Mahui

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.

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.

The photochemistry of halide‐related defects affects the optoelectronic properties of lead–halide perovskite semiconductors and their reactivity to external stimuli such as light and environmental molecules.

Abstract

The presence of various types of chemical interactions in metal‐halide perovskite semiconductors gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

The factors that limit the luminescence efficiency (LE) of metal halide perovskite (MHP) light‐emitting diodes (PeLEDs) are reviewed by categorizing them into i) photophysical properties of MHPs, ii) morphological factors, and iii) problems caused by device architectures. Various strategies to overcome those LE‐limiting factors in MHPs and PeLEDs, and research directions to further increase the LE of MHPs are discussed.

Abstract

Metal‐halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light‐emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence‐limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next‐generation displays and solid‐state lightings are suggested.

The approaches and the consequences of lead replacement in lead halide perovskite solar cells are summarized. The theoretical understanding of the electronic, optical, and defect properties of lead and lead‐free halide perovskites and perovskite derivatives is reviewed, explaining why all reported lead‐free perovskite solar cells underperform compared to lead halide perovskite solar cells.

Abstract

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb‐free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb‐free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb‐free perovskites and perovskite derivatives do not.

The factors that limit the luminescence efficiency (LE) of metal halide perovskite (MHP) light‐emitting diodes (PeLEDs) are reviewed by categorizing them into i) photophysical properties of MHPs, ii) morphological factors, and iii) problems caused by device architectures. Various strategies to overcome those LE‐limiting factors in MHPs and PeLEDs, and research directions to further increase the LE of MHPs are discussed.

Abstract

Metal‐halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light‐emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence‐limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next‐generation displays and solid‐state lightings are suggested.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Lead‐halide perovskites have demo‐nstrated rapid rises in optoelectronic device performance, which directly links to their efficient luminescence properties. The current understanding of the physics of light emission in perovskites is discussed, along with current outstanding challenges and opportunities to push device performances beyond existing technologies.

Abstract

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light‐emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state‐of‐the‐art metal‐halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Perovskites are versatile ABX₃ crystals, hosting many intriguing physical properties. While most are inorganic compounds with cationic A‐ and B‐, and anionic X‐sites, recently, the introduction of organic ions (hybrid perovskites) and structures with inverted ionic charges (inverse (hybrid) perovskites) have been explored. Thus, the combinatorial space for design with optimized properties has new dimensions.

Abstract

Materials science evolves to a state where the composition and structure of a crystal can be controlled almost at will. Given that a composition meets basic requirements of stoichiometry, steric demands, and charge neutrality, researchers are now able to investigate a wide range of compounds theoretically and, under various experimental conditions, select the constituting fragments of a crystal. One intriguing playground for such materials design is the perovskite structure. While a game of mixing and matching ions has been played successfully for about 150 years within the limits of inorganic compounds, the recent advances in organic–inorganic hybrid perovskite photovoltaics have triggered the inclusion of organic ions. Organic ions can be incorporated on all sites of the perovskite structure, leading to hybrid (double, triple, etc.) perovskites and inverse (hybrid) perovskites. Examples for each of these cases are known, even with all three sites occupied by organic molecules. While this change from monatomic ions to molecular species is accompanied with increased complexity, it shows that concepts from traditional inorganic perovskites are transferable to the novel hybrid materials. The increased compositional space holds promising new possibilities and applications for the universe of perovskite materials.

The narrow emission linewidths, easily tunable emission wavelengths, low cost, and high charge‐carrier mobility of metal‐halide perovskites make them well suited to act as natural light emitters. In article https://doi.org/10.1002/adma.2018045951804595, Tae‐Woo Lee and co‐workers discuss the progress made in metalhalide perovskite light‐emitting diodes since their appearance in 2014. The limiting factors of this technology—the photophysical properties of the crystals, morphological factors in layers made from them, and limitations in device architectures—and stategies to overcome these factors are explored.

