~2021~

218. "D-A-π-A-D-type Dopant-free Hole Transport Material for Low-Cost, Efficient, and Stable Perovskite Solar Cells"

Tianqi Niu, Weiya Zhu, Yiheng Zhang, Qifan Xue, Xuechen Jiao, Zijie Wang, Yue-Min Xie, Ping Li, Runfeng Chen, Fei Huang, Yuan Li, Hin-Lap Yip, Yong Cao

Joule.,  2021, DOI:https://doi.org/10.1016/j.joule.2020.12.003

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The development of low-cost and efficient hole transport materials (HTMs) is important for the commercialization of perovskite solar cells (PSCs). Comparing with the widely studied D-A-D and D-π-D linear-type small molecule HTMs, DTB-FL with a D-A-π-A-D molecular design is proposed, featuring facile synthesis and excellent optoelectronic properties. Moreover, the HTM with efficient surface passivation effects and proper energy level alignment at the hole extraction interface effectively inhibits recombination loss and improves the charge collection property. As a result, the champion efficiencies of 21.5% and 19.6% for active areas of 0.09 and 1.0 cm 2, respectively, with superior operational stability are achieved by using DTB-FL HTM. In addition, DTB-FL can also be used as efficient HTM for all-inorganic PSCs, producing an impressive PCE of 17.0% with a high V oc of 1.30 V. These results underscore the promising potential of the D-A-π-A-D molecular design in preparing low-cost dopant-free HTMs toward stable and efficient PSCs.

219. "Utilization of Trapped Optical Modes for White Perovskite Light-Emitting Diodes with Efficiency over 12%"

Ziming Chen, Zhenchao Li, Zhen Chen, Ruoxi Xia, Guangruixing Zou, Linghao Chu, Shi-Jian Su, Junbiao Peng, Hin-Lap Yip, Yong Cao

Joule.,  2021, DOI:https://doi.org/10.1016/j.joule.2020.12.008

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The inferior light extraction efficiency (LEE), which is generally less than 20%, based on optical modeling, and the difficulty in achieving white emission are the two main challenges in the metal-halide-perovskite light-emitting diode (PeLED) field. Herein, we report a simple and efficient approach to construct high-performance white PeLEDs with much-enhanced LEE by coupling a blue PeLED with a layer of red perovskite nanocrystal (PeNC) down-converter through a rationally designed multilayer semitransparent electrode (LiF/Al/Ag/LiF). The red PeNC layer allows the extraction of the trapped waveguide mode and surface plasmon polariton mode in a blue PeLED and converts them to red emission, resulting in over 50% LEE improvement. Simultaneously, the complementary emission spectrum of blue photons and down-converting red photons contributes to a white PeLED with a high external quantum efficiency and luminance of more than 12% and approximately 2,000 cd m −2, respectively, which represent state-of-the-art results in this field.

220. "Monolithic perovskite/organic tandem solar cells: Developments, prospects, and challenges"

Yue-Min Xie, Qifan Xue, Qin Yao, Shenkun Xie, Tianqi Niu, Hin-Lap Yip

Nano Select.,  2021, https://doi.org/10.1002/nano.202000287

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The recent progress made in perovskite‐based monolithic perovskite/organic tandem devices and perovskite/bulk‐heterojunction devices is systematically reviewed in terms of the sub‐cell and interconnecting layer properties. Specifically, the shortcomings in terms of the device photovoltaic parameters (PCE, FF, Jsc, and Voc) of perovskite/organic tandem devices compared with all‐perovskite tandem devices are systematically demonstrated.

221. "Synthesis and photovoltaic performance of a non-fullerene acceptor comprising siloxane-terminated alkoxyl side chain"

 Zhuhao Wu, Rihang Qiu, Haiying Jiang, Qian Wang, Yinchu Chen, Haizhen Liu, Shenkun Xie, Hin-Lap Yip, Lianjie Zhang, Junwu Chen

Organic Electronics.,  2021, 106087

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As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 105 M−1 cm−1 in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.

222. "High‐Performance Semi‐Transparent Organic Photovoltaic Devices via Improving Absorbing Selectivity"

Yaokai Li, Chengliang He, Lijian Zuo, Feng Zhao, Lingling Zhan, Xin Li, Ruoxi Xia, Hin‐Lap Yip, Chang‐Zhi Li, Xu Liu, Hongzheng Chen

Advanced Energy Materials.,  2021, 11, 2003408

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High‐performance organic semi‐transparent photovoltaic (ST‐OPV) devices are achieved by improving the light‐absorbing selectivity, that is, the light‐absorbing capability in invisible regions and light transmission in the visible region. Systematic optimization, including developing a numerical method for photo‐active layer screening, interface engineering, and optical manipulation, enables high‐performance ST‐OPVs with the best light utilization efficiency of 4.1%, ranking among the highest for ST‐OPVs.

