Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U33: Focus Session: Organic Electronics and Photonics - Organic Photovoltaics I - Theory and Processing |
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Sponsoring Units: DMP Chair: Michael Chabinyc, University of California at Santa Barbara Room: 341 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U33.00001: David Adler Lectureship Award in the Field of Materials Physics Lecture Invited Speaker: Jean-Luc Bredas We first review the current state-of-the-art in the field of organic electronics and then focus on organic solar cells, which we define as solid-state cells in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules. We describe the optical and electronic processes that take place in such cells and turn our attention briefly to: (i) optical absorption and exciton formation; (ii) exciton migration to the electron donor -- electron acceptor interface; (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor component and electrons in the acceptor component; (iv) charge carrier mobility; and (v) charge collection at the electrodes [1-3]. In the second part of the presentation, we underline the complexity of the processes taking place at the nanoscale at the donor/acceptor interfaces and highlight the molecular understanding that comes from a computational approach combining electronic-structure theory calculations, molecular mechanics / molecular dynamics simulations, and Monte Carlo simulations [4-6].\\[4pt] References:\\[0pt] [1] B. Kippelen and J.L. Bredas , Energy {\&} Environmental Science \underline {2}, 251 (2009).\\[0pt] [2] J.L. Bredas, J. Norton, J. Cornil, and V. Coropceanu, Accounts of Chemical Research \underline {42}, 1691 (2009).\\[0pt] [3] Y. Zhou \textit{et al.}, Science \underline {336}, 327 (2012).\\[0pt] [4] N.C. Miller \textit{et al.}, Advanced Materials, 2012 (DOI: 10.1002/adma.201202293).\\[0pt] [5] N.C. Miller \textit{et al.}, Advanced Energy Materials, 2012 (DOI: 10.1002/aenm.201200392).\\[0pt] [6] Y.T. Fu, C. Risko, and J.L. Bredas, Advanced Materials, 2012 (DOI: 10.1002/adma.2012 03412). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U33.00002: An ab initio approach to organic photovoltaics Vincent Gosselin, Nicolas B\'erub\'e, Josiane Gaudreau, Michel C\^ot\'e Within the recent years, we have witnessed continual improvements in the Power Conversion Efficiencies (PCE) of organic photovoltaic devices. These improvements have been achieved by the discovery of new polymers which are being syntesised and their performance assessed experimentally. Scharber has introduced a simple model which determines the desired properties of polymers in order to achieve high PCE. An appealing alternative to the lengthy process of polymer synthesis consists in using ab initio calculations in order to predict the electronic structure of polymer candidates and evaluate the relevant properties in the determination of their PCE. In this work, Density Functional Theory (DFT) is being used to compute the optical band gap and HOMO / LUMO levels which, in conjunction with Scharber's model, allows to predict the efficiency of various polymer - fullerene blends. In order to assess the quality of such calculations and the validity of the model, we first compare the predictions with experimental device performances. We find that the model offers an indication as to what one should expect in terms of the maximum efficiency attainable experimentally. Lastly, we present new unsynthesised polymers which have shown promising results within this framework. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U33.00003: First principles modeling of donor materials for organic solar cells: where theory complements experiment Andriy Zhugayevych, Sergei Tretiak, Guillermo Bazan We discuss the predictive power and accuracy of first principles modeling of small-molecule crystalline donors for organic solar cells. First of all, in order to understand where the theory can help us in improving the performance of photovoltaic devices, we clarify what factors constituting power conversion efficiency needed to be improved. We argue these are short circuit current and fill factor, rather than bandgap and open circuit voltage. This implies that the optimization of intramolecular properties (e.g. HOMO/LUMO), which is best suitable for theoretical search, will not give the anticipated gain in efficiency. The intermolecular properties are amenable to first principles modeling on a single-crystallite scale and we discuss some challenges in this avenue. As an example of how theory can provide design rules for architecturing small-molecule crystals we analyze the dependence of charge carrier mobility on the intermolecular geometry of a pi-stack. In the other case study we show that changes in device performance due to small changes in chemical composition can be well tracked by the theory. Finally, we analyze the performance of commonly used density functionals for typical molecular systems used in organic electronics (oligomers, polymers, dimers, crystals). [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U33.00004: A Dynamic Monte Carlo Model with an Improved Charge Injection Mechanism for the Photocurrent Generation of Organic Solar Cells Dylan Kipp, Venkat Ganesan Previous dynamic Monte Carlo studies have made great strides in connecting organic solar cell device microstructure to final properties. One challenge still remaining is to capture the full illuminated and dark current-voltage curves and their dependencies on the charge injection