Chen Weijie

Laminated perovskite layers with different crystal sizes and optical and electrical characteristics are achieved by using aniline as the solvent in the perovskite precursor solution. Inverted planar perovskite solar cells with the laminated films as active layers achieve an average power conversion efficiency of 20.65%, originating from the high V _OC 1.112 V and fill factor of 80.8%.

Abstract

Laminated multilayers of perovskite films with different optical and electronic characteristics will easily realize high‐performance optoelectronic devices because it is widely demonstrated that differential distribution of film properties in the vertical direction of devices plays particularly important roles in device performance. However, the existing laminated perovskite films are hardly prepared by a solution process because there is no solvent with sufficient selectivity of solubility for different perovskite materials. Here, it is demonstrated that aniline (AN) has a largely different solubility toward the perovskite MAPbI₃ and the MAPbI₃ blend with an additive of hydrochloride diethylammonium chloride. By using AN as the solvent in the perovskite precursor solution, two laminated perovskite layers with different crystal size and optical and electrical characteristics are achieved. Inverted perovskite solar cells with the laminated films as active layers achieve an averaged power conversion efficiency of 20.65% originating from the high V _OC 1.112 V and fill factor of 80.8%. The devices maintain 98% efficiency after 400 h under 65% RH. This work provides a very simple and feasible method for production of laminated perovskite films to achieve high‐performance perovskite solar cells.

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

In article number https://doi.org/10.1002/aenm.2019012571901257, Yana Vaynzof and co‐workers introduce π‐extended phosphoniumfluorene electrolytes as hole‐blocking layers in planar perovskite solar cells. The electrolytes drastically alter the energetic landscape of the device, introducing a strong dipole between the fullerene electron extraction layer and the silver electrode. This results in a substantial enhancement in the built‐in potential of the device, increasing its open‐circuit voltage by up to 120 meV.

Recent progress of inorganic perovskite materials and photovoltaic solar cells is summarized, including materials design, methods for preparing high‐quality perovskite films, phase instabilities, nanocrystals, quantum dots, lead‐free perovskites, device process, and upscaling. In addition, the energy loss mechanisms within the device are discussed and relevant methods are proposed accordingly.

Abstract

All‐inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. The power‐conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high‐quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead‐free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low‐bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge‐carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

The changes in photophysical properties of mixed‐halide perovskite films under solar‐equivalent illumination are studied. The illumination generates localized low‐bandgap surface domains, onto which photoexcited charge carriers transfer and recombine with high radiative efficiency. The fraction of radiative and nonradiative (Auger) recombination bandgap can be balanced to achieve extremely high photoluminescence quantum yields at low excitation densities.

Abstract

Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs_0.06MA_0.15FA_0.79)Pb(Br_0.4I_0.6)₃ perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI₂ material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.

This review provides a systematic overview of self‐powered integrated systems based on perovskite solar cells, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices are discussed to further the understanding of fundamental device physics. The current challenges and future perspective are provided.

Integrated smart portable devices (e.g., self‐powered devices) that utilize the environment‐friendly energy (e.g., solar energy) by means of photovoltaic technology (e.g., solar cell) are a popular concept in the current technological development trend. As a key component of integrated devices, photovoltaic devices acting as a bridge between solar energy and working devices play an important role in the whole system performance. The emergence of perovskite solar cells (PSCs) with high power conversion efficiencies (over 25%) allows for the possibility and appearance of many multifunctional self‐powered integrated devices. In this review, a systematic overview of self‐powered integrated devices based on PSCs that are reported so far is provided, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices and performance are also discussed to further the understanding of fundamental device physics. Finally, the current challenging issues and future perspective are provided to promote the development of self‐powered integrated devices based on PSCs in the near future.

Herein, the electronic and optical properties of the Janus Ga₂SeTe monolayer are calculated via first principles. The multilayer Janus Ga₂SeTe solar cells give rise to a photocurrent exceeding that of thin‐film silicon devices at a phonon energy below 2.5 eV, indicating that Janus Ga₂SeTe is a potential material that can be used in photovoltaic devices.

The electronic and optical properties of Janus Ga₂SeTe monolayer are calculated using first‐principles calculations and it is found that it has potential in solar cells. It is found that ultrathin cross‐plane pn‐junctions are obtained by stacking Ga₂SeTe structures. The graphene‐Ga₂SeTe‐graphene sandwich‐structured solar cells are configured to explore the device performance of Ga₂SeTe solar cells. The photocurrent and the power conversion efficiency of the Janus Ga₂SeTe solar cells are evaluated. The results show that multilayer Janus Ga₂SeTe solar cells give rise to a photocurrent exceeding that of thin‐film silicon devices, indicating that Ga₂SeTe is a potential material that could be used in photovoltaics devices.