223. "Materials, Photophysics and Device Engineering of Perovskite Light-Emitting Diodes"

Ziming Chen, Zhenchao Li, Tom Hopper, Artem A Bakulin, Hin-Lap Yip

Reports on Progress in Physics,  2021, https://doi.org/10.1088/1361-6633/abefba

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Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

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Low-dimensional metal halide perovskites have emerged as promising alternatives to the traditional three-dimensional (3D) components, due to their greater structural tunability and environmental stability. Dion-Jacobson (DJ) phase two-dimensional (2D) perovskites, which are formed by incorporating bulky organic diammonium cations into inorganic frameworks that comprises a symmetrically layered array, have recently attracted increasing research interest. The structure-property characteristics of DJ phase perovskites endow them with a unique combination of photovoltaic efficiency and stability, which has led to their impressive employment in perovskite solar cells (PSCs). Here, we review the achievements that have been made to date in the exploitation of DJ phase perovskites in photovoltaic applications. We summarize the various ligand designs, optimization strategies and applications of DJ phase PSCs, and examine the current understanding of the mechanisms underlying their functional behavior. Finally, we discuss the remaining bottlenecks and future outlook for these promising materials, and possible development directions of further commercial processes.

225. "Blue perovskite light-emitting diodes: opportunities and challenges"
        Guangruixing Zou, Ziming Chen, Zhenchao Li, Hin-Lap Yip
        Acta Phys. -Chim. Sin.,  2021, https://doi: 10.3866/PKU.WHXB202009002

Metal halide perovskites are considered as promising candidates for lighting applications owing to their excellent optoelectronic properties, such as high electron/hole mobility, high photoluminescence quantum yield, high color purity, and facile color tunability. In recent years, perovskite light-emitting diodes (LEDs) have developed rapidly, and their external quantum efficiencies (EQEs) have exceeded 20% for green and red emissions. However, the EQEs and stabilities of blue (particularly deep-blue) perovskite LEDs are still inferior to the green and red counterparts, which severely restricts the application of perovskite LEDs in high-performance and wide color gamut displays as well as white light illumination. Therefore, summarizing the development of blue perovskite LEDs and discussing the opportunities and challenges associated with their future applications will help to guide the further development of the entire perovskite LED field.

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226. "Metal-Halide Perovskite Crystallization Kinetics: A Review of Experimental and Theoretical Studies"
        Yue-Min Xie, Qifan Xue, and Hin-Lap Yip
        Adv. Energy Mater.,  2021,  https://doi.org/10.1002/aenm.202100784

Metal-halide perovskites (MHPs) are regarded as ideal photovoltaic materials because of their variable crystal material composition and superb optoelectronic performance. However, this compositional variability results in a complicated crystallization process during MHP film fabrication, leading to reduced MHP film crystallinity and decreased performance of devices containing such films. The crystallization kinetics of MHPs have therefore been extensively explored in efforts to determine the effect of crystallization properties on MHP film properties and figure out the corresponding modulating strategies. Here, the first comprehensive review of reported studies on the crystallization properties of 3D MHPs is presented. The experimental and theoretical research on 3D MHP crystallization kinetics is systematically surveyed, and the methods that are used for modulating MHP crystallization are summarized, namely, solution engineering, compositional engineering, interfacial engineering, and additive passivation. Meanwhile, the prospects and current challenges in revealing perovskite crystallization kinetics are suggested.

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227. "Tandem Organic Solar Cells with 18.7% Efficiency Enabled by Suppressing the Charge Recombination in Front Sub‐Cell"
        Gongchu Liu, Ruoxi Xia, Qiri Huang, Kai Zhang, Zhicheng Hu, Tao Jia, Xiang Liu, Hin‐Lap Yip, Fei Huang
        Adv. Func. Mater.,  2021,  https://doi.org/10.1002/adfm.202103283

The maximum photocurrent in tandem organic solar cells (TOSCs) is often obtained by increasing the thicknesses of sub-cells, which leads to recombination enhancement of such devices and compromises their power conversion efficiency (PCE). In this work, an efficient interconnecting layer (ICL) is developed, with the structure ZnO NPs:PEI/PEI/PEDOT:PSS, which enables TOSCs with very good reproducibility. Then, it is discovered that the optimal thickness of the front sub-cell in such TOSCs can be reduced by increasing the proportion of a non-fullerene acceptor in the active layer. The non-fullerene acceptor used in this work has a much larger absorption coefficient than the donor in the front sub-cell, and the absorption reduction of donor can be well complemented by that of the acceptor when increasing the acceptor proportion, thus leading to a significant overall absorption enhancement even with a thinner film. As a result, the optimal

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thickness of the front sub-cell is reduced and its charge recombination is suppressed. Ultimately, the use of this ICL combined with fine-turning of the composition in the front sub-cell enables an efficient TOSC with a very high fill factor of 78% and an excellent PCE of 18.71% (certified by an accredited institute to be 18.09%) to be obtained.

