Chen Weijie

The photoinitiator bifunctional bis‐benzophenone is introduced into non‐fullerene solar cells as a multifunctional solid additive for the first time. The doping of this solid additive could not only modify the polymer order and firm morphology of active layer to improve device performance, but also to achieve better reproducibility, thickness insensitivity, and thermal stability for the non‐fullerene solar cells.

Abstract

Simultaneously improving efficiency and stability is critical for the commercial application of non‐fullerene acceptor polymer solar cells (NFA‐PSCs). Multifunctional solid additives have been considered as a potential route to tune the morphology of the active layer and optimize performance. In this work, photoinitiator bifunctional bis‐benzophenone (BP‐BP) is used as a solid additive, replacing solvent additives, in the PBDB‐T:ITIC NFA system. With the addition of BP‐BP, the intermolecular π–π stacking of PBDB‐T and morphology is improved, leading to more balanced carrier transport and more effective exciton dissociation. Devices fabricated with BP‐BP show a power conversion efficiency (PCE) of 11.89%, with enhanced short‐circuit current (J _sc), and fill factor (FF). Devices optimized with BP‐BP show excellent reproducibility, insensitivity to thickness, and an improved thermal stability under atmospheric conditions without encapsulation. This work provides a new strategy for the application of solid additives in NFA‐PSCs.

9.5% semitransparent solar cells with ultrahigh transmission in the visible range (50% AVT) are fabricated via inkjet printing. The effect of different photoactive layer ink solvents on the vertical stratification and performance is explored. The formulation of transport layer inks compatible with highly hydrophobic active layers and with scalable printing processes permits the use of a semitransparent electrode grid.

Abstract

New polymer donors and nonfullerene acceptors have elevated the performance and stability of solar cells to higher grounds. To achieve their full potential, they require their adaptation to scalable and cost‐effective solution manufacturing techniques for large area deposition. Likewise, formulating scalable solution‐based transport layer inks that are compatible with the photoactive layer is imperative. This manuscript reports the full integration of solution‐based transport layers and electrode alongside a PTB7‐Th:IEICO‐4F bulk heterojunction in inverted architecture through inkjet‐printing, resulting in power conversion efficiencies up to 12.4% opaque devices and 9.5% semitransparent devices with average visible transmittance values of 50.1%, including hole transport layer. The wetting envelope of the highly‐hydrophobic photoactive layer alongside the surface energy of candidate solutions and solvents allows the formulation of thick transport layer inks that are compatible with the drop‐on‐demand inkjet‐printing process and yield uniform and homogenous films. Moreover, the surface energy components of the donor and acceptor serves as a fingerprint to assess the vertical stratification of the photoactive layer with the inclusion of different solvents. This methodology addresses a scale‐up bottleneck of solution‐based transport layers for high‐efficiency organic cells, enabling its adaptation to high‐throughput techniques including slot‐die and roll‐to‐roll coating.

Publication date: 16 December 2020

Source: Joule, Volume 4, Issue 12

Author(s): Nicholas Rolston, William J. Scheideler, Austin C. Flick, Justin P. Chen, Hannah Elmaraghi, Andrew Sleugh, Oliver Zhao, Michael Woodhouse, Reinhold H. Dauskardt

Nature Communications, Published online: 26 November 2020; doi:10.1038/s41467-020-19853-z

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Aili Wang, Xiaoyu Deng, Jianwei Wang, Shurong Wang, Xiaobin Niu, Feng Hao, Liming Ding

By introducing moderate phenylethylammonium iodide and lead acetate in CsPbI₃ perovksite, moiture resistance and charge recombination are optimized. The device achieves a 17% power conversion efficiency, a 1.33 V open‐circuit voltage (V _OC) and the smallest 0.38 V V _OC deficit. Meanwhile, the device maintains 94% of its efficiency after 2000 h storage in ambient environment.

