Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D41: Focus Session: Polymers for Solar Energy Conversion - Morphological Impacts |
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Sponsoring Units: DPOLY Chair: Bryan Boudouris, Purdue University Room: 214A |
Monday, March 2, 2015 2:30PM - 3:06PM |
D41.00001: Fully conjugated block copolymers for organic photovoltaics Invited Speaker: Enrique Gomez Weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. Our approach has focused on utilizing block copolymer architectures --where critical interfaces are controlled and stabilized by covalent bonds-- to provide the hierarchical structure needed for high-performance organic electronics from self-assembled soft materials. For example, we have demonstrated control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers to achieve 3{\%} power conversion efficiencies in non-fullerene photovoltaics. Characterization through X-ray scattering and electron microscopy reveals that the efficient performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Furthermore, incorporating the donor-acceptor interface within the molecular structure facilitates studies of charge transfer processes, where we can systematically modulate the chemical structure and energetics to perturb exciton dissociation and charge recombination. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D41.00002: Computational Description of Donor-Acceptor pi-Conjugated Materials for Organic Photovoltaics Applications Jean-Luc Bredas This presentation will focus on the following topics, related to organic solar cell applications: 1) We will describe our recent work on the electronic structure and local morphology of donor-acceptor interfaces in bulk-heterojunction solar cells. 2) We will discuss how we can evaluate the polarization effects, which play a critical role in the charge-separation process at donor-acceptor interfaces in organic solar cells. 3) Finally, we will describe the peculiarities of the electronic structure of a new pi-conjugated polymer with very high hole mobilities over 20 cm2/Vs. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D41.00003: New Insight into Morphology of High Performance BHJ Photovoltaics Using High Resolution AFM Feng Liu, Dong Wang, Ken Nakajima, Thomas Russell Direct imaging of the bulk BHJ thin film morphology in OPV is essential to understand device function and optimize efficiency. While transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy in the tapping and nano-mechanical modes to investigate the BHJ active layer morphology which, when combined with Ar ion etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a face-on orientation, and the morphology at the anode interface was markedly different. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D41.00004: Creating Efficient Quasi-3D Transport Pathways With Crossed-Chain Polymer Interfaces Christopher Takacs, Michael Brady, Neil Treat, Michael Chabinyc While our understanding of the local molecular packing in many well-performing polymer semiconductors has improved, many open questions regarding the molecular level details of long-range connectivity and the best strategies for optimizing electronic functionality remain. Here we focus on the possible benefits of epitaxy in polymer semiconductors for improving nano-scale connectivity, a concept particularly useful for systems where charge-transport is expected to be highly anisotropic. The periodic crossing of the non-parallel chains at the crystal-crystal interfaces may enable efficient coupling across grain-boundaries and increase the effective dimensionality of the charge transport processes. Using a combination of high-resolution transmission electron microscopy, statistical analysis of the electron micrographs, and a variety of molecular simulation methods, we will demonstrate that such epitaxy relationships can be predicted and observed in a variety of well-performing polymer semiconductors. The results suggest that further engineering of epitaxy may lead to substantial advances in both control of self-assembly and electronic performance. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D41.00005: Small Molecule Bulk Heterojuction: Impact of two thermodynamically stable morphologies on the efficiency of organic photovoltaics devices Nuradhika Herath, Valeria Lauter, Jim Browning, IlIa Ivanov, Jong Keum, Kai Xiao, Jiahua Zhu, Sanjib Das, Gong Gu Structural characteristics of device active layers play a critical role in charge generation, separation and transport in organic photovoltaics (OPVs). Here we report on morphology and structure control of $p$-DTS(FBTTh$_{\mathrm{2}})_{\mathrm{2}}$:PC$_{\mathrm{71}}$BM films by means of thermal annealing and solvent additive processing. \textit{Depth-sensitive} neutron reflectometry and X-ray diffraction are employed to characterize the thin film structures, and are correlated to the device performance. The neutron reflectometry results reveal that $p$-DTS(FBTTh$_{\mathrm{2}})_{\mathrm{2}}$:PC$_{\mathrm{71}}$BM films fabricated with 1,8-diiodooctane (DIO) reproducibly exhibit a 3-sublayer morphology similar to thermally annealed films without DIO. Moreover, DIO promotes the formation of a large population of $p$-DTS(FBTTh$_{\mathrm{2}})_{\mathrm{2}}$ nanocrystals leading to a device efficiency (PCE) of 5.9 {\%}. The thermally annealing generate $p$-DTS(FBTTh$_{\mathrm{2}})_{\mathrm{2}}$ crystallites one order of magnitude larger than those formed with the DIO. Our results provide direct evidence that delicate size control of crystal domains closer to the optimum exciton diffusion length is as important as a high crystallinity of charge transport layers. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D41.00006: Vertical Phase Separation in Bulk-Heterojunction Polymer Solar Cells Yueh-Lin Loo, He Wang, Jongbok Kim With soft-contact lamination and delamination, we have elucidated whether and how vertical phase separation of active layers effects solar cell performance. We constructed conventional bulk-heterojunction solar cells comprising P3HT, PCPDTBT and 7,7$\prime $-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b$\prime $]dithiophene-2,6-diyl)bis(6-fluoro-4-(5$\prime $-hexyl-[2,2$\prime $-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), or T1, with fullerene derivatives. To exaggerate the influence of vertical phase separation, we laminated in each case a thin layer of electron donor on the bulk-heterojunction active layer prior to cathode deposition. While devices with PCPDTBT and T1 exhibit drastically reduced current compared to devices without the additional wetting layer, devices with P3HT exhibit only a marginal drop in its current compared to devices without a P3HT wetting layer. We ascribe this difference in the reduction of current to intrinsic differences in the ionization potential and tail state distribution of the electron donors. Relative to PCPDTBT and T1, P3HT has a HOMO energy level closer to vacuum and a substantially broader tail state distribution. Against the energy levels of fullerene, both these factors support electron-hole recombination, followed by electron injection from the cathode into the tail states of P3HT under device operation. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D41.00007: Morphology and performance of organic photovoltaics containing a small-molecule acceptor Kathryn O'Hara, David Ostrowski, Christopher Takacs, Unsal Koldemir, Sean Shaheen, Alan Sellinger, Michael Chabinyc Fullerene derivatives are widely used as acceptor materials in organic photovoltaics (OPVs). However, they have a high cost, low absorption in the visible range and limited synthetic variability compared to small molecule alternatives, which generally underperform PCBM, but it is unclear if the reason is morphological or due to the electronic structure of the acceptor. A promising fullerene alternative, HPI-BT, is blended with P3HT to achieve a power conversion efficiency (PCE) of 2.1{\%}, which is lower than for fullerene OPVs (10{\%}), but an understanding of the morphology could improve the efficiency of future small-molecule based devices. The active layer microstructure is probed complimentary techniques of atomic force microscopy (AFM), grazing incidence wide angle x-ray scattering (GIWAXS), and scanning transmission electron microscopy (STEM). STEM indicates that HPI-BT crystals are buried in the film upon casting and AFM shows they grow to the film surface upon annealing. GIWAXS reveals the acceptor is crystalline, which should improve charge transport, but film texturing suggests that crystals nucleate off the substrate, cover the anode and ultimately limit cell performance. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D41.00008: Direct internuclear distance measurements of P3HT/PCBM interfaces in bulk heterojunction thin films using $^{13}$C \textbraceleft $^{2}$H\textbraceright REDOR NMR Ryan Nieuwendaal, Dean Delonghcamp, Alex Sieval, J.C. Hummelen, Martin Heeney, Zhuping Fei Robust structure/function relationships are generally lacking in organic photovoltaic (OPV) thin film active layers. Structural complexity has contributed to a lack of performance predictability, so there exists a need for measurement tools that can unveil fine details of the bulk heterojunction (BHJ) thin film structure. Optical methods, microscopy (AFM, TEM), and scattering techniques are useful for coarse morphological assessment, but their lack of sub-nm spatial resolution has obscured perhaps the most pertinent morphological quality of the BHJ: the donor/acceptor interface. In this contribution, I will discuss the results of solid state NMR measurements performed in our laboratory to characterize the donor/acceptor interface. $^{13}$C \textbraceleft $^{2}$H\textbraceright REDOR experiments on isotopically-enriched thin film blends are used to determine distances between nuclei in the donor molecules ($^{2}$H on P3HT main chain) and acceptor molecules ($^{13}$C-enriched C$_{60}$ cage). Experiments are performed on $\sim$ 100 nm thin film samples utilizing casting recipes that are typical for fabricating real OPV devices. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D41.00009: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 4:42PM - 4:54PM |
