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Publication date: June 2021

Source: Nano Energy, Volume 84

Author(s): Jian Xiong, Zhongjun Dai, Shiping Zhan, Xiaowen Zhang, Xiaogang Xue, Weizhi Liu, Zheling Zhang, Yu Huang, Qilin Dai, Jian Zhang

This Minireview describes developments in all‐polymer solar cells containing a new type of n‐type conjugated polymer, polymerized small‐molecule acceptors (PSMAs). PSMAs combine the merits of small‐molecule acceptors (narrow band gap, strong absorption, and suitable electronic energy levels) with the good film formation, higher morphology and light‐irradiation stability of polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have drawn tremendous research interest in recent years, due to their inherent advantages of good film formation, stable morphology, and mechanical flexibility. The most representative and most widely used n‐CP acceptor was the naphthalene diimide based D‐A copolymer N2200 before 2017, and the power conversion efficiency (PCE) of the all‐PSCs based on N2200 reached over 8% in 2016. However, the low absorption coefficient of N2200 in the near‐infrared (NIR) region limits the further increase of its PCE. In 2017, we proposed a strategy of polymerizing small‐molecule acceptors (SMAs) to construct new‐generation polymer acceptors. The polymerized SMAs (PSMAs) possess low band gap and strong absorption in the NIR region, which attracted great attention and drove the PCE of the all‐PSCs to over 15% recently. In this Minireview we explain the design strategies of the molecular structure of PSMAs and describe recent research progress. Finally, current challenges and future prospects of the PSMAs are analyzed and discussed.

Publication date: June 2021

Source: Nano Energy, Volume 84

Author(s): Jiehao Fu, Haiyan Chen, Peihao Huang, Qingqing Yu, Hua Tang, Shanshan Chen, Sungwoo Jung, Kuan Sun, Changduk Yang, Shirong Lu, Zhipeng Kan, Zeyun Xiao, Gang Li

Publication date: June 2021

Source: Nano Energy, Volume 84

Author(s): Hailiang Wang, Huicong Liu, Zijing Dong, Weiping Li, Liqun Zhu, Haining Chen

The molecular orientation, charge transfer behavior, and dipole formation dynamic of a kind of water/alcohol soluble amino‐functionalized polyelectrolyte (PFN) in organic solar cells are investigated by X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. A large absorption energy and charge transfer between PFN and different substrates are verified for the first time.

Water‐/alcohol‐soluble polyelectrolyte poly[(9, 9‐bis (3′‐(N,N‐dimethylamino) propyl)‐2, 7‐fluorene)‐alt‐2, 7‐(9, 9‐dioctylfluorene)] (PFN) used in organic solar cells (OSCs) reduces the work function of the electrode due to the effect of an interfacial dipole, which is beneficial for the energy‐level alignment between the electrode and the active layer. To date, the studies on the working mechanism of PFN are mainly conducted through topographical and electronic research. Herein, a dynamic insight into the formation mechanisms of the PFN interlayer at the molecular structural level is established. The charge transfer between PFN and the substrates is verified for the first time by X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) studies, which results in chemisorption dipoles with their direction aligned with the intrinsic dipole of the PFN molecule, thereby reducing the work function of the substrate. The larger adsorption energy in the substrates of the nitrogen‐containing side chains of PFN is also identified, which induces the preferential orientation of PFN molecule to reduce the work function of the substrate. By incorporating this interlayer, high efficiency in single‐junction OSCs is achieved using commercial materials. The findings are of great significance for understanding and optimizing the polymer dipole interlayers for OSCs.

Harmful UV light and surface defects accelerate the degradation of perovskite solar cells (PSCs). A tautomeric “sunscreen” molecule can be used to protect the PSC from UV degradation and enable molecular defect passivation (defect formation energy: −1.35 eV) through interactions between functional groups and defects. This strategy provides high‐efficiency PSCs with long‐term UV stability.

