University of Texas at El Paso
Faculty Sponsor's Department(s):
Exciton Diffusion Length in P3HT with the Presence of Additive F4TCNQ
Exciton diffusion is an imperative process for organic solar cells. Excitons are excited states of molecules, Coulombically-bound electron-hole pairs, created upon the absorption of light. These pairs must be separated into free charges in order to generate current in solar cells. In order for an exciton to be separated in a bulk heterojunction, excitons must diffuse to a donor-acceptor interface. Thus, exciton diffusion is a critical, but often overlooked, process in organic solar cells. This study aims to explore (i) the effects of morphology on exciton diffusion and (ii) how the presence of a molecular dopant affects exciton diffusion. Doping of organic semiconductors has been shown to moderately improve solar cell performance, but characterization has not been done on how the presence of dopants affects exciton diffusion. We measure the exciton diffusion in Poly-3-hexylthiophene (P3HT) using a method developed by Dr. Oleksander Mikhneko. This method measures fluorescence bulk heterojunction films with Time-Correlated Single Photon Counting and a Monte Carlo Simulation to model exciton diffusion from experimental parameters. Thermal Annealing was done on the films to influence the degree of crystallinity in P3HT and these morphological changes were observed through UV Vis Absorption Spectroscopy. 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ) was the molecular dopant used for this analysis. The studies done show that exciton diffusion length of P3HT increases slightly by annealing, which is thought to be caused by increases in crystallinity and reduction in amorphous domains. Even at low concentrations of F4TCNQ, the lifetime of excitons is reduced significantly, which correlated to reduction in exciton diffusion.