Chen Weijie

A remarkable high efficiency of 17.34% is achieved for low‐dimensional Ruddlesden–Popper perovskite (RPP) solar cells (n ≤ 5) by using a fluorinated phenylethalammonium (4‐fluoro‐phenethylammonium (4FPEA)) organic cation. These fluorinated devices also show the better humidity and thermal stability as compared to nonfluorinated phenylethlammonium (PEA) devices. These findings provide a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

Abstract

Low‐dimensional Ruddlesden–Popper perovskites (RPPs) exhibit excellent stability in comparison with 3D perovskites; however, the relatively low power conversion efficiency (PCE) limits their future application. In this work, a new fluorine‐substituted phenylethlammonium (PEA) cation is developed as a spacer to fabricate quasi‐2D (4FPEA)₂(MA)₄Pb₅I₁₆ (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a J _sc of 19.00 mA cm⁻², a V _oc of 1.16 V, and a fill factor (FF) of 79%, which are among the best results for low‐dimensional RPP solar cells (n ≤ 5). The enhanced device performance can be attributed as follows: first, the strong dipole field induced by the 4‐fluoro‐phenethylammonium (4FPEA) organic spacer facilitates charge dissociation. Second, fluorinated RPP crystals preferentially grow along the vertical direction, and form a phase distribution with the increasing n number from bottom to the top surface, resulting in efficient charge transport. Third, 4FPEA‐based RPP films exhibit higher film crystallinity, enlarged grain size, and reduced trap‐state density. Lastly, the unsealed fluorinated RPP devices demonstrate superior humidity and thermal stability. Therefore, the fluorination of the long‐chain organic cations provides a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

Nature Energy, Published online: 05 August 2019; doi:10.1038/s41560-019-0434-y

Publication date: October 2019

Source: Nano Energy, Volume 64

Author(s): Zhenye Li, Wenkai Zhong, Lei Ying, Feng Liu, Ning Li, Fei Huang, Yong Cao

Graphical abstract

Thionation is a straightforward strategy to dramatically boost the performance of dopant‐free polymeric hole‐transporting materials (HTMs) for perovskite solar cells. Upon HTM thionation, a nearly 40% enhancement in the power conversion efficiency of the corresponding devices is observed. Such an increase is attributed to the enhancement of both the hole transport within the HTM and the interfacial hole transfer dynamics.

Abstract

To date, the most efficient perovskite solar cells (PSCs) require hole‐transporting materials (HTMs) that are doped with hygroscopic additives to improve their performance. Unfortunately, such dopants negatively impact the overall PSCs stability and add cost and complexity to the device fabrication. Hence, there is a need to investigate new strategies to boost the typically modest performance of dopant‐free HTMs for efficient and stable PSCs. Thionation is a simple and single‐step approach to enhance the carrier‐transport capability of organic semiconductors, yet still completely unexplored in the context of HTMs for PSCs. In this work, a novel polymeric semiconductor, P1, based on a diketopyrrolopyrrole (DPP) moiety, is proposed as a dopant‐free HTM. Its modest performance in PSCs (power conversion efficiency (PCE) = 7.1%) is significantly enhanced upon thionation of the DPP moiety. The resulting dithioketopyrrolopyrrole‐based HTM, P2, leads to PSCs with nearly 40% performance improvement (PCE = 9.7%) compared to devices based on the nonthionated HTM (P1). Furthermore, thionation also remarkably boosts the shelf‐storage and thermal stability with respect to traditional 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene‐based PSCs. This work provides useful insights to further design effective dopant‐free HTMs employing the straightforward one‐step thionation strategy for efficient and stable PSCs.

Efficient Zn‐SnO _x electron transport layers (ETLs) by the low‐temperature (100 °C) electron beam (E‐beam) method are prepared. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved power conversion efficiency from 18.95% to 20.16%. The low‐temperature preparation of ETLs and the excellent performance of devices present great commercial potential for future applications.

Perovskite solar cells (PSCs) attract tremendous interest due to their feasibility, high power conversion efficiency (PCE), light weight, and flexible architecture. However, some challenges are still present for cheap mass fabrication in commercial applications. Herein, efficient Zn‐SnO _x electron transport layers (ETLs) are used by the low‐temperature (100 °C) electron beam (E‐beam) method. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved PCE from 18.95% to 20.16%. More importantly, the PCE of the modified device is more than 80% of its initial values for 800 h in ambient air with a relative humidity of ≈40%. The flexible device exhibits a PCE of 15.25% and remains at an initial PCE of 83% after 100 bending cycles. The efficient and flexible PSCs are potentially used as wearable energy power sources. The low‐temperature preparation of ETL and the excellent performance of devices present great commercial potential for future applications.

Herein, Pb‐site doping in organic–inorganic hybrid perovskite (OIH‐LHP) and inorganic CsPbX₃‐based materials is discussed, elucidating the functions of doping on lead halide perovskite (LHP) crystallization, optoelectronic property, and stability. Perspectives for further investigation are also presented.

Although great success has been achieved in perovskite solar cells (PSCs), it still suffers from several drawbacks in terms of stability and higher efficiency. Doping as an effective method to modify the optical and electronic properties of the materials is extensively studied in lead halide perovskites (LHPs). Herein, Pb‐site doping in organic–inorganic hybrid perovskites (OIH‐LHPs) and inorganic CsPbX₃‐based materials is discussed. Doping has three functions toward PSCs: participating in the crystalline process, modifying the energy states in LHPs, and contributing to the stability of PSCs. Issues about further improvements are raised, and perspectives for further investigation are presented.

The location and distribution of fullerenes in the perovskite:fullerene hybrid phase are confirmedly visualized by the conductive atomic force microscopy and Kelvin probe force microscopy measurements. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current is avoided in perovskite solar cells based on the hybrid perovskite:fullerene phases.

In perovskite solar cells (PSCs), hybrid perovskite:fullerene phases are proposed to suppress macroscopic current hysteresis behavior by alleviating ion migration. However, the understanding of how fullerenes exactly alleviate the current hysteresis and what is the influence of fullerenes in such hybrid phases are still unclear from a microscopic viewpoint. Herein, the intentional incorporation of fullerene into perovskite is used to examine how fullerene exactly reduces the macroscopic current hysteresis. The location and distribution of fullerenes in the hybrid phase are confirmedly visualized using conductive atomic force microscopy and Kelvin probe force microscopy measurements. Fullerenes located at grain boundaries function as a source of beneficial effect on choking the channels of ion migration and also as the electron traps that compromise the photocarrier extraction. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current signals is avoided in PSCs based on the hybrid perovskite:fullerene phases. These results not only provide a strong correlation between nanoscopic and macroscopic current hysteresis behaviors but also clearly clarify how fullerenes play a role in reducing the current hysteresis in hybrid phases and thus prototype devices.

A comprehensive theoretical analysis of two‐terminal and four‐terminal perovskite/copper indium gallium selenide (CIGS) tandem solar cells is investigated from optical and electrical aspects. According to different optical absorptions, the current matching points of different halide components are obtained. Under the condition of current matching, an optimal performance up to 31.13% can be obtained by using two‐terminal CH₃NH₃PbI₂Br/CIGS tandem structure.

Perovskite/copper indium gallium selenide (CIGS) tandem solar cells represent an attractive configuration to obtain ultrahigh efficiency. A detailed theoretical analysis is crucial for further improving the performance of tandem solar cells. Herein, four‐terminal and two‐terminal perovskite/CIGS tandem solar cells are intensively researched. For four‐terminal perovskite/CIGS tandem solar cell, the optimal thicknesses of CH₃NH₃PbI₃ and CIGS are 0.5 and 3 μm, respectively, according to the simulation result. Reducing the thickness of TiO₂ and Spiro‐OMeTAD can minimize the short‐wavelength parasitic absorption and long‐wavelength parasitic absorption, respectively. Meanwhile, using antireflection coating, such as 100 nm MgF₂, is beneficial to increase the photon absorption. For two‐terminal perovskite/CIGS tandem solar cells, the thicknesses of perovskite and CIGS are tuned to meet the current matching. To further improve the efficiency of two‐terminal tandem cells, FTO thickness is reduced to minimize reflection, and the optimal doping concentration of CIGS (1 × 10¹⁸ cm⁻³) is used. In addition, results show that the quality of perovskite films should be improved by enlarging the grain size to decrease the trap states at grain boundary. Finally, the optimal efficiency of two‐terminal CH₃NH₃PbI₂Br/CIGS tandem solar cells reaches 31.13%.

