Photophysics and solar cell application of a benzodithiophene conjugated polymer containing cyclometalated platinum units

Abstract

Cyclometalated platinum(II) chromophores were combined with benzodithiophene (BDT) units to form a conjugated molecule and polymer in order to ascertain their photophysical and electrochemical properties, as well as their potential for use in bulk heterojunction organic solar cells. Weak fluorescence and room temperature phosphorescence were observed for both the small molecule model complex and polymer, leading to the assumption that the presence of platinum metal centers serves to enhance intersystem crossing (ISC). Nanosecond and picosecond transient absorption spectroscopy were employed to study the excited state dynamics which revealed ISC rates of the polymers (3 × 1012 s−1) is faster than that of the model (2.5 × 1011 s−1). These fast ISC rates ensure a high population of triplet excited states, which was confirmed by singlet oxygen sensitization measurements. A laser power dependence of the triplet yield was observed to be more pronounced for the polymer, likely due to triplet-triplet annihilation within single polymer chains. Unfortunately, energy level estimations from electrochemistry and emission spectra placed the triplet state at or below the energy of the charge transfer state to PCB71M acceptor. Organic solar cells were constructed using the model or polymer as donor in conjunction with PC71BM as acceptor. The polymer donor provided the higher photocurrent efficiency at just over 1% in a non-optimized device.