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Publication date: July 2021

Source: Nano Energy, Volume 85

Author(s): Jianxing Xia, Ruiling Zhang, Junsheng Luo, Hua Yang, Hongyu Shu, Haseeb Ashraf Malik, Zhongquan Wan, Yu Shi, Keli Han, Ruilin Wang, Xiaojun Yao, Chunyang Jia

Publication date: July 2021

Source: Nano Energy, Volume 85

Author(s): Cheng Chen, Tongtong Xuan, Wenhao Bai, Tianliang Zhou, Fan Huang, An Xie, Le Wang, Rong-Jun Xie

Gradient 1D/3D perovskite bilayer using 4-tert-butylpyridinium cation (TBP⁺) as a cation for 1D perovskite layer is introduced. The crystal structure and fundamental properties of 1D perovskite (TBPPbI₃) are investigated. Gradient 1D/3D perovskite-based devices show an improved power conversion efficiency from 18.3% to 19.3% due to the enhanced hole extraction process and suppressed carrier recombination.

To achieve both high efficiency and long-term stability of perovskite solar cells (PSCs), it is effective to use a perovskite layer in which a low-dimensional perovskite layer is stacked on a 3D perovskite layer. However, the guidelines for the effective structure of these perovskite layers remain unclear. Herein, the gradient structured 1D perovskite layer formed on top of a 3D perovskite layer using 4-tert-butylpyridinium iodide (TBPI) as a capping layer is introduced. It is demonstrated that the gradient structured 1D perovskite layer on the 3D perovskite improves the conversion efficiency of PSCs despite the lateral orientation of the (PbI₃ ⁻) _n linear chain of the 1D perovskite, which is responsible for electronic conduction. In addition, it is found that the hydrophobic organic unit of TBPI protects the 3D perovskite layer, which enhances its long-term stability.

Polymer Solar Cells

In article number 2000661, Jijun Zhao and co‐workers authors designed 2D Ruddlesden‐Popper Perovskites to simultaneously enhance the stability and efficiency. The (FPEA)₂(FA)₈Pb₉I₂₈ film is exceptionally vertically orientated and exhibits an extended absorption edge. A high‐performance solar cell with 16.15% efficiency is achieved. Its unencapsulated device maintains 95% of its stating PCE after 2112 h (30–70% RH).

In article number 2004092, Shigeo Maruyama, Phillip Lee, Il Jeon, and co‐workers report carbon nanotube‐embedded ultra‐thin polyimide conductor‐based foldable perovskite solar cells. The foldable photovoltaic device exhibits 15.2% of power output and withstands 10 000 cycles of folding test at a bending radius of 0.5 mm. Such high efficiency and mechanical robustness are attributed to 7‐µm‐thin and smooth carbon nanotube‐polyimide complex film, which also induces strong and stable MoOx doping.

A new urea-ammonium thiocyanate (UAT) molten salt was introduced as the additive in all-inorganic cesium lead triiodide solar cell, as a modification strategy to fully release and exploit coordination activities of SCN⁻ to deposit high-quality CsPbI₃ film. Thus, the UAT-based devices can provide an encouraging PCE up to 20.08 % with excellent operational stability of over 1000 h.

Abstract

Besides widely used surface passivation, engineering the film crystallization is an important and more fundamental route to improve the performance of all-inorganic perovskite solar cells. Herein, we have developed a urea-ammonium thiocyanate (UAT) molten salt modification strategy to fully release and exploit coordination activities of SCN⁻ to deposit high-quality CsPbI₃ film for efficient and stable all-inorganic solar cells. The UAT is derived by the hydrogen bond interactions between urea and NH₄ ⁺ from NH₄SCN. With the UAT, the crystal quality of the CsPbI₃ film has been significantly improved and a long single-exponential charge recombination lifetime of over 30 ns has been achieved. With these benefits, the cell efficiency has been promoted to over 20 % (steady-state efficiency of 19.2 %) with excellent operational stability over 1000 h. These results demonstrate a promising development route of the CsPbI₃ related photoelectric devices.

UV light and surface defects accelerate the degradation process of perovskite solar cells (PSCs). In their Communication on page 8673, Mingzhu Li, Yanlin Song, and co‐workers utilize the tautomerism of “sunscreen” molecules under UV light illumination to protect the PSC from degradation and enable molecular defect passivation, achieving high efficiency and long‐term UV stability of PSCs.

An easily synthesized building block BNT and a relevant polymer PBNT‐BDD featuring B‐N covalent bond were synthesized for application in solar cells. The polymer offered a power conversion efficiency of 16.1 %, a nonradiative recombination energy loss of 0.19 eV, and a singlet‐triplet gap as low as 0.15 eV, demonstrating the promising prospect of B‐N‐containing materials in organic photovoltaics.

Abstract

High‐efficiency organic solar cells (OSCs) largely rely on polymer donors. Herein, we report a new building block BNT and a relevant polymer PBNT‐BDD featuring B‐N covalent bond for application in OSCs. The BNT unit is synthesized in only 3 steps, leading to the facile synthesis of PBNT‐BDD. When blended with a nonfullerene acceptor Y6‐BO, PBNT‐BDD afforded a power conversion efficiency (PCE) of 16.1 % in an OSC, comparable to the benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based counterpart. The nonradiative recombination energy loss of 0.19 eV was afforded by PBNT‐BDD. PBNT‐BDD also exhibited weak crystallinity and appropriate miscibility with Y6‐BO, benefitting of morphological stability. The singlet–triplet gap (ΔE _ST) of PBNT‐BDD is as low as 0.15 eV, which is much lower than those of common organic semiconductors (≥0.6 eV). As a result, the triplet state of PBNT‐BDD is higher than the charge transfer (CT) state, which would suppress the recombination via triplet state effectively.

We developed a size effect that controls the crystallization of perovskite and enhances the passivation effect with g-C₃N₄ additive. The similarity in value of lattice constant and distance between double hydrogen binding sites affects crystallization. The combination allows g-C₃N₄ to cover tin-based perovskite, thus improves hydrophobicity and oxidation resistance of films and leads to splendid PCE of wearable device with excellent stability.

Abstract

Tin-based perovskite solar cells (PSCs) demonstrate a potential application in wearable electronics due to its hypotoxicity. However, poor crystal quality is still the bottleneck for achieving high-performance flexible devices. In this work, graphite phase-C₃N₄ (g-C₃N₄) is applied into tin-based perovskite as a crystalline template, which delays crystallization via a size-effect and passivates defects simultaneously. The double hydrogen bond between g-C₃N₄ and formamidine cation can optimize lattice matching and passivation. Moreover, the two-dimensional network structure of g-C₃N₄ can fit on the crystals, resulting an enhanced hydrophobicity and oxidation resistance. Therefore, the flexible tin-based PSCs with g-C₃N₄ realize a stabilized power conversion efficiency (PCE) of 8.56 % with negligible hysteresis. In addition, the PSCs can maintain 91 % of the initial PCE after 1000 h under N₂ environment and keep 92 % of their original PCE after 600 cycles at a curvature radius of 3 mm.

The work presents a detailed understanding of solution-processing-dependent quantum well growth and its impact on charge transport and photovoltaic performance for Dion–Jacobson perovskite. Faster solvent removal during film formation leads to a gradient distribution of the quantum wells and a preferential perpendicular orientation. The highest efficiency of 15.81% for aromatic spacer-based Dion–Jacobson perovskite solar cells is achieved.

Abstract

Dion–Jacobson (DJ) 2D hybrid perovskite semiconductors offer improved environmental stability and higher structural diversity in comparison with their 3D analogous. However, lacking of controlled perovskite crystallization makes it a challenge to achieve high charge transport for photovoltaic devices. Here, a detailed understanding of effects on film formation during different solution-casting processes for the DJ perovskite (PDMA)(MA) _{n −1}Pb _n I_{3 n +1} (<n> = 4, PDMA refers to 1,4-phenylenedimethanammonium) in the final film structure and photovoltaic outcomes is presented. Faster removal of solvent from solution via hot-casting or antisolvent dripping results in a more uniform thickness distribution of quantum wells. This eventually enhances carrier transport greatly along perpendicular direction and increases power conversion efficiencies. A high efficiency of 15.81% is achieved for the hot-casting devices, which is also the highest for aromatic spacer-based DJ perovskite solar cells. This work helps to better understand the control of film formation during solution-casting for high performance solar cells.

