Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

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.

Nongeminate and Geminate Recombination in PTB7:PCBM Solar Cells

“…The externally applied voltage is supplied to the solar cell by a Keithley 2602 in combination with a fast digital/analog switch. The derived experimental data were treated as reported elsewhere, [ 18,34 ] while all charges were corrected for recombination losses during extraction as mentioned above.…”

“…According to the literature, [ 17 ] an activation energy E u characterizing the exponential trap distribution can be estimated from the parameter λ . [ 18 ] For low charge carrier densities, we fi nd E u ≈ 28 kT/n J = 51 meV. This indicates trap limited recombination for the device processed without additive and in the low charge carrier regime of the device prepared from CB/DIO.…”

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

“…For thicker absorber layers, we model the charge extraction to be spatially uniform, as no significant concentration of recombination centers is present in the bulk of the neat absorber. [20][21][22] …”

Ionic high-performance light harvesting and carrier transporting OPV materials

“…[46][47][48] Although this has afforded a lot of useful insights, it is limited to materials that can be sublimed, excluding polymers and ionic materials. [48][49][50][51][52] The preparation of neat bi-layers from solution is not trivial as both donor and acceptor materials are usually soluble in the same solvents. 53 A few previous reports have mentioned bi-layer heterostructures prepared from solution using ionic donor molecules in combination with evaporated acceptor molecules.…”