Fullerenes and derivatives as electron transport materials in perovskite solar cells

Rational Design of Fullerene Derivatives for Improved Stability of p-i-n Perovskite Solar Cells

Perovskite solar cells (PSCs) with p-i-n architecture attracted particular attention from the research community due to their simple and scalable fabrication at low temperatures. However, the operational stability of p-i-n PSCs has to be improved, which requires the development of advanced charge transport interlayers. Fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PC61BM) are commonly used as electron transport layer (ETL) materials in PSCs, though they strongly compromise the device stability. Indeed, it has been shown that PC61BM films actively absorb volatile products resulting from photodegradation of lead halide perovskites and transport them towards top metal electrode. Thus, there is an urgent need for development of new fullerene-based electron transport materials with improved properties, in particular the ability to heal defects on the perovskite films surface and block the diffusion of volatile perovskite photodegradation products. To address this challenge, a systematic variation of organic addends structure should be performed in order to tailor the properties of fullerene derivatives. Herein, we rationally designed a series of fullerene derivatives with different side chains and explored their performance as ETL materials in perovskite solar cells. It has been shown that among all studied compounds, a methanofullerene with thiophene pendant group enables both high efficiency and improved device operational stability. The obtained results suggest that further engineering of fullerene-based materials could pave a way for the development of advanced ETL materials enabling long lifetimes of p-i-n perovskite solar cells.

One-step direct oxidation of fullerene-fused alkoxy ethers to ketones for evaporable fullerene derivatives

Ketones are widely applied moieties in designing functional materials and are commonly obtained by oxidation of alcohols. However, when alcohols are protected/functionalized, the direct oxidation strategies are substantially curbed. Here we show a highly efficient copper bromide promoted one-step direct oxidation of benzylic ethers to ketones with the aid of a fullerene pendant. Mechanistic studies unveil that fullerene can serve as an electron pool proceeding the one-step oxidation of alkoxy group to ketone. In the absence of the fullerene pendant, the unreachable activation energy threshold hampers the direct oxidation of the alkoxy group. In the presence of the fullerene pendant, generated fullerene radical cation can activate the neighbour C-H bond of the alkoxy moiety, allowing a favourable energy barrier for initiating the direct oxidation. The produced fullerene-fused ketone possesses high thermal stability, affording the pin-hole free and amorphous electron-transport layer with a high electron-transport mobility.

On demand synthesis of hollow fullerene nanostructures

Hollow nanostructures are widely used in chemistry, materials, bioscience, and medicine, but their fabrication remains a great challenge. In particular, there is no effective strategy for their assembly and interconnection. We bring pottery, the oldest and simplest method of fabricating hollow containers, into the nanoscale. By exploiting the liquid nature of the xylene template, fullerene hollow nanostructures of tailored shapes, such as bowls, bottles, and cucurbits, are readily synthesized. The liquid templates permit stepwise and versatile manipulation and hence, modular assembly of nodes and junctions leads to interconnected hollow systems. As a proof-of-concept, we create multi-compartment nano-containers, with different nanoparticles isolated in the separate pockets. This methodology expands the synthetic freedom for hollow nanostructures, building a bridge from isolated hollow units to interconnected hollow systems.

At nanoscale, it is synthetically very difficult to increase the structural complexity of hollow structures. Here, the authors use a stepwise liquid templating strategy to build, assemble, and interconnect fullerene hollow nanostructures, just like the synthetic freedom one could have with pottery.

Achieving High Efficiency in Solution-Processed Perovskite Solar Cells Using C60/C70 Mixed Fullerenes

Fullerenes have attracted considerable interest as an electron-transporting layer in perovskite solar cells. Fullerene-based perovskite solar cells produce no hysteresis and do not require high-temperature annealing. However, high power conversion efficiency has been only achieved when the fullerene layer is thermally evaporated, which is an expensive process. In this work, the limitations of a solution-processed fullerene layer have been identified as high crystallinity and the presence of remnant solvents, in contrast to a thermally deposited C₆₀ film, which has low crystallinity and no remaining solvents. As a solution to these problems, a mixed C₆₀ and C₇₀ solution-processed film, which exhibits low crystallinity, is proposed as an electron-transporting layer. The mixed-fullerene-based devices produce power conversion efficiencies as high as that of the thermally evaporated C₆₀-based device (16.7%) owing to improved fill factor and open-circuit voltage. In addition, by vacuum-drying the mixed fullerene film, the power conversion efficiency of the solution-processed perovskite solar cells is further improved to 18.0%. This improvement originates from the enhanced transmittance and charge transport by removing the solvent effect. This simple and low-cost method can be easily used in any type of solar cells with fullerene as the electron-transporting layer.

