Chen Weijie

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Guanshui Xie, Zheling Zhang, Zhenying Su, Xiaoling Zhang, Jian Zhang

Graphical abstract

Moisture Stability

In article number 1900331, Baomin Xu, Chun Cheng, and co‐workers show that the cascade‐aligned energy levels and the defect‐passivation achieved by combining commercially accessible SnO₂ and home‐made TiO₂ nanoparticles effectively reduce energy loss and inhibit defects in the device. Consequently, the perovskite solar cell delivers a high power conversion efficiency (PCE) of 20.50%, which is superior to that of control devices based on SnO₂ with a PCE of 18.09%.

Synergistic effects originate from a dual functionalization of the electron transport layer/perovskite interface in solar cells designed on TiO₂ nanorods. TiCl₄ treatment combined with PC₆₁BM monolayer deposition leads to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to the suppression of hysteresis.

Abstract

The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO₂ nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl₄ treatment combined with a subsequent PC₆₁BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show J _SC, V _OC, and fill factor as high as 23.2 mA cm⁻², 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO₂ NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.

The efficiency of colloidal quantum dot solar cells (CQDSCs) is improved by employing molecularly engineered π‐conjugated polymer‐based organic hole transport materials (HTMs). Their optical and charge generation/collection properties are optimized, and the CQDSC with an efficiency of 11.53%, which is the best among CQDSCs using organic HTMs, is achieved.

Abstract

Organic p‐type materials are potential candidates as solution processable hole transport materials (HTMs) for colloidal quantum dot solar cells (CQDSCs) because of their good hole accepting/electron blocking characteristics and synthetic versatility. However, organic HTMs have still demonstrated inferior performance compared to conventional p‐type CQD HTMs. In this work, organic π‐conjugated polymer (π‐CP) based HTMs, which can achieve performance superior to that of state‐of‐the‐art HTM, p‐type CQDs, are developed. The molecular engineering of the π‐CPs alters their optoelectronic properties, and the charge generation and collection in CQDSCs using them are substantially improved. A device using PBDTTPD‐HT achieves power conversion efficiency (PCE) of 11.53% with decent air‐storage stability. This is the highest reported PCE among CQDSCs using organic HTMs, and even higher than the reported best solid‐state ligand exchange‐free CQDSC using pCQD‐HTM. From the viewpoint of device processing, device fabrication does not require any solid‐state ligand exchange step or layer‐by‐layer deposition process, which is favorable for exploiting commercial processing techniques.

Chain gang: Regulating the alkyl‐chain length of quantum dots attached to inorganic CsPbBr₃ perovskites, maximizes charge extraction and transfer at the perovskite/carbon interface. The optimized inorganic CsPbBr₃ perovskite solar cell (PSC) with C₁₂ alkyl chain QDs yields an efficiency of up to 10.85 %.

Abstract

Improved charge extraction and wide spectral absorption promote power conversion efficiency of perovskite solar cells (PSCs). The state‐of‐the‐art carbon‐based CsPbBr₃ PSCs have an inferior power output capacity because of the large optical band gap of the perovskite film and the high energy barrier at perovskite/carbon interface. Herein, we use alkyl‐chain regulated quantum dots as hole‐conductors to reduce charge recombination. By precisely controlling alkyl‐chain length of ligands, a balance between the surface dipole induced charge coulomb repulsive force and quantum tunneling distance is achieved to maximize charge extraction. A fluorescent carbon electrode is used as a cathode to harvest the unabsorbed incident light and to emit fluorescent light at 516 nm for re‐absorption by the perovskite film. The optimized PSC free of encapsulation achieves a maximum power conversion efficiency up to 10.85 % with nearly unchanged photovoltaic performances under 80 %RH, 80 °C, or light irradiation in air.

An all‐inorganic mixed‐halide perovskite solar cell with a power conversion efficiency of 16.42% is realized by using a Cs₂CO₃‐doped ZnO electron transport layer, which ascribes to the interfacial energy level tuning for reducing ohmic loss at the contact and enlarging the built‐in potential. A high thermostability is simultaneously obtained via surface defect passivation for improving the CsPbI₂Br film against phase transformation.

