JIANG ZHI XUAN

Sodium dodecyl benzene sulfonate (SDBS) is used as a multifunctional chemical additive for efficient and stable planar fully air‐processed perovskite solar cells (PSCs). The introduction of SDBS can promote the preferential growth of crystal orientation, reduce defects, inhibit the migration of iodide ions, enhance the built‐in potential, and improve the water resistance of perovskite films.

The device performance of organic–inorganic hybrid halide perovskite solar cells (PSCs) is highly dependent on the quality of perovskite layer. Herein, a multifunctional chemical additive strategy is reported to simultaneously improve the efficiency and stability of fully air‐processed PSCs. The planner methylammonium lead trihalide (MAPbI₃)‐based PSCs incorporating sodium dodecyl benzene sulfonate (SDBS) exhibit a champion power conversion efficiency (PCE) of 19.20% and negligible hysteresis, which is one of the top efficiencies of MAPbI₃‐based PSCs made in air. The increased efficiency is due to the reduction of defects and inhibition of ion migration in the perovskite films. Furthermore, the enhancement of device performance and stability can also be ascribed to highly preferred and efficient perovskite crystals protecting the perovskite films from humidity. The corresponding unencapsulated device retains 92.34% of its initial efficiency after 90 days (>2100 h) storage in air and maintains 85.20% of its original PCE after being exposed to 85 °C for 27 h. The results indicate that SDBS is a promising chemical additive to enhance the performance of air‐processed PSCs for future applications.

SnO₂ has been applied as an efficient electron transport layer for perovskite solar cells over the past few years. In this progress report, recent advances in SnO₂ modification toward high efficiency and stability are summarized from the perspective of the optimization strategies, and the remaining challenges as well as opportunities for future research are also discussed.

Abstract

The electron transport layer plays a key role in affecting the charge dynamics and photovoltaic parameters in perovskite solar cells. Compared to other counterparts, SnO₂ has unique advantages such as low temperature fabrication and high electron extraction ability, and it receives extra attentions from the research community since the first report. Planar‐type perovskite solar cells based on SnO₂ exhibit a simple architecture and state of art device can achieve a power conversion efficiency of over 23%, which can compete with traditional devices using mesoporous TiO₂. The modification engineering of SnO₂ has contributed significantly to the enhanced device performance during the past years. There is still great potential for further improvement in the efficiency and long‐term stability. Herein recent advances toward modifying the optoelectronic properties of SnO₂ from the perspective of the optimization strategies are summarized and the remaining challenges as well as opportunities for future research are discussed. The continuous efforts dedicated to this exciting field may pave the way for developing commercial perovskite solar cells.

An N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (CSCO), is prepared for the first time as an electron transport layer for n‐i‐p planar perovskite solar cells, which leads to one of the highest power conversion efficiencies of 22.21%, and to remarkable stability, retaining over 83% of its initial power conversion efficiency without encapsulation after 130 days of storage in ambient conditions.

Abstract

The electron transport layer (ETL) has an important influence on the power conversion efficiency (PCE) and stability of n‐i‐p planar perovskite solar cells (PSCs). This paper presents an N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (abbreviated as CSCO) that is synthesized and prepared for the first time as an ETL for n‐i‐p planar PSCs, which leads to a high PCE of 22.21% after KCl treatment, one of the highest PCEs of n‐i‐p planar PSCs to date. Further analysis reveals that the high PCE is attributed to the excellent conductivity of CSCO because of its more delocalized electron cloud distribution due to its unique −O=C−O− group, and to the defect passivation of the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (denoted as CsFAMA) perovskite through the interaction between the O (Sn) atoms of CSCO and the Pb (halogen) atoms of CsFAMA at CSCO/CsFAMA interface, while the traditional ETL materials such as SnO₂ film lack this function. In addition to the high PCE, the optimal PSCs using CSCO as ETL show remarkable stability, retaining over 83% of its initial PCE without encapsulation after 130 days of storage in ambient conditions (≈25 °C at ≈40% humidity), much better than the traditional SnO₂‐based n‐i‐p PSCs.

A bilinkable contact passivation strategy is developed for modifying charge kinetics at the charge transport layer:active layer interface in solar cells. The use of the bifunctional molecule 3‐thiophenecarboxylic acid (TCA) passivates undercoordinated Ti (ETL‐side) and Pb (perovskite‐side), enabling efficient electron extraction through the interface. TCA‐treated films show an increase of PCE of 21.2% compared to 19.8% for reference devices.

Abstract

Charge recombination due to interfacial defects is an important source of loss in perovskite solar cells. Here, a two‐sided passivation strategy is implemented by incorporating a bilinker molecule, thiophene‐based carboxylic acid (TCA), which passivates defects on both the perovskite side and the TiO₂ side of the electron‐extracting heterojunction in perovskite solar cells. Density functional theory and ultrafast charge dynamics reveal a 50% reduction in charge recombination at this interface. Perovskite solar cells made using TCA‐passivated heterojunctions achieve a power conversion efficiency of 21.2% compared to 19.8% for control cells. The TCA‐containing cells retain 96% of initial efficiency following 50 h of UV‐filtered MPP testing.

This article reports the interfacial modification of compact TiO₂ ETL with Au NPs functionalized with fully conjugated fullerene C₆₀ derivative. This interlayer facilitates charge transfer and reduces charge carrier recombination pathways at interfaces. Consequently, the performance and stability of the modified devices are improved as compared to the reference devices.

Abstract

Titanium dioxide (TiO₂) is an extensively used electron transporting layer (ETL) in n–i–p perovskite solar cells (PSCs). Although, TiO₂ ETL experiences the high surface defect together with low electron extraction ability, which causes severe energy loss and poor stability in the PSC. In this study, a new intermediate layer consisting of gold nanoparticles functionalized with fully conjugated fullerene C₆₀ derivative (C₆₀‐BCT@Au NPs) that enhances the interfacial contact at ETL/perovskite interface leading to a perovskite film with improved crystallinity and morphology is reported. Moreover, the studies demonstrate that the interface modification of the TiO₂ ETL with C₆₀‐BCT@Au NPs substantially improves the charge extraction efficiency from the perovskite layer and suppresses charge recombination processes. Consequently, the resulting device yields a champion efficiency of 19.08% as well as devaluation in hysteresis. In addition, the unencapsulated PSCs with c‐TiO₂/C₆₀‐BCT@Au NPs ETL retain 83% and 90% of their original PCEs after 500 h storage in air and exposure to continuous UV illumination for 200 h, respectively. This study provides an effective method to address the electron transporting issues between perovskite and c‐TiO₂ ETL for developing stable and efficient PSCs.

The film quality of perovskite active layer is crucial for achieving high efficiency of perovskite solar cells. The antisolvent treatment method is a successful technique to improve the film quality. The fundamentals of antisolvent treatment, various antisolvent treatment methods, issues with antisolvents, and alternative methods are discussed.

Abstract

Organic–inorganic metal halide perovskite solar cells are emerging as potential solar energy harvesting tools and can be a tough competitor to already matured solar cell technologies. The success of perovskite solar cells is attributed to superior optoelectronic properties of perovskites, feasible synthesis process, and low fabrication cost. Though perovskite solar cells confront perovskite film quality related issues, such as rough surface, pinholes (which result in poor device performance) at the initial stages, many techniques have been developed to improve the perovskite film quality. Among these developed techniques, the antisolvent treatment method is certainly one of the most successful techniques till date. Antisolvent treatment increases the nucleus density during film formation to produce uniform and pinhole‐free perovskite film, which facilitates improved solar cell efficiency, low hysteresis, and stability. Interestingly, many of the best efficiency perovskite solar cells till date have been produced by the antisolvent treatment. This review discusses the fundamentals of antisolvent treatment, various aspects of antisolvent application on perovskite film, different issues with antisolvent usage, and alternatives techniques for perovskite film quality improvement.

