ZHANG JIAJIA

Applied Physics Letters, Volume 117, Issue 20, November 2020.
In this report, we discuss the 22% efficiency improvement of solar cells based on the MAPbI3 perovskite film extracted with a mixed anti-solvent. The film quality of MAPbI3 extracted from the mixed anti-solvent of ether and isopropanol is improved greatly. The average grain size of the film may be enlarged twice. We argue that some solvents residing in the precursor may effectively promote the crystallization process of MAPbI3 to form large grains. We believe that this study may open a method to fabricate high-quality MAPbI3 perovskite films for highly efficient solar cells.

An additive‐involved leaching method is proposed to reduce the preparation temperature of CsPbI₃ to 100 °C. The CsPbI₃ perovskite film with high crystallinity is formed by an ion exchange reaction between DMAPbI₃ and Cs₄PbI₆. More than 16% photoelectric conversion efficiency can be achieved and the inencapsulation device exhibits remaekable stability.

Abstract

Inorganic CsPbI₃ perovskite with an optical bandgap ranging from 1.67 to 1.75 eV is a promising light‐harvesting material as a top cell in tandem solar cells, but its high fabrication temperature can damage the middle layers or the bottom subcells. Here, an additive‐involved leaching method to fabricate CsPbI₃ perovskite films is demonstrated, which can decrease the preparation temperature to 100 °C. The CsPbI₃ perovskite films with high crystallinity are achieved by a solution assisted reaction between DMAPbI₃ and Cs₄PbI₆ with the leaching of DMA⁺, Cs⁺, and I⁻. The as‐prepared CsPbI₃ perovskite films exhibit much superior stability compared to their high‐temperature counterparts. As a result, a power conversion efficiency of over 16% is obtained, and the unencapsulated device maintains over 93% of the initial efficiency after aging for 30 days in air with a relative humidity of 10%.

Great progress has been made in the field of inorganic CsPbX₃ perovskite solar cells (PSCs), and organic molecule engineering has been playing a vital role in improving device performance. In this review, the roles of organic molecules in inorganic CsPbX₃ PSCs are systematically reviewed and discussed, and future research directions are suggested to further improve the performance of inorganic PSCs.

Abstract

Over 25% efficiencies have been achieved by organic–inorganic hybrid perovskite solar cells (PSCs). However, their practical applications are limited by the instability of the hybrid perovskite materials. Replacing hybrid perovskites with inorganic CsPbX₃ perovskites shows great promise to address the above issue and much progress has been made. To achieve high efficiency and stable inorganic CsPbX₃ PSCs, organic molecular engineering has been playing a vital role. Herein, the progress of the organic molecular engineering in inorganic CsPbX₃ PSCs is systematically reviewed. First, structure evolution induced by organic molecular engineering for inorganic CsPbX₃ perovskites is demonstrated. Then, organic molecular engineering in CsPbX₃ PSCs is categorized and reviewed (alloying in perovskite structures, as sacrificial agents, forming 2D structures, and modifying surfaces and interfaces). Finally, future research directions are suggested to further improve the performance of inorganic PSCs.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

The capability to deposit perovskite materials by either thermal evaporation or solution processing offers intriguing possibilities for mass production of perovskite solar cells. This Progress Report describes the current state of research in both fields, discusses the challenges faced by these methods and their future opportunities.

Abstract

The last decade has seen remarkable advancements in the field of perovskite materials and photovoltaic technologies. One of their most extraordinary characteristics is the high quality of layers that can be obtained by “dirty processing” from solution at low temperatures. Alternatively, perovskites can also be deposited by thermal evaporation, a clean, solvent‐free process, which is well established for many industrial applications. Although the vast majority of research reports focus on solution‐processing as the deposition method for perovskite solar cells, thermally evaporated perovskite solar cells are closing in the performance gap with several reports of efficiencies above 20%. In this Progress Report, the two deposition methods are briefly introduced, the key developments in photovoltaic devices based on each deposition technique are outlined, and the challenges and future possibilities are discussed.

Organic solar cells have the potential to become the cheapest form of electricity, even beating silicon solar cells, at least in principle. This article summarizes where the field is on its way towards successful large‐scale commercialization, highlighting research challenges, discussing the status of current and future applications as well as the environmental footprint of this renewable energy technology.

Abstract

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

A combination of in‐ and ex situ measurements are used to monitor the degradation processes in mixed‐halide perovskite light‐emitting devices. Device performance degradation is found to arise from accumulation of bromide at the interface between the perovskite and electron injecting layer, causing nonradiative recombination. Potassium halides mitigate the ionic movement and degradation, leading to better stability of devices under operation.

Abstract

Halide perovskites have attracted substantial interest for their potential as disruptive display and lighting technologies. However, perovskite light‐emitting diodes (PeLEDs) are still hindered by poor operational stability. A fundamental understanding of the degradation processes is lacking but will be key to mitigating these pathways. Here, a combination of in operando and ex situ measurements to monitor the performance degradation of (Cs_0.06FA_0.79MA_0.15)Pb(I_0.85Br_0.15)₃ PeLEDs over time is used. Through device, nanoscale cross‐sectional chemical mapping, and optical spectroscopy measurements, it is revealed that the degraded performance arises from an irreversible accumulation of bromide content at one interface, which leads to barriers to injection of charge carriers and thus increased nonradiative recombination. This ionic segregation is impeded by passivating the perovskite films with potassium halides, which immobilizes the excess halide species. The passivated PeLEDs show enhanced external quantum efficiency (EQE) from 0.5% to 4.5% and, importantly, show significantly enhanced stability, with minimal performance roll‐off even at high current densities (>200 mA cm⁻²). The decay half‐life for the devices under continuous operation at peak EQE increases from <1 to ≈15 h through passivation, and ≈200 h under pulsed operation. The results provide generalized insight into degradation pathways in PeLEDs and highlight routes to overcome these challenges.

A rerouting crystallization pathway (RCP) is developed to suppress defects in vertically oriented 2D perovskites. Lower trap states, better homogeneity, and higher charge transport/collection efficiency are obtained due to the improved film quality. Solar cells using these RCP‐2D perovskite films show a highest efficiency of 18.5% with a high fill factor of 83.4% and exhibit superior environmental stability.

Abstract

Vertically oriented 2D perovskites exhibit promising optoelectronic properties and intrinsic stability, but their photovoltaic application is still limited by the low power conversion efficiency (PCE) compared to 3D analogs. Here, a new crystallization pathway (RCP) is reported to suppress defects in vertically oriented 2D perovskite caused by its over‐rapid self‐assembly behavior. By controlling the specific adsorption of an ammonium halide additive on different perovskite crystal planes, the dynamic preferred growth of (111) plane is intentionally restrained, and the minority (202) planes emerge as secondary nucleation sites to stimulate the creation of large grains. As the halogen‐regulated deprotonation of ammonium proceeds, the (111) crystal plane gradually recovers its growth dominance, and a vertically oriented 2D perovskite film finally forms with high homogeneity, reduced trap density of states, and desired carrier transport/collection kinetics. Solar cells using RCP‐2D films show a highly reproducible and stable PCE reaching 18.5% with a high fill factor of 83.4%. These findings provide critical missing information on simultaneously achieving highly oriented and less defective 2D perovskite films for excellent device performance.

Lead‐free perovskite‐inspired materials (PIMs) provide a particularly attractive route to low‐toxicity indoor photovoltaics (IPV). Two exemplar PIMs, bismuth oxyiodide (BiOI) and Cs₃Sb₂Cl _x I_{9‐ x} , deliver an IPV efficiency of 4–5%, and can power thin‐film‐transistor electronics. Loss analyses and calculations of the optically limited efficiency reveal that further efficiency increases are possible, encouraging future efforts for the exploration of PIMs for powering Internet of Things (IoT) devices.

Abstract

With the exponential rise in the market value and number of devices part of the Internet of Things (IoT), the demand for indoor photovoltaics (IPV) to power autonomous devices is predicted to rapidly increase. Lead‐free perovskite‐inspired materials (PIMs) have recently attracted significant attention in photovoltaics research, due to the similarity of their electronic structure to high‐performance lead‐halide perovskites, but without the same toxicity limitations. However, the capability of PIMs for indoor light harvesting has not yet been considered. Herein, two exemplar PIMs, BiOI and Cs₃Sb₂Cl _x I_{9‐ x} are examined. It is shown that while their bandgaps are too wide for single‐junction solar cells, they are close to the optimum for indoor light harvesting. As a result, while BiOI and Cs₃Sb₂Cl _x I_{9‐ x} devices are only circa 1%‐efficient under 1‐sun illumination, their efficiencies increase to 4–5% under indoor illumination. These efficiencies are within the range of reported values for hydrogenated amorphous silicon, i.e., the industry standard for IPV. It is demonstrated that such performance levels are already sufficient for millimeter‐scale PIM devices to power thin‐film‐transistor circuits. Intensity‐dependent and optical loss analyses show that future improvements in efficiency are possible. Furthermore, calculations of the optically limited efficiency of these and other low‐toxicity PIMs reveal their considerable potential for IPV, thus encouraging future efforts for their exploration for powering IoT devices.

Significant progress has been made in non‐fullerene organic solar cells (OSCs) in recent years, including in materials development, device engineering, and mechanistic understanding. This review summarizes progress and offers some reflections on the emerging methods for enabling high efficiency and improved stability for non‐fullerene OSCs.

Abstract

The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.

The chemical composition modulation and electric field‐induced ion migration of organic‐inorganic hybrid perovskites are utilized to fabricate performance‐enhanced triboelectric nanogenerators (TENGs). The chemical composition modulation induced conductive type conversion and electric field‐induced self‐doping on the surfaces enable controlled performance of the TENGs.

Abstract

In this paper, new strategies are proposed to design high‐performance organic–inorganic hybrid perovskite (PVK)‐based triboelectric nanogenerators (TENGs) via both chemical composition modulation and electric field‐induced ion migration in the films. Both composition variation and ion migration under electric field are found to change the type of conductivity of the perovskite films, then modify their surface potentials and electron affinities. These are utilized to fabricate PVK‐based TENGs in pairs with poly‐tetrafluoroethylene (PTFE) or nylon films, respectively. Results show that PVK films are able to work as either a positive or a negative tribo‐material depending on the tribo‐material pair used; the optimal performances are obtained for PTFE/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 2 and forward polarization, and for nylon/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 0.4 and reverse polarization, respectively. The maximum output voltage and peak power density of PTFE/PVK TENGs are about 979 V and 24 W m⁻², 2.5 and 6.5 times higher than those of TENGs with nonoptimal composition ratio or that are poorly polarized. This work provides a new material design method for high‐performance TENGs and a novel polarization strategy for TENG performance enhancement.

In article number 2001759, Kilwon Cho and co‐workers report the novel design of an organic spacer as a multifunctional additive for formamidinium lead tri‐iodide (FAPbI₃) perovskite solar cells. Low dimensional (LD) perovskites assembled by organic spacers not only protect the grain boundary of FAPbI₃ from moisture but also facilitate the nucleation and growth of FAPbI₃ at low temperature. An LD/ FAPbI₃ composite based solar cell exhibits power conversion efficiency of 21.25% retaining 80% of the initial efficiency after 500 hours without encapsulation.

The capability to deposit perovskite materials by either thermal evaporation or solution processing offers intriguing possibilities for mass production of perovskite solar cells. This Progress Report describes the current state of research in both fields, discusses the challenges faced by these methods and their future opportunities.

Abstract

The last decade has seen remarkable advancements in the field of perovskite materials and photovoltaic technologies. One of their most extraordinary characteristics is the high quality of layers that can be obtained by “dirty processing” from solution at low temperatures. Alternatively, perovskites can also be deposited by thermal evaporation, a clean, solvent‐free process, which is well established for many industrial applications. Although the vast majority of research reports focus on solution‐processing as the deposition method for perovskite solar cells, thermally evaporated perovskite solar cells are closing in the performance gap with several reports of efficiencies above 20%. In this Progress Report, the two deposition methods are briefly introduced, the key developments in photovoltaic devices based on each deposition technique are outlined, and the challenges and future possibilities are discussed.

Nature Energy, Published online: 09 November 2020; doi:10.1038/s41560-020-00721-5

Two-dimensional Ruddlesden–Popper layered metal-halide perovskites show better performance over three-dimensional versions, but are typically based on quantum wells with random width distribution. Liang et al. show that introducing molten salt spacers gives phase-pure quantum wells and improved solar cell performance.

Nature Energy, Published online: 11 November 2020; doi:10.1038/s41560-020-00732-2

The power conversion efficiency of organic solar cells has rapidly increased, yet significantly less attention has been paid to materials stability and device longevity. For organic solar cells to make an impact in the marketplace, researchers, funding agencies and journals should do more to address this crucial gap.

Applied Physics Letters, Volume 117, Issue 13, September 2020.
Organic solar cells (OSCs) have been attracting considerable interest due to their unique advantages of low cost, light weight, and especially mechanical flexibility. The low-cost and high-throughput techniques matching with the large-scale and roll-to-roll (R2R) process for fabricating efficient OSCs in the ambient condition would greatly accelerate the potential commercialization of OSCs. Herein, we demonstrate that the fabrication processes of OSCs using the bulk heterojunction (BHJ) composed of poly[(2,6–(4,8-bis(5–(2-ethylhexy)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5–(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′5′-c′]dithiophene-4,8-dione))] (PBDB-T) and 3,9-bis(2-methylene-(3–(1,1-dicyanomethylene)-5-methylindanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (IT-M) are transferred from a glovebox to the ambient condition, where the deposition of doctor blading instead of conventional spin coating is investigated. The morphology, microphase separation, and crystallinity of BHJ PBDB-T:IT-M are dramatically influenced by the fabrication processes. The OSCs with a structure of ITO/ZnO/PBDB-T:IT-M/MoO3/Ag fabricated via doctor-blading in the ambient condition show a power conversion efficiency (PCE) of 9.0% as compared to conventional spin-coated OSCs in a glovebox with a PCE of 11.91% and in the ambient condition with a PCE of 9.91%. These results suggest that efficient OSCs could be processed in the ambient condition by large-scale and low-cost doctor-blading, which can be compatible with the R2R manufacturing process.

A facile and effective additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM) molecules into perovskite (PVK) layer. The synergistic effect of the SM⁻ anions and the Gua⁺ cations are demonstrated, which effectively reduces the trap density and the recombination in PVK, so that the photovoltaic performance and stability of the perovskite solar cells are improved noticeably.

Abstract

High efficiency and good stability are the challenges for perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high defect density and internal nonradiative recombination of perovskite (PVK) limit its development. In this work, a facile additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM; CH₆N₃ ⁺, Gua⁺; H₂N−SO₃ ⁻, SM⁻) into PVK. The size of Gua⁺ ion is suitable with Pb(BrI)₂ cavity relatively, so it can participate in the formation of low‐dimensional PVK when mixed with Pb(BrI)₂. The O and N atoms of SM⁻ can coordinate with Pb²⁺. The synergistic effect of the anions and cations effectively reduces the trap density and the recombination in PVK, so that it can improve the efficiency and stability of PSCs. At an optimal concentration of GuaSM (2 mol%), the PSC presents a champion power conversion efficiency of 21.66% and a remarkably improved stability and hysteresis. The results provide a novel strategy for highly efficient and stable PSCs by bifunctional additive.

Nature Energy, Published online: 02 November 2020; doi:10.1038/s41560-020-00714-4

Ultrathin solar cells attract interest for their relatively low cost and potential novel applications. Here, Massiot et al. discuss their performance and the challenges in the fabrication of ultrathin absorbers, patterning of light trapping structures and ensuring efficient charge-carrier collection.

Nature Energy, Published online: 02 November 2020; doi:10.1038/s41560-020-00716-2

Lead leakage from damaged perovskite photovoltaic modules poses health and environmental risks limiting the potential use of the technology. Now Chen et al. show that the encapsulation of perovskite modules with a cation-exchange resins reduces lead leakage to 14.3 ppb in waste water.

PbS nanoplatelets (NPLs) used as an external additive with (100) preferential crystal orientation improve a formamidinium‐based perovskite material and solar cell stability. A stable current density of 23 mA cm⁻² for 4 months is recorded along with an improved reproducibility, demonstrating the potential of the interaction between the (100) facets of the NPLs and the perovskite α‐phase.

Abstract

Formamidinium‐based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm⁻², and a stable incident photon to current efficiency in the infrared region of the spectrum for 4 months is obtained, one of the best stability achievements for planar PSCs. Moreover, a better reproducibility than the control device, by optimizing the PbS concentration in the perovskite matrix, is achieved. These outcomes validate the synergistic use of PbS nanoplatelets to improve formamidinium long‐term stability and performance reproducibility, and pave the way for using metastable perovskite active phases preserving their light harvesting capability.

A new narrow bandgap nonfullerene acceptor featuring 3,6‐dimethoxylthieno[3,2‐b]thiophene and alkylthio side‐chains is synthesized as a higher lowest unoccupied molecular orbital level guest acceptor of PTB7‐Th:IEICO‐4F to realize simultaneous improvement of solar cell performance, yielding 12.5% efficiency in a ternary solar cell.

Abstract

Obtaining a large open‐circuit voltage (V _OC) and high short‐circuit current density (J _SC) simultaneously is important in improving power conversion efficiency (PCE) of organic photovoltaics. The ternary strategy with using a higher lowest unoccupied molecular orbital (LUMO) level nonfullerene acceptor (NFA) guest can achieve increased V _OC, yet J _SC is decreased or maintained, so it's still a challenge to offer increased V _OC and J _SC values concurrently via the newly presented V _OC‐increased ternary strategy. To overcome this issue, a new narrow bandgap NFA TT‐S‐4F is reported by introducing 3,6‐dimethoxylthieno[3,2‐b]thiophene (TT) as π‐spacers to connect electron‐rich core with terminal groups, so as to upshift the LUMO level and extend π‐system. When adding 10% TT‐S‐4F into binary system based on PTB7‐Th:IEICO‐4F, the higher‐LUMO‐level of TT‐S‐4F, the increased charge mobilities, the reduced trap‐assisted combination loss, and a finer nanofiber structure and increased phase separation size are obtained, which simultaneously promotes J _SC, V _OC, and fill factor (FF), thus obtaining an optimal PCE (12.5% vs 11.5%). This work illustrates that an extending conjugated backbone with large π‐spacers and inclusion of alkylthiophenyl side‐chains is a concept to synthesize NFA guests for use on the V _OC‐increased ternary strategy that enables to realize simultaneously increased J _SC, V _OC, and FF.

In this work, two highly crystalline polymer donors with isomers are synthesized for polymer solar cells. A systematic investigation is figured out to get a deep insight into the relationship between isomer structure and performance.

Abstract

Two highly crystalline polymer donors (PBTz4T2C‐a, PBTz4T2C‐b) with isomers (4T2C‐a, 4T2C‐b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C‐b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C‐a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C‐b:IT2F blend film. In contrast, PBTz4T2C‐a with 4T2C‐a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high‐performance polymer donors.

Applied Physics Letters, Volume 117, Issue 16, October 2020.
A low-temperature fabrication routine is developed for hole-conductor-free and mesoscopic perovskite solar cells using a TiO2 nanoparticle-binding carbon electrode as the top electrode. Vacuum treatment is adopted to help the infiltration and formation processes of the organic–inorganic hybrid perovskite crystallites. It is observed that such treatment not only condenses the mesoporous skeleton and improves film conductance of the carbon electrode but also makes the perovskite crystallites grow in the core part of the mesoporous skeleton. As such, the extraction process of photogenerated charge carriers is accelerated due to the strengthened interfacial contact between the perovskite crystallites and the skeleton. Accordingly, the photo-to-electric power conversion efficiency of the low-temperature devices is upgraded from 7.38 (±1.40)% to 10.17 (±0.86)% (optimized at 12.29%, AM 1.5 G, 100 mW/cm2). In addition, prolonged stability is observed. Due to the condensed device structure, storage stability of 225 days has been achieved in ambient air (with relative humidity of about 40–60%), even without encapsulation. The proposed strategy is helpful in further reducing the production cost.

Applied Physics Letters, Volume 117, Issue 16, October 2020.
We propose a mechanism based on thermally assisted charge transfer (CT) to study the thermal effect on the formation of CT states and subsequent charge separation in an organic donor–acceptor solar cell. We reveal that the difference between phonons in the donor and acceptor caused by elastic energy acts as a thermally assisted driving force for charge transfer. It is found that the system exhibits a quite different CT process in the high and low temperature regions. Remarkably, combined with the entropy driving mechanics, the thermally assisted CT yields charge separation probability as high as 70% at room temperature. Our model and results provide a microscopic quantum understanding of the relevant recent experiments and open up a route to realize high-efficiency organic solar cells by effectively taking advantage of the thermal effect.

Nature Energy, Published online: 05 October 2020; doi:10.1038/s41560-020-00705-5

Ensuring both stability and efficiency in mixed lead–tin perovskite solar cells is crucial to the development of all-perovskite tandems. Xiao et al. use an antioxidant zwitterionic molecule to suppress tin oxidation thus enabling large-area tandem cells with 24.2% efficiency and operational stability over 500 hours.

Nature Energy, Published online: 15 October 2020; doi:10.1038/s41560-020-00692-7

Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

The recent progress in potentially low‐cost polythiophene:nonfullerene‐based solar cells is reviewed from the viewpoints of molecular engineering and morphology control. The molecular design strategies of polythiophenes and nonfullerene acceptors are discussed, followed by the recent achievements in understanding and controlling the morphology of polythiophene:nonfullerene blends. Finally, the future challenges are delineated for advancing the commercial applications of polythiophenes in solar cells.

