Elementary Processes in Organic Photovoltaics

Frontmatter

In situ Studies of Morphology Formation in Solution-Processed Polymer–Fullerene Blends

Abstract

Control of the blend nanomorphology in bulk heterojunctions (BHJs) is still a challenge that demands more fundamental knowledge of the mechanism of phase separation and crystallization during solvent drying. In this review we show that in situ studies using combined laser reflectometry and grazing-incidence wide-angle X-ray scattering provide a fundamental understanding on how the nanomorphology develops dynamically during film drying. We identify influencing parameters for controlled film formation in order to obtain optimized solar cell performance. We review here our results on BHJs of poly(3-hexylthiophene)–[6,6]-phenyl-C61-butyric acid methyl ester and poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3 benzothidiazole)-4,7-diyl} with [6,6]-phenyl-C71-butyric acid methyl ester.

Esther Barrena, Felix Buss, Ana Perez-Rodriguez, Monamie Sanyal, Benjamin Schmidt-Hansberg, Michael F. G. Klein, Philip Scharfer, Wilhelm Schabel, Uli Lemmer

Organic and Hybrid Solar Cells Based on Well-Defined Organic Semiconductors and Morphologies

Abstract

Organic and hybrid bulk heterojunction (BHJ) solar cells are investigated. We describe the synthesis and solar cell characteristics of well-defined functional thiophene dendrimers. Three-dimensional morphologies of the polymer–metal oxide BHJs are analyzed with electron tomography and stochastic models, and are simulated with the latter, to establish the effect of processing on morphology. Device models based on stochastically simulated data of morphologies and transport parameters are used to provide accurate descriptions of solar cell performance.

Amaresh Mishra, Volker Schmidt, René A. J. Janssen, Peter Bäuerle

Dicyanovinylene-Substituted Oligothiophenes for Organic Solar Cells

Abstract

We investigate dicyanovinyl-substituted oligothiophene derivatives as absorber materials in organic solar cells. We determine structure–property relationships, which are important for materials design. We demonstrate the influence of those structural changes on the processing ability, energy levels, optical properties, thin-film morphology, and charge transport. Furthermore, we give a detailed picture of the microscopic processes between photon absorption and charge carrier generation, in particular, the importance of triplet exciton losses and a relationship between the yield of charge carrier generation and macroscopic charge-transport properties.

Christian Koerner, Hannah Ziehlke, Roland Fitzner, Moritz Riede, Amaresh Mishra, Peter Bäuerle, Karl Leo

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Abstract

Planar and bulk heterojunctions of organic donor and acceptor molecules are used to understand elementary processes in photovoltaic cells. The electronic structure, interface and film morphology, excitonic behavior, device characteristics, and correlations between these properties are reviewed here using a wide range of material combinations.

Andreas Opitz, Rupak Banerjee, Stefan Grob, Mark Gruber, Alexander Hinderhofer, Ulrich Hörmann, Julia Kraus, Theresa Linderl, Christopher Lorch, Andreas Steindamm, Anna Katharina Topczak, Andreas Wilke, Norbert Koch, Jens Pflaum, Frank Schreiber, Wolfgang Brütting

Optoelectronic Properties of PCPDTBT for Photovoltaics: Morphology Control and Molecular Doping

Abstract

Donor–acceptor copolymers have recently been recognized as excellent materials for organic photovoltaic applications. Because of complex film formation properties, however, direct correlations between morphology and optical and electrical properties have yet to be established. Within our collaborative project within the German Science Foundation–funded program “Elementary Processes in Photovoltaics,” we focused on the morphology control and molecular doping of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] to control the optoelectronic properties. The main results are highlighted in this chapter.

