Chen Weijie

Film fabrication environment and anti‐solvent properties strongly influence the microstructure evolution of perovskite films. An ambient fabrication environment induces anisotropies in crystallization. The choice of antisolvent is critical to alleviating these anisotropies. The key is to induce uniform spherulitic crystallization to achieve robust pinhole‐free films possessing grains, crystallinity, crystallographic phases, and crystallite orientations unaffected by the processing environment.

Abstract

The influence of precursor solution properties, fabrication environment, and antisolvent properties on the microstructural evolution of perovskite films is reported. First, the impact of fabrication environment on the morphology of methyl ammonium lead iodide (MAPbI₃) perovskite films with various Lewis‐base additives is reported. Second, the influence of antisolvent properties on perovskite film microstructure is investigated using antisolvents ranging from nonpolar heptane to highly polar water. This study shows an ambient environment that accelerates crystal growth at the expense of nucleation and introduces anisotropies in crystal morphology. The use of antisolvents enhances nucleation but also influences ambient moisture interaction with the precursor solution, resulting in different crystal morphology (shape, size, dispersity) in different antisolvents. Crystal morphology, in turn, dictates film quality. A homogenous spherulitic crystallization results in pinhole‐free films with similar microstructure irrespective of processing environment. This study further demonstrates propyl acetate, an environmentally benign antisolvent, which can induce spherulitic crystallization under ambient environment (52% relative humidity, 25 °C). With this, planar perovskite solar cells with ≈17.78% stabilized power conversion efficiency are achieved. Finally, a simple precipitation test and in situ crystallization imaging under an optical microscope that can enable a facile a priori screening of antisolvents is shown.

A simple perovskite solar cell architecture, which is based on dopant‐free electron and hole conductors and carbon back contact deposited at room temperature, is demonstrated. The resulting architecture leads to the fabrication of cheap and highly efficient perovskite solar cells exhibiting unprecedented long‐term operational and UV stability thus hold immense potential for large‐scale deployment.

Abstract

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large‐scale deployment of PSCs, these components need to be replaced with cost‐effective and robust ones that maintain high efficiency while ascertaining long‐term operational stability. Here, a simple and low‐cost PSC architecture employing dopant‐free TiO₂ and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain ≈95% of their initial efficiencies for >2000 h at the maximum power point under full‐sun illumination at 60 °C. In addition, the CuSCN/carbon‐based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro‐MeOTAD/Au based devices degrade severely.

The current status of polaron physics in metal‐halide perovskites is reviewed based on a first‐principles computational perspective. The static and dynamic properties of polarons are discussed focusing on the impact of the chemical composition of the perovskite on their stabilization energy, extension, and mobility.

Abstract

The peculiar optoelectronic properties of metal‐halide perovskites, partly underlying their success in solar cells and light emitting devices, are likely related to the complex interplay of electronic and structural features mediated by formation of polarons. In this paper the current status of polaron physics in metal‐halide perovskites is reviewed based on a first‐principles computational perspective, which has delivered hitherto noaccessible insights into the electronic and structural features associated with polaron formation in this materials class. The role of organic (dipolar) versus inorganic (spherical) A‐site cations is extensively analyzed, these cations are related to modulation of the energetics and structural extension of polarons in lead‐halide perovskites. Further tuning of polaron energetics is achieved by individual variations in metal (e.g., Pb → Sn) and halide (e.g., I → Br), showing a transition from a semilocalized to a localized polaron regime in which charge holes can be trapped at isolated Sn centers. The vastly varying and tunable nature of charge lattice interactions represents a peculiarity of metal‐halide perovskites that should be taken into account when designing novel materials or targeting specific compositional engineering of existing perovskites.

Perovskite/Si tandem solar cells offer a feasible and promising approach to further reduce solar electricity costs by promising higher efficiency than their single‐junction counterparts. Prospects for achieving over 30% efficient monolithic perovskite/Si tandems in the near term using a combination of literature review with original results from numerical optoelectronic simulations are presented in this progress report.