Abstract

Cesium lead iodide (CsPbI₃) perovskite has gained great attention due to its potential thermal stability and appropriate bandgap (≈1.73 eV) for tandem cells. However, the moisture‐induced thermodynamically unstable phase and large open‐circuit voltage (V _OC) deficit and also the low efficiency seriously limit its further development. Herein, long chain phenylethylammonium (PEA) is utilized into CsPbI₃ perovskite to stabilize the orthorhombic black perovskite phase (γ‐CsPbI₃) under ambient condition. Furthermore, the moderate lead acetate (Pb(OAc)₂) is controlled to combine with phenylethylammonium iodide to form the 2D perovskite, which can dramatically suppress the charge recombination in CsPbI₃. Unprecedentedly, the resulted CsPbI₃ solar cells achieve a 17% power conversion efficiency with a record V _OC of 1.33 V, the V _OC deficit is only 0.38 V, which is close to those in organic‐inorganic perovskite solar cells (PSCs). Meanwhile, the PEA modified device maintains 94% of its initial efficiency after exceeding 2000 h of storage in the low‐humidity controlled environment without encapsulation.

Photoinduced degradation can happen in each functional layer in perovskite solar cells, including the active layer, electronic transport layer, hole transport layer and their interfaces. An overview of these degradation categories and the corresponding solutions is proposed in this review, in the hope of encouraging further research and optimization of the devices.

Abstract

Solar cells based on metal halide perovskites have reached a power conversion efficiency as high as 25%. Their booming efficiency, feasible processability, and good compatibility with large‐scale deposition techniques make perovskite solar cells (PSCs) desirable candidates for next‐generation photovoltaic devices. Despite these advantages, the lifespans of solar cells are far below the industry‐needed 25 years. In fact, numerous PSCs throughout the literature show severely hampered stability under illumination. Herein, several photoinduced degradation mechanisms are discussed. With light radiation, the organic–inorgainc perovskites are prone to phase segregation or chemical decomposition; the oxide electron transport layers (ETLs) tend to introduce new defects at the interface; the commonly used small molecules‐based hole transport layers (HTLs) typically suffer from poor photostability and dopant diffusion during device operation. It has been demonstrated the photoinduced degradation can take place in every functional layer, including the active layer, ETL, HTL, and their interfaces. An overview of these degradation categories is provided in this review, in the hope of encouraging further research and optimization of relevant devices.

A series of PM6 polymers with different weight‐average molecular weights and polydispersity index are synthesized, and the effects of PM6 polymerization degree on the efficiency and degradation behaviors of the Y6‐based photovoltaic system are systematically studied.

Abstract

The degree of polymerization can cause significant changes in the blend microstructure and physical mechanism of the active layer of non‐fullerene polymer solar cells, resulting in a huge difference in device performance. However, the diversity of stability issues, including photobleaching stability, storage stability, photostability, thermal stability, and mechanical stability, and more, poses a challenge for the degree of polymerization to comprehensively address the trade‐off between device efficiency and stability and reasonably evaluate the application potential of polymer materials. Herein, a series of PM6 polymers with different weight‐average molecular weights (M _w) and polydispersity index (PDI) are synthesized. The effects of the degree of PM6 polymerization on the efficiency and degradation behaviors of the photovoltaic systems based on Y6 as acceptor are investigated systematically. The findings regarding stability issues, together with the trade‐offs in the efficiency‐stability gap, formulate a complete guideline for the material design and performance evaluation in a way that relies much less on trial‐and‐error efforts.

The present work deconvolutes the electronic processes in organic solar cells under short‐circuit conditions by combining readily available experimental methods (current‐voltage characteristics, external quantum efficiency) with optical simulations. The proposed method allows the quantification of geminate recombination, to determine the mobility‐lifetime product, and to quantify extraction. The applicability of this new approach is demonstrated in three different organic photovoltaic systems.

Abstract

The short‐circuit current (J _sc) of organic solar cells is defined by the interplay of exciton photogeneration in the active layer, geminate and non‐geminate recombination losses and free charge carrier extraction. The method proposed in this work allows the quantification of geminate recombination and the determination of the mobility‐lifetime product (µτ) as a single integrated parameter for charge transport and non‐geminate recombination. Furthermore, the extraction efficiency is quantified based on the obtained µτ product. Only readily available experimental methods (current‐voltage characteristics, external quantum efficiency measurements) are employed, which are coupled with an optical transfer matrix method simulation. The required optical properties of common organic photovoltaic (OPV) materials are provided in this work. The new approach is applied to three OPV systems in inverted or conventional device structures, and the results are juxtaposed against the µτ values obtained by an independent method based on the voltage–capacitance spectroscopy technique. Furthermore, it is demonstrated that the new method can accurately predict the optimal active layer thickness.

Photo‐oxidation of the dangling hydroxyl group on ZnO surface under visible light illumination leads to the formation of hydroxyl radicals, which decompose the acceptor molecule IT‐4F and consequently decrease PM6:IT‐4F solar cell performance.

Power conversion efficiencies (PCEs) of polymer solar cells (PSCs) have exceeded 18% in the last few years. Stability has therefore become the next most important issue before commercialization. Herein, the degradation behaviors of the inverted PM6:IT‐4F (PBDB‐T‐2F:3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene) solar cells with different ZnO layers are systematically investigated. The PCE decay rates of the cells and the photobleaching process of the IT‐4F containing organic films on ZnO surface are directly correlated with the light‐absorption ability of the ZnO layer in the visible light range, indicating that photochemical decomposition of IT‐4F is initiated by the light absorption of ZnO layer. By analyzing the products of the aged ZnO/IT‐4F films with matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS), it is confirmed that photochemical reactions at the IT‐4F/ZnO interface include de‐electron‐withdrawing units and dealkylation on the side‐phenyl ring. Hydroxyl radicals generated by the photo‐oxidation of dangling hydroxide by ZnO are confirmed by electron spin resonance (ESR) spectroscopy measurements, which is attributed as the main reason causing the decomposition of IT‐4F. Surface treatment of ZnO with hydroxide and/or hydroxyl radical scavenger is found to be able to improve the stability of the PSCs, which further supports the proposed degradation mechanism.

Strain tuning in Sb‐Chs was demonstrated by simultaneously replacing Sb and S with larger Bi and I ions, respectively. This strategy has been applied in Sb₂(S _x Se_{1– x} )₃ solar cells, and the champion cell exhibits a PCE of 7.05% under the standard illumination conditions (100 mW cm⁻²), which is one of the top efficiencies in solution processing Sb₂(S _x Se_{1– x} )₃ solar cells.

Abstract

Strain induced by lattice distortion is one of the key factors that affect the photovoltaic performance via increasing defect densities. The unsatisfied power conversion efficiencies (PCEs) of solar cells based on antimony chalcogenides (Sb‐Chs) are owing to their photoexcited carriers being self‐trapped by the distortion of Sb₂S₃ lattice. However, strain behavior in Sb‐Chs‐based solar cells has not been investigated. Here, strain tuning in Sb‐Chs is demonstrated by simultaneously replacing Sb and S with larger Bi and I ions, respectively. Bi/I codoped Sb₂S₃ cells are fabricated using poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b']dithiophene)‐alt‐4,7‐(2,1,3‐enzothiadiazole)] as the hole‐transporting layer. Codoping reduced the bandgap and rendered a bigger tension strain (1.76 × 10⁻⁴) to a relatively smaller compression strain (−1.29 × 10⁻⁴). The 2.5 mol% BiI3 doped Sb₂S₃ cell presented lower trap state energy level than the Sb₂S₃ cell; moreover, this doping amount effectively passivated the trap states. This codoping shows a similar trend even in the low bandgap Sb₂(S_xSe_1‐x)₃ cell, resulting in 7.05% PCE under the standard illumination conditions (100 mW cm⁻²), which is one of the top efficiencies in solution processing Sb₂(S_xSe_1‐x)₃ solar cells. Furthermore, the doped cells present higher humidity, thermal and photo stability. This study provides a new strategy for stable Pb‐free solar cells.

A series of Donor–π–Acceptor porphyrins coded as CS0, CS1, and CS2 that can effectively passivate the perovskite surface, increase V _OC and FF, reduce the hysteresis effect, enhance power conversion efficiency to be higher than 22%, and improve the device stability have been developed.

Abstract

In recent years, hybrid perovskite solar cells (PSCs) have attracted much attention owing to their low cost, easy fabrication, and high photoelectric conversion efficiency. Nevertheless, solution‐processed perovskite films usually show substantial structural disorders, resulting in ion defects on the surface of lattice and grain boundaries. Herein, a series of D–π–A porphyrins coded as CS0, CS1, and CS2 that can effectively passivate the perovskite surface, increase V _OC and FF, reduce the hysteresis effect, enhance power conversion efficiency to be higher than 22%, and improve the device stability is developed. The results in this study demonstrated that the donor–π–acceptor type porphyrin derivatives are promising passivators that can improve the cell performance of PSCs.

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Xunchang Wang, Jianxiao Wang, Jianhua Han, Da Huang, Pengchao Wang, Lixue Zhou, Chunming Yang, Xichang Bao, Renqiang Yang

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

Abstract

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

Kesterite Cu₂ZnSnSe₄ (CZTSe) thin film solar cells with independently confirmed 12.5% total area efficiency have been demonstrated with a novel strategy to effectively control the formation of intrinsic defects and defect clusters in CZTSe by carefully engineering the local chemical environment (e.g., suitable local chemical composition, oxidation states of cations) during film growth.

Abstract

Kesterite‐based Cu₂ZnSn(S,Se)₄ semiconductors are emerging as promising materials for low‐cost, environment‐benign, and high‐efficiency thin‐film photovoltaics. However, the current state‐of‐the‐art Cu₂ZnSn(S,Se)₄ devices suffer from cation‐disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high‐quality Cu₂ZnSnSe₄ absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high V _OC of 491 mV, which is the new record efficiency of pure‐selenide Cu₂ZnSnSe₄ cells with lowest V _OC deficit in the kesterite family by E _g/q‐Voc. These encouraging results demonstrate an essential route to overcome the long‐standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.

To fine‐tune the energy levels of polymer donors, a family of random polymers is synthesized, which shows favorable properties of aggregation and morphology. The performance of these polymers is less sensitive to their molecular weights compared with PM7. Thus, multiple cases of highly efficient nonfullerene organic solar cells are achieved with efficiencies between 16.0% and 17.1%.

Abstract

Developing high‐performance donor polymers is important for nonfullerene organic solar cells (NF‐OSCs), as state‐of‐the‐art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF‐OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well‐controlled morphology and high‐performance with a variety of chemical structures and molecular weights. The polymer donors are D–A1–D–A2‐type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron‐deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF‐OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron‐deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high‐performance donor polymers for NF‐OSCs.

A naphthalene diimide based double‐cable conjugated polymer provided a record efficiency of 8.4 % in single‐component organic solar cells. It simultaneously facilitates exciton separation and charge transport via miscibility control.

Abstract

A record power conversion efficiency of 8.40 % was obtained in single‐component organic solar cells (SCOSCs) based on double‐cable conjugated polymers. This is realized based on exciton separation playing the same role as charge transport in SCOSCs. Two double‐cable conjugated polymers were designed with almost identical conjugated backbones and electron‐withdrawing side units, but extra Cl atoms had different positions on the conjugated backbones. When Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This improves the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These findings reveal the importance of charge generation process in SCOSCs and suggest a strategy to improve this process: controlling miscibility between conjugated backbones and aromatic side units in double‐cable conjugated polymers.

Two-dimensional lead halide perovskites with confined excitons have shown exciting potentials in optoelectronic applications. It is intriguing but unclear how the soft and polar lattice redefines excitons in layered perovskites. Here, we reveal the intrinsic exciton properties by investigating exciton spin dynamics, which provides a sensitive probe to exciton coulomb interactions. Compared to transition metal dichalcogenides with comparable exciton binding energy, we observe orders of magnitude smaller exciton-exciton interaction and, counterintuitively, longer exciton spin lifetime at higher temperature. The anomalous spin dynamics implies that excitons exist as exciton polarons with substantially weakened inter- and intra-excitonic interactions by dynamic polaronic screening. The combination of strong light matter interaction from reduced dielectric screening and weakened inter-/intra-exciton interaction from dynamic polaronic screening explains their exceptional performance and provides new rules for quantum-confined optoelectronic and spintronic systems.