Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R33: Organic Electronics and Photonics - Organic PhotovoltaicsFocus
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Sponsoring Units: DPOLY Chair: Daniel Sinkovits, University of Wisconsin, Stout Room: 336 |
Thursday, March 17, 2016 8:00AM - 8:36AM |
R33.00001: ABSTRACT MOVED TO V4.002 |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R33.00002: Multiple Charge Transfer States at Ordered and Disordered Donor/Acceptor Interfaces Michael Fusella, Bregt Verreet, YunHui Lin, Alyssa Brigeman, Geoffrey Purdum, Yueh-Lin Loo, Noel Giebink, Barry Rand The presence of charge transfer (CT) states in organic solar cells is accepted, but their role in photocurrent generation is not well understood. Here we investigate solar cells based on rubrene and C$_{60}$ to show that CT state properties are influenced by molecular ordering at the donor/acceptor (D/A) interface. Crystalline rubrene films are produced with domains of 100s of microns adopting the orthorhombic phase, as confirmed by grazing incidence XRD, with the (h00) planes parallel to the substrate. C$_{60}$ grown atop these films adopts a highly oriented face-centered cubic phase with the (111) plane parallel to the substrate. For this highly ordered system we have discovered the presence of four CT states. Polarized external quantum efficiency (EQE) measurements assign three of these to crystalline origins with the remaining one well aligned with the disordered CT state. Varying the thickness of a disordered blend of rubrene:C$_{60}$ atop the rubrene template modulates the degree of crystallinity at the D/A interface. Strikingly, this process alters the prominence of the four CT states measured via EQE, and results in a transition from single to multiple electroluminescence peaks. These results underscore the impact of molecular structure at the heterojunction on charge photogeneration. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R33.00003: Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge Transfer Excitons Kenan Gundogdu, Bhoj Gautam, Robert Younts, liang yan, Robert Younts, Harald Ade, Wei You The role of excess excitation energy on long-range charge separation in organic donor/acceptor bulk heterojunctions continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high energy CT states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in bulk heterojunction (BHJ) devices. Here we studied the population dynamics of charge separated polarons upon excitation of high energy polymer states and low energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond (ns) time scales. We observed that the charge separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R33.00004: Fully conjugated donor-acceptor block copolymers as model systems for studies of charge transfer Melissa Aplan, Youngmin Lee, Christopher Gray, Thomas Mallouk, Enrique Gomez Fully conjugated block copolymers, consisting of an electron donor and an electron acceptor block, can serve as the active layer in organic photovoltaic devices. Incorporating the donor-acceptor interface within the chemical structure enables model studies of charge transfer. We synthesized a series of block copolymers consisting of a P3HT electron donor and a push-pull polymer electron acceptor, yielding: poly(3-hexylthiophene)$-$block$-$poly-((9-(9-heptadecanyl)-9H-carbazole)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PCT6BT), poly(3-hexylthiophene)$-$block$-$poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PFT6BT), and poly(3-hexylthiophene)$-$block$-$poly-((2,5-dihexylphenylene)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PPT6BT). By altering the electron rich unit of the acceptor, we adjust the energy difference between donor and acceptor HOMOs. Using photoluminescence, we observe no evidence of exciton dissociation to a charge transfer state in P3HT-b-PCT6BT. In P3HT-b-PFT6BT and P3HT-b-PPT6BT we observe varying degrees of intrachain charge transfer. These results measure the critical driving force needed for charge transfer. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:48AM |
