Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study

Highlights

• Effect of external static electric field on ground and excited state properties in organic semiconductor systems.

• Investigation of pure and bulk-heterojunction semiconductor systems.

• Two-dimensional contour plot visualizing the electric-field effect on electron-hole coherence.

• Thermodynamically favored charge separation process.

Abstract

In this work, a detailed comparison of optical and electronic properties in bulk and interfaces of well-known organic semiconductor systems in presence of an external electric field is reported. We have used density functional theory (DFT) to model organic solar cell systems. The study promotes a deeper understanding of the connection between the chemical structures and the optical and electronic properties in the well-known organic solar cell systems based on thiophene and fullerene polymers. We have performed a vibration-mode analysis by simulating Raman spectra in presence of external electric fields. Time-dependent DFT has been used to investigate the effect of an external electric field on excited state properties. The charge-transfer rate controlled by the external electric field has been quantitatively extracted using the simulated excited state dipole moment, Gibbs free energy, and Marcus theory. Our results provide a detailed characterization of the effect of the external electric field on the neat polymers (bulk) and on the donor-acceptor heterojunctions (interfaces) in organic solar cell systems. This theoretical approach not only helps to understand the effect of an external field on bulk and interfaces in organic semiconductors, but it also supports the design of novel devices.

Introduction

Organic solar cells have attracted a lot of attention in the past few years for their interesting advantages such as low cost, easy processing and flexibility [1], [2], [3]. The organic solar cells based on poly (3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as acceptor have been studied widely in both experiments [4], [5], [6], [7], [8], [9] and theory [10], [11], [12], [13], [14]. In general, a sandwich configuration of the blend of donor and acceptor, which is connected to two electrodes, is used to fabricate an organic solar cell. The light-to-current conversion happens in these solar cells through different steps; at first, light excites the polymer blend and immediately the primary excitons are generated. Then, the exciton migrates towards the donor-acceptor interfaces where charge separation (CS) happens due to the differences of the work functions of the two materials. The localized electron from the LUMO of the donor is transferred to the LUMO of the acceptor and forms a charge transfer (CT) state.

A uniform external electric field can control the charge-transfer processes. Numerous studies have been dedicated to the investigation of the charge-transfer process in presence of an external electric field [15], [16], [17], [18], [19], [20], [21], [22]. Two competitive processes, charge separation and charge recombination, contribute to the efficiency of the solar cell. Marcus theory [23] has been used widely to investigate the charge-transfer process in donor-acceptor systems. Various factors influence the charge-transfer rate including the vibrational frequency and the external electric fields, as has been already reported [15], [16]. The power-conversion efficiencies of organic solar cells based on P3HT:PCBM have exceeded 11% [24], [25]. It is expected that the global demand of energy will be increased by 35% in the coming 24 years. This has motivated scientists to perform research on new technologies that provide efficient renewable energy sources. Very recently, Biswas et al. [26] have presented quantitative information regarding the photovoltaic performance of S,N-heteroacene dyes (SN_n). They have reported that an even-odd relationship for different lengths n holds for the charge separation process of the heteroacenes; for odd values of n a higher rate is found for the charge separation process. The field of organic photovoltaics has reached a milestone with the invention of C₆₀ and C₇₀ polymers and their analogues PC₆₁BM and PC₇₁BM. Using a similar polymer as an acceptor, Biswas et al. [27] have recently reported the electron transfer rates of regioisomeric small organic molecules. They have replaced the terminal thiophene donor parts of the system by a methylidene rhodanine group and found that this more efficiently electron-withdrawing group lowered the HOMO level, which improves the photovoltaic properties. Also an external electric field can influence the hot-exciton dissociation process and as a result it enhances the probability of collecting more charge carriers.

In the study reported here, we have mainly focused on the effect of the external electric field on structural properties as well as charge-transfer processes in bulk and bulk-heterojunction systems. Up to now, the P3HT:PCBM-based organic solar cells have been considered to be very prominent candidates in device physics. Even when other polymers in the meantime show better efficiencies, a basic understanding of the P3HT-based material is still of interest. A device based on only neat bulk P3HT polymer does not perform with high efficiency compared to the bulk heterojunction system. Nevertheless, due to our interest in fundamental aspects, we have investigated both bulk and bulk-heterojunction systems. This research shows the electron-hole coherence in neat polymers and also in a practical device. Marcus theory is also applied additionally on the donor-acceptor system to get a complete story on charge-transfer processes. Song et al. [15] have already discussed the charge-transfer processes in a similar kind of system where fullerene was linked with a quarter thiophene. Liu et al. [28] have also reported the absolute rate of charge-separation processes in P3HT:PCBM systems. In our case, we have kept a distance between the donor and acceptor; the donor is modeled with seven monomer units of the thiophene ring. The exciton separation and charge recombination in P3HT:PCBM systems have also been investigated experimentally earlier in presence of an external electric field [6], [29], [30].

On the basis of the earlier studies, it is clear that exciton dissociation into charges across the donor-acceptor interfaces can occur spontaneously with the help of the intrinsic interfacial field without an additional external electric field. Nevertheless, although, an external electric field is not required for the spontaneous generation of charges in donor-acceptor systems, the overall charge generation will be affected by the presence of an electric field in a working device. Very recently, we have also investigated the effect of an external electric field in neat P3HT polymer using ultrafast spectroscopy and found that the external electric field helps to dissociate the bound charge-pair [31], [32], [33]. In parallel, we have found bimolecular recombination processes, which act as a loss channel. So far, theoretical investigations have been more focused on the donor-acceptor-based bulk-heterojunction systems; there, a broad overview of the charge-transfer processes can be found in literature. However, a study of the external-field effect on pure organic semiconductor systems is still missing and is not completely understood. Therefore, we have first performed a theoretical investigation of the bulk material, and then also studied the bulk-heterojunction organic semiconductor. Several parameters are extracted from the theoretical simulations.

The presentation of our work is divided into three sections; at first, we concentrate on ground-state properties, then we investigate the structural properties (mainly based on a vibrational-mode analysis), and, finally, we explore the excited-state properties and the electron-hole-coherence in organic semiconductor systems.

This paper is dedicated to Prof. Wolfgang Kiefer on the occasion of his 80th birthday. His pioneering work in the field of linear and nonlinear Raman spectroscopy [34], [35], [36], [37] contributed significantly to the nowadays great popularity of these techniques in many fields of research. Part of his extensive work also gave valuable insights into structural and dynamic properties of inorganic semiconductor nanostructures [38], [39], [40] and organic semiconductor materials [41], [42].