Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend

Abstract

We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]-thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C-61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using time-resolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet-singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a more complete description of the photophysical processes taking place in this photoactive blen

Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

“…Previous studies of triplet excitons in donor/fullerene blends allow us to assign the narrow 25 mT signal to triplet excitons residing on PC 61 BM and the 90 mT signal to triplet excitons on the donor. 37, 44, 63–64 The width of the triplet spectrum is determined by the zero field splitting (ZFS) tensor describing the energetics of the three triplet sublevels, where D x, D y and D z are the eigenvalues of the ZFS tensor. Since the ZFS tensor is traceless (D x +D y +D z = 0), the splitting can be described by only two parameters D and E: 65 …”

“…66 The ZFS parameters of triplet excitons on the PC 61 BM are similar to previously reported values. 37, 44, 63 …”

ESR spectroscopy for monitoring the photochemical and thermal degradation of conjugated polymers used as electron donor materials in organic bulk heterojunction solar cells

“…4 35 Here we have applied ESR spectroscopy for monitoring photochemical and thermal degradation of several different conjugated polymers in the absence of oxygen and moisture. The light-induced ESR (LESR) spectroscopy was used previously for investigation of the photoinduced charge 40 separation in the fullerene/polymer blends, 5 charge recombination kinetics, 6 distribution of trap states in conjugated polymers 7 and their accumulation during aging of P3DDT/[60]PCBM blends. 8 The ESR spectra obtained in the absence of light (dark ESR) were used to studydegradation of 45 conjugated polymers in air 9 and determination of defects in inorganic semiconductors.…”

Morphology Effect on Geometry of Photoinduced Charge-Separated State in P3HT:PCBM Blend Films as Studied by Time-Resolved EPR Spectroscopy

“…Around the center field regions (338 mT), however, additional spectrum components are superimposed. The broad A/A/E/E component is assigned to a fine structure of the excited triplet state of 3 PC 60 BM* generated in the isolated PC 60 BM-rich region by the S 1 -T 1 intersystem crossing of PC 60 BM [7]. In order to obtain the spectrum components at the center-field positions, we have performed the baseline corrections by the spectrum shapes of the 3 PC 60 BM* around the center field regions as shown in the right of Fig.…”