Ligang Yuan

Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites

Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites, Published online: 10 January 2019; doi:10.1038/s41467-018-07655-3

Intermediate-band solar cell presents a possible route to break the Shockley-Queisser limit but the fabrication has been difficult. Here Hosokawa et al. exhibit working cells at room temperature in solution processed quantum dot-perovskite system with carefully designed miscibility and energy levels.

Herein, significantly improved ambient operational stability, including air processability and long‐term stability in polymer‐polymer solar cells relative to polymer‐PCBM devices is demonstrated. It is shown that all‐polymer blends exhibit excellent stability, with an efficiency approaching 9% despite being processed under high‐humidity conditions. Additionally, the all‐polymer cell shows improved stability under thermal stress and ambient conditions without encapsulation.

Abstract

In this work, the way in which ambient moisture impacts the photovoltaic performance of conventional PCBM and emerging polymer acceptor–based organic solar cells is examined. The device performance of two representative p‐type polymers, PBDB‐T and PTzBI, blended with either PCBM or polymeric acceptor N2200, is systemically investigated. In both cases, all‐polymer photovoltaic devices processed from high‐humidity ambient conditions exhibit significantly enhanced moisture‐tolerance compared to their polymer–PCBM counterparts. The impact of moisture on the blend film morphology and electronic properties of the electron acceptor (N2200 vs PCBM), which results in different recombination kinetics and electron transporting properties, are further compared. The impact of more comprehensive ambient conditions (moisture, oxygen, and thermal stress) on the long‐term stability of the unencapsulated devices is also investigated. All‐polymer solar cells show stable performance for long periods of storage time under ambient conditions. The authors believe that these findings demonstrate that all‐polymer solar cells can achieve high device performance with ambient processing and show excellent long‐term stability against oxygen and moisture, which situate them in an advantageous position for practical large‐scale production of organic solar cells.

A perovskite solar cell with both high efficiency and high thermal stability is examined. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% RH while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm²). These results reveal a great potential for future practical use.

Abstract

Perovskite solar cells have received great attention because of their rapid progress in efficiency, with a present certified highest efficiency of 23.3%. Achieving both high efficiency and high thermal stability is one of the biggest challenges currently limiting perovskite solar cells because devices displaying stability at high temperature frequently suffer from a marked decrease of efficiency. In this report, the relationship between perovskite composition and device thermal stability is examined. It is revealed that Rb can suppress the growth of PbI₂ even under PbI₂‐rich conditions and decreasing the Br ratio in the perovskite absorber layer can prevent the generation of unwanted RbBr‐based aggregations. The optimized device achieved by engineering perovskite composition exhibits 92% power conversion efficiency retention in a stress test conducted at 85 °C/85% relative humidity (RH) according to an international standard (IEC 61215) while exceeding 20% power conversion efficiency (certified efficiency of 20.8% at 1 cm²). These results reveal the great potential for the practical use of perovskite solar cells in the near future.

Foldable paper‐based perovskite solar cells (PSCs) with high power conversion efficiency of 13.19% and robust foldability are demonstrated. Beneficial from ultrathin cellophane substrates combined with foldable TiO₂/ultrathin Ag/TiO₂ electrodes, the solar cells exhibit 50 single folding stability at full angle range from −180° to 180° and 10 dual folding stability, enabling size compactness and shape transformation of paper‐based PSCs.

Foldable paper‐based solar cells are attractive power sources for wearable and portable applications. Currently, low power conversion efficiency (PCE) and degradation under different folding conditions restrict practical applications of paper‐based solar cells. Herein are constructed solar cells on cellophane paper using oxide/ultrathin Ag/oxide (OMO) and perovskite as electrodes and absorbers, respectively. The perovskite solar cell (PSC) on cellophane exhibits a PCE of 13.19%, the highest among all the paper‐based solar cells. More importantly, beneficial from ultrathin cellophane substrates combined with foldable OMO electrodes, PSCs on paper exhibit 50 single folding and 10 dual folding stability: they preserve 85.3 and 84.1% of the initial PCE after −180° and +180° single folding for 50 cycles, respectively; and they remain 67.2 and 55.3% of the initial PCE after 10 inner and outer dual folding cycles, respectively. Furthermore, the solar cells after dual folding show serious cracks and delamination, leading to faster degradation than single folding. The highly efficient, foldable, and lightweight PSCs on cellophane are promising for future self‐powered paper‐based electronic applications.

Compared with polycrystalline films, grain‐boundary‐free perovskite single‐crystal films are expected to significantly boost the optoelectronic performance of devices because of their higher carrier mobility, longer diffusion length, and better stability. This review is intended to provide a timely overview of the preparation methods, inherent properties, and state‐of‐the‐art applications of perovskite single‐crystal thin films and offers an outlook for future research.

Metal‐halide perovskites have aroused intense interest in the scientific community by virtue of their numerous remarkable optoelectronic properties, which render them promising candidates for applications in various optoelectronic fields, such as solar cells, light‐emitting diodes, photodetectors, and lasers. Compared with perovskite polycrystalline films and nanocrystals, grain‐boundary‐free single‐crystal perovskites possess lower trap‐state densities, higher carrier mobilities, and longer diffusion lengths, which are supposed to deliver superior optoelectronic performance. However, the thickness (a few millimeters) of typical bulk single‐crystal perovskites are much greater than their carrier diffusion length (e.g., MAPbI₃ SC, ≈175 µm), which results in severe charge accumulation and significantly hinders their development. To this end, fabricating single‐crystal thin films is of great importance for further exploring the potential of single‐crystal perovskites in various applications. In this paper, the rapid and prosperous developments in the fabrication methods of perovskite single‐crystal thin films are systematically summarized and recent encouraging progress in their physical and chemical properties as well as the optoelectronic applications (solar cells, photodetectors, and light‐emitting diodes) of single‐crystal thin films are reviewed. Finally, the challenges and a brief outlook for further improving the quality of perovskite single‐crystal thin films and optimizing the device design are highlighted.

A sequential slot‐die (SSD) coating technique can lead the polymer to form pre‐aggregates, resulting in enhanced crystallinity and face‐on orientation which is superior for charge transport. As a result, large area (1 cm²) flexible devices with a power conversion efficiency of 7.11% are obtained. Therefore this approach offers a new strategy for the fabrication of large area flexible organic solar cells.

For the preparation of flexible organic solar cells (OSCs), the Roll‐to‐Roll slot‐die coating technique is preferable. Herein, a sequential slot‐die (SSD) coating strategy to fabricate flexible OSCs using non halogenated solvent under ambient atmosphere, is developed. The coating order of the active layer materials shows great influence on the performance of OSCs. It is found that, compared with the one‐step coating, the power conversion efficiency (PCE) of devices with an area of 0.75 cm², and fabricated by SSD coating process with the polymer as the first layer, is enhanced from 4.86 to 5.51% for the binary system, whereas from 6.09 to 7.32% for the ternary system, showing an increase of 13 and 20% in PCE, respectively. For the devices with a standard small area of 4 mm² and large area of 1 cm², PCE as high as 9.36 and 7.11% are obtained, respectively, which are among the top value for flexible devices fabricated by slot‐die coating. It turns out that the SSD coating process with the polymer as the first layer assists pre‐aggregation of the polymer to form better crystal domains with face‐on orientation. Therefore, a sequential deposition strategy could provide a new means for manufacturing the high efficiency flexible OSCs.

Various novel photovoltaic devices with enhanced stability, high power conversion efficiency, thick films, and semitransparent features can be achieved via a ternary approach. In article no. 1800263, Xiaotao Hao and Pengqing Bi summarize the recent excellent work on various functional organic solar cells fabricated using a ternary strategy.

Quaternary organic solar cells based on PBDB‐T:PTB7‐Th:ITIC:FOIC are reported to deliver a parallel‐alloy morphology mode in non‐fullerene‐based devices. In the quaternary system, the parallel‐like donors and alloy‐like acceptors together facilitate transfer kinetics, optimize blend morphology, and drive the device efficiency toward over 12.5%, which indicates the great potential of quaternary organic solar cells.

Abstract

Two compatible donors (PBDB‐T and PTB7‐Th) and two miscible acceptors (ITIC and FOIC) are employed to deliver a parallel‐alloy morphology model in non‐fullerene‐based quaternary organic solar cells. PBDB‐T and PTB7‐Th form a parallel link with a slight adjustment of molecular packing into enhanced face‐on crystallites while ITIC disperses into discontinuous FOIC microcrystal regions to form continuous and ordered alloy‐like acceptor phases. Characterization of blend morphology highlights the parallel‐alloy model—enabled by the introduction of PBDB‐T and ITIC, which contributes to improved molecular packing and reduced domain size resulting in efficient charge generation and consistent transport channels. This successful parallel‐alloy quaternary blend morphology demonstrates an enhanced optical absorption, optimized domain size, and nanostructures toward simultaneous improvement in charge transfer and transport. Therefore, a power conversion efficiency of 12.52% is realized for a quaternary device which is 6% higher than the ternary device (PBDB‐T:PTB7‐Th:FOIC) and 12% higher than the binary device (PTB7‐Th:FOIC). Domination of quaternary devices over ternary and binary blends, which is another feasible way to realize highly efficient devices through further investigation of quaternary OSCs, is presented.

Environmentally friendly 2,2,2‐trifluoroethylamine hydrochloride (TFEACl) is used in synergy with SnF₂ to enhance the efficiency and stability of FASnI₃‐based solar cells, due to the improvements in film quality, suppression of the Sn²⁺ oxidation and more favorable energy band alignment.

The environmentally friendly additive 2,2,2‐trifluoroethylamine hydrochloride (TFEACl) is used in synergy with SnF₂ to enhance the efficiency and stability of FASnI₃‐based solar cells. Both TFEA⁺ and Cl⁻ are present in the films, but only Cl⁻ is incorporated into the crystal lattice of the perovskite. The addition of TFEACl suppresses the segregation of SnF₂, resulting in improvements in film morphology, in addition to a more favorable energy band alignment, and improved suppression of the formation of Sn⁴⁺. Consequently, reduced charge recombination and improved charge collection result in an efficiency enhancement from 3.63 to 5.30%. The stability of the devices is also significantly enhanced, with devices with TFEACl retaining over 60% of initial PCE after 350 h of light soaking in ambient, while devices without TFEACl experience failure in 120 h under the same testing condition.

Various deposition methods to satisfy the application of SnO₂ in different configuration‐based PSCs are summarized. Moreover, the efforts aimed at improving the performance of PSCs are cataloged according to different purposes and methods.

Organic‐inorganic hybrid perovskite solar cells (PSCs) have developed rapidly in recent years owing to the low cost and high power conversion efficiency achieved. The excellent performance of PSCs is attributed to the superior electrical properties of each layer, including the electron transport layer (ETL), light‐harvest layer, hole transport layer. As one of the most promising ETL materials for PSCs, SnO₂ shows excellent transmission, an appropriate energy band gap, a deep conduction band level, and high electron mobility, leading to efficient electron extraction and transport. Here, recent advancements in the PSCs with SnO₂ ETLs and endeavors aimed at improving the performance of this photovoltaic device are reviewed. Several typical configurations of SnO₂ based PSCs are discussed, including the planar structure, mesoporous structure, inverted structure and flexible PSCs. The efforts of modification and composite SnO₂ with other metal oxides are also assessed. Finally, an overview of the perspectives and challenges for the future of SnO2 based PSCs is provided.

Reliability of maximum power point tracking measurement for perovskite solar cells is analyzed, concluding that measurement delay and voltage step size are crucial parameters that can change measured performance up to 30% for specific perovskite architectures. This work proposes a two‐step measuring procedure for accurate and unbiased comparison of different perovskite architectures.

Performance comparison of perovskite solar cells with different architectures is nontrivial. J–V sweeps can be unreliable due to stack‐dependent output variations affected by sample preconditioning, scan rate, scan direction, and temperature. Maximum power point tracking is regarded as a more reliable performance measurement. In this study, a two‐step measuring procedure is proposed for quick and unbiased comparison of perovskite‐based solar cells. In the proposed procedure, the transient current density characterization step is used to determine optimal parameters for maximum power point tracking performance characterization. Moreover, factors affecting time limited maximum power point tracking by comparing performance and output of three different maximum power point tracking algorithms applied to devices, from three planar n‐i‐p perovskite stacks, are analyzed and identified. Numerical simulation is used to model differences and define limitations of the algorithms. As a result, it is shown that measurement delay and voltage step can have a crucial effect on maximum power point tracking output, regardless of the chosen algorithm.

SnO₂ QDs and CsMBr₃ (M = Sn, Bi, Cu) QDs are applied as ETMs and HTMs for all‐inorganic CsPbBr₃ PSCs, respectively. Arising from high optical transmittance and electron mobility of SnO₂ QDs ETL as well as hole extraction of CsMBr₃ QD HTL, the device achieves a good PCE of 10.60% and improved stability.

The power conversion efficiency (PCE) of state‐of‐the‐art perovskite solar cells (PSCs) with mesoscopic titanium dioxide (TiO₂) has rushed to 23.7% in recent years. However, photodegradation of perovskites under illumination (including ultraviolet light), assisted by TiO₂, significantly reduces the long‐term stability of the corresponding device, which in turn limits the commercialization of PSCs. Owing to the advantages of high electron mobility, wide bandgap, high transparency, and good photostability, nanostructured tin oxide (SnO₂) is demonstrated to be a more promising electron‐transporting material for planar PSCs. Herein, low‐temperature solution‐processed SnO₂ quantum dots (QDs) are employed as the electron transport layer (ETL) for all‐inorganic cesium lead bromide (CsPbBr₃) PSC applications. Through optimizing the aging time of SnO₂ QDs and adding a hole transport layer (HTL) of CsMBr₃ (M = Sn, Bi, Cu) QDs between the CsPbBr₃ layer and carbon electrode, the all‐inorganic PSC with a structure of FTO/SnO₂/CsPbBr₃/CsMBr₃/carbon achieves a good PCE of 10.60% with an ultrahigh open‐circuit voltage up to 1.610 V. These optimized devices, free of encapsulation, present excellent stability in 80% humidity or temperature of 80 °C. The maximized PCE report to date and improved environmental‐tolerance for all‐inorganic CsPbBr₃ solar cells provide new opportunities to dramatically promote the commercialization of PSC platforms.

Polymethyl methacrylate is coated on a perovskite grain boundary, blocking moisture penetration. The distributed C60 clusters create a dipole‐like electric field inside the perovskite layer, which favors exciton dissociation, and improves conversion efficiency of perovskite solar cells.

Abstract

Lead halide perovskite solar cells (PSCs) have demonstrated great potential for realizing low‐cost and easily fabricated photovoltaics. At this juncture, power conversion efficiency and long‐term stability are two important factors limiting their transition. PSCs exhibit rapid environmental degradation since the perovskite layer is very sensitive to factors such as humidity, temperature, and ultraviolet light. Here, a novel successful approach is demonstrated that simultaneously improves the efficiency and stability of PSCs. This approach relies on incorporation of a dual‐functional polymethyl methacrylate (PMMA)–fullerene complex into the perovskite layer. The fullerene within perovskite layer forms a localized dipole‐like electric field that favors electron–hole separation, resulting in significant improvement in current density and fill factor with conversion efficiency reaching 18.4%. The molecular‐scale coating of hydrophobic PMMA on the perovskite grain boundary effectively blocks moisture penetration into the perovskite, thereby, significantly improving the stability against moisture, heat, and light. The PSCs with PMMA–fullerene complex showed no photovoltaic performance degradation for 250 d and exhibited 60 times higher stability compared to the state‐of‐the‐art devices under continuous 1 sun illumination in ambient air.

An intriguing maze‐like CH₃NH₃PbI₃ film featuring a bilayer structure with a dense bottom layer and a porous top layer is judiciously designed for electron transport layer‐free perovskite solar cells (PSCs). Such maze‐like perovskite film shows high crystallinity, superior light‐harvesting capability, and enables facilitated hole extraction at the perovskite/hole transport layer interface, thus leading to a PCE of 18.5% with negligible hysteresis.

Perovskite solar cells (PSCs) without an electron transport layer (ETL) exhibit fascinating advantages such as simplified configuration, low cost, and facile fabrication process. However, the performance of ETL‐free PSCs has been hampered by severe charge carrier recombination induced either by current leakage (insufficient perovskite film coverage) or inferior charge extraction. Herein, an additive‐assisted morphological engineering strategy is used to construct an intriguing bilayer perovskite film featuring a dense bottom layer and a maze‐like top layer. Such maze‐like perovskite films enable the construction of ETL‐free PSCs with a PCE of 18.5% and negligible hysteresis, which can be attributed to the higher crystallinity and superior light‐harvesting capability of the resultant perovskite film, as well as facilitated hole extraction at the hole transport layer (HTL)/perovskite interface. This work provides a simple approach to modify the perovskite film morphology and demonstrates the correlation between facilitated charge‐carrier extraction and high‐performance ETL‐free perovskite photovoltaics.

Kelvin probe force microscopy and conductive atomic force microscopy are applied to explore local contact potential difference (CPD), current, and charge activities of the perovskite films with different contact layers. The average difference of the CPD values of the perovskite films obtained in dark and under illumination signifies the efficacy of the perovskite/contact layer heterojunctions.

Abstract

Hybrid halide perovskite based on CH₃NH₃PbI₃ and related materials has emerged as the most exciting development in the next generation photovoltaic technologies. There is still requirement for an effective method to establish a relationship between the charge transfer behaviors and photovoltaic properties. This study presents Kelvin probe force microscopy and conductive atomic force microscopy measurements of versatile perovskite films that participate in the formation of different heterojunctions, exploring local current, contact potential difference (CPD), and charge activities at the nanoscale. By comparing the values of CPD and current of these perovskite films in dark and under illumination, the charge transfer behaviors are locally illustrated, suggesting that the perovskite roles in these heterojunctions are strictly dependent on the contact layers. Furthermore, the average difference (ΔV) of the CPD values obtained in dark and under illumination for each heterojunction can be set to analyze the efficacy of the perovskite/contact layer interfaces. The ΔV polarity is related to the type of charge carrier (hole or electron), while the ΔV magnitude is related to the number of charge carrier. These results emphasize the importance of understanding of these heterojunction systems that could guide the design and optimization of the photovoltaic configuration.

Advanced Materials Interfaces, Volume 5, Issue 22, November 23, 2018.

A simple and generally applicable method to fabricate efficient and stable Pb‐Sn binary perovskite solar cells (PVSCs) based on a galvanic displacement reaction (GDR) is demonstrated. With optimizing the ratio of Pb and Sn, high PCEs of 15.85% and 18.21% are achieved for Pb‐Sn binary based PVSCs. Moreover, these Pb‐Sn based PVSCs exhibit improved stability with encapsulation.

Abstract

Here, a simple and generally applicable method of fabricating efficient and stable Pb‐Sn binary perovskite solar cells (PVSCs) based on a galvanic displacement reaction (GDR) is demonstrated. Different from the commonly used conventional approaches to form perovskite precursor solutions by mixing metal halides and organic halides such as PbI₂, SnI₂, MAI, FAI, etc., together, the precursor solutions are formulated by reacting pure Pb‐based perovskite precursor solutions with fine Sn metal powders. After the ratios between Pb and Sn are optimized, high PCEs of 15.85% and 18.21% can be achieved for MAPb_0.4Sn_0.6I₃ and (FAPb_0.6Sn_0.4I₃)_0.85(MAPb_0.6Sn_0.4Br₃)_0.15 based PVSCs, which are the highest PCEs among all values reported to date for Pb‐Sn binary PVSCs. Moreover, the GDR perovskite‐based PVSCs exhibit significantly improved ambient and thermal stability with encapsulation, which can retain more than 90% of their initial PCEs after being stored in ambient (relative humidity (RH) ≈50%) for 1000 h or being thermal annealed at 80 °C for more than 120 h in ambient conditions. These results demonstrate the advantage of using GDR to prepare tunable bandgap binary perovskites for devices with greatly improved performance and stability.

A low‐temperature solution‐processed TFB is demonstrated as an ideal hole‐transporting layer to push the PCE of the inverted perovskite solar cells (PVSCs) up to 20.2%. Moreover, this TFB‐based inverted PVSC exhibits good stability, retaining 90% of its original efficiency after storage for 30 days in ambient air.

Abstract

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.

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). Here, recent progress in interface modification for OSCs and PSCs aimed to improve interfacial charge extraction and mitigate surface recombination, and to enhance 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.

The role of ligands in perovskite photovoltaics is discussed from the perspective of film formation, passivation, and stability. Ligands can be used to improve the morphology and film quality by modulating the crystallization kinetics of perovskite precursors and passivating the defects via coordinate bonding. Besides, the stability of perovskites can be enhanced by ligand engineering.

Abstract

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. The latest certificated efficiency of 23.3% recently achieved in perovskite solar cells (PVSCs) enables them to be used as a very promising candidate for next‐generation photovoltaics. The morphology, defect density, and water resistance of perovskite films have an enormous impact on the performance and stability of PVSCs. Ligands, with coordinating capability, have been widely developed to improve the quality and stability of perovskite materials significantly. In the first section of this review, the role of ligands in fabricating perovskite films by different methods (one‐step, two‐step, and postdeposition treatment) is discussed. In the second section, the progress on ligand‐passivated perovskites via post‐treatment, in situ passivation during perovskite formation, and modifying the substrates before perovskite formation is reviewed. In the third section, a discussion of ligand‐stabilized perovskite films from the perspectives of crystal crosslinking, dimensionality engineering, and interfacial modification is presented. Finally, a summary and an outlook are given.

Halide perovskites exhibit extraordinary properties of slow hot‐carrier cooling; long‐range hot‐carrier transport; and efficient hot‐carrier extraction that are capable of unlocking disruptive high‐efficiency hot‐carrier photovoltaics which will overcome the Shockley–Queisser limit. Herein, the intricate photophysical mechanisms behind the novel phenomenon are distilled; an engineering and developmental toolkit is assembled; and the challenges and opportunities in this fledging area are examined.

Abstract

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.

In article number 1801954, Wallace C.H. Choy and co‐workers reveal the thermionic‐emission assisted electron transport mechanism theoretically and experimentally in a novel interconnecting structure of perovskite‐perovskite tandem solar cells. This interconnecting layer features solvent‐resistance, is free of indium tin oxide, and facilitates the fabrication of perovskite films by solution‐process.

Replacement of alkyl chains with alkoxyl chains of the backbone of nonfullerene acceptors successfully increases the energy levels, enhances the light absorption, and improves charge transport mobility, which simultaneously enhances three parameters (short circuit current density (J _SC), open circuit voltage (V _OC), and fill factor (FF)), resulting in a highest efficiency for poly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) (6.6%).

Abstract

Poly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) have attracted much attention due to their advantages of low‐cost production and matured roll‐to‐roll manufacture. However, the efficiency of P3HT‐based OSCs lag much behind the non‐P3HT ones due to their negligible absorption of long wavelengths of light over 650 nm, high‐lying highest occupied molecular orbitals (HOMO), and difficulty of controlling morphology. In this study, the alkyl chains of the nonfullerene acceptors are replaced with alkoxy chains to achieve synergistic enhancement of all three parameters ( short circuit current density (J _SC), open circuit voltage (V _OC), and fill factor (FF)) and thus significant increase of power conversion efficiency for P3HT‐based OSCs. As a result, the OSCs exhibit a maxima efficiency of 6.6%. The P3HT‐based systems are systematically studied with optical spectroscopy, photoluminescence, cyclic voltametry, space charge limit current, grazing incident wide‐angle X‐ray scattering, transient absorption spectroscopy, transmission electron microscope, and atomic force microscopy to probe the mechanism, which reveal that introducing alkoxy chains simultaneously increases the energy levels of the HOMO and the lowest unoccupied molecular orbitals, enhances the light absorption, improves the rigidity of the backbone and charge transport mobility, and tunes the molecular orientation and film morphology, thus improving the photovoltaic performance. This contribution provides an important guidance in the design of novel nonfullerene acceptors for high‐performance P3HT‐based OSCs.

Incorporation of indium(III) chloride is directly shown to induce the structural reconstruction of CsPbI₂Br perovskite at the microscopic level, which allows the stabilization of the α‐phase perovskite by means of increasing the structure tolerance factor and decreasing the grain size. Consequently, the square‐centimeter all‐inorganic InCl₃:CsPbI₂Br perovskite solar cells yield a power conversion efficiency of 11.4% with high stability.

Abstract

Although all‐inorganic perovskite solar cells (PSCs) demonstrate high thermal stability, cesium‐lead halide perovskites with high iodine content suffer from poor stability of the black phase (α‐phase). In this study, it is demonstrated that incorporating InCl₃ into the host perovskite lattice helps to inhibit the formation of yellow phase (δ‐phase) perovskite and thereby enhances the long‐term ambient stability. The enhanced stability is achieved by a strategy for the structural reconstruction of CsPbI₂Br perovskite by means of In³⁺ and Cl⁻ codoping, which gives rise to a significant improvement in the overall spatial symmetry with a closely packed atom arrangement due to the crystal structure transformation from orthorhombic (Pnma) to cubic (Pm‐3m). In addition, a novel thermal radiation heating method that further improves the uniformity of the perovskite thin films is presented. This approach enables the construction of all‐inorganic InCl₃:CsPbI₂Br PSCs with a champion power conversion efficiency of 13.74% for a small‐area device (0.09 cm²) and 11.4% for a large‐area device (1.00 cm²).

By introducing phenylethylammonium iodide (PhEtNH₃I) treatment can significantly enhance film and device stability under light and oxygen stress. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowering the yield of superoxide formation and improving stability.

Organic–inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cells, but device stability remains a concern. Recent research has demonstrated that the formation of superoxide species under exposure of the perovskite to light and oxygen leads to the degradation of CH₃NH₃PbI₃ perovskites. In particular, it has been revealed that iodide vacancies in the perovskite are key sites in facilitating superoxide formation from oxygen. This paper shows that passivation of CH₃NH₃PbI₃ films with an iodide salt, namely phenylethylammonium iodide (PhEtNH₃I) can significantly enhance film and device stability under light and oxygen stress, without compromising power conversion efficiency. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowers the yield of superoxide formation and improves stability. The present study elucidates a pathway to the future design and optimization of perovskite solar cells with greater stability.

The surface treatment of the electron transport layer, PCBM, is done using stearic acid. The treated surface consists of perpendicularly aligned monolayers of stearic acid which repel water, creating a hydrophobic film on top of PCBM. Amide linkages which crosslink stearic acid and the methyl ester group of PCBM, act as a barrier by preventing iodine ions which migrate from the active layer, reacting with the aluminum electrode.

Having achieved power conversion efficiencies higher than 22%, perovskite solar cells (PSCs) look set to be game changers in the field of photovoltaics. Their instability in humid environments, however, reduces their potential for commercialization. In this study, the role chemical degradation plays in moisture‐affected devices is investigated, and, based on this concept, a technique that enhances the device stability of p‐i‐n PSCs is developed. By surface treatment of the [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer with hydrophobic stearic acid and ethylenediamine, increased moisture resistivity of PCBM is achieved. The treated surface of the PCBM layer improves hydrophobicity, with a contact angle of 108°, and also prevents water ingress in the perovskite layer longer than non‐treated surfaces. In addition, interfacial stability is enhanced by the suppressed interaction between the ions and the electrodes, resulting in treated devices exhibiting improved stability in their photovoltaic parameters compared to non‐treated devices when exposed to a dark environment with a relative humidity of 45%.