Abstract

Inorganic mixed‐halide CsPbX₃‐based perovskite solar cells (PeSCs) are emerging as one of the most promising types of PeSCs on account of their thermostability compared to organic–inorganic hybrid counterparts. However, dissatisfactory device performance and high processing temperature impede their development for viable applications. Herein, a facile route is presented for tuning the energy levels and electrical properties of sol–gel‐derived ZnO electron transport material (ETM) via the doping of a classical alkali metal carbonate Cs₂CO₃. Compared to bare ZnO, Cs₂CO₃‐doped ZnO possesses more favorable interface energetics in contact with the CsPbI₂Br perovskite layer, which can reduce the ohmic loss to a negligible level. The optimized PeSCs achieve an improved open‐circuit voltage of 1.28 V, together with an increase in fill factor and short‐circuit current. The optimized power conversion efficiencies of 16.42% and 14.82% are realized on rigid glass substrate and flexible plastic substrate, respectively. A high thermostability can be simultaneously obtained via defect passivation at the Cs₂CO₃‐doped ZnO/CsPbI₂Br interface, and 81% of the initial efficiency is retained after aging for 200 h at 85 °C.

Synergistic effects originate from a dual functionalization of the electron transport layer/perovskite interface in solar cells designed on TiO₂ nanorods. TiCl₄ treatment combined with PC₆₁BM monolayer deposition leads to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to the suppression of hysteresis.

Abstract

The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO₂ nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl₄ treatment combined with a subsequent PC₆₁BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show J _SC, V _OC, and fill factor as high as 23.2 mA cm⁻², 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO₂ NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.

Nature Communications, Published online: 10 January 2020; doi:10.1038/s41467-019-13909-5

Chain gang: Regulating the alkyl‐chain length of quantum dots attached to inorganic CsPbBr₃ perovskites, maximizes charge extraction and transfer at the perovskite/carbon interface. The optimized inorganic CsPbBr₃ perovskite solar cell (PSC) with C₁₂ alkyl chain QDs yields an efficiency of up to 10.85 %.

Abstract

Improved charge extraction and wide spectral absorption promote power conversion efficiency of perovskite solar cells (PSCs). The state‐of‐the‐art carbon‐based CsPbBr₃ PSCs have an inferior power output capacity because of the large optical band gap of the perovskite film and the high energy barrier at perovskite/carbon interface. Herein, we use alkyl‐chain regulated quantum dots as hole‐conductors to reduce charge recombination. By precisely controlling alkyl‐chain length of ligands, a balance between the surface dipole induced charge coulomb repulsive force and quantum tunneling distance is achieved to maximize charge extraction. A fluorescent carbon electrode is used as a cathode to harvest the unabsorbed incident light and to emit fluorescent light at 516 nm for re‐absorption by the perovskite film. The optimized PSC free of encapsulation achieves a maximum power conversion efficiency up to 10.85 % with nearly unchanged photovoltaic performances under 80 %RH, 80 °C, or light irradiation in air.

The systematic working principle study of fluorination on fused‐ring electron acceptors paired with an oligothiophene donor is described, including the quantification of the key parameters to unveil the involving photophysics process, and enables top‐performing oligothiophene‐based all‐small‐molecule photovoltaics.