228. "Surpassing 13% Efficiency for Polythiophene Organic Solar Cells Processed from Nonhalogenated Solvent"
        Jingyang Xiao, Xiao'e Jia, Chunhui Duan, Fei Huang, Hin‐Lap Yip, Yong Cao
        Adv. Mater.,  2021, https://doi.org/10.1002/adma.202008158

Benefiting from low cost and simple synthesis, polythiophene (PT) derivatives are one of the most popular donor materials for organic solar cells (OSCs). However, polythiophene-based OSCs still suffer from inferior power conversion efficiency (PCE) than those based on donor–acceptor (D–A)-type conjugated polymers. Herein, a fluorinated polythiophene derivative, namely P4T2F-HD, is introduced to modulate the miscibility and morphology of the bulk heterojunction (BHJ)-active layer, leading to a significant improvement of the OSC performance. The Flory–Huggins interaction parameters calculated from the surface energy and differential scanning calorimetry results suggest that P4T2F-HD shows moderate miscibility with the popular nonfullerene acceptor Y6-BO (2,2′-((2Z,2′Z)-((12,13-bis(2-butyloctyl) -3,9-diundecyl-12,13-dihydro-[1,2,5] thiadiazolo [3,4 e] thieno [2′,3′:4′,5′] thieno [2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis (methanylylide-ne)) bis (5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), while poly(3-hexylthiophene) (P3HT) is very miscible with Y6-BO. As a result, the P4T2F-HD case forms desired nanoscale phase separation in the BHJ film while the P3HT case forms a completely mixed BHJ film, as revealed by transmission electron microscopy (TEM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). By optimizing the cathode interface and the morphology of the P4T2F-HD:Y6-BO films processed from nonhalogenated solvents, a new record PCE of 13.65% for polythio-phene -based OSCs is demonstrated. This work highlights the importance of controlling D/A interactions for achieving desired morphology and also demonstrates a promising OSC system for potential cost-effective organic photovoltaics.

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229. "Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells"
        Tianqi Niu, Qifan Xue, Hin-Lap Yip
        J. Phys. Chem. Lett.,  2021, https://doi.org/10.1021/acs.jpclett.1c00954

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Perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation thin-film photovoltaic technology owing to their excellent optoelectronic properties and cost-effectiveness. To gain the full potential of device performance, an in-depth understanding of the surface/interface science is an urgent need. Here, we present a review of molecularly engineered studies on interface modifications of PSCs. We elaborate a systematic classification of the existing optimization techniques employed in molecularly engineered perovskite and interface materials and analyze the insights underlying the reliability issues and functional behaviors. The achievements allow us to highlight the crucial strengths of molecular design for further tailoring of the interfacial properties, mitigating the nonradiative losses, optimizing the device performance, and retarding the degradation process of PSCs. Finally, the remaining challenges and potential development directions of molecularly engineered interfaces for high-performance and stable PSCs are also proposed.

230. "Architecturing 1D‐2D‐3D Multidimensional Coupled CsPbI2Br Perovskites toward Highly Effective and Stable Solar Cells"
        Kun Liu, Songyang Yuan, Yeming Xian, Yi Long, Qin Yao, Naveed Ur Rahman, Yang Guo, Mingyuan Sun, Qifan Xue, Hin‐Lap            Yip, Andreu Cabot, Wenzhe Li, Jiandong Fan
        Small,  2021, https://doi.org/10.1002/smll.202100888

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Despite the rapid development of CsPbIxBr3−x (0 ≤ x ≤ 3) inorganic perovskite solar cells, associated with their superior thermal stability, their low moisture stability limits their commercial deployment. In this study, 1D-2D-3D multidimensional coupled perovskites are prepared by means of an in situ self-integration approach. This pioneering method allows incorporating thus far unreported 1D-Tpy2Pb3I6 and 2D-TpyPb3I6 (Tpy; terpyridine) perovskites. Heterojunction perovskites demonstrate superior stability against water in comparison with control 3D CsPbI2Br, which is related to the hydrophobicity of low-dimension (LD) perovskites. Remarkably, the spontaneous involvement of LD perovskites can adjust/reconstruct the interfacial structure. This modification allows releasing the residual strain, establishing effective charge transfer channels that increase the carrier transport ability. Accordingly, 1D-2D-3D hybrid CsPbI2Br perovskite solar cells demonstrate a stabilized power conversion efficiency as high as 16.1%, which represents a very significant improvement, by a factor of 43%, with respect to control 3D CsPbI2Br perovskite solar cell. Equally importantly, the multidimensional coupled perovskite solar cells exhibit extraordinary stability, well above 1000 h in ambient atmosphere.

231. "Semitransparent organic solar cells based on all-low-bandgap donor and acceptor materials and their performance potential"
        Ting Jiang, Guichuan Zhang, Ruoxi Xia, Jun Huang, Xin Li, Ming Wang, Hin-Lap Yip, Yong Cao
        Materials Today Energy,  2021, https://doi.org/10.1016/j.mtener.2021.100807

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Recent advances in organic solar cells (OSCs) based on large-bandgap donors and low-bandgap non-fullerene acceptors (NFAs) have increased the power conversion efficiency (PCE) of OSCs to ~18%. However, these state-of-the-art OSCs have strong absorption in the visible region, limiting their application in semitransparent organic solar cells (STOSCs). In this study, an all-low-bandgap system based on a low-bandgap polymer donor (PM2) and a low-bandgap NFA (Y6-BO), was introduced as the light-harvesting layer for STOCSs with absorption mainly localized in the near-infrared (NIR) spectrum from 600 to 900 nm. The corresponding opaque OSCs exhibited the highest PCE among reported all-low-bandgap OSC systems, and the corresponding STOSCs showed higher visible light transmittances (VLTs) and light utilization efficiencies (LUEs) than the reference devices based on state-of-the-art PM6:Y6-BO OSC system with broad range absorption from visible to NIR. Optical simulations predicted that the PM2:Y6-BO-based STOSCs have a greater potential to realize higher VLTs and PCEs and better PCE retention (PCEsemitransparent/PCEopaque) than those from the PM6:Y6-BO-based STOSCs. Guided by these simulations, PM2-based STOSCs with VLTs exceeding 40% and PCEs of ~6% were achieved. Further recombination analysis suggested that the PM2-based devices experienced more severe charge recombination and energy losses, indicating there is further room for PCE improvement by designing new all-low-bandgap systems. Overall, this work shows the great potential of all-low-bandgap systems in realizing STOSCs with high PCEs and VLTs, which is promising for the commercialization of OSCs as power-generating window applications.

