The Role of Charge Transfer States in Organic Photovoltaic Cells

Having the potential for low-cost and high-efficiency, organic photovoltaic (OPV) cellshave attracted lots of attention during the last decade. Although OPV has alreadyachieved power conversion efficiency (PCE) comparable to that of the amorphous-silicon(a-Si) based PV, challenges still remain on transferring the laboratory to manufacturingprocesses and further improving the device efficiency. For the later, understanding theloss mechanism in OPV is of the utmost importance for developing more efficientmaterials and device architectures.In OPVs, the electronic properties at the donor-acceptor interface play a significant rolein determining the device efficiency. It is recently proposed that charge-transfer (CT)states at the organic heterojunctions are necessary and intermediate steps for chargedissociation. Depending on the energy profile of the CT states, the excitons will either berecombined or dissociated into free charges. However, correlation between the energyprofile of CT states and the charge dissociation rate in PV devices has never beenreported. The challenges mainly due to the lack of standard experimental approach toprobe the energy profile of such low-density CT states, and the technique to pinpointthe states those are responsible for efficient charge dissociation.In this proposed research, we are aiming at determining the role of charge-transferstates on charge dissociation in organic photovoltaic materials. We will develop a newapproach by combing the charge-modulated electroabsorption spectroscopy (CMEAS)and photothermal deflection spectroscopy (PDS) to probes the CT states in OPV devices.We will then correlate the spectroscopy results with the charge dissociation results on aseries of OPV materials. Our objectives are to: 1) Develop a methodology to investigatethe CT states in organic heterojunction devices; 2) Establish a correlation between theenergy profile of CT states and the charge dissociation efficiency in devices; 3) Establisha correlation between material properties and CT states in different organicheterojunctions; 4) Explore the theoretical model of charge dissociation in OPVs.The success of this project would not only bring insight into developing more efficientphotovoltaic devices, but more fundamentally the charge transfer process in organicelectronic materials.