Zidan Liu

Publication date: November 2019

Source: Solar Energy Materials and Solar Cells, Volume 202

Author(s): Gengyao Wei, Dongya Yang, Tao Zhang, Xuejie Yue, Fengxian Qiu

Graphical abstract

Publication date: November 2019

Source: Solar Energy Materials and Solar Cells, Volume 202

Author(s): Amin Abbasiyan, Mina Noori, Hamed Baghban

Publication date: November 2019

Source: Solar Energy Materials and Solar Cells, Volume 202

Author(s): Hyunjin Jo, Jo-Hwa Yang, Soo-Won Choi, Jaeho Park, Eun Jin Song, Myunhun Shin, Ji-Hoon Ahn, Jung-Dae Kwon

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods used in the attempt to reduce the hysteresis, potassium‐ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.

Vacuum‐deposited methylammonium lead iodide can adopt a perovskite structure with a stable cubic lattice at room temperature. Reducing the metallic salt evaporation rate leads to a tetragonal phase structure. This room‐temperature cubic perovskite circumvents the tetragonal to cubic phase transition resulting at ≈55 °C, and leads to photovoltaic devices with efficiencies above 19%.

Abstract

Methylammonium lead triiodide (MAPI) has emerged as a high‐performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50–60 °C. Most MAPI films are prepared using a solution‐based coating process, yet they can also be obtained by vapor‐phase deposition methods. Vapor‐phase‐processed MAPI films have significantly different characteristics than their solvent‐processed analogous, such as relatively small crystal‐grain sizes and short excited‐state lifetimes. However, solar cells based on vapor‐phase‐processed MAPI films exhibit high power‐conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor‐phase‐processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.

An efficient and neutral colored semitransparent organic photovoltaic cell (ST‐OPV) is realized by utilizing a near‐infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high‐conductivity Cu–Ag alloy electrode. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed.

Abstract

Building‐integrated photovoltaics employing transparent photovoltaic cells on window panes provide an opportunity to convert solar energy to electricity rather than generating waste heat. Semitransparent organic photovoltaic cells (ST‐OPVs) that utilize a nonfullerene acceptor‐based near‐infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high conductivity Cu‐Ag alloy electrode are demonstrated. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed. This combination of coatings results in doubling of the light utilization efficiency (LUE), which is equal to the product of the power conversion efficiency (PCE) and the average photopic transparency, compared with a conventional semitransparent cell lacking these coatings. A maximum LUE = 3.56 ± 0.11% is achieved for an ST‐OPV with a PCE = 8.0 ± 0.2% at 1 sun, reference AM1.5G spectrum. Moreover, neutral colored ST‐OPVs are also demonstrated, with LUE = 2.56 ± 0.2%, along with Commission Internationale d'Eclairage chromaticity coordinates of CIE = (0.337, 0.349) and a color rendering index of CRI = 87.

Highly efficient photoluminescence (PL‐QY = 68%), amplified spontaneous emission, and low‐threshold lasing in thin films of cesium lead bromide at room temperature are shown. Importantly, the films are not based on nanocrystals or quantum dots but consist of extended continuous layers, that are formed upon recrystallization of as‐deposited layers by thermal imprint.

Abstract

Cesium lead halide perovskites are of interest for light‐emitting diodes and lasers. So far, thin‐films of CsPbX₃ have typically afforded very low photoluminescence quantum yields (PL‐QY < 20%) and amplified spontaneous emission (ASE) only at cryogenic temperatures, as defect related nonradiative recombination dominated at room temperature (RT). There is a current belief that, for efficient light emission from lead halide perovskites at RT, the charge carriers/excitons need to be confined on the nanometer scale, like in CsPbX₃ nanoparticles (NPs). Here, thin films of cesium lead bromide, which show a high PL‐QY of 68% and low‐threshold ASE at RT, are presented. As‐deposited layers are recrystallized by thermal imprint, which results in continuous films (100% coverage of the substrate), composed of large crystals with micrometer lateral extension. Using these layers, the first cesium lead bromide thin‐film distributed feedback and vertical cavity surface emitting lasers with ultralow threshold at RT that do not rely on the use of NPs are demonstrated. It is foreseen that these results will have a broader impact beyond perovskite lasers and will advise a revision of the paradigm that efficient light emission from CsPbX₃ perovskites can only be achieved with NPs.

A multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) between SnO₂ and a perovskite layer improves the average power conversion efficiency from 18.60% to 20.22% due to the upward shift of band position, reduced nonradiative recombination, and improved carrier lifetime. In addition, interfacial engineering improves thermal and moisture stability.

Abstract

Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis‐less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO₂ and perovskite through an ester bond with SnO₂ via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The introduction of ImAcHCl onto SnO₂ realigns the positions of the conduction and valence bands upwards, reduces nonradiative recombination, and improves carrier life time. As a consequence, average power conversion efficiency (PCE) is increased from 18.60% ± 0.50% to 20.22% ± 0.34% before and after surface modification, respectively, which mainly results from an enhanced voltage from 1.084 ± 0.012 V to 1.143 ± 0.009 V. The best PCE of 21% is achieved by 0.1 mg mL⁻¹ ImAcHCl treatment, along with negligible hysteresis. Moreover, an unencapsulated device with ImAcHCl‐modified SnO₂ shows much better thermal and moisture stability than unmodified SnO₂.

Lasing and loss dynamics of mechanically exfoliated, homologous 2D Ruddlesden–Popper perovskite (RPP, (C₄H₉NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1}) crystals are reported. The Auger recombination and the electron–phonon coupling are the major loss channels, and they increase with the decreasing of inorganic layer thickness (n), leading to larger threshold in smaller‐n RPPs, and even the absence of lasing action for n < 3 above 78 K.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)₂(MA) _{n −1}Pb _n I_{3 n +1} RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large‐n RPPs (n ≥ 3) in the spectral range of 620–680 nm but not from small‐n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n‐engineered lasing behaviors are attributed to the stronger Auger recombination and exciton–phonon interaction as a result of the enhanced quantum confinement in the smaller‐n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low‐threshold, substrate‐free, and multicolor 2D semiconductor microlasers.

PEDOT:PSS hydrogels are an important framework for creating conductive porous materials that are of broad interest to researchers in the fields of bioelectronics, tissue engineering, stretchable electronics, and energy. To incorporate these materials into devices, a novel patterning method is presented that uses electrochemically produced ions to rapidly generate PEDOT:PSS hydrogel patterns with high spatial resolution.

Abstract

Due to their high water content and macroscopic connectivity, hydrogels made from the conducting polymer PEDOT:PSS are a promising platform from which to fabricate a wide range of porous conductive materials that are increasingly of interest in applications as varied as bioelectronics, regenerative medicine, and energy storage. Despite the promising properties of PEDOT:PSS‐based porous materials, the ability to pattern PEDOT:PSS hydrogels is still required to enable their integration with multifunctional and multichannel electronic devices. In this work, a novel electrochemical gelation (“electrogelation”) method is presented for rapidly patterning PEDOT:PSS hydrogels on any conductive template, including curved and 3D surfaces. High spatial resolution is achieved through use of a sacrificial metal layer to generate the hydrogel pattern, thereby enabling high‐performance conducting hydrogels and aerogels with desirable material properties to be introduced into increasingly complex device architectures.

Publication date: October 2019

Source: Solar Energy Materials and Solar Cells, Volume 201

Author(s): Sixuan Chen, Jingjing Dong, Jian Wu, Shaochuan Hou, Jie Xing, Hao Liu, Huiying Hao

Publication date: October 2019

Source: Solar Energy Materials and Solar Cells, Volume 201

Author(s): Tao Ye, Guifang Han, Abhijith Surendran, Jia Li, Teck Ming Koh, Subodh G. Mhaisalkar, Wei Lin Leong

Graphical abstract

Lasing and loss dynamics of mechanically exfoliated, homologous 2D Ruddlesden–Popper perovskite (RPP, (C₄H₉NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1}) crystals are reported. The Auger recombination and the electron–phonon coupling are the major loss channels, and they increase with the decreasing of inorganic layer thickness (n), leading to larger threshold in smaller‐n RPPs, and even the absence of lasing action for n < 3 above 78 K.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)₂(MA) _{n −1}Pb _n I_{3 n +1} RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large‐n RPPs (n ≥ 3) in the spectral range of 620–680 nm but not from small‐n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n‐engineered lasing behaviors are attributed to the stronger Auger recombination and exciton–phonon interaction as a result of the enhanced quantum confinement in the smaller‐n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low‐threshold, substrate‐free, and multicolor 2D semiconductor microlasers.

Nonconjugated multi‐zwitterionic small‐molecule electrolyte (NSE) molecules in perovskite solar cells (PSCs) act not only as both charge‐extracting layers for barrier‐free cathode charge collection but also as charged defect fillers in perovskite bulk and interfaces by spontaneous bottom‐up passivation. Thus, the NSE‐based PSCs deliver PCEs as high as 21.18% with an ultrahigh V _OC of 1.19 V, suppressed hysteresis, and enhanced stability.