mechanism. By modifying the injection mechanism of previous algorithms, we have developed an improved model for the simulation of photocurrent generation in organic solar cells. We include and utilize an injection rate prefactor to control the portion of dark current attributed to each of 4 kinds of charge injection. By shifting the dark current between electrode-polymer pairs, the injection timescales are aligned even when modeling ohmic contacts. Using our model, we are able to generate charge density and potential profiles that better agree with theory and better reproduce experimental results as compared to previous dynamic Monte Carlo methods. We are able to demonstrate how charge accumulation and band bending effects the shape and placement of the various current-voltage regimes. Finally, we are able to demonstrate how various parameters influence the current-voltage characteristics. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U33.00005: Computational materials design for bulk heterojunction solar cells Xi Lin, Yongwoo Shin The adapted Su-Schrieffer-Heeger Hamiltonian is further developed in this work to predict the optical bandgaps of more than 200 different $\pi $-conjugated systems. Insights on the structure-property relationship of these $\pi $-conjugated systems lead to guiding rules for new photovoltaic materials design. A copolymer of parallel and perpendicular benzodithiophenes, differing only in sulfur atom locations, is proposed as a candidate to achieve the optimal 1.2 eV donor optical gap for organic photovoltaics. The charge transfer mechanisms and the exciton and charge carrier mobilities are computed and compared for various bulk-heterojunction structures to improve the overall power convention efficiency. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U33.00006: New way of polymer design for organic solar cells using the quinoid structure Nicolas Berube, Josiane Gaudreau, Michel Cote Research in organic photovoltaic applications are receiving a great interest as they offer an environmentally clean and low-cost solution to the world's rising energy needs. Controlling the device's active polymer optical bandgap is an important step that affects its absorption of the solar spectrum, and ultimately, its power conversion efficiency. The use of fused heterocycles that favors the polymer's quinoid structure has been a known method to lower the bandgap, for example, with isothianapthene, but there is a lack of quantifiable data on this effect. Density functional theory (DFT) calculations were done on over 60 polymers with bandgaps between 0.5 eV and 4 eV. They clearly show that low bandgaps are observed in copolymers that carefully stands between their quinoid and aromatic structures. Such balance can be obtained by mixing monomer units with quinoid characteristics with aromatic ones. Time-dependant DFT results also links low bandgaps with lower reorganization energy, which means that polymers with this structural form could possess higher charge mobilities. This link between the geometrical structure and the bandgap is compatible with a vast variety of polymers and is more convincing than the commonly used donor-acceptor method of polymer design. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:27PM |
U33.00007: Morphology-property insights into high-performance organic photovoltaics Invited Speaker: Seth Darling Organic solar cells have attracted increasing attention as potential low-cost alternatives to traditional inorganic photovoltaic (PV) technologies. Additional advantages of OPVs include the use of earth-abundant materials, mechanical flexibility, light weight, rapid energy payback time, and the option for tunable coloring for aesthetic architectural installation. Key to their low-cost is solution-based high-throughput processing. Power conversion efficiency (PCE) of organic photovoltaics (OPVs) has steadily improved, with PTB series polymers exhibiting some of the highest PCEs. Using a suite of advanced characterization techniques, it is possible to decipher the morphology of OPV active layers across length scales from the molecular to the mesoscopic. Correlating these structural features with optoelectronic function leads to morphology-performance relationship insights, which in turn can be utilized as the foundation for a rational design of improved performance in OPV devices. Initial results from this methodology are encouraging, suggesting a viable alternative to the traditional Edisonian approach to device performance improvement. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U33.00008: The impact of miscibility on organic solar cell performance and stability Brian A. Collins, John R. Tumbleston, Jon A. Bartelt, Michael D. McGehee, Christopher R. McNeill, Harald Ade The recent demonstration of molecular miscibility/solubility between polymers and fullerenes [1] has revealed a much more complex picture of nanostructure, charge dynamics, and device stability -- aspects that are all entangled. Here we show that miscibility is important in several ways that depends on the particular material blend. For example, recent absolute measurements on domain size and composition [2] have revealed nanostructure in PTB7:PC$_{71}$BM blends that is controlled by miscibility and that well-mixed regions likely hinder charge separation in this system. On the other hand, PBDTTPD:PC$_{61}$BM blends rely on high levels of mixing for electron percolation [3]. Such evidence leads to a complex interplay between charge separation, electron trapping, and percolation. Miscibility, a