Jens Czolk

Morphology Change and Improved Efficiency in Organic Photovoltaics via Hexa-peri-hexabenzocoronene Templates

The morphology of the active layer in organic photovoltaics (OPVs) is of crucial importance as it greatly influences charge generation and transport. A templating interlayer between the electrode and the active layer can change active layer morphology and influence the domain orientation. A series of amphiphilic interface modifiers (IMs) combining a hydrophilic polyethylene-glycol (PEG) oligomer and a hydrophobic hexabenzocoronene (HBC) were designed to be soluble in PEDOT:PSS solutions, and surface accumulate on drying. These IMs are able to self-assemble in solution. When IMs are deposited on top of a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) film, they induce a morphology change of the active layer consisting of discotic fluorenyl-substituted HBC (FHBC) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). However, when only small amounts (0.2 wt %) of IMs are blended into PEDOT:PSS, a profound change of the active layer morphology is also observed. Morphology changes were monitored by grazing incidence wide-angle X-ray scattering (GIWAXS), transmission electron microscopy (TEM), TEM tomography, and low-energy high-angle angular dark-field scanning transmission electron microscopy (HAADF STEM). The interface modification resulted in a 20% enhancement of power conversion efficiency.

p-Doping of polystyrene polymers with attached functional side-groups from solution

Electrically doped buffer layers are often employed in organic optoelectronic devices to improve charge carrier injection or extraction. We study p-doping of non-conjugated polymers with attached hole transport moieties by intermixing strong molecular acceptors. Polymers and dopants are processed from solution and their electrical properties are benchmarked against non-polymerized, p-doped molecular materials. The equilibrium charge carrier density is estimated by electron paramagnetic resonance (EPR) measurements that show an increased number of unpaired spins upon doping. We find an increase in the conductivity of the electrically doped polymer and the corresponding molecular materials by several orders of magnitude by studying hole-only devices and employing charge extraction by linearly increasing voltage (CELIV). Altogether, the increased number of unpaired spins and the enhanced conductivity indicate the presence of free holes. The p-doping of the polymers with attached hole transport moieties is equally efficient as the p-doping of the corresponding molecular materials.

Photochemical Upconversion Applied to Organic and Thin Film Silicon Solar Cells

We will review triplet-triplet annihilation upconversion (TTA-UC) demonstrating a solar cell spectral response peak enhancement of 13% and a photocurrent increase of 0.28mA/cm2 under moderate concentration. We will discuss the potential of TTA-UC for device application.