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

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.

Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells

“…Furthermore, embedding ZnPc into media m of a greater optical gap, p-BF-DPB with 3.0eV and C60, allows neglecting FRET dynamics and thus circumvents additional PL-studies to determine the Förster radius [15] : kFRET ZnPc,m ≈ 0. Moreover, within the framework of interference based thin-film-optics [13] , we find the spatial distribution of the optical field, and thus the exciton generation, to be approximately parabolic…”

“…As shown in Figure 1 (top), we center the maximum of the optical field in the ZnPc layer for 680nm, a wavelength close to which ZnPc has its peak absorption (details in SI.3b). For centering, we adjust the thickness of HTL and ETL for all ZnPc thicknesses xo by means of numerical transfer-matrixsimulations [13] .…”

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…75 Scharber as well as Armin et al commented on the IQE measurement by Grancini et al, and claimed that the IQE is independent of the wavelength across the absorption spectrum of the blend. 76,77 Grancini et al acknowledged the difficulty in obtaining accurate IQE due to interference and parasitic absorptions.…”

New generation solar cells: concepts, trends and perspectives

“…19 Fullerenes promote ultrafast charge separation and long exciton diffusion length, and are also characterized by their small reorganization energy and high electron affinity, which renders them ideal electron acceptors. 20 Numerous derivatives have been prepared for OPV applications, 21 but the best performing, and certainly the most popular, is highly soluble phenyl-C61-butyric acid methyl ester (PCBM), 22 a benchmark n-type structure for OPVs (Fig. 2).…”