Chen Weijie

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Wei Wang, Qiang Wu, Rui Sun, Jing Guo, Yao Wu, Mumin Shi, Wenyan Yang, Hongneng Li, Jie Min

A narrow‐bandgap polymer acceptor PF3‐DTCO is developed by increasing the conjugation of the acceptor unit from the five‐ring‐fused IDIC16 to the seven‐ring‐fused ITIC16 and enhancing the electron‐donating ability of the donor unit from carbon‐bridged DTC to carbon–oxygen‐bridged DTCO, and a high power conversion efficiency of 10.13% with a high J _sc of 15.75 mA cm⁻² in all‐polymer solar cells is achieved.

The limited light absorption capacity for most polymer acceptors hinders the improvement of the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs). Herein, by simultaneously increasing the conjugation of the acceptor unit and enhancing the electron‐donating ability of the donor unit, a novel narrow‐bandgap polymer acceptor PF3‐DTCO based on an A–D–A‐structured acceptor unit ITIC16 and a carbon–oxygen (C–O)‐bridged donor unit DTCO is developed. The extended conjugation of the acceptor units from IDIC16 to ITIC16 results in a red‐shifted absorption spectrum and improved absorption coefficient without significant reduction of the lowest unoccupied molecular orbital energy level. Moreover, in addition to further broadening the absorption spectrum by the enhanced intramolecular charge transfer effect, the introduction of C–O bridges into the donor unit improves the absorption coefficient and electron mobility, as well as optimizes the morphology and molecular order of active layers. As a result, the PF3‐DTCO achieves a higher PCE of 10.13% with a higher short‐circuit current density (J _sc) of 15.75 mA cm⁻² in all‐PSCs compared with its original polymer acceptor PF2‐DTC (PCE = 8.95% and J _sc = 13.82 mA cm⁻²). Herein, a promising method is provided to construct high‐performance polymer acceptors with excellent optical absorption for efficient all‐PSCs.

A new strategy is established to improve the performance of perovskite solar cells, which sheds more light on the currently proposed mechanism governing the action of moisture on the quality of perovskite film. Self‐passivated perovskite solar cells show an extraordinary V_OC of 1.17 V and the highest efficiency of 21.38%.

The performance of perovskite solar cells (PSCs) is known to be extremely sensitive to humidity in the preparation environment. However, the main mechanism by which the moisture influences the quality of the perovskite film and the device performance is not yet fully understood. Herein, a new strategy is established to obtain inverted PSCs with a remarkabll high V _OC by including a high‐humidity treatment and sufficient DMSO‐atmosphere annealing in the preparation process. It is found that the lattice distortion on the surface of perovskite grains caused by the high‐humidity treatment plays a key role in the self‐passivation of perovskite. Inverted (p‐i‐n) PSCs based on the self‐passivated perovskite films show effective suppression of nonradiative recombination, which increase the device V _OC to 1.17 V and achieve the highest efficiency of 21.38%. It is expected that the findings of this work shed more light on the currently proposed mechanism governing the action of moisture on the performance of the PSCs.

The highest power conversion efficiency of the perovskite solar cells based on poly (3‐hexylthiophene)/gold nanorod (P3HT/AuNR) composite hole‐transporting material reaches up to 16.88%, which is higher than that of a pristine P3HT‐based device (13.40%). The enhancement can be attributed to the higher carrier mobility and increased light utilization efficiency induced by addition of AuNRs with localized surface plasmon resonance effect in the polymer matrix.

Poly(3‐hexylthiophene)/gold nanorod (P3HT/AuNR) composites are developed and introduced as hole‐transporting materials (HTMs) to fabricate mixed‐ion perovskite solar cells (PSCs). The highest power conversion efficiency of the optimized devices based on the composite HTM reaches up to 16.88%, which is an increase of 26% from that of a pristine P3HT‐based device (13.40%). The enhanced performance can be attributed to the increased crystallinity of P3HT induced by the addition of AuNRs in the polymer matrix and the localized surface plasmon resonance effect of AuNRs, which lead to higher carrier mobility and increased light utilization efficiency. This work provides a comprehensive understanding of the effect of plasmonic AuNRs in PSCs application and a useful method to further improve the performance of PSCs.