Abstract

The article commences with a review focusing on three critical aspects of the perovskite/Si tandem technology: the evolution of efficiencies to date, comparisons of Si subcell choices, and the interconnection design strategies. Building on this review, a clear route is provided for minimizing optical losses aided by optical simulations of a recently reported high‐efficiency perovskite/Si tandem system, optimizations which result in tandem current densities of ≈20 mAcm⁻² with front‐side texture. The primary focus is on electrical modeling on the Si‐subcell, in order to understand the efficiency potential of this cell under filtered light in a tandem configuration. The possibility of increasing the Si subcell efficiency by 1% absolute is offered through joint improvements to the bulk lifetime, which exceeds 4 ms, and improves surface passivation quality to saturation current densities below 10 fA cm⁻². Polycrystalline‐Si/SiO _x passivating contacts are proposed as a promising alternative to partial‐area rear contacts, with the potential for further simplifying cell fabrication and improving device performance. A combination of optical modeling of the complete tandem structure alongside electrical modeling of the Si‐subcell, both with state‐of‐the‐art modeling tools, provides the first complete picture of the practical efficiency potential of perovskite/Si tandems.

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Cong Li, Zhinan Zhu, Yongmei Wang, Qiang Guo, Chenyun Wang, Peng Zhong, Zhan'ao Tan, Rusen Yang

Graphical abstract

The crystallization process of perovskite inside a mesoscopic scaffold is revealed and a possible model of crystallization is proposed. By applying a solvent evaporation controlled crystallization method, ideal crystallization in the mesoscopic structure is achieved. As a result, a stabilized power conversion efficiency of 16.26% based on a printable mesoscopic perovskite solar cell is achieved.

Abstract

Controlling the crystallization of organic–inorganic hybrid perovskite is of vital importance to achieve high performing perovskite solar cells. The growth mechanism of perovskites has been intensively studied in devices with planar structures and traditional structures. However, for the printable mesoscopic perovskite solar cells, it is difficult to study the crystallization mechanism of perovskite owing to the complicated mesoporous structure. Here, a solvent evaporation controlled crystallization method to achieve ideal crystallization in the mesoscopic structure is provided. Combining results of scanning electron microscope and X‐ray diffraction, it is found that adjusting the evaporation rate of solvent can control the crystallization rate of perovskite and a model for the crystallization process during annealing in mesoporous structures is proposed. Finally, a homogeneous pore filling in the mesoscopic structure without any additives is successfully achieved and a stabilized power conversion efficiency of 16.26% using ternary‐cation perovskite absorber is realized. The findings will provide better understanding of perovskite crystallization in printable mesoscopic perovskite solar cells and pave the way for the commercialization of perovskite solar cells.

Oxo‐functionalized graphene/dodecylamine is used to solve ion migration in cesium‐formamidinium‐methylammonium triple cation‐base perovskites. The ultra‐thin two‐dimensional network structure can wrap the crystals and reduce the ion migration of the perovskite film. The resulting devices deliver a power conversion efficiency of 21.1%, and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability.

Abstract

Mixed cation/halide perovskites have led to a significant increase in the efficiency and stability of perovskite solar cells. However, mobile ionic defects inevitably exacerbate the photoinduced phase segregation and self‐decomposition of the crystal structure. Herein, ultrathin 2D nanosheets of oxo‐functionalized graphene/dodecylamine (oxo‐G/DA) are used to solve ion migration in cesium (Cs)‐formamidinium (FA)‐methylammonium (MA) triple‐cation‐based perovskites. Based on the superconducting carbon skeleton and functional groups that provide lone pairs of electrons on it, the ultrathin 2D network structure can fit tightly on the crystals and wrap them, isolating them, and thus reducing the migration of ions within the built‐in electric field of the perovskite film. As evidence of the formation of sharp crystals with different orientation within the perovskite film, moiré fringes are observed in transmission electron microscopy. Thus, a champion device with a power conversion efficiency (PCE) of 21.1% (the efficiency distribution is 18.8 ± 1.7%) and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability, is reported. This work provides a simple method for the improvement of the structural stability of perovskite in solar cells.

High‐efficiency organic solar cells are achieved through the use of a new electron acceptor AQx‐2 with a quinoxaline‐containing fused core. The increase in performance is attributed to the optimized phase separation morphology that significantly boosts hole transfer and suppresses geminate recombination. The power conversion efficiency of these devices, 16.4%, is the highest certified value for binary organic solar cells.

