Electronic properties of small molecule-diketopyrrolopyrrole derivatives and a correlation with the open circuit voltage in organic solar cells

The electronic properties of five diketopyrrolopyrrole (DPP) derivatives as a function of alkyl chains and conjugated frameworks were investigated. The results of ultraviolet photoelectron spectroscopy (UPS) and UV–vis absorption measurements show different ionization potentials (IPs), and optical band gaps. To investigate the relation between the IP and the open circuit voltage (VOC) of bulk heterojunction (BHJ) solar cells, the VOC of organic solar cells fabricated with DPP derivatives as the donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the acceptor were measured. The values between the IP of the DPP derivatives and VOC in BHJ devices exhibit linear relationship.