Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Optical spectroscopy has been proven to be a powerful tool to probe the charge photogeneration process in hybrid and organic photovoltaic cells (PVs). In most of the spectroscopic techniques that are conventionally used to study organic PV material systems electrons and holes are generated simultaneously upon light excitation. In this work we demonstrated the use of charge modulation spectroscopy (CMS), which measures the optical absorption spectra of field-induced (rather than photo-induced) charge carriers, to study charge localization in donor-acceptor heterojunctions for organic PVs. A heterojunction field-effect transistor (FET) structure is used as the testing configuration (Fig.1), allowing us to easily probe the material system in the electron-only or hole-only regime by changing the polarity of the gate bias. This approach enables acquisition of spectra that separately represent the electronic states of electron or hole, without the interference from its counter charge. Using CMS, we studied two promising copolymer systems, poly(N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b]thiophene) (DPP-DTT), which exhibits record high hole mobility in FETs, and a thieno[3,4-b]thiophene-alt-benzodithiophene copolymercopolymer (PTB7), which yields high power conversion efficiency in a PV structure. By comparing the CMS spectra of pure copolymers and their blends with (6,6)-phenyl-C61-butyric acid methylester (PCBM), this work reveals how molecular packing and interactions at the donor-acceptor interfaces affect charge localization. This property is crucial to the PV performance because it influences both the charge separation and charge transport processes, which in turn determine the efficiency of photocurrent generation in the devices.
(a) Device configurations donor-acceptor heterojunction FETs. (b) Energy band diagram illustrating hole (left) and electron (right) injection from Au electrode to donor and acceptor materials, respectively.
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