Abstract

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene‐electron‐acceptor‐based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx‐2 with quinoxaline‐containing fused core and PBDB‐TF as donor. The significant increase in photovoltaic performance of AQx‐2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx‐1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure–morphology–property relationship is established. The stronger π–π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx‐2‐based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next‐generation high‐performance OSCs.

Nonfullerene all‐small‐molecule organic solar cells (NFSM‐OSCs) have shown a promising potential towards the commercialization of OSCs, owing to their unique advantages of high purity, easy synthesis and good reproducibility. However, great challenges in the modulation of phase separation morphology have limited their future development. Herein, two novel small molecular donors of BTEC‐1F and BTEC‐2F, derived from the small molecule DCAO3TBDTT, were designed and synthesized. While using Y6 as the acceptor, the devices based on non‐fluorinated DCAO3TBDTT showed an open circuit voltage ( V oc ) of 0.804 V and a power conversion efficiency (PCE) of 10.64%. Mono‐fluorinated BTEC‐1F showed an increased V oc of 0.870 V and a PCE of 11.33%. More impressively, the fill factor (FF) of di‐fluorinated BTEC‐2F based NFSM‐OSC was largely improved to 72.35% resulting in an impressive PCE of 13.34%, which was much higher than that of BTEC‐1F (61.35%) and DCAO3TBDTT (60.95%). To the best of our knowledge, this is the highest reported PCE to date for NFSM‐OSCs . BTEC‐2F depicted a more compact molecular stacking and a lower crystallinity as revealed from characterization studies, which was beneficial for enhancing phase separation and carrier transport. Those results demonstrated an effective strategy to improve the performance of NFSM‐OSCs via fluorination of small molecular donors and modulation of crystallinity deviation between donors and acceptors.

A strategy for introducing the additive 1,4,7,10,13,16‐hexaoxacyclooctadecane (18C6) into the triple cation perovskite precursor solution is demonstrated, and its influence in precursor and perovskite crystals is thoroughly investigated with simultaneous experimental and theoretical methods. It is found that the formation of the 18C6/Pb complex plays a significant role in the enhanced precursor stability and defect passivation effect within the crystal surface.

Abstract

Triple cation perovskites (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃) have received lots of attention owing to the excellent stability and photovoltaic performance. However, the development toward efficient solar cells has been significantly impeded by its intrinsic precursor instability, as well as defective crystal surface. Herein, a strategy for introducing the additive of 1,4,7,10,13,16‐hexaoxacyclooctadecane (18C6) in the precursor solution, rendering an excellent stability of more than one month, and the defect passivation effect on the crystal surface are demonstrated. In those perovskite solar cells, a power conversion efficiency of 20.73% has been achieved with a substantially improved open‐circuit voltage and fill factor. As evidenced by the density functional theory calculations, the fundamental reason relating to the enhanced performance is found to be the interaction effect between the 18C6 and cations, and in particular the formation of the 18C6/Pb complex. This finding represents an alternative strategy for achieving a stable precursor solution and efficient perovskite solar cells.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Chunyu Xu, Jian Wang, Qiaoshi An, Xiaoling Ma, Zhenghao Hu, Jinhua Gao, Jian Zhang, Fujun Zhang

Graphical abstract

A power conversion efficiency of 12.84% is achieved in small molecules organic photovoltaics by incorporating PC₇₁BM as morphology regulator.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Zhaozhao Bi, Hafiz Bilal Naveed, Xinyu Sui, Qinglian Zhu, Xianbin Xu, Lu Gou, Yanfeng Liu, Ke Zhou, Lei Zhang, Fengling Zhang, Xinfeng Liu, Wei Ma

Graphical abstract

Organic solar cells based on DR3TBDTT:PTB7-Th:FOIC:PC₇₁BM are designed to deliver an individually optimization nanostructure for donor and acceptor phases in quaternary devices. In the quaternary system, the modified acceptor packing and domains together with improved donor crystallinity contribute to facilitated carrier kinetics, and drive the device efficiency towards over 13.5%, which indicates the great potential of quaternary organic solar cells.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Zhenghui Luo, Tao Liu, Yiqun Xiao, Tao Yang, Zhanxiang Chen, Guangye Zhang, Cheng Zhong, Ruijie Ma, Yuzhong Chen, Yang Zou, Xinhui Lu, He Yan, Chuluo Yang

Graphical abstract

Three isomeric ending-group based small molecular acceptors are designed to get deep insight into the ending-group engineering from theoretical simulation and experimental evidence. The ITC-2Br2-based polymer solar cells (PSCs) yields a significantly higher power energy efficiency (PCE) of 13.1% than those of the device base on ITC-2Br (10.9%) and ITC-2Br1 (11.9%).

