Relating the open-circuit voltage to interface molecular properties of donor:acceptor bulk heterojunction solar cells

TLDR

The open‑circuit voltage of polymer:fullerene bulk heterojunction solar cells is governed by interfacial charge‑transfer states between the polymer and fullerene. Using Fourier‑transform photocurrent spectroscopy and electroluminescence, the authors extracted interfacial molecular parameters and derived an analytical expression linking these to Voc, validated for devices with three common conjugated polymers blended with PCBM. They found that Voc scales with the CT state energy, that the energetic loss qΔV vanishes at 0 K, depends linearly on temperature and logarithmically on illumination, can be reduced by weakening polymer–fullerene coupling or nonradiative recombination, and amounts to ~0.6 eV at room temperature—split into ~0.25 eV radiative and ~0.35 eV nonradiative components.

Abstract

The open-circuit voltage $({V}_{oc})$ of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer (CT) states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to ${V}_{oc}$ is deduced and shown to be valid for photovoltaic devices comprising three commonly used conjugated polymers blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). ${V}_{oc}$ is proportional to the energy of the CT states ${E}_{CT}$. The energetic loss $q\ensuremath{\Delta}V$ between ${E}_{CT}$ and $q{V}_{oc}$ vanishes when approaching 0 K. It depends linearly on $T$ and logarithmically on illumination intensity. Furthermore $q\ensuremath{\Delta}V$ can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss $q\ensuremath{\Delta}V$ of $\ensuremath{\sim}0.6\text{ }\text{eV}$ at room temperature and under solar illumination conditions, of which $\ensuremath{\sim}0.25\text{ }\text{eV}$ is due to radiative recombination via the CT state and $\ensuremath{\sim}0.35\text{ }\text{eV}$ is due to nonradiative recombination.