The Effect of Solvent Additives on Morphology and Excited-State Dynamics in PCPDTBT:PCBM Photovoltaic Blends

Abstract

The dependence of the thin film morphology and excited-state dynamics for the low-bandgap donor-acceptor copolymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) in pristine films and in blends (1:2) with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) on the use of the solvent additive 1,8-octanedithiol (ODT) is studied by solid-state nuclear magnetic resonance (NMR) spectroscopy and broadband visible and near-infrared pump-probe transient absorption spectroscopy (TAS) covering a spectral range from 500-2000 nm. The latter allows monitoring of the dynamics of excitons, bound interfacial charge-transfer (CT) states, and free charge carriers over a time range from femto- to microseconds. The broadband pump-probe experiments reveal that excitons are not only generated in the polymer but also in PCBM-rich domains. Depending on the morphology controlled by the use of solvent additives, polymer excitons undergo mainly ultrafast dissociation (<100 fs) in blends prepared without ODT or diffusion-limited dissociation in samples prepared with ODT. Excitons generated in PCBM diffuse slowly to the interface in both samples and undergo dissociation on a time scale of several tens of picoseconds up to hundreds of picoseconds. In both samples a significant fraction of the excitons creates strongly bound interfacial CT states, which exhibit subnanosecond geminate recombination. The total internal quantum efficiency loss due to geminate recombination is estimated to be 50% in samples prepared without ODT and is found to be reduced to 30% with ODT, indicating that more free charges are generated in samples prepared with solvent additives. In samples prepared with ODT, the free charges exhibit clear intensity-dependent recombination dynamics, which can be modeled by Langevin-type recombination with a bimolecular recombination coefficient of 6.3 × 10(-11) cm(3) s(-1). In samples prepared without ODT, an additional nanosecond recombination of polaron pairs is observed in conjunction with an increased intensity-independent trap-assisted nongeminate recombination of charges. Furthermore, a comparison of the triplet-induced absorption spectra of PCPDTBT with the charge-induced absorption in PCPDTBT:PCBM blends reveals that triplets have a very similar excited-state absorption spectrum compared to the free charge carriers, however, in contrast have a distinct intensity-independent lifetime. Overall, our results suggest that whether free charges or strongly bound CT states are created upon dissociation of excitons at the PCPDTBT:PCBM interface is determined instantaneously upon exciton dissociation and that once formed strongly bound CT states rapidly recombine and thus are unlikely to dissociate into free charges. The observation of a significantly larger bimolecular recombination coefficient than previously determined for poly(3-hexylthiophen-2,5-diyl):PCBM (P3HT:PCBM) and PCDTBT:PCBM samples indicates that nongeminate recombination of free charges considerably ...

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

“…[9][10][11]13,17,41 We are not able to distinguish charges that are bound in the CT state from free ones using the broad polaron absorption band (similar spectral signatures have also been noted in TA studies of some other polymers). 9,10 On the basis of a model by Laquai et al, 9−11 both are expected even at the earliest time delays, since they are instantaneously generated and do not interconvert. A recent pump-push photoconductivity experiment confirms that free charge carriers are generated from "hot" delocalized states populated directly following CS.…”

“…A similar flat band extending next to the GSB with a peak at lower wavelengths has been observed in blends of other donor− acceptor copolymers with PCBM, and was also assigned to the charged polymer. 9,17 The anion of PCBM is known to absorb in the 1030 nm region, and is thus outside our measuring window. 13 It has often been reported that CS occurs quantitatively in less that 100 fs in polymer:fullerene bulk heterojunctions.…”

“…9,11 More probable is geminate charge recombination from an interfacial CT state. Time scales of hundreds of picoseconds to nanoseconds have been reported for this process in various polymers.…”

Charge Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

“…40,59 However, the presence of these domains also plays a key role in reducing geminate and non-geminate charge recombination losses within the blend, as we discuss below, such that optimum device performance is achieved at higher (1:3 or 1:4) blend ratios. After concluding that polymer excitons are efficiently separated in all SiIDT-DTBT:PC70BM…”

Effect of Crystallizable Solvent on Phase Separation and Charge Transport in Polymer-fullerene Films

“…[1][2][3] To improve the photovoltaic performance of OPVs, morphology and charge transport in bulk heterojunction (BHJ) solar cells are important. Many methods were used to optimize the morphology of polymers and fullerenes in BHJ such as thermal annealing [4,5], solvent annealing [6,7] and addition of solvent additives [8,9]. One effective method to control the morphology of polymer-fullerene is adding high boiling point additive in polymer blend such as 1,8-diiodooctane (DIO) [10], 1,8-dibromooctane [11] and chloronaphthalene [12].…”