Publication date: February 2020

Source: Nano Energy, Volume 68

Author(s): Wanpei Hu, Xin He, Zhimin Fang, Weitao Lian, Yanbo Shang, Xingcheng Li, Weiran Zhou, Mengmeng Zhang, Tao Chen, Yalin Lu, Lijun Zhang, Liming Ding, Shangfeng Yang

Graphical abstract

Bulk heterojunction (BHJ) bismuth-based perovskite solar cells consisting of in-situ phase-separated Cs₃Bi₂I₉ and Ag₃Bi₂I₉ components as the photoactive layer are constructed for the first time, achieving a record efficiency of approximate 3.6% and an unprecedented open-circuit voltage reaching 0.89 V. Formation of Cs₃Bi₂I₉–Ag₃Bi₂I₉ BHJ structure effectively promotes the ambient and thermal stability of device, retaining ~90% of the initial efficiency after 450 h heating under 85 °C in glove box.

Publication date: February 2020

Source: Nano Energy, Volume 68

Author(s): Wei Chen, Yongqiang Shi, Yang Wang, Xiyuan Feng, Aleksandra B. Djurišić, Han Young Woo, Xugang Guo, Zhubing He

Graphical abstract

Chiral-induced spin selectivity (CISS) occurs when the chirality of the transporting medium selects one of the two spin 1/2 states to transport through the media while blocking the other. Monolayers of chiral organic molecules demonstrate CISS but are limited in their efficiency and utility by the requirement of a monolayer to preserve the spin selectivity. We demonstrate CISS in a system that integrates an inorganic framework with a chiral organic sublattice inducing chirality to the hybrid system. Using magnetic conductive-probe atomic force microscopy, we find that oriented chiral 2D-layered Pb-iodide organic/inorganic hybrid perovskite systems exhibit CISS. Electron transport through the perovskite films depends on the magnetization of the probe tip and the handedness of the chiral molecule. The films achieve a highest spin-polarization transport of up to 86%. Magnetoresistance studies in modified spin-valve devices having only one ferromagnet electrode confirm the occurrence of spin-dependent charge transport through the organic/inorganic layers.

This study reports a simultaneous contact and grain‐boundary passivation strategy in planar perovskite solar cells using SnO₂‐KCl composite as the electron transport layer. When applied to perovskite solar cells employing a composition of (FAPbI₃)_0.95(MAPbBr₃)_0.05, this strategy increases the open‐circuit voltage from 1.077 to 1.137 V and the corresponding efficiency from 20.2% to 22.2%.

Abstract

The performance of perovskite solar cells is sensitive to detrimental defects, which are prone to accumulate at the interfaces and grain boundaries of bulk perovskite films. Defect passivation at each region will lead to reduced trap density and thus less nonradiative recombination loss. However, it is challenging to passivate defects at both the grain boundaries and the bottom charge transport layer/perovskite interface, mainly due to the solvent incompatibility and complexity in perovskite formation. Here SnO₂‐KCl composite electron transport layer (ETL) is utilized in planar perovskite solar cells to simultaneously passivate the defects at the ETL/perovskite interface and the grain boundaries of perovskite film. The K and Cl ions at the ETL/perovskite interface passivate the ETL/perovskite contact. Meanwhile, K ions from the ETL can diffuse through the perovskite film and passivate the grain boundaries. An enhancement of open‐circuit voltage from 1.077 to 1.137 V and a corresponding power conversion efficiency increasing from 20.2% to 22.2% are achieved for the devices using SnO₂‐KCl composite ETL. The composite ETL strategy reported herein provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.