Christoph J. Brabec

Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Bavaria, Germany

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Publications (147)704.37 Total impact

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    ABSTRACT: The charge carrier lifetime in small molecule: C60 photovoltaic devices is increased by reducing the physical interface area availabe for recombination. For donor contents below 10%, the gain in open-circuit voltage (Voc ) depends logarithmically on the interface area while the energy of the interfacial charge-transfer state ECT remains invariant.
    Advanced Materials 03/2014; · 14.83 Impact Factor
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    ABSTRACT: Long-term stability of polymer solar cells is determined by many factors, one of which is thermal stability. Although many thermal stability studies occur far beyond the operating temperature of a solar cell which is almost always less than 65 °C, thermal degradation is studied at temperatures that the solar cell would encounter in real-world operating conditions. At these temperatures, movement of the polymer and fullerenes, along with adhesion of the polymer to the back contact, creates a barrier for electron extraction. The polymer barrier can be removed and the performance can be restored by peeling off the electrode and depositing a new one. X-ray photoelectron spectroscopy measurements reveal a larger amount of polymer adhered to electrodes peeled from aged devices than electrodes peeled from fresh devices. The degradation caused by hole-transporting polymer adhering to the electrode can be suppressed by using an inverted device where instead of electrons, holes are extracted at the back metal electrode. The problem can be ultimately eliminated by choosing a polymer with a high glass transition temperature.
    Advanced Functional Materials 03/2014; · 9.77 Impact Factor
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    ABSTRACT: The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. It is strongly suggested that the roll-to-roll production of organic solar cells will employ the tandem concept to enhance the power conversion efficiency (PCE). However, due to the undeveloped deposition techniques, the challenges in ink formulation as well as the lack of commercially available high performance active materials, roll-to-roll fabrication of highly efficient organic tandem solar cells currently presents a major challenge. The reported high PCE values from lab-scale spin-coated devices are, of course, representative, but not helpful for commercialization. Here, organic tandem solar cells with exceptionally high fill factors and PCE values of 7.66% (on glass) and 5.56% (on flexible substrate), which are the highest values for the solution-processed tandem solar cells fabricated by a mass-production compatible coating technique under ambient conditions, are demonstrated. To predict the highest possible performance of tandem solar cells, optical simulation based on experimentally feasible values is performed. A maximum PCE of 21% is theoretically achievable for an organic tandem solar cell based on the optimized bandgaps and achieved fill factors.
    Advanced Energy Materials 03/2014; · 10.04 Impact Factor
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    ABSTRACT: Replacing halogenated solvents in the processing of organic solar cells by green solvents is a required step before the commercialization of this technology. With this purpose, some attempts have been made, although a general method is yet to be developed. Here, the potential of the Hansen solubility parameters (HSP) analysis for the design of green ink formulations for solution-processed active layer in bulk heterojunction photovoltaic devices based on small molecules is demonstrated. The motivation of moving towards organic small molecules stems from their lower molecular weight and more definite structure which makes them more likely to be dissolved in a wider variety of organic solvents. In the first step, the HSP of selected active materials are determined, namely, the star-shaped D-π-A tris{4-[5′′-(1,1-dicyanobut-1-en-2-yl)-2,2′-bithiophen-5-yl]phenyl}amine N(Ph-2T-DCN-Et)3 small molecule and fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). Secondly, computer simulations based on HSP allow the prediction of suitable green solvents for this specific material system. The most promising green solvents, according to the simulations, are then used to fabricate solar cell devices using pristine solvents and two solvents mixtures. These devices show power conversion efficiencies around 3.6%, which are comparable to those obtained with halogenated solvents. This good performance is a result of the sufficient solubility achieved after a successful prediction of good (green) solvents.
