perovskite active layers are grown on MoS₂ flakes coated on hole‐transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high‐performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.

To determine the exact phase‐transition temperatures of lead‐halide perovskite solar‐cell materials, thermal expansion using a capacitive dilatometer is employed. This strategy leads to the discovery of unexplored structural characteristics in formamidinium (FA)‐based compounds providing a platform to predict changes in the optical and charge‐transport properties relevant for applications.

Abstract

The extraordinary properties of lead‐halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX₃ (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high‐resolution capacitive dilatometer to determine the phase transition temperatures. For δ‐FAPbI₃, two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr₃ that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X‐ray diffraction. Moreover, it is demonstrated that transport under steady‐state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead‐halide perovskite materials.

A novel strategy for achieving efficient and stable CsPbI₃ solar cells via regulating lattice distortion with a surface organic terminal group (OTG) is proposed. As the steric hindrance of the OTG increases, the CsPbI₃ perovskite will be more stable under ambient conditions. Consequently, the optimized CsPbI₃ device shows negligible degradation of efficiency after 30 day aging in ambient air.

Abstract

All‐inorganic cesium lead iodide perovskites (CsPbI₃) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI₆]⁴⁻). Here, a novel strategy to suppress the octahedral tilting of [PbI₆]⁴⁻ units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.

A thermodynamically favored crystal preferable orientation growth along the (001) crystal plane is explored in formamidinium/methylammonium mixed perovskites, and the origin is found to be the reduction of surface energy. Combined with the (001) plane lying parallel to the substrate, it affects the charge transportation and collection in the resultant perovskite solar cells, resulting in a power conversion efficiency of 21.2%.

Abstract

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed‐cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed‐cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed‐cation perovskite (FA_{1– x} MA _x PbI_2.87Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed‐cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

Inorganic and layered perovskites have broadened research paradigm for a range of optoelectronic devices. Their unique electronic and photophysical properties show that they are an excellent material, leading forefronts of solar cells, light‐emitting diodes, photodetectors, lasers, and beyond. An overview of key research activities for these devices is provided and challenges for their future research are identified.

Abstract

Organic–inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light‐emitting diodes (LEDs), continuous‐wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long‐term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current–voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single‐crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Microcavity‐integrated monolithic flexible perovskite artificial human photoreceptors are demonstrated. The artificial cones and rods exhibit a true similarity with the natural human retina and exhibit excellent specific detectivity, large linear dynamic range, short response time, and low noise current. The potential of these versatile structures is manifested by reproducing a realistic full‐color image.

Abstract

Electronic device versions of the neural functions of the human retina have high potential for use in artificial vision. This study demonstrates halide perovskite artificial human photoreceptors with specific photoresponses to red, green, and blue colors, which are consistent with human retinal photoreceiving cones and rods. In contrast to the current programmable spectral‐response technologies, a novel microcavity structure is combined in this study with a perovskite absorber to achieve a targeted spectrum without using external optical filters. The fabricated artificial photoreceptors exhibit excellent performance including a high detectivity of more than 10¹³ Jones, a large linear dynamic range of 154 dB, and a short response time of 580 ns. These values are equal to or better than those of the natural human retina. These devices can easily be monolithically integrated on a single flexible substrate by using vacuum deposition, and a true proof‐of‐concept full‐color image reconstruction is demonstrated.

Triimide‐functionalized n‐type polymers are synthesized and used as the acceptor materials in all‐polymer solar cells (all‐PSCs), and a remarkable power conversion efficiency of 8.98% is achieved, which is among the highest values in all‐PSCs. The results demonstrate the positive effects of increasing imide number in polymer acceptors on improving all‐polymer solar cell performance.

Two triimide‐functionalized n‐type acceptor polymers are designed and synthesized, which show narrower bandgap, lower‐lying frontier molecular orbital energy levels, and improved film morphology than the diimide‐functionalized analogue polymers. When blended with a p‐type donor polymer semiconductor PTB7‐Th, an outstanding power conversion efficiency of 8.98% with a remarkable open‐circuit voltage of 1.03 V is attained. This efficiency is among the highest values in all‐polymer solar cells (all‐PSCs) reported till today, surpassing that (6.85%) of the diimide‐functionalized analogue polymers by a big margin and even higher than that (8.69%) of the fullerene‐based solar cells. The results demonstrate that the triimide‐functionalized f‐BTI3 is an excellent building block for developing n‐type polymer semiconductors, and the polymer f‐BTI3‐T is among the best‐performing n‐type polymers for applications in all‐PSCs. The structure–property correlations of these imide‐functionalized polymer semiconductors offer important guides for developing high‐performance n‐type polymer semiconductors.

