A new multiscale modeling method for simulating the loss processes in polymer solar cell nanodevices

Abstract

The photoelectric power conversion efficiency of polymer solar cells is till now, compared to conventional inorganic solar cells, still relatively low with maximum values ranging from 7% to 8%. This essentially relates to the existence of exciton and charge carrier loss phenomena, reducing the performance of polymer solar cells significantly. In this paper we introduce a new computer simulation technique, which permits to explore the causes of the occurrence of such phenomena at the nanoscale and to design new photovoltaic materials with optimized opto-electronic properties. Our approach consists in coupling a mesoscopic field-theoretic method with a suitable dynamic Monte Carlo algorithm, to model the elementary photovoltaic processes. Using this algorithm, we investigate the influence of structural characteristics and different device conditions on the exciton generation and charge transport efficiencies in case of a novel nanostructured polymer blend. More specifically, we find that the disjunction of continuous percolation paths leads to the creation of dead ends, resulting in charge carrier losses through charge recombination. Moreover, we observe that defects are characterized by a low exciton dissociation efficiency due to a high charge accumulation, counteracting the charge generation process. From these observations, we conclude that both the charge carrier loss and the exciton loss phenomena lead to a dramatic decrease in the internal quantum efficiency. Finally, by analyzing the photovoltaic behavior of the nanostructures under different circuit conditions, we demonstrate that charge injection significantly determines the impact of the defects on the solar cell performance.

A multiscale modeling study of loss processes in block-copolymer-based solar cell nanodevices

“…To simulate the photovoltaic process, 5 we assumed that three types of elementary particles are present in the polymer solar cell device, i.e., electrons, holes, and excitons. Depending on the nature and environment of the elementary particles, they can exhibit the following algorithmic steps: (1) exciton generation by light absorption; (2) exciton motion within the material through diffusion or exciton annihilation; (3) exciton dissociation at the DA heterojunction with creation of an electron and a hole; (4) charge recombination or charge motion under the influence of the built-in electric field resulting from the difference between the electrode work functions; (5) charge collection at the appropriate electrodes.…”

“…5 To keep the problem computationally tractable, we determine the equilibrium morphologies of the block-copolymer systems by approximating the corresponding polymer-field theories at the MF level. An efficient numerical strategy, relying on this approximation, is the SCFT method.…”

“…Such a phenomenon has been investigated, e.g., by time-resolved electrostatic force microscopy and scanning Kelvin probe imaging by Reid et al 18 Due to the complex relationship of these loss mechanisms within the photovoltaic process and their strong dependence on the structural-dynamical characteristics of the material, their impact on the photovoltaic properties has not been minimized in a satisfactory way for the polymer solar-cell materials currently available. 5,19 Therefore, it is not surprising that their power conversion efficiency is still relatively low, compared to their inorganic counterparts. For a simultaneous optimization of the photovoltaic steps previously mentioned, the generation of a clearly defined morphology with optimized film as well as phase thickness is of crucial importance, since it ensures that the electrons and holes can be separated and transported to the electrodes without losses.…”

“…22,32,33 Methods going beyond the MF level have been introduced recently and may be used for computing the structural-dynamical characteristics of polymer-solvent systems with strong repulsive interactions between monomers, which may arise, e.g., in cases where the polymers possess polar or ionic groups in the semi-dilute or dilute concentration regime. 22,[33][34][35][36] In a recent work, 5 we have extended the range of application of the field-theoretic methods mentioned previously, to investigate the loss processes of charge carriers and excitons in defect structures of nanostructured polymer solar cells. This approach is based on combining either the timedependent Ginzburg-Landau method or the SCFT method, to generate morphologies of different scale of phase separation and degree of defect formation, with a first reaction (FR)-type dynamic Monte Carlo (DMC) method, to simulate the elementary photovoltaic processes involving the charge carriers and excitons within a particle description.…”

“…1,3 This has mainly been attributed to small-scale loss phenomena of the elementary particles occurring in the photovoltaic process, such as photon loss, exciton loss, and charge carrier loss. 4,5 These loss processes typically emerge at characteristic stages of the photovoltaic process, such as photon absorption, exciton generation and diffusion to the donor-acceptor (DA) heterojunction, exciton separation, and charge carrier generation at the DA heterojunction, diffusion of the charge carriers to the respective electrodes and/or collection of the charge carriers at the electrodes. 5,6 Exciton loss, 7 for instance, can take place through radiative recombination of an exciton via a firsta) Electronic mail: stephan.baeurle@chemie.uni-regensburg.…”