Simultaneous photoluminescence (PL) and external quantum efficiency (EQE) confocal mapping is used to investigate the correlation between the local PL and the EQE in a regioregular poly(3-exylthiophene):poly(9,9-dioctylfluorene-co-benzothiadiazole) inverted bulk heterojunction solar cell. We show that the charge generation and charge collection are strongly non-uniform on a length scale up to 100 μm. Our results evidence that organic solar cells optimization requires not only the control of the submicrometric active materials arrangement but also the control of the large scale device uniformity.
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The presence of bright coincident F8BT and rrP3HT regions showing low PC could be simply explained by a locally higher film thickness resulting in the observed PL enhancement and PC collection decrease. However it is easy to estimate that the observed PL variation of about 20% would be given by a film thickness increase of about 40 nm, which would result in a PC decrease of the order of 40%,14 being much lower than the observed one. We can also exclude that the observed EQE non uniformity can be related to the roughness of both the blend and the ZnO, as a uniform roughness of about 10 nm has been measured by Atomic Force Microscopy for both layers, which does not affect the overall device performances, as shown by Maet al.26
