Donor-acceptor-donor modelled donor targets based on indoline and naphthalene diimide functionalities for efficient bulk-heterojunction devices

Highlights

• Donor–acceptor–donor modelled, small molecule donor semiconductors, W5 and W6.

• The first connection of indoline and naphthalene diimide units to generate W5 and W6.

• W5 and W6 comprised phenyl and thiophene π-linkers, respectively.

• Improvement of short-circuit current density and device performance with the design of W6.

• PCE > 6.5% (W6:PC₇₁BM) is among the leading efficiency numbers for the investigated format.

Abstract

Two new organic semiconductors based on indoline and naphthalene diimide building blocks are reported for solution-processed bulk-heterojunction devices. Both materials, coded as W5 and W6, were based on the donor–acceptor–donor structural format, where indoline was used as a terminal donor and the naphthalene diimide unit was the central acceptor. Both W5 and W6 were designed to be the structural analogues where a phenyl ring was used as a π-linker in the former, and the latter comprised an additional thiophene ring. The π-linkers were induced between the donor (indoline) and acceptor (naphthalene diimide) functionalities. As a result of adding a thiophene ring in W6, its thin film demonstrated improved light-harvesting properties together with a narrower optical band gap. The photovoltaic properties were evaluated using solution-processed, bulk-heterojunction devices and W6 afforded a promising power conversion efficiency of 6.71% when tested as a donor material together with the conventional acceptor ([6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM)). The efficiency exhibited by W6: PC₇₁BM is approximately 50% higher than the device based on the W5 blend (4.49%). To our knowledge, W5 and W6 are the first reported examples in the literature where indoline and naphthalene diimide units are linked together, and the optoelectronic and photovoltaic properties have been altered with the incorporation of a thiophene ring within the investigated molecular backbone.

Introduction

The renewable energy technologies, such as organic bulk-heterojunction (BHJ), and dye-sensitized solar cells, are promising as they offer unique advantages such as flexibility, low-cost, light-weight and solution processability [[1], [2], [3], [4]]. BHJ devices in particular are attractive as these devices can be fabricated using cheaply synthesized materials, easily available substrates such as plastic and paper, and with the use of environmentally friendly and industrially viable solvents. The main focus of research in the area of BHJ solar cells has been either the development of novel semiconducting materials (donors or acceptors), or expansion of new fabrication strategies to achieve optimum device outcome. The active layer of such devices comprises a blend of donor and acceptor materials, in which fullerene derivatives such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) and PC₇₁BM have dominated as the acceptor domains for more than two decades [[5], [6], [7]]. In terms of donor materials, conventional polymers such as P3HT have been investigated in detail and have encouraged the development of new and more efficient polymeric semiconducting materials such as PTB7 and PTB7-Th. Although the polymeric materials offer advantages, including structural flexibility, high charge carriers’ mobility, and broad absorption, it is argued that small molecules provide potential advantages over their polymeric counterparts. These advantages are straightforward synthetic routes, evaluation of structure-property relationships, simple purification strategies, and well-defined structures [2,8]. Moreover, small molecule formats, including donor–acceptor (D–A), D–A–D and A–D–A, can be developed as either donor or acceptor target materials. Though it is notable that small molecules offer multiple advantages over polymeric counterparts, it is apparent that in line with the design of high-performing polymers, the design requirements of small molecules include efficient light-harvesting, good chemical and thermal stabilities, excellent solubility, and appropriately positioned energy levels [highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)]. Therefore, it is challenging to design a small molecule that offers optimal performance not only using a simple device architecture, but without any significant treatment, such as annealing, to its active layer to improve device outcome.

The current literature reveals remarkable advancements in the research area of organic solar cells, whether it is the design and development of new materials [[9], [10], [11], [12]], organization of blend film morphology [[13], [14], [15]], interfacial modification [[16], [17], [18], [19]], or the use of new device architectures [20]. The power conversion efficiencies (PCEs) for single junction and tandem solar cells have been reported over 15% and 17%, respectively [21,22]. Though these numbers are encouraging, there are still a number of undesirable procedures that can be improved, including the utilization of halogenated solvents, poor reproducibility of device fabrication strategies, and use of non-scalable processes, such as annealing, that limit the practical applications of organic solar cells. One of the strategies to meet these weaknesses is to carry out the fundamental research on the structural design and synthetic development of small molecule targets, given the advantages they offer. In this paper, we have focused our attention on the D–A–D format – a format offering a great deal of advantages in terms of solubility, wide selection of donor and acceptor blocks, and chemical and thermal stabilities – to design new donor target materials. It is noteworthy to mention that the D–A–D format has been reported in the literature with a limited number of central accepting units, including naphthalene diimide [23], diketopyrrolopyrrole [24], thiazolothiazole [25] and 2-pyran-4-ylidenemalononitrile [26]. Not only the use of such accepting units makes the D–A–D format lucrative, but suggests that it can be investigated with a variety of D and A units, though the selection and choice of the latter is still somewhat restricted. That being said, it is clear that there exists an extensive opportunity to explore the D–A–D format and to generate novel light-harvesting materials. However, it is imperative to mention that in-line with the requirements of efficient donor targets, the developed materials should possess excellent solubility, high charge carrier mobility, and thermal stability. Not only is it our motivation behind the present work, but a continuous effort on the development of novel materials for organic electronic applications, and photovoltaics in particular [[27], [28], [29], [30]].

Herein we report two new small molecules, W5 and W6, based on the D–A–D format that have been used as donor semiconductors in solution-processed BHJ devices. Both W5 and W6 used trimethylindoline and naphthalene diimide units as donor and acceptor, respectively. While designing W5 and W6, we took into account (1) the literature precedence, (2) our detailed understanding of the design modules of oligothiophene donors, and (3) the advantages exerted by the D–A–D design, including simple and scalable synthesis. We used naphthalene diimide (NDI) as the central accepting unit as not only is it a strong acceptor group and a chromophore with appropriate physical and electronic properties, but helps to improve solubility of a given target by allowing substitutions at its imide positions. The incorporation of lipophilic chains at imide positions enhances the solubility of a proposed target, a result that is of great significance to generate excellent pristine and blend films without crystallization. W5 and W6 were designed to be the structural analogues where different π-linkers were used. In W6, an additional thiophene ring was incorporated next to the central NDI unit so as to alter the optoelectronic and photovoltaic properties (Fig. 1). As a result of the extended conjugation, W6 films showed superior light-harvesting properties and a reduced optical band gap. The W6-based device (W6: PC₇₁BM 1: 1) exhibited an impressive PCE of 6.71%, an approximate 50% increase when compared with the device based on W5 blend (4.49%). Given the literature precedence, W5 and W6 are the first reported examples where indoline and NDI building blocks have been conjoined to generate new small molecule donors for organic photovoltaic applications (see Scheme 1).