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  • 1.
    Bergqvist, Jonas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Lindqvist, Camilla
    Chalmers, Sweden .
    Backe, Olof
    Chalmers, Sweden .
    Ma, Zaifei
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tang, Zheng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Gustafsson, Stefan
    Chalmers, Sweden .
    Wang, Ergang
    Chalmers, Sweden .
    Olsson, Eva
    Chalmers, Sweden .
    Andersson, Mats R.
    Chalmers, Sweden .
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Müller, Christian
    Chalmers, Sweden .
    Sub-glass transition annealing enhances polymer solar cell performance2014In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 17, p. 6146-6152Article in journal (Refereed)
    Abstract [en]

    Thermal annealing of non-crystalline polymer: fullerene blends typically results in a drastic decrease in solar cell performance. In particular aggressive annealing above the glass transition temperature results in a detrimental coarsening of the blend nanostructure. We demonstrate that mild annealing below the glass transition temperature is a viable avenue to control the nanostructure of a non-crystalline thiophene-quinoxaline copolymer: fullerene blend. Direct imaging methods indicate that coarsening of the blend nanostructure can be avoided. However, a combination of absorption and luminescence spectroscopy reveals that local changes in the polymer conformation as well as limited fullerene aggregation are permitted to occur. As a result, we are able to optimise the solar cell performance evenly across different positions of the coated area, which is a necessary criterion for large-scale, high throughput production.

  • 2.
    Bergqvist, Jonas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Tress, Wolfgang
    Laboratory of Photonics and Interfaces, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland.
    Forchheimer, Daniel
    Nanostructure Physics, KTH Royal Institute of Technology, Stockholm, Sweden.
    Melianas, Armantas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Tang, Zheng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Haviland, David
    Nanostructure Physics, KTH Royal Institute of Technology, Stockholm, Sweden.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    New method for lateral mapping of bimolecular recombination in thin film organic solar cells2016In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 24, no 8, p. 1096-1108Article in journal (Refereed)
    Abstract [en]

    The best organic solar cells are limited by bimolecular recombination. Tools to study these losses are available; however, they are only developed for small area (laboratory-scale) devices and are not yet available for large area (production-scale) devices. Here we introduce the Intermodulation Light Beam-Induced Current (IMLBIC) technique, which allows simultaneous spatial mapping of both the amount of extracted photocurrent and the bimolecular recombination over the active area of a solar cell. We utilize the second-order non-linear dependence on the illumination intensity as a signature for bimolecular recombination. Using two lasers modulated with different frequencies, we record the photocurrent response at each modulation frequency and the bimolecular recombination in the second-order intermodulation response at the sum and difference of the two frequencies. Drift-diffusion simulations predict a unique response for different recombination mechanisms. We successfully verify our approach by studying solar cells known to have mainly bimolecular recombination and thus propose this method as a viable tool for lateral detection and characterization of the dominant recombination mechanisms in organic solar cells. We expect that IMLBIC will be an important future tool for characterization and detection of recombination losses in large area organic solar cells.

  • 3.
    Gao, Feng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. University of Cambridge, England.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Wang, Jianpu
    University of Cambridge, England; Nanjing Technical University, Peoples R China; Nanjing Technical University, Peoples R China.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 12, p. 128701-Article in journal (Refereed)
    Abstract [en]

    The charge generation mechanism in organic photovoltaics is a fundamental yet heavily debated issue. All the generated charges recombine at the open-circuit voltage (VOC), so that investigation of recombined charges at VOC provides a unique approach to understanding charge generation. At low temperatures, we observe a decrease of VOC, which is attributed to reduced charge separation. Comparison between benchmark polymer: fullerene and polymer: polymer blends highlights the critical role of charge delocalization in charge separation and emphasizes the importance of entropy in charge generation.

  • 4.
    Schwarze, Martin
    et al.
    Technical University of Dresden, Germany.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. Swiss Federal Institute Technology EPFL, Switzerland.
    Beyer, Beatrice
    Fraunhofer Institute Electron Beam Plasma Technology and CO, Germany.
    Gao, Feng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Scholz, Reinhard
    Technical University of Dresden, Germany; Technical University of Dresden, Germany.
    Poelking, Carl
    Max Planck Institute Polymer Research, Germany.
    Ortstein, Katrin
    Technical University of Dresden, Germany.
    Guenther, Alrun A.
    Technical University of Dresden, Germany.
    Kasemann, Daniel
    Technical University of Dresden, Germany.
    Andrienko, Denis
    Max Planck Institute Polymer Research, Germany.
    Leo, Karl
    Technical University of Dresden, Germany.
    Band structure engineering in organic semiconductors2016In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 352, no 6292, p. 1446-1449Article in journal (Refereed)
    Abstract [en]

