Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications

Abstract

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This “two-step” charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.

Microstructure Formation of Functional Polymers by Evaporative Self-Assembly under Flexible Geometric Confinement

“…Poly-methylmethacrylate (PMMA) as a kind of structural polymer, and regioregular-poly(3-hexylthiophene) (RR-P3HT) as a functional polymer with semiconducting property, were commonly used [ 9 , 14 , 15 , 16 , 17 , 18 ]. For instance, PMMA gratings could be used as diffractive components for optical devices or as a template for replicating other functional patterns [ 11 , 19 ].…”

“…For instance, PMMA gratings could be used as diffractive components for optical devices or as a template for replicating other functional patterns [ 11 , 19 ]. The latter one, RR-P3HT, is widely used in organic solar cells and often used as an electron donor in the blend of P3HT: PCBM with PCBM as the acceptor [ 18 , 20 , 21 ]. Han et al have reported highly oriented nanofibrils of such semiconducting materials by blading deposition in a microfluidic dragging manner [ 22 , 23 ].…”

Understanding the low-loss mechanism of general organic–inorganic perovskites from first-principles calculation

“…The other approach entails separating the electron and hole in real space. An example is the extensively investigated polymer-fullerene blend system, in which ultrafast charge transfer from the polymer to the fullerene occurs after light excitation [13], leading to spatially separated electron and hole transport channels and very low carrier recombination rate (3 to 4 orders lower than the expected Langevin recombination rate) [14][15][16].…”

Impedance Spectroscopy for Annealing-Induced Change of Molybdenum Oxide in Organic Photovoltaic Cell

“…In parallel to the synthesis of new polymers, molecular stacking in the active layer, and optimized device structures, evaluation methods of carrier dynamics in OPVs have been developed by many researchers [8] [9] [10] [11]. Several characterization techniques as carrier mobility of organic materials [12] [13], energy level at the interfaces [14] [15], and transient absorption spectroscopy [16] [17] have been investigated to understand carrier dynamics in the OPVs. Impedance spectroscopy is an important tool to discuss carrier dynamics including carrier mobility and density through the equivalent circuit of the device.…”