Modeling photocurrent action spectra of photovoltaic devices based on organic thin films

TLDR

The model assumes exciton generation and diffusion with dissociation at the PEOPT/C60 interface, incorporates the internal optical electric field derived from complex refractive indices and layer thicknesses measured by spectroscopic ellipsometry, and accounts for absorption in both polymer and fullerene layers to reproduce the experimental photocurrent action spectra and assess geometric effects on device efficiency. The model reproduces the experimental short‑circuit photocurrent spectra, yielding exciton diffusion lengths of 4.7 nm for PEOPT and 7.7 nm for C60, and demonstrates that optimizing the photocurrent is achievable through device geometry adjustments.

Abstract

We have modeled experimental short-circuit photocurrent action spectra of poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and C60, respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in order to study the influence of the geometrical structure with respect to the efficiency of the thin film devices. In this way the photocurrent was optimized.