Chen Weijie

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Ankur Solanki, Pankaj Yadav, Silver-Hamill Turren-Cruz, Swee Sien Lim, Michael Saliba, Tze Chien Sum

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Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Minwoo Nam, Hye Yeon Noh, Joo-Han Kang, Junhee Cho, Byoung Koun Min, Jae Won Shim, Doo-Hyun Ko

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Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Guangbao Wu, Jiyu Zhou, Jianqi Zhang, Rui Meng, Boxin Wang, Baoda Xue, Xuanye Leng, Dongyang Zhang, Xuning Zhang, Shiqing Bi, Qian Zhou, Zhixiang Wei, Huiqiong Zhou, Yuan Zhang

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The management on the orientational crystallization in 2-dimensional BA₂MA₃Pb4I₁₃ perovskite solar cells based on the polymeric PTAA hole transporting layer leads to boosted power conversion efficiency (PCE) to 14.28% with a small hysteresis at room temperature. The device also exhibits a negative temperature dependence of PCE reaching 15% at 210 K with supreme stability.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Nannan Zheng, Khalid Mahmood, Wenkai Zhong, Feng Liu, Peng Zhu, Zhenfeng Wang, Boming Xie, Zhiming Chen, Kai Zhang, Lei Ying, Fei Huang, Yong Cao

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A fluorinated spiro[fluorene‐9,9′‐xanthene] based hole transport material (HTM), 2mF‐X59, is designed and synthesized within two steps for sensitive‐dopant‐free, high efficient, and stable perovskite solar cells (PSCs). 2mF‐X59 shows the lowered HOMO level, improved hole mobility and hydrophobicity, compared to its nonfluorinated counterpart X59. Without the use of any sensitive‐dopants, the optimized 2mF‐X59‐based PSCs exhibit a power conversion efficiency up to 18.13% with impressive long‐term stability.

Despite the substantial development of efficient hole transporting materials (HTMs) for high‐performance perovskite solar cells (PSCs), optimization of the HTMs to sensitive‐dopant‐free HTMs for high efficient PSCs with prominent stability have rarely been reported. Herein, a low‐cost fluorinated spiro[fluorene‐9,9′‐xanthene] based HTM termed 2mF‐X59 is designed and synthesized. In comparison with its reported nonfluorinated counterpart X59, 2mF‐X59 shows lowered highest occupied molecular orbital (HOMO) level, improved hole mobility, and hydrophobicity. Aided by 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) to further lower the HOMO level of 2mF‐X59 and improve its hole transfer, the optimized 2mF‐X59 based PSCs show a maximum power conversion efficiency (PCE) of 18.13% without the use of any sensitive‐dopants (e.g., lithium salt/4‐tert‐butylpyridine), which is comparable to the Spiro‐OMeTAD based PSCs (18.22%) with sensitive dopants. More importantly, the sensitive‐dopant‐free 2mF‐X59 based PSCs maintain 95% of their initial performance for more than 500 h under air exposure, showing much better long‐term stability than control PSCs based on Spiro‐OMeTAD with sensitive dopanst. This is the first case that a sensitive‐dopant‐free HTM is demonstrated in PSCs with a high PCE (>18%) and good stability by optimizing the literature HTM. This work could pave a new way to develop low‐cost sensitive‐dopant‐free HTMs for highly efficient and stable PSCs for practical applications.

A low temperature–processed metal oxide with excellent mechanical properties and thickness‐insensitivity is exploited as an electron transporting layer for high‐efficiency robust flexible polymer solar cells (PSCs). A record efficiency of 11.5% is achieved for the flexible PSCs, and over 91% of initial efficiency is well maintained after 1500 bending cycles.

Abstract

Landmark power conversion efficiency (PCE) over 14% has been accomplished for single‐junction polymer solar cells (PSCs). However, the inevitable fracture of inorganic transporting layers and deficient interlayer adhesion are critical challenges to achieving the goal of flexible PSCs. Here, a bendable and thickness‐insensitive Al‐doped ZnO (AZO) modified by polydopamine (PDA) has emerged as a promising electron transporting layer (ETL) in PSCs. It has special ductility and adhesion to the active layer for improving the mechanical durability of the device. Nonfullerenes PSCs based on PBDB‐T‐2F:IT‐4F with AZO:1.5% PDA (80 nm) ETL yield the best PCE of 12.7%. More importantly, a prominent PCE, approaching 11.5%, is reached for the fully flexible device based on Ag‐mesh flexible electrode, and the device retains >91% of its initial PCE after bending for 1500 cycles. Such thickness insensitivity, mechanical durability, and interfacial adhesion properties for the inorganic ETLs are desired for the development of flexible and wearable PSCs with reliable photovoltaic performance and large‐area roll‐to‐roll printing manufacture.

