Comparison of Electroluminescence Intensity and Photocurrent of Polymer Based Solar Cells

Mass production of organic donor–acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes, will be realized by high throughput coating and printing technologies. This requires novel quality control techniques preceding the state of the art for vacuum based photovoltaic technologies. In this article we demonstrate that electroluminescence imaging of organic solar cells has the potential to visualize the photocurrent distribution signifi cantly faster than standard laser beam induced current mapping (LBIC) techniques. Here, we demonstrate that the emission of the polymer-fullerene charge transfer complex is related to the short circuit current of the solar cell in agreement with the assumptions of the detailed balance theory. In recent years huge improvements in the understanding of optical devices based on conjugated polymers have been made. Continuously higher effi ciencies are reported for the transformation of electricity into light [ 1 ] as well as for the transformation of light into electricity. [ 2 ] Recently, the principles of light generation (as known for organic light emitting diodes (OLEDs)) and light conversion (as known from organic solar cells (OPV)) were put in relation by the detailed balance limit. [ 3–6 ] It was shown that the electroluminescence radiation in bulk heterojunction solar cells is not the sum of the recombination radiation of the single components. Instead, spectral analysis of the electroluminescence (EL) of OPV cells with different semiconductors cleared that a new electronic state, formed between a donor and an acceptor molecule, and referred to as charge transfer state (CTS), is responsible for the EL radiation. The CT state is energetically lowered compared to the lowest unoccupied molecular orbital of the acceptor. Therefore the emission spectrum is redshifted relative to the emission from the pristine semiconductor components. In the case of poly(3-hexylthiophene)/[6,6]phenyl C61 butyric acid methyl ester (P3HT–PCBM), the peak emission of the CT state is located at wavelengths around ∼ 1.2 μ m. [ 3 ] For another polymer with even lower bandgap, poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b ′ ]dithiophene)alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) the peak emission is located at 0.9 μ m. [ 7 ]