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Dyes and Pigments

Volume 198, February 2022, 109963
Dyes and Pigments

The evolution of small molecular acceptors for organic solar cells: Advances, challenges and prospects

https://doi.org/10.1016/j.dyepig.2021.109963Get rights and content

Highlights

  • This review presents a complete summary of recent advances in efficient small molecular acceptors.

  • The relationship between molecular structures and photovoltaic performance are thoroughly analyzed and summarized.

  • Key issues, challenges and design principles are discussed.

Abstract

Organic solar cells (OSCs) have attracted lots of research attention and significant developments have been achieved in the last decade. Small molecular acceptors (SMAs) have played an important role. During the last five years, non-fullerene small molecular acceptors (NFSMAs) have replaced fullerene ones and become the research focus of OSCs, due to their excellent optoelectronic properties, such as wide and strong absorption, easily adjustable energy levels and stable morphology. This review highlights the recent advances of SMAs, especially NFSMAs including perylenediimide, naphthalenediimide, diketopyrrolopyrrole, indacenodothiophene, and Y6 series. The relationships among molecular structures, optical properties, energy levels, device processing methods and photovoltaic performance are systematically analyzed and summarized. Challenges, strategies and prospects are also discussed. This review may provide new guidances for designing high-performance SMAs.

Introduction

With continuous development of the society, the energy demand for human life, industries and commercial activities has been increased rapidly. Currently, fossil energy is the main energy source. However, fossil energy resources like coal, oil and natural gas are non-renewable. Furthermore, the exploitation and consumption of fossil energy produce lots of pollutants, which may lead to a serious damage on the environment, climate and human health. Therefore, the development of renewable and clean energy had become a worldwide common concern [[1], [2], [3]]. However, non-renewable energy resources are limited. Among all the new energy sources, solar energy is considered as one of the most promising energies due to its outstanding features, such as abundant reserves, wide distribution, easy access, safe and clean in use [[4], [5], [6], [7]].

One way to utilize the solar energy is to convert it into electric power, which is convenient for storage, transport and utilization [8]. Solar cells have gone through three generations, namely, the first generation, the second generation and the third generation (Fig. 1). The first generation of solar cells is represented by monocrystalline silicon devices. Silicon solar cell was firstly manufactured in 1954 and had a photoelectric conversion efficiency of up to 6% [9]. Since then, inorganic semiconductor-based solar cell devices have been developed rapidly, and play a leading role in the industrial applications [10,11]. But the development of the first generation solar cells is limited by several shortcomings, such as complex manufacturing process, high cost and energy consumption, causing serious pollution, and large efficiency attenuation effect [12]. The second generation is thin-film solar cells, which are represented by amorphous silicon, GeAs etc. [13,14] Thin-film solar cells have the advantages of good stability, but the scarcity of raw materials and toxicity limit their further application [15,16]. In recent years, the new third generation solar cells have emerged and developed rapidly, including dye sensitization solar cells (DSSCs) [[17], [18], [19]], quantum dots solar cells (QDSCs) [[19], [20], [21], [22]], organic solar cells (OSCs) [[23], [24], [25], [26], [27]] and perovskite solar cells (PSCs) [[28], [29], [30]]. With lots of research attention paid in the third generation solar cells, the energy conversion efficiency of various third generation solar cells had been increased significantly [[31], [32], [33], [34], [35]]. The champion certified conversion efficiencies for various solar cells released by the National Renewable Energy Laboratory (NREL) of the United States can help understand the development process of various photovoltaic technologies (Fig. 2).

Compared with the inorganic solar cells, OSCs have the following distinct characteristics: a) Organic materials are light in weight and flexible [[36], [37], [38]]; b) The structure of organic materials can be easily and finely tuned; c) The device preparation process is simple and can be prepared by solution processing with low cost, such as screen printing, inkjet printing and spinning coating [39]. Based on these advantages, OSCs come into researcher's sight and have attracted lots of attention [[40], [41], [42], [43], [44], [45]].

The structures of OSC devices can be divided in to single layer, bilayer heterojunction, bulk heterojunction (BHJ), and tandem types (Fig. 3a). The single layer structure, in which there is no distinction between donor and acceptor, is composed of a photovoltaic material sandwiched between different work function electrodes [46]. The efficiency of single layer OSCs is usually less than 0.5% [47]. Later, the concept of heterojunction is introduced into OSCs by inserting a p-type material (donor) and a n-type material (acceptor) with different energy levels between the two electrodes to improve the dissociation efficiency of excitons [48]. To further increase the donor-acceptor interface area, the concept of BHJ was proposed [49]. Compared with the single layer and bilayer heterojunction device, the exciton dissociation efficiency of BHJ device was greatly improved [[50], [51], [52], [53], [54], [55], [56]]. Then, tandem solar cells were developed in order to further improve the efficiency. Tandem solar cells can well exploit the sunlight with large bandgap materials absorbing high energy photons and narrow bandgap materials absorbing low energy photons [43,57]. Moreover, the structure of tandem solar cells is like the series connection of two or more subcells, and thus the voltage of the tandam solar cells can be largely improved.

