Moritz Riede

Abstract:

We observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current-voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power-generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage. The spatial absorption profile in the bulk heterojunction of an organic solar cell is varied by employing different thicknesses of an optical spacer layer. The fill factor correlates with the profile and the imbalance in charge carrier mobilities. These results indicate that extraction of holes in competition with bimolecular recombination limits the device performance. Therefore, the spectral shape of the external quantum efficiency and, in turn, the spectral mismatch change with applied voltage. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C as model system. Analyzing the open-circuit voltage (V oc) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing V oc at higher light intensities. © 2013 AIP Publishing LLC.

Abstract:

Trap states are known to be one of the key parameters limiting charge transport in organic semiconductors and hence the performance of organic solar cells. Here, small-molecule organic solar cells based on a bulk heterojunction between zinc-phtalocyanine (ZnPc) and the fullerene C60 are characterized according to their trapping nature by noninvasive methods and under ambient conditions. We show how impedance spectroscopy, applied to systematically varied device structures, reveals the trap localization as well as its occupation mechanisms. Further insight is given from investigations of different device working points and illumination intensities. Thus, we find the traps to be bulk states in the active layer with an electron-trapping nature. They can be described by a Gaussian energy distribution of 55 meV width, centered at 0.46 eV below the electron transport level and with a concentration of 3.5 × 1016 cm-3. Moreover, the trap states act as recombination centers in the presence of injected or photogenerated charge carriers. The results are confirmed by electrical simulations. © 2013 American Physical Society.