Bernhard Ecker

Polymer/cathode interface barrier limiting the open circuit voltage in polymer:fullerene organic bulk heterojunction solar cells: A quantitative analysis

By exploring the origin of the saturation current (J0), we show that the polymer/cathode interface determines the open circuit voltage (Voc) in polymer:fullerene bulk heterojunction solar cells. We fabricate and characterize P3HT:PCBM solar cells and compare their electrical characteristics using as reference P3HT/cathode Schottky diodes. We demonstrate with thermionic emission theory that J0 has the same origin in both devices. Thus, a general barrier-dependent expression for Voc, valid for non-ohmic cathode contacts is proposed and discussed.

Increasing organic solar cell efficiency with polymer interlayers

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer:fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface.

Consequence of thermal annealing on PCDTBT-based solar cells performance and composition profile (presentation video)

As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.