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

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 poly͑p-phenylenevinylene͒ ͑PPV͒ and Cu-phthalocyanine ͑CuPc͒, respectively, together with C 60 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 C 60 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 ͑12Ϯ3͒ nm in PPV and ͑68Ϯ20͒ nm in CuPc. By systematically varying the layer thickness of the C 60 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.

Efficient organic solar cells by penetration of conjugated polymers into perylene pigments

“…28 ) and 12± 3 nm. 29 If the effect of the exciton quenching by other layers can be eliminated, the diffusion length of PPV in the previously reported cells may increase, like in our case.…”

Ambipolar organic semiconductor blends for photovoltaic cells

“…Due to this mixture of donor and acceptor materials the photon-to-current conversion efficiency and the power conversion efficiency were found to increase. [4][5][6] One molecular material system of interest for photovoltaic cells is the combination of copper-phthalocyanine (CuPc) as p-conducting donor and fullerene (C 60 ) as n-conducting acceptor.…”

Phthalocyanine Nano Colloids in Water Prepared by Reprecipitation Method

“…In general, organic devices consist of an organic layer with thickness less than 100 nm 17,18,19 due to the short exciton diffusion lengths of common organic materials. 20,21 Therefore, the optimal size of phthalocyanine colloid materials for organic devices should be 100 nm or less. The average diameter calculated from the DLS results are summarized in Table II.…”