Determining the effect of selenium substitution on low band-gap conjugated polymer-fullerene solar cells by optoelectronic characterization

The design of new, low band-gap polymers is a major part of organic photovoltaic research. Atomic substitution in a ring structure is one way to alter the electronic properties of a polymer (such as substituting the sulfur in a thiophene ring for selenium to make a selenaphene ring). We show that, starting with a benzodithiophene donor, that substituting benzoselenadiazole for benzothiodiazole leads to a polymer with a lower band-gap as characterized by the onset of optical absorption. Photovoltaic devices, optimized for 1,2-dichlorobenzene as the solvent, are comparable except that the polymer containing benzoselenidiazole collects over 1 mA/cm² less short circuit current than the polymer containing benzothiodiazole, a counter intuitive result. Similar fill factors suggest that film morphology is not a contributor to this discrepancy. This result is further strengthened by optical time of flight measurements that show a better hole mobility for the benzoselenidiazole polymer (at room temperature and over a range of external fields). We then conducted further test in order to find the cause of this discrepancy.