Fig. 3b shows the working mechanism of BHJ OSCs, which includes five steps. Firstly, the active layer materials absorb photons and thus electrons are excited to form excitions which are bounded electron-hole pairs by van der Waals forces. Secondly, the excitons diffuse to the donor-acceptor interface. Thirdly, excitons dissociate into free electrons and holes at the interface, by the driving force of the lowest unoccupied molecular orbital (LUMO) energy differences between donor and acceptor. Then, the electrostatic field of the device promotes the free electrons to move to the cathode through the acceptor and the free holes to move to the anode through the donor. Lastly, the free electrons and holes are collected at the cathode and anode, respectively.

The efficiency of converting the input solar power (Pin) to the output electric power is defined as power conversion efficiency (PCE), which correlates to the open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) (PCE= Voc × Jsc × FF/Pin) [58]. PCE is the key index to measure the performance of OSCs.

Acceptors can be divided into fullerene and non-fullerene acceptors (NFAs). In a long stage, fullerene derivatives have dominated OSC acceptors [[59], [60], [61]]. However, fullerene derivatives have some drawbacks, such as weak absorption in visible light region, poor light stability, and limited adjustment of their molecular energy levels through chemical modification [[62], [63], [64]]. Those drawbacks limit the PCE improvement of fullerene derivatives based OSCs.

NFAs have gradually become the focus of research on OSCs due to their superior advantages, such as wide and strong absorption in the visible-near infrared region, easy adjustment of energy levels and simple synthesis and purification process [[65], [66], [67]]. The highest proportion of the photons in the sunlight is distributed in the visible and near-infrared region (Fig. 4a) [68]. The NFAs have much wider and stronger absorption in the visible-near infrared region than fullerene acceptors (Fig. 4b), which is favorable to improving the Jsc [69,70].

NFAs include polymer acceptors and non-fullerene small molecular acceptors (NFSMAs). Compared with polymer acceptors, NFSMAs have the following advantages:

  • a)

    The molecular weight of NFSMAs is defined and clear. Therefore, the photovoltaic performance differences are much smaller from batch and batch [[71], [72], [73]].

  • b)

    The synthesis and purification processes are simpler. NFSMAs are easy to obtained [74,75].

  • c)

    NFSMAs have higher crystallinity and phase purity, which is beneficial to improving charge mobility and reducing energy loss [76,77].

Todate, various types of NFSMAs have been developed, and the corresponding PCEs have improved termendously [[78], [79], [80], [81], [82]]. We summarized recently reported high-performance NFSMAs and classified them into five categories, which are perylene diimide (PDI), indacenodithiophene (IDT), naphthalene diimide (NDI), diketopyrrolopyrrole (DPP), and Y6 based small molecular acceptors (SMAs). The structure–property relationships were detailed analyzed and summarized. The challenges and design principles of NFSMAs were also discussed.

Section snippets

Fullerene acceptors

The representative structures of Fullerene acceptors are showed in Fig. 5, and the optoelectronic and photovoltaic properties are summarized in Table 1. Fullerene and its derivatives are a closed cage molecule composed of five and six-membered rings. The unique structure is conducive to electron transport [[83], [84], [85]]. OSCs based on fullerene derivative acceptors have made rapid progress, with device efficiency rising from less than 1% in 1992 to 11.7% in 2018 [86,87]. In 1992, Sariciftci

Summary and comparisons of SMAs

From fullerene derivatives to Y6 series, the PCE has increased from less than 1% to more than 18%. Each type of the acceptors has unique structure and performance characteristics. The evolution of the structures pof SMAs is shown in Fig. 27.

Fullerene derivatives have good electron affinity, high electron mobility and unique spherical structure. Fullerene acceptors can be well miscible with donor materials, which is beneficial to obtaining a good active layer morphology. Reducing the number of π

Challenges and strategies

NFSMAs have become and will still be the research focus in OSCs in the near future. Despite the great success of NFSMAs, there are still some challenges faced to be solved. The following issues need to be focused on in the future studies of NFSMAs.

  • (a)

    NFSMAs with high electron mobility and proper aggregation. The electron mobility of the acceptor is one of the determinants of the photovoltaic performance. And it can be improved by extending the molecular conjugation area and enhancing the

Conclusions and future prospects

The emerging NFSMAs have exhibited outstanding photovoltaic performance and become the hottest research direction in the field of OSCs in recent years. Their excellent advantages, such as wide and strong absorption, easily adjustable energy levels and stable morphology, enable them much more promising than fullerene counterparts.

Overall, this review systematically discussed and summarized the main advances of PDI, NDI, IDT, DPP and Y6 based NFSMAs for photovoltaic applications. To better

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51703104, 51878361), the Shandong Provincial Natural Science Foundation (ZR2017BEM035, ZR2019MEM048, ZR2020MB085), the China Postdoctoral Science Foundation (2017M612198), the State Key Project of International Cooperation Research (2017YFE0108300, 2016YFE0110800), National Key R&D Program of China (2021YFE0190400), the Shandong Double-Hundred Project (2018), the National Plan for Introducing Talents of Discipline to

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