The whole crystallization pathways and mechanism of two‐step fabricated perovskites are unveiled by in situ grazing‐incidence wide‐angle X‐ray scattering measurements and density functional theory calculations. Sequential A‐site doping of Cs⁺ and GA⁺ is found to alter the crystallization kinetics and improves the perovskite film morphology, giving rise to device efficiency as high as 23.5%.

Abstract

Two‐step‐fabricated FAPbI₃‐based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI₃‐based perovskite film with sequential A‐site doping of Cs⁺ and GA⁺ is revealed by in situ X‐ray scattering and first‐principles calculations. Incorporating Cs⁺ in the first step induces an alternative pathway from δ‐CsPbI₃ to perovskite α‐phase, which is energetically more favorable than the conventional pathways from PbI₂. However, pinholes are formed due to the nonuniform nucleation with sparse δ‐CsPbI₃ crystals. Fortunately, incorporating GA⁺ in the second step can not only promote the phase transition from δ‐CsPbI₃ to the perovskite α‐phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two‐step process over the one‐step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.

A biological polymer is employed to regulate the arrangement of SnO₂ nanocrystals on a substrate and induce vertical crystal growth of a perovskite layer on top. The enhanced interface contact between the electron‐transport layer and the perovskite layer significantly contributes to the improvement of efficiency and stability of derived planar perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar‐structured PSCs can be fabricated with low‐temperature (≤150 °C) solution‐based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO₂ electron‐transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO₂ nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO₂–HP‐based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open‐circuit voltage (V _OC) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP‐modified SnO₂ ETL exhibits promising potential for application in flexible and large‐area devices.

A narrow‐bandgap polymer acceptor L14 with an acceptor–acceptor (A–A) backbone is synthesized, showing lower‐lying frontier molecular orbitals, higher electron mobility, and larger absorption coefficient without sacrificing photovoltage compared to its donor–acceptor (D–A) analog polymer, L11. When applied in all‐polymer solar cells, L14 yields an outstanding efficiency of 14.3%.

Abstract

Narrow‐bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow‐bandgap polymer acceptor L14, featuring an acceptor–acceptor (A–A) type backbone, is synthesized by copolymerizing a dibrominated fused‐ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A–A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low‐lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open‐circuit voltage (V _oc), which is attributed to a small nonradiative recombination loss (E _loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V _oc, an excellent efficiency of 14.3% is achieved, which is among the highest values for all‐polymer solar cells (all‐PSCs). The results demonstrate the superiority of narrow‐bandgap A–A type polymers for improving all‐PSC performance and pave a way toward developing high‐performance polymer acceptors for all‐PSCs.

Three benzo[1,2‐b:4,5‐b']dithiophene‐thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione‐based polymer donors named as PBDT‐X (X=H, F, Cl) are developed. While a planar accepting unit helps improve the crystallinity, all three photovoltaic parameters are simultaneously increased with the introduction of halogen atoms. PBDT‐Cl:Y6‐based devices yield an efficiency of 15.63%, attributed to the enhanced crystallinity, hole mobility, and domain purity.

Abstract

In this work, a new series of polymer donors consisting of thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione (8ttTPD) and benzo[1,2‐b:4,5‐b']dithiophene (BDT) units for producing highly efficient organic solar cells (OSCs) paired with a Y6 acceptor is developed. The incorporation of the highly planar 8ttTPD unit enhances crystalline properties as well as hole mobilities of the BDT‐based polymers that typically have amorphous features. Further, the 2D side chains with halogen atoms (fluorine and chlorine) are designed as another handle to control the crystallinity and energy levels of the BDT‐based polymer donors: PBDT‐X (X = H, F, or Cl). Synergistic effects of incorporated 8ttTPD unit and the halogenated 2D side chain generate significantly enhanced charge transport and recombination properties of the OSCs, which is mainly attributed to optimized crystallinity and hole mobility of the polymer donors. Therefore, the PBDT‐Cl:Y6‐based OSCs exhibit the highest power conversion efficiency (PCE) of 15.63% with simultaneous improvements of open‐circuit voltage, short‐circuit current density, and fill factor, which outperforms the PCEs of PBDT‐H:Y6 (11.84%) and PBDT‐F:Y6 (14.86%).

