Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L21: Focus Session: Polymers for Energy Storage and Conversion II - Photovoltaics |
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Sponsoring Units: DPOLY GERA Chair: Brian Collins, National Institute of Standards and Technology Room: 406 |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L21.00001: Expanded Morphological Paradigm of Polymeric Solar Cells: Contributions by Soft X-ray Methods Invited Speaker: Harald Ade The complex three dimensional morphology of polymeric/organic donor:fullerene bulk heterojunction solar cells and the structure of the discrete and recently inferred dispersed interfaces are critical to performance, yet have been very difficult to study due to a paucity of adequate characterization methods. Recently developed soft X-ray microscopy and scattering tools and methods can provide new avenues and contribute substantially to infer the number of phases present in a device, determine the minimum fullerene content in mixed domains and to provide a quantitative statistical measurement of the composition variations and size distribution. This contributed to the realization that mixed domains are prevalent and rather than just being detrimental can have important beneficial contributions for charge generation and charge transport as such mixed domains represent a special form of hierarchical structures (E.g. [1-3]). Furthermore, polarized x-ray scattering can reveal preferential orientation of the donor polymer/small molecule (edge-on or face-on) relative to the fullerene aggregate interface. Such ordering has previously not been observed nor controlled in fullerene-based solar cells and is shown here to be a critical factor for high performance in a number of systems.\\[4pt] [1] Collins, B. A. \textit{et al.} \textit{Nat. Mater.} \textbf{11}, 1--8 (2012).\\[0pt] [2] Collins, B. A.\textit{ et al.} \textit{Adv. Energy Mater.} \textbf{3}, 65-74 (2013).\\[0pt] [3] Ma, W.\textit{ et al.} \textit{Adv. Energy Mater.}, \textbf{3}, 864 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L21.00002: Understanding the Role of Additives in Improving the Performance of Polymer:Fullerene Bulk Heterojunction Solar Cells Wei Chen Solar cells based on the polymer:fullerene bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last fifteen years, research efforts have led to organic photovoltaic (OPV) devices with power conversion efficiencies (PCEs) $\sim$ 12{\%}, but these values are still insufficient for the devices to become widely marketable. To further improve solar cell performance, a thorough understanding of the complex processing-structure-performance relationships in OPV devices is required. Recently, the use of processing additives have been proved to be one of the most effective methods to tune the nanomorphology of polymer:fullerene active layer, as the incorporation of a small percentage of solvent additives results in a nearly doubling of device efficiency. However, the physics behind these improved performances by processing additives still remains unclear. In this work, by taking advantage of resonant soft x-ray scattering (RSoXS) and energy-filtered transmission electron microscopy (EFTEM), we have determined that the solvent additives induce the change in the formation mechanism of polymer:fullerene nanomorphologies in the process of film casting. Progress established in the course of these studies on structural and morphological characterizations will serve as the foundation for further improving the efficiency of polymer solar cells to realize their large-scale commercial use. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L21.00003: High molecular weight insulating polymers can improve the performance of molecular solar cells Ye Huang, Wen Wen, Edward Kramer, Guillermo Bazan Solution-processed molecular semiconductors for the fabrication of solar cells have emerged as a competitive alternative to their conjugated polymer counterparts, primarily because such materials systems exhibit no batch-to-batch variability, can be purified to a greater extent and offer precisely defined chemical structures. Highest power conversion efficiencies (PCEs) have been achieved through a combination of molecular design and the application of processing methods that optimize the bulk heterojunction (BHJ) morphology. However, one finds that the methods used for controlling structural order, for example the use of high boiling point solvent additives, have been inspired by examination of the conjugated polymer literature. It stands to reason that a different class of morphology modifiers should be sought that address challenges unique to molecular films, including difficulties in obtaining thicker films and avoiding the dewetting of active photovoltaic layers. Here we show that the addition of small quantities of high molecular weight polystyrene (PS) is a very simple to use and economically viable additive that improves PCE. Remarkably, the PS spontaneously accumulates away from the electrodes as separate domains that do not interfere with charge extraction and collection or with the arrangement of the donor and acceptor domains in the BHJ blend. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L21.00004: Suppressing intermolecular charge recombination in photovoltaics through conjugated block copolymer architectures Hao Kuang, Enrique Gomez, Michael Janik Block copolymers have the potential to control the interfacial and mesoscopic morphology of the active layer of organic photovoltaics and consequently enhance device performance. For example, the self-assembly of conjugated block copolymers into periodic microstructures with nanometer length scales could facilitate exciton dissociation by creating large amounts of donor-acceptor interfaces. Furthermore, the interfacial structure may strongly affect charge transfer processes. Using Density Functional Theory, we have examined charge transfer rates in model interfaces of poly(3-hexylthiophene)$-$block$-$poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2$\prime $,2?-diyl) donor-acceptor block copolymers which yield 3{\%} efficient devices when incorporated into solar cells. Our results demonstrate that intermolecular charge recombination can depend on the interfacial breadth, where sharp interfaces (ca. 1 nm) suppress intermolecular charge recombination by orders of magnitude. Furthermore, we compare intramolecular and intermolecular charge transfer rates in donor-acceptor block copolymers through Constrained Density Functional Theory calculations. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L21.00005: Solvent Annealing in Selective Solvents: A Novel Method to Tune the Morphology of Low Band Gap Polymer:Bis-Fullerene Heterojunctions Mark Dadmun, Huipeng Chen, Yu-Che Hsiao, Bin Hu One of the most important challenges facing our society is the development of technologies for renewable energy conversion. Polymeric bulk-heterojunction (BHJ) photovoltaics, based on conjugated polymers and fullerenes, are an economically viable option for low cost renewable power generation. The most promising conjugated polymer:fullerene active layers in organic photovoltaics now utilize low band-gap (LBG) copolymers. Unfortunately, for most of these LBG devices, the as-cast film is not usually optimal, and there are few further treatment available after film deposition to optimize the morphology. To address this problem, we have exploited the selective solubility of the LBG:fullerene nanocomposite components to direct the assembly of these mixtures by annealing in the vapor of a selective solvent. Our recent work demonstrates that annealing in a solvent that is selective to the fullerene forms a sample with fullerene aggregation, while annealing in a solvent vapor that is selective to the polymer forms a thin film with polymer precipitation. There is also a direct correlation between the resultant morphology and OPV performance, increasing PCE by 190{\%}. These results indicate that solvent annealing and solvent choice provides a unique tool to precisely tune the morphology of CP:Fullerene BHJ systems, optimizing the morphology and performance of the active layer. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L21.00006: Using Molecular Simulations to Link Chemical and Physical Features of Conjugated Polymers and Fullerene Derivatives to Bulk Heterojunction Morphology for Organic Photovoltaics Hilary Marsh, Eric Jankowski, Arthi Jayaraman The morphology of blends of conjugated polymers (electron donors) and fullerene derivatives (electron acceptors) strongly affects the charge transport, charge separation and the overall efficiency of organic photovoltaic devices. In this talk we will present coarse-grained molecular simulation studies to understand how molecular-level features such as alkyl side chain length, alkyl side chain spacing along thiophene polymer backbone and fullerene functionalization (and in turn miscibility with the conjugated polymer) affect the blend morphology. Our coarse-grained models are validated by reproducing neat polymer (without acceptors) morphologies observed in experiments, such as lamellae and hexagonally packed cylinders. Furthermore, for blends of conjugated polymers and fullerene derivatives, this work shows how conjugated polymer architecture and acceptor miscibility can be tuned to obtain new blend morphologies with features that are known to enhance efficiency of organic solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L21.00007: Phase Behavior of Polymer Blends for Organic Photovoltaic Applications Jillian Emerson, Eric Furst, Thomas Epps, III Polymer blends offer a promising and economically-viable route to creating organic photovoltaic (OPV) devices, as blends can form bicontinuous domains via spinodal decomposition. Understanding the phase behavior of conjugated polymer blends commonly used in