Geminate Polaron Pairs Research Articles

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

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics within molecularly intermixed donor–acceptor domains versus the dynamics between phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC70BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair r...

Particle plasmon‐induced charge trapping at heterointerfaces in PCDTBT:PC70BM blends

ABSTRACTWe investigate the influence of particle plasmons on exciton and charge generation and recombination processes in the blend of poly (9‐(1‐octylnonyl)‐9H‐carbazole‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) (PCDTBT) and [6,6]‐phenyl‐C70butyric acid methyl ester (PC70BM). The particle plasmons are generated from gold nanoparticles, which are embedded into PCDTBT:PC70BM blend. For the blend with gold nanoparticles, we observe enhance light harvesting. Despite the enhanced light collection, we find that the quasi‐steady‐state charge generation has not been influenced by the particle plasmons. However, the generation and recombination of long‐lived (sub‐millisecond) polaron paris have been significantly enhanced: from untrapped state in the pristine blend to the trapped state in the gold nanoparticle‐embedded blend. This result implies that the plasmon‐influenced polarons are trapped at the broadband geminate polaron pair (GPP) state. This state acts as an intermediate state, which either leads to the formation of charge transfer excitons (CTXs) or free charge carriers. In our case, the particle plasmon‐influenced polarons are trapped in the GPP state, which leads to the formation of CTXs. For this reason, we do not observe the enhanced charge generation in PCDTBT:PC70BM blend with particle plasmon resonance. Finally, we revealed that the long‐lived polarons mainly resulted from the localization by particle plasmons. The macroscopic modification in the blend film made negligible contributions to this influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 940–947

Slow geminate‐charge‐pair recombination dynamics at polymer: Fullerene heterojunctions in efficient organic solar cells

AbstractWe explore charge recombination dynamics at electron donor‐acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC70BM), by means of combined time‐resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron‐hole pairs recombines with a temperature‐independent rate distribution spanning submicrosecond timescales to produce luminescent charge‐transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge‐pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long‐lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate‐charge recombination highlights the importance of the molecular nature of specific donor–acceptor electronic interactions in defining the relaxation pathways of trapped GPP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Kinetic Studies of Geminate Polaron Pair Recombination, Dissociation, and Efficient Triplet Exciton Formation in PC:PCBM Organic Photovoltaic Blends

The recombination and dissociation of geminate polaron pairs (GPPs) in bulk-heterojunction blends of the dye molecule 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (PC) with the electron a...

Photophysics of Charge Generation in Organic Photovoltaic Materials: Kinetic Studies of Geminate and Free Polarons in a Model Donor/Acceptor System

Photoinduced charge generation is studied in a model donor/acceptor blended system, consisting of bulk-heterojunction blends of the conjugated polymer regioregular poly(3-hexylthiophene) (P3HT) and the dye molecule phthalocyanine (PC). Excitation at 780 nm, into the PC NIR absorption band, creates excitons in PC domains without populating the P3HT excited state. The kinetics of geminate polaron pairs (GPP) and free polarons can then be followed without the interference from the signatures of directly created polarons, energy transfer, or significant exciton migration. The donor/acceptor film morphology has a significant impact on the ratio of free polarons and GPP that are formed: increased phase segregation leads to a pronounced increase on the fraction of free polarons formed directly. Unlike the free polaron population, GPPs are affected by fast recombination directly to the ground state, occurring with a time constant of 170 ps. Both GPP and free polarons are formed at the P3HT/PC interfaces. Free polarons are, however, able to migrate from these interfaces into the bulk, quasi-crystalline P3HT regions. Finally, the results obtained with our model system were compared with the photoinduced charge generation in P3HT/PCBM 1:1 blend films. Indication for free polaron formation yield following electron transfer of ∼100% was obtained; no GPP formation was observed. This is in agreement with previous studies of both the photophysics and OPV device characterization based on these blends.

Localized trions in conjugated polymers

Experimental and theoretical evidence for charged trions in films of poly[2-(N-carbazolyl)-5-(${2}^{\ensuremath{'}}$-ethyl)-hexoxy-1,4-phenylenevinylene] consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign, which can be metastable with respect to both dissociation and recombination, is described. Such trions can be generated by fusion of a free polaron with a neutral polaron pair; this process eliminates the high Coulomb barrier for fusion of two like-charge carriers into a bipolaron (BP). We also argue that trions can be created by photoexcitation of a conjugated polymer, which may create a high density of geminate polaron pairs. Since the metastable trion is anchored by a deeply trapped charge, it is immobile and can be considered as a Coulombically trapped BP, even if the BP may not exist as a free particle. Such localized trions can account for the reversible photoinduced fatigue of the thermally stimulated luminescence (TSL) and the negative [photoluminescence (PL) quenching] spin $1∕2$ PL-detected magnetic resonance (PLDMR) of these films. We also show that the model of reversible UV photoinduced cyclization, recently suggested to explain metastable charged states in substituted poly(phenylene vinylene) derivatives, is inconsistent with the obtained TSL and PLDMR results.