Room-Temperature and Aqueous Solution-Processed Two-Dimensional TiS2 as an Electron Transport Layer for Highly Efficient and Stable Planar n-i-p Perovskite Solar Cells

In this study, a room-temperature and aqueous solution-processed two-dimensional (2D) transition-metal dichalcogenide TiS₂ was applied as an electron transport layer (ETL) in planar n-i-p perovskite solar cells (Pero-SCs). Upon insertion of the 2D TiS₂ ETL with UV-ozone (UVO) treatment, the power conversion efficiency (PCE) of the planar Pero-SCs was optimized to 18.79%. To the best of our knowledge, this value should be the highest efficiency to date among those PCEs of the n-i-p Pero-SCs with room-temperature-processed metal compound ETLs. More importantly, the n-i-p Pero-SCs with the UVO-treated 2D TiS₂ as an ETL also show extremely high stability, where the average PCE remained over 95% of its initial value after 816 h storage without encapsulation.

Enhancing Performance and Uniformity of Perovskite Solar Cells via a Solution-Processed C70 Interlayer for Interface Engineering

Although some kinds of semiconductor metal oxides (SMOs) have been applied as electron selective layers (ESLs) for planar perovskite solar cells (PSCs), electron transfer is still limited by low electron mobility and defect film formation of SMO ESLs fabricated via low-temperature solution process. Herein, the C₇₀ interlayer between TiO₂ and (HC(NH₂)₂PbI₃)_x(CH₃NH₃PbCl₃)_1-x is prepared by spin-coating and low-temperature annealing for planar n-i-p PSCs. The resultant TiO₂/C₇₀ ESL shows good surface morphology, efficient electron extraction, and facilitation of high-quality perovskite film formation, which can be attributed to the suitable nanosize and the superior electronic property of C₇₀ molecules. In comparison with pristine TiO₂-based PSCs, the efficiency and hysteresis index are, respectively, enhanced 28% and reduced 76% by adding the C₇₀ interlayer between TiO₂ and perovskite on the basis of statistical data of more than 50 cells. With the main advantages of low-temperature process and optimized interface, the champion efficiency of PSCs on flexible substrates could exceed 12% in contrast with the above 18% on rigid substrate.

Comprehensive Study of Sol-Gel versus Hydrolysis-Condensation Methods To Prepare ZnO Films: Electron Transport Layers in Perovskite Solar Cells

Owing to the high charge mobility and low processing temperature, ZnO is regarded as an ideal candidate for electron transport layer (ETL) material in thin-film solar cells. For the film preparation, the presently dominated sol-gel (SG) and hydrolysis-condensation (HC) methods show great potential; however, the effect of these two methods on the performance of the resulting devices has not been investigated in the same frame. In this study, the ZnO films made through SG and HC methods were applied in perovskite solar cells (Pero-SCs), and the performances of corresponding devices were compared under parallel conditions. We found that the surface morphologies and the conductivities of the films prepared by SG and HC methods showed great differences. The HC-ZnO films with higher conductivity led to relatively higher device performance, and the best power conversion efficiencie (PCE) of 12.9% was obtained; meanwhile, for Pero-SCs based on SG-ZnO, the best PCE achieved was 10.9%. The better device performance of Pero-SCs based on HC-ZnO should be attributed to the better charge extraction and transportation ability of HC-ZnO film. Moreover, to further enhance the performance of Pero-SCs, a thin layer of pristine C₆₀ was introduced between HC-ZnO and perovskite layers. By doing so, the quality of perovskite films was improved, and the PCE was elevated to 14.1%. The preparation of HC-ZnO film involves relatively lower-temperature (maximum 100 °C) processing; the films showed better charge extraction and transportation properties and can be a more promising ETL material in Pero-SCs.

Water-Soluble 2D Transition Metal Dichalcogenides as the Hole-Transport Layer for Highly Efficient and Stable p-i-n Perovskite Solar Cells

As a hole-transport layer (HTL) material, poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) was often criticized for its intrinsic acidity and hygroscopic effect that would in the long run affect the stability of perovskite solar cells (Pero-SCs). As alternatives, herein water-soluble two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS₂ and WS₂ were used as HTLs in p-i-n Pero-SCs. It was found that the content of 1T phase in 2D TMDs HTLs is centrally important to the power conversion efficiencies (PCEs) of Pero-SCs, and the 1T-rich TMDs (as achieved from exfoliation and without postheating) lead to much higher PCEs. More importantly, as PEDOT:PSS was replaced by 2D TMDs, both the PCEs and stability of Pero-SCs were significantly improved. The highest PCEs of 14.35 and 15.00% were obtained for the Pero-SCs with MoS₂ and WS₂, respectively, whereas the Pero-SCs with PEDOT:PSS showed a highest PCE of only 12.44%. These are up to date the highest PCEs using 2D TMDs as HTLs. After being stored in a glovebox for 56 days, PCEs of the Pero-SCs using MoS₂ and WS₂ HTLs remained 78 and 72%, respectively, whereas the PCEs of Pero-SCs with PEDOT:PSS almost dropped to 0 over 35 days. This study demonstrates that water-soluble 2D TMDs have great potential for application as new generation of HTLs aiming at high performance and long-term stable Pero-SCs.