Metal‐halide perovskites have attracted much attention in recent years due to their use as light‐harvesting and light‐emitting semiconductors. Their superior optical and electronic properties, low processing cost, and the ease with which their bandgaps can be tuned suggest that they will be useful in various optoelectronic applications. This special issue aims to address the fundamentals of perovskite materials and devices, to explore recent progress in this exciting field, and to overcome the hurdles that remain to commercialization of optoelectronic perovskite devices.

Publication date: Available online 11 November 2019

Source: Nano Energy

Author(s): Haomiao Yu, Miaosheng Wang, Changfeng Han, Kai Wang, Bin Hu

Graphical abstract

Publication date: Available online 12 November 2019

Source: Nano Energy

Author(s): Yuanyuan Zhao, Jialong Duan, Yudi Wang, Xiya Yang, Qunwei Tang

Graphical abstract

Precise stress control of CsPbBr₃ perovskite film maximizes grain size and minimizes grain boundary defects. The all-inorganic CsPbBr₃ solar cell achieves a champion PCE of 10.71% with an ultrahigh V_oc of 1.622 V.

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.

Herein, it is demonstrated that solution‐processed dopant‐free spiro molecules can serve as superior hole‐transport materials (HTMs) to fabricate efficient inverted (p‐i‐n) perovskite solar cells. An entirely solution process is achieved by rational choice of orthogonal solvent, which allows to deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro interlayers.

Spiro‐linked compounds have been used as benchmark hole‐transport materials (HTMs) for the construction of efficient normal architecture (n‐i‐p) perovskite solar cells (PSCs). However, the heavy reliance on the use of dopants not only complicates the device fabrication but imposes long‐term stability concern of the devices. Herein, it is reported that solution‐processed dopant‐free spiro molecules can serve as superior HTMs to fabricate efficient inverted (p‐i‐n) PSCs. Rational choice of orthogonal solvent allows us to solution deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro‐based interface layers. To illustrate the generality of the strategy, three spiro‐linked molecules are investigated side by side as HTMs in one‐step solution‐processed CH₃NH₃PbI₃ PSCs. Due to the favored energy‐level alignment and high hole mobility, solar cells based on the HTM of spiro‐TTB yield a high efficiency of 18.38% with open‐circuit voltages (V _OC) up to 1.09 V. These results suggest that small molecular HTMs commonly developed for normal structure devices can be of great potential to fabricate cost‐effective and highly efficient inverted PSCs.

Ternary organic solar cells based on nonfullerene 3TP3T‐4F and 3TP3T‐IC guest acceptors and PM:Y6 binary host are investigated. The incorporation of 15% 3TP3T‐4F leads to an impressive efficiency of 16.7% (certified as 16.2%) for PM6:Y6:3TP3T‐4F ternary organic solar cells, higher than that (15.6%) of PM6:Y6:3TP3T‐4F devices, which is mainly ascribed to the compatibility between the third component and the host materials.

Abstract

A ternary structure has been demonstrated as being an effective strategy to realize high power conversion efficiency (PCE) in organic solar cells (OSCs); however, general materials selection rules still remain incompletely understood. In this work, two nonfullerene small‐molecule acceptors 3TP3T‐4F and 3TP3T‐IC are synthesized and incorporated as a third component in PM6:Y6 binary blends. The photovoltaic behaviors in the resultant ternary OSCs differ significantly, despite the comparable energy levels. It is found that incorporation of 15% 3TP3T‐4F into the PM6:Y6 blend results in facilitating exciton dissociation, increasing charge transport, and reducing trap‐assisted recombination. All these features are responsible for the enlarged PCE of 16.7% (certified as 16.2%) in the PM6:Y6:3TP3T‐4F ternary OSCs, higher than that (15.6%) in the 3TP3T‐IC containing ternary devices. The performance differences are mainly ascribed to the compatibility between the third component and the host materials. The 3TP3T‐4F guest acceptor exhibits an excellent compatibility with Y6, tending to form well‐mixed phases in the ternary blend without disrupting the favored bicontinuous transport networks, whereas 3TP3T‐IC displays a morphological incompatibility with Y6. This work highlights the importance of considering the compatibility for materials selection toward high‐efficiency ternary organic OSCs.