Highly efficient perovskite light‐emitting diodes are achieved by implementing a simple and cost‐effective method for efficient outcoupling of waveguided light. A record external quantum efficiency of 28.2% is realized for the device based on cesium lead bromide (CsPbBr₃), while retaining the same spectral response for broad viewing angles.

Abstract

Perovskite light‐emitting diodes (PeLEDs) show great application potential in high‐quality flat‐panel displays and solid‐state lighting due to their steadily improved efficiency, tunable colors, narrow emission peak, and easy solution‐processing capability. However, because of high optical confinement and nonradiative charge recombination during electron–photon conversion, the highest reported efficiency of PeLEDs remains far behind that of their conventional counterparts, such as inorganic LEDs, organic LEDs, and quantum‐dot LEDs. Here a facile route is demonstrated by adopting bioinspired moth‐eye nanostructures at the front electrode/perovskite interface to enhance the outcoupling efficiency of waveguided light in PeLEDs. As a result, the maximum external quantum efficiency and current efficiency of the modified cesium lead bromide (CsPbBr₃) green‐emitting PeLEDs are improved to 20.3% and 61.9 cd A⁻¹, while retaining spectral and angular independence. Further reducing light loss in the substrate mode using a half‐ball lens, efficiencies of 28.2% and 88.7 cd A⁻¹ are achieved, which represent the highest values reported to date for PeLEDs. These results represent a substantial step toward achieving practical applications of PeLEDs.

A new strategy toward the trichromatic patterning of perovskites, involving a photo‐activated and surface‐mediated halide exchange reaction between perovskite nanocrystals and haloalkanes, is reported. It is shown that the inkjet printing of haloalkanes under mild visible‐light illumination can trigger a green‐emitting bromide perovskite to undergo rapid halide exchange, and give rise to functional blue and red emissions at a resolution of less than 50 µm.

Abstract

Lead halide perovskite possesses a semiconductor bandgap that is readily tunable by a variation in its halide composition. Here, a photo‐activated halide exchange process between perovskite nanocrystals and molecular haloalkanes is reported, which enables the perovskite luminescence to be controllably shifted across the entire visible spectrum. Mechanistic investigations reveal a mutual exchange of halogens between the perovskite crystal surface and a chemisorbed haloalkane, yielding nanocrystals and haloalkanes with mixed halide contents. Exchange kinetics studies involving primary, secondary, and tertiary haloalkanes show that the rate of reaction is governed by the activation barrier in the breakage of the covalent carbon–halogen (CX) bond, which is a function of the CX bond energy and carbon radical stability. Employing this halide exchange approach, a micrometer‐scale trichromatic patterning of perovskites is demonstrated using a light‐source‐integrated inkjet printer and tertiary haloalkanes as color‐conversion inks. The haloalkanes volatilize after halide exchange and leave no residues, thereby offering significant processing advantage over conventional salt‐based exchange techniques. Beyond the possible applications in new‐generation micro‐LED and electroluminescent quantum dot displays, this work exemplifies the rich surface and photochemistry of perovskite nanocrystals, and could lead to further opportunities in perovskite‐based photocatalysis and photochemical sensing.

A thermodynamically favored crystal preferable orientation growth along the (001) crystal plane is explored in formamidinium/methylammonium mixed perovskites, and the origin is found to be the reduction of surface energy. Combined with the (001) plane lying parallel to the substrate, it affects the charge transportation and collection in the resultant perovskite solar cells, resulting in a power conversion efficiency of 21.2%.

Abstract

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed‐cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed‐cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed‐cation perovskite (FA_{1– x} MA _x PbI_2.87Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed‐cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

Inorganic and layered perovskites have broadened research paradigm for a range of optoelectronic devices. Their unique electronic and photophysical properties show that they are an excellent material, leading forefronts of solar cells, light‐emitting diodes, photodetectors, lasers, and beyond. An overview of key research activities for these devices is provided and challenges for their future research are identified.

Abstract

Organic–inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light‐emitting diodes (LEDs), continuous‐wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long‐term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current–voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single‐crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

A novel strategy for achieving efficient and stable CsPbI₃ solar cells via regulating lattice distortion with a surface organic terminal group (OTG) is proposed. As the steric hindrance of the OTG increases, the CsPbI₃ perovskite will be more stable under ambient conditions. Consequently, the optimized CsPbI₃ device shows negligible degradation of efficiency after 30 day aging in ambient air.

Abstract

All‐inorganic cesium lead iodide perovskites (CsPbI₃) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI₆]⁴⁻). Here, a novel strategy to suppress the octahedral tilting of [PbI₆]⁴⁻ units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.