TY - JOUR TI - Organic Solar Cells: Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells (Adv. Energy Mater. 1/2013) AU - Collins, Brian A. AU - Li, Zhe AU - Tumbleston, John R. AU - Gann, Eliot AU - McNeill, Christopher R. AU - Ade, Harald T2 - Advanced Energy Materials AB - The front cover represents the morphology and resulting device dynamics in organic solar cell blend films of PTB7 and PC71BM, as revealed by combined resonant X-ray scattering and microscopy. Harald Ade and co-workers find on page 65 that the fullerene molecules (red) are miscible in the polymer (blue) up to 30 wt.%, above which they begin to agglomerate (bottom). This agglomeration is important for the optoelectronic processes within the device, but the agglomerates must be kept to small sizes by the solvent processing additive diiodooctane (DIO). Correlation of this morphology with the spectrally resolved quantum efficiency shows that the yellow excitons created upon photoabsorption must arrive at the agglomerate interface for charge separation to occur. The blue electrons and green holes can then percolate through appropriate molecules in the mixed matrix to the electrodes for harvesting of electrical energy. DA - 2013/1// PY - 2013/1// DO - 10.1002/aenm.201370001 VL - 3 IS - 1 SP - 1-1 J2 - Adv. Energy Mater. LA - en OP - SN - 1614-6832 UR - http://dx.doi.org/10.1002/aenm.201370001 DB - Crossref ER - TY - JOUR TI - Organic Solar Cells: Domain Purity, Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement (Adv. Energy Mater. 7/2013) AU - Ma, Wei AU - Tumbleston, John R. AU - Wang, Ming AU - Gann, Eliot AU - Huang, Fei AU - Ade, Harald T2 - Advanced Energy Materials AB - The importance of domain purity and molecular orientation are investigated for solar cell devices based on naphtha[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) or 2,1,3-benzothiadiazole-based conjugated polymers. Harald Ade and co-workers show on page 864 that the domain purity, domain size and the nature of the interface structure in BHJ organic solar cells are critical for their performance and provide for the first time a quantitative comparison of domain purity between devices based on polymers with a different backbone. DA - 2013/7// PY - 2013/7// DO - 10.1002/AENM.201370029 VL - 3 IS - 7 SP - 826-826 J2 - Adv. Energy Mater. LA - en OP - SN - 1614-6832 UR - http://dx.doi.org/10.1002/AENM.201370029 DB - Crossref ER - TY - JOUR TI - A systematic study on efficiency enhancements in phosphorescent green, red and blue microcavity organic light emitting devices AU - Xiang, Chaoyu AU - Koo, Wonhoe AU - So, Franky AU - Sasabe, Hisahiro AU - Kido, Junji T2 - Light: Science & Applications AB - A systematic study has been conducted on microcavity organic light emitting diodes (OLEDs) based on green, red and blue phosphorescent emitters to elucidate the microcavity effects for different color emitters. We found that the luminance output is determined by the reflectivity of the semitransparent electrode and the photopic response of the green, red and blue emitters. While the luminance enhancements of blue and red phosphorescent microcavity devices are small, a current efficiency as high as 224 cd A−1 is obtained in the green phosphorescent microcavity OLEDs. The role that microcavities play in the output of an organic light-emitting device (OLED) has now been studied in detail. Chaoyu Xiang and co-workers from the University of Florida in the USA and Yamagata University in Japan investigated the efficiency and spectral characteristics of phosphorescent green, red and blue microcavity OLEDs. They concluded that the luminance output of a device is strongly affected by the reflectivity of its semitransparent electrode and its spectral match to the organic material’s electroluminescence. They fabricated device electrodes from indium-tin-oxide-coated distributed Bragg reflectors comprising two separated stacks of alternating layers of titanium oxide and silicon dioxide. They recorded a luminous efficiency of up 224 cd A–1 for an optimized green microcavity device, with smaller values achieved for blue and red devices. DA - 2013/6// PY - 2013/6// DO - 10.1038/LSA.2013.30 VL - 2 IS - 6 SP - e74-e74 J2 - Light Sci Appl LA - en OP - SN - 2047-7538 UR - http://dx.doi.org/10.1038/LSA.2013.30 DB - Crossref KW - efficiency enhancements KW - microcavity KW - organic light emitting devices KW - phosphorescence KW - photopic luminosity ER - TY - JOUR TI - Investigation of the Role of the Acceptor