Abstract

Recent perovskite solar cell (PSC) advances have pursued strategies for reducing interfacial energetic mismatches to mitigate energy losses, as well as to minimize interfacial and bulk defects and ion vacancies to maximize charge transfer. Here nonconjugated multi‐zwitterionic small‐molecule electrolytes (NSEs) are introduced, which act not only as charge‐extracting layers for barrier‐free charge collection at planar triple cation PSC cathodes but also passivate charged defects at the perovskite bulk/interface via a spontaneous bottom‐up passivation effect. Implementing these synergistic properties affords NSE‐based planar PSCs that deliver a remarkable power conversion efficiency of 21.18% with a maximum V _OC = 1.19 V, in combination with suppressed hysteresis and enhanced environmental, thermal, and light‐soaking stability. Thus, this work demonstrates that the bottom‐up, simultaneous interfacial and bulk trap passivation using NSE modifiers is a promising strategy to overcome outstanding issues impeding further PSC advances.

Publication date: October 2019

Source: Solar Energy Materials and Solar Cells, Volume 201

Author(s): Lethy Krishnan Jagadamma, Oskar Blaszczyk, Muhammad T. Sajjad, Arvydas Ruseckas, Ifor D.W. Samuel

Graphical abstract

Nature, Published online: 08 August 2019; doi:10.1038/d41586-019-02409-7

Highly efficient photoluminescence (PL‐QY = 68%), amplified spontaneous emission, and low‐threshold lasing in thin films of cesium lead bromide at room temperature are shown. Importantly, the films are not based on nanocrystals or quantum dots but consist of extended continuous layers that are formed upon recrystallization of as‐deposited layers by thermal imprint.

Abstract

Cesium lead halide perovskites are of interest for light‐emitting diodes and lasers. So far, thin‐films of CsPbX₃ have typically afforded very low photoluminescence quantum yields (PL‐QY < 20%) and amplified spontaneous emission (ASE) only at cryogenic temperatures, as defect related nonradiative recombination dominated at room temperature (RT). There is a current belief that, for efficient light emission from lead halide perovskites at RT, the charge carriers/excitons need to be confined on the nanometer scale, like in CsPbX₃ nanoparticles (NPs). Here, thin films of cesium lead bromide, which show a high PL‐QY of 68% and low‐threshold ASE at RT, are presented. As‐deposited layers are recrystallized by thermal imprint, which results in continuous films (100% coverage of the substrate), composed of large crystals with micrometer lateral extension. Using these layers, the first cesium lead bromide thin‐film distributed feedback and vertical cavity surface emitting lasers with ultralow threshold at RT that do not rely on the use of NPs are demonstrated. It is foreseen that these results will have a broader impact beyond perovskite lasers and will advise a revision of the paradigm that efficient light emission from CsPbX₃ perovskites can only be achieved with NPs.

A multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) between SnO₂ and a perovskite layer improves the average power conversion efficiency from 18.60% to 20.22% due to the upward shift of band position, reduced nonradiative recombination, and improved carrier lifetime. In addition, interfacial engineering improves thermal and moisture stability.

Abstract

Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis‐less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO₂ and perovskite through an ester bond with SnO₂ via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The introduction of ImAcHCl onto SnO₂ realigns the positions of the conduction and valence bands upwards, reduces nonradiative recombination, and improves carrier life time. As a consequence, average power conversion efficiency (PCE) is increased from 18.60% ± 0.50% to 20.22% ± 0.34% before and after surface modification, respectively, which mainly results from an enhanced voltage from 1.084 ± 0.012 V to 1.143 ± 0.009 V. The best PCE of 21% is achieved by 0.1 mg mL⁻¹ ImAcHCl treatment, along with negligible hysteresis. Moreover, an unencapsulated device with ImAcHCl‐modified SnO₂ shows much better thermal and moisture stability than unmodified SnO₂.

The latest research advances in the chemical vapor deposition (CVD) synthesis of 2D transition metal dichalcogenides and related heterostructures/superlattices are comprehensively summarized. The controlled growth behavior, preparation strategies, and breakthroughs regarding their synthesis are also discussed. Finally, recent progress on the application of CVD‐grown 2D materials is presented with emphasis on the future prospects of these materials.

