Modeling the Effects of Molecular Length Scale Electrode Heterogeneity in Organic Solar Cells

Abstract

Kinetic Monte Carlo simulations of planar heterojunction (PHJ) organic solar cells (OPVs) constructed with electronically heterogeneous electrodes are presented which correlate the extent and length scale of electrode heterogeneity with their capacity for collecting photogenerated charge carriers. The PHJ OPV is modeled as an ensemble of discrete 1 nm3 molecular sites and 1 nm2 electrode sites for which we individually assign various effective activation energies for charge hopping. Utilizing Marcus theory to describe charge transfer reactions within the energetically disordered lattice, described by a Gaussian distribution of discrete site energies, we demonstrate the sensitivity of solar cell device performance to nanometer length scale heterogeneity at the electrode-active layer contact. Such sensitivity is reflected in the steady-state charge density profiles in the vicinity of the electrode-active material interface, charge collection efficiencies, and rates of recombination at the donor–acceptor (D/A) interface. Additionally, we demonstrate how implementation of idealized interlayers placed between the electrode and active layer functions to mitigate the negative effects of electrode heterogeneity.

Nanoscale Visualization of Electrochemical Activity at Indium Tin Oxide Electrodes

“…There is increasing interest as to how heterogeneity in the electrical and electrochemical properties of ITO impacts its performance for the aforementioned applications. 14 − 16 While the morphology, 8 , 15 , 17 conductivity, 15 , 17 , 18 spectroscopic behavior, 17 , 19 and composition 17 , 20 of (modified) ITO surfaces have been characterized down to the nanometer scale, electrochemical measurements have been predominantly performed on the macroscale. 4 , 21 , 22 This “bulk” macroscale electrochemical characterization (usually voltammetry) gives the average activity of the entire electrode surface, although there have been attempts to interpret macroscopic measurements in terms of nanoscale heterogeneous activity, by adopting a partially blocked-electrode model of the surface.…”

High efficiency arrays of polymer solar cells fabricated by spray‐coating in air

“…It is well known that organic solvents and process additive can change both the degree of phaseseparation between PEDOT and PSS within a PEDOT: PSS film, and increase the molecular orientation of the PEDOT component, leading to an increase in electrical conductivity of several orders of magnitude [26,27]. For a single pixel, high conductivity is beneficial for the extraction of charge carriers [28,29]. However, in the pixelated device arrays explored here, the high lateral conductivity of PEDOT:PSS layer is likely to result in significant current-spreading, with lateral charge transport resulting in charges traveling outside the region defined by the aperture mask through which each individual device is illuminated.…”

Simulation of Charge Carriers in Organic Electronic Devices: Methods with their Fundamentals and Applications

“…The impact of such spatially varying hopping rates across the contact was investigated by Zacher and Armstrong. [ 126 ] After assigning various effective activation energies for charge hopping on quadratically arranged contact sites in a solar cell, a fraction of sites was found not to participate in charge extraction. As illustrated by selected trajectories in Figure , the charges are not necessarily extracted when they hit the contact the first time.…”

“…[ 82 ] For example, electron and holes are randomly placed at sites selected from the entire volume [ 120 ] or related to the donor:acceptor interface. [ 126 ]…”

“…For the calculation of transient properties, the simulation starting out from the same initial configuration has to be run multiple times. [82,126,129] Each trajectory is simulated until a certain number of steps passed or until the charge (or exciton) leaves the simulation, be that via escape through the contact or via recombination (dissociation for excitons). The trajectories are averaged to obtain a mean value for an observable value at a specific time.…”