Charge-carrier generation in organic solar cells using crystalline donor polymers

Abstract

Charge generation and recombination dynamics in a blend film of a crystalline low-bandgap polymer, poly[(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied by transient absorption spectroscopy. Upon photoexcitation of the PSBTBT absorption band at 800 nm, singlet excitons were promptly generated, and then rapidly converted into polarons in a few picoseconds. We found that there are two different polarons in PSBTBT: one is ascribed to polarons generated in the disorder phase and the other is ascribed to polarons in the crystalline phase. On a time scale of nanoseconds, ∼50% of polarons in the disorder phase recombined geminately to the ground state. On the other hand, such geminate recombination was negligible for polarons in the crystalline phase. As a result, the overall charge dissociation efficiency is as high as ∼75% for PSBTBT/PCBM blend films. On the basis of these analyses, we discuss the role of polymer crystallinity in the charge-carrier generation in organic solar cells.

Local Intermolecular Order Controls Photoinduced Charge Separation at Donor/Acceptor Interfaces in Organic Semiconductors

“…Time-resolved microwave conductivity, time-resolved photoluminescence, and transient absorption spectroscopy measurements show that while ultrafast charge transfer occurs in all samples, long-lived charge carriers are only produced in fi lms with intermolecular aggregates of either RR-P3HT or PC 61 BM, and that polymer aggregates are just as effective in this regard as those of fullerenes. more crystalline polymers, [ 6,[13][14][15][16] or more crystalline regions of a single polymer, [ 17 ] have the same effect. Even for ground-state electron transfer, the effi ciency of free charge generation has been associated with local molecular order.…”

On the role of local charge carrier mobility in the charge separation mechanism of organic photovoltaics

“…The degree of fluorescence quenching is direct evidence, for instance, the value of Z ED of amorphous regiorandom P3HT, PCPDTBT, and semi-crystalline PSBTBT are B1, while that of crystalline P3HT is B0.9. 45 As mentioned previously, the hierarchical phase and energy gradient in the polymer:fullerene mixed domain, amorphous, and crystallite domains are important for efficient charge separation. 41 In addition to the presence of high local mobility probed using terahertz, 20 electric field-induced second harmonic (EFISH), 50,51 and gigahertz electromagnetic waves, 52 control of the polymer/fullerene interface is presumed to be of particular importance with respect to the energetics [53][54][55][56] and morphology suitable for efficient charge separation.…”

Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement in Photovoltaic Conversion Efficiency

“…[63][64][65] Rather, such amorphous polymer solar cells suffer from the lower fill factor (FF) for the thicker active layer because of severe non-geminate charge recombination. 64, 66 As a result, the power conversion efficiency (PCE) is typically optimized with optically thin (~100 nm) films.…”