232. "Color‐Stable Deep‐Blue Perovskite Light‐Emitting Diodes Based on Organotrichlorosilane Post‐Treatment"
        Guangruixing Zou, Zhenchao Li, Ziming Chen, Linghao Chu, Hin‐Lap Yip, Yong Cao
        Adv. Func. Mater.,  2021, https://doi.org/10.1002/adfm.202103219

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Recent studies of sky-blue perovskite light-emitting diodes (PeLEDs) have extensively promoted optimal device design to achieve an external quantum efficiency (EQE) above 12%. However, the development of thin-film deep-blue PeLEDs lags dramatically behind, especially with regards to meeting the latest Rec. 2020 standard. A trichloro(3,3,3-trifluoropropyl) silane post-treatment that drives the emission of perovskite into the deep-blue region, ranging from 440 to 460 nm, which meets the Rec. 2020 standard, is proposed. The chlorine ions released from the organotrichlorosilane molecules during their polycondensation reaction provide an addition halide source to fine tune the composition of the mixed halide perovskite films, leading to increase of bandgap and deep-blue emission. In addition, hydrogen bonds between the hydroxy groups of silane molecules and halide anions in perovskite can suppress ion migration for improving emission stability.

As a result, an optimal PeLED is developed with deep-blue emission at 458 nm and excellent color stability, which yields an EQE and luminance of 1.1% and 130 cd m−2, respectively, representing a state-of-the-art result for thin-film PeLEDs in this emission region. This work paves the way to achieve high-performance deep-blue PeLEDs with stable emissions to meet the demand for potential applications such as full-color display.

233. "Quantification of Temperature‐Dependent Charge Separation and Recombination Dynamics in Non‐Fullerene Organic                     Photovoltaics"
        Christopher CS Chan, Chao Ma, Xinhui Zou, Zengshan Xing, Guichuan Zhang, Hin‐Lap Yip, Robert A Taylor, Yan He, Kam              Sing Wong, Philip CY Chow
        Adv. Func. Mater.,  2021, https://doi.org/10.1002/adfm.202107157

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Transient optical spectroscopy is used to quantify the temperature-dependence of charge separation and recombination dynamics in P3TEA:SF-PDI2 and PM6:Y6, two non-fullerene organic photovoltaic (OPV) systems with a negligible driving force and high photocurrent quantum yields. By tracking the intensity of the transient electroabsorption response that arises upon interfacial charge separation in P3TEA:SF-PDI2, a free charge generation rate constant of ≈2.4 × 1010 s−1 is observed at room temperature, with an average energy of ≈230 meV stored between the interfacial charge pairs. Thermally activated charge separation is also observed in PM6:Y6, and a faster charge separation rate of ≈5.5 × 1010 s−1 is estimated at room temperature, which is consistent with the higher device efficiency. When both blends are cooled down to cryogenic temperature, the reduced charge separation rate leads to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OPV blends can achieve high photocurrent generation yields given that the thermal dissociation rate of charges outcompetes the recombination rate.

234. "High‐Performance Upscaled Indium Tin Oxide–Free Organic Solar Cells with Visual Esthetics and Flexibility"
        Feng Zhao, Lijian Zuo, Yaokai Li, Lingling Zhan, Shuixing Li, Xin Li, Ruoxi Xia, Hin–Lap Yip, Hongzheng Chen
        Sol. RRL,  2021, https://doi.org/10.1002/solr.202100339

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Organic solar cells (OSCs) show great promise for future applications due to their merits of low cost, flexibility, and vivid colors, etc. However, the “conventional” device architecture with a brittle and expensive glass/indium tin oxide (ITO) transparent electrode weakens these potential advantages and restricts it to small areas for high performance. Herein, a device architecture simultaneously combining the advantages of high performance, superior flexibility, diverse colors, and low-cost upscaling production is developed. The device structure features a top-illumination geometry with a thermally evaporated ultrathin Ag film as transparent electrode for ITO replacement. The formation of optical microcavity and high conductance of ultrathin Ag enables high performance for upscaled OSCs. Moreover, this device architecture further enables diverse colors via tuning the TeO2 layer atop of the ultrathin Ag transparent electrode. This top-illumination structure is more tolerant for substrates and enables wider flexible substrates. As a result, the flexible OSCs with upscaled areas of 1.05 cm2 exhibit a best performance of 13.09% (certified 11.9%) with superior flexibility, diverse colors, representing one of the best ITO-free upscaled flexible OSCs. This work provides a versatile device structure to highlight the merits of OSCs, and paves the way for the future commercialization and practical applications.