thermodynamic parameter, can, furthermore, determine the thermal stability of device active layers, which we show varies widely between materials systems. This suggests tailoring of the molecular interactions between donor and acceptor materials in solar cells may be the key to high-performing, highly stable and, therefore, economically viable organic electronics technologies. [1] B. A. Collins et al., J Phys. Chem. Lett. 1, 3160, (2010). [2] B. A. Collins et al., Adv. Energy Materials DOI: 10.1002/aenm.201200377 [3] J. A. Bartelt et al., Adv. Energy Materials DOI: 10.1002/aenm.201200637 [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U33.00009: An Alternative Processing Strategy for Polymer-Fullerene Organic Photovoltaic Devices Using Supercritical Carbon Dioxide Jojo Amonoo, Emmanouil Glynos, Chelsea Chen, Anton Li, Bong-Gi Kim, Jinsang Kim, Peter Green Bulk heterojunction thin film polymer solar cells based on poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC$_{61}$BM) donor/acceptor blends have received extensive attention in recent years. Well-established processing protocols, such as heating to elevated temperatures, have been employed to obtain optimum three-dimensional nano-scale morphologies critical for enhanced device performance. We show for the first time that supercritical carbon dioxide (scCO$_{2})$ processing provides a viable alternative strategy to achieve same or better power conversion efficiencies and short circuit currents compared to high temperature thermal annealing. Furthermore, energy-filtered transmission electron microscopy, and electron energy loss spectroscopy studies show that the same nano-scale morphologies are achieved using scCO$_{2}$, at an optimized temperature and pressure as those achieved using thermal annealing. Photoconductive atomic force microscopy revealed that the higher efficiency devices possessed larger fractions of photoactive regions throughout the active layer. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U33.00010: Optimization of low band gap polymer photovoltaics through structure modification Feng Liu, Yu Gu, Alejandro Briseno, Thomas Russell, Cheng Wang In BHJ-type solar cells, the ability to control and optimize the active layer morphology is a critical issue to improve device efficiency, and this is usually achieved by optimizing the processing conditions, eg. using varied annealing procedures and choosing the right solvent additive. In this work, we shown that device performance of DPP based low band gap polymers should be optimized both in processing and structural optimization approach. Without the use of chemical additive in blended thin film preparation, large size-scaled phase separation, up to several hundred of nanometers exist. This morphology is due to the surface aggregation of phenyl-C71-butyric acid methyl ester (PCBM), which forms large oval structures and then buried by a polymer-PCBM mixture thin film. In this process, the miscibility of polymer matrix plays an important role. While using chemical additive processing method can tune the general morphology to a more fibril network texture, fine-tuning of fibril dimensions and domain size needs delicate chemical structure modification. Through this modification, a 30{\%} device performance enhancement was observed, which mostly came from an enhancement of short circuit current, thus strongly related to the morphological details. Besides conventional morphology characterizations, an initiative effort of understanding the domain interface structure was also carried out by using polarized soft x-ray scattering, in which we observed polymer crystal orientation plays an important role. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U33.00011: Relating Organic Solar Cell Fabrication Methods to Internal Electronic Properties Using Impedance Spectroscopy James Basham, David Gundlach, Thomas Jackson We report on the use of impedance spectroscopy to quantify the effect of processing on an array of important OPV device metrics. Interestingly, extract modeled mobilities over the range of 2x10$^{\mathrm{-3}}$ to 1x10$^{\mathrm{-2}}$ cm$^{\mathrm{2}}$/Vs by changing the spinning recipe. We find fast carrier relaxation times of 1x10$^{\mathrm{-4}}$ s for 3{\%} efficiency cells vs 3x10$^{\mathrm{-6}}$ s for a 1.8{\%} efficiency cell, possibly demonstrating reduced recombination in more efficient devices. Devices made via slowly dried films exhibit repressed recombination compared to quickly dried films. Measurements are taken across a bias range of -1 to 1 volt with illumination intensities spanning .001 to 3 suns, in order to test under conditions which are most relevant to real device operation. Impedance spectra are analyzed through the use of a 5 element compact model based upon the work of Bisquert et al [1,2]. We report an array of device metrics measured via impedance spectroscopy including shunt resistance, effective carrier lifetime, mobility, and capacitance for P3HT:PCBM devices with efficiencies of 3.5{\%} to \textless 1{\%}, fabricated via several common recipes, in an effort to elucidate the varied and complex interplay between processing and device physics, and the overall effect on solar cell efficiency. [1] Fabregat-Santaigo, F., Garcia-Belmonte, G., Mora-Sero, I., and Bisquert, J. Phys. Chem. Chem. Phys., 2011, 13, 9083--9118 [2] Garcia-Belmonte, G.,Boix, P.P., Bisquert, J., Sessolo, M., and Bolink, H.J. Solar Energy Materials {\&} Solar Cells 94(2010)366--375 [Preview Abstract] |
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