Abstract

UV light always does great harm to perovskite solar cells, relentlessly degrading perovskites and shortening the lifetime of perovskite devices. Meanwhile, surface defects in perovskite films further accelerate the degradation process and serve as nonradiative charge recombination centers to deteriorate device efficiency. Herein, we demonstrate that a “sunscreen” molecule, 2‐hydroxy‐4‐methoxybenzophenone, not only protects the perovskite solar cell from UV degradation but also enables molecular defect passivation through interaction between functional groups and defects by molecular tautomerism under UV light illumination. Therefore, the sunscreen strategy efficiently enhances the UV endurance of PSCs and improves defect formation energy to −1.35 eV. The perovskite solar cell with sunscreen (sunscreen PSC) exhibits outstanding efficiencies of up to 23.09 % (0.04 cm²) and 19.73 % (1.00 cm²) as well as long‐term UV (UVa: 365 nm and UVb: 285 nm) stability.

A Ni phthalocyanine (NiPc) decorated by four methoxyethoxy units with a strong intramolecular electric field is prepared and used as hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). The best PSCs with NiPc as dopant‐free HTM show a record efficiency of 21.23 % (certified 21.03 %). The PSCs also exhibit the excellent stability.

Abstract

Low conductivity and hole mobility in the pristine metal phthalocyanines greatly limit their application in perovskite solar cells (PSCs) as the hole‐transporting materials (HTMs). Here, we prepare a Ni phthalocyanine (NiPc) decorated by four methoxyethoxy units as HTMs. In NiPc, the two oxygen atoms in peripheral substituent have a modified effect on the dipole direction, while the central Ni atom contributes more electron to phthalocyanine ring, thus efficiently increasing the intramolecular dipole. Calculation analyses reveal the extracted holes within NiPc are mainly concentrated on the phthalocyanine core induced by the intramolecular electric field, and further to be transferred by π‐π stacking space channel between NiPc molecules. Finally, the best efficiency of PSCs with NiPc as dopant‐free HTMs realizes a record value of 21.23 % (certified 21.03 %). The PSCs also exhibit the good moisture, heating and light stabilities. This work provides a novel way to improve the performance of PSCs with free‐doped metal phthalocyanines as HTMs.

An ionic liquid, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆), was used to passivate a perovskite to decrease the defects of Pb‐cluster and Pb‐I antisite, thereby reducing the energy barrier between the perovskite and hole transport layer. A perovskite solar cell attained a 23.25 % efficiency with a high stability due to hydrophobic DMIMPF₆.

Abstract

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆) ionic liquid (IL) is adopted to passivate the surface of a formamidinium‐cesium lead iodide perovskite (Cs_0.08FA_0.92PbI₃) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb‐cluster and Pb‐I antisite defects can be effectively passivated by [DMIM]⁺ bonding with the Pb²⁺ ion on the perovskite surface, leading to significantly suppressed non‐radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF₆‐treated perovskite device demonstrated long‐term stability because the hydrophobic DMIMPF₆ layer blocked moisture permeation.

Bridge‐jointed 2D nanosheets are inserted between the methylammonium‐free perovskite and the dopant‐free hole transport layer (HTL) to form a scalable heterostructure, which preserves p‐type semiconduction of HTL and suppresses nonradiative‐recombination. Further, a perovskite solar module with an area of 35.80 cm² shows a certified efficiency of 15.3% and encapsulated modules retain over 91% of initial efficiency after damp heat test for 1000 h.

Abstract

Perovskite solar cell (PSC) modules employing a hole transport layer (HTL) without unstable dopants possess high potential for improving operational stability. However, the low efficiencies of the devices greatly limit their commercial applications owing to the lower efficacy of the dopant‐free HTL, introduced by the unintentional n‐doping effect of volatile ions from the halide‐rich perovskite surface. Here, a scalable heterostructure integrated by a methylammonium‐free perovskite film with an iodide‐rich surface, an ultrathin interlayer of bridge‐jointed graphene oxide nanosheets (BJ‐GO), and an HTL without additional ionic dopants is developed. In this heterostructure, the iodide ions are physically immobilized by the compact 2D network, and lead defects are chemically passivated by multiple coordination bonds. Moreover, the BJ‐GO with tunable surface energy enables a highly ordered HTL a considerably improved carrier mobility by an order of magnitude. Finally, the PSC module with an area of 35.80 cm² employing this heterostructure shows a certified efficiency of 15.3%. The encapsulated PSC modules retain over 91% of initial efficiency after the damp heat test at 85 °C and ≈85% relative humidity for 1000 h, while maintaining 90% of the initial value for 1000 h at the maximum power point under continuous 1‐Sun illumination at 60 °C.