235. "Inkjet-Printed Full-Color Matrix Quasi-Two-Dimensional Perovskite Light-Emitting Diodes"
        Junjie Wang, Danyang Li, Lan Mu, Miaozi Li, Yu Luo, Binbin Zhang, Chaohuang Mai, Biao Guo, Linfeng Lan, Jian Wang,          Hin-Lap Yip, Junbiao Peng
        ACS Appl. Mater. & Inter.,  2021, https://doi.org/10.1021/acsami.1c07526

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Full-color matrix devices based on perovskite light-emitting diodes (PeLEDs) formed via inkjet printing are increasingly attractive due to their tunable emission, high color purity, and low cost. A key challenge for realizing PeLED matrix devices is achieving high-quality perovskite films with a favorable emission structure via inkjet printing techniques. In this work, a narrow phase distribution, high-quality quasi-two-dimensional (quasi-2D) perovskite film without a “coffee ring” was obtained via the introduction of a phenylbutylammonium cation into the perovskite and the use of a vacuum-assisted quick-drying process. Relatively efficient emissions of red, green, and blue (RGB) uniform quasi-2D perovskite films with high photoluminescence quantum yields were cast by the inkjet printing technique. The RGB monochrome perovskite matrix devices with 120 pixel-per-inch resolution exhibited electroluminescence, with maximum external quantum efficiencies of 3.5, 3.4, and 1.0% (for red, green, and blue light emissions, respectively). Furthermore, a full-color perovskite matrix device with a color gamut of 102% (NTSC 1931) was realized. To the best of our knowledge, this is the first report of a full-color perovskite matrix device formed by inkjet printing.

236. "Emission Wavelength Tuning via Competing Lattice Expansion and Octahedral Tilting for Efficient Red Perovskite Light‐             Emitting Diodes"
        Guanwei Sun, Xinyan Liu, Zhe Liu, Denghui Liu, Fanyuan Meng, Zhenchao Li, Linghao Chu, Weidong Qiu, Xiaomei Peng,
        Wentao Xie, Chenyang Shen, Jiting Chen, Hin‐Lap Yip, Shi‐Jian Su

        Adv. Func. Mater.,  2021,  https://doi.org/10.1002/adfm.202106691

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The band-edge electronic structure of lead halide perovskites (ABX3) is composed of the orbitals of B and X components and can be tuned through the composition and structure of the BX6 octahedron. Although A-site cations do not directly contribute to near-edge states, the bandgap of 3D metal halide perovskites can be affected by A-cations through BX6 octahedron tilting or lattice size variation. Here, as confirmed by the Rietveld refinement results of X-ray diffraction characterization, the competition between lattice expansion and octahedral tilting is identified for the first time in emission wavelength tuning when introducing a large A-site cation (C2H5NH3+, EA+) into 1-naphthylmethylammonium iodide-passivated CsPbI3 system. The former dominates spectral redshift, while the latter leads to a blueshift of emission peak, which broadens the way to tune the emission wavelength. In addition, excess cations can also passivate the perovskites,

leading to a photoluminescence (PL) quantum yield as high as 61%, increased average PL lifetime of 74.7 ns, and a high radiative and non-radiative recombination ratio of 15.7. Eventually, spectral-stable deep-red perovskite light-emitting diode with a maximum external quantum efficiency of 17.5% is realized, which is one of the highest efficiencies without using any light outcoupling and anti-solvent techniques.

237. "Flexibility of Room-Temperature-Synthesized Amorphous CdO-In2O3 Alloy Films and Their Application as Transparent             Conductors in Solar Cells"
        Ying Wang, Menglin Li, Baobing Fan, Yeung Sum Wong, Chung Yan Lo, Cheuk Kai Gary Kwok, Sujit Kumer Shil, Hin-Lap          Yip, Alex K-Y Jen, Sai-Wing Tsang, Kin Man Yu
        ACS Appl. Mater. & Inter.,  2021,  https://doi.org/10.1021/acsami.1c14722

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Due to their low-temperature deposition, high mobility (>10 cm2/V·s), and electrical conductivity, amorphous ionic oxide semiconductors (AIOSs) have received much attention for their applications in flexible and/or organic electro-optical devices. Here, we report on a study of the flexibility of CdO-In2O3 alloy thin films, deposited on a polyethylene terephthalate (PET) substrate by radio frequency magnetron sputtering at room temperature. Cd1–xInxO1+δ alloys with the composition of x > 0.6 are amorphous, exhibiting a high electron mobility of 40–50 cm2/V·s, a low resistivity of ∼3 × 10–4 Ω·cm, and high transmittance over a wide spectral window of 350 to >1600 nm. The flexibility of both crystalline and amorphous Cd1–xInxO1+δ films on the PET substrate was investigated by measuring their electrical resistivity after both compressive and tensile bending with a range of bending radii and repeated bending cycles. Under both compressive and tensile bending with Rb = 16.5 mm, no significant degradation was observed for both the crystalline and amorphous films up to 300 bending cycles. For a smaller bending radius, the amorphous film shows much less electrical degradation than the crystalline films under compressive bending due to less film delamination at the bending sites. On the other hand, for a small bending radius (<16 mm), both crystalline and amorphous films degrade after repeated tensile bending, most likely due to the development of microcracks in the films. To demonstrate the application of amorphous Cd1–xInxO1+δ alloy in photovoltaics, we fabricated perovskite and bulk-heterojunction organic solar cells (OSCs) on glass and flexible PET utilizing amorphous Cd1–xInxO1+δ layers as transparent electrodes. The organic–inorganic hybrid perovskite solar cells (PSCs) exhibit a power conversion efficiency (PCE) of ∼11 to 12% under both front and back illumination, demonstrating good bifacial performance with bifaciality factor >90%. The OSCs fabricated on an amorphous Cd1–xInxO1+δ-coated flexible PET substrate achieve a promising PCE of 12.06%. Our results strongly suggest the technological potentials of amorphous Cd1–xInxO1+δ as a reliable and effective transparent conducting material for flexible and organic optoelectronic devices.

