PC 07


A. Purkrta, A. Porubaa, M. Vaněčeka, L. Gorisb, J. Mancab

aInstitute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 6, CZ-16253 Czech Republic

bInstitute for Materials Research, Hasselt University, Wetenschapspark 1, B-3590 Belgium

Organic bulk heterojunction solar cells, based on nanostructured phase-separated blends, are well established subject of study now, with

conversion efficiencies reaching 5%. Post-production annealing was found to be a key step in attaining high efficiency, when P3HT:PCBM blend is used as the active absorber layer. Nevertheless, detailed information on sub-gap absorption processes in the absorber layer, as well as on changes in the photocurrent spectrum during the annealing process, which could provide insight into the efficiency boost, are still lacking.

We report on a photoelectric study of fully encapsulated bulk heterojunction solar cells with an active absorber layer of P3HT:PCBM. We have used Fourier transform photocurrent spectroscopy (FTPS) [1] to measure the optical absorption processes, connected with the collection of free photogenerated charge carriers in these thin film organic solar cells. FTPS is very sensitive, dynamic range obtained with this method reaches 8 orders of magnitude in organic solar cells [2].

We have studied these processes in the 0.8 - 3 eV spectral range, for three different weight ratios of P3HT:PCBM mixture (55%, 66.66% and 75% wt. content of PCBM in the mixture), with step-wise thermal annealing of the cells in the range of 80-200 °C.

Observed changes depend heavily on the absorber composition. There are profound changes in the spectra for the "standard, optimal" 1:2 (66.66% PCBM) composition, whereas for 55% and 75% the changes are much smaller, if any. Moreover, for the 1:2 ratio, we observed two isosbestic points in measured spectra, at approximately 1.35eV and 1.95eV during annealing at lower temperatures (up to 140°C). These results will be related with the nanoscale morphology changes occurring in the cell's active layer during the annealing procedure for this composition.

[1] M. Vanecek, A. Poruba, APL 80 (2002) 719

[2] L. Goris, A. Poruba, L. Hodakova, M. Vanecek, K. Haenen, M. Nesládek,P. Wagner, D. Vanderzande, L. De Schepper, J. Manca, APL 88 (2006) 052113