Chen Weijie

This work introduces an ultra‐thin MgO layer between SnO₂ electron transport layer and CsPbIBr₂ perovskite layer to reduce the interface and nonradiative recombination. Meanwhile, the MgO provides better substrate for pure α‐phase perovskite growth. As a result, it achieves power conversion efficiency of 11.04% and maintains ≈90% after 1250 hours, with an open‐circuit voltage up to 1.36 V.

Although the power conversion efficiency (PCE) of thermally stable inorganic CsPbIBr₂ perovskite solar cells (PSCs) is over 10%, the severe interfacial and nonradiative recombination deteriorates the open‐circuit voltage (V _oc). Herein, an ultrathin wideband MgO is mediated between the SnO₂ electron transport layer (ETL) and the CsPbIBr₂ photoabsorber to passivate the undesirable recombination, thereby enhancing the V _oc. Meanwhile, the δ‐phase perovskite located at the interface between SnO₂ ETL and CsPbIBr₂ film is reduced after MgO modification, because the MgO layer provides a substrate for perovskite growth and reduces vacancy. Moreover, the tunneling effect and better band alignment effectively block holes and accelerate electrons to the electrode. Consequently, for optimal MgO‐modified devices, a shining improvement of V _oc from 1.25 to 1.36 V without short‐circuit current losses boosts the champion CsPbIBr₂ PSCs to obtain a PCE of 11.04%, which is the highest value among the pure‐CsPbIBr₂ PSCs. However, the MgO layer significantly reduces severe hysteresis and increases device stability.

Simplified perovskite solar cells without employing electron/hole‐transport layers are attractive owing to their reduced fabrication process and low cost. The developments of simplified perovskite solar cells are discussed systematically. Device structure design, perovskite‐film optimization, interface modification, the mechanism, and other research hotspots are considered to present the bright future of simplified perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) are considered one of the most promising next‐generation examples of high‐tech photovoltaic energy converters, as they possess an unprecedented power conversion efficiency with low cost. A typical high‐performance PSC generally contains a perovskite active layer sandwiched between an electron‐transport layer (ETL) and a hole‐transport layer (HTL). The ETL and HTL contribute to the charge extraction in the PSC. However, these additional two layers complicate the manufacturing process and raise the cost. To extend this technology for commercialization, it is highly desired that the structure of PSCs is further simplified without sacrificing their photovoltaic performances. Thus, ETL‐free or/and HTL‐free PSCs are developed and attract more and more interest. Herein, the commonly used methods in reducing the defect density and optimizing the energy levels in conventional PSCs in order to simplify their structures are summarized. Then, the development of diverse ETL‐free or/and HTL‐free PSCs is discussed, with the PSCs classified, including their working principles, implemented technologies, remaining challenges, and future perspectives. The aim is to redirect the way toward low‐cost and high‐performance PSCs with the simplest possible architecture.

Purely vibrationally excited lead–iodide perovskites are prepared using off‐resonance, infrared optical excitation far below the bandgap. The transient optical response manifested as bandgap oscillations below and above the static bandgap is attributed to the A _g optical phonon mode at 25 cm⁻¹. This mode, arising from antiphase octahedral rotations, is observed in both 3D perovskite CH₃NH₃PbI₃ and layered 2D perovskite [CH₃(CH₂)₃NH₃]₂PbI₄.

Abstract

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the A _g vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.

A fibril network strategy is demonstrated to fabricate semitransparent organic solar cells (ST‐OSCs). An effective hole transport pathway can be maintained even when a small amount of PBT1‐C‐2Cl donor is incorporated in the blends due to the well distributed fibril nanostructure formed by PBT1‐C‐2Cl. And a high efficiency of 9.1% with an average visible transmittance of over 40% is achieved for ST‐OSCs.