    A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

  • 5.
    Tang, Zheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Elfwing, Anders
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Bergqvist, Jonas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Light Trapping with Dielectric Scatterers in Single- and Tandem-Junction Organic Solar Cells2013In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 12, p. 1606-1613Article in journal (Refereed)
    Abstract [en]

    Efficient dielectric scatterers based on a mixture of TiO2 nanoparticles and polydimethylsiloxane are demonstratedfor light trapping in semitransparent organic solar cells. An improvement of 80% in the photocurrent of an optimized semitransparent solar cell is achieved with the dielectric scatterer with approximate to 100% diffuse reflectance for wavelengths larger than 400 nm. For a parallel tandem solar cell, the dielectric scatterer generates 20% more photocurrent compared with a silver mirror beneath the cell; for a series tandem solar cell, the dielectric scatterer can be used as a photocurrent balancer between the subcells with different photoabsorbing materials. The power conversion efficiency of the tandem cell in series configuration with balanced photocurrent in the subcells exceeds that of an optimized standard solar cell with a reflective electrode. The characteristics of polydimethylsiloxane, such as flexibility and the ability to stick conformably to surfaces, also remain in the dielectric scatterers, which makes the demonstrated light trapping configuration highly suitable for large scale module manufacturing of roll-to-roll printed organic single- or tandem-junction solar cells.

  • 6.
    Tang, Zheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Bao, Qinye
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
    Jafari, Mohammad Javad
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, The Institute of Technology.
    Bergqvist, Jonas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Ederth, Thomas
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, The Institute of Technology.
    Andersson, Mats R.
    , Chalmers University of Technology, Göteborg, Sweden; University of South Australia, Australia.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Improving Cathodes with a Polymer Interlayer in Reversed Organic Solar Cells2014In: Advanced Energy Materials, ISSN 1614-6832, Vol. 4, no 15, article id 1400643Article in journal (Refereed)
    Abstract [en]

    The effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer, which is further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found to be significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep highest occupied molecular orbital (HOMO) of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor.

  • 7.
    Tang, Zheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Bao, Qinye
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
    Jafari, Mohammad Javad
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, Faculty of Science & Engineering.
    Bergqvist, Jonas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Ederth, Thomas
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, Faculty of Science & Engineering.
    Andersson, Mats R.
    Polymer Technology, Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Universal modification of poor cathodes into good ones by a polymer interlayer for high performance reversed organic solar cells2014Manuscript (preprint) (Other academic)
    Abstract [en]

    In organic bulk-heterojunction solar cells, energy losses at the active layer/electrode interface are often observed. Modification of these interfaces with organic interlayers optimizes charge carrier injection and extraction and thus improves device performance. In this work, the effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer; further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep HOMO of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor. We demonstrate a possibility of improving photovoltaic performance of reversed solar cells via a simple and universal interface modification and provide the basic guidelines for development and characterization of interface materials for organic solar cells in general.

  • 8.
    Tang, Zheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Light trapping in thin film organic solar cells2014In: Materials Today, ISSN 1369-7021, E-ISSN 1873-4103, Vol. 17, no 8, p. 389-396Article, review/survey (Refereed)
    Abstract [en]

    A major issue in organic solar cells is the poor mobility and recombination of the photogenerated charge carriers. The active layer has to be kept thin to facilitate charge transport and minimize recombination losses. However, optical losses due to inefficient light absorption in the thin active layers can be considerable in organic solar cells. Therefore, light trapping schemes are critically important for efficient organic solar cells. Traditional light trapping schemes for thick solar cells need to be modified for organic thin film solar cells in which coherent optics and wave effects play a significant role. In this review, we discuss the light trapping schemes for organic thin film solar cells, which includes geometric engineering of the structure of the solar cell at the micro and nanoscale, plasmonic structures, and more.

  • 9.
    Tress, Wolfgang
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Organic Solar Cells: Theory, Experiment, and Device Simulation2014Book (Refereed)
    Abstract [en]

    This book covers in a textbook-like fashion the basics or organic solar cells, addressing the limits of photovoltaic energy conversion and giving a well-illustrated introduction to molecular electronics with focus on the working principle and characterization of organic solar cells. Further chapters based on the author’s dissertation focus on the electrical processes in organic solar cells by presenting a detailed drift-diffusion approach to describe exciton separation and charge-carrier transport and extraction. The results, although elaborated on small-molecule solar cells and with focus on the zinc phthalocyanine: C60 material system, are of general nature. They propose and demonstrate experimental approaches for getting a deeper understanding of the dominating processes in amorphous thin-film based solar cells in general.