R33.00005: Morphology-insensitive Performance Facilitates Transition from Spin-Coating to Roll-to-Roll Coating For High-Performance, Solution-Processed Solar Cells Invited Speaker: Dean DeLongchamp Solution processing via roll-to-roll (R2R) coating promises a low cost, low thermal-budget, sustainable revolution for the production of solar cells. Yet virtually all high efficiency solution processed research cells have been demonstrated by spin-coating, a low-volume deposition process. We present detailed device and morphology studies of an organic photovoltaic (OPV) system deposited by a high volume manufacturing technique, blade-coating, that achieves greater than 9.5 {\%} power conversion efficiency (PCE). The average crystal domain orientation and characteristic phase separation length distribution are markedly different when deposited by blade-coating rather than spin-coating,. This result allows us to determine which aspects of morphology are not relevant to the PCE of this system. Whether the crystallites are ``face on'' or ``edge on'' does not appear to impact the PCE of system, nor does the length scale or ``hierarchical'' nature of the phase length scale. Persistent morphological qualities that may be associated with high PCE in this system are relatively pure phases and relatively strong diffraction. We posit that OPV systems in which the PCE is less sensitive to morphology may also be less sensitive to film thickness, enabling some to maintain high PCE in active layers thicker than greater than $\approx $200 nm. We confirm that blade-coating is a suitable prototyping technique for R2R coating by demonstrating nominally identical morphologies for both piece blade-coating and continuous-web, slot-die coating. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R33.00006: Contorted hexabenzocoronene derivatives enable fullerene-free, semi-transparent solar cells with record-breaking single-junction photovoltage Nicholas Davy, Melda Sezen, Yueh-Lin Loo Recent work on tuning the chemical structure of contorted hexabenzocoronene (cHBC) in our group has yielded derivatives with a spectrum of energy levels and absorption profiles, greatly improving the utility of these materials as donor and/or acceptor constituents in organic solar cells. Here, we report planar-heterojunction solar cells comprising an extended heterocyclic cHBC donor and a halogenated cHBC acceptor. By harvesting primarily near-UV light, these devices exhibit a record open-circuit voltage of 1.5 V; this value is higher than any previously reported value for a single-junction organic solar cell. Our active layers are molecularly smooth and pinhole-free; these devices should be scalable to large areas without incurring substantial loss to performance. With a transmittance of 79{\%} across the visible, our devices can be vertically integrated to directly drive the switching of electrochromic windows, where existing prototypes depend on tandem solar cells having near-infrared absorbers. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R33.00007: Layer-by-layer fabrication of supramolecular dyes on TiO$_{\mathrm{2}}$ surfaces for optoelectronic applications Xiaoqing Kong, Shawn Maguire, Diane Lye, Marcus Weck, Stephanie Lee We present a modular layer-by-layer approach based on metal coordination chemistry to assemble supramolecular dyes exhibiting increased absorption in the visible range on electrode surfaces. Specifically, palladiated bis-pincer complexes (Pd-BPCs) were employed as linkers between pyridyl-terminated organic molecules via dative bonding. By alternately immersing mesoporous TiO$_{\mathrm{2}}$-coated glass substrates in solutions containing dissolved zinc porphyin (ZnP) and Pd-BPCs, supramolecular dyes were assembled layer-by-layer on the TiO$_{\mathrm{2}}$ surfaces. UV-visible absorption spectra of these assembled structures revealed a linear increase in the Soret and Q bands of ZnP after each immersion of the substrate in the ZnP solution. Coordination of the ZnP layers with Pd-BPC resulted in a slight red shift (\textless 10 nm) of the absorption bands. The modular nature of the assembly process afforded the incorporation of other pyridyl-terminated organic molecules in specific layers of the supramolecular assemblies. By assembling unique organic dyes that absorb different wavelengths of light, we expect to expand light absorption across the visible region of the solar spectrum for solar cell applications. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R33.00008: Surprising increase in photostability of organic amorphous materials by efficient molecular packing Yue Qiu, Lucas Antony, Juan de Pablo, Mark Ediger Photochemically robust materials are desired for organic electronics. Previous work has demonstrated that crystal packing can strongly influence photochemical reactivity. In amorphous materials, however, similar efforts to tune photostability have not been successful. In this work, we show that organic glasses prepared by physical vapor deposition can be highly stable against photo-isomerization. Disperse orange 37 (DO37), an azobenzene derivative, is studied as a model molecule. The thickness and molecular orientation of DO37 thin films can be altered by the photo-isomerization reaction. We use spectroscopic ellipsometry to measure sample thickness and molecular orientation during light irradiation. By changing the substrate temperature during the deposition, photostability can increase 2 to 3 orders of magnitude relative to the liquid-cooled glass. We find that photostability correlates with density of packing, with density increases of up to 1.3\%. Simulations also show that glasses with higher density can be significantly more photo-stable. These results show for the first time that photostability of glasses can be significantly modulated by molecular packing. And they may provide insight in designing organic photovoltaics and light emission devices with longer lifetimes. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R33.00009: Fixed Junction Photovoltaic Devices Based On Polymerizable Ionic Liquids Austin Limanek, Dr. Janelle Leger Recently, polymer-based photovoltaic devices (PPVs) have received significant attention as a possible affordable, large area and flexible solar energy technology. In particular, research on chemically fixed p-i-n junctions in polymer photovoltaic devices has shown promising results. These devices are composed of ionic monomers in a polymer matrix sandwiched between two electrodes. When a potential is applied, the ionic monomers migrate towards their corresponding electrodes, enabling electrochemical doping of the polymer. This leads to the formation of bonds between the polymer and ionic monomers, resulting in the formation of a chemically fixed p-i-n junction. However, early devices suffered from long charging times and low overall response. This has been attributed to the low phase compatibility between the ionic monomers and the polymer. It has been shown for light-emitting electrochemical cells, replacing the ionic monomers with polymerizable ionic liquids (PILs) mitigates these challenges. We will present the use of PILs as the dopant in fixed junction PPV devices. Preliminary devices demonstrate significantly improved performance, decreased charging times, and high open circuit voltages. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R33.00010: Physical and electrical models for interpreting AC and DC transport measurements in polymer solar cells Max McIntyre, Marian Tzolov, Raquel Cossel, Seth Peeler We have fabricated and studied bulk heterojunction solar cells using a mixture of the low bandgap material PCPDTBT and PCBM-C60. Our transport studies show that the devices in dark have good rectification and they respond to AC voltage as a simple RC circuit. The illumination causes an additional contribution to the impedance, which varies with the level of illumination. One proposed model is that photo-generated charges can become trapped in potential wells. These charges then follow a Debye relaxation process, which contributes to a varying dielectric constant. Another proposed model is based on a RC circuit model with two capacitors which can describe the varying capacitance behavior. The physical mechanism for this model is that photo-generated charges become accumulated at the interface between PCPDTBT and PCBM-C60 and form an additional layer of charge. We will show that our circuit models and their analogous physical models can predict the AC and DC responses of polymer solar cells. [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R33.00011: Dye-Sensitized Carbon Nano-Yarn Based Photovoltaic Cells with Enhanced Electron-Hole Separation and Barrier Characteristics H. Justin Moore, Miguel Leal, Glenn Grissom, Tarek Trad, Nazmul Islam, Ahmed Touhami, M. Jasim Uddin Over the last 30 years dye-sensitized solar cells have received considerable interest as an alternative energy source due to their low-cost, environmental sustainability, flexibility, and an abundant number of other practical applications. Flexible carbon nanotube-yarn based photo voltaic cells have shown considerable advantages over metal wire based solar cells or non-flexible substrates like indium-doped tin oxide glass. Carbon nanotubes are superior for photo voltaic cells due to their lower electrical resistance, excellent electrocatalytic activity, and high mechanical integrity. Here, we introduce the use of poly(3-hexylthiophene-2,5-diyl), [6.6] diphenyl C$_{62}$ bis(butyric acid methyl ester), cadmium sulfide-cadmium selenide quantum dots, and ruthenium-based dye N719 to locally increase electron generation, decrease electron-hole pair recombination, as well as enhancing barrier characteristics. Our prototype 3-dimensional carbon nano-yarn based photovoltaic cells show an enhancement in photon to energy conversion efficiency (\textgreater 6.5{\%}). This along with prolonged environmental stability makes for a very promising solar cell. [Preview Abstract] |
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