    Advanced Functional Materials 03/2014; 24(10). · 9.77 Impact Factor
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    ABSTRACT: A perylene diimide type small molecule (BI-PDI) has been synthesized through Suzuki coupling reaction between N,N'-bis(2,6-diisopropylphenyl)-1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimide and 2-(2-hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-boronic acid. BI-PDI small molecule has showed an absorption band between 350 and 750nm on thin films. HOMO and LUMO energy levels of BI-PDI dye have been calculated to be about -5.92eV and -3.82eV, respectively. Solution-processed bulk heterojunction (BHJ) solar cells have been constructed using BI-PDI as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor or poly(3-hexylthiophene) (P3HT) as donor and BI-PDI as acceptor. The external quantum efficiencies (EQE) of the devices cover the most of the visible region between 400 and 700nm for both configurations. Photovoltaic performances of BI-PDI-based organic solar cells are limited by the aggregation tendency of PDI structure and poor hole/electron mobilities of the active layer.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 01/2014; 128:197–206. · 1.98 Impact Factor
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    ABSTRACT: Two different types of aluminum-doped zinc oxide (AZO) thin films were fabricated using low cost sol–gel technique. By applying damp heat testing, the optical and electrical properties of those films were investigated under the influence of accelerated degradation from moisture or moisture vapor. Complementary measurements of optical transmittance, work function, and conductivity allowed exploring the degradation of AZO thin films and the corresponding OPV devices. We found that optical properties like transmittance, absorption coefficient, and band gap are not influenced by temperature and moisture. However, an increase in the work function, and a decrease in the conductivity of AZO films were observed upon damp heat exposure indicating the formation of a barrier or depletion layer at the metal oxide semiconductor interface.
    Organic Electronics 01/2014; 15(2):569–576. · 3.84 Impact Factor
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    Solar Energy Materials and Solar Cells 01/2014; 120:701-708. · 4.63 Impact Factor
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    Ivan Litzov, Christoph J Brabec
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    ABSTRACT: Solution-processed inverted bulk heterojunction (BHJ) solar cells have gained much more attention during the last decade, because of their significantly better environmental stability compared to the normal architecture BHJ solar cells. Transparent metal oxides (MeO x) play an important role as the dominant class for solution-processed interface materials in this development, due to their excellent optical transparency, their relatively high electrical conductivity and their tunable work function. This article reviews the advantages and disadvantages of the most common synthesis methods used for the wet chemical preparation of the most relevant n-type-and p-type-like MeO x interface materials consisting of binary compounds A x B y . Their performance for applications as electron transport/extraction layers (ETL/EEL) and as hole transport/extraction layers (HTL/HEL) in inverted BHJ solar cells will be reviewed and discussed.
    Materials. 12/2013; 6:5796-5820.
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    ABSTRACT: Photovoltaic tandem technology has the potential to boost the power conversion efficiency of organic photovoltaic devices. Here, a reliable and efficient fully solution-processed intermediate layer (IML) consisting of ZnO and neutralized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is demonstrated for series-connected multi-junction organic solar cells (OSCs). Drying at 80 °C in air is sufficient for this solution-processed IML to obtain excellent functionality and reliability, which allow the use of most of high performance donor materials in the tandem structure. An open circuit voltage (Voc) of 0.56 V is obtained for single-junction OSCs based on a low band-gap polymer, while multi-junction OSCs based on the same absorber material deliver promising fill factor values along with fully additive Voc as the number of junctions increase. Optical and electrical simulations, which are reliable and promising guidelines for the design and investigation of multi-junction OSCs, are discussed. The outcome of optical and electrical simulations is in excellent agreement with the experimental data, indicating the outstanding efficiency and functionality of this solution-processed IML. The demonstration of this efficient, solution-processed IML represents a convenient way for facilitating fabrication of multi-junction OSCs to achieve high power conversion efficiency.
    Advanced Energy Materials 12/2013; 3(12). · 10.04 Impact Factor
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    ABSTRACT: The spectral conversion of incident sunlight by appropriate photoluminescent materials has been a widely studied issue for improving the efficiency of photovoltaic solar energy harvesting. By using phosphors with suitable excitation/emission properties, also the light conditions for plants can be adjusted to match the absorption spectra of chlorophyll dyes, in this way increasing the photosynthetic activity of the plant. Here, we report on the application of this principle to a high plant, Spinacia oleracea. We employ a calcium strontium sulfide phosphor doped with divalent europium (Ca<sub>0.4</sub>Sr<sub>0.6</sub>S:Eu<sup>2+</sup>, CSSE) on a backlight conversion foil in photosynthesis experiments. We show that this phosphor can be used to effectively convert green to red light, centering at a wavelength of ~650 nm which overlaps the absorption peaks of chlorophyll a/b pigments. A measurement system was developed to monitor the photosynthetic activity, expressed as the CO<sub>2</sub> assimilation rate of spinach leaves under various controlled light conditions. Results show that under identical external light supply which is rich in green photons, the CO<sub>2</sub> assimilation rate can be enhanced by more than 25% when the actinic light is modified by the CSSE conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity.
    Optics Express 11/2013; 21 Suppl 6:A909-16. · 3.55 Impact Factor
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    ABSTRACT: Cu2ZnSnS4 (CZTS) nanoparticles have shown promising properties to be used as an energy harvesting material. They are usually synthesised under inert atmosphere or vacuum, whereas the subsequent step of film formation is carried out under an atmosphere of sulphur and/or Sn in order to avoid the decomposition of CZTS nanoparticles into binary and ternary species as well as the formation of the corresponding oxides. In the present paper we show that both the synthesis of CZTS nanoparticles and the film formation from the corresponding suspension can be considerably simplified. Namely, the synthesis can be carried out without controlling the atmosphere, whereas during the film annealing a nitrogen atmosphere is sufficient to avoid the depletion of the CZTS kesterite phase. Furthermore, an integrated approach including in-depth Raman analysis is developed in order to deal with the challenges associated with the characterization of CZTS nanoparticles in comparison to bulk systems. The formation of competitive compounds during the synthesis such as binary and ternary sulphides as well as metal oxides nanoparticles is discussed in detail. Finally, the as-produced films have ten times higher conductivity than the state of the art.
    Journal of Nanoparticle Research 08/2013; · 2.18 Impact Factor
  • Philipp Schwamb, Thilo C.G. Reusch, Christoph J. Brabec
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    ABSTRACT: The authors investigated passive cooling of large-area organic light-emitting diodes (OLEDs) with special focus on convective cooling. Electro-optical and thermal behaviour of large-area OLED devices are therefore modelled using finite element method (FEM) and computational fluid dynamics (CFD) simulations. Resulting temperature and luminance distributions are compared with measurement data at different driving conditions and test setups. The investigation yields that including laterally resolved convection coefficients from CFD simulations greatly improves model accuracy compared to simpler convection estimations. These findings are important for OLED and their heat spreader design especially for features like flexible, transparent, high-power and or large-area due to their specific limitations for heat spreading and or their high heat spreading requirements.
    Organic Electronics 08/2013; 14(8):1939–1945. · 3.84 Impact Factor
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    Christoph J. Brabec, N. Serdar Sariciftci
    Materials Today 07/2013; 3(2):5–8. · 6.07 Impact Factor
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    ABSTRACT: Recently, the concept of near-infrared sensitization is successfully employed to increase the light harvesting in large-bandgap polymer-based solar cells. To gain deeper insights into the operation mechanism of ternary organic solar cells, a comprehensive understanding of charge transfer-charge transport in ternary blends is a necessity. Herein, P3HT:PCPDTBT:PCBM ternary blend films are investigated by transient absorption spectroscopy. Hole transfer from PCPDTBT-positive polarons to P3HT in the P3HT:PCPDTBT:PCBM 0.9:0.1:1 blend film can be visualized. This process evolves within 140 ps and is discussed with respect to the proposed charge-generation mechanisms.
    Macromolecular Rapid Communications 07/2013; 34(13):1090-1097. · 4.93 Impact Factor
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    Advanced Energy Materials 07/2013; · 10.04 Impact Factor
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    ABSTRACT: Sustainable biomass production is expected to be one of the major supporting pillars for future energy supply, as well as for renewable material provision. Algal beds represent an exciting resource for biomass/biofuel, fine chemicals and CO2 storage. Similar to other solar energy harvesting techniques, the efficiency of algal photosynthesis depends on the spectral overlap between solar irradiation and chloroplast absorption. Here we demonstrate that spectral conversion can be employed to significantly improve biomass growth and oxygen production rate in closed-cycle algae reactors. For this purpose, we adapt a photoluminescent phosphor of the type Ca0.59Sr0.40Eu0.01S, which enables efficient conversion of the green part of the incoming spectrum into red light to better match the Qy peak of chlorophyll b. Integration of a Ca0.59Sr0.40Eu0.01S backlight converter into a flat panel algae reactor filled with Haematococcus pluvialis as a model species results in significantly increased photosynthetic activity and algae reproduction rate.
