Probing the effect of acceptor engineering in benzothiadiazole-based D-A-D-typed hole-transporting materials for perovskite solar cells

Highlights

• Six D-A-D HTMs are tailored and their electronic and charge transport properties are characterized.

• Strategy of extending and rigidifying the acceptor unit in D-A-D-typed HTMs is evaluated theoretically.

• All predicted HTMs display suitable energy levels and good optical properties.

• The π-extension and bent molecular structure are favorable to improve the carrier transport in HTMs.

Abstract

Hole-transporting materials (HTMs) is very important for improving the stability and efficiency of perovskite solar cells (PSCs) because it plays a crucial role for the exciton dissociation at the interface and the following hole transport. To improve the performance of HTMs, the strategy of extension and rigidification of the π-conjugated acceptor in donor-acceptor-donor-typed HTMs is evaluated in this work. Theoretical calculations indicate that all the predicted HTMs display the suitable energy levels to ensure the effortless hole transfer at the perovskite/HTM interface. More importantly, our results reveal that extending the acceptor group is a good strategy to promote the hole transport in HTMs. Meanwhile, the bent molecular conformation by rigidifying the conjugated acceptor can also improve the hole mobility of HTMs due to the better molecular planarity and enhanced intermolecular stacking and orbital overlapping. All the predicted HTMs display the higher hole mobility than that of the YN1. The higher mobility and matched energy levels of predicted HTMs are favorable for improving the performance of PSCs. In addition, the better optical property and solution property can also be anticipated for the new tailored HTMs. In summary, this work provides a useful strategy for the design of high-efficient HTMs, and three new designed HTMs are proposed as the potential candidates toward more efficient dopant-free HTMs.

Introduction

Hole-transporting materials (HTMs), as one of the indispensable buffer layers in well-performing n-i-p typed perovskite solar cells (PSCs), play very important roles in extracting and transporting photo-generated holes from the perovskite into the metal electrode and reducing the undesired electron-hole recombination to acquire a higher open-circuit photovoltage ($V_{\mathit{OC}}$) [1], [2], [3]. To improve the cell efficiency and stability, a large variety of HTMs, from inorganic complexes to organic semiconductor materials, have been utilized in PSCs, and especially, the small molecule HTMs have attracted great interest due to their rich raw materials and easy structural engineering [4], [5], [6], [7], [8], [9].

Compared with the spiro-typed and π-extended planar-typed HTMs, the linear- conjugated donor-π-donor (D-π-D) or donor-acceptor-donor (D-A-D) conformations were often testified to have the advantages of simple structure, low cost and effortless synthesis [10], [11], [12], [13], [14], [15], [16], [17]. Furthermore, the D-A-D-typed small molecule HTMs usually display a lower highest occupied molecular orbital (HOMO) energy level, stable electron-rich arylamine system, and strong intramolecular charge delocalization and intermolecular interactions [18], [19], [20], [21], [22]. Besides two-dimensionally extended HTMs, design of D-A-typed or D-A-D-typed scaffold is another important strategy to develop dopant-free HTMs [23], [24]. Due to the electron-deficient property of acceptor group, the strong charge transfer is presented in D-A-D-typed HTMs, which is believed to be beneficial for enhancing the intermolecular π-π stacking and promoting hole carrier mobility [25], [26]. Rational tailoring of acceptor unit can effectively regulate the frontier molecular orbitals (FMOs) energy levels, optical property as well as hole mobility [27], [28], [29]. Up to now, a number of electron- deficient moieties, such as benzothiadiazole [30], [31], quinoxaline [18], thienopyrazine [26], and dithienopyrrolobenzothiadiazole [32] have been used to construct the high-efficient HTMs. Meanwhile, the π-extended influences of the spacer moiety in D-π-D or D-A-D-typed HTMs have been researched in previous reports on the basis of the anthradithiophene, dimethoxyselenophene and the EDOT-based conjugated linkers [33], [34], [35]. Very recently, Wu et al. reported a new dopant-free HTM (TQ4) with coplanar π-extended quinoxaline as acceptor and p-methoxy-substituted triphenylamine (MeOTPA) as donor [36]. The n-i-p structured PSCs employing TQ4 exhibit a high efficiency of over 21% and excellent long-term stability [36]. Meanwhile, Li and co-workers designed two dopant-free HTMs (Y6-T and Y-T) with a bulky electron-deficient backbone (dithienothiophen[3,2-b]- pyrrolobenzothiadiazole, DTTPBT) as acceptor core [37]. By combing the coplanar and electron-deficient properties of acceptor, the Y-T delivers an impressive efficiency of 20.29% [37]. Due to the π-conjugated structure and abundant heteroatoms, DTTPBT and its derivatives are extensively investigated in the field of organic solar cells [38], [39], [40]. From the above, we can find that design of coplanar π-extended acceptor in D-A-D-typed scaffold may be a good strategy for developing dopant-free HTMs.

To obtain a fundamental understanding for the influence of π-extended acceptor on the performance of HTMs, in this work, six new D-A-D HTMs (SM21~SM26, as displayed in Fig. 1) are investigated by extending (SM21 and SM22) and rigidifying (SM23~SM26) of the benzothiadiazole (BT, YN1) acceptor [26], respectively. The large conjugated acceptor is considered to be helpful for constructing ordered π-π stacking and increasing hole hopping channels [18], [25], [36]. Therefore, the BT was firstly extended by adding two thiophene and thieno[3,2-b]thiophene units, and secondly, the conjugated core was further rigidified by nitrogen (N) and sulfur (S) atoms, respectively. Besides, the incorporation of acceptor can easily regulate the electronic and optical property of HTMs. The focus of this study is upon the impact of the extension and rigidification of acceptor unit on the electronic, optical and hole transport properties of HTMs. The calculational details are provided in the ESI. The results show that, the rigidification with the N atom can make the energy levels up-shifted, whereas the S atom induces the energy levels down-shifted slightly. Accordingly, the optical absorption spectra of designed HTMs exhibit red-shift or blue-shift respectively compared to that of YN1. The transfer integral of the main hole hopping route plays an important role for carrier mobility. On the whole, the designed HTMs display the higher hole mobility than that of the referenced YN1. We hope our work can provide some insights and guidance for the design of dopant-free D-A-D-typed HTMs.