High‐performance inverted lead‐free perovskite solar cells (PVSCs) with enhanced UV stability are demonstrated via grain boundaries modification by PTN‐Br. The gradient band alignment of FASnI₃ films with a PEDOT:PSS hole‐transport layer ensures excellent hole transportation and higher open‐circuit voltage. This study provides a strategy to develop high‐performance tin‐based PVSCs based on balanced charge transportation and reduced trap states.

Abstract

High electronic quality perovskite films with a balanced charge transportation is critical for satisfying high‐performance for perovskite solar cells (PVSCs). However, the inferior band alignment of tin‐based perovskite films with an adjacent hole‐transport layer (HTL) leads to a poor hole transportation and collection. In this work, the semiconducting molecule poly[tetraphenylethene 3,3′‐(((2,2‐diphenylethene‐1,1‐diyl)bis(4,1‐phenylene))bis(oxy))bis(N,N‐dimethylpropan‐1‐amine)tetraphenylethene] (PTN‐Br) is introduced into a lead‐free perovskite precursor to form a bulk heterojunction film. In addition, the PTN‐Br molecule with the suitable highest occupied molecular orbital energy level (−5.41 eV) can fill into the grain boundaries of the perovskite film, serving as a hole‐transport medium between grains. The gradient band alignment of the perovskite film with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL ensures excellent hole transportation and higher open‐circuit voltage. In addition, the π‐conjugated polymer PTN‐Br can passivate trap states within the perovskite film due to the formation of Lewis adducts between uncoordinated Sn atoms and the dimethylamino of PTN‐Br. Consequently, a champion efficiency of 7.94% is achieved with significant enhancements in the open‐circuit voltage and fill factor. Furthermore, the PTN‐Br incorporated device shows better ultra violet (UV) stability because of the UV barrier and passivating effect of PTN‐Br, retaining about 66% of its initial efficiency after 5 h of continuous UV light irradiation.

The [7]helicenes with stable partial open‐shell biradical ground states are demonstrated as effective surface modifiers of the inorganic NiO _x hole‐transporting layer in p–i–n perovskite solar cells. Their nonpolar feature improves the crystallinity of the perovskite films grown on them. Meanwhile, their biradical character provides a certain defect passivation function to facilitate charge transfer/extraction across the perovskite interface.

Abstract

Organic–inorganic hybrid perovskites have realized a high power conversion efficiency (PCE) in both n–i–p and p–i–n device configurations. However, since the p–i–n structure exempts the sophisticated processing of charge‐transporting layers, it seems to possess better potential for practical applications than the n–i–p one. Currently, the inorganic NiO _x is the most prevailing hole‐transporting layer (HTL) used in p–i–n perovskite solar cells. Nevertheless, defects might exist on its surface to influence the charge transfer/extraction across the interface with perovskite and to affect the quality of the perovskite film grown on it. Herein, two novel [7]helicenes with stable open‐shell singlet biradical ground states at room temperature are demonstrated as an effective surface modifier of the NiO _x HTL. Their nonpolar feature effectively promotes the crystallinity of the perovskite film grown on them; meanwhile, their unique partial biradical character seems to provide a certain degree of defect passivation function at the perovskite interface to facilitate interfacial charge transfer/extraction. As a result, both 1ab‐ and 1bb‐modifed devices yield a PCE of >18%, exceeding the value (15.6%) of the control device using a sole NiO _x HTL, and the maximum PCE can reach 19%. Detailed characterizations are carefully conducted to understand the underlying reasons behind such enhancement.

Fluorinated aromatic cations (FPEAI) can react with the excess PbI₂ in a 3D perovskite film to form a capping 2D perovskite layer. Compared to the control device, the resulting multidimensional perovskite shows enhanced environmental stability with equally superior device performances. Judicious optimization of the perovskite precursor recipe realizes a power conversion efficiency of 20.54% for mesoporous perovskite solar cells.

Abstract

Supported by the density functional theory (DFT) calculations, for the first time, a fluorinated aromatic cation, 2‐(4‐fluorophenyl)ethyl ammonium iodide (FPEAI), is introduced to grow in situ a low dimensional perovskite layer atop 3D perovskite film with excess PbI₂. The resulted (p‐FC₆H₄C₂H₄NH₃)₂[PbI₄] perovskite functions as a protective capping layer to protect the 3D perovskite from moisture. In the meantime, the thin layer facilitates charge transfer at the interfaces, thereby reducing the nonradiative recombination pathways. Laser scanning confocal microscopy unveils visually the distribution of the 2D perovskite layer on top of the 3D perovskite. When employing the 3D–2D perovskite as the absorbing layer in the photovoltaic cells, a high power conversion efficiency of 20.54% is realized. Superior device performance and moisture stability are observed with the modified perovskite over the whole stability test period.

One of the problems that restrict the further development of perovskite solar cells (PSCs) is hysteresis, making it difficult to evaluate the reliable performance of PSCs. Recent process regarding the strategies to efficiently reduce hysteresis in PSCs is reviewed. The influential factors and possible reasons are also outlined.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) have become a promising candidate in the photovoltaic field due to their high power conversion efficiency and low material cost. However, the development of PSCs is limited by their poor stability under practical conditions in the presence of oxygen, moisture, sunlight, heat, and the current–voltage (I–V) hysteresis. In particular, the hysteretic I–V issue casts doubt on the validity of the photovoltaic performance results that are achieved, making it difficult to evaluate the authentic performance of PSCs. This review article focuses on understanding the I–V hysteresis behavior in PSCs and on exploring the possible reasons leading to this hysteresis phenomenon. The various strategies attempted to suppress the I–V hysteresis in PSCs are summarized, and a brief future recommendation is provided.

By using the new electron‐rich heptacyclic anthracene(cyclopentadithiophene) (AT) core, together with energy level modulations by end‐group optimizations enabling the match with polymer donors, two new nonfullerene small molecule acceptors AT‐NC and AT‐4Cl are synthesized. With both halogenated donor and acceptor, the organic photovoltaics device based on AT‐4Cl achieves a high power conversion efficiency of 13.27% with simultaneously improved J _sc and fill factor.

Abstract

Two new nonfullerene small molecule acceptors (NF‐SMAs) AT‐NC and AT‐4Cl based on heptacyclic anthracene(cyclopentadithiophene) (AT) core and different electron‐withdrawing end groups are designed and synthesized. Although the two new acceptor molecules use two different end groups, naphthyl‐fused indanone (NINCN) and chlorinated INCN (INCN‐2Cl) demonstrate similar light absorption. AT‐4Cl with chlorinated INCN as end groups are shifted significantly due to the strong electron‐withdrawing ability of chlorine atoms. Thus, desirable V _oc and photovoltaic performance are expected to be achieved when polymer PBDB‐T is used as the electron donor with AT‐NC as the acceptor, and fluorinated analog PBDB‐TF with down‐shifted energy levels is selected to blend with AT‐4Cl. Consequently, the device based on PBDB‐TF:AT‐4Cl yields a high power conversion efficiency of 13.27% with a slightly lower V _oc of 0.901 V, significantly enhanced J _sc of 19.52 mA cm⁻² and fill factor of 75.5% relative to the values based on PBDB‐T:AT‐NC. These results demonstrate that the use of a new electron‐rich AT core, together with energy levels modulations by end‐group optimizations enabling the match with polymer donors, is a successful strategy to construct high‐performance NF‐SMAs.

Nonfullerene‐based small‐molecule organic solar cells with a new record efficiency of 12.08% are achieved by first incorporation of near‐infrared absorbing molecules and by tuning the sequentially evolved crystalline morphology. The improved crystallinity of both donor and acceptor materials facilitates the formation of multilength scale morphologies, which further enhance charge mobility and extraction, and reduce the nongeminate recombination.

Abstract

In this paper, two near‐infrared absorbing molecules are successfully incorporated into nonfullerene‐based small‐molecule organic solar cells (NFSM‐OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP‐TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP‐TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J _sc. These result in a very respectable PCE, which is the highest among all NFSM‐OSCs and all small‐molecule binary solar cells reported so far.

An in situ back‐contact passivation strategy is adopted to optimize the photovoltaic performance of n–i–p planar perovskite solar cells. Devices with a flat‐band alignment between the perovskite and polymer passivation layer achieve a high photovoltage of 1.15 V and fill factor of 83% with 1.53 eV bandgap perovskite, leading to a stabilized power conversion efficiency of 21.6%.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Coherent spin dynamics of electrons and holes in CsPbBr₃ perovskite crystals

Coherent spin dynamics of electrons and holes in CsPbBr₃ perovskite crystals, Published online: 08 February 2019; doi:10.1038/s41467-019-08625-z