Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D16: Focus Session: Organic Electronics and Photonics: Solar Cells and Photophysics |
Hide Abstracts |
Sponsoring Units: DMP DPOLY Chair: Jenny Nelson, Imperial College London Room: B115 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D16.00001: Charge transfer excitons in codeposited organic films Xiaosheng Wang, Ajward Milhan, Hans Peter Wagner Charge transfer (CT) excitons play an important role in solar cells and organic light emitting devices. Such excitons are usually formed at the hetero-interfaces with an electron and hole residing in different materials. We studied the formation and recombination dynamics of CT excitons in co-deposited Alq3/PTCDA, TPD/Alq3, and TPD/PTCDA thin films that are grown by organic molecular beam deposition with various co-deposition ratios. (Alq3: \textit{Tris(8-hydroxyquinolinato)aluminium}, PTCDA: \textit{perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride, }TPD: \textit{N,N'-Bis(3-methylphenyl)- N,N'-diphenylbenzidine.}) The formation of CT excitons is associated with a quenching of the photoluminescence (PL) of the individual material. The PL spectrum shows a new emission peak at $\sim $750 nm for Alq3/PTCDA films and $\sim $700 nm for TPD/Alq3 which we tentatively attribute to the radiative recombination of CT excitons. The formation and decay time of the CT excitons is studied by time-resolved PL measurements using the technique of time-correlated single photon counting. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D16.00002: Energy and charge transfer in select organic semiconductor composites Andrew Platt, Mark Kendrick, John Anthony, Oksana Ostroverkhova We report on the photophysical properties of high-performance solution-processable functionalized anthradithiophene (ADT) composites. The time-resolved fluorescence and photoconductivity of drop-cast films of a fluorinated derivative, ADT-TES-F, with guest molecules of C$_{60}$, or another functionalized ADT derivative, ADT-TIPS-CN, are measured after excitation with $\sim$ 100~fs laser pulses and suggest strong charge and energy transfer in ADT-TES-F/C60 and ADT-TES-F/ADT-TIPS-CN composites, respectively. An addition of 2 and 5 wt~\% of C$_{60}$ to the host resulted in transient photocurrent amplitude enhancement by a factor of 3 and 10, respectively, and a partial quenching of fluorescence. An addition of 0.05\% wt of ADT-TIPS-CN guest resulted in strong energy transfer from host to guest, as observed from fluorescence spectral shift, and a change in transient photocurrent dynamics. We summarize these results from our studies involving time-resolved fluorescence and photoconductivity techniques coupled with temperature control to probe the processes responsible. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D16.00003: Resolving the charge transfer complex (CTC) state in organic heterojunction solar cell using capacitive photocurrent technique Hemant Shah, Aditya Mohite, Tanesh Bansal, Bruce Alphenaar The dissociation of photo-excited states in organic heterojunction solar cells is thought to occur through the formation of a charge transfer complex (CTC) at the bi-layer interface. Absorbance and luminescence from the CTC is usually very weak, making direct observation difficult. Here we describe the use of a capacitive photocurrent technique to directly probe the CTC. The capacitive photocurrent signal is sensitive to photo-generated charge only when it is physically separated across the bi-layer interface. Since the CTC leads directly to exciton dissociation, the signal observed using the capacitive photocurrent technique is greatly amplified by the high dissociation fraction of the excited carriers. Measurements of a MDMO-PPV/PCBM heterojunction solar cell shows a strong peak at around 650 nm, that is observed as a much weaker feature in standard photocurrent and absorbance measurements. Excitation at 650 nm causes a reduction in the background photocurrent signal. This bleaching of the photocurrent suggests that occupation of the CTC states blocks the dissociation of higher energy excitonic states. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D16.00004: Molecular Control of Charge Transfer from Organic Semiconductors to ZnO Josef Spalenka, Peerasak Paoprasert, Ryan Franking, Dominic Bindl, Michael Arnold, Robert Hamers, Padma Gopalan, Paul Evans The transfer of charge at organic-inorganic semiconductor interfaces is relevant for a range of applications such as organic light emitting diodes and photovoltaics. We investigate exciton dissociation and charge transfer using a novel probe based on a modified pentacene phototransistor with ZnO nanoparticles at the gate dielectric interface. We observe a large threshold voltage shift (+33 V) upon illumination of samples with ZnO, and no comparably large shift in samples without ZnO. This large threshold voltage shift arises from photoinduced charge transfer at the pentacene-ZnO interface and charge trapping on the nanoparticles. We further find that modifying the ZnO surface with a layer of organic molecules using a carboxylic acid binding chemistry can tune the threshold voltage, and therefore the density of charge transferred to ZnO. The effect of absorbing incident photons with varying energy will also be discussed. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D16.00005: Ultrafast Charge Separation at Organic Photovoltaic Interfaces:Time-Resolved Photoemission Studies of CuPc/C$_{60}$ G.J. Dutton, W. Jin, J.E. Reutt-Robey, S.W. Robey We discuss first results from TR-2PPES studies of exciton dynamics and charge separation at well-characterized organic photovoltaic heterointerfaces. Organic MBE was used to engineer interfaces between phthalocyanines and C$_{60}$. These interfaces were characterized using STM, STS, and UPS to reveal molecular structure and band alignment. Ultrafast TR-2PPES measurements for CuPc/ 20 MLC$_{60}$ structures then provide a detailed picture of charge separation and recombination processes at the CuPc/C$_{60}$ heterointerfaces. The Ti:sapphire fundamental wavelength at 750 nm (1.65 eV) was used to pump the lowest $\pi$-$\pi$* transitions (Q band) in CuPc. A frequency tripled beam at 250 nm (4.95 eV) was employed to probe subsequent dynamics of the CuPc exciton population at sub- picosecond timescales. The initial population decays on picosecond to sub-picosecond timescales. Charge separation at the interface significantly enhances the decay rate of the CuPc exciton population. We will discuss dominant charge separation and recombination processes at and near CuPc/C$_{60}$ interfaces and evidence for recombination of charge separated carriers to lower lying triplet states determined from time dependent spectra and cross-correlation data, coupled with rate-equation modeling. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D16.00006: Theory of interfacial charge-transfer complex photophysics in $\pi$-conjugated polymer-fullerene blends K. Aryanpour, D. Psiachos, S. Mazumdar We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes [1]. Within the same Coulomb-correlated Hamiltonian applied previously to interacting chains of single-component $\pi$-conjugated polymers [2], we find an exciplex state that occurs below the polymer's optical exciton. Weak absorption from the ground state occurs to the exciplex. We explain transient photoinduced absorptions in the blend [3], observed for both above-gap and below-gap photoexcitations, within our theory. Photoinduced absorptions for above-gap photoexcitation are from the optical exciton as well as the exciplex, while for below-gap photoexcitation induced absorptions are from the exciplex alone. In neither case are free polarons generated in the time scale of the experiment. Importantly, the photophysics of films of single-component $\pi$-conjugated polymers and blends can both be understood by extending Mulliken's theory of ground state charge-transfer to the case of excited state charge-transfer. [1] K.~Aryanpour, D.~Psiachos, and S.~Mazumdar, arXiv:0908.0366 [2] D.~Psiachos and S.~Mazumdar, Phys. Rev. B. {\bf 79} 155106 (2009) [3] T.~Drori {\it et al}., Phys. Rev. Lett. {\bf 101}, 037402 (2008) [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D16.00007: Origin and impact of recombination via charge transfer excitons in polymer/fullerene solar cells Markus Hallermann, Enrico Da Como, Jochen Feldmann To further advance the performances of organic photovoltaic cells a thorough understanding of loss mechanisms in polymer/fullerene blends is mandatory. Recombination via charge transfer excitons (CTEs) appears to be a fundamental loss, potentially impacting the open circuit voltage (V$_{OC})$ and the short circuit current (I$_{SC})$ of cells. We unravel the origin of CTEs forming in polymer/fullerene blends and discuss their importance in recombination processes considering binding energy [1], polymer conformation [2], and energetic position. CTE photoluminescence (PL) is observed in material combinations such as P3HT and PPV blended with fullerene acceptors. By combining electron microscopy and PL spectroscopy, we show that CTE recombination is only slightly influenced