Three new dithieno[2,3‐d;2ʹ,3ʹ‐dʹ]benzo[1,2‐b;4,5‐bʹ]dithiophene based small‐molecule donors with different branching points for alkyl side chains are designed and synthesized for all small molecular organic solar cells. Modifying the branching points tunes the properties in the aggregation state, and an optimal nanofiber‐based hierarchical morphology for efficient charge separation and transport is successfully demonstrated.

Abstract

The optimization of bulk heterojunction morphology is one of the most challenging topics in all‐small‐molecule organic solar cells. Herein, three small molecular donors based on dithieno[2,3‐d;2′,3′‐d′]benzo[1,2‐b;4,5‐b′]dithiophene (DTBDT) unit by systematically moving the branching point of the alkyl chain have been designed, synthesized, and applied in organic solar cells. Modifying the branching points enables the properties of the aggregation state to be tuned, and an efficient nanofiber‐based hierarchical morphology is successfully demonstrated by combining with different nonfullerene acceptors. The molecules with far branching points can form nanofibers in active layers, and theses nanofibers help the charge separation and charge transport in a large donor‐rich or acceptor‐rich domain of approximately 100 nm. Using nonfullerrene Y6 as an acceptor, the highest power conversion efficiency of 14.78% is obtained, which is one of the highest efficiencies in all‐small‐molecule organic solar cells. The strategy of modification of alkyl side chain branching points can be a practical way to actualize crystallinity control and active layer morphology for improving the performance of all‐small‐molecule organic solar cells.

N‐functionalized conjugation engineering is explored for the design of 2D asymmetric nonfullerene acceptors (NFAs). Endowed with high charge transport and good nanofibrous phase separation with PBDB‐T, the best‐performing NFA contributes a champion power conversion efficiency of 14.02% in solar cells. Excellent thermal aging and light soaking stability are observed for the corresponding solar cells.

Abstract

The charge transport and morphology of active layers are key considerations for device performance and stability in organic solar cells (OSCs). Such properties can be fine‐tuned via elaborate molecular design of fused‐ring electron acceptors (FREAs), especially conjugation extension and side chain engineering. In this work, N‐functionalized conjugation is explored in the design of high‐efficient asymmetric FREAs. The twisting of N‐conjugated side chains from backbone endows three FREAs with similar energy levels and light absorptions (≈850 nm edge). Their blends with PBDB‐T exhibit high charge carrier mobility and ordered phase separation. Excitingly, IPT2F‐TT based OSCs yield a champion power conversion efficiency (PCE) of 14.02% with a fill factor (FF) of 75.06%, outperforming PBDB‐T devices with IPT2F‐Th (12.52%, 71.20%), IPT2F‐Ph (13.13%, 72.11%), and octylated IPT‐2F (13.70%, 71.50%). The PCE over 14% and FF over 75% are among the highest values for 2D FREAs OSCs reported to date. More importantly, outstanding thermal stability and light soaking stability are observed with PCE over 12% maintained after thermal or light aging for 100 h. This work demonstrates N‐conjugated FREAs design as an effective strategy to simultaneously improve the photovoltaic performance and device stability for the OSCs.

Defect tolerance of metal‐halide perovskites is a commonly evoked concept to explain the development of high efficiency solar cells upon solution processing. However, moving the attention to solar cell stability, these materials seem to be defect intolerant. Further material engineering is needed to obtain a 100% defect tolerant materials platform.

Abstract

Metal‐halide perovskites present exceptional optoelectronic properties such as large light absorption coefficients, long free charge carrier diffusion lengths with ambipolar character. They are apparently protected by what is often described as a “defect tolerance” which has allowed to achieve, relatively quickly, highly performing devices. Nevertheless, there also exists a “defect intolerance” when it is dealt with stability. Further rationalization of the passivation strategies, especially for complex chemical systems, will be beneficial to achieve a full materials library which can be the platform for an efficient and reliable technology.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

In article number https://doi.org/10.1002/aenm.2020004532000453, Weili Yu, Chunlei Guo and co‐workers demonstrate that by combining the space‐limiting technique and the anti‐temperature crystallization method, millimeter sized MAPbI₃ perovskite monocrystals with thickness from tens of nanometers to micrometers can be fabricated. Solar cells based on the 300 nm thick perovskite monocrystal are prepared, which show 3% enhancement in power conversion efficiency (PCE) compared to their polycrystalline counterparts. This work provides a scalable way to synthesize high quality perovskite crystals with less grains and grain boundaries, is believed a key step to develop perovskite single crystal solar cells with high performance.

