The role of exciton delocalization in the performance of Bulk-Heterojunction polymer solar cells

Abstract

Due to the advantages of low cost, easy fabrication, mechanical flexibility, the research on organic solar cells has attracted a lot of attentions in the last decades. Among countless organic materials, the most successful ones are conjugated polymers, which were widely used in bulk heterojunction (BHJ) in conjugation with the fullerene [1]. In view of the fact the interpenetratedphase-separated p-n composites offer more chances for excitons to reach the p-n interfaces, BHJ greatly facilitates the dissociation of photogenerated excitons before decaying to ground state. During exciton dissociation process, the exciton delocalization at an ultra fast timescale has been observed recently, which has considerable effects on the charge generation and device performance. In this work, based on the recent studies [2–4], we established a theoretical model of polymer solar cells (PSCs) incorporating the exciton delocalization process. Through the model, we found the increase of delocalized excitons (compared to the total generated excitons) will significantly improve the short-circuit current, open-circuit voltage, fill factor and power conversion efficiency of PSCs. Particularly, the saturations of fill factor and open-circuit voltage are also observed, and the saturation points depend on the active layer thickness and recombination rate. For instance,the device model of 200nm thickness active layer has the saturated fill factor at 70% delocalization ratio, which is close to the conventional ratio of practical devices. The device configuration studied comprises a BHJ active layer sandwiched between a transparent anode and a metal cathode. Moreover, the active material is a mixture of OC 1 C 10 -PPV, as the donor, and PCBM, as the acceptor [5]. The generation rate of excitons is assumed to be homogeneous. The model proposed involves both the drift-diffusion model for photocarriers (Eq. (1)) and the diffusion and dissociation model for excitons (Eq. (2)). The governing equations are given as follows equation where q is elementary charge, n(p) is the electron (hole) density, and r G , r R are the parametersfor controlling exciton generation and bulk recombination, respectively. E is the internal E-field. µ and D are the mobility and diffusion coefficients. The exciton density is denoted by X diff and k diss is the exciton dissociation rate. The recombination R is taken to be a bimolecular form,with the rate given by Langevin [6], and η s is 0.25 for the singlet excitons. Using the physical parameters given by experiments [5], the nonlinear governing equation sets are solved self-consistently. The excitons are delocalized at an ultra fast timescale to form electron-hole pairs, which is much faster than the photo carrier relaxation time and exciton diffusion time. Consequently, the delocalization process was simulated by directly inserting the delocalization ratio η del into the equations.