by the mesoscopic morphology, whereas strongly by the polymer chain conformation [2]. By shifting the orbital energies of the fullerene, we tune the CTE PL characteristics. High energy CTE emission results in cells with a beneficial increase in V$_{OC}$. On the other hand, high energy CTE emission leads to a more efficient recombination impacting directly the I$_{SC}$. The results highlight a fundamental limit in the efficiency of organic solar cells with CTE recombination. [1] Hallermann et al. APL 2008 [2] Hallermann et al. AFM 2009 [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D16.00008: Experimental Visualization of Singlet and Triplet Photovoltaic Processes in Organic Solar Cells Huidong Zang This presentation will report the singlet and triplet photovoltaic processes in organic solar cells based on the studies of magnetic field dependence of photocurrent. We found that magnetic field dependence of photocurrent can be used to experimentally visualize the singlet and triplet photovoltaic processes in polymer bulk-heterojunction solar cells based on poly[3-methylthiophene] (P3HT) and surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6] C61 (PCBM). We observed that singlet and triplet excitons undergo two different channels: dissociation and charge reaction, respectively, in the generation of photocurrent in organic semiconductors. When bulk-heterojunctions are formed between donor and acceptor molecules in organic solar cells, the weak donor-acceptor interaction mainly affects the dissociation channel but has little influence on charge reaction channel. Moreover, the strong donor-acceptor interaction can directly separate the electrons and holes in singlet and triplet excitons to generate photocurrent before they experience dissociation and charge reaction. Furthermore, the magnetic field dependence of photocurrent clearly indicates that the dissociated electrons and holes can recombine into charge- transfer complexes at the donor-acceptor intermolecular interfaces. This presentation will discuss the control of useful and non-useful photovoltaic processes based on the studies of magnetic field effects of photocurrent. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D16.00009: Ultrafast energy transfer in organic-inorganic hybrid nanostructures Evgeny Danilov, Hue Nguyen, Amandeep Sra, Anton Malko The integration of organic and inorganic materials at the nanoscale offers the possibility of developing new photonic devices that could potentially combine the advantages of both classes of materials. Such optoelectronic structures could work both in photovoltaic as well as in light emitting modes depending on the direction of \textit{exciton} energy transfer. In the present work, we studied hybrid film structures consisting of a thin layer of colloidal CdSe nanocrystals (NCs) ``anchored'' to a monolayer of J-aggregates (JA) of a cyanine dye (TDBC) with polyelectrolyte (PDDA) acting as a molecular ``glue''. We performed time-resolved and steady-state photoluminescence (PL) measurements to quantify the energy transfer (ET) rates from NCs to JA layer. Systematic study of ET rates as a function of donor-acceptor distance revealed that even at separations approaching 100 A, energy transfer is still an efficient process. Additionally, we present our initial results for ET rates and efficiencies from NCs to patterned GaAs multiquantum well structures. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:54PM |
D16.00010: Transient Microwave Studies of Organic Photovoltaics Invited Speaker: Certified power conversion efficiencies of excitonic solar cells based on the polymer:fullerene (donor:acceptor) bulk heterojunction have increased from 2.5{\%} back in 2001, to 7.7{\%} in 2009. This impressive increase can be attributed to new polymer designs, combined with improved device engineering. As impressive as this increase is, it is the push towards a 15{\%} efficient device that will have the impact required for the effective harvesting of solar energy. This increase will still require a continued effort towards new polymers, which act as the primary donor, but will also need a step towards new acceptors too. However, our knowledge of how the bulk heterojunction works is still in its infancy creating hurdles that we must overcome. It is this basic understanding that motivates the work that will be described. Few acceptors have proven to be as good as C60 or C70, and therefore this presentation will examine a number of new systems that will include molecules, quantum dots and single-walled carbon nanotubes. To examine their effectiveness as acceptors, results from studies using the electrodeless technique of transient microwave conductivity will be discussed and, where appropriate, a comparison with device performance will be examined. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D16.00011: Direct observation of Frenkel and charge transfer excitons in single crystal organic semiconductor of pentacene D.-C. Qi, A.T.S. Wee, A. Rusydi, H.B. Su, M. Bastjan, M. Ruebhausen, O.D. Jurchescu, T.T.M. Palstra The understanding of lowest-lying electronic excitations in organic semiconductor solids is fundamentally important. In this paper, we report on the emerging of Frenkel and CT excitons in pentacene single crystals. We discover three novel low-energy electronic excitations below 2.0 eV using high energy resolution spectroscopic generalized ellipsometry with full polarization dependence. Surprisingly, these excitations depend strongly on polarization of the incident light, and they are discussed in terms of intramolecular Frenkel excitons and intermolecular CT excitons. In particular, the controversial description of the energetically lowest electronic excitation in pentacene crystals is explicitly clarified as a pure Frenkel exciton. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D16.00012: Magnetic field effects in $\pi $-conjugated polymer/fullerene blends; evidence for multiple components Fujian Wang, Heinz B\"assler, Z. Valy Vardeny We studied magnetoconductance (MC) in organic diodes of blends of $\pi $-conjugated polymers and fullerene molecules at various concentrations, $c$. The MC response is composed of several components that depend on the applied bias voltage and $c$. A dominant positive low-field (LF) component, which also governs the magneto-electroluminescence response dramatically decreases and broadens with $c$, thus unraveling a negative LF and positive high-field (HF) components. Our results attest that the positive LF component is caused by Coulombically bound electron-hole (e-h) pairs in the active layer. This is confirmed by studying MC in electron- and hole-unipolar devices, which shows small negative LF that might be due to e-e and h-h pairs (bipolaron), respectively; but lacks the positive LF component. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D16.00013: Trap States in Copper Phthalocyanine Thin Films using Photogenerated Currents Thomas Gredig, Jorge Guerra, Matthew Byrne, Evan Silverstein The efficiency of organic solar cells is limited by several factors including the photocurrent generation process. Copper phthalocyanine thin films with different grain structures are prepared via thermal evaporation onto interdigitated gold electrodes. The samples are analyzed with atomic force microscopy and then exposed to light pulses to explore the time dependence of photogenerated currents in phthalocyanine thin films. The average grain size is obtained from the correlation length of the height-height correlation function and varies from 30-200nm. The dependence of the recombination of photo-excited, dissociated charge pairs on the electric field is compared with the Onsager mechanism and a simple dual trap state model from which relevant time scales are extracted. [Preview Abstract] |
Monday, March 15, 2010 5:30PM - 5:42PM |
D16.00014: Electronic couplings in organic/ZnO hybrid structures for photovoltaics Na Sai, Kevin Leung, James R. Chelikowsky Organic-inorganic hybrid structures are promising for photovoltaic applications. Interfacial charge separation and charge transfer must be optimized for efficient power conversion in these systems. While the role of these fundamental processes in interfacial structures has been recognized, the complex interplay between the crystal structure, interfacial molecular orientations, and electronic structure in organic-inorganic interfaces is not well understood at the atomic level. We have carried out large scale first principles calculations of the interfacial energy level alignment and electronic coupling between oligothiophene and ZnO for different molecular orientations at the interface. This allows us to elucidate the geometric dependence of the electronic interactions between the organic molecule and the substrate. The work is supported by EFRC:CST Energy Frontier Research Center funded by the U.S. DOE under Award number DE-SC0001091 and Texas Advanced Computing Center. KL is also supported by the DOE under Contract DE-AC04-94AL85000. Sandia is a multiprogram operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Deparment of Energy. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700