Semiconducting oxide overlayer materials (SOOMs) can offer a new way for low-cost and highly-stable halide perovskite solar cells (HPSCs) compared to organic semiconducting overlayer materials. The effective deposition of SOOMs on top of the perovskite layer is expected to contribute to the commercialization of single-junction as well as multi-junction HPSCs.

Abstract

Halide perovskite solar cells (HPSCs) contain charge transport layers (CTLs) both above and below the photoactive perovskite layer. These semiconducting CTLs are just as important as the perovskite layer to fully realizing the potential of perovskite materials. In particular, semiconducting oxide overlayer materials (SOOMs) are expected to lower costs and provide better long-term stability compared to the organic semiconducting materials commonly used for the upper layer. However, SOOM-based HPSCs are currently less efficient than conventional devices owing to SOOM's deposition constraints imposed by the underlying perovskite layer. This progress report focuses on the recent evolution of SOOM-based HPSCs by describing the key issues and recent advances in SOOM deposition methods. Finally, remaining challenges and future research directions for SOOMs are discussed to provide guidance toward the commercialization of HPSCs.

The effect of Pd cross‐coupling catalyst traces on the physical processes in a non‐fullerene bulk‐heterojunction solar cell is investigated. The drop of the solar cell performance upon addition of systematically added amounts of tetrakis(triphenylphosphine)palladium(0) is explained by alteration of the morphology, charge carrier generation, recombination, and charge extraction.

Abstract

The effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh₃)₄) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th) donor and 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 (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh₃)₄ on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh₃)₄ on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh₃)₄ in small amounts.

Aided by theoretical calculations, a multifunctional 2,2‐difluoropropanediamide (DFPDA) molecule that bears carbonyl, amino, and fluorine groups is first introduced into the perovskite precursor, serving as a crystal growth mitigator, grain boundaries passivator, and surface protection material. With the help of the combined effects of multifunctional groups in DFPDA, the perovskite cells deliver an efficiency of 22.21% and improved stability.

Abstract

With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb²⁺ and passivate under‐coordinated Pb²⁺ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.

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.

An in‐situ formed polymeric interlayer enables enhanced photovoltaic performance of the methylammonium‐, cesium‐, and bromide‐free perovskite solar cells with superior photo‐ and thermal‐stability. The polymeric interlayer promotes growth of perovskite crystals with reduced defect density and improves the contact between the perovskite and hole transporting layers to assists in photo‐excited charge extraction.

Abstract

The vast majority of high‐performance perovskite solar cells (PSCs) are based on multi‐cation mixed‐anion compositions incorporating methylammonium (MA) and bromide (Br). Nevertheless, the thermal instability of MA and the tendency of mixed halide compositions to phase segregate limit the long‐term stability of PSCs. However, reports of MA‐free and/or Br‐free compositions are rare in the community since their performance is generally inferior. Here, a strategy is presented to achieve highly efficient and stable PSCs that are altogether cesium (Cs)‐free, MA‐free and Br‐free. An antisolvent quenching process is used to in‐situ deposit a polymeric interlayer to promote the growth of phase‐pure formamidinium lead tri‐iodide perovskite crystals with reduced defect density and to assist in photo‐excited charge extraction. The PSCs developed are among the best‐performing reported for such compositions. Moreover, the PSCs show superior stability under continuous exposure to both illumination and 85 °C heat.

Using newly developed high‐quality FlexAgNEs, flexible OPV devices are fabricated and studied with the newly emerging star acceptor Y6 and its derivatives. Comparable performance with rigid counterparts is achieved for all the tested materials. The flexible devices display superior and robust mechanical stability under extreme bending or even folding conditions. Furthermore, the mechanism underlying the super mechanical robustness of these flexible devices is thoroughly investigated.

