Guang Zeng

Two novel donor–acceptor‐type hole‐transporting materials are developed and characterized. Due to the good energy level alignment, appropriate hole‐transporting ability, and most importantly, the excellent film morphology, the MPA‐BTTI‐based dopant‐free inverted perovskite solar cell exhibits a remarkable power conversion efficiency of 21.17% with negligible hysteresis and long‐time operational stability.

Abstract

Hole‐transporting materials (HTMs) play a critical role in realizing efficient and stable perovskite solar cells (PVSCs). Considering their capability of enabling PVSCs with good device reproducibility and long‐term stability, high‐performance dopant‐free small‐molecule HTMs (SM‐HTMs) are greatly desired. However, such dopant‐free SM‐HTMs are highly elusive, limiting the current record efficiencies of inverted PVSCs to around 19%. Here, two novel donor–acceptor‐type SM‐HTMs (MPA‐BTI and MPA‐BTTI) are devised, which synergistically integrate several design principles for high‐performance HTMs, and exhibit comparable optoelectronic properties but distinct molecular configuration and film properties. Consequently, the dopant‐free MPA‐BTTI‐based inverted PVSCs achieve a remarkable efficiency of 21.17% with negligible hysteresis and superior thermal stability and long‐term stability under illumination, which breaks the long‐time standing bottleneck in the development of dopant‐free SM‐HTMs for highly efficient inverted PVSCs. Such a breakthrough is attributed to the well‐aligned energy levels, appropriate hole mobility, and most importantly, the excellent film morphology of the MPA‐BTTI. The results underscore the effectiveness of the design tactics, providing a new avenue for developing high‐performance dopant‐free SM‐HTMs in PVSCs.

Eco‐compatible solvent‐processed organic photovoltaic cells with over 16% power conversion efficiency are achieved via modifying the flexible alkyl chains of BTP‐4F‐8. Combining with the polymer donor T1, over 14% power conversion efficiencies are obtained not only for using several kinds of greener solvents like o‐xylene, 1,2,4‐trimethylbenzene, and tetrahydrofuran but also for 1.07 cm² cells by the blade‐coating method.

Abstract

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large‐area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco‐compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco‐compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP‐4F‐8 (also known as Y6) are modified and BTP‐4F‐12 is synthesized. Combining with the polymer donor PBDB‐TF, BTP‐4F‐12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB‐TF is replaced by T1 with better solubility, various eco‐compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm² OPV cells by the blade‐coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

Nonconjugated multi‐zwitterionic small‐molecule electrolyte (NSE) molecules in perovskite solar cells (PSCs) act not only as both charge‐extracting layers for barrier‐free cathode charge collection but also as charged defect fillers in perovskite bulk and interfaces by spontaneous bottom‐up passivation. Thus, the NSE‐based PSCs deliver PCEs as high as 21.18% with an ultrahigh V _OC of 1.19 V, suppressed hysteresis, and enhanced stability.

Abstract

Recent perovskite solar cell (PSC) advances have pursued strategies for reducing interfacial energetic mismatches to mitigate energy losses, as well as to minimize interfacial and bulk defects and ion vacancies to maximize charge transfer. Here nonconjugated multi‐zwitterionic small‐molecule electrolytes (NSEs) are introduced, which act not only as charge‐extracting layers for barrier‐free charge collection at planar triple cation PSC cathodes but also passivate charged defects at the perovskite bulk/interface via a spontaneous bottom‐up passivation effect. Implementing these synergistic properties affords NSE‐based planar PSCs that deliver a remarkable power conversion efficiency of 21.18% with a maximum V _OC = 1.19 V, in combination with suppressed hysteresis and enhanced environmental, thermal, and light‐soaking stability. Thus, this work demonstrates that the bottom‐up, simultaneous interfacial and bulk trap passivation using NSE modifiers is a promising strategy to overcome outstanding issues impeding further PSC advances.

A multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) between SnO₂ and a perovskite layer improves the average power conversion efficiency from 18.60% to 20.22% due to the upward shift of band position, reduced nonradiative recombination, and improved carrier lifetime. In addition, interfacial engineering improves thermal and moisture stability.