The inverse temperature‐dependencies of the photovoltaic parameters in MAPbI₃ perovskite solar cells lead to obtaining a peak efficiency of 21.4% at 220 K. These T ‐varied behaviors are related to combined properties of improved interfacial charge extraction, reduced charge trap density, and suppressed nonradiative recombination at lower temperatures.

Abstract

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T )‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T ‐dependent photovoltaic properties are explored and negative T ‐coefficients for the three device parameters (V _OC, J _SC, and FF) are observed within a wide low T ‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T ‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T . By comparing the T ‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T ‐dependencies of J _SC and FF, which eventually leads to nearly T ‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T ‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.

Herein, 17% efficient and stable ternary organic solar cells are realized by introducing a delayed fluorescence material 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in a non‐fullerene system. Long‐lifetime singlet excitons on APDC‐TPDA can transfer to the polymer donor to prolong the excitons lifetime and suppress the reverse energy transfer from charge transfer state to triplet state, and then reduce the recombination energy loss of the device.

Abstract

Charge transfer state (CT) plays an important role in exciton diffusion, dissociation, and charge recombination mechanisms. Enhancing the utilization and suppressing the recombination process of CT excitons is a promising way to improve the performance of organic solar cells (OSCs). Here, an effective method is presented via introducing a delayed fluorescence (DF) emitter 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in OSCs. The long‐lifetime singlet excitons on APDC‐TPDA can transfer to polymer donors to prolong exciton lifetime, which ensures sufficient time for diffusion and dissociation. Concurrently, the high triplet energy level (T₁) of the DF material can also prevent the reverse energy transfer from CT to T₁. APDC‐TPDA‐containing ternary OSCs shows a high PCE of 16.96% with a reduced recombination energy loss of 0.46 eV. It is noteworthy that the ternary OSC also exhibits superior storage stability. After 55 days of storage, the PCE of the ternary OSC still retains about 96% of its primitive state. Furthermore, this ternary strategy is efficient and universally applicable to OSCs, and positive results can be obtained in different systems with different DF emitters. These results indicate that the ternary strategy provides a new design idea to realize high performance OSCs.

Graphitic carbon nitride (g‐C₃N₄) is doped into PEDOT:PSS to improve the conductivity by weakening the shield effect of PSS on conductive PEDOT. Employing g‐C₃N₄ doped PEDOT:PSS as a hole transport layer for PM6:Y6‐based organic solar cells, a device efficiency of up to 16.4% is achieved, partly as a result of improved charge transport and suppressed charge recombination at the interface.

Abstract

The power‐conversion efficiency (PCE) of single‐junction organic solar cells (OSCs) has exceeded 16% thanks to the development of non‐fullerene acceptor materials and morphological optimization of active layer. In addition, interfacial engineering always plays a crucial role in further improving the performance of OSCs based on a well‐established active‐layer system. Doping of graphitic carbon nitride (g‐C₃N₄) into poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for PM6:Y6‐based OSCs is reported, boosting the PCE to almost 16.4%. After being added into the PEDOT:PSS, the g‐C₃N₄ as a Bronsted base can be protonated, weakening the shield effect of insulating PSS on conductive PEDOT, which enables exposures of more PEDOT chains on the surface of PEDOT:PSS core‐shell structure, and thus increasing the conductivity. Therefore, at the interface between g‐C₃N₄ doped HTL and PM6:Y6 layer, the charge transport is improved and the charge recombination is suppressed, leading to the increases of fill factor and short‐circuit current density of devices. This work demonstrates that doping g‐C₃N₄ into PEDOT:PSS is an efficient strategy to increase the conductivity of HTL, resulting in higher OSC performance.

