Chen Weijie

The perovskite solar cell (PSC) has boosted its power conversion efficiency along with the application of inorganic hole transport layer (HTL). The presence of inorganic HTL assist the carrier transport and improve the stability. Wide variety of inorganic HTLs are reviewed in this report along with their properties, synthesis technique and interfacial chemistry and carrier dynamic.

Abstract

This review presents various hole transport layers (HTLs) employed in perovskite solar cells (PSCs) in pursuing high power conversion efficiency (PCE) and functional stability. The PSCs have achieved high PCE (over 23%, certified by NREL) and more efforts have been devoted into research for stability enhancement. Inorganic HTLs become a popular choice as selective contact materials because of their intrinsic chemical stability and low cost. HTLs and electron transport layers (ETLs) are critical components of PSCs due to the requirement to create charge collection selectivity. Herein the authors provide an overview on inorganic HTLs synthesis, properties, and their application in various PSCs for both mesoporous and planar architectures. Inorganic HTLs with appropriate properties, such as proper energy level and high carrier mobility, can not only assist with charge transport, but also improve the stability of PSCs under ambient conditions. The importance of interfacial chemistry and interfacial charge transport is further addressed to understand the underlying mechanism of related degradation and carrier dynamic. It is expected that the success of the inorganic HTL in PSCs can stimulate further research and bring real impact for future photovoltaic technologies.

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Weijie Chen, Haiyang Chen, Guiying Xu, Rongming Xue, Shuhui Wang, Yaowen Li, Yongfang Li

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Conjugated polymers act as a hydrophobic interlayer with superb hole mobility between perovskite and doped spiro‐OMeTAD, enhancing the device stability and performance.

Hybrid organic–inorganic perovskite (HOIP) solar cells have achieved a certified power conversion efficiency (PCE) of 22.7%, which commonly use doped spiro‐OMeTAD as hole transport materials (HTMs). However, the additives in spiro‐OMeTAD can absorb moisture and cause the degradation of HOIP layers, leading to severe air‐instability of devices. Herein, conjugated polymers of PD‐10‐DTTE‐7 as a new effective interlayer between perovskite and doped spiro‐OMeTAD to achieve air‐stable efficient perovskite solar cells are reported. Its hydrophobic nature can effectively prevent the penetration of moisture and additives. Its superb hole mobility (9.54 cm² V⁻¹ s⁻¹, ≈10⁵ times higher than spiro‐OMeTAD) and suitable highest occupied molecular orbital level (−5.33 eV) are preferable to the hole injection and transport at the interface thus enhancing the device PCE. As a result, the MAPbI₃ solar cells with the PD‐10‐DTTE‐7 interlayer achieve remarkable device air‐stability and enhanced PCE, compared with the devices without the interlayer. These results provide a feasible approach to enhance solar cell stability and performance simultaneously.

A novel TTPTTT building block as an eight‐membered indacanodithiophene derivative is developed. The effect of nonfluorinated, momofluorinated, and difluorinated end‐capping group on photovoltaic performance of asymmtry TTPTTT non‐fullerene acceptors (NFAs) is investigated. As a result, organic solar cells based on TTPTTT‐4F exhibit a high efficiency of 12.05%, indicating that the asymmetric TTPTTT unit is a promising central core unit for designing efficient A‐D‐A type NFAs.

Indacenodithiophene (IDT) has been widely used as the central core to design high‐performance acceptor‐donor‐acceptor (A‐D‐A)‐type non‐fullerene acceptors (NFAs). NFAs based on five‐, six‐, seven‐, and nine‐membered IDT have been successfully prepared. However, less research attention has been paid to the eight‐membered IDT derivative. In this study, a novel asymmetric TTPTTT building block as an eight‐membered IDT unit are designed and synthesized. The effect of nonfluorinated, monofluorinated, and difluorinated end‐capping groups on the photovoltaic performance of TTPTTT‐based NFAs are specifically investigated. By blending with the polymer donor PBT1‐C, organic solar cells (OSCs) based on TTPTTT‐IC, TTPTTT‐2F, and TTPTTT‐4F exhibited power conversion efficiencies (PCEs) of 7.91, 11.52, and 12.05%, respectively. Our results indicate that the asymmetric TTPTTT building block as an eight‐membered IDT derivative is a promising central core unit for designing high‐performance A‐D‐A type NFAs.

Electron backscatter diffraction (EBSD) combined with carrier lifetime, mobility, and diffusion length measurements, shows that optical and scanning electron microscopy images cannot accurately predict grain boundary positions or their resulting properties. Grain boundaries passivated by amorphous perovskite display increased photoluminescence lifetime and intensity. This suggests that crystallographic (not only chemical) effects play an important role in halide perovskite grain boundary properties.

Abstract

Grain boundaries play a key role in the performance of thin‐film optoelectronic devices and yet their effect in halide perovskite materials is still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Noncrystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard – electron backscatter diffraction (EBSD) – destroys halide perovskite thin films. Here, this problem is solved by using a solid‐state EBSD detector with 6000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length, and mobility shows that grain boundaries are not benign but have a recombination velocity of 1670 cm s⁻¹, comparable to that of crystalline silicon. Amorphous grain boundaries are also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halide perovskite thin films. It also suggests a new approach for passivating grain boundaries, independent of surface passivation, to lead to even better performance in optoelectronic devices.

Novel technological applications significantly favor alternatives to electrons toward constructing low power–consuming, high-speed all-optical integrated optoelectronic devices. Polariton condensates, exhibiting high-speed coherent propagation and spin-based behavior, attract considerable interest for implementing the basic elements of integrated optoelectronic devices: switching, transport, and logic. However, the implementation of this coherent polariton condensate flow is typically limited to cryogenic temperatures, constrained by small exciton binding energy in most semiconductor microcavities. Here, we demonstrate the capability of long-range nonresonantly excited polariton condensate flow at room temperature in a one-dimensional all-inorganic cesium lead bromide (CsPbBr₃) perovskite microwire microcavity. The polariton condensate exhibits high-speed propagation over macroscopic distances of 60 μm while still preserving the long-range off-diagonal order. Our findings pave the way for using coherent polariton condensate flow for all-optical integrated logic circuits and polaritonic devices operating at room temperature.

Publication date: December 2018

Source: Nano Energy, Volume 54

Author(s): Jianming Yang, Xianjie Liu, Yuexing Zhang, Xuerong Zheng, Xiaoxiao He, Han Wang, Fangyu Yue, Slawomir Braun, Jinquan Chen, Jianhua Xu, Yanqing Li, Yizheng Jin, Jianxin Tang, Chungang Duan, Mats Fahlman, Qinye Bao

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Publication date: December 2018

Source: Nano Energy, Volume 54

Author(s): Kyungeun Jung, Jae-Ho Lee, Kwonwoo Oh, Chan Im, Junghwan Do, Joosun Kim, Weon-Sik Chae, Man-Jong Lee

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Publication date: 19 December 2018

Source: Joule, Volume 2, Issue 12

Author(s): Tiankai Zhang, Mingzhu Long, Minchao Qin, Xinhui Lu, Si Chen, Fangyan Xie, Li Gong, Jian Chen, Ming Chu, Qian Miao, Zefeng Chen, Wangying Xu, Pengyi Liu, Weiguang Xie, Jian-bin Xu

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Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Bo Chen, Zhengshan Yu, Kong Liu, Xiaopeng Zheng, Ye Liu, Jianwei Shi, Derrek Spronk, Peter N. Rudd, Zachary Holman, Jinsong Huang

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Fine-tuning of the chemical structure of photoactive materials for highly efficient organic photovoltaics

Fine-tuning of the chemical structure of photoactive materials for highly efficient organic photovoltaics, Published online: 22 October 2018; doi:10.1038/s41560-018-0263-4

Zn_0.8Cd_0.2S nanoparticles (ZCS) are doped in [6,6]‐phenyl‐C61‐butyric acid methyl (PCBM) to form an inorganic/organic hybrid as an efficient electron transport layer (ETL) in p‐i‐n planar perovskite solar cells. The ZCS@PCBM interlayer improves electron extraction, enhances electron transportation, and suppresses charge recombination. The shielding effect of ZCS nanoparticles can keep the perovskite from erosion by ambient moisture, thus improving the device stability.

In this study, an inorganic/organic hybrid, Zn_0.8Cd_0.2S nanoparticles (ZCS) embedded in [6,6]‐phenyl‐C61‐butyric acid methyl (PCBM), as an efficient electron transport layer (ETL) for air‐processed p‐i‐n perovskite solar cells (PSCs) has been demonstrated, and the doping effect and doping mechanism are systematically studied. As compared to PCBM, ZCS@PCBM ETL exhibits improved electron extraction at the perovskite/ETL interface, increased electron transportation within the ETL, enhanced charge collection efficiency, and suppressed interfacial charge recombination, resulting in significantly improved power conversion efficiency (PCE) from 14.41 to 17.18% by 19.2%. Interestingly, the ZCS nanoparticles can protect the perovskite layer from erosion by ambient moisture, and 82% of the initial PCE for the non‐encapsulated devices with ZCS@PCBM ETL is retained after 500 h storage in the atmosphere (humidity 30–60%) versus only 13% of the initial PCE for the PCBM ETL without ZCS doping.

An effective strategy of promoting grain growth and defects passivation simultaneously for perovskite film by using Ni²⁺ addition is demonstrated. An appreciated efficiency of 20.6% can be achieved for an inverted planar perovskite solar cells device based on a CH₃NH₃PbI₃ (Ni²⁺) film.

Abstract

Today's state‐of‐the‐art perovskite solar cells (PSCs) are utilizing polycrystalline perovskite thin films via solution‐processing at low temperature (<150 °C). It is extremely significant to enlarge grain size and passivate trap states for perovskite thin films to achieve high power conversion efficiency. Herein, a strategy for defect passivation of perovskite films via metal ion Ni²⁺ is for the first time reported. It is found that addition of Ni²⁺ can significantly generate polyporous PbI₂ films due to a different solubility between NiCl₂ and PbI₂ which benefits penetration of MAI and thus formation of large grain perovskite films eventually. It further demonstrated that Ni²⁺ ions can effectively passivate PbI₃ ⁻ antisite defects and restrain the generation of Pb⁰ by interacting with the under‐coordinated halide anions and halide‐rich antisites. Therefore, introducing moderate Ni²⁺ ions result in a significant increase in photoluminescence lifetime from 285 to 732 ns. Accordingly, a power conversion efficiency of 20.61% can be achieved for the 3% Ni²⁺ addition‐based PSCs with an enhanced cell stability under ambient conditions. This work provides a promising route toward perovskite films featuring with high crystallinity and low trap‐density.