238. "Perovskite Light‐Emitting Diodes with EQE Exceeding 28% through a Synergetic Dual‐Additive Strategy for Defect                   Passivation and Nanostructure Regulation"
        Zhe Liu, Weidong Qiu, Xiaomei Peng, Guanwei Sun, Xinyan Liu, Denghui Liu, Zhenchao Li, Fangru He, Chenyang Shen,          Qing Gu, Fulong Ma, Hin‐Lap Yip, Lintao Hou, Zhengjian Qi, Shi‐Jian Su
        Adv. Mater.,  2021,  https://doi.org/10.1002/adma.202103268

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Quasi-2D perovskites have long been considered to have favorable “energy funnel/cascade” structures and excellent optical properties compared with their 3D counterparts. However, most quasi-2D perovskite light-emitting diodes (PeLEDs) exhibit high external quantum efficiency (EQE) but unsatisfactory operating stability due to Auger recombination induced by high current density. Herein, a synergetic dual-additive strategy is adopted to prepare perovskite films with low defect density and high environmental stability by using 18-crown-6 and poly(ethylene glycol) methyl ether acrylate (MPEG-MAA) as the additives. The dual additives containing COC bonds can not only effectively reduce the perovskite defects but also destroy the self-aggregation of organic ligands, inducing the formation of perovskite nanocrystals with quasi-core/shell structure. After thermal annealing, the MPEG-MAA with its CC bond can be polymerized to obtain a comb-like polymer, further protecting the passivated perovskite nanocrystals against water and oxygen. Finally, state-of-the-art green PeLEDs with a normal EQE of 25.2% and a maximum EQE of 28.1% are achieved, and the operating lifetime (T50) of the device in air environment is over ten times increased, providing a novel and effective strategy to make high efficiency and long operating lifetime PeLEDs.

239. "Development and Challenges of Metal Halide Perovskite Solar Modules"
         Yuanhang Cheng, Yong Peng, Alex KY Jen, Hin-Lap Yip
         Sol. RRL.,  2021,   https://doi.org/10.1002/solr.202100545

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Metal halide perovskites are considered as game changers for future solar cell technology due to their rapidly increasing device efficiencies and potential for manufacturing at low cost. In view of their promising commercial potential, increasing research efforts are now dedicated to the development of large-area perovskite solar modules. Herein, the motivation for developing perovskite solar modules and the challenges to fabricate large-area perovskite solar cells with high efficiency are discussed. The important thin-film processing methods including solution-based and vacuum-based deposition technologies for scaling up the fabrication of perovskites are reviewed. In addition, other key challenges that need to be overcome to bring perovskite solar modules into photovoltaic (PV) markets are also discussed, including module stability, potential lead leakage issue, and outdoor field testing of the perovskite modules. Finally, opinions on the future development of perovskite PV modules are shared.

240. "Emissive Charge-Transfer States at Hybrid Inorganic/Organic Heterojunctions Enable Low Non-Radiative Recombination
         and High-Performance Photodetectors"

         Flurin Eisner, Georgie Foot, Jun Yan, Mohammed Azzouzi, Dimitra G Georgiadou, Wai Yu Sit, Yuliar Firdaus, Guichuan               Zhang, Yen-Hung Lin, Hin-Lap Yip, Thomas D Anthopoulos, Jenny Nelson
         Adv. Mater.,  2021,  https://doi.org/10.1002/adma.202104654

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Hybrid devices based on a heterojunction between inorganic and organic semiconductors have offered a means to combine the advantages of both classes of materials in optoelectronic devices, but, in practice, the performance of such devices has often been disappointing. Here, it is demonstrated that charge generation in hybrid inorganic–organic heterojunctions consisting of copper thiocyanate (CuSCN) and a variety of molecular acceptors (ITIC, IT-4F, Y6, PC70BM, C70, C60) proceeds via emissive charge-transfer (CT) states analogous to those found at all-organic heterojunctions. Importantly, contrary to what has been observed at previous organic–inorganic heterojunctions, the dissociation of the CT-exciton and subsequent charge separation is efficient, allowing the fabrication of planar photovoltaic devices with very low non-radiative voltage losses (0.21 ±  0.02 V). It is shown that such low non-radiative recombination enables the fabrication of simple and cost-effective near-IR (NIR) detectors with extremely low dark current (4 pA cm−2) and noise spectral density (3 fA Hz−1/2) at no external bias, leading to specific detectivities at NIR wavelengths of just under 1013 Jones, close to the performance of commercial silicon photodetectors. It is believed that this work demonstrates the possibility for hybrid heterojunctions to exploit the unique properties of both inorganic and organic semiconductors for high-performance opto-electronic devices.

241. "Spacer Engineering of Diammonium‐Based 2D Perovskites toward Efficient and Stable 2D/3D Heterostructure     
         Perovskite Solar Cells"

         Tianqi Niu, Yue‐Min Xie, Qifan Xue, Sangni Xun, Qin Yao, Fuchao Zhen, Wenbo Yan, Hong Li, Jean‐Luc Brédas,
         Hin‐LapYip, Yong Cao

         Advanced Energy Materials,  2021,   https://doi.org/10.1002/aenm.202102973

Perovskite solar cells (PSCs) based on 2D/3D heterostructures show great potential to combine the advantages of the high efficiency of 3D perovskites and the high stability of 2D perovskites. However, an in-depth understanding of the organic-spacer effects on the 2D quantum well (QW) structures and electronic properties at the 2D/3D interfaces is yet to be fully achieved, especially in the case of 2D perovskites based on diammonium spacers/ligands. Here, a series of diammonium spacers is considered for the construct ion 2D/3D perovskite heterostructures. It is found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers, including their phase purity and orientation. Density functional theory calculations indicate that the spacer modifications can induce shifts in the energy-level alignments at the 2D/3D interfaces and therefore influence the charge-transfer characteristics. The strong intermolecular interactions between the 2,2-(ethylenedioxy)bis(ethylammonium) (EDBE) cations and inorganic [PbI6]4− slabs facilitate a controlled deposition of a phase-pure QW structure (n = 1) with a horizontal orientation, which leads to better surface passivation and carrier extraction. These benefits endow the EDBE-based 2D/3D devices with a high power conversion efficiency of 22.6% and remarkable environmental stability, highlighting the promise of spacer-chemistry design for high-performance 2D/3D PSCs.

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242. "Color‐Stable Deep‐Blue Perovskite Light‐Emitting Diodes Based on Organotrichlorosilane Post‐Treatment"
         Guangruixing Zou, Zhenchao Li, Ziming Chen, Linghao Chu, Hin‐Lap Yip, Yong Cao
         Advanced Functional Materials,  2021,  https://doi.org/10.1002/adfm.202103219

Recent studies of sky-blue perovskite light-emitting diodes (PeLEDs) have extensively promoted optimal device design to achieve an external quantum efficiency (EQE) above 12%. However, the development of thin-film deep-blue PeLEDs lags dramatically behind, especially with regards to meeting the latest Rec. 2020 standard. A trichloro(3,3,3-trifluoropropyl) silane post-treatment that drives the emission of perovskite into the deep-blue region, ranging from 440 to 460 nm, which meets the Rec. 2020 standard, is proposed. The chlorine ions released from the organotrichlorosilane molecules during their polycondensation reaction provide an addition halide source to fine tune the composition of the mixed halide perovskite films, leading to increase of bandgap and deep-blue emission. In addition, hydrogen bonds between the hydroxy groups of silane molecules and halide anions in perovskite can suppress ion migration for improving emission stability. As a result, an optimal PeLED is developed with deep-blue emission at 458 nm and excellent color stability, which yields an EQE and luminance of 1.1% and 130 cd m−2, respectively, representing a state-of-the-art result for thin-film PeLEDs in this emission region. This work paves the way to achieve high-performance deep-blue PeLEDs with stable emissions to meet the demand for potential applications such as full-color display.

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243. "Quantification of Temperature‐Dependent Charge Separation and Recombination Dynamics in Non‐Fullerene Organic           Photovoltaics"
         Christopher CS Chan, Chao Ma, Xinhui Zou, Zengshan Xing, Guichuan Zhang, Hin‐Lap Yip, Robert A Taylor, Yan He,
         Kam Sing Wong, Philip CY Chow

         Advanced Functional Materials,  2021,   https://doi.org/10.1002/adfm.202107157

Transient optical spectroscopy is used to quantify the temperature-dependence of charge separation and recombination dynamics in P3TEA:SF-PDI2 and PM6:Y6, two non-fullerene organic photovoltaic (OPV) systems with a negligible driving force and high photocurrent quantum yields. By tracking the intensity of the transient electroabsorption response that arises upon interfacial charge separation in P3TEA:SF-PDI2, a free charge generation rate constant of ≈2.4 × 1010 s−1 is observed at room temperature, with an average energy of ≈230 meV stored between the interfacial charge pairs. Thermally activated charge separation is also observed in PM6:Y6, and a faster charge separation rate of ≈5.5 × 1010 s−1 is estimated at room temperature, which is consistent with the higher device efficiency. When both blends are cooled down to cryogenic temperature, the reduced charge separation rate leads to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OPV blends can achieve high photocurrent generation yields given that the thermal dissociation rate of charges outcompetes the recombination rate.

