Origin of nanoscale variations in photoresponse of an organic solar cell.

Photogenerated charge transport in bulk heterojunction (BHJ) solar cells is strongly dependent on the active layer nanomorphology resulting from phase segregation. Here, we systematically study the nanoscale photocurrent response from BHJs based on poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT-PCBM) with a photoconductive atomic force microscope (PCAFM). The photocurrent is either collected directly by the tip or through nanopatterned metal contacts. The photoresponse measured at the top surface shows significant inhomogeneity on the length scale of 100-500 nm with large low-efficiency regions, consistent with existence of a P3HT-rich skin layer of approximately 10 nm thick. The measurements with the nanocontacts validate the PCAFM results and demonstrate that the inhomogeneity averages to the conventional device result. Additionally, we use an ultralow angle microtomy (ULAM) technique to slice the active layer and create wedges along these cuts for probing of nanomorphology in the bulk. AFM images show a striking contrast between the top surface and the ULAM exposed material, revealing much finer features related to phase segregation below the skin layer and sub-100 nm length scales for charge transport.