A strong fluorine‐containing Lewis acid tris(pentafluorophenyl) phosphine (TPFP) is developed to passivate mixed perovskite solar cells, achieving a champion efficiency of 22.02% and a high stability under 85% relative humidity. The moisture degradation mechanism is phase segregation of I‐rich black phase and Cs/Br‐rich yellow phase resulting from water‐assisted synergistic Cs and halide ion migrations.

Abstract

Multiple‐cation lead mixed‐halide perovskites (MLMPs) have been recognized as ideal candidates in perovskite solar cells in terms of high efficiency and stability due to decreased open‐circuit voltage loss and suppressed yellow phase formation. However, they still suffer from an unsatisfactory long‐term moisture stability. In this study, phosphorus‐containing Lewis acid and base molecules are employed to improve device efficiency and stability based on their multifunction including recombination reduction, phase segregation suppression, and moisture resistance. The strong fluorine‐containing Lewis acid treatment can achieve a champion PCE of 22.02%. Unencapsulated and encapsulated devices retain 63% and 80% of the initial efficiency after 14 days of aging under 75% and 85% relative humidity, respectively. The better passivation of Lewis acid implies more halide defects than Pb defects at the MLMP surface. This unbalanced defect type results from phase segregation that is the synergistic effect of Cs and halide ion migrations. Identifying defect type based on different passivation effects is beneficial to not only choose suitable passivators to boost the efficiency and slow down the moisture degradation of MLMP solar cells, but also to understand the mechanism of defect‐assisted moisture degradation.

Novel fullerene dimers are designed and employed as interfacial materials in perovskite solar cells, and shown to be effective at passivating and stabilizing devices with a maximum efficiency of 22.3% without any hysteresis and with 98% retained efficiency after ambient storage for 1000 h.

Abstract

A major limit for planar perovskite solar cells is the trap‐mediated hysteresis and instability, due to the defective metal oxide interface with the perovskite layer. Passivation engineering with fullerenes has been identified as an effective approach to modify this interface. The rational design of fullerene molecules with exceptional electrical properties and versatile chemical moieties for targeted defect passivation is therefore highly demanded. In this work, novel fulleropyrrolidine (NMBF‐X, XH or Cl) monomers and dimers are synthesized and incorporated between metal oxides (i.e. TiO₂, SnO₂) and perovskites (i.e. MAPbI₃ and (FAPbI₃) _x (MAPbBr₃)_{1‐ x} ). The fullerene dimers provide superior stability and efficiency improvements compared to the corresponding monomers, with chlorinated fullerene dimers being most effective at coordinating with both metal oxides and perovskite via the chlorine terminals. The non‐encapsulated planar device delivers a maximum power conversion efficiency of 22.3% without any hysteresis, while maintaining over 98% of initial efficiency after ambient storage for 1000 h, and exhibiting an order of magnitude improvement of the T₈₀ lifetime.

Efficient and robust interconnection in perovskite‐based tandem solar cells is necessary to ensure high performances. This review summarizes the functions, requirements, and recent advances of interconnecting layers in perovskite‐based tandem solar cells.

Abstract

Organic–inorganic hybrid perovskite materials are excellent candidates as light absorbers in tandem solar cells with advantages of tunable bandgaps, high absorption coefficients, facile fabrication processes, and low costs. Tandem devices offer a route to further improve the efficiency and reduce the cost for the solar cell practical applications. One critical challenge that limits the development of two‐terminal perovskite‐based tandem devices is the interconnection between two subcells. To achieve efficient interconnection in the tandem devices, it is required to simultaneously fulfill the high electrical, optical, and chemical requirements. In particular, chemical protection requirement is necessary to enable a tandem device in the case of solution‐processed perovskite–perovskite tandem solar cells. In this work, recent advances of interconnection in perovskite‐based two‐terminal tandem solar cells are reviewed. A brief introduction to the topic is first given. The definition, functions, and requirements of interconnecting layers in two‐terminal tandem devices are then discussed. Next, the insights into recent advances of interconnecting layers in two‐terminal perovskite‐based tandem solar cells (perovskite–perovskite, perovskite–polymer, perovskite–inorganic tandem solar cells) are further described. Finally, an outlook of the future research directions and a brief summary are drawn.

