Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B25: Focus Session: OPV Device Physics and Charge Transport |
Hide Abstracts |
Sponsoring Units: GERA Chair: Dana Olson, National Renewable Energy Laboratory Room: 503 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B25.00001: Is there any Exciton (bottleneck) in an Excitonic Solar Cell: Revisiting the Prospects of Single-Semiconductor OPV Invited Speaker: Muhammad Alam The discovery dye sensitized and bulk heterojunction (BHJ) solar cells in early 1990s introduced a new class of PV technology that rely on (i) distributed photogeneration of excitons, (ii) dissociation of excitons into free carriers by the heterojunction between two organic semiconductors (OSC), and (iii) collection of free carriers through electron and hole transport layers. The success of the approach is undisputed: the highest efficiency OPV cells have all relied on variants of BHJ approach. Yet, three concerns related to the use of a pair of OSCs, namely, low Voc, process sensitivity, and reliability, suggest that the technology may never achieve efficiency-variability-reliability metrics comparable to inorganic solar cells. This encourages a reconsideration of the prospects of Single semiconductor OPV (SS-OPV), a system presumably doomed by the exciton bottleneck. In this talk, we use an inverted SS-OPV to demonstrate how the historical SS-OPV experiments may have been misinterpreted. No one disputes the signature of excitons in polymer under narrowband excitation, but our experiments show that exciton dissociation need not be a bottleneck for OPV under broadband solar illumination. We demonstrate that an alternate collection-limited theory consistently interprets the classical and new experiments, resolves puzzles such as efficiency loss with increasing light intensity, and voltage-dependent reverse photo-current, etc. The theory and experiments suggest a new ``perovskite-like'' strategy to efficiency-variability-reliability of organic solar cells.\\[4pt] In collaboration with Biswajit Ray, Purdue University. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B25.00002: Power conversion efficiency enhancement in OPV devices using spin 1/2 molecular additives Tek Basel, Valy Vardeny, Luping Yu We investigated the power conversion efficiency of bulk heterojunction OPV cells based on the low bandgap polymer PTB7, blend with C61-PCBM. We also employed the technique of photo-induced absorption, PA; electrical and magneto-PA (MPA) techniques to understand the details of the photocurrent generation process in this blend. We found that spin 1/2 molecular additives, such as Galvinoxyl (Gxl) radicals dramatically enhance the cell efficiency; we obtained 20{\%} increase in photocurrent upon Gxl doping with 2{\%} weight. We explain our finding by the ability of the spin 1/2 radicals to interfere with the known major loss mechanism in the cell due to recombination of charge transfer exciton at the D-A interface via triplet excitons in the polymer donors. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B25.00003: Effect of Copolymer Chain Architecture on Active Layer Morphology and Device Performance Jojo Amonoo, Anton Li, Matthew Sykes, Bingyuan Huang, Edmund Palermo, Anne McNeil, Max Shtein, Peter Green The optimum morphological structure that determines the device performance of bulk heterojunction thin film polymer solar cells is greatly influenced by the extent of phase separation between the polymer and fullerene components, which ultimately defines the length scales and purity of the donor- and acceptor-rich phases. Block copolymer thin films have been widely studied for their ability to microphase separate into well-defined nanostructures. Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined $\pi $-conjugated copolymers of poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) to achieve diblock, random and gradient copolymer chain architectures. This allowed us to study the effect of copolymer sequence and nanoscale morphology of P3HT-P3HS copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PC$_{\mathrm{61}}$BM) on device performance. With the use of energy-filtered transmission electron microscopy and conductive and photoconductive atomic force microscopy we found that copolymer sequence strongly influences the phase separation capabilities of the copolymer-fullerene blend in bulk heterojunction organic photovoltaic devices. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B25.00004: Computational design of co-polymer electron donors for bulk heterojunction photovoltaic solar cells Yongwoo Shin, Jiakai Liu, Xi Lin In this work, our recently developed adapted Su-Schrieffer-Heeger Hamiltonian is used to systematically explore the optoelectronic properties of thousands of pi-conjugated structures. New physical insights on the structure-property relationship are extracted and transformed into practical guiding rules in the donor materials design. For the power-efficient copolymer structures, we find that the energy variation of frontier orbitals can be controlled either independently or collectively, depending on their specific donor or acceptor structures. In particular, we find that having five-membered conjugated carbon rings in the acceptor units is essential