     The main focus is on the interpretation of the current-voltage characteristics (J-V curve). This very standard measurement technique for a solar cell reflects the electrical processes in the device. Comparing experimental to simulation data, the author discusses the reasons for S-Shaped J-V curves, the role of charge carrier mobilities and energy barriers at interfaces, the dominating recombination mechanisms, the charge carrier generation profile, and other efficiency-limiting processes in organic solar cells. The book concludes with an illustrative guideline on how to identify reasons for changes in the J-V curve.

     This book is a suitable introduction for students in engineering, physics, material science, and chemistry starting in the field of organic or hybrid thin-film photovoltaics. It is just as valuable for professionals and experimentalists who analyze solar cell devices.

  • 10.
    Tress, Wolfgang
    et al.
    Technical University of Dresden, Germany .
    Corvers, Steef
    Technical University of Dresden, Germany .
    Leo, Karl
    Technical University of Dresden, Germany .
    Riede, Moritz
    Technical University of Dresden, Germany .
    Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S-Shaped Current-Voltage Characteristics2013In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 7, p. 873-880Article in journal (Refereed)
    Abstract [en]

    The role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S-shaped current-voltage (J-V) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric field and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the s to ms regime. In combination with drift-diffusion simulation data, we demonstrate a pile-up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S-kinks in the J-V curve and can be distinguished from each other and from other reasons for S-kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S-shaped J-V curve close to open circuit to evaluate whether their electrodes are optimized.

  • 11.
    Tress, Wolfgang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Simple experimental test to distinguish extraction and injection barriers at the electrodes of (organic) solar cells with S-shaped current–voltage characteristics2013In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 117, no SI, p. 599-603Article in journal (Refereed)
    Abstract [en]

    Adjusting the work function of the two electrodes to the energy levels of the intrinsic active materials of an organic solar cell is crucial for a good device performance. Often, injection barriers (in combination with selective contacts blocking one charge carrier species) caused by a misaligned metal work function or extraction barriers resulting from insulating intentional or unintentional interlayers between metal and active layers, result in a decrease in fill factor seen in the extreme case in S-shaped current–voltage (J–V) characteristics. To avoid this S-kink, it is essential to identify its origin, desirably applying a simple experimental method. We propose an approach based on analyses of current–voltage data as a function of illumination intensity. A normalization of the J–V curves at the saturated photocurrent reveals distinctive features for each type of barrier. We apply the method to planar heterojunction small-molecule and bulk heterojunction polymer solar cells with oxidized metal electrode or plasma-treated active layer and explain the theory with a drift-diffusion model.

  • 12.
    Tress, Wolfgang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. Technical University of Dresden, Germany .
    Leo, Karl
    Technical University of Dresden, Germany .
    Riede, Moritz
    Technical University of Dresden, Germany .
    Dominating recombination mechanisms in organic solar cells based on ZnPc and C-602013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 16Article in journal (Refereed)
    Abstract [en]

    We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C-60 as model system. Analyzing the open-circuit voltage (V-oc) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing Voc at higher light intensities.

  • 13.
    Tress, Wolfgang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Leo, Karl
    Technical University of Dresden, Germany .
    Riede, Moritz
    Technical University of Dresden, Germany .
    Photoconductivity as loss mechanism in organic solar cells2013In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 7, no 6, p. 401-405Article in journal (Refereed)
    Abstract [en]

    We observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current-voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power-generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices.

  • 14.
    Tress, Wolfgang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. Swiss Federal Institute Technology EPFL, Switzerland.
    Marinova, Nevena
    Swiss Federal Institute Technology EPFL, Switzerland.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Nazeeruddin, Mohammad. K.
    Swiss Federal Institute Technology EPFL, Switzerland.
    Zakeeruddin, Shaik M.
    Swiss Federal Institute Technology EPFL, Switzerland.
    Graetzel, Michael
    Swiss Federal Institute Technology EPFL, Switzerland.
    Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination2015In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 5, no 3, p. 1400812-Article in journal (Refereed)
    Abstract [en]

    n/a

  • 15.
    Tress, Wolfgang
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. Technical University of Dresden, Germany .
    Merten, Andre
    Technical University of Dresden, Germany .
    Furno, Mauro
    Technical University of Dresden, Germany .
    Hein, Moritz
    Technical University of Dresden, Germany .
    Leo, Karl
    Technical University of Dresden, Germany .
    Riede, Moritz
    Technical University of Dresden, Germany .
    Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance2013In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 5, p. 631-638Article in journal (Refereed)
    Abstract [en]

    We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.

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