Abstract

Among the various advantages of organic photovoltaics (OPVs), the key one is their ability to be a highly flexible renewable energy source. However, the power conversion efficiencies for flexible OPV devices still lag behind those of their rigid counterparts, and their mechanical stability cannot meet the requirements for practical applications at present. These, in particular, depend on flexible transparent electrodes (FTEs). Here, a high‐quality FTE (called FlexAgNE), with the simultaneously combined excellent characteristics, has been tested with a series of efficient active materials for flexible OPV devices, and high performance comparable with rigid counterparts has been achieved. In addition, due to the synergistic effect of FlexAgNE and the upper ZnO transport layer, including strong binding between the polyethylene terephthalate substrate and a hydrophilic polyelectrolyte (the key component of FlexAgNE), together with the capillary force effect of crossed silver nanowires and tight filling of ZnO, the flexible devices demonstrate robust mechanical stability even under extreme bending or folding conditions.

Sunlight can be converted to electricity via solar cells, with which then light can be generated the other way around. In this regard, skyscrapers performing light show during night or the bright Karst landscape under the ground are exemplified, echoing the geometry of the TiO₂ nanopillars which are fabricated via a low‐temperature dry process and work as the efficient electron‐transporting layer in flexible perovskite solar cells. More details can be found in article number 2001512 by Zhifeng Huang, Zijian Zheng, and co‐workers.

CsBr dual‐interface modification is employed in CsPbIBr₂ perovskite solar cells to facilitate crystallization and passivate surface defects and the synergistic interface modification finally generates the improved power conversion efficiency and stability.

Abstract

The organic‐inorganic hybrid perovskite solar cell has been a rising star in photovoltaics (PV) in the last decade due to its high efficiency and the fastest efficiency‐rise among all known materials in the PV history. The newly developed all‐inorganic perovskite, for its high stability against thermal and light irradiation stresses, is recognized as a promising material for both PV and general optoelectronic applications. Interface and its modification have been proven to play an important role in the solar cell performance. However, all previous research on the all‐inorganic CsPbIBr₂ based solar cells limits their scope to only one surface/interface while ignoring the other. Herein, synergistic effect is discovered when proper amount of CsBr is introduced on both sides of the perovskite active layer. It is found that the TiO₂/perovskite interface modification reduces pinhole and trap‐state densities while modification on perovskite/Spiro‐OMeTAD promotes smoother surface and better crystallinity. The synergistic effect of both modifications leads to increased efficiency to 10.33% with V _OC of 1.24 V, both are among the highest for these types of solar cells. In addition, the optimized device retains 60% of its initial efficiency after 60 h of aging in ambient atmosphere.

A multi-functional interfacial layer composed of a mixture of a poly(oxyethylene tridecyl ether) surfactant and an ethanolamine compound is introduced between a CH₃NH₃PbI₃ perovskite light-harvesting layer and a nickel oxide hole transport layer. Due to the improved film-forming and hole-extracting capabilities, excellent photovoltaic performance is successfully realized together with reduced recombination losses.

Abstract

Recently, hybrid organic–inorganic perovskite solar cells (PVSCs) have attracted significant attention owing to their simple solution processability and high efficiency for the next generation of low-cost solar cell technology. Herein, a multi-functional interfacial layer (IFL) composed of a mixture of poly(oxyethylene tridecyl ether) (PTE) and ethanolamine (EA) is introduced between a CH₃NH₃PbI₃ perovskite light-absorbing layer and a nickel oxide (NiO _x ) hole transport layer to improve the photovoltaic (PV) performance of PVSCs. With the solution-coated IFL of mixed PTE:EA, a highly improved film-forming capability of the perovskite layer is realized together with large-sized grains and fewer film defects. Moreover, the IFL also improved the charge carrier separation and hole-extraction capabilities at the interface between the CH₃NH₃PbI₃ and the NiO _x layers. The results here successfully demonstrate that the CH₃NH₃PbI₃ PVSC with IFL exhibits greatly improved PV performance, in this case a much higher power conversion efficiency (15.1%), greatly exceeding that (12.3%) of a reference device without an IFL. The author's study demonstrates that a multi-functional mixed IFL can be used as a solid foundation for efficient and cost-effective PVSCs, thus providing a platform for the realization of a new generation of highly efficient solution-processable PVSCs.