Fluorination has proven effective in increasing the absorption, downshifting the energy levels, and enhancing the crystallinity of high‐performance fused‐ring electron acceptors (FREAs). However, an in‐depth understanding of the effects of fluorination is still lacking, as research efforts have mainly focused on increasing the power conversion efficiency (PCE). In addition, fluorination on FREAs has rarely been reported in all‐small‐molecule organic solar cells (ASM OSCs). Herein, fluorination on FREAs is systematically studied in ASM OSCs using the popular FREA 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile and its fluorinated analog paired with DRCN5T, an oligothiophene donor seldom investigated in ASM OSCs to date. (Photo)physical studies are conducted on both systems and it is identified that, along with the aforementioned ones, fluorination exerts several additional effects, including the following. First, it optimizes the morphology, thereby accelerating charge separation and reducing geminate recombination charge pairs; second, it suppresses energetic disorder; and third, it prolongs the carrier lifetime and thus aids charge extraction. Consequently, the short‐circuit current density and fill factor are significantly enhanced, and in turn, the PCE yields a 36% improvement, climbing to 9.25% and rivaling that of the current state‐of‐the‐art oligothiophene‐donor/nonfullerene ASM OSCs. The insights decipher the working mechanism of ASM OSCs that use fluorinated FREAs, paving the way toward high‐performance ASM OSCs.

Cubic zinc metatitanate (ZTO) is identified as an excellent electron transport material with interface engineering treatment of dezincification for high efficient perovskite solar cells (PSCs). By integrating an inorganic hole transport layer and rGO protection, the ZTO electron transport layer‐based PSCs exhibit strong resistance to moisture, heat, and ultraviolet light, demonstrating high efficiency and stability toward practical applications.

Perovskite solar cells (PSCs) have experienced considerable development in the past few years. The stability issue has become a focus of research efforts toward their commercial applications. The development and interface engineering of electron transport materials (ETMs) to build up stable interfaces with perovskites has been emerging as a powerful strategy to enhance PSCs' stability. Herein, cubic zinc metatitanate (ZTO) is identified as an excellent ETM with interface engineering treatment of dezincification for fabricating PSCs with much better overall performances than those fabricated from TiO₂, a popularly used ETM. The high electron mobility of ZTO helps minimize the hysteresis. Together with the use of CuSCN as inorganic hole transport material and further protecting the PSCs with reduced graphene oxide, the ZTO‐based PSCs exhibit remarkable enhancement in stability, retaining 95% of initial power conversion efficiency under AM 1.5 G illumination at 85 °C and 85% relative humidity in air for 1000 h at open circuit.

Piecewise spray is used to control the component distribution of bulk heterojunction for nonfullerene polymer solar cells. Assisted by 1,3,5‐trimethylbenzene‐based solvent engineering, a solvent driving force is formed, resulting in a continuous interface for piecewise spray‐coated active layers. PBDB‐T‐2Cl:IT4F photovoltaic device then shows a notable improvement in conversion efficiency from 10.78% to 12.29%.

Piecewise spray has the advantage of controlling the component distribution, i.e., constructing vertical phase‐separated active layers, to improve the charge generation efficiency of large‐scale polymer solar cells. However, it brings a connection problem because of the drastic difference in the donor/acceptor ratio of continuous coatings using the conventional solvent. Assisted by 1,3,5‐trimethylbenzene‐based solvent engineering, a solvent driving force is formed, resulting in a continuous interface for piecewise spray‐coated active layers. The blend film forms a bicontinuous interpenetrating network in the active layer and provides efficient percolation pathways for charge carrier transport, resulting in a considerable improvement in the photovoltaic performance. When the donor/acceptor ratio is 1:2 in the first coating and 2:1 in the second coating, and the thickness of each coating is 90 nm, the performance of the PBDB‐T‐2Cl:IT4F photovoltaic device shows a notable conversion efficiency of 12.29% with a high short‐circuit current density of 21.55 mA cm⁻². Importantly, this piecewise spray technique can be used for large‐scale module fabrication.

The role of Mn substitution in sulfide and sulfoselenide films are investigated in terms of optoelectronic properties and device performances by having double‐layered structures. The sulfoselenide devices show higher improvement due to larger improved carriers’ collection, separation, and transport improvement. Meanwhile, the higher Mn in sulfide films induces reduction in acceptor defect level.

