awn

Herein, the dipolar polarization in a quasi‐2D organic–inorganic hybrid‐perovskite nanorod network–based solar cell using impedance spectroscopy is studied. Electric field and photoinduced dipole–dipole interaction plays an important role for the solar cell working at steady states.

Layered quasi‐2D organic–inorganic hybrid perovskites (OIHPs) prevent oxygen and moisture permeation, for long‐lifetime photovoltaic performance. Unfortunately, the electrical and photoinduced surface and dipolar polarizations caused due to the presence of the organic cation spacer in the structure remain unclear. Herein, a high‐performance planar quasi‐2D OIHP solar cell comprising (PEA)₂(MA)₃Pb₄I₁₃ (n = 4) is designed. It displays a large area coverage and an interconnected nanorod network, which contributes to efficient light absorption and charge carrier transport. The surface and dipolar polarizations exhibit remarkable light intensity and electric field–dependent characteristics at short‐circuit‐current (J _sc) and steady‐state (i.e., V _oc) conditions. More importantly, Voc exhibits a nonlinear behavior at steady states. Such a unique feature is in accordance with the dipolar polarization measured at the same condition. The phenomenon can be explained by the significant dipole–dipole interaction at lower electric field strengths. At higher field strengths, the screen of the dipoles due to charge accumulation at the surface of the organic cation spacer leads to slower increment of Voc. Thus, carefully designing the quasi‐2D perovskite nanostructure, together with the dielectric property of the organic cation spacer, may play an exceptionally important role for future high‐performance quasi‐2D perovskite solar cells.

Quantum dots are regarded as neutralized charged intermedia to transfer ligands for interfacial modification, which can significantly adjust surface electric properties to reduce V _OC loss and improve device performance. A stable V _OC enhancement with excellent reproducibility is fulfilled by simple solution‐processed QDs modification, achieving 20.6% power conversion efficiency (PCE) and enhanced stability.

The tremendous passion for inverted planar heterojunction perovskite solar cells (PSCs) is originated from their great tendency in the roll‐to‐roll process‐compatible fabrication and huge potential for integration into tandem solar cells. But the device efficiency is still lower than regular structured PSCs. Engineering of the cathode interface to efficiently control and reduce V _OC loss lights a lamp for increasing electrochemical properties and boosting overall performance. Herein, a simple interfacial modification strategy is developed by introducing a hybrid ligand interfacial layer to reduce V _OC loss in PSCs with inverted planar structure. Heavily washed QDs are used as neutral charged intermedia to enable alloying reaction to transfer ligands without damage to perovskite (PVK). A band bending is immediately generated on the top surface of PVK film after QDs modification, which is directly confirmed by ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe force microscopy (KPFM). This contributes to ≈50 mV reduced V _OC loss, leading to a V _OC of 1.15 V and a power conversion efficiency (PCE) of 20.6% in inverted PSCs. Meanwhile, enhanced stability is achieved for these devices after QDs modification, in which PCE is maintained at >90% of initial value after 1000 h storage.

Operationally stable mixed‐cation‐halide perovskite solar cells are fabricated by halogen‐engineering concept via a Br‐rich seeding growth method. Bromine anions are effectively incorporated into the final perovskite film with larger grains and better vertical columnar alignment. Photovoltaic devices based on the film show a power conversion efficiency (PCE) of 21.5% and significantly enhanced operational stability for over 500 h.

Abstract

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_{1− x} (MAPbBr₃) _x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.

Various definitions of band gaps are used in the perovskite solar cell community as a reference to analyze losses in open‐circuit voltage. This essay proposes a band‐gap independent method to reference voltages that is easy to implement and a meta‐analysis of literature data to illustrate the state of the art and development of voltage losses in perovskite solar cells.

Abstract

Open‐circuit voltages of lead‐halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open‐circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open‐circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta‐analysis of methods to determine the bandgap and a radiative limit for open‐circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.

A carrier‐resolved photo‐Hall characterization technique is employed to simultaneously access majority/minority carrier properties as a function of light intensity for CH₃NH₃PbI₃ perovskite films processed without and with photovoltaic performance‐enhancing additives. Measurements on films with variable grain size reveal the passivation of bulk defects and n‐doping effect with PbI₂ excess and relative insensitivity to grain boundary density and thiocyanate additive concentration.