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

Publication date: 20 November 2019

Source: Joule, Volume 3, Issue 11

Author(s): Enbing Bi, Wentao Tang, Han Chen, Yanbo Wang, Julien Barbaud, Tianhao Wu, Weiyu Kong, Peng Tu, Hong Zhu, Xiaoqin Zeng, Jinjin He, Shin-ichi Kan, Xudong Yang, Michael Grätzel, Liyuan Han

Context & Scale

Summary

Graphical Abstract

A donor derivative is incorporated in benzodithiophene terthiophene rhodanine (BTR)‐based thick‐film all‐small‐molecule (ASM) organic solar cells (OSC), which achieves power conversion efficiency of 10.14% and fill factor of 74.2%, outperforms its binary counterparts, and stands the record value for thick‐film dual‐donor ternary ASM OSCs. The results demonstrate that the donor derivative is a promising third component to boost the performance of ASM OSCs.

Abstract

Thick‐film all‐small‐molecule (ASM) organic solar cells (OSCs) are preferred for large‐scale fabrication with printing techniques due to the distinct advantages of monodispersion, easy purification, and negligible batch‐to‐batch variation. However, ASM OSCs are typically constrained by the morphology aspect to achieve high efficiency and maintain thick film simultaneously. Specifically, synchronously manipulating crystallinity, domain size, and phase segregation to a suitable level are extremely challenging. Herein, a derivative of benzodithiophene terthiophene rhodanine (BTR) (a successful small molecule donor for thick‐film OSCs), namely, BTR‐OH, is synthesized with similar chemical structure and absorption but less crystallinity relative to BTR, and is employed as a third component to construct BTR:BTR‐OH:PC₇₁BM ternary devices. The power conversion efficiency (PCE) of 10.14% and fill factor (FF) of 74.2% are successfully obtained in ≈300 nm OSC, which outperforms BTR:PC₇₁BM (9.05% and 69.6%) and BTR‐OH:PC₇₁BM (8.00% and 65.3%) counterparts, and stands among the top values for thick‐film ASM OSCs. The performance enhancement results from the enhanced absorption, suppressed bimolecular/trap–assisted recombination, improved charge extraction, optimized domain size, and suitable crystallinity. These findings demonstrate that the donor derivative featuring similar chemical structure but different crystallinity provides a promising third component guideline for high‐performance ternary ASM OSCs.

Recent progress of inorganic perovskite materials and photovoltaic solar cells is summarized, including materials design, methods for preparing high‐quality perovskite films, phase instabilities, nanocrystals, quantum dots, lead‐free perovskites, device process, and upscaling. In addition, the energy loss mechanisms within the device are discussed and relevant methods are proposed accordingly.

Abstract

All‐inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. The power‐conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high‐quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead‐free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low‐bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge‐carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

A low bandgap nonfullerene acceptor (NFA) is incorporated into fullerene electron‐transporting layer (ETL) of an inverted perovskite solar cell aiming to intercept the NIR light passing through the device. However, it cannot enhance the device's NIR photoresponse. Further adding a p‐type polymer effectively enhances the device's NIR photoresponse due to better cascade energy‐level alignment and increased hole mobility.

Abstract

How to extend the photoresponse of perovskite solar cells (PVSCs) to the region of near‐infrared (NIR)/infrared light has become an appealing research subject in this field since it can better harness the solar irradiation. Herein, the typical fullerene electron‐transporting layer (ETL) of an inverted PVSC is systematically engineered to enhance device's NIR photoresponse. A low bandgap nonfullerene acceptor (NFA) is incorporated into the fullerene ETL aiming to intercept the NIR light passing through the device. However, despite forming type II charge transfer with fullerene, the blended NFA cannot enhance the device's NIR photoresponse, as limited by the poor dissociation of photoexciton induced by NIR light. Fortunately, it can be addressed by adding a p‐type polymer. The ternary bulk‐heterojunction (BHJ) ETL is demonstrated to effectively enhance the device's NIR photoresponse due to the better cascade‐energy‐level alignment and increased hole mobility. By further optimizing the morphology of such a BHJ ETL, the derived PVSC is finally demonstrated to possess a 40% external quantum efficiency at 800 nm with photoresponse extended to the NIR region (to 950 nm), contributing ≈9% of the overall photocurrent. This study unveils an effective and simple approach for enhancing the NIR photoresponse of inverted PVSCs.

Application of the proposed slow post‐annealing for layered 2D perovskite solar cells based on BA₂MA₃Pb₄I₁₃ photo‐absorber leads to a favorable alignment on the multi‐perovskite phases and resultant champion power conversion efficiency to 17.26%, showing simultaneously enhanced open‐circuit voltage and short‐circuit current.

Abstract

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA₂MA₃Pb₄I₁₃ (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N₂ environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.

Nature Materials, Published online: 02 September 2019; doi:10.1038/s41563-019-0465-6