Elizabeth von Hauff, Enrico da Como, Sabine Ludwigs

Interplay Between Microscopic Structure and Intermolecular Charge-Transfer Processes in Polymer–Fullerene Bulk Heterojunctions

Abstract

We provide an overview of the development and application of nonconventional techniques that allowed us to probe the molecular excited states in conjugated polymers with respect to their molecular orientation and the nanomechanical properties of the topmost surface layer. To probe triplet and charge-transfer excited states and their fate, we applied the optically detected magnetic resonance technique extended for angular resolution. The surface morphology and three-dimensional depth profiles of the topmost surface layer were obtained with multi-set point intermittent contact mode atomic force microscopy. These studies were performed on model systems such as poly(3-hexylthiophene) as well as on the novel, low-bandgap copolymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b9)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)].

Vladimir Dyakonov, Hannes Kraus, Andreas Sperlich, Robert Magerle, Mario Zerson, Martin Dehnert

Nanoscale Morphology from Donor–Acceptor Block Copolymers: Formation and Functions

Abstract

General design principles of donor–acceptor block copolymers are reviewed and specific results arising from block copolymers consisting of semicrystalline poly(3-hexylthiophene-2,5-diyl) (P3HT) blocks of appreciably high molecular weight and acceptor blocks carrying pendant perylene bisimides or fullerene derivatives are summarized. The chapter is structured according to the building blocks P3HT, poly(perylene bisimide acrylate), and a polystyrene copolymer grafted with phenyl-C61-butyric acid methyl ester used for the synthesis of the corresponding block copolymers, and in each part the synthetic challenges, structure formation, and consequences for charge transport, and in some cases photovoltaic properties, are addressed.

David Heinrich, Martin Hufnagel, Chetan Raj Singh, Matthias Fischer, Shahidul Alam, Harald Hoppe, Thomas Thurn-Albrecht, Mukundan Thelakkat

Donor–Acceptor Dyes for Organic Photovoltaics

Abstract

Small-molecule π-systems bearing donor (D) and acceptor (A) groups constitute an interesting class of dyes because of their tunable strong absorption, which covers the visible and near-infrared range. The dipolarity associated with D–A structures directs antiparallel stacking arrangements in the solid state, thus reducing the dipolar disorder at the supramolecular level. Their straightforward synthesis and purification make them good candidates for photovoltaic application with power conversion efficiencies >6 %. This review summarizes the application of D–A dyes, and in particular merocyanines, in organic photovoltaics in recent years.

Alhama Arjona-Esteban, Martin Robert Lenze, Klaus Meerholz, Frank Würthner

Controlled Morphologies by Molecular Design and Nano-Imprint Lithography

Abstract

In this review we report on the recent advances concerning molecular design and nano-imprint techniques to improve the morphology of organic solar cells. Advanced scattering techniques allow us to resolve issues pertaining to the alignment and crystallization of the photoactive materials of nanostructured solar cells. Interfacial design at a donor–acceptor heterojunction is one of the key issues for improved device performance.

Thomas Pfadler, Claudia M. Palumbiny, Wojciech Pisula, Holger C. Hesse, Xinliang Feng, Klaus Müllen, Peter Müller-Buschbaum, Lukas Schmidt-Mende

Tuning Side Chain and Main Chain Order in a Prototypical Donor–Acceptor Copolymer: Implications for Optical, Electronic, and Photovoltaic Characteristics

Abstract

The recent development of donor–acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure–property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.

Marcel Schubert, Johannes Frisch, Sybille Allard, Eduard Preis, Ullrich Scherf, Norbert Koch, Dieter Neher

Charge Carrier Generation, Recombination, and Extraction in Polymer–Fullerene Bulk Heterojunction Organic Solar Cells

Abstract

In this chapter we review the basic principles of photocurrent generation in bulk heterojunction organic solar cells, discuss the loss channels limiting their efficiency, and present case studies of several polymer–fullerene blends. Using steady-state and transient, optical, and electrooptical techniques, we create a precise picture of the fundamental processes that ultimately govern solar cell efficiency.

Frédéric Laquai, Denis Andrienko, Carsten Deibel, Dieter Neher

Controlling the Electronic Interface Properties in Polymer–Fullerene Bulk Heterojunction Solar Cells

Abstract

This work covers the use of solution-processