Microphase-separated donor-acceptor diblock copolymers: Influence of HOMO energy levels and morphology on polymer solar cells

The synthesis of novel semiconducting donor-acceptor (D-A) diblock copolymers by means of nitroxide-mediated polymerization (NMP) is reported. The copolymers contain functional moieties for hole transport, electron transport, and light absorption. The first block, representing the donor, is made up of either substituted triphenylamines (poly(bis(4-methoxyphenyl)-4'-vinylphenylamine), PvDMTPA) or substituted tetraphenylbenzidines (poly(N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-(1,1'-biphenyl)-4,4'-diamine), PvDMTPD). The second block consists of perylene diimide side groups attached to a polyacrylate backbone (PPerAcr) via a flexible spacer. This block is responsible for absorption in the visible range and for electron-transport properties. The electrochemical properties of these fully functionalized diblock copolymers, PvDMTPA-b-PPerAcr and PvDMTPD-b-PPerAcr, are investigated by cyclic voltammetry (CV), and their morphology is investigated by transmission electron microscopy (TEM). All diblock copolymers exhibit microphase-separated domains in the form of either wire- or wormlike structures made of perylene diimide embedded in a hole-conductor matrix. In single-active-layer organic solar cells, PvDMTPD-b-PPerAer reveals a fourfold improvement in power conversion efficiency (eta = 0.26%, short-circuit current (I-SC) 1.21 mA cm(-2)), and PvDMTPA-b-PPerAcr a fivefold increased efficiency (eta = 0.32%, I-SC = 1.14 mA cm(-2)) compared with its unsubstituted analogue PvTPA-b-PPerAcr (eta = 0.065%, I-SC = 0.23 mA cm(-2)).