Nature Materials, Published online: 14 September 2020; doi:10.1038/s41563-020-0784-7

The development of perovskite emitters, their use in light-emitting devices, and the challenges in enhancing the efficiency and stability, as well as reducing the potential toxicity of this technology are discussed in this Review.

Nature Communications, Published online: 09 September 2020; doi:10.1038/s41467-020-18373-0

Simultaneously achieving high efficiency and mechanical robustness is challenging for ultraflexible organic solar cells. Here, Qin et al. present a robust interlayer of Zinc-chelated polyethylenimine (PEI-Zn) to facilitate the demonstration of efficient and mechanically robust ultraflexible solar cells.

The incorporation of various lanthanides ions in perovskite films (perovskite solar cells (PSCs)) and Ce³⁺ doping achieves the best performance, with a champion power conversion efficiency of 21.67% in contrast to 18.50% for pristine PSCs and outstanding long‐term and UV stability that originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region.

Abstract

Since Yan's work, incorporation of some lanthanide elements, such as Eu and Nd, into MAPbI₃ layer has been proven to be a powerful strategy on improving the permanence of the perovskite solar cells (PSCs). However, a comprehensive configuration has not been given for different lanthanide elements doping while the mechanism has not been clarified. Herein, the incorporation of various lanthanides ions (Ln³⁺ = Ce³⁺, Eu³⁺, Nd³⁺, Sm³⁺, or Yb³⁺) into perovskite films to largely enhance the performance of PSCs is presented. Arising from the enlarged grain size and crystallinity of perovskite film upon Ln³⁺ ions doping, the efficiency and stability of PSCs are significantly improved. Extraordinarily, PSCs with Ce³⁺ doping achieve the best performance, with a champion power conversion efficiency (PCE) of 21.67% in contrast to 18.50% for pristine PSCs, and outstanding long‐term and UV irradiation stability. Such high performance of PSCs after Ce³⁺ doping originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region. Finally, the flexible PSCs with low‐temperature preparation are explored. Considering the richer deposition of cerium element in the earth and lower price, the findings may provide new opportunities for developing low‐cost, highly efficient, air/UV stable, and flexible PSCs.

Herein, the recent research advances in 2D phosphorene‐analog in‐plane structural anisotropic group IV_A–VI metal monochalcogenides (MMCs) are presented, with a focus on their unique wavy crystal structures, their exotic physical properties and applications. The in‐depth synthesis and characterization methods as well as applications related to the anisotropic response of 2D MMCs in emerging electronics and photonics are concretely summarized.

Abstract

The family of emerging low‐symmetry and structural in‐plane anisotropic two‐dimensional (2D) materials has been expanding rapidly in recent years. As an important emerging anisotropic 2D material, the black phosphorene analog group IV_A–VI metal monochalcogenides (MMCs) have been surged recently due to their distinctive crystalline symmetries, exotic in‐plane anisotropic electronic and optical response, earth abundance, and environmentally friendly characteristics. In this article, the recent research advancements in the field of anisotropic 2D MMCs are reviewed. At first, the unique wavy crystal structures together with the optical and electronic properties of such materials are discussed. The Review continues with the various methods adopted for the synthesis of layered MMCs including micromechanical and liquid phase exfoliation as well as physical vapor deposition. The last part of the article focuses on the application of the structural anisotropic response of 2D MMCs in field effect transistors, photovoltaic cells nonlinear optics, and valleytronic devices. Besides presenting the significant research in the field of this emerging class of 2D materials, this Review also delineates the existing limitations and discusses emerging possibilities and future prospects.