to break the electron-hole charge conjugation symmetry, so that the LUMO levels of the copolymer can be reduced dramatically while holding the HOMO energy levels in the donor units constant. On the other hand, by incorporating heteroatoms into the donors units, we can vary the HOMO levels of the copolymers independently. Effects of introducing various side groups (-R, -O, -CO, -COO, and thiophene) on the primitive donor and acceptor structures are investigated and their results are discussed in details. Finally, unexpected localized states are found, for the first time, in our calculations for a few special co-polymer structures. These localized states, with electrons localized on one end of the copolymer chain and holes on the other end, contain large dipole moments and therefore may be treated as a new design dimension when these copolymers are placed in polar and non-polar solvent environments. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B25.00005: Measuring and Modeling Exciton Dynamics in Multichromophore Macromolecules Daniel Weingarten, Nan Hu, Michael LaCount, Andrew Ferguson, Daniel Dessau, David Walba, Jao VanDeLagemaat, Mark Lusk, Garry Rumbles, Sean Shaheen Attaining specific control over the dynamics of exciton movement in organic photovoltaics (OPV) has, thus far, been a largely unachieved goal of OPV design. Such an understanding of exciton transfer dynamics would allow for the design of macromolecules whose energetics, bandgaps, and conformational properties allow for control of exciton flow toward specific reaction site chromophores, potentially enabling non-linear improvements in energy harvesting. To better understand exciton movement we synthesized and characterized a multi-chromophoric macromolecule and measured the dynamics of exciton transfer across coupled chromophores. Our model system is a hexabenzocoronene molecule attached to six oligothiophene. We developed a kinetic model and by fitting it to the decay rates of excited states measured via time-correlated single photon counting, we were able to extract rates for exciton transfer between chromophores. Since this macromolecule exhibits liquid crystalline aggregation behavior, observing the dependence of exciton transfer rates on solution concentration yields an improved understanding of exciton movement within a single molecule as well as the dependence of that transfer process on local material structure. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B25.00006: Design principle for efficient charge separation at the donor-acceptor interface for high performance organic solar cell device Wanyi Nie, Gautam Gupta, Brian Crone, Hsing-lin Wang, Aditya Mohite The performance of donor (D) /acceptor (A) structure based organic electronic devices, such as solar cell, light emitting devices etc., relays on the charge transfer process at the interface dramatically. In organic solar cell, the photo-induced electron-hole pair is tightly bonded and will form a charge transfer (CT) state at the D/A interface after dissociation. There is a large chance for them to recombine through CT state and thus is a major loss that limit the overall performance. Here, we report three different strategies that allow us to completely suppress the exciplex (or charge transfer state) recombination between any D/A system. We observe that the photocurrent increases by 300{\%} and the power conversion efficiency increases by 4-5 times simply by inserting a spacer layer in the form of an a) insulator b) Oliogomer or using a c) heavy atom at the donor-acceptor interface in a P3HT/C60 bilayer device. By using those different functional mono layers, we successfully suppressed the exciplex recombination in evidence of increased photocurrent and open circuit voltage. Moreover, these strategies are applicable universally to any donor-acceptor interface. And we demonstrated such strategies in a bulk-heterojunction device which improved the power conversion efficiency from 3.5{\%} up to 4.6{\%}. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:27PM |
B25.00007: Understanding and Mitigating Recombination Loss in Organic Solar Cells Invited Speaker: David Ginger We study recombination losses in organic photovoltaics occurring at both the donor/acceptor interface within a bulk heterojunction, as well as at the electrode contacts. In the bulk, we discuss how the interplay of energetics and morphology, particularly the size and connectivity of domains, can alter the ratio of free carriers to triplets. Turning to the active layer/electrode interface, we also show how surface fields generated at interfaces can be used to modulate recombination rates, and probe the role of structural heterogeneity in recombination at interfaces. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B25.00008: Small Molecule Acceptors for Organic Photovoltaics David P. Ostrowski, Unsal Koldemir, Alan Sellinger, Sean E. Shaheen Organic photovoltiacs (OPVs) have demonstrated solar power conversion efficiencies in the regime of 10-12{\%} from several classes of materials, including conjugated polymers and small-molecules. Of note, in each of the classes, the electron-accepting molecule is based on C$_{\mathrm{60}}$. While C$_{\mathrm{60\thinspace }}$is a very effective electron-acceptor and transporter, it has low optical absorption strength in the solar spectrum and it is difficult to tune its optoelectronic properties. Here we present results on small molecule acceptors based on a core unit of benzothiodiazole (BT) whose optoelectronic properties are readily tunable. A library of these small molecule acceptors has been synthesized with a variety of absorbance bands in order for OPV technologies to harness a greater amount of the solar spectrum. Through utilization of a range of solvents, co-solvent mixtures and orthogonal solvents, devices are fabricated with contrasting bulk heterojunction (BHJ) morphologies or bi-layer architectures. Device performance is compared over a range of active layer morphologies, with particular emphasis on the effectiveness of photocurrent generation when light is absorbed in the acceptor molecule with subsequent charge (hole) transfer to the donor (channel 2 photocurrent generation). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B25.00009: The influence of Morphology on Charge Transport Properties in P3HT Alex Dixon, Nikos Kopidakis, Sean Shaheen The field of Organic Photovoltaic (OPV) has been growing quickly, yet there are still several questions about the underlying physics that remain poorly understood. One question is the nature of the relationship between active layer microstructure and charge transport. To investigate this, we fabricated devices using a range of molecular weights (from 13kDa to 331kDa) of poly 3-hexothyophene (P3HT). Varying the molecular weight of P3HT causes the films to exhibit changes in microstructure, with low molecular weights forming a paraffinic-like structure and higher molecular weights forming a semi-crystalline structure. Using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique, we determined the hole mobility and recombination factor for theses devices. We found that the mobility in the devices peaked at 47kDa and the recombination rate decreased with increasing molecular weight. We hypothesize that the decrease in recombination is due to spacial separation of charge carrier in the semi-crystalline regions, with the holes populating the crystalline regions and the electrons populating the amorphous areas. This improves mobility for mid rage molecular weights but defects cause it to dip at higher molecular weights. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B25.00010: Ionically Gated Hybrid Tandem Organic Photovoltaics Alexander Cook, Jonathan Yuen, Joeseph Michelli, Anvar Zakhiov In our work, `Electrochemically gated organic photovoltaics with tunable carbon nanotube electrodes', recently published in Applied Physics Letters, and our previous APS presentation, we demonstrated a hybrid device comprised of an organic photovoltaic (OPV) monolithically attached to a supercapacitor via a common transparent carbon nanotube (CNT) electrode. This structure may also be viewed as an electrochemically gated OPV in which the gate voltage gradually shifts a resistor-like device into a high efficiency photovoltaic. We have extended this concept to an electrochemically-gated, parallel tandem, organic photovoltaic device, which features two photoactive layers in addition to the supercapacitive cell. This device can be produced entirely in ambient conditions via spin-coating and lamination, and avoids many processing difficulties associated with tandem architectures. Additionally, this architecture allows us to perform experiments to better understand the undelaying phenomena in this system such as whether the electrochemical charging in the OPV device extends to the semiconducting photoactive layers or is constrained entirely to the carbon nanotube electrodes. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B25.00011: Study of free carrier recombination in carbon nanotubes photovoltaic materials Meng-Yin Wu, Thomas McDonough, Matthew Shea, Yumin Ye, Martin Zanni, Michael Arnold Semiconducting single-walled carbon nanotubes are promising photoabsorbers for future solar cells and photodetectors. Previously, we have put nanotubes in contact with electronegative accepting semiconductors such as C60-fullerenes, which spontaneously drive photoexcited electron transfer from the nanotubes to the C60. One important part of making efficient CNT/C60 photovoltaics is that the free carrier lifetime has to be longer than the charge collection time. Here, we analyze CNT/C60 planar heterojunction devices for simplicity to study the free carrier recombination lifetime. We use three measurements: photocurrent-voltage characteristics, photovoltage decay following transient optical excitation, and simple charge extraction to determine how charge density and lifetime are interrelated and how they vary under different illumination intensities. These dependencies also allow us to uncover recombination mechanisms. The results showed the free carrier lifetime decays from 130 us to 280 ns as the free carrier density increases from 3.4E13 cm$^{\mathrm{-3}}$ to 1.3E17 cm$^{\mathrm{-3}}$, with a power law dependence. These measurements of free carrier lifetime and concentration will help us in future investigations and modeling of the C60-nanotube heterointerface. [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