Rational design of naphthalimide based small molecules non-fullerene acceptors for organic solar cells

Highlights:

• Four new three-dimensional (3D) D-π-A type small molecules were designed computationally.

• All the molecules exhibited excellent absorption properties as well as proper frontier molecular orbital alignment.

• M3 is the champion molecule with exceptional optoelectronic properties.

Abstract

To explore optical electronic and charge transfer properties, a series of D-π-A type of molecules with central core of 9,9-dimethyl-9H-fluorene as donor linked by 6-fluoro-4-(prop-1-yn-1-yl)benzo[1,2,5]thiadiazole as π-bridge to variable end group acceptor materials have been designed for organic solar cells (OSCs). Optoelectronic properties of designed molecules M1-M4 with similar central core and π-bridge but with different end groups were compared with R as representative of our system with 1,8-naphthalimide as end group. These optoelectronic properties are influenced by different end groups. Lower band gaps and longer wavelength of absorption have been observed for molecules by analyzing their frontier molecular orbitals. Furthermore, the computed reorganization energies for designed molecules are also comparable to the R so these molecules can be used as electron and hole transport materials. Among designed molecules M3 has higher wavelength of absorption along with minimum band gap and suitable distribution pattern of HOMO and LUMO during transition. The results presented display that varying the end groups is a highly promising approach in order to develop a series of D-π-A type of materials for organic photovoltaics. So, our computed results display that these designed molecules can be used as an excellent candidates for OSCs.

Introduction

To overcome existing and future energy crisis the most important technology that can serve as alternative energy source is Organic photo-voltaics (OPVs). Conversion of sunlight directly into electricity have been made possible by utilizing either polymers or small molecules based on organic semiconducting moieties. There are many potential advantages associated with OPVs, which make them easier for commercialization as compared to the inorganic counterparts. Among different advantages most important are, higher flexibility of device with cost-effective fabrication, semitransparency and relatively simple processing techniques [1]. In addition to these benefits, greater than 10% power conversion efficiency (PCE) have been reported for single-junction organic solar cells (OSCs). Organic photo-voltaics have the blends of donor and acceptor molecules. So, overall performance, light harvesting ability and controlled morphology of OSCs are strictly governed by optical, electrochemical and structural properties of both acceptor and donor materials [2].

In the past few years, fullerene and its derivatives have emerged as the most promising candidates to be used as electron acceptor materials due to higher electron affinity as well as greater electron mobility [3]. The reported PCE for fullerene based device is 11%. However, further increase in PCE is limited because of relatively poor absorption in visible region of solar spectrum and less photochemical stability of fullerene based devices. Moreover, it is very difficult to chemically modify fullerene and its derivatives as well as expensive fabrication process have significantly reduced the use of fullerenes in OSCs [4]. Recently, most of the research is being done on the development of non-fullerene based acceptors to overcome the shortcomings of fullerene based OSCs.

Tunable LUMO level as well as greater molar absorption coefficient in visible region make the non-fullerene (NF) acceptors an excellent functional materials that can significantly improve PCE of OSC devices. Most of the NF based OSCs tends to exhibit lower electron mobility and higher hole mobility, of acceptors and donor respectively, leading to the recombination of charges which in turn decrease fill factor and short circuit current [5]. Therefore design of non-fullerene molecule having higher electron mobility is needed, for its implementation in OSCs. So, in order to increase the electron mobility, various electron withdrawing species have been incorporated to small molecules NF acceptors.

Among different designed molecules of n-type for OSCs applications, 1,8-naphthalimide (NI) is a promising candidate. This NI fragment incorporated as end group to central core of 9,9-dimethyl-9H-fluorene with 6-fluoro-4-(prop-1-yn-1-yl)benzo[1,2,5]thiadiazole as π-bridge [6] is the representative of our system R. NI based small molecules usually possess higher LUMO level which in turn provide higher open circuit voltage (Voc) and lower energy loss [7]. NI and its derivatives generally have central ring that is electron deficient that results in higher electron affinity.

Herein, from theoretical study we have investigated further charge transfer, optical and electronic properties for our own designed molecular system comprising of similar central core and conjugated π-bridges of R with different designed small molecules as end groups replacing NI fragment. These end groups include 1a (2-ethylidenemalonitrile), 1b (6-ethylidene-3-methyl-2-thioxo-1,3-thiazinan-4-one), 1c (2-(2-ethylidene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile) and 1d (5-ethylidene-3-methyl-2-thioxothiazolidin-4-one). These acceptor moieties with good performance have been effectively used in many recent OSC devices. The logic behind using such different end groups is that we need to tune the energy levels of molecules and to analyze their effect on overall photovoltaic parameters. Moreover, the selection of the these four end groups provided a facile strategy for the designing of series of small molecules with certain functionalities as well as to predict the variations in the properties and molecular structure of selected reference molecule. Structure of these end groups leading to the construction of M1, M2, M3 and M4 are shown in Fig. 1. Furthermore, we investigated absorption region, charge carrier mobilities and the relationship between optoelectronic properties as well as the effect of D–π–A on these properties. We have provided a brief demonstration about our own designed candidates that can serve as multifunctional materials in organic solar cells.