Molecule in Bulk Heterojunction Photovoltaic Cells Using Impedance Spectroscopy AU - Casalini, Riccardo AU - Tsang, Sai Wing AU - Deininger, James J. AU - Arroyave, Frank A. AU - Reynolds, John R. AU - So, Franky T2 - The Journal of Physical Chemistry C AB - An investigation of the recombination kinetics and the density of states distribution in bulk heterojunction organic photovoltaic cells was undertaken using impedance spectroscopy measurements under varying levels of illuminations. Three organic polymer solar cells were investigated which differed only by the p-type polymer used in the active layers while the device architecture and acceptor molecule (PC70BM) were kept the same. We found that the density of states profiles for the three cells are similar and can be superimposed by a horizontal shift due to a difference between the polymers’ HOMO and the PC70BM’s LUMO levels. The recombination mechanisms for the three cells showed the same behavior, suggesting an important role of the acceptor in this process; however, large differences in the absolute value of carrier lifetime τeff related to the choice of the polymer are evident, indicating that the recombination kinetics depends on a combination of the properties of both materials and the morphology of their blend. DA - 2013/6/26/ PY - 2013/6/26/ DO - 10.1021/JP401435S VL - 117 IS - 27 SP - 13798-13804 J2 - J. Phys. Chem. C LA - en OP - SN - 1932-7447 1932-7455 UR - http://dx.doi.org/10.1021/JP401435S DB - Crossref ER - TY - JOUR TI - Defect-Induced Loss Mechanisms in Polymer–Inorganic Planar Heterojunction Solar Cells AU - Hartel, Michael AU - Chen, Song AU - Swerdlow, Benjamin AU - Hsu, Hsien-Yi AU - Manders, Jesse AU - Schanze, Kirk AU - So, Franky T2 - ACS Applied Materials & Interfaces AB - The effect of ZnO defects on photoexcited charge carrier recombination and forward induced charge transfer was studied in organic–inorganic bilayer organic heterojunction solar cells. Decreased bimolecular recombination via passivation of ZnO nanopariticle defects resulted in longer carrier lifetime as determined by transient photovoltage (TPV) measurements. It was also found by time-resolved photoluminescence (TRPL) measurements that defect passivation decreased the fluorescence lifetime which indicated higher exciton dissociation efficiency. Through passivation of the ZnO nanoparticles defects, the two loss mechanisms were reduced and the power conversion efficiency (PCE) is significantly enhanced. DA - 2013/7/23/ PY - 2013/7/23/ DO - 10.1021/AM4015605 VL - 5 IS - 15 SP - 7215-7218 J2 - ACS Appl. Mater. Interfaces LA - en OP - SN - 1944-8244 1944-8252 UR - http://dx.doi.org/10.1021/AM4015605 DB - Crossref KW - organic solar cell KW - ZnO nanoparticles KW - PCDTBT KW - defects KW - transient photovoltage KW - PL lifetime ER - TY - JOUR TI - Hole injection polymer effect on degradation of organic light-emitting diodes AU - Chen, Song AU - Jiang, Xuezhong AU - So, Franky T2 - Organic Electronics AB - Polythienothiophene:poly(perfluoroethylene-perfluoroethersulfonic acid) (PTT:PFFSA) has been used to enhance hole injection into small molecule OLEDs. Compared to devices with polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) as the hole injection layer (HIL), the OLED using PTT:PFFSA as a HIL gives enhanced efficiency and a slower luminance decay as well as a slower rise in operating voltage. Further studies of capacitance–voltage characteristics reveal that positive trapped charges accumulate in the hole transporting layer during operation. These results thus highlight the significance of hole injection layer to OLED operational stability. DA - 2013/10// PY - 2013/10// DO - 10.1016/J.ORGEL.2013.06.023 VL - 14 IS - 10 SP - 2518-2522 J2 - Organic Electronics LA - en OP - SN - 1566-1199 UR - http://dx.doi.org/10.1016/J.ORGEL.2013.06.023 DB - Crossref KW - Organic light emitting diode KW - Hole injection layer KW - Degradation KW - Lifetime KW - Capacitance ER - TY - JOUR TI - Properties of interlayer for organic photovoltaics AU - Lai, Tzung-Han AU - Tsang, Sai-Wing AU - Manders, Jesse R. AU - Chen, Song AU - So, Franky T2 - Materials Today AB - Interfacial materials play an important role in determining the efficiency of an organic photovoltaic (OPV) cell. They are not only responsible for establishing ohmic contact, but also determining different device parameters such as the internal electric field, the film morphology, and the carrier recombination rate which are important to