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Wei Hui, Ying Xu, Fei Xia, Hui Lu, Bixin Li, Lingfeng Chao, Tingting Niu, Bin Du, Haiyan Du, Xueqin Ran, Yingguo Yang, Yingdong Xia, Xingyu Gao, Yonghua Chen, Wei Huang

Publication date: 15 April 2020

Source: Joule, Volume 4, Issue 4

Author(s): Xiangyue Meng, Yanbo Wang, Jianbo Lin, Xiao Liu, Xin He, Julien Barbaud, Tianhao Wu, Takeshi Noda, Xudong Yang, Liyuan Han

An oxidized phenothiazine‐based (OPTZ) hole transporting material (HTM) synthesized from its neutral form (NPTZ) is used to accurately tune the concentration of radical cations in HTMs via its stoichiometric ratio. Using the optimized ratio of OPTZ as the dopant in the HTM, the hole transporting mobility is effectively enhanced, due to the intra‐and intermolecular charge transfer process, thus increasing the fill‐factor of perovskite solar cells.

In traditional n‐i‐p‐type perovskite solar cells (PSCs), most hole transporting materials (HTMs) rely on an uncontrolled oxidative process using Li salt and Co (III) complex to achieve sufficient hole mobilities. Herein, a stabilized oxidized phenothiazine‐based HTM (OPTZ) synthesized from its neutral form (NPTZ) through a photoredox reaction is demonstrated. This controllable and stable oxidation state is mainly derived from the planar structure and π conjugation of phenothiazine core in OPTZ. The energy gap between the singly occupied molecular orbital (SOMO) of OPTZ and highest occupied molecular orbital (HOMO) of NPTZ suitably promotes hole hopping in hole transporting layers. Using an optimized ratio of OPTZ as the dopant in NPTZ, the hole transporting mobility is effectively enhanced due to an intra‐ and intermolecular charge transfer process, resulting in an enhancement in the fill factor of the PSCs. Herein, a new strategy to obtain stabilized oxidized HTMs, which deliver significantly enhanced hole mobilities of HTMs in PSCs, is provided.

Combined data of transient optical and electro‐optical experiments reveals the efficiency determining processes in all‐polymer solar cells and precisely quantifies their yields. For the test system presented here, field‐dependent charge separation limits the fill factor and thus the performance evident by comparing the experimentally measured current–voltage characteristics to those reproduced by drift‐diffusion simulations using the spectroscopically determined kinetic parameters.

All‐polymer solar cells lag behind the state‐of‐the‐art in small molecule nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells (OSCs) for reasons still unclear. Herein, the efficiency‐limiting processes in all‐polymer solar cells are investigated using blends of the common donor polymer PBDT‐TS1 with different acceptor polymers, namely P2TPD[2F]T and P2TPDBT[2F]T. Combining data from steady‐state optical spectroscopy and time‐resolved photoluminescence, transient absorption, and time‐delayed collection field experiments, provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and nongeminate charge carrier recombination, field‐dependent charge generation, and inefficient carrier extraction. Although both systems exhibit a similar charge separation efficiency in the absence of external bias, charge separation is significantly enhanced in P2TPDBT[2F]T‐based blends when biased. Kinetic parameters obtained via pulsed laser spectroscopy are used to reproduce the experimentally measured device current–voltage (J –V ) characteristics and indicate that low fill factors originate either from nongeminate recombination competing with charge extraction, or from a pronounced field dependence of charge generation, depending on the acceptor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs including both all‐polymer and small molecule NFA systems.