This work presents a comprehensive review on the current understanding, and apparent contradictions, of experimental observation, interpretation, and theoretical hypotheses presented in the state-of-the-art mixed dimensional 2D-3D perovskite literature and identifies promising future research directions for enhancing the stability and performance of such devices.

Abstract

Perovskite solar cells are a potential game changer for the photovoltaics industry, courtesy of their facile fabrication and high efficiency. Despite this, commercialization is being held back by poor stability. To become economically feasible for commercial production, perovskite solar cells must meet or exceed industry standards for operational lifetime and reliability. In this regard, mixed dimensional 2D-3D perovskite solar cells, incorporating long carbon-chain organic spacer cations, have shown promising results, with enhancement in both device efficiency and stability. Dimensional engineering of perovskite films requires a delicate balance of 2D and 3D perovskite composition to take advantage of the specific properties of each material phase. This review summarizes and assesses the current understanding, and apparent contradictions in the state-of-the-art mixed dimensional perovskite solar cell literature regarding the origin of stability and performance enhancement. By combining and comparing results from experimental and theoretical studies it is focused on how the perovskite composition, film formation methods, additive and solvent engineering influence efficiency and stability, and identify future research directions to further improve both key performance metrics.

Bromide-rich regions with a higher Young's Modulus form mechanical boundaries that define mechanical domains in mixed-halide perovskite films. These mechanical domains are smaller than the morphological grains. Light soaking induces chemical reorganization that leads to a homogenization of the mechanical properties in the film, with photobrightening processes occurring concomitantly.

Abstract

Halide perovskites are a versatile class of semiconductors employed for high performance emerging optoelectronic devices, including flexoelectric systems, yet the influence of their ionic nature on their mechanical behavior is still to be understood. Here, a combination of atomic-force, optical, and compositional X-ray microscopy techniques is employed to shed light on the mechanical properties of halide perovskite films at the nanoscale. Mechanical domains within and between morphological grains, enclosed by mechanical boundaries of higher Young's Modulus (YM) than the bulk parent material, are revealed. These mechanical boundaries are associated with the presence of bromide-rich clusters as visualized by nano-X-ray fluorescence mapping. Stiffer regions are specifically selectively modified upon light soaking the sample, resulting in an overall homogenization of the mechanical properties toward the bulk YM. This behavior is attributed to light-induced ion migration processes that homogenize the local chemical distribution, which is accompanied by photobrightening of the photoluminescence within the same region. This work highlights critical links between mechanical, chemical, and optoelectronic characteristics in this family of perovskites, and demonstrates the potential of combinational imaging studies to understand and design halide perovskite films for emerging applications such as photoflexoelectricity.

In this progress report, the recent developments of halide perovskites in powder form and their film processing approaches are focused on. The current limitations of these methods are shown, and the advantages and opportunities of powder‐based halide perovskite processing are also highlighted.

Abstract

Halide perovskites have undergone an impressive development and could be used in a wide range of optoelectronic devices, where some of them are already at the edge of commercialization, e.g., perovskite solar cells. Recently, interest in perovskites in powder form has increased, as for example, they are found to exhibit high stability and allow for easy production of large quantities. Accordingly, also the topic of processing thin and thick films on the basis of perovskite powders is currently gaining momentum. Here, perovskite powder can form the basis for both, typical wet and solvent‐based processing approaches, as well as for dry processes. In this Progress Report, the recent developments of halide perovskites in powder form and of film processing approaches are summarized that are based on them. The advantages and opportunities of the different processing methods are highlighted, but their individual drawbacks and limitations are also discussed. Prospects are also pointed out and possible steps necessary to unlock the full potential of powder‐based processing methods for producing high quality thick and thin perovskite layers in the future are discussed.