Highly efficient dopant‐free hole‐transporting materials based on random copolymers of dialkoxybenzene and bithiophene are presented. By replacing 3 mol% of the alkyl side chains with diethylene glycol groups, the polymer yields a doubled hole mobility, an increased fill factor, and a correspondingly enhanced power conversion efficiency of 20.19% (certified: 20.10%).

Abstract

A variety of dopant‐free hole‐transporting materials (HTMs) is developed to serve as alternatives to the typical dopant‐treated ones; however, their photovoltaic performance still falls far behind. In this work, the side chain of a polymeric HTM is engineered by partially introducing diethylene glycol (DEG) groups in order to simultaneously optimize the properties of both the bulk of the HTM layer and the HTM/perovskite interface. The intermolecular π–π stacking interaction in the HTM layer is unexpectedly weakened after the incorporation of DEG groups, whereas the lamellar packing interaction is strengthened. A doubled hole mobility is obtained when 3% of the DEG groups replace the original alkyl side chains, and a champion power conversion efficiency (PCE) of 20.19% (certified: 20.10%) is then achieved, which is the first report of values over 20% for dopant‐free organic HTMs. The device maintains 92.25% of its initial PCE after storing at ambient atmosphere for 30 d, which should be due to the enhanced hydrophobicity of the HTM film.

Crosslinked perovskite crystals: Incorporation of π‐conjugated organic crosslinkers into the organic‐inorganic hybrid perovskite structure imparts stability towards light and high moisture degradation without compromising power conversion efficiencies.

Abstract

Methylammonium lead halide perovskite‐based solar cells have demonstrated efficiencies as high as 24.2 %, highlighting their potential as inexpensive and solution‐processable alternatives to silicon solar cell technologies. Poor stability towards moisture, ultraviolet irradiation, heat, and a bias voltage of the perovskite layer and its various device interfaces limits the commercial feasibility of this material for outdoor applications. Herein, we investigate the role of hydrogen bonding interactions induced when metal halide perovskite crystals are crosslinked with alkyl or π‐conjugated boronic acid small molecules (‐B(OH)₂). The crosslinked perovskite crystals are investigated under continuous light irradiation and moisture exposure. These studies demonstrate that the origin of the interaction between the alkyl or π‐conjugated crosslinking molecules is due to hydrogen bonding between the ‐B(OH)₂ terminal group of the crosslinker and the I of the [PbI₆]⁴⁻ octahedra of the perovskite layer. Also, this interaction influences the stability of the perovskite layer towards moisture and ultraviolet light irradiation. Morphology and structural analyses, as well as IR studies as a function of aging under both dark and light conditions show that π‐conjugated boronic acid molecules are more effective crosslinkers of the perovskite crystals than their alkyl counterparts thus imparting better stability towards light and moisture degradation.

Watching the defects: Defects play a pivotal role in the overall performance of perovskite solar cells. This Review focuses on central questions of “what defects exist in metal halide perovskites” and “how can one reduce detrimental defects towards high‐performance perovskite solar cells”.