    Nature Communications 06/2013; 4:2047. · 10.02 Impact Factor
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    ABSTRACT: Ternary composite inverted organic solar cells based on poly(3-hexylthiophen-2,5-diyl) (P3HT), phenyl-C61-butyric acid methyl ester (PCBM) blended with two different near-infrared absorbing benzannulated aza-BODIPY dyes, Difluoro-bora-bis-(1-phenyl-indoyl)-azamethine (1) or Difluoro-bora-bis-(1-(5- methylthiophen)-indoyl)-azamethine (2), were constructed and characterized. The amount of these two aza-BODIPY dyes, within the P3HT and PCBM matrix were systematically varied and the characteristics of the respective devices were recorded. Although the addition of both aza-BODIPY dyes enhanced the absorption of the blends, only the addition of 1 improved the overall power conversion efficiency (PCE) in the near-IR region. The present work paves the way for the integration of near-infrared absorbing aza-BODIPY derivatives as sensitizers in ternary composite solar cells.
    ACS Applied Materials & Interfaces 05/2013; · 5.01 Impact Factor
  • Tayebeh Ameri, Ning Li, Christoph J Brabec
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    ABSTRACT: Multi-junction solar cell configuration, where two or further sub-cells with complementary absorption are stacked and connected in series or parallel, offer an exciting approach to tackle the single junction limitations of organic solar cells and further improve their power conversion efficiency. In this article we aim to follow up our previews work and review the most important and novel developments that have been recently reported on organic tandem solar cells. In addition, some brief theoretical considerations addressing the potential of single and tandem solar cells, the working principles of the intermediate layer, the importance and benefits of optical simulation and finally the intricacies of a precise performance measurement of bulk-heterojunction organic tandem solar cells based on complementary absorber materials are presented.
    Energy & Environmental Science 05/2013; · 11.65 Impact Factor
  • Tayebeh Ameri, Parisa Khoram, Jie Min, Christoph J Brabec
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    ABSTRACT: Recently, researchers have paid a great deal of attention to the research and development of organic solar cells, leading to a breakthrough of over 10% power conversion efficiency. Though impressive, further development is required to ensure a bright industrial future for organic photovoltaics. Relatively narrow spectral overlap of organic polymer absorption bands within the solar spectrum is one of the major limitations of organic solar cells. Among different strategies that are in progress to tackle this restriction, the novel concept of ternary organic solar cells is a promising candidate to extend the absorption spectra of large bandgap polymers to the near IR region and to enhance light harvesting in single bulk-heterojunction solar cells. In this contribution, we review the recent developments in organic ternary solar cell research based on various types of sensitizers. In addition, the aspects of miscibility, morphology complexity, charge transfer dynamics as well as carrier transport in ternary organic composites are addressed.
    Advanced Materials 05/2013; · 14.83 Impact Factor
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    ABSTRACT: This article presents a smart strategy to successfully tackle two main limitations of organic solar cells (OSCs) in one step: the narrow absorption window of polymers as well as the Voc limitation related to the mono-PCBM. We demonstrate the high OSC performance of >5% in the combination of a fullerene multiadduct, i.e. indene-C60 bisadduct (ICBA) with a low bandgap polymer employing the concept of ternary OSCs. Solar cells achieving a fill factor (FF) of >60% along with high quantum efficiency in the near IR region are demonstrated for ternary composites consisting of P3HT, the fullerene-bisindene adduct of ICBA and a low bandgap copolymer sensitizer. Using P3HT as an efficient transport matrix with non-Langevin recombination dynamics allowed us to limit and overcome the otherwise dominant recombination losses of the indene-C60 bisadduct – low bandgap copolymer blends. The success of this strategy resulted in a relative efficiency improvement of over 25%.
    Energy & Environmental Science 05/2013; 6(6):1796-1801. · 11.65 Impact Factor

Publication Stats

2k Citations
704.37 Total Impact Points


  • 2009–2014
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      • Department of Materials Science and Engineering
      Erlangen, Bavaria, Germany
  • 2013
    • Universitätsklinikum Jena
      Jena, Thuringia, Germany
  • 1995–2013
    • Johannes Kepler University Linz
      • • Institute of Physical Chemistry
      • • Linzer Institut für Organische Solarzellen
      Linz, Upper Austria, Austria
  • 2011
    • Bayerisches Zentrum für angewandte Energieforschung
      Würzburg, Bavaria, Germany
  • 2010
    • Universitätsklinikum Erlangen
      Erlangen, Bavaria, Germany
  • 2008
    • University of California, Santa Barbara
      • Center for Polymers and Organic Solids
      Santa Barbara, CA, United States
  • 2004
    • Carl von Ossietzky Universität Oldenburg
      • Working Group of Energy and Semiconductor Research
      Oldenburg, Lower Saxony, Germany
  • 2003
    • University of Groningen
      Groningen, Groningen, Netherlands