Cation substitution is one of the effective ways to improve Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic performance. However, the commonly reported substitutes, Ag and Cd, are not ideal as they detract from the earth‐abundant and nontoxic motivation of CZTSSe. Herein, the role of Mn substitution in sulfide and sulfoselenide films are compared in terms of optoelectronic properties and device performance. CZT(S,Se) + CMZT(S,Se) double‐layered structures are fabricated by a sol–gel spin‐coating method with variations in the CMZT(S,Se) layer thickness. It is found that a smaller amount of Mn is required to achieve the highest photovoltaic performance in sulfoselenide films in comparison with sulfide‐based films. All device parameters (particularly V _oc and fill factor) of the sulfoselenide films are improved as compared with the sulfide system. Using a combination of capacitance–voltage, drive‐level capacitance profiling, and photoluminescence (PL), it is found that the sulfoselenide film has a smaller interface defect density and higher hole mobility and PL intensity, which suggest much more effective charge separation and transport. In contrast, in double‐layer sulfide films, Mn reduces the acceptor defect level of the absorber.

An effective polar molecule of (2‐aminothiazole‐4‐yl)acetic acid (ATAA) is incorporated onto a ZnO electron transport layer to simultaneously achieve defect passivation and work function modulation by forming permanent interface dipoles. It minimizes the charge recombination loss in perovskite solar cells, and the ZnO–ATAA‐based device ultimately achieves an enhanced efficiency of 19.74% while suppressing the device hysteresis.

The intrinsic characteristics of a ZnO electron transport layer (ETL) lead to severe charge loss in perovskite solar cells (PSCs), such as photogenerated charge accumulation recombination in the perovskite layer due to the low electron extraction capacity, and defect‐induced charge recombination at the interface due to the unfavorable defects, causing efficiency loss and device hysteresis. Here, the polar molecule of (2‐aminothiazole‐4‐yl)acetic acid (ATAA) is self‐assembled onto a ZnO layer with the help of oxygen vacancy defects, combining the advantages of lowering the work function by forming the permanent interface dipole and simultaneously passivating defect states. It effectively strengthens the electron extraction capacity and reduces the density of defect states. Therefore, the resulting PSCs with a ZnO–ATAA ETL yield an enhanced efficiency of 19.74% with evidently reduced device hysteresis.

A simple low‐temperature‐processed In₂O₃/SnO₂ bilayer electron‐transport layer (ETL) is used for fabricating efficient perovskite solar cells (PSCs). The bilayer ETL with appropriate energy alignment is beneficial for charge transfer, thus minimizing open‐circuit voltage (V _OC) loss. An optimized planar PSC with a power conversion efficiency (PCE) of 23.24% is obtained. In contrast, devices based on single SnO₂ only achieve efficiency of 21.42%.

Abstract

An electron‐transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open‐circuit voltage (V _OC). Herein, a simple low‐temperature‐processed In₂O₃/SnO₂ bilayer ETL is developed and used for fabricating a new PSC device. The presence of In₂O₃ results in uniform, compact, and low‐trap‐density perovskite films. Moreover, the conduction band of In₂O₃ is shallower than that of Sn‐doped In₂O₃ (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing V _OC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high V _OC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO₂ layers achieve 21.42% efficiency with a V _OC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N₂ without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices.

A novel wide bandgap polymer donor PNDT‐ST and a near infrared nonfullerene acceptor Y6‐T are developed for highly efficient organic solar cells. The high lowest unoccupied molecular orbital energy of Y6‐T and the high crystallinity of PNDT‐ST as well as the compatible PNDT‐ST:PNDT‐ST:Y6‐T ternary blend enable the significantly improved power conversion efficiency of 16.57% with minimal energy loss of 0.521 eV.

Abstract

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V _oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E _loss) of 0.521 eV. Such small E _loss is the best record for polymer solar cells with PCEs over 16% to date.

Publication date: 19 February 2020

Source: Joule, Volume 4, Issue 2

Author(s): Rui Sun, Qiang Wu, Jie Guo, Tao Wang, Yao Wu, Beibei Qiu, Zhenghui Luo, Wenyan Yang, Zhicheng Hu, Jing Guo, Mumin Shi, Chuluo Yang, Fei Huang, Yongfang Li, Jie Min