Abstract

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD‐grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

Publication date: October 2019

Source: Solar Energy Materials and Solar Cells, Volume 201

Author(s): Lili Zhang, Sun-an Ding, Guogang Qin

A remarkable high efficiency of 17.34% is achieved for low‐dimensional Ruddlesden–Popper perovskite (RPP) solar cells (n ≤ 5) by using a fluorinated phenylethalammonium (4‐fluoro‐phenethylammonium (4FPEA)) organic cation. These fluorinated devices also show the better humidity and thermal stability as compared to nonfluorinated phenylethlammonium (PEA) devices. These findings provide a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

Abstract

Low‐dimensional Ruddlesden–Popper perovskites (RPPs) exhibit excellent stability in comparison with 3D perovskites; however, the relatively low power conversion efficiency (PCE) limits their future application. In this work, a new fluorine‐substituted phenylethlammonium (PEA) cation is developed as a spacer to fabricate quasi‐2D (4FPEA)₂(MA)₄Pb₅I₁₆ (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a J _sc of 19.00 mA cm⁻², a V _oc of 1.16 V, and a fill factor (FF) of 79%, which are among the best results for low‐dimensional RPP solar cells (n ≤ 5). The enhanced device performance can be attributed as follows: first, the strong dipole field induced by the 4‐fluoro‐phenethylammonium (4FPEA) organic spacer facilitates charge dissociation. Second, fluorinated RPP crystals preferentially grow along the vertical direction, and form a phase distribution with the increasing n number from bottom to the top surface, resulting in efficient charge transport. Third, 4FPEA‐based RPP films exhibit higher film crystallinity, enlarged grain size, and reduced trap‐state density. Lastly, the unsealed fluorinated RPP devices demonstrate superior humidity and thermal stability. Therefore, the fluorination of the long‐chain organic cations provides a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

A general strategy based on an “anti‐colloidal‐solution” is shown to embed a variety of laser‐generated nanocrystals in a solution processed perovskite film. The capability of such universal embedding is also demonstrated for improved film performance in photovoltaics, e.g., power conversion efficiency of up to 21.41%, and moisture stability over 5000 h in relative humidity of 40%.

Abstract

Regulating the chemical/physical features of solution processed metal halide perovskite films by integrating sub‐10 nm nanocrystals is a highly promising strategy to advance their outstanding optoelectronic performance. However, significant challenges remain for the universal embedding of the well‐defined nanocrystals in the film matrix. By generating nanocrystals in desired solvents via pulsed laser irradiation in liquid, the authors demonstrate the effective decoration of sub‐10 nm nanocrystals in perovskite films for enhanced optoelectronic performance. It is believed that this improved performance is due to the modification of the widely adopted “antisolvent” to a novel “anti‐colloidal‐solution” (ACS). Exemplified by a typical ACS; carbon dots in chlorobenzene, its encouraging superiority in regulating, not only the films morphology, but also the electronic structure, is demonstrated. This results in perovskite solar cells with a champion efficiency of 21.41% as well as a pronounced stability over 5000 h in relative humidity of 40%. The capability of nanocrystal embedding for boosted photovoltaic performance is further exploited by employing other laser generated ACSs. Such a strategy may open up a route to regulating hybrid perovskite film performance via nanocrystal embedding for photovoltaics or even beyond optoelectronic applications.

A facile low‐energy‐cost one‐pot scalable preparation strategy is developed to achieve homogeneously dispersed organic–inorganic perovskites nanoparticles in a freestanding gel with superior stability and high color purity even in water. The modular material design allows for a broad range of mechanical properties tunable from high elasticity stretchable gel in LEDs to rigid arbitrary 2D/3D structures printed by fast 3D‐printing technology.

Abstract

Metal‐halide perovskites have become appealing materials for optoelectronic devices. While the fast advancing stretchable/wearable devices require stability, flexibility and scalability, current perovskites suffer from ambient‐environmental instability and incompatible mechanical properties. Recently perovskite−polymer composites have shown improved in‐air stability with the protection of polymers. However, their stability remains unsatisfactory in water or high‐humidity environment. These methods also suffer from limited processability with low yield (2D film or beads) and high fabrication cost (high temperature, air/moisture‐free conditions), thereby limiting their device integration and broader applications. Herein, by combining facile photo‐polymerization with room‐temperature in‐situ perovskite reprecipitation at low energy cost, a one‐step scalable method is developed to produce freestanding highly‐stable luminescent organogels, within which CH₃NH₃PbBr₃ nanoparticles are homogeneously distributed. The perovskite‐organogels present a record‐high stability at different pH and temperatures, maintaining their high quantum yields for > 110 days immersing in water. This paradigm is universally applicable to broad choices of polymers, hence casting these emerging luminescent materials to a wide range of mechanical properties tunable from rigid to elastic. With intrinsically ultra‐stretchable photoluminescent organogels, flexible phosphorous layers were demonstrated with > 950% elongation. Rigid perovskite gels, on the other hand, permitted the deployment of 3D‐printing technology to fabricate arbitrary 2D/3D luminescent architectures.

The nanoscale probing of concurrent ionic migration, spontaneous polarization, and photocurrent of triple‐cation mixed‐halide perovskites by dynamic‐strain‐based scanning probe microscopy reveals photoenhanced polarization that does not contribute to the photovoltaic hysteresis, supporting the concept that the primary mechanism responsible for photovoltaic hysteresis is ionic migration while polarization is harmless.

Abstract

Triple‐cation mixed‐halide perovskites of composition Cs _x (FA _y MA_{1− y} )_{1 −x} Pb(I _z Br_{1 −z} )₃ (CsFAMA) have been reported to possess excellent photovoltaic efficiency with minimal hysteresis; in this work, nanoscale insight is shed into the roles of illumination‐induced polarization and ionic migration in photovoltaic hysteresis. By examining the concurrent evolution of ionic distribution and spontaneous polarization of CsFAMA under light illumination using dynamic‐strain‐based scanning probe microscopy, strong linear piezoelectricity arising from photoenhanced polarization is observed, while ionic migration is found to be not significantly increased by lightening. Nanoscale photocurrents are mapped under a series of biases using conductive atomic force microscopy, revealing negligible difference between forward and backward scans, and local IV curves reconstructed from principal component analysis show minimal hysteresis of just 1%. These observations at the nanoscale are confirmed in a macroscopic perovskite solar cell made of CsFAMA, exhibiting a high efficiency of 20.11% and with hysteresis index as small as 3%. Ionic migration, polarization, and photocurrent hysteresis are thus directly correlated at the nanoscale, and photoenhanced polarization in triple‐cation mixed‐halide perovskites is established, which does not contribute to the photovoltaic hysteresis.

Perovskite solar cells based on polarized ferroelectric polymers are fabricated by doping the ferroelectric polymer into the perovskite layer with different polarizing electric fields and different doping concentrations, different polarized ferroelectric polymers' interlayers between the perovskite and the hole‐transporting layer, and both doping and interlayer. After these treatments, the fabricated devices show a maximum power conversion efficiency of 21.38%.

Abstract

In hybrid organic–inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI₃) layer and/or inserted between the perovskite and the hole‐transporting layers to enhance the build‐in field (BIF), improve the crystallization of MAPbI₃, and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer‐doped MAPbI₃ shows an orderly arrangement of MA⁺ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open‐circuit voltage of 1.14 V.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

Perovskite solar cells based on polarized ferroelectric polymers are fabricated by doping the ferroelectric polymer into the perovskite layer with different polarizing electric fields and different doping concentrations, different polarized ferroelectric polymers' interlayers between the perovskite and the hole‐transporting layer, and both doping and interlayer. After these treatments, the fabricated devices show a maximum power conversion efficiency of 21.38%.

Abstract

In hybrid organic–inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI₃) layer and/or inserted between the perovskite and the hole‐transporting layers to enhance the build‐in field (BIF), improve the crystallization of MAPbI₃, and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer‐doped MAPbI₃ shows an orderly arrangement of MA⁺ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open‐circuit voltage of 1.14 V.

The power conversion efficiency of perovskite colloidal quantum dot (CQD) solar cells is improved using a conjugated small molecule, ITIC. The carrier dynamics of this unique perovskite CQD/ITIC system are investigated, showing an effective carrier transfer from the perovskite CQDs to the ITIC, which provides an additional driving force for charge separation in perovskite CQDs photovoltaic devices and boosts the efficiency up to 12.7%.

Abstract

Halide perovskite colloidal quantum dots (CQDs) have recently emerged as a promising candidate for CQD photovoltaics due to their superior optoelectronic properties to conventional chalcogenides CQDs. However, the low charge separation efficiency due to quantum confinement still remains a critical obstacle toward higher‐performance perovskite CQD photovoltaics. Available strategies employed in the conventional CQD devices to enhance the carrier separation, such as the design of type‐Ⅱ core–shell structure and versatile surface modification to tune the electronic properties, are still not applicable to the perovskite CQD system owing to the difficulty in modulating surface ligands and structural integrity. Herein, a facile strategy that takes advantage of conjugated small molecules that provide an additional driving force for effective charge separation in perovskite CQD solar cells is developed. The resulting perovskite CQD solar cell shows a power conversion efficiency approaching 13% with an open‐circuit voltage of 1.10 V, short‐circuit current density of 15.4 mA cm⁻², and fill factor of 74.8%, demonstrating the strong potential of this strategy toward achieving high‐performance perovskite CQD solar cells.