Hysteresis in perovskite solar cells is suppressed with the insertion of a phenyl‐C61‐butyric acid methyl ester (PCBM) layer. In situ PL imaging is employed to observe the ionic migration in the perovskite layer, perovskite/PCBM bilayer and PPCBM bilayer. The mobilizable PCBM molecules are able to diffuse into the perovskite and therein passivate iodine ions/vacancies, thus reducing the hysteresis.

Abstract

The power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.

Herein, 17% efficient and stable ternary organic solar cells are realized by introducing a delayed fluorescence material 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in a non‐fullerene system. Long‐lifetime singlet excitons on APDC‐TPDA can transfer to the polymer donor to prolong the excitons lifetime and suppress the reverse energy transfer from charge transfer state to triplet state, and then reduce the recombination energy loss of the device.

Abstract

Charge transfer state (CT) plays an important role in exciton diffusion, dissociation, and charge recombination mechanisms. Enhancing the utilization and suppressing the recombination process of CT excitons is a promising way to improve the performance of organic solar cells (OSCs). Here, an effective method is presented via introducing a delayed fluorescence (DF) emitter 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in OSCs. The long‐lifetime singlet excitons on APDC‐TPDA can transfer to polymer donors to prolong exciton lifetime, which ensures sufficient time for diffusion and dissociation. Concurrently, the high triplet energy level (T₁) of the DF material can also prevent the reverse energy transfer from CT to T₁. APDC‐TPDA‐containing ternary OSCs shows a high PCE of 16.96% with a reduced recombination energy loss of 0.46 eV. It is noteworthy that the ternary OSC also exhibits superior storage stability. After 55 days of storage, the PCE of the ternary OSC still retains about 96% of its primitive state. Furthermore, this ternary strategy is efficient and universally applicable to OSCs, and positive results can be obtained in different systems with different DF emitters. These results indicate that the ternary strategy provides a new design idea to realize high performance OSCs.

Graphitic carbon nitride (g‐C₃N₄) is doped into PEDOT:PSS to improve the conductivity by weakening the shield effect of PSS on conductive PEDOT. Employing g‐C₃N₄ doped PEDOT:PSS as a hole transport layer for PM6:Y6‐based organic solar cells, a device efficiency of up to 16.4% is achieved, partly as a result of improved charge transport and suppressed charge recombination at the interface.

Abstract

The power‐conversion efficiency (PCE) of single‐junction organic solar cells (OSCs) has exceeded 16% thanks to the development of non‐fullerene acceptor materials and morphological optimization of active layer. In addition, interfacial engineering always plays a crucial role in further improving the performance of OSCs based on a well‐established active‐layer system. Doping of graphitic carbon nitride (g‐C₃N₄) into poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for PM6:Y6‐based OSCs is reported, boosting the PCE to almost 16.4%. After being added into the PEDOT:PSS, the g‐C₃N₄ as a Bronsted base can be protonated, weakening the shield effect of insulating PSS on conductive PEDOT, which enables exposures of more PEDOT chains on the surface of PEDOT:PSS core‐shell structure, and thus increasing the conductivity. Therefore, at the interface between g‐C₃N₄ doped HTL and PM6:Y6 layer, the charge transport is improved and the charge recombination is suppressed, leading to the increases of fill factor and short‐circuit current density of devices. This work demonstrates that doping g‐C₃N₄ into PEDOT:PSS is an efficient strategy to increase the conductivity of HTL, resulting in higher OSC performance.

By employing HOOCCH₂NH₃ ⁺ (Gly⁺) with its outstanding additive effect, self‐additive low‐dimensional Ruddlesden–Popper perovskites are first designed. As a result, the Gly‐based self‐additive low‐dimensional RP perovskites with large grain sizes exhibit remarkable photoelectric properties, yielding the highest power conversion efficiency of 18.06% with negligible hysteresis. More importantly, Gly‐based devices exhibit markedly improved stability against humidity, heat, and UV light.