Abstract

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film‐growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Publication date: October 2019

Source: Nano Energy, Volume 64

Author(s): Rong Tang, Zhuang-Hao Zheng, Zheng-Hua Su, Xue-Jin Li, Ya-Dong Wei, Xiang-Hua Zhang, Yong-Qing Fu, Jing-Ting Luo, Ping Fan, Guang-Xing Liang

Graphical abstract

Publication date: October 2019

Source: Nano Energy, Volume 64

Author(s): Yuan Chang, Xin Zhang, Yabing Tang, Monika Gupta, Dan Su, Jiaen Liang, Dong Yan, Kun Li, Xuefeng Guo, Wei Ma, He Yan, Chuanlang Zhan

Graphical abstract

This review aims at presenting a comprehensive overview of the latest progress on perovskite solar cells (PSCs), especially the strategies toward enhancing their near‐infrared light harvesting. An in‐depth understanding of the working mechanism of tandem solar cells (TSCs) and integrated perovskite/organic solar cells (IPOSCs) is presented, and the recent developments of perovskite/Si, perovskite/Cu(In_1–x , Ga _x )Se₂ TSCs, and IPOSCs are further highlighted.

The emerging perovskite materials present great opportunities for cost‐saving and efficient photovoltaic devices. However, perovskite solar cells (PSCs) suffer from the limitation of short optical absorption edge, resulting in most of the near‐infrared (NIR) light being wasted. Recently, strategies toward broadening the NIR spectra response and further improve the power conversion efficiency of PSCs have attracted extensive attention. In this review, the unique features of perovskite materials are first introduced; subsequently, the current developments of organic–inorganic hybrid PSCs and all‐inorganic PSCs are highlighted. Then, a detailed summary of the strategies toward enhancing the NIR light harvesting of PSCs, namely, perovskite/Si and perovskite/Cu(In_1–x , Ga _x )Se₂ tandem solar cells (TSCs) and the integrated perovskite/organic solar cells (IPOSCs), is presented. After an in‐depth understanding of the working mechanism of TSCs and IPOSCs, a comprehensive overview about their recent developments, key detrimental factors restricting their further performance enhancement, and feasible countermeasures to conquer these scientific and technological problems are given. In the end, the perspectives on the related materials and devices are addressed.

The introduction of funtional molecular self‐assembled monolayers (SAMs) atop of zinc oxide (ZnO) effectively optimizes the energetic and heterojunction properties of the organic–metal oxide interface to improve the performance and photostability of nonfullerene polymer solar cells.

Abstract

Charge events across organic–metal oxide heterointerfaces routinely occur in organic electronics, yet strongly influence their overall performance and stability. They become even more complicated and challenging for the heterojunction conditions in polymer solar cells (PSCs), especially when nonfullerene acceptors with varied energetics are employed. In this work, an effective interfacial strategy that utilizes novel small molecule self‐assembled monolayers (SAMs) is developed to improve the electronic and electric, as well as chemical properties of organic–zinc oxide (ZnO) interfaces for nonfullerene PSCs. It is revealed that the tailored SAMs with well‐controlled energy levels and molecular dipoles can effectively optimize the energetic barrier and work function (WF) of heterointerface for optimal electron extraction. In addition, the introduction of SAMs atop of ZnO facilitates not only acceptor segregation near the n‐contact interface, but also passivation of the photocatalytic activities for ZnO, to improve overall performance and photo stability of the derived nonfullerene PSCs. Overall, the methodology and structure–property relationship revealed herein would be beneficial for a wide range of hybrid electronics.

A photoconductive ZnO interlayer was generated by coordinative binding of zinc ions to perylene bisimide dyes bearing four hydroxy groups in bay area. With this dye‐sensitized interlayer, improvement of polymer solar cells was achieved, leading to power conversion efficiencies close to 16 %.

Abstract

By introduction of four hydroxy (HO) groups into the two perylene bisimide (PBI) bay areas, new HO‐PBI ligands were obtained which upon deprotonation can complex Zn^II ions and photosensitize semiconductive zinc oxide thin films. Such coordination is beneficial for dispersing PBI photosensitizer molecules evenly into metal oxide films to fabricate organic–inorganic hybrid interlayers for organic solar cells. Supported by the photoconductive effect of the ZnO:HO‐PBI hybrid interlayers, improved electron collection and transportation is achieved in fullerene and non‐fullerene polymer solar cell devices, leading to remarkable power conversion efficiencies of up to 15.95 % for a non‐fullerene based organic solar cell.