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244. "Interface Engineering for All‐Inorganic CsPbIBr2 Perovskite Solar Cells with Enhanced Power Conversion Efficiency     
         over 11%"

         Jing Wang, Xin Wu, Yizhe Liu, Qifan Xue, Hin-Lap Yip, Alex KY Jen, Zonglong Zhu
         Energy Technology,  2021,   https://doi.org/10.1002/ente.202100562

Perovskite solar cells (PVSCs) receive great attention due to their excellent photovoltaic performance. Recently, all-inorganic PVSCs have been extensively studied owing to their superior thermal and photo stability. Among them, CsPbIBr2 perovskite stands out due to its superb phase stability in ambient environment. However, the severe energy loss caused by non-radiative recombination limits its development. Herein, a facile interface engineering method is employed to modify the electron transporting interface and reduce the energy loss. The insertion of a thin polyethylenimine ethoxylated (PEIE) film between SnO2 and the perovskite can simultaneously tune the work function of SnO2 and passivate the defects of the perovskite by the amino group in PEIE. Meanwhile, the PEIE interface serves as a modifier to enhance the crystallinity of the perovskite film, leading to enlarged grain size and reduced grain boundaries. As a result, the power conversion efficiency was enhanced from 8.7% for the SnO2-based device to 11.2% for the SnO2/PEIE-based device, with an open-circuit voltage of 1.29 V, a short-circuit current of 11.0 mA/cm2, and a fill factor of 78.6%. Moreover, the photostability of devices were improved, which retained over 80% of its initial efficiency under continuous one sun illumination for 500 h. This work proves the effectiveness of interface engineering to boost the efficiency and stability of all-inorganic PVSCs.

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245. "Emission Wavelength Tuning via Competing Lattice Expansion and Octahedral Tilting for Efficient Red Perovskite Light‐         Emitting Diodes"
         Guanwei Sun, Xinyan Liu, Zhe Liu, Denghui Liu, Fanyuan Meng, Zhenchao Li, Linghao Chu, Weidong Qiu, Xiaomei     
         Peng, Wentao Xie, Chenyang Shen, Jiting Chen, Hin‐Lap Yip, Shi‐Jian Su

         Advanced Functional Materials,  2021,   https://doi.org/10.1002/adfm.202106691

The band-edge electronic structure of lead halide perovskites (ABX3) is composed of the orbitals of B and X components and can be tuned through the composition and structure of the BX6 octahedron. Although A-site cations do not directly contribute to near-edge states, the bandgap of 3D metal halide perovskites can be affected by A-cations through BX6 octahedron tilting or lattice size variation. Here, as confirmed by the Rietveld refinement results of X-ray diffraction characterization, the competition between lattice expansion and octahedral tilting is identified for the first time in emission wavelength tuning when introducing a large A-site cation (C2H5NH3+, EA+) into 1-naphthylmethylammonium iodide-passivated CsPbI3 system. The former dominates spectral redshift, while the latter leads to a blueshift of emission peak, which broadens the way to tune the emission wavelength. In addition, excess cations can also passivate the perovskites, leading to a photoluminescence (PL) quantum yield as high as 61%, increased average PL lifetime of 74.7 ns, and a high radiative and non-radiative recombination ratio of 15.7. Eventually, spectral-stable deep-red perovskite light-emitting diode with a maximum external quantum efficiency of 17.5% is realized, which is one of the highest efficiencies without using any light outcoupling and anti-solvent techniques.

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246. "The evolution and future of metal halide perovskite-based optoelectronic devices"
         Shengfan Wu, Ziming Chen, Hin-Lap Yip, Alex K-Y Jen
         Matter,  2021,  https://doi.org/10.1016/j.matt.2021.10.026

Metal halide perovskites have emerged as a class of promising semiconductors for high-performance optoelectronics in the last decade. Their unique optical and electrical properties render them with great potential for applications in photovoltaics, light-emitting diodes, lasers and photodetectors. The highest power conversion efficiency of 25.5% and external quantum efficiency of over 20% have been demonstrated for perovskite solar cells and light-emitting diodes, respectively. Perovskite photodetectors have also exhibited impressive device performance for broad- and narrow-band, and hard radiation detection. Although substantial performance improvements have been realized, there are still many open questions and challenges. For instance, their long-term stability under different external stresses needs to be further improved to meet the requirements for real-world applications. More importantly, the toxic element Pb used in devices may leak from the degraded devices to contaminate the environment. Therefore, it is important to develop effective strategies to prevent the Pb leakage during their life cycle.

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247. "Device Performance of Emerging Photovoltaic Materials (Version 2)"
         Osbel Almora, Derya Baran, Guillermo C Bazan, Christian Berger, Carlos I Cabrera, Kylie R Catchpole, Sule Erten‐Ela,
         Fei Guo, Jens Hauch, Anita WY Ho‐Baillie, T Jesper Jacobsson, Rene AJ Janssen, Thomas Kirchartz, Nikos Kopidakis,
         Yongfang Li, Maria A Loi, Richard R Lunt, Xavier Mathew, Michael D McGehee, Jie Min, David B Mitzi, Mohammad K
         Nazeeruddin, Jenny Nelson, Ana F Nogueira, Ulrich W Paetzold, Nam‐Gyu Park, Barry P Rand, Uwe Rau, Henry J
         Snaith, Eva Unger, Lídice Vaillant‐Roca, Hin‐Lap Yip, Christoph J Brabec

         Advanced Energy Materials,  2021,  https://doi.org/10.1002/aenm.202102526

Following the 1st release of the “Emerging photovoltaic (PV) reports”, the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2020. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application and are put into perspective using, e.g., the detailed balance efficiency limit. The 2nd instalment of the “Emerging PV reports” extends the scope toward tandem solar cells and presents the current state-of-the-art in tandem solar cell performance for various material combinations.

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