Limits to accumulation of electric-field-stabilized geminate polaron pairs in an organic semiconductor thin film

We study the dynamics of geminate polaron-pair accumulation using time-dependent electroabsorption spectroscopy in a guest-host system composed of the laser dye, coumarin 6 (C6), doped into the host, $4,{4}^{\ensuremath{'}}$-bis($N$-carbazolyl)biphenyl. Probing the Stark-shifted absorption of C6 at a wavelength of $496\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, we determine the decrease of the internal electric field resulting from buildup of a polarizing charge distribution that accompanies optically induced geminate pair accumulation in an applied electric field. Our model suggests that the drop in internal field qualitatively leads to the observed time-dependent roll-off of accumulated geminate pair density. The results highlight the importance of both nongeminate recombination and reduced geminate pair stability resulting from the decrease in internal electric field in ultimately limiting geminate pair densities in doped organic semiconductors under continuous illumination.

Accumulation of electric-field-stabilized geminate polaron pairs in an organic semiconductor to attain high excitation density under low intensity pumping

The recombination dynamics of geminate polaron pair (PP) states are investigated by monitoring electric-field-induced delayed fluorescence in thin films consisting of the green laser dye, Coumarin-6 (C6) doped at 1wt% into 4,4′-bis(N-carbazolyl)biphenyl. We find that the PP decay follows τ−m (with m∼0.1), where τ is the time that the PPs are held in the field. This sublinear decay suggests the possibility for accumulation of PPs over time that can then be reconverted into excitons upon field removal. We demonstrate the generation of short (∼50ns full width at half maximum) bursts of C6 fluorescence with peak intensities >20 times the steady-state fluorescence intensity (corresponding to a C6 singlet exciton density NS>4×1015cm3) when pumped continuously by a low intensity (<1W∕cm2) laser in the presence of a pulsed electric field.

Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer:fullerene solar cells

We investigate the nature of ultrafast exciton dissociation and carrier generation in acceptor-doped conjugated polymers. Using a combination of two-pulse femtosecond spectroscopy with photocurrent detection, we compare the exciton dissociation and geminate charge recombination dynamics in blends of two conjugated polymers, MeLPPP [methyl-substituted ladder-type poly($p$-phenylene)] and MDMO-PPV [poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], with the electron accepting fullerene derivative PCBM [1-(3-methoxycarbonyl)-propyl-1-phenyl-$(6,6){\mathrm{C}}_{61}$]. This technique allows us to distinguish between free charge carriers and Coulombically bound polaron pairs. Our results highlight the importance of geminate pair recombination in photovoltaic devices, which limits the device performance. The comparison of different materials allows us to address the dependence of geminate recombination on the film morphology directly at the polymer:fullerene interface. We find that in the MeLPPP:PCBM blend exciton dissociation generates Coulombically bound geminate polaron pairs with a high probability for recombination, which explains the low photocurrent yield found in these samples. In contrast, in the highly efficient MDMO-PPV:PCBM blend the electron transfer leads to the formation of free carriers. The anisotropy dynamics of electronic transitions from neutral and charged states indicate that polarons in MDMO-PPV relax to delocalized states in ordered domains within $500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The results suggest that this relaxation enlarges the distance of carrier separation within the geminate pair, lowering its binding energy and favoring full dissociation. The difference in geminate pair recombination concurs with distinct dissociation dynamics. The electron transfer is preceded by exciton migration towards the PCBM sites. In MeLPPP:PCBM the exciton migration time decays smoothly with increasing PCBM concentration, indicating a trap-free exciton hopping. In MDMO-PPV:PCBM, however, the exciton migration time is found to show a threshold-like dependence on the PCBM concentration. This observation is explained by efficient interchain energy transfer in ordered MDMO-PPV domains in conjunction with exciton trapping.

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers.

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.

Film morphology and ultrafast photoexcitation dynamics in polyfluorene

We study the effect of sample morphology on ultrafast dynamics of photoexcited species in poly(9,9-dioctyl)fluorene (PFO). We show that within 200 fs both excitons and geminate polaron pairs with distinctly different absorption bands in the visible-near infrared spectral range are photogenerated in PFO solutions, pristine films, thermal and vapor treated films. However the branching ratio between the excitons and polaron pairs, as well as the polaron pair recombination kinetics depends on film morphology; polaron pair generation is enhanced and their recombination kinetics is faster in a solid phase, called the β phase, where the polymer chains are more planar.

Photoexcitation dynamics in regioregular and regiorandom polythiophene films

Using a variety of optical probes techniques we studied the photoexcitation dynamics in thin films of poly-3-hexyl thiophene with regioregular order that forms lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chainlike morphology. In regiorandom films we found that intrachain excitons with correlated induced absorption and stimulated emission bands are the primary excitations; they give rise to a moderately strong photoluminescence band. In regioregular films, on the contrary, we found that the primary excitations are excitons with a much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and very weak photoluminescence band. In regioregular films we also measured photogenerated geminate polaron pairs with ultrafast dynamics that are precursor to long-lived polaron excitations.