Abstract

The development of semitransparent organic solar cells (ST‐OSCs) represents a significant step toward the commercialization of OSCs. However, the trade‐off between power conversion efficiency (PCE) and average visible transmittance (AVT) restricts further improvements of ST‐OSCs. Herein, it is demonstrated that a fibril network strategy can enable ST‐OSCs with a high PCE and AVT simultaneously. A wide‐bandgap polymer PBT1‐C‐2Cl that can self‐assemble into a fibril nanostructure is used as the donor and a near‐infrared small molecule Y6 is adopted as the acceptor. It is found that a tiny amount of PBT1‐C‐2Cl in the blend can form a high speed pathway for hole transport due to the well distributed fibril nanostructure, which increases the transmittance in the visible region. Meanwhile, the acceptor Y6 guarantees sufficient light absorption. Using this strategy, the optimized ST‐OSCs yield a high PCE of 9.1% with an AVT of over 40% and significant light utilization efficiency of 3.65% at donor/acceptor ratio of 0.25:1. This work demonstrates a simple and effective approach to realizing high PCE and AVT of ST‐OSCs simultaneously.

Sm³⁺‐ and Eu³⁺‐doped calcium scandate (CaSc₂O₄) is an emerging deep‐red‐emitting material with promising light absorption, enhanced emission properties, and excellent thermal stability that make it a promising candidate with potential applications in emission display, solid‐state white lighting, and the device performance of perovskite solar cells.

Abstract

The luminous efficiency of inorganic white light‐emitting diodes, to be used by the next generation as light initiators, is continuously progressing and is an emerging interest for researchers. However, low color‐rendering index (Ra), high correlated color temperature (CCT), and poor stability limit its wider application. Herein, it is reported that Sm³⁺‐ and Eu³⁺‐doped calcium scandate (CaSc₂O₄ (CSO)) are an emerging deep‐red‐emitting material with promising light absorption, enhanced emission properties, and excellent thermal stability that make it a promising candidate with potential applications in emission display, solid‐state white lighting, and the device performance of perovskite solar cells (PSCs). The average crystal structures of Sm³⁺‐doped CSO are studied by synchrotron X‐ray data that correspond to an extremely rigid host structure. Samarium ion is incorporated as a sensitizer that enhances the emission intensity up to 30%, with a high color purity of 88.9% with a 6% increment. The impacts of hosting the sensitizer are studied by quantifying the lifetime curves. The CaSc₂O₄:0.15Eu³⁺,0.03Sm³⁺ phosphor offers significant resistance to thermal quenching. The incorporation of lanthanide ion‐doped phosphors CSOE into PSCs is investigated along with their potential applications. The CSOE‐coated PSCs devices exhibit a high current density and a high power conversion efficiency (15.96%) when compared to the uncoated control devices.

One multifunctional molecule, 5‐amino‐2,4,6‐triiodoisophthalic acid (ATPA), is used as an interfacial layer between CsPbI₃ and TiO₂. The ATPA not only results in cascade energy‐level alignment, but also interacts strongly with the CsPbI₃ layer and effectively passivates the defects. Optimized devices based on the ATPA‐modified CsPbI₃ deliver a high efficiency of 18.12% with excellent stability.

A highly effective interface engineering approach uses a multifunctional molecule, 5‐amino‐2,4,6‐triiodoisophthalic acid (ATPA), to anchor on TiO₂ and CsPbI₃ simultaneously by reacting with dangling hydroxyl groups on TiO₂ surfaces and passivating the defects of CsPbI₃ films. In addition, the introduction of ATPA results in cascade energy‐level alignment between the perovskite and TiO₂ electron‐transporting layer (ETL) to improve the electron extraction property. Based on the ATPA‐modified TiO₂ substrates, optimized CsPbI₃ perovskite solar cells (PVSCs) deliver the highest power conversion efficiency (PCE) of over 18% with suppressed hysteresis. Moreover, the unencapsulated TiO₂/ATPA‐based devices exhibit much better long‐term stability and photostability than the only TiO₂‐based devices.

Morphology of an efficient ternary polymer blend is manipulated and the impacts on photovoltaic properties are explored. The morphology of ternary all‐polymer blending is determined by the interplay of the heterogeneous components from solution to solid state. Morphology with nanosized crystallite fibers in a mixing matrix assists in enhancing the solar cell performances.

Morphology control in multiblend all‐polymer solar cells is crucial for improving charge generation processes. Herein, it is demonstrated that the film morphology of the light‐harvesting layer of all‐polymer solar cells can be manipulated by incorporating a copolymer as the compatibilizer. Through in situ grazing‐incidence wide‐angle X‐ray scattering characterization, the insights of the crystallization kinetics of the polymer blends from solution to thin‐film state are provided. Of particular importance is that by kinetic and thermodynamic control of the film‐processing conditions, an optimal morphology with appropriate nanoscale fibrillar structure in a well‐mixed matrix is achieved. These findings indicate that the interplay between the crystalline regions and weakly/noncrystalline regions that are induced by the compatibilizer polymer plays a critical role in determining the morphology in multicomponent blend all‐polymer solar cells.