the device performance. Here, we will present the material properties and requirements for these interlayers used in high efficiency OPV cells. This paper aims to reveal the different roles of interlayers, introduce techniques for characterizing their properties, and provide an insight into the future development of novel interlayers for high efficiency organic photovoltaic cells. DA - 2013/11// PY - 2013/11// DO - 10.1016/J.MATTOD.2013.10.001 VL - 16 IS - 11 SP - 424-432 J2 - Materials Today LA - en OP - SN - 1369-7021 UR - http://dx.doi.org/10.1016/J.MATTOD.2013.10.001 DB - Crossref ER - TY - JOUR TI - Charge transport study of semiconducting polymers and their bulk heterojunction blends by capacitance measurements AU - Chan, Kevin K. H. AU - Tsang, S. W. AU - Lee, Harrison K. H. AU - So, F. AU - So, S. K. T2 - Journal of Polymer Science Part B: Polymer Physics AB - Abstract We use frequency dependent capacitance measurements to probe carrier mobilities and transport parameters of six representative semiconducting polymers and some of their bulk heterojunction (BHJ) blends. With a suitable choice of a hole injection layer, well‐defined signals for hole transport characterization can be obtained for the pristine polymers [ J. Appl. Phys . 99, 013706 (2006)]. However, ill‐defined signals with negative capacitances, arising from undesirable electron leakages, are obtained for the BHJ blends. The problem of electron leakage can be circumvented by inserting an electron blocking and trapping layer under the cathode. As a result, hole transport properties of BHJ blends can be obtained. For the BHJ of poly(3‐hexylthiophene) blended with [6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC 61 BM), the hole mobilities seem to be insensitive to the composition of the BHJ, indicating the P3HT component in the BHJ is well connected. On the other hand, for poly[ N ‐9“‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadia zole)] doped with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCDTBT:PC 71 BM), a clear reduction of the hole mobility is observed as the polymer composition is reduced. Temperature dependent experiments were performed. The data are analyzed by the Gaussian Disorder Model. We found that the energetic disorder is independent of the composition of the BHJ. Organic photovoltaic performances of BHJ blends are also measured in this contribution. The correlation between device performance and energetic disorder of the BHJ will be discussed. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 DA - 2013/1/8/ PY - 2013/1/8/ DO - 10.1002/polb.23236 VL - 51 IS - 8 SP - 649-658 J2 - J. Polym. Sci. B Polym. Phys. LA - en OP - SN - 0887-6266 UR - http://dx.doi.org/10.1002/polb.23236 DB - Crossref ER - TY - JOUR TI - Loss Mechanisms in Thick-Film Low-Bandgap Polymer Solar Cells AU - Small, Cephas E. AU - Tsang, Sai-Wing AU - Chen, Song AU - Baek, Sujin AU - Amb, Chad M. AU - Subbiah, Jegadesan AU - Reynolds, John R. AU - So, Franky T2 - Advanced Energy Materials AB - Abstract Polymer bulk heterojunction solar cells based on low bandgap polymer:fullerene blends are promising for next generation low‐cost photovoltaics. While these solution‐processed solar cells are compatible with large‐scale roll‐to‐roll processing, active layers used for typical laboratory‐scale devices are too thin to ensure high manufacturing yields. Furthermore, due to the limited light absorption and optical interference within the thin active layer, the external quantum efficiencies (EQEs) of bulk heterojunction polymer solar cells are severely limited. In order to produce polymer solar cells with high yields, efficient solar cells with a thick active layer must be demonstrated. In this work, the performance of thick‐film solar cells employing the low‐bandgap polymer poly(dithienogermole‐thienopyrrolodione) (PDTG‐TPD) was demonstrated. Power conversion efficiencies over 8.0% were obtained for devices with an active layer thickness of 200 nm, illustrating the potential of this polymer for large‐scale manufacturing. Although an average EQE > 65% was obtained for devices with active layer thicknesses > 200 nm, the cell performance could not be maintained due to a reduction in fill factor. By comparing our results for PDTG‐TPD solar cells with similar P3HT‐based devices, we investigated the loss mechanisms associated with the limited device performance observed for thick‐film low‐bandgap polymer solar cells. DA - 2013/3/19/ PY - 2013/3/19/ DO - 10.1002/AENM.201201114 