Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells

TLDR

Exciton dissociation at the donor–acceptor interface is the key step in photocurrent generation after light absorption and exciton diffusion in the bulk of the absorbing material. The study investigates how the thickness of the organic layers (PPV, CuPc, and C60) affects short‑circuit photocurrent spectra and device efficiency in heterojunction photovoltaic cells. By systematically varying the donor and acceptor layer thicknesses, the authors examined exciton diffusion, photocurrent spectra, and optical interference from back‑reflected light at the Al electrode. The optimum donor and acceptor thicknesses depend on exciton diffusion lengths (12 ± 3 nm for PPV, 68 ± 20 nm for CuPc), with significant photocurrent from C60 and optical interference effects, yielding a maximum power‑conversion efficiency of about 0.5 % under white light.

Abstract

The influence of the organic layer thickness on short-circuit photocurrent spectra and efficiency is investigated in heterojunction photovoltaic cells with the electron donor materials poly(p-phenylenevinylene) (PPV) and Cu-phthalocyanine (CuPc), respectively, together with C60 as electron acceptor material. The main process of photocurrent generation after light absorption, exciton generation, and exciton diffusion in the bulk of the absorbing material is given by the exciton dissociation at the donor–acceptor interface. We determined a strong dependence of the optimum layer thickness of the absorbing material on the exciton diffusion length by systematically varying the layer thickness of the electron donor material. Additionally, a significant photocurrent contribution occurred due to light absorption and exciton generation in the C60 layer with a subsequent hole transfer to PPV, respectively, CuPc at the dissociation interface. Using a simple rate equation for the exciton density we estimated the exciton diffusion lengths from the measured photocurrent spectra yielding (12±3) nm in PPV and (68±20) nm in CuPc. By systematically varying the layer thickness of the C60 layer we were able to investigate an optical interference effect due to a superposition of the incident with backreflected light from the Al electrode. Therefore both the layer thickness of the donor and of the acceptor layer significantly influence not only the photocurrent spectra but also the efficiencies of these heterolayer devices. With optimized donor and acceptor layer thicknesses power conversion efficiencies of about 0.5% under white light illumination were obtained.