Abstract

With power conversion efficiencies now exceeding 25%, hybrid perovskite solar cells require deeper understanding of defects and processing to further approach the Shockley‐Queisser limit. One approach for processing enhancement and defect reduction involves additive engineering—, e.g., addition of MASCN (MA = methylammonium) and excess PbI₂ have been shown to modify film grain structure and improve performance. However, the underlying impact of these additives on transport and recombination properties remains to be fully elucidated. In this study, a newly developed carrier‐resolved photo‐Hall (CRPH) characterization technique is used that gives access to both majority and minority carrier properties within the same sample and over a wide range of illumination conditions. CRPH measurements on n‐type MAPbI₃ films reveal an order of magnitude increase in carrier recombination lifetime and electron density for 5% excess PbI₂ added to the precursor solution, with little change noted in electron and hole mobility values. Grain size variation (120–2100 nm) and MASCN addition induce no significant change in carrier‐related parameters considered, highlighting the benign nature of the grain boundaries and that excess PbI₂ must predominantly passivate bulk defects rather than defects situated at grain boundaries. This study offers a unique picture of additive impact on MAPbI₃ optoelectronic properties as elucidated by the new CRPH approach.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

Ionic conduction in the perovskite oxide La_0.9Sr_0.1Ga_0.95Mg_0.05O_{3– δ} (LSGM) is found to be strongly correlated with crystal structure. A structural design with simultaneously large unit‐cell volume and octahedral rotations for fast ionic conduction is proposed and realized in LSGM superlattice thin films, where the ionic conductivity is tuned with structure alone by a factor of ≈2.5 at 600 °C.

Abstract

Solid‐oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure–property relationships that would enable the rational design of better materials. Here, using epitaxial thin‐film growth, synchrotron radiation, impedance spectroscopy, and density‐functional theory, the impact of structural parameters (i.e., unit‐cell volume and octahedral rotations) on ionic conductivity is delineated in La_0.9Sr_0.1Ga_0.95Mg_0.05O_{3– δ} . As compared to the zero‐strain state, compressive strain reduces the unit‐cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit‐cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit‐cell volumes and octahedral rotations decrease migration barriers and create low‐energy migration pathways, respectively. The desired combination of large unit‐cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit‐cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion‐conducting perovskite electrolytes.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

A‐site management by introducing an A‐site placeholder cation, NH₄ ⁺, during the perovskite crystallization process is proposed to balance the supersaturation discrepancy between AX and BX₂ so as to improve its crystal quality without any residue. Most importantly, the sharply decreased A‐site‐related defect I_MA indicates that it is responsible for such crystalline optimization.

Abstract

An in‐depth understanding and effective suppression of nonradiative recombination pathways in perovskites are crucial to their crystallization process, in which supersaturation discrepancies at different time scales between CH₃NH₃I (MAI, methylammonium iodide) and PbI₂ remain a key issue. Here, an A‐site management strategy via the introduction of an A‐site placeholder cation, NH₄ ⁺, to offset the deficient MA⁺ precipitation by occupying the cavity of Pb–I framework, is proposed. The temporarily remaining NH₄ ⁺ is substituted by subsequently precipitated MA⁺. The temperature‐dependent crystallization process with the generation and consumption of a transient phase is sufficiently demonstrated by the dynamic changes in crystal structure characteristic peaks through in situ grazing‐incidence X‐ray diffraction and the surface potential difference evolution through temperature‐dependent Kelvin probe force microscopy. A highly crystalline perovskite is consequently acquired, indicated by the enlarged grain size, lowered nonradiative defect density, prolonged carrier lifetime, and fluorescence lifetime imaging. Most importantly, it is identified that the A‐site I_MA defect is responsible for such crystal quality optimization based on theoretical calculations, transient absorption, and deep‐level transient spectroscopy. Furthermore, the universality of the proposed A‐site management strategy is demonstrated with other mixed‐cation perovskite systems, indicating that this methodology successfully provides guidance for synthesis route design of highly crystalline perovskites.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

The rapid development in large‐area organic solar cells (OSCs) is reviewed. Materials requirements, modular designs, and printing methods for large‐area OSCs are discussed. By combining thick‐film material systems with efficient modular designs, and then by employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Abstract

The printing of large‐area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small‐area devices is approaching 15%, whereas the PCE for large‐area devices has also surpassed 10% in a single cell with an area of ≈1 cm². Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large‐area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large‐area fabrication, including knife coating, slot‐die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick‐film material systems with efficient modular designs exhibiting low‐efficiency losses and employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

The latest progress in exciton–photon coupling of perovskite materials is reviewed. Polaritons in planar and nanowire Fabry–Pérot microcavities are discussed predominantly in terms of materials and photophysics. Large Rabi‐splitting energy (≈656 meV) is achieved in CsPbBr₃. These large values enable polariton condensation and polariton lasers to be realized at high temperature or in low‐Q cavities.

Abstract

The semiconductor exciton–polariton, arising from the strong coupling between excitons and confined cavity photon modes, is not only of fundamental importance in macroscopic quantum effects but also has wide application prospects in ultralow‐threshold polariton lasers, slowing‐light devices, and quantum light sources. Very recently, metallic halide perovskites have been considered as a great candidate for exciton–polariton devices owing to their low‐cost fabrication, large exciton oscillator strength, and binding energy. Herein, the latest progress in exciton–polaritons and polariton lasers of perovskites are reviewed. Polaritons in planar and nanowires Fabry–Pérot microcavities are discussed with particular reference to material and photophysics. Finally, a perspective on the remaining challenges in perovskite polaritons research is given.

Aromatic‐diimide‐based polymers have emerged as the most promising n‐type semiconductors and their photovoltaic performance has been significantly improved in the past decade. The recent exciting progress is highlighted and the structure–property relationship of aromatic‐diimde‐based photovoltaic polymers is revealed, which could provide important guidelines for the further design of n‐type photovoltaic polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n‐type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all‐PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure–property relationships of ADI‐based CPs are revealed.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

As described by Zhixiang Wei and co‐workers in article number https://doi.org/10.1002/adma.2018050891805089, for large‐area organic solar cells, high active‐layer thickness tolerability is generally required, the methods to reduce power conversion efficiency losses are critical, and printing methods suitable for roll‐to‐roll printing are highly important. By combining material requirements, modular designs, and printing methods, the application of organic solar cells will be successfully realized in the near future.

A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared through defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, but with intrinsic defect concentration largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere. By completely eliminating the labile and expensive components in traditional perovskite solar cells (PSCs), these all‐inorganic PSCs exhibit high photovoltaic performances.

Abstract

The emergence of cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared by defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all‐inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibit PCE and open‐circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI₃ can survive only a few days in air. The progress reported represents a major leap for all‐inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

The latest progress in exciton–photon coupling of perovskite materials is reviewed. Polaritons in planar and nanowire Fabry–Pérot microcavities are discussed predominantly in terms of materials and photophysics. Large Rabi‐splitting energy (≈656 meV) is achieved in CsPbBr₃. These large values enable polariton condensation and polariton lasers to be realized at high temperature or in low‐Q cavities.

Abstract

The semiconductor exciton–polariton, arising from the strong coupling between excitons and confined cavity photon modes, is not only of fundamental importance in macroscopic quantum effects but also has wide application prospects in ultralow‐threshold polariton lasers, slowing‐light devices, and quantum light sources. Very recently, metallic halide perovskites have been considered as a great candidate for exciton–polariton devices owing to their low‐cost fabrication, large exciton oscillator strength, and binding energy. Herein, the latest progress in exciton–polaritons and polariton lasers of perovskites are reviewed. Polaritons in planar and nanowires Fabry–Pérot microcavities are discussed with particular reference to material and photophysics. Finally, a perspective on the remaining challenges in perovskite polaritons research is given.

Aromatic‐diimide‐based polymers have emerged as the most promising n‐type semiconductors and their photovoltaic performance has been significantly improved in the past decade. The recent exciting progress is highlighted and the structure–property relationship of aromatic‐diimde‐based photovoltaic polymers is revealed, which could provide important guidelines for the further design of n‐type photovoltaic polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n‐type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all‐PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure–property relationships of ADI‐based CPs are revealed.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

As described by Zhixiang Wei and co‐workers in article number https://doi.org/10.1002/adma.2018050891805089, for large‐area organic solar cells, high active‐layer thickness tolerability is generally required, the methods to reduce power conversion efficiency losses are critical, and printing methods suitable for roll‐to‐roll printing are highly important. By combining material requirements, modular designs, and printing methods, the application of organic solar cells will be successfully realized in the near future.