Monte Carlo Simulation of Charge Transport in Organic Solar Cells

Organic solar cells have advantages over the traditional inorganic solar cells. The flexibility and light-weight of an organic solar device lead to novel deployment unattainable by inorganic solar devices. There is a wide range of materials that can be used to fabricate an organic solar cell with many suitable conducting polymers and small molecules. The large number of appropriate materials means that it is possible to tune an organic cell to target specific wavelengths of light by targeting specific band gaps. Despite all these benefits, the commercialization of organic solar devices has been limited by their low efficiency. One path to raising the efficiency is to better understand how the morphology or internal structure of the solar cell influences charge transport. The work presented here is a Dynamical Monte Carlo (DMC) simulation that can model the processes that govern the conversion of light to electrical power. The Monte Carlo simulation describes how the interacting particles in the solar cell move and behave. This simulation can provide insight to how the structure of the cell can influence both charge creation and charge collection. The DMC simulations were performed for two archetypal organic solar cell material systems: CuPc-C60 and P3HT-PCBM. Various morphologies were used for these material systems to examine the effect of morphology on both internal and external quantum efficiency. Introduction Since the introduction of the bilayer donor-acceptor heterojunction organic solar cell by C.W. Tang in 1986 [1], organic solar cell efficiencies have been rising. However, the efficiencies of organic photovoltaic (OPV) cells still lag behind their inorganic counterparts, with the most efficient OPV cell rated at 10% efficiency [2]. The challenges to successful organic solar cell design are numerous. Chief among these is that organic solar cell physics differs greatly from that of inorganic solar cells. Thus, the theoretical framework developed to describe inorganic solar cells cannot be directly applied to organics. A new approach for organic solar cells is needed. The Organic Solar Cell The physics and analysis of organic solar cells differ greatly from their inorganic counterparts. The energy states are distributed differently in an organic vs. inorganic solar cell. For an inorganic solar cell, the periodicity of the inorganic semiconductor crystal lattice leads to the formation of conduction bands and valence bands. The charges move along these bands and carrier movement can be solved analytically with Bloch wave functions [3]. Organic solar cells have two important energy states: highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Rather than form energy bands, the disorder of the organic cell leads to highly localized states. Charge movement is described by hopping from localized state to localized state [3]. Figure 1(a) and (b) show the movement of charges in the energy structure of an inorganic semiconductor and an organic semiconductor, respectively. The hopping behavior of charge movement in the organic semiconductor lends itself well to be described by the Dynamic Monte Carlo method. The basic organic solar cell is a stacked structure. Starting from the bottom, there is the glass substrate, the indium tin oxide (ITO) layer, the PEDOT-PSS layer, the active layer, and lastly the aluminum top contact. An example of this structure can be seen in Figure 2. The ITO is a transparent layer which serves as the hole contact. The PEDOT-PSS layer is a hole transporting layer, which matches well energetically with the ITO contact, serves to smooth the rough ITO layer for better active layer deposition, and blocks electrons to prevent recombination at the electrode. The active region layer is where absorbed photons are converted into holes and electrons and consists of two types of materials, an electron conducting material and a hole conducting material. Hereafter, the electron and hole conducting materials will be referred to as the acceptor and donor materials, respectively. For this work, three different active region morphologies are considered: the bilayer which is a layer of acceptor material on top of a layer of donor material, the chessboard which is a morphology with alternating columns of acceptor and donor materials, and the bulk heterojunction (BHJ) which is a mixture of acceptor and donor material. Lastly, the aluminum layer is the electron contact. Figure 1: Charges moving through the energy structure of an inorganic semiconductor and an organic semiconductor [23] (a): Inorganic Semiconductor (b): Organic Semiconductor Photovoltaic Process The photovoltaic process starts with the absorption of a photon in the active region layer which generates an exciton. The exciton is a quasiparticle representing a tightly bound electron and hole pair. This exciton diffuses through the cell until it reaches a donor-acceptor material interface where it dissociates into the electron and hole. The exciton can also recombine before it reaches an interface. An exciton that recombines does not contribute to the photovoltaic effect. The generated electron and hole will only travel in the acceptor and donor material respectively. These charge carriers move through their respective material type until they reach a contact, from which they can provide power to an external circuit. A diagram of this process can be seen in Figure 3. Figure 2: The structure of the organic solar cell. Figure from [24]. Figure 3: The photovoltaic process in an organic solar cell. Figure from [25] Two figures of merit for the organic solar cell is the external quantum efficiency (EQE), denoted as , and the internal quantum efficiency (IQE) denoted as . The EQE is the number of carriers collected at the contacts of the solar cell divided by the number of incident photons. To determine the EQE, the efficiencies of four processes need to be considered: the absorption efficiency describing the number of excitons generated for a given number of incident photons, the exciton diffusion efficiency describing the number of excitons that reach a donor-acceptor interface without recombination, the charge transfer efficiency describing the number of dissociated electrons and holes generated for a given number of excitons that reach a donor-acceptor interface, and the charge collection efficiency describing the number of charges collected at the contact for a given number of dissociated electrons and holes. The calculation of EQE is as follows: