Charge photogeneration in organic donor/acceptor blends

Abstract

Polymer-small molecule blend films are of increasing interest in the field of organic solar cells. This thesis employs transient absorption spectroscopy as a mean to study charge photogeneration in these blend films. These studies allow identifying and addressing the charge photogeneration efficiency-limiting processes in polymer:perylene diimide organic solar cells. We approach the question by considering the influence of nanomorphology and phase segregation on charge photogeneration and recombination dynamics. We further report on yield of charge separated species in polythiophene / perylene diimide blend films as a function of electron acceptor’s energy levels. We find that, compared to polythiophene / PCBM blend films, charge photogeneration is significantly enhanced. Correlations between free energy for charge dissociation and charge photogeneration yield within different polymer:acceptor series are observed and indicate the generality of this relationship. Furthermore, the energetic model proposed to account for these results is consistent with the well-established Onsager and Marcus theories. It can therefore be concluded that the yield of photogenerated charges in polymer/acceptor systems is likely to be dependent upon the excess vibrational energy of the bound radical pair, such that the key kinetic competition in the photogeneration process is between vibrational relaxation and dissociation of this species Simultaneously, we investigate photoinduced charge separation in solid films of two perylene diimides and a donor-bridge-acceptor (D-B-A)molecule, exTTF-pcp-C60 relative to solution. First we find Intramolecular charge separation and recombination is correlated with a reduction in the yield of long-lived, intermolecular charge-separated species in the perylene diimide dyad. In the D-B-A system we observe the exTTF-pcp-C60 motif in this case leads to more charges than the reference compounds or a mixture of them but that the excited state of the electron acceptor, the fullerene, suffers from concentration self-quenching which severely affect the charge yield in solid films.