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.

Lead–tin‐based perovskites represent a promising route for achieving low‐gap perovskite solar absorbers and light‐emitters. Films composed of heterostructures of 2D and 3D lead–tin perovskite domains are fabricated with distinctively unique optoelectronic properties. These tunable structures enhance the understanding of the growth and optoelectronic properties of 2D/3D perovskite heterojunctions, and will see use in energy funneling and passivating structures.

Abstract

Halide perovskites are emerging as valid alternatives to conventional photovoltaic active materials owing to their low cost and high device performances. This material family also shows exceptional tunability of properties by varying chemical components, crystal structure, and dimensionality, providing a unique set of building blocks for new structures. Here, highly stable self‐assembled lead–tin perovskite heterostructures formed between low‐bandgap 3D and higher‐bandgap 2D components are demonstrated. A combination of surface‐sensitive X‐ray diffraction, spatially resolved photoluminescence, and electron microscopy measurements is used to reveal that microstructural heterojunctions form between high‐bandgap 2D surface crystallites and lower‐bandgap 3D domains. Furthermore, in situ X‐ray diffraction measurements are used during film formation to show that an ammonium thiocyanate additive delays formation of the 3D component and thus provides a tunable lever to substantially increase the fraction of 2D surface crystallites. These novel heterostructures will find use in bottom cells for stable tandem photovoltaics with a surface 2D layer passivating the 3D material, or in energy‐transfer devices requiring controlled energy flow from localized surface crystallites to the bulk.

Lead–tin‐based perovskites represent a promising route for achieving low‐gap perovskite solar absorbers and light‐emitters. Films composed of heterostructures of 2D and 3D lead–tin perovskite domains are fabricated with distinctively unique optoelectronic properties. These tunable structures enhance the understanding of the growth and optoelectronic properties of 2D/3D perovskite heterojunctions, and will see use in energy funneling and passivating structures.

Abstract

Halide perovskites are emerging as valid alternatives to conventional photovoltaic active materials owing to their low cost and high device performances. This material family also shows exceptional tunability of properties by varying chemical components, crystal structure, and dimensionality, providing a unique set of building blocks for new structures. Here, highly stable self‐assembled lead–tin perovskite heterostructures formed between low‐bandgap 3D and higher‐bandgap 2D components are demonstrated. A combination of surface‐sensitive X‐ray diffraction, spatially resolved photoluminescence, and electron microscopy measurements is used to reveal that microstructural heterojunctions form between high‐bandgap 2D surface crystallites and lower‐bandgap 3D domains. Furthermore, in situ X‐ray diffraction measurements are used during film formation to show that an ammonium thiocyanate additive delays formation of the 3D component and thus provides a tunable lever to substantially increase the fraction of 2D surface crystallites. These novel heterostructures will find use in bottom cells for stable tandem photovoltaics with a surface 2D layer passivating the 3D material, or in energy‐transfer devices requiring controlled energy flow from localized surface crystallites to the bulk.

In article number https://doi.org/10.1002/adma.2019035591903559, Edward H. Sargent, Zhijun Ning, and co‐workers intentionally include a secondary amine, dimethylamine, in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. NiO _x ‐based inverted perovskite solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% with excellent operational stability under continuous light soaking.

A new small molecule IBC‐F as the third component to improve efficiency and stability of ternary polymer solar cells is developed. The ternary device with 20% IBC‐F exhibits a higher efficiency of 15.06% compared with the host binary PBDB‐T:IE4F‐S‐based device with an efficiency of 13.70%. Furthermore, the ternary devices show better thermal and photoinduced stability compared the binary devices.