VL - 3 IS - 7 SP - 909-916 J2 - Adv. Energy Mater. LA - en OP - SN - 1614-6832 UR - http://dx.doi.org/10.1002/AENM.201201114 DB - Crossref KW - organic solar cell KW - thickness dependence KW - dithienogermole ER - TY - JOUR TI - Direct Fabrication of Organic Light-Emitting Diodes on Buckled Substrates for Light Extraction AU - Koo, Won Hoe AU - Zhe, Yin AU - So, Franky T2 - Advanced Optical Materials AB - Organic light-emitting diodes fabricated directly on a buckled PDMS surface with ultraviolet/ozone treatment without using lithography and imprinting processes are effective for the extraction of waveguide modes and are fully compatible with large-area manufacturing. DA - 2013/4/22/ PY - 2013/4/22/ DO - 10.1002/ADOM.201300030 VL - 1 IS - 5 SP - 404-408 J2 - Advanced Optical Materials LA - en OP - SN - 2195-1071 UR - http://dx.doi.org/10.1002/ADOM.201300030 DB - Crossref ER - TY - JOUR TI - Energy Level Alignment and Sub-Bandgap Charge Generation in Polymer:Fullerene Bulk Heterojunction Solar Cells (Adv. Mater. 17/2013) AU - Tsang, Sai-Wing AU - Chen, Song AU - So, Franky T2 - Advanced Materials AB - A novel technique – charge modulated electro-absorption spectroscopy (CEMAS) – is reported by Franky So and co-workers on page 2434 and used to determine the energy-level alignment and the effective bandgap of organic heterojunctions. What makes CMEAS unique is that it can directly determine the interface energetics of donor–acceptor blends, not possible using conventional electro-chemical methods. DA - 2013/5/2/ PY - 2013/5/2/ DO - 10.1002/ADMA.201370113 VL - 25 IS - 17 SP - 2433-2433 J2 - Adv. Mater. LA - en OP - SN - 0935-9648 UR - http://dx.doi.org/10.1002/ADMA.201370113 DB - Crossref ER - TY - JOUR TI - Energy Level Alignment and Sub-Bandgap Charge Generation in Polymer:Fullerene Bulk Heterojunction Solar Cells AU - Tsang, Sai-Wing AU - Chen, Song AU - So, Franky T2 - Advanced Materials AB - Using charge modulated electroabsorption spectroscopy (CMEAS), for the first time, the energy level alignment of a polymer:fullerene bulk heterojunction photovoltaic cell is directly measured. The charge-transfer excitons generated by the sub-bandgap optical pumping are coupled with the modulating electric field and introduce subtle changes in optical absorption in the sub-bandgap region. This minimum required energy for sub-bandgap charge genreation is defined as the effective bandgap. DA - 2013/2/18/ PY - 2013/2/18/ DO - 10.1002/ADMA.201204495 VL - 25 IS - 17 SP - 2434-2439 J2 - Adv. Mater. LA - en OP - SN - 0935-9648 UR - http://dx.doi.org/10.1002/ADMA.201204495 DB - Crossref KW - polymer solar cells KW - bulk heterojunction KW - electroabsorption KW - effective bandgap KW - energy level alignment ER - TY - JOUR TI - Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells AU - Manders, Jesse R. AU - Tsang, Sai-Wing AU - Hartel, Michael J. AU - Lai, Tzung-Han AU - Chen, Song AU - Amb, Chad M. AU - Reynolds, John R. AU - So, Franky T2 - Advanced Functional Materials AB - Abstract The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA) 2 (OAc)] + ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni 3+ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC 71 BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS. DA - 2013/1/16/ PY - 2013/1/16/ DO - 10.1002/adfm.201202269 VL - 23 IS - 23 SP - 2993-3001 J2 - Adv. Funct. Mater. LA - en OP - SN - 1616-301X UR - http://dx.doi.org/10.1002/adfm.201202269 DB - Crossref KW - polymer photovoltaics KW - organic electronics KW - hole transport layer KW - nickel oxide ER - TY - JOUR TI - On the role of intermixed phases in organic photovoltaic blends AU - Westacott, Paul AU - Tumbleston, John R. AU - Shoaee, Safa AU - Fearn, Sarah AU - Bannock, James H. AU - Gilchrist, James B. AU - Heutz, Sandrine AU - deMello, John AU - Heeney, Martin AU - Ade, Harald AU - Durrant, James AU - McPhail, David S. AU - Stingelin, Natalie T2 - Energy & Environmental Science AB - Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor–acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer–fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction. DA - 2013/// PY - 2013/// DO - 10.1039/C3EE41821A VL - 6 IS - 9 SP - 2756 J2 - Energy Environ. Sci. LA - en OP - SN - 1754-5692 1754-5706 UR - http://dx.doi.org/10.1039/C3EE41821A DB - Crossref ER - TY - JOUR TI - Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells AU - Stuart, Andrew C. AU - Tumbleston, John R. AU - Zhou, Huaxing AU - Li, Wentao AU - Liu, Shubin AU - Ade, Harald AU - You, Wei T2 - Journal of the American Chemical Society AB - Three structurally identical polymers, except for the number of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF) of related solar cells. While the enhanced Voc can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in Jsc and FF are likely due to suppressed charge recombination. While the reduced bimolecular recombination with raising fluorine concentration is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states (Δμge) for these polymers as the number of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π–π stacking directions, observed with GIWAXS. This indicates that the addition of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an observed decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed “fluorine” impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers. DA - 2013/1/25/ PY - 2013/1/25/ DO - 10.1021/ja309289u VL - 135 IS - 5 SP - 1806-1815 J2 - J. Am. Chem. Soc. LA - en OP - SN - 0002-7863 1520-5126 UR - http://dx.doi.org/10.1021/ja309289u DB - Crossref ER - TY - JOUR TI - Thermally Induced Dewetting in Ultrathin C-60 Films on Copper Phthalocyanine AU - McAfee, T. AU - Gann, E. AU - Ade, H. AU - Dougherty, D. B. T2 - JOURNAL OF PHYSICAL CHEMISTRY C AB - The evolution of thermally annealed ultrathin fullerene–C60 layers on copper phthalocyanine is examined by atomic force microscopy and near-edge X-ray absorption fine structure spectroscopy. Annealing causes C60 films to dewet the copper phthalocyanine substrate surface via lateral surface mass transport. Coarsening of C60 clusters is observed that creates mounds that exceed the nominal C60 thickness by more than an order of magnitude and is consistent with surface diffusion-mediated mass transport. Implications for thermal morphology control in organic solar cells, such as the destabilization of multilayered C60:CuPc with individual layers only ∼5–10 nm thick, are discussed. DA - 2013/12/12/ PY - 2013/12/12/ DO - 10.1021/jp4067372 VL - 117 IS - 49 SP - 26007-26012 SN - 1932-7447 ER - TY - JOUR TI - Fluorinated Polymer Yields High Organic Solar Cell Performance for a Wide Range of Morphologies AU - Tumbleston, John R. AU - Stuart, Andrew C. AU - Gann, Eliot AU - You, Wei AU - Ade, Harald T2 - Advanced Functional Materials AB - Abstract Device performance is recognized to be generally sensitive to morphology in bulk heterojunction solar cells. Through the use of quantitative morphological measurements, it is demonstrated that devices based on benzodithiophene and fluorinated benzotriazole moieties constitute an exception to this design rule and exhibit a range of morphologies that yield similar high performance. In particular, the fill factor (FF) remains above 65% even with factor of two changes in domain size and factor of two changes in relative domain purity. Devices with active layer thicknesses of 250 nm are employed, which are capable of increasing optical absorption to produce high photocurrent. The general insensitivity to both morphology and thickness is likely related to the measured low equilibrium miscibility of fullerene in the polymer of 3‐4%. The materials and processes investigated therefore provide insights into functional material design that yield increased processing latitude and may be more amenable to roll‐to‐roll processing. DA - 2013/2/15/ PY - 2013/2/15/ DO - 10.1002/adfm.201300093 VL - 23 IS - 27 SP - 3463-3470 J2 - Adv. Funct. Mater. LA - en OP - SN - 1616-301X UR - http://dx.doi.org/10.1002/adfm.201300093 DB - Crossref KW - organic solar cells KW - soft X-ray scattering KW - molecular miscibility KW - roll-to-roll processing KW - morphology KW - PBnDT-FTAZ ER - TY - JOUR TI - Domain Purity, Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement AU - Ma, Wei AU - Tumbleston, John R. AU - Wang, Ming AU - Gann, Eliot AU - Huang, Fei AU - Ade, Harald T2 - ADVANCED ENERGY MATERIALS AB - Abstract Domain purity and interface structure are known to be critical for fullerene‐based bulk heterojunction (BHJ) solar cells, yet have been very difficult to study. Using novel soft X‐ray tools, we delineate the importance of these parameters by comparing high performance cells based on a novel naphtha[1,2‐c:5,6‐c]bis[1,2,5]thiadiazole (NT) material to cells based on a 2,1,3‐benzothiadiazole (BT) analogue. BT‐based devices exhibit ∼15 nm, mixed domains that differ in composition by at most 22%, causing substantial bimolecular recombination. In