OPVs is vital to producing more efficient devices. In this work, we determined the Flory-Huggins solvent--polymer and polymer--polymer interaction parameters for a model system of poly(3-hexylthiophene) (P3HT) and polystyrene (PS) through solvent vapor swelling of thin polymer films. From these interaction parameters, we constructed a polymer/polymer/solvent phase diagram. The phase diagram was validated experimentally with solution-based transmission measurements of PS/P3HT. This work highlights a method to determine the phase behavior in polymer/polymer/solvent blends that can be extended to other combinations of macromolecules relevant to organic photovoltaics, composites, and other materials systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L21.00008: Solvent-Polarity-Induced Active Layer Morphology Control in Crystalline Diketopyrrolopyrrole-Based Low Band Gap Polymer Photovoltaics Sunzida Ferdous, Feng Liu, Dong Wang, Thomas Russell The effects of various processing solvents on the morphology of diketopyrrolopyrrole (DPP)-based low band gap polymer (PDPPBT) and phenyl-C71-butyric acid methyl ester (PC71BM) blends are studied. The quality of the processing solvents was varied systematically using a mixture of a non-aromatic polar primary solvent with high boiling point secondary solvents of increasing polarities. An unfavorable solvent-PC71BM interaction affects the growth process of polymer crystallites inside the blend. When non-aromatic polar solvent was used, large PC71BM aggregates were formed that increase in size with the addition of non-polar secondary solvents. When polar solvents were instead used as the secondary solvents, the size scales of the aggregates decrease markedly, creating a percolated fibrillar network. Power conversion efficiencies of 0.03{\%} to 5{\%} are obtained, depending on the solvent system used. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L21.00009: Synthesis, Morphology, and Optoelectronic Properties of All-Conjugated Block Copolymers Kendall Smith, Rafael Verduzco, Yen-Hao Lin, Jorge Mok Recent work has demonstrated the potential of all-conjugated block copolymers for solution-processed photovoltaic devices, with power conversion efficiencies near 3{\%}. However, optoelectronic properties and structure-property-processing relationships are poorly understood for this class of materials. Here, we present systematic studies on the processing, morphology, and optoelectronic properties of model all-conjugated block copolymer systems. All-conjugated block copolymer poly(3-dodecylthiophene)-\textit{block}-poly(9,9-dioctylfluorene) (P3DDT-$b$-PF) exhibit simultaneous crystallization of both blocks but no clear evidence of microphase segregation. By contrast, under solvent annealing, poly(3-hexylthiophene) --$b$--poly(9,9-dioctylfluorene) (P3HT-$b$-PF) exhibit lamellar ordering, evidenced by multiple reflections under GIWAXS and GISAXS analysis, including an in-plane reflection indicative of strong $\pi $-$\pi $ stacking for both P3HT and PF blocks. The characteristic lamellar domain spacing (4.2 nm) is found to be independent of block ratio or total molecular weight. Optoelectronic measurements and photovoltaic device results are presented for all-conjugated block copolymers that incorporate ambipolar PFTBT polymer block and high-performance $p$-type PTB7 polymer. These results provide guidelines for optimizing the morphology of all-conjugated block copolymers through materials design and processing. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L21.00010: Improving the performance of All-Polymer Solar Cells Yan Jin, Fei Yu, Vikram Kuppa We find that the power conversion efficiency (PCE) of photovoltaic devices based on conjugated polymer blends is dramatically improved by the addition of small quantities of pristine -unmodified- graphene in the active layer. The graphene is obtained by solvent exfoliation from graphite, and is spin-coated along with the conjugated polymer blend from solution to make cells. The PCE as well as short circuit current (Jsc) show an approximately three-fold increase with increasing graphene concentration. The incorporation of graphene changes the recombination mechanism in such cells from monomolecular (geminate) to bimolecular (non-geminate) recombination, as revealed from current-light intensity studies. In contrast to neat devices, the addition of graphene leads to an increase in the thickness of the active layer, which also influences performance. These investigations reveal three major effects of graphene on polymer blend solar cells: the incorporation of graphene (i) enhances exciton dissociation, (ii) increases the charge transport, and (iii) modifies the polymer morphology. The results demonstrate the potential for graphene in improving OPV performance by addressing poor charge mobilities, which are a fundamental drawback of OPV cells. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L21.00011: Role of Domain Size and Phase Purity on Charge Carrier Density, Mobility and Recombination in P3HT:PC$_{61}$BM Devices Bingyuan Huang, Jojo Amonoo, Anton Li, Chelsea Chen, Peter Green From an experimental perspective, understanding the interrelationships between the morphological structure, transport properties and device performance remains an important question. We designed and fabricated active material morphologies that possess dissimilar domain sizes/phase purities using different processing strategies: organic solvent casting, supercritical carbon dioxide (scCO$_{2})$ processing and thermal annealing. The short circuit currents of the as-cast samples, J$_{\mathrm{as-cast}}$, were appreciably lower than those in the scCO$_{2}$ processed samples, J$_{\mathrm{scCO2}}$, and the thermally annealed samples, J$_{\mathrm{thermal}}$. While J$_{\mathrm{scCO2}}$ $\sim$ J$_{\mathrm{thermal}}$, the initial carrier densities in the scCO$_{2}$ processed samples, n(0)$_{\mathrm{scCO2}}$, and the carrier recombination coefficients,~$\alpha _{\mathrm{scCO2}}$, were significantly higher than those in the thermally annealed samples (n(0)$_{\mathrm{scCO2}}$ $\sim$ 5n(0)$_{\mathrm{thermal}}$; $\alpha_{\mathrm{scCO2}}$ $\sim$ 2$\alpha_{\mathrm{thermal}})$. It is also shown that while J$_{\mathrm{scCO2}}$ $\sim$ 3J$_{\mathrm{as-cast}}$, the n(0)$_{\mathrm{scCO2}}$ $\sim$ n(0)$_{\mathrm{as-cast}}$, yet $\alpha_{\mathrm{scCO2}}$\textgreater $\alpha_{\mathrm{as-cast}}$. These observations are reconciled on the basis of details of the morphologies of these systems. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L21.00012: Morphology Development During Deposition in OPV Low Band Gap Polymer:Bis-Fullerene Heterojunctions: Effect of a Second Solvent Huipeng Chen, Yu-Che Hsiao, Bin Hu, Mark Dadmun Polymer based bulk-heterojunction solar cells, based on blends of conjugated polymers and fullerenes are one potential option for low cost renewable power generation. One way to improve power conversion efficiency (PCE) of this cell is to increase the open-circuit voltage (V$_{\mathrm{oc}})$. It has been reported that replacing PCBM with bis-adduct fullerenes (i.e. ICBA) significantly improves V$_{\mathrm{oc}}$ and PCE in P3HT device. However, for the most promising low band-gap polymer (LBP) system, replacing PCBM with ICBA gives very poor short-circuit current (J$_{\mathrm{sc}})$ and PCE although V$_{\mathrm{oc\thinspace }}$is significantly improved. As J$_{\mathrm{sc}}$ and PCE strongly depend on the morphology, we therefore tried to optimize the morphology of as-cast LBP/ICBA mixture by adding a second solvent with varying solubility to LBP and ICBA to the deposition solution. The results show that there is no change of LBP ordering by adding the second solvent regardless of its solubility. The morphology of all the as-cast samples is then determined by neutron scattering. A homogenous dispersion of ICBA in LBP is found in the sample where the second solvent is selective to LBP, giving poor PCE. Aggregates of ICBA are formed in those samples where the second solvent is selective to ICBA. The resultant morphology improves PCE by up to 246{\%}. A quantitative analysis of neutron data shows that the interfacial area between ICBA aggregates and LBP/ICBA mixed phase is improved in these samples, which appears to facilitate charge transport and reduce the recombination of free charge carriers. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L21.00013: Modifying growth of perylene diimide nanocrystals with poly(3-hexyl thiophene) as additives Laju Bu, Ryan Hayward The shape, size, and crystallinity of organic semiconductors play vital roles in their applications in optoelectronics. Various methods to control crystallization of organic semiconductors, including thermal/solvent annealing, addition of poor solvents, and chemical structure modification, have been applied to improve the performance of organic photovoltaics. While soluble additives controlled crystallization are commonly found in biomineralization, pharmaceutics, and food science, they have rarely been applied to organic semiconductors. Here, we show that a p-type polymer, P3HT, serves as a soluble additive in crystallization of a n-type semiconductor, perylene diimide (PDI), by preferentially adsorbing on lateral crystal faces, which reduce lateral growth of PDI crystals relative to longitudinal growth, yielding extended 1-D nanofibers. Upon subsequent crystallization of P3HT, the PDI nanofibers serve as efficient nucleation sties, resulting in shish-kebab like p/n heterostuctures. Using ultrasound to enhance nucleation of PDI crystals, variations in P3HT molecular weight and concentration, and sonication temperature, allow PDI nanocrystal size and uniformity to be tuned. The uniform PDI nanocrystals can act as seeds to crystallize additional PDI to get segmented nanocrystals. [Preview Abstract] |
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