D41.00010: Two-Dimensional Effects on Lateral Organic Bulk Heterojunction Devices Kelly Liang, Eric Danielson, Zien Ooi, Ananth Dodabalapur For moderately thick (50 nm) lateral organic bulk heterojunction (OBHJ) photovoltaic devices, a one-dimensional space charge limited current model with electric field independent mobility accurately simulates key device metrics, like photocurrents, mobilities, and bimolecular recombination coefficients. However, as the thickness of these lateral organic devices approach thinner and thicker limits, two-dimensional effects greatly influence device characteristics, and a one-dimensional approximation is no longer sufficient. Both the two-dimensional electric field spreading in the OBHJ semiconductor and the electric field dependent mobility of the material system need to be considered in these devices. Introducing a geometric prefactor--dependent on the thickness of the organic semiconductor, to the one-dimensional space charge limited model accounts and adjusts for the two-dimensional effects. Using this modified and more accurate model, we further examined the photoconduction and photoconductive gain in lateral OBHJ devices and reaffirmed that high gains originate from contact injection rather than bulk photoconduction. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D41.00011: High-Performance All-Polymer Solar Cells Based on Face-on Stacked Polymer Blends with Low Interfacial Tension Bumjoon Kim, Hyunbum Kang, Changyeon Lee, Taesu Kim We report highly-efficient all-polymer solar cells with power conversion efficiencies of over 4.5{\%} by highly-intermixed blends of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-$b$:4,5-$b$']dithiophene-\textit{alt}-3-fluorothieno[3,4-$b$]thiophene-2-carboxylate] (PTB7-Th) donor and poly[[$N,N'$-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-\textit{alt}-5,5'-(2,2'-bithiophene)] P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on $\pi $-$\pi $ stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency and $J_{SC}$ values. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D41.00012: All-polymer photovoltaics: Correlating Efficiency and Morphology Yan Jin, Jong K. Keum, Kunlun Hong, James F. Browning, Gregory S. Smith, Vikram K. Kuppa We have recently demonstrated how the efficiency of devices fabricated from a blend of the polymers P3HT and F8BT increases three-fold by incorporating pristine graphene into the active layer. The fundamental mechanisms underlying this enhancement are investigated, and are shown to arise from improvements in both charge transport and morphology. We investigate the structure via small angle neutron scattering (SANS) studies of the deuterated-P3HT/F8BT system with and without graphene. SANS reveals the existence of disk-like P3HT crystallites distributed in an amorphous miscible blend of P3HT and F8BT. P3HT crystallinity was enhanced upon graphene addition, resulting in larger crystallites and a higher degree of ordering. These structural changes are accompanied by better charge transport, resulting in a peak improvement of over 200{\%} in the short-circuit current of the devices. Results on cell characteristics and recombination mechanisms are also reported, and indicate means of addressing fundamental problems in OPV systems. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D41.00013: Role of Molecular Linker on Charge Separation and Photovoltaic Performance in All-Conjugated Block Copolymers Jorge Wu Mok, Yen-Hao Lin, Kendall Smith, Rafael Verduzco Recent studies have demonstrated the potential of all-conjugated donor-acceptor block copolymer for organic photovoltaics, but it remains unclear how~molecular structure, morphology, and electronic properties of conjugated block copolymers influence performance. Here, we study the role of chemical linker between donor and acceptor polymers on photovoltaic performance and optoelectronic properties. Two poly(3-hexylthiophene)-poly(2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl-(9,9-dioctylfluorene)) (P3HT-PTBTF) donor-acceptor block copolymers which differ only in the chemistry of linking group are studied through device measurements, GIXS, and steady-state and time-resolved absorbance and photoluminescence. Device studies show that power conversion efficiencies decrease by one order of magnitude by changing the linking group. X-ray analysis shows that the morphology is virtually identical in both samples, as expected. Transient absorption measurements reveal charge separation in block copolymers which contain a wide~bandgap monomer at the donor-acceptor interface, but charge separation is suppressed when donor and acceptor blocks are~directly linked without this spacer present. This work demonstrates that the linking group~chemistry influences charge separation in all-conjugated block copolymer systems, and also suggests that all-conjugated block copolymers can be used as model systems for the donor-acceptor interface in bulk heterojunction blends. [Preview Abstract] |
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