An environmentally stable acetamidinium (Aa⁺)‐incorporated MAPbI₃ film is successfully fabricated via hot casting in air. The large Aa⁺ immobilizes ions and improves crystal structure of MAPbI₃ through strong coordination bonds. The corresponding Aa–MAPbI₃ device shows 20.68% power conversion efficiency. Its 80% is maintained after 1300 h testing at 85 °C and 85 relative humidity (RH)%.

Ion migration in organometal halide perovskite solar cell (OHPSC) and crystal structure evolution of organometal halide perovskites (OHPVSKs) in air are considered as one of the critical factors for unstable performance and of the urgent issues for the reliability of OHPSCs. Herein, a novel cation of acetamidinium (Aa⁺) with stronger coordinated bond with I⁻ than methylammonium is induced into OHPVSK to stabilize its crystal structure. By incorporating Aa⁺ ions into OHPVSKs, the power conversion efficiency (PCE) of OHPSC without an encapsulation can maintain higher than 75% of its initial PCE after a 200 h humidity (60–80% relative humidity (RH) in air) or a 24 h thermal stress test (85 °C in dry N₂). The Aa–MAPbI₃ device exhibits an outstanding efficiency of 20.68%, and over 80% of initial PCE is maintained after a 1300 h damp heat as encapsulated. This novel cation can be easily incorporated into OHPVSK via a hot casting process in air with a high environmental tolerance as compared with that from the conventional coating process, which suffers from sophisticated crystallization steps and a strict processing atmosphere. It extends processing windows for OHPVSK fabrication and provides a promising path toward mass production and further commercialization.

Two isomeric crossconjugated polymer hole‐transporting materials (HTMs) are developed to demonstrate significantly distinct device power conversion efficiencies (PCEs) under the same device fabrication conditions, 11.1% PPE1 and 19.3% for PPE2 , which is found to be due to the improved quality of perovskite films made on top of PPE2 . More excitingly, the PPE2 ‐based perovskite solar cells (PVSCs) can further achieve a more impressive PCE of 21.3% through suitable surface passivation.

Abstract

Currently, there are only very few dopant‐free polymer hole‐transporting materials (HTMs) that can enable perovskite solar cells (PVSCs) to demonstrate a high power conversion efficiency (PCE) of greater than 20%. To address this need, a simple and efficient way is developed to synthesize novel crossconjugated polymers as high performance dopant‐free HTMs to endow PVSCs with a high PCE of 21.3%, which is among the highest values reported for single‐junction inverted PVSCs. More importantly, rational understanding of the reasons why two isomeric polymer HTMs (PPE1 and PPE2 ) with almost identical photophysical properties, hole‐transporting ability, and surface wettability deliver so distinctly different device performance under similar device fabrication conditions is manifested. PPE2 is found to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization. These results help unveil the new HTM design rules to influence the perovskite growth/crystallization for improving the performance of inverted PVSCs.

Two critical issues of interfacial charge separation in near‐infrared nonfullerene acceptor organic solar cells based on the paradigm PTB7–Th:IEICO–4F blend are unraveled as the long‐lived charge‐transfer states and the lack of a long‐range charge transportation pathway. These issues are resolved by additional incorporation of either ITIC or PC₇₁BM, which remarkably enhances charge separation and hence afford improved efficiencies beyond 10% any post‐treatment.

Abstract

Organic solar cells (OSCs) consisting of an ultralow‐bandgap nonfullerene acceptor (NFA) with an optical absorption edge that extends to the near‐infrared (NIR) region are of vital interest to semitransparent and tandem devices. However, huge energy‐loss related to inefficient charge dissociation hinders their further development. The critical issues of charge separation as exemplified in NIR‐NFA OSCs based on the paradigm blend of PTB7–Th donor (D) and IEICO–4F acceptor (A) are revealed here. These studies corroborate efficient charge transfer between D and A, accompanied by geminate recombination of photo‐excited charge carriers. Two key factors restricting charge separation are unveiled as the connection discontinuity of individual phases in the blend and long‐lived interfacial charge‐transfer states (CTS). By incorporation of a third‐component of benchmark ITIC or PC₇₁BM with various molar ratios, these two issues are well‐resolved accordingly, yet in distinctly influencing mechanisms. ITIC molecules modulate film morphology to create more continuous paths for charge transportation, whereas PC₇₁BM diminishes CTS and enhances electron transfer at the D/A interfaces. Consequently, the optimal untreated ternary OSCs comprising 0.3 wt% ITIC and 0.1 wt% PC₇₁BM in the blend deliver higher J _SC values of 21.9 and 25.4 mA cm^‐2, and hence increased PCE of 10.2% and 10.6%, respectively.

An effective strategy is demostrated to create a double barrier that not only blocks the invasion of the moisture but also takes advantage of the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days in a relative humidity of 70% and a champion efficiency up to 21.34%.

Abstract

The rapid growth in the device efficiency of perovskite solar cells (PSCs) has raised great demands for tackling their long‐term stability upon external environmental stimuli that restricts the commercialization of PSCs, in which the instability upon exposure to moisture has been one of the major obstacles. Herein, an effective way of building up double barriers for moisture degradation of the perovskite films is demonstrated by modifying them with rationally selected hydrolyzable hydrophobic molecules (1H,1H,2H,2H‐perfluorooctyl trichlorosilane, PFTS). The layer of oligomer derived from the hydrolyzed PFTS at the surface that increases the hydrophobicity of perovskite film could serve as an efficient wall preventing the moisture invasion. The long‐term exposure of the film upon moisture allows for the formation of a secondary wall that employs the hydrolyzation of PFTS at grain boundaries, favoring defects passivation to further improve the humidity stability. Such gradual hydrolyzation is encouragingly helpful for the enhancement of the open‐circuit voltage of the PSCs from the original 1.136 up to 1.205 V. The PSCs constructed with the double barriers demonstrate excellent stability upon moisture and improved thermal and light stabilities, as well as a champion power conversion efficiency up to 21.34%.

Superior thermo‐tolerant and highly emissive perovskite solids are attained via a straightforward heterogeneous interfacial method. The resultant materials maintain high quantum efficiency even after heating over 150 °C for 22 h. The constructed w‐LED exhibits recorded long temperature sustainable lifetime over 1100 h and its working current can go up to 300 mA.

Abstract

By virtue of their narrow emission bands, near‐unity quantum yield, and low fabrication cost, metal halide perovskites hold great promise in numerous aspects of optoelectronic applications, including solid‐state lighting, lasing, and displays. Despite such promise, the poor temperature tolerance and suboptimal quantum yield of the existing metal halide perovskites in their solid state have severely limited their practical applications. Here, a straightforward heterogeneous interfacial method to develop superior thermotolerant and highly emissive solid‐state metal halide perovskites is reported and their use as long‐lasting high‐temperature and high‐input‐power durable solid‐state light‐emitting diodes is illustrated. It is found that the resultant materials can well maintain their superior quantum efficiency after heating at a temperature over 150 °C for up to 22 h. A white light‐emitting diode (w‐LED) constructed from the metal halide perovskite solid exhibits superior temperature sustainable lifetime over 1100 h. The w‐LED also displays a highly durable high‐power‐driving capability, and its working current can go up to 300 mA. It is believed that such highly thermotolerant metal halide perovskites will unleash the possibility of a wide variety of high‐power and high‐temperature solid‐state lighting, lasing, and display devices that have been limited by existing methods.

Both single‐junction perovskite solar cells and perovskite/Si tandem devices are approaching the Shockley–Queisser limit. Recombination and hysteresis offer significant challenges for efficiency advancement, and therefore their mechanisms and promising solutions are focused. The contributions of materials development to the efficiencies of single‐junction and tandem cells are reviewed and analyzed, providing valuable insights into device design and further performance improvement.

Abstract

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single‐junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single‐junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Ziqi Liang, Miaomiao Li, Qi Wang, Yunpeng Qin, Sam J. Stuard, Zhongxiang Peng, Yunfeng Deng, Harald Ade, Long Ye, Yanhou Geng

The power conversion efficiency of inorganic perovskite solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. This work brings new thoughts on the critical factors that lead to high efficiency in inorganic perovskite solar cells.

Inorganic perovskites with cesium (Cs⁺) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI_1.8Br_1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T _S80, of ≈300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI_3 − x Br _x perovskite solar cells.

Simplified perovskite solar cells without employing electron/hole‐transport layers are attractive owing to their reduced fabrication process and low cost. The developments of simplified perovskite solar cells are discussed systematically. Device structure design, perovskite‐film optimization, interface modification, underlying mechanism, and other research hotspots are considered to present the bright future of simplified perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) are considered one of the most promising next‐generation examples of high‐tech photovoltaic energy converters, as they possess an unprecedented power conversion efficiency with low cost. A typical high‐performance PSC generally contains a perovskite active layer sandwiched between an electron‐transport layer (ETL) and a hole‐transport layer (HTL). The ETL and HTL contribute to the charge extraction in the PSC. However, these additional two layers complicate the manufacturing process and raise the cost. To extend this technology for commercialization, it is highly desired that the structure of PSCs is further simplified without sacrificing their photovoltaic performances. Thus, ETL‐free or/and HTL‐free PSCs are developed and attract more and more interest. Herein, the commonly used methods in reducing the defect density and optimizing the energy levels in conventional PSCs in order to simplify their structures are summarized. Then, the development of diverse ETL‐free or/and HTL‐free PSCs is discussed, with the PSCs classified, including their working principles, implemented technologies, remaining challenges, and future perspectives. The aim is to redirect the way toward low‐cost and high‐performance PSCs with the simplest possible architecture.

An autonomously longitudinal scaffold constructed by the interspersion of in situ polymerized methyl methacrylate in PbI₂ is introduced to effectively eliminate the dependence of sequential deposition on mesoporous TiO₂, and is applied in planar perovskite solar cells, with excellent performance. Moreover, this scaffold's cross‐linking grains are capable of releasing mechanical stress, impeding ion migration, and water/oxygen permeation.

Abstract

Sequential deposition is certified as an effective technology to obtain high‐performance perovskite solar cells (PVSCs), which can be derivatized into large‐scale industrial production. However, dense lead iodide (PbI₂) causes incomplete reaction and unsatisfactory solution utilization of perovskite in planar PVSCs without mesoporous titanium dioxide as a support. Here, a novel autonomously longitudinal scaffold constructed by the interspersion of in situ self‐polymerized methyl methacrylate (sMMA) in PbI₂ is introduced to fabricate efficient PVSCs with excellent flexural endurance and environmental adaptability. By this strategy perovskite solution can be confined within an organic scaffold with vertical crystal growth promoted, effectively inhibiting exciton accumulation and recombination at grain boundaries. Additionally, sMMA cross‐linked perovskite network can release mechanical stress and occupy the main channels for ion migration and water/oxygen permeation to significantly improve operational stability, which opens up a new strategy for the commercial development of large‐area PVSCs in flexible electronics.

The efficiency and operational stability of MA‐free FA_0.95Cs_0.05PbI₃ perovskite solar cells can be simultaneously enhanced by the incorporation of the β‐guanidinopropionic acid (β‐GUA) molecule. The introduction of β‐GUA forms a 2D/3D hybrid perovskite phase, which effectively passivates the surface defects, resulting in an impressive power conversion efficiency of 22.2% with a substantial increase in V _oc (from 1.01 to 1.14 V).

Abstract

Almost all highly efficient perovskite solar cells (PVSCs) with power conversion efficiencies (PCEs) of greater than 22% currently contain the thermally unstable methylammonium (MA) molecule. MA‐free perovskites are an intrinsically more stable optoelectronic material for use in solar cells but compromise the performance of PVSCs with relatively large energy loss. Here, the open‐circuit voltage (V _oc) deficit is circumvented by the incorporation of β‐guanidinopropionic acid (β‐GUA) molecules into an MA‐free bulk perovskite, which facilitates the formation of quasi‐2D structure with face‐on orientation. The 2D/3D hybrid perovskites embed at the grain boundaries of the 3D bulk perovskites and are distributed through half the thickness of the film, which effectively passivates defects and minimizes energy loss of the PVSCs through reduced charge recombination rates and enhanced charge extraction efficiencies. A PCE of 22.2% (certified efficiency of 21.5%) is achieved and the operational stability of the MA‐free PVSCs is improved.