contrast, NT‐based devices have more pure domains that are >80 nm in size, yet the polymer‐rich phase still contains at least 22% fullerene. Power conversion efficiency >6% is achieved for NT devices despite a domain size much larger than the nominal exciton diffusion length due to a favourable trade‐off in the mixed domain between exciton harvesting, charge transport, and bimolecular recombination. The miscibility of the fullerene with the NT and BT polymer is measured and correlated to the purity in devices. Importantly, polarized x‐ray scattering reveals preferential face‐on orientation of the NT polymer relative to the PCBM‐rich domains. Such ordering has previously not been observed in fullerene‐based solar cells and is shown here to be possibly a controlling or contributing factor to high performance. DA - 2013/7// PY - 2013/7// DO - 10.1002/aenm.201200912 VL - 3 IS - 7 SP - 864-872 SN - 1614-6840 KW - domain purity KW - miscibility KW - morphology KW - organic solar cells KW - interface structure KW - orientational ordering KW - soft x-ray scattering KW - R-SoXS KW - P-SoXS ER - TY - JOUR TI - Differences in NEXAFS of odd/even long chain n-alkane crystals AU - Swaraj, Sufal AU - Ade, Harald T2 - JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA AB - We present the near edge X-ray absorption fine structure (NEXAFS) spectra of several long chain n-alkanes crystallites formed on Silicon nitride (Si3N4) windows. Dichroic signature was investigated with the CC backbone aligned perpendicular to the substrate. Significant changes in the dichroic signature of spectral intensities at energies below the ionization edge (287.5 and 288.1 eV) have been observed. While the dichroic ratio corresponding to the spectral feature at 287.5 eV remains relatively unaffected by the overall length of CC backbone, it is noticeably affected by the parity (odd or even) of the number of Carbon atoms in the n-alkane backbone. Data obtained provide evidence of the influence of interaction of molecular orbitals with periodic lattice structure. DA - 2013/12// PY - 2013/12// DO - 10.1016/j.elspec.2013.10.006 VL - 191 SP - 60-64 SN - 1873-2526 KW - NEXAFS KW - n-Alkanes KW - Odd/even effect KW - Dichroism KW - STXM KW - Spectroscopy ER - TY - JOUR TI - PDT-S-T: A New Polymer with Optimized Molecular Conformation for Controlled Aggregation and pi-pi Stacking and Its Application in Efficient Photovoltaic Devices AU - Wu, Yue AU - Li, Zhaojun AU - Ma, Wei AU - Huang, Ye AU - Huo, Lijun AU - Guo, Xia AU - Zhang, Maojie AU - Ade, Harald AU - Hou, Jianhui T2 - ADVANCED MATERIALS AB - The correlation among molecular conformation, the crystallinity of the morphology, propensity for π-π stacking, J- versus H-aggregation, and photovoltaic performance have been studied based on two newly designed polymers, PBDTTT-S-T and PDT-S-T. The results show that more linear backbone structure is helpful to improve photovoltaic properties of the polymer, and therefore, molecular conformation should be considered for molecular design of photovoltaic polymers. DA - 2013/7/5/ PY - 2013/7/5/ DO - 10.1002/adma.201301174 VL - 25 IS - 25 SP - 3449-3455 SN - 1521-4095 KW - low bandgap polymers KW - polymer solar cells KW - molecular conformation KW - crystallinity ER - TY - JOUR TI - Molecular Design toward Efficient Polymer Solar Cells with High Polymer Content AU - Qian, Deping AU - Ma, Wei AU - Li, Zhaojun AU - Guo, Xia AU - Zhang, Shaoqing AU - Ye, Long AU - Ade, Harald AU - Tan, Zhan'ao AU - Hou, Jianhui T2 - JOURNAL OF THE AMERICAN CHEMICAL SOCIETY AB - A novel polythiophene derivative, PBT1, was designed, synthesized, and applied in polymer solar cells (PSCs). This work provides a successful example of using molecular structure as a tool to realize optimal photovoltaic performance with high polymer content, thus enabling the realization of efficient photoabsorption in very thin films. As a result, an efficiency of 6.88% was recorded in a PSC with a 75 nm active layer. DA - 2013/6/12/ PY - 2013/6/12/ DO - 10.1021/ja402971d VL - 135 IS - 23 SP - 8464-8467 SN - 1520-5126 ER - TY - JOUR TI - Modifications in Morphology Resulting from Nanoimprinting Bulk Heterojunction Blends for Light Trapping Organic Solar Cell Designs AU - Tumbleston, John R. AU - Gadisa, Abay AU - Liu, Yingchi AU - Collins, Brian A. AU - Samulski, Edward T. AU - Lopez, Rene AU - Ade, Harald T2 - ACS APPLIED MATERIALS & INTERFACES AB - Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption. DA - 2013/8/28/ PY - 2013/8/28/ DO - 10.1021/am402363r VL - 5 IS - 16 SP - 8225-8230 SN - 1944-8252 KW - organic solar cells KW - light trapping KW - bulk heterojunction morphology KW - X-ray microscopy KW - X-ray diffraction KW - nanoimprinting ER - TY - JOUR TI - Competition between morphological attributes in the thermal annealing and additive processing of polymer solar cells AU - Ma, Wei AU - Ye, Long AU - Zhang, Shaoqing AU - Hou, Jianhui AU - Ade, Harald T2 - Journal of Materials Chemistry C AB - Thermal annealing and additive processing are employed and compared using alkoxy substituted (QxO) and extended π conjugated alkythienyl substituted (QxT) benzo[1,2-b:4,5 b′]dithiophene based heterojunction (BHJ) solar cells. The characteristic median length of the morphology, average composition fluctuations, interface structure, crystallinity and molecular miscibility are investigated based on these two processes. Our results suggest that focusing on single structural, morphological or thermodynamic measurements is not sufficient to explain differences in device performance. In the current work, no blends are close to the ideal morphology containing either domains that are too large, too mixed or too pure. An optimization strategy is proposed to improve those devices. Importantly, we find that domain size and relative domain purity are overall correlated with molecular miscibility, i.e. the more immiscible system induces larger and purer domains irrespective of the processing and even in non-equilibrium structures. This indicates that the relative domain size and purity, and device performance can be potentially predicted by the donor–acceptor molecular miscibility, a factor not yet widely considered when designing new materials for BHJ devices. DA - 2013/// PY - 2013/// DO - 10.1039/C3TC30679H VL - 1 IS - 33 SP - 5023 J2 - J. Mater. Chem. C LA - en OP - SN - 2050-7526 2050-7534 UR - http://dx.doi.org/10.1039/C3TC30679H DB - Crossref ER - TY - JOUR TI - Soft X-ray characterisation of organic semiconductor films AU - McNeill, Christopher R. AU - Ade, Harald T2 - JOURNAL OF MATERIALS CHEMISTRY C AB - Organic semiconductor devices such as organic solar cells and organic field-effect transistors are based on blended and/or multilayered structures. Many organic semiconductors used in high-performance devices are also semicrystalline or liquid crystalline with a complex relationship between film microstructure and device performance. Unravelling structure–function relationships in organic semiconductor devices therefore requires structural probes that have high chemical specificity, sensitivity to molecular orientation and order and high spatial resolution. Soft X-rays have proven to be versatile in spectroscopy, microspectroscopy and scattering experiments providing contrast derived from differences in the near-edge X-ray absorption spectra of different organic semiconductors. Furthermore, the sensitivity of polarised soft X-ray beams to the orientation of bonds in organic materials makes them a unique probe of molecular orientation. This feature article provides an overview of the range of microscopy, reflectivity and scattering techniques based on soft X-rays that have been developed in recent years and their utility for providing new insight into the complex structure of organic semiconductor thin films. DA - 2013/// PY - 2013/// DO - 10.1039/c2tc00001f VL - 1 IS - 2 SP - 187-201 SN - 2050-7534 ER - TY - JOUR TI - The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells AU - Bartelt, Jonathan A. AU - Beiley, Zach M. AU - Hoke, Eric T. AU - Mateker, William R. AU - Douglas, Jessica D. AU - Collins, Brian A. AU - Tumbleston, John R. AU - Graham, Kenneth R. AU - Amassian, Aram AU - Ade, Harald AU - Frechet, Jean M. J. AU - Toney, Michael F. AU - McGehee, Michael D. T2 - ADVANCED ENERGY MATERIALS AB - Abstract Most optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐ b :4,5‐ b ′]dithiophene‐ co ‐octylthieno[3,4‐ c ]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells. DA - 2013/3// PY - 2013/3// DO - 10.1002/aenm.201200637 VL - 3 IS - 3 SP - 364-374 SN - 1614-6840 KW - organic electronics KW - photovoltaic devices KW - solar cells KW - bulk heterojunctions KW - charge transport ER - TY - JOUR TI - Accurate and Facile Determination of the Index of Refraction of Organic Thin Films Near the Carbon 1s Absorption Edge AU - Yan, Hongping AU - Wang, Cheng AU - McCarn, Allison R. AU - Ade, Harald T2 - PHYSICAL REVIEW LETTERS AB - A practical and