In this study, the importance of terminal group match in the design of polymer donor and small-molecule acceptor for optimal blend morphology, reduced voltage loss, and high device performances are demonstrated.

Abstract

As a variety of non-fullerene small molecule acceptors (SMAs) have been developed to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the pairing of the SMAs with optimal polymer donors (P _Ds) is an important issue. Herein, a systematic investigation is conducted with the development of the SMA series, named C6OB-H, C6OB-Me, and C6OB-F, which contain distinctive terminal substituents –H, –CH₃, and –F, respectively. These SMAs are paired with two P _Ds, PBDT-H and PBDT-F. Interestingly, the P _D/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. In particular, the OSC based on the PBDT-F:C6OB-F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT-H:C6OB-F (10.1%) and PBDB-F:C6OB-H OSCs (11.2%). Furthermore, the PBDT-F:C6OB-F OSC maintains high PCEs with active layer thicknesses between 85 and 310 nm. In contrast, the PCE of PBDT-H:C6OB-F-based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. This study establishes an important P _D/SMA pairing rule in terms of terminal functional groups for achieving high-performance OSC.

A new series of conjugated molecules with spatially 2D sidechains are designed and utilized as the non-fullerene acceptors in all-small-molecule organic solar cells. The multi-dimensional lamellar packing induced by the orthogonal sidechains is able to tune the morphology as effective as the stacking of conjugated backbones, thus providing an impressive power conversion efficiency of 15.67%.

Abstract

Organic semiconductors consist of a conjugated backbone and flexible sidechains. Compared to the meticulous design of backbones, less attention has been paid to the investigation of sidechains, in particular their spatial orientation. Herein, three non-fullerene acceptors, anti-PDFC, syn-PDFC, and PDFC-Ph, are applied in all-small-molecule organic solar cells (ASM-OSCs) to reveal the varied effects of sidechains on morphology and device performance. With spatially orthogonal alkyl chains, anti-PDFC and syn-PDFC show unique bimodal lamellar packing and moderate crystallinity. When blending with an efficient binary BTR-Cl/Y6 system, anti-PDFC as well as syn-PDFC not only form their own crystal phase but also improve the packing order of BTR-Cl, consequently enhancing the power conversion efficiency (PCE) of ternary ASM-OSC to be 14.56%. However, although PDFC-Ph has an identical backbone with anti-PDFC, the alternated sidechains make it relatively amorphous, which is prone to damage the original packing of the host donor/acceptor, and thus deteriorating the device performance. When PC₇₁BM is added to optimize the morphology further, the triple-acceptor device involving anti-PDFC realizes a PCE of 15.67%, which is among the best efficiencies in ASM-OSCs. This study demonstrates that a multi-dimensional sidechain can optimize the morphology of a bulk heterojunction as effective as a conjugated backbone.

High quality, all inorganic perovskite crystalline grain can be grown by a solvent-free chemical vapor deposition method. Carefully controlling the growth conditions allows for single crystalline grain growth on a large scale. The single grain exhibits excellent opto-electronic properties which makes it an ideal candidate for devices like photo and radiation detectors.

Abstract

The chemical vapor deposition (CVD) method is a dry approach that can produce high quality crystals and thin films at large scale which can be easily adapted by industry. In this work, CVD technology is employed to grow high quality, large size all-inorganic cesium lead bromide perovskite crystalline film for the first time. The obtained films have millimeter size crystalline domains with high phase purity. The growth kinetics are examined in detail by optical microscopy and X-ray diffraction. The deposition rate and growth temperature are found to be the key parameters allowing to achieve large scale crystal growth. The large crystalline grains exhibit exceptional optical properties including negligible Stokes shift and uniform photoluminescence over a large scale. This suggests a high degree of crystallinity free from internal strain or defects. A lateral diode within one large crystalline grain is further fabricated and significant photo-generated voltage and short circuit current are observed, suggesting highly efficient carrier transport and collections without scattering within the grain. This demonstration suggests that the CVD grown all-inorganic perovskite thin films enable a promising fabrication route suitable for photovoltaic or photo-detector applications.