Abstract

The recent rise of low‐dimensional Ruddlesden–Popper (RP) perovskites is notable for superior humidity stability, however they suffer from low power conversion efficiency (PCE). Suitable organic spacer cations with special properties display a critical effect on the performance and stability of perovskite solar cells (PSCs). Herein, a new strategy of designing self‐additive low‐dimensional RP perovskites is first proposed by employing a glycine salt (Gly⁺) with outstanding additive effect to improve the photovoltaic performance. Due to the strong interaction between CO and Pb²⁺, the Gly⁺ can become a nucleation center and be beneficial to uniform and fast growth of the Gly‐based RP perovskites with larger grain sizes, leading to reduced grain boundary and increased carrier transport. As a result, the Gly‐based self‐additive low‐dimensional RP perovskites exhibit remarkable photoelectric properties, yielding the highest PCE of 18.06% for Gly (n = 8) devices and 15.61% for Gly (n = 4) devices with negligible hysteresis. Furthermore, the Gly‐based devices without encapsulation show excellent long‐term stability against humidity, heat, and UV light in comparison to BA‐based low‐dimensional PSCs. This approach provides a feasible design strategy of new‐type low‐dimensional RP perovskites to obtain highly efficient and stable devices for next‐generation photovoltaic applications.

State‐of‐the‐art perovskite solar cells generally consist of an excess of lead iodide (PbI₂) as passivator. In this work, ligand‐modulation technology is demonstrated to fabricate vertically distributed PbI₂ nanosheets between the perovskite grain boundaries, which enhances the passivation effect of PbI₂ and improves the power conversion efficiency and stability of perovskite solar cells.

Abstract

Excess lead iodide (PbI₂), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high‐efficiency perovskite solar cells. However, the random distribution of excess PbI₂ also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand‐modulation technology is developed to modulate the shape and distribution of excess PbI₂ in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI₂ nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI₂ and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.

Nanopyramid arrays of 1D highly oriented anatase TiO₂ present an oriented electric field distribution, which is favorable for charge separation and transport. An impressive power conversion efficiency of approximately 22.5 % was achieved, which is the highest efficiency reported for perovskite solar cells consisting of 1D electron transport materials to date.

Abstract

One‐dimensional (1D) nanostructured oxides are proposed as excellent electron transport materials (ETMs) for perovskite solar cells (PSCs); however, experimental evidence is lacking. A facile hydrothermal approach was employed to grow highly oriented anatase TiO₂ nanopyramid arrays and demonstrate their application in PSCs. The oriented TiO₂ nanopyramid arrays afford sufficient contact area for electron extraction and increase light transmission. Moreover, the nanopyramid array/perovskite system exhibits an oriented electric field that can increase charge separation and accelerate charge transport, thereby suppressing charge recombination. The anatase TiO₂ nanopyramid array‐based PSCs deliver a champion power conversion efficiency of approximately 22.5 %, which is the highest power conversion efficiency reported to date for PSCs consisting of 1D ETMs. This work demonstrates that the rational design of 1D ETMs can achieve PSCs that perform as well as typical mesoscopic and planar PSCs.

State‐of‐the‐art perovskite solar cells generally consist of an excess of lead iodide (PbI₂) as passivator. In this work, ligand‐modulation technology is demonstrated to fabricate vertically distributed PbI₂ nanosheets between the perovskite grain boundaries, which enhances the passivation effect of PbI₂ and improves the power conversion efficiency and stability of perovskite solar cells.

Abstract

Excess lead iodide (PbI₂), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high‐efficiency perovskite solar cells. However, the random distribution of excess PbI₂ also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand‐modulation technology is developed to modulate the shape and distribution of excess PbI₂ in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI₂ nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI₂ and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.