accurate method to obtain the index of refraction, especially the decrement $\ensuremath{\delta}$, across the carbon $1s$ absorption edge is demonstrated. The combination of absorption spectra scaled to the Henke atomic scattering factor database, the use of the doubly subtractive Kramers-Kronig relations, and high precision specular reflectivity measurements from thin films allow the notoriously difficult-to-measure $\ensuremath{\delta}$ to be determined with high accuracy. No independent knowledge of the film thickness or density is required. High confidence interpolation between relatively sparse measurements of $\ensuremath{\delta}$ across an absorption edge is achieved. Accurate optical constants determined by this method are expected to greatly improve the simulation and interpretation of resonant soft x-ray scattering and reflectivity data. The method is demonstrated using poly(methyl methacrylate) and should be extendable to all organic materials. DA - 2013/4/23/ PY - 2013/4/23/ DO - 10.1103/physrevlett.110.177401 VL - 110 IS - 17 SP - SN - 1079-7114 ER - TY - JOUR TI - Synthesis, solid-state, and charge-transport properties of conjugated polythiophene-S,S-dioxides AU - Cochran, Justin E. AU - Amir, Elizabeth AU - Sivanandan, Kulandaivelu AU - Ku, Sung-Yu AU - Seo, Jung Hwa AU - Collins, Brian A. AU - Tumbleston, John R. AU - Toney, Michael F. AU - Ade, Harald AU - Hawker, Craig J. AU - Chabinyc, Michael L. T2 - JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS AB - Abstract An alkylated semiconducting polymer comprising alternating bithiophene‐[all]‐ S , S ‐dioxide and aromatic monothiophene units in the polymer backbone was synthesized with the intent of modifying the energy gap and lowest unoccupied molecular orbital for use as a stable n ‐type semiconductor. Films spun from this semiconducting polymer were characterized utilizing X‐ray scattering, near edge X‐ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and thin‐film field effect transistors to determine how oxidation of the thiophene ring systems impacts the structural and electronic properties of the polymer. The thiophene‐ S,S ‐dioxide polymers have lower optical and electrical band gaps than corresponding thiophene polymers. X‐ray scattering results indicate that the polymers are well ordered with the π–π stacking distances increased by 0.4 Å relative to analogous thiophene polymers. The electrical stability of these polymers is poor in transistors with a drop in the field effect mobility by approximately one order of magnitude upon addition of just 5% of the thiophene‐ S , S ‐dioxide unit in a copolymer with thiophene. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 DA - 2013/1/1/ PY - 2013/1/1/ DO - 10.1002/polb.23167 VL - 51 IS - 1 SP - 48-56 SN - 1099-0488 KW - polythiophene KW - polythiophene-S KW - S-dioxides KW - semiconducting polymers KW - thin film KW - X-ray scattering ER - TY - JOUR TI - Disentangling the impact of side chains and fluorine substituents of conjugated donor polymers on the performance of photovoltaic blends AU - Yang, Liqiang AU - Tumbleston, John R. AU - Zhou, Huaxing AU - Ade, Harald AU - You, Wei T2 - ENERGY & ENVIRONMENTAL SCIENCE AB - Side chains and fluorine (F) substituents on conjugated polymers have shown significant impact on the photovoltaic properties of polymer-based bulk heterojunction (BHJ) solar cells, but their respective impact is largely studied independently. In order to disentangle the effect of side chains and F substituents, we comprehensively investigate a series of conjugated polymers with an identical backbone (PNDT–DTBT) but different combinations of side chains and F substituents. Surprisingly, these seemingly marginal changes to the polymer backbone strongly influence the morphology and structure in BHJ thin films (e.g., domain size/purity and the relative orientation of polymer crystallites), as manifested by resonant soft X-ray scattering (R-SoXS) and grazing-incidence wide-angle X-ray scattering (GI-WAXS), thereby exerting significant impact on the photovoltaic properties of these conjugated polymer-based BHJ cells. Devices based on the polymer with long bulky side chains and F substituents (C8,4-C6,2F) simultaneously exhibit large open circuit voltage (Voc), high short circuit current (Jsc) and good fill factor (FF), with an efficiency as high as 5.6% for this series of PNDT–DTBT polymers. DA - 2013/1// PY - 2013/1// DO - 10.1039/c2ee23235a VL - 6 IS - 1 SP - 316-326 SN - 1754-5706 ER -