Abstract

Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis‐less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4‐imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO₂ and perovskite through an ester bond with SnO₂ via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The introduction of ImAcHCl onto SnO₂ realigns the positions of the conduction and valence bands upwards, reduces nonradiative recombination, and improves carrier life time. As a consequence, average power conversion efficiency (PCE) is increased from 18.60% ± 0.50% to 20.22% ± 0.34% before and after surface modification, respectively, which mainly results from an enhanced voltage from 1.084 ± 0.012 V to 1.143 ± 0.009 V. The best PCE of 21% is achieved by 0.1 mg mL⁻¹ ImAcHCl treatment, along with negligible hysteresis. Moreover, an unencapsulated device with ImAcHCl‐modified SnO₂ shows much better thermal and moisture stability than unmodified SnO₂.

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Fengyou Wang, Yuhong Zhang, Meifang Yang, Jinyue Du, Leilei Xue, Lili Yang, Lin Fan, Yingrui Sui, Jinghai Yang, Xiaodan Zhang

Graphical abstract

Taking the lead: This Review summarizes recent advances in widely studied lead‐free halide perovskite nanocrystals, centering on understanding their crystal structures, synthesis methods, environmental stability, and optical properties. The challenges in this rapidly evolving field and opportunities to further improve the quality and stability of these nanocrystals are also provided.

Abstract

In recent years, there have been rapid advances in the synthesis of lead halide perovskite nanocrystals (NCs) for use in solar cells, light emitting diodes, lasers, and photodetectors. These compounds have a set of intriguing optical, excitonic, and charge transport properties, including outstanding photoluminescence quantum yield (PLQY) and tunable optical band gap. However, the necessary inclusion of lead, a toxic element, raises a critical concern for future commercial development. To address the toxicity issue, intense recent research effort has been devoted to developing lead‐free halide perovskite (LFHP) NCs. In this Review, we present a comprehensive overview of currently explored LFHP NCs with an emphasis on their crystal structures, synthesis, optical properties, and environmental stabilities (e.g., UV, heat, and moisture resistance). In addition, strategies for enhancing optical properties and stabilities of LFHP NCs as well as the state‐of‐the‐art applications are discussed. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high‐quality LFHP NCs for a broader range of fundamental research and practical applications.

Triple whammy: Reported here is the fabrication of 2D perovskites (n=5) with triple cations. The resulting perovskites feature longer carrier lifetime, greater mobility, and higher conductivity. The efficiency of 2D perovskite solar cells (PSCs) with triple cations was enhanced by more than 80 % (from 7.80 to 14.23 %) compared to PSCs fabricated with a monocation. The efficiency is also higher than that of PSCs based on binary cation 2D structures.

Abstract

Organic‐inorganic hybrid two‐dimensional (2D) perovskites (n≤5) have recently attracted significant attention because of their promising stability and optoelectronic properties. Normally, 2D perovskites contain a monocation [e.g., methylammonium (MA⁺) or formamidinium (FA⁺)]. Reported here for the first time is the fabrication of 2D perovskites (n=5) with mixed cations of MA⁺, FA⁺, and cesium (Cs⁺). The use of these triple cations leads to the formation of a smooth, compact surface morphology with larger grain size and fewer grain boundaries compared to the conventional MA‐based counterpart. The resulting perovskite also exhibits longer carrier lifetime and higher conductivity in triple cation 2D perovskite solar cells (PSCs). The power conversion efficiency (PCE) of 2D PSCs with triple cations was enhanced by more than 80 % (from 7.80 to 14.23 %) compared to PSCs fabricated with a monocation. The PCE is also higher than that of PSCs based on binary cation (MA⁺‐FA⁺ or MA⁺‐Cs⁺) 2D structures.

Perovskite solar cells based on polarized ferroelectric polymers are fabricated by doping the ferroelectric polymer into the perovskite layer with different polarizing electric fields and different doping concentrations, different polarized ferroelectric polymers' interlayers between the perovskite and the hole‐transporting layer, and both doping and interlayer. After these treatments, the fabricated devices show a maximum power conversion efficiency of 21.38%.

Abstract

In hybrid organic–inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI₃) layer and/or inserted between the perovskite and the hole‐transporting layers to enhance the build‐in field (BIF), improve the crystallization of MAPbI₃, and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer‐doped MAPbI₃ shows an orderly arrangement of MA⁺ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open‐circuit voltage of 1.14 V.