Abstract

The use of a ternary active layer offers a promising approach to enhance the power conversion efficiency (PCE) of polymer solar cells (PSCs) via simply incorporating a third component. Here, a ternary PSC with improved efficiency and stability facilitated by a new small molecule IBC‐F is demonstrated. Even though the PBDB‐T:IBC‐F‐based device gives an extremely low PCE of only 0.21%, a remarkable PCE of 15.06% can be realized in the ternary device based on PBDB‐T:IE4F‐S:IBC‐F with 20% IBC‐F, which is ≈10% greater than that (PCE = 13.70%) of the control binary device based on PBDB‐T:IE4F‐S. The improvement in the device performance of the ternary PSC is mainly attributed to the enhancement of fill factor, which is due to the improved charge dissociation and extraction, suppressed bimolecular and trap‐assisted recombination, longer charge‐carrier lifetime, and enhanced intermolecular interactions for preferential face‐on orientation. Additionally, the ternary device with 20% IBC‐F shows better thermal and photoinduced stability over the control binary device. This work provides a new angle to develop the third components for building ternary PSCs with enhanced photovoltaic performance and stability for practical applications.

Four low‐cost copolymer donors of poly(thiophene‐quinoxaline) (PTQ) derivatives are developed to investigate the effect of their fluorination forms on charge‐separation and voltage loss (V _loss) of their polymer solar cells. The device based on the PTQ derivative with a bifluorine substituent on its quinoxaline A‐unit demonstrates a high power conversion efficiency of 16.21%, benefitting from the efficient charge separation and low V _loss.

Abstract

Four low‐cost copolymer donors of poly(thiophene‐quinoxaline) (PTQ) derivatives are demonstrated with different fluorine substitution forms to investigate the effect of fluorination forms on charge separation and voltage loss (V _loss) of the polymer solar cells (PSCs) with the PTQ derivatives as donor and a A–DA'D–A‐structured molecule Y6 as acceptor. The four PTQ derivatives are PTQ7 without fluorination, PTQ8 with bifluorine substituents on its thiophene D‐unit, PTQ9, and PTQ10 with monofluorine and bifluorine substituents on their quinoxaline A‐unit respectively. The PTQ8‐ based PSC demonstrates a low power conversion efficiency (PCE) of 0.90% due to the mismatch in the highest occupied molecular orbital (HOMO) energy levels alignment between the donor and acceptor. In contrast, the devices based on PTQ9 and PTQ10 show enhanced charge‐separation behavior and gradually reduced V _loss, due to the gradually reduced nonradiative recombination loss in comparison with the PTQ7‐based device. As a result, the PTQ10‐based PSC demonstrates an impressive PCE of 16.21% with high open‐circuit voltage and large short‐circuit current density simultaneously, and its V _loss is reduced to 0.549 V. The results indicate that rational fluorination of the polymer donors is a feasible method to achieve fast charge separation and low V _loss simultaneously in the PSCs.

A power conversion efficiency of 8.25% for poly(3‐hexylthiophene)‐based polymer solar cells is realized by pairing a novel star‐shaped small‐molecular acceptor 2,7,12‐tris((2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐7‐benzothiadiazole‐2‐)truxene with a smart solution‐processing technology in the green solvent 1,2,4‐trimethylbenzene.

Abstract

A novel molecular acceptor of TrBTIC (2,7,12‐tris((2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐7‐benzothiadiazole‐2‐)truxene) is designed by attaching the 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐benzothiadiazole (BTIC) electron‐deficient unit to an electron‐rich truxene core. TrBTIC has excellent solubility in common solvents and features good energy level matching with poly(3‐hexylthiophene) (P3HT). Interestingly, P3HT can be readily dissolved in warm 1,2,4‐trimethylbenzene (TMB), a green solvent, but crystallizes slowly with long‐term aging in TMB at room temperature. A prephase separation can thus occur before active blend film deposition, and the separation degree can be easily controlled by varying the aging time. After 40 min of aging, the resulting active blend has the most appropriate phase separation with uniform nanowires, which forms favorable interpenetrating networks for exciton dissociation and charge transport. As a result, the device performance is improved from 6.62% to 8.25%. Excitingly, 8.25% is a new record for P3HT‐based solar cells. The study not only provides an efficient nonfullerene acceptor for matching P3HT donors but also develops a promising processing technology to realize high‐performance P3HT‐based polymer solar cells with an efficiency over 8%.