Abstract

With the advances in organic photovoltaics (OPVs), the development of low‐cost and easily accessible polymer donors is of vital importance for OPV commercialization. Polythiophene (PT) and its derivatives stand out as the most promising members of the photovoltaic material family for commercial applications, owing to their low cost and high scalability of synthesis. In recent years, PTs, paired with nonfullerene acceptors, have progressed rapidly in photovoltaic performance. This Review gives an overview of the strategies in designing PTs for nonfullerene OPVs from the perspective of energy level modulation. A survey of the typical classes of nonfullerene acceptors designed for pairing with the benchmark PT, i.e., poly(3‐hexylthiophene) (P3HT) is also presented. Furthermore, recent achievements in understanding and controlling the film morphology for PT:nonfullerene blends are discussed in depth. In addition to the effects of molecular weight and blend ratio on film morphology, the crucial roles of miscibility between PT and nonfullerene and processing solvent in determining film microstructure and morphology are highlighted, followed by a discussion on thermal annealing and ternary active layers. Finally, the remaining questions and the prospects of the low‐cost PT:nonfullerene systems are outlined. It is hoped that this review can guide the optimization of PT:nonfullerene blends and advance their commercial applications.

Herein, novel Ruddlesden–Popper Cs₂PbI₂Cl₂ nanosheets are synthesized and creatively employed as a multifunctional interface optimization material to improve the performance of CsPbI₂Br solar cells. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic film, an efficiency of 16.65% is obtained, which is one of the best reported for CsPbI₂Br solar cells, along with much‐enhanced UV, air, and thermal stabilities.

Abstract

Inorganic CsPbI₂Br perovskite solar cells (PSCs) have gained enormous research interest due to their excellent thermal and light stabilities. However, their unsatisfactory power‐conversion efficiency and poor intrinsic phase stability remain roadblocks to their further development. Herein, Cs₂PbI₂Cl₂ nanosheets (NSs) with the Ruddlesden–Popper (RP) structure are synthesized, and an NSs/CsPbI₂Br/NSs heterostructure is employed to enhance both the stability and efficiency of CsPbI₂Br solar cells. The novel Cs₂PbI₂Cl₂ NSs can not only passivate the top and bottom surfaces of the perovskite film and top surface of the TiO₂ film but also enhance the stability of the perovskite film. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic perovskite film, the efficiency of the CsPbI₂Br PSCs is improved from 15.02% to 16.65%. Moreover, the unencapsulated CsPbI₂Br devices with the NSs/CsPbI₂Br/NSs heterostructure sustain over 90% of their original efficiencies after being exposed to ambient conditions (≈25 °C and ≈35% RH) for 648 h. Both the UV‐light‐soaking stability (100 mW cm⁻¹ 365 nm UV light) and thermal stability (T = 85 °C) of the optimized devices are dramatically improved in comparison with their counterparts with only a 3D active layer. Therefore, this work promotes the application of RP inorganic perovskite nanocrystals in a range of perovskite optoelectronic devices.

A newly conceived n‐type small molecule (Y‐Th2) is incorporated as an efficient additive in perovskite solar cells, achieving simultaneous improvements in device performance and stability. Y‐Th2 effectively passivates defects in perovskite crystals by Lewis acid–base interactions and intermolecular hydrogen bonds, obtaining high‐quality perovskite film. The inverted structure device exhibits a power conversion efficiency of 21.5% with notably enhanced operational stability.

Abstract

Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high‐performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene‐based n‐type conjugated small molecule (Y‐Th2) is introduced to PSCs, which simultaneously enhances the performance and stability of the cell. The Y‐Th2 effectively passivates the defect states in perovskite through Lewis acid–base interactions, increasing the grain size and quality of the perovskite absorber. An inverted PSC containing the Y‐Th2 additive achieves a power conversion efficiency of 21.5%, versus 18.3% in the reference device. The operational stability is also considerably enhanced by the improved hydrophobicity and intermolecular hydrogen bonds in the perovskite.

The precise composition of the intermixed phase in bulk heterojunction structures with device‐relevant size is determined via the analysis of the glass transition temperatures proven by fast scanning calorimetry. A relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads already to efficient charge generation. However, charge transport can only be sustained in blend morphologies with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%).

Abstract

The relation of phase morphology and solid‐state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase‐pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device‐relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk‐heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology–function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high‐performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.