Spectroscopic, imaging, and doping studies of push-pull conjugated polymer solar cells

Organic Photovoltaics (OPV) have the potential to provide inexpensive, renewable electric power to satisfy increasing global energy demands due to their low materials and processing costs. Poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H-cyclopenta [2,1- b;3,4-b&feet;]dithiophene)- alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) is a promising candidate for use in solar cell active layers, and has produced OPV devices with power conversion efficiencies (PCEs) exceeding 5% when blended with fullerene electron acceptors. Although these results are encouraging, a molecular-level understanding of photophysical processes involved in photocurrent production is needed to propel the use of PCPDTBT and other conjugated molecules into widespread use in solar cells. The research presented herein is aimed at improving our understanding of the electronic processes that allow PCPDTBT to outperform its homopolymer counterparts in OPV devices. Electronic absorption and Raman spectroscopies are used to probe the morphology-dependent optical properties of PCPDTBT in pristine nanoparticles and thin films and in thin film blends with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and 1,8-octanedithiol (ODT). Absorption spectra suggest decreased polymer order in nanoparticles and increased polymer ordering in thin film blends with PCBM and ODT. Raman spectra collected at wavelengths spanning the entire absorption spectrum of PCPDTBT show selective excitation of the benzothiadiazole (BT) vibrational modes at low excitation energies and cyclopentadithiophene (CPDT) vibrational modes at high excitation energies. In addition, density functional theory (DFT) calculations provide a means of assigning specific molecular vibrational modes, and provide more insight into the interactions between PCPDTBT and PCBM in functional solar cell devices. Raman intensity and frequency imaging of PCPDTBT:PCBM and PCPDTBT:PCBM:ODT blend thin films show correlations between areas of decreased BT vibrational activity and increased PCBM vibrational mode activity, suggesting preferential interaction between the acceptor moiety and PCBM. Photocurrent imaging of PCPDTBT:PCBM:ODT solar cell devices shows areas of higher photocurrent production in well-blended regions of the active layers, suggesting some degree of disorder and more thorough intercalation of PCBM into PCPDTBT domains are advantageous for increased photocurrent production.;Doping PCPDTBT with the strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) allows devices based on these blends to show improved power conversion efficiencies (PCEs), as well as improvements in other solar cell operating parameters and figures of merit. Blends of PCPDTBT with the electron acceptors 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and F4-TCNQ are investigated with absorption and Raman spectroscopies in an effort to better understand the reasons behind these performance enhancements and the preferential interactions between these dopant molecules and the PCPDTBT backbone. Absorption spectra of doped PCPDTBT thin films show red-shifts in the low-energy PCPDTBT absorption peak and decreased intensity of the high-energy absorption peak, suggesting increased polymer ordering upon addition of electron-accepting dopant species. Raman spectra of doped PCPDTBT thin films reveal increasing BT vibrational activity with increasing doping levels of both DDQ and F 4-TCNQ, suggesting preferential interaction between the dopants and the polymer acceptor moiety. DFT calculations provide additional insight into the interactions between PCPDTBT and these dopant molecules, showing varying degrees of charge transfer between the polymer and dopants as a function of the position of the dopant molecule relative to the CPDT and BT moieties.