The significant advances in efficient photoabsorbent materials have been instrumental in the performance enhancement of semitransparent organic solar cells (ST‐OSCs) from <7% to 12–14% (with good visible transmittance) only in the last 3 years. This study reviews the progress of photoabsorbent materials for ST‐OSCs, and discusses the structure–property relationships and future perspectives for the development of multifunctional ST‐OSCs.

Abstract

Semi‐transparent organic solar cells (ST‐OSCs) have revolutionized the field of photovoltaics (PVs) due to their unique abilities, such as transparency and color tunability, and have transformed normal power‐harvesting OSC devices into multifunctional devices, such as building‐integrated photovoltaics, agrivoltaics, floating photovoltaics, and wearable electronics. Very recently, ST‐OSCs have seen remarkable progress, with a rapid increase in power conversion efficiency from below 7% to 12–14%, with an average visible transparency of 9–25%, especially due to the use of low bandgap semiconductors including polymer donors and non‐fullerene acceptors that exhibit absorption in the near‐infrared region as photoabsorbent materials. From this perspective, the latest developments in ST‐OSCs stemming from the innovations in photovoltaic materials that delivered multifunctional ST‐OSCs with top‐of‐the‐line power conversion efficiencies are discussed to shed light on the structure‐property relationship between molecular design and current challenges in this cutting‐edge research field. Finally, personal perspectives, including research directions for the future use of ST‐OSCs in multifunctional applications, are also proposed.

A self-sufficient micrometer-level vacuum growth chamber is proposed for MAPbBr₃ microcrystals to effectively prevent water and oxygen, and to greatly improve the thermal and optical stability by the reduction of deep level trap states.

Abstract

Perovskite materials and their optoelectronic devices have attracted intensive attentions in recent years. However, it is difficult to further improve the performance of perovskite devices due to the poor stability and the intrinsic deep level trap states (DLTS), which are caused by surface dangling bonds and grain boundaries. Herein, the CH₃NH₃PbBr₃ perovskite microcrystal is encapsulated by a dense Al₂O₃ layer to form a microenvironment. Through optical measurement, it is found that the structure of perovskite can be healed by itself even under high temperature and long-time laser illumination. The DLTS density decreases nearly an order of magnitude, which results in 4–14 times enhancement of light emission. The observation is ascribed to the micron-level environment, which serves as a self-sufficient high-vacuum growth chamber, where the components of the perovskite are completely retained when sublimated and the decomposed atoms can re-arrange after thermal treatment. The modified structure showing high thermal stability is able to maintain excellent optical and lasing stability up to 2 years. This discovery provides a new idea and perspective for improving the stability of perovskite and can be of practical interest for perovskite device application.

Reducing and mitigating non‐radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light‐emitting applications. Ethoxylated trimethylolpropane triacrylate is introduced in antisolvent to passivate surface and bulk defects during the spinning process, and external quantum efficiency of quasi‐2D perovskite light‐emitting diodes as high as 22.49% is demonstrated.

Abstract

Perovskite light‐emitting diodes (PeLEDs) are considered as particularly attractive candidates for high‐quality lighting and displays, due to possessing the features of wide gamut and real color expression. However, most PeLEDs are made from polycrystalline perovskite films that contain a high concentration of defects, including point and extended imperfections. Reducing and mitigating non‐radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light‐emitting applications. Here, ethoxylated trimethylolpropane triacrylate (ETPTA) is introduced as a functional additive dissolved in antisolvent to passivate surface and bulk defects during the spinning process. The ETPTA can effectively decrease the charge trapping states by passivation and/or suppression of defects. Eventually, the perovskite films that are sufficiently passivated by ETPTA make the devices achieve a maximum external quantum efficiency (EQE) of 22.49%. To our knowledge, these are the most efficient green PeLEDs up to now. In addition, a threefold increase in the T ₅₀ operational time of the devices was observed, compared to control samples. These findings provide a simple and effective strategy to make highly efficient perovskite polycrystalline films and their optoelectronics devices.