Session Q49: Focus Session: Organic Electronics and Photonics - Photophysics and Excited State Dynamics

Hybrid Phonoriton in Organic-Semiconductor Materials

In this work electronic structures and optical properties of organic-inorganic phonoriton, a new elementary excitation existing in heterostructures combining both organic and semiconductor materials, are studied. In those systems, Wannier-Frenkel hybrid exciton has unique and interesting properties that can improve the efficiency of optical materials. When an organic-semiconductor combined heterostructure is illuminated by high-intensity electromagnetic radiation with the frequency of the photons at or near the resonance frequency of the Wannier-Frenkel exciton, we obtain a macroscopically occupied system of hybrid polaritons that further interacts with phonons which will in turn generate the hybrid phonoriton. Electronic structure, energy and dispersion relation of phonoritons are theoretically determined. By analyzing the interactions between the hybrid exciton, photons and phonons, the conditions for phonoriton formation are discussed.   

Effect of structural distortion and polarization in localization of electronic excitations in organic semiconductor materials

Organic polymers find varied applications in optoelectronic devices such as solar cells, light emitting diodes and lasers. Detailed understanding of charge carrier transport by polarons and excitonic energy transfer producing singlet and triplet excitations is critical to improve their efficiency. We benchmarked the ability of current functional models to describe the spatial extent of self-trapped neutral and charged excitations for MEH-PPV owing to its superior luminescence and experimental evidence. Now we are interested in distinguishing between two distinct origins leading to localization; spatial localization of the wavefunction by itself on the undistorted geometry and localization of the wavefunction assured by distortion of the structure during its relaxation. We suggest localization is produced by electronic rearrangements and character of the functional. We also observe that different functionals place the highest occupied and lowest virtual orbitals at different positions in the energy band diagram based on their ability to predict the extent of localization of these states.   

The Effect of LUMO Level Offset on the Electron Dissociation Rates in Low Bandgap Polymer Heterostructures

In order to maximize the efficiency of polymer/fullerene bulk heterojunction solar cells, the voltage lost when the electron transfers from the polymer to the fullerene must be minimized. While the magnitude of this loss will significantly impact the maximum attainable efficiency of this technology, there have been relatively few attempts to quantify the dependence of the electron transfer rate and yield on the driving force for electron transfer. In order to isolate the effect of electrochemical potential difference on the exciton dissociation rate, we present results of photophysical measurements of a low bandgap copolymer mixed with a series of fullerene based acceptor materials in a bulk heterojunction geometry. The LUMO level of the acceptor material is varied relative to the polymer's so that the effect of the energy offset on the electron dissociation rate can be determined. Using photoluminescence and transient absorption measurements, we find that the exciton quenching rate varies systematically with increasing energy offset. We examine the mechanism of charge carrier generation by correlating the exciton quenching with charge carrier generation.   

First principles study of optical and electronic properties of anthradithiophene based organic conductors

Functionalized anthradithiophene (ADT) derivatives are high performance organic conductors where the addition of side groups such as triethylsilylethynyl (TES) to the ADT backbone induces a change in the morphology from a herringbone to a planar crystal structure in which improved intermolecular $\pi$-orbital overlap increases carrier mobility. Bulk type-II heterojunctions can be formed using matched pairs such as ADT-TES-F (donor) and ADT-TIPS-CN (acceptor). We report electronic and optical properties calculated from ab initio density functional theory (DFT) calculations for ADT derivatives. Exciton and exciplex formation and charge separation in ADT bulk heterojunctions is studied using 2 molecule model calculations for ADT-TES-F (donor) and ADT-TIPS-CN or C$_{60}$ (acceptor). We compare our results to available experimental results such as photo luminescence and photocurrent measurements.   

Ultrafast exciton energy transfer from giant nanocrystals to layered J-aggregate films

The integration of organic and inorganic materials at the nanoscale offers the possibility of developing new photonic devices that could combine the advantages of both classes of materials. Particularly interesting for such applications is a new class of core/shell CdSe/CdS nanocrystals (NQDs) with large number of shell monolayers (MLs) that are photostable, non-blinking and have an advantage of suppressed non-radiative Auger recombination leading to the existence of bright multiexcitonic (MX) states. However, due to large MLs thicknesses, the extraction of charge through such shells may pose considerable problems. In this work we studied hybrid structures composed of ``giant'', ({\#}ML$>$10) CdSe/CdS NQDs anchored on top of thin layers of strongly absorbing J-aggregates (JA) of cyanine dye (TDBC). We performed time-resolved and steady-state photoluminescence (PL) measurements to quantify the \textit{excitonic energy transfer }(ET) rates from the gNQDs to JA layer. By varying temperature (from RT to 80K) we observed change in ET rates in accordance with the overlap integral between NQD PL emission and JA absorption. In all cases, ET transfer rates exceeded 99{\%}. Hence, we foresee the utilization of gNQDs in applications in hybrid systems based on energy transfer.   

Ultrafast multidimensional spectroscopy of P3HT thin films

We report on measurements of morphology dependence of exciton/polaron dynamics in P3HT thin films. Intrachain and interchain electronic coupling has a significant impact on optical and electronic properties of polymers. Due to flexibility of polymers, slight differences in processing results in a variation of morphologies and electronic coupling between the chains. We employ ultrafast multidimensional spectroscopy techniques to resolve the resulting polaron formation dynamics in different polymer thinfilms spin casted from different solvents. Our results suggest, depending on the average conjugation length and crystallinity of the thin film, polaron formation dynamics exhibit spectrally homogeneous or inhomogeneous behavior.   

Charge transfer complex in diketopyrrolopyrrole polymers and fullerene blends: Implication for organic solar cell efficiency

Copolymers based on diketopyrrolopyrrole (DPP) have recently gained potential in organic photovoltaics. When blended with another acceptor such as PCBM, intermolecular charge transfer occurs which may result in the formation of charge transfer (CT) states. We present here the spectral photocurrent characteristics of two donor-acceptor DPP based copolymers, PDPP-BBT and TDPP-BBT, blended with PCBM to identify the CT states. The spectral photocurrent measured using Fourier-transform photocurrent spectroscopy (FTPS) and monochromatic photocurrent (PC) methods are compared with P3HT:PCBM, where the CT state is well known. PDPP-BBT:PCBM shows a stable CT state while TDPP-BBT does not. Our analysis shows that the larger singlet state energy difference between TDPP-BBT and PCBM along with the lower optical gap of TDPP-BBT obliterates the formation of a midgap CT state resulting in an enhanced photovoltaic efficiency over PDPP-BBT:PCBM.   

Dynamic Monte Carlo Modeling of Exciton Dissociation in Organic Donor-Acceptor Solar Cells

A general dynamic Monte Carlo model for exciton dissociation at a donor-acceptor interface including exciton delocalization and hot geminate pair dissociation has been developed to model the experimental behavior observed for the P3HT:PCBM system and predict the theoretical performance of future materials systems. The presence of delocalized excitons and the direct formation of separated charge pairs has been recently measured by transient photo-induced absorption experiments, and has been proposed to facilitate charge separation efficiency. The excess energy of the exciton dissociation process has also been observed to have a strong correlation with the charge separation yield for a range of thiophene polymer:PCBM systems, suggesting that a hot charge separation process is also occurring. Hot geminate pair dissociation has been previously theorized as a cause for highly efficient charge separation, however a detailed model for this process has not been implemented and tested. Here, both conceptual models have been implemented into a dynamic Monte Carlo simulation and tested using a model bilayer donor-acceptor system.   

Ultrafast Photo Physics of P3HT/PCBM blends for Organic Photovoltaic applications

We studied the ultrafast dynamics of photoexcitations in pristine polymer films of regio-regular polythiophene, regio-random polythiophene, and their blends with the fullerene derivative C$_{61}$-PCBM using the pump-probe photomodulation (PM) spectroscopy with $\sim $150 fs time resolution. Our transient PM spectrum covers the broad spectral range of 0.25 -- 2.4 eV using two different laser systems; which allows us to simultaneously monitor the dynamics of various photoinduced absorption bands such as intrachain excitons, charge transfer excitons, and polaron-pairs. Surprisingly, we have been able to monitor the decay of intrachain exciton on the polymer chains in films of polymer/fullerene blends, but unable to detect the subsequent generation of polarons in the donor (D) and acceptor (A) materials up to $\sim $ 1 ns. We explain this finding considering that the excitons in the polymer chains form charge transfer excitons upon reaching the D-A interface, rather than undergo a more direct dissociation on the D-A materials. The understanding of charge separation at the D-A interface is crucial for improving the power conversion efficiency of organic solar cell devices. Supported in part by the DOE grant No. DE-FG02-04ER46109.   

Can Singlet Fission Enhance the Performance of Organic Solar Cells?

The high efficiency of pentacene-fullerene (Pc-C$_{60}$) donor-acceptor solar cells has been ascribed to singlet fission, which generates two spin triplet excitons that each undergo ionization to give two pairs of electrons and holes [1,2]. For triplet ionization to give charge generation, the charge-transfer exciplex in the Pc-C$_{60}$ heterostructure should be energetically below the the molecular triplet state in Pc.Our initial calculations show that this is not a plausible scenario. We propose an alternate mechanism for the relatively high efficiencies of solar cells constructed from donors such as Pc, based on correlated-electron configuration interaction calculations [3] of ground state and photoinduced charge-transfer. \\[4pt] [1] Wilson M. W. B.; et al., J. Am. Chem. Soc, v133, 31, 11830-11833 (2011)\\[0pt] [2] Rao A.; et al., J. Am. Chem. Soc, v132, 36, 12698-12703 (2010)\\[0pt] [3] Yi Y.; Coropceanu V.; Br??das J. L.; J. Am. Chem. Soc, v131, 43, 15777-15783 (2009)   

Exciton self-trapping and Stark effect in the optical response of pentacene crystals from first principles

Pentacene is a prototypical organic semiconductor with optoelectronic and photovoltaic applications. It is known that the lowest-energy singlet excitation has a Stokes shift between absorption and emission of about 0.14 eV, but the deformation associated with this self-trapped exciton remains unknown. We begin with a calculation of the optical properties via the first-principles GW/Bethe-Salpeter (BSE) theory [ML Tiago, JE Northrup, and SG Louie, Phys. Rev. B 67, 115212 (2003); S Sharifzadeh, A Biller, L Kronik, and JB Neaton, arXiv:1110.4928 (2011)]. We then study the self-trapping phenomenon via our reformulation of the Bethe-Salpeter excited-state forces approximation of Ismail-Beigi and Louie [Phys. Rev. Lett. 90, 076401 (2003)], which can describe the structural relaxation after optical excitation. Whether excitons in pentacene have charge-transfer character has been controversial in electro-absorption experiments. We use the same BSE analytic derivatives approach to calculate the changes in excitation energies due to an applied electric field to understand this experimental controversy.   

Singlet Fission and Multi-Exciton Generation in Organic Systems

Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2$\times$T0). D satisfies the energy requirement for SF ($E_{D}>2E_{T0}$) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into triplets proceeds through the excimer. The predicted energetics, wavefunctions and excimer interaction support the proposed mechanism, which accounts for the observed rapid, unactivated SF in pentacene. Results for SF in polyacenes, grapheme nanoribbons, rubrene and carbon nanotubes will be presented.