Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

TL;DR

This study compares nongeminate recombination in planar and bulk heterojunction organic solar cells based on CuPc and C60, revealing how device architecture influences charge dynamics and performance.

Contribution

It provides a direct comparison of recombination and charge carrier densities in PHJ and BHJ devices using combined experimental and simulation methods.

Findings

BHJ has higher open circuit voltage at low light intensities

PHJ benefits from longer charge carrier lifetimes at higher light intensities

Recombination dynamics differ significantly between PHJ and BHJ architectures

Abstract

We investigate nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C$_{60}$. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. We apply a combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density. In organic solar cells, charge photogeneration and recombination primarily occur at the donor--acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two-dimensional heterointerface. To study recombination dynamics in PHJ devices most relevant is the charge accumulation at this interface. As from extraction techniques only the spatially averaged carrier concentration can be determined, we derive the charge carrier density at the interface $n_{int}$ from the open circuit voltage. Comparing the experimental results with macroscopic device simulation we discuss the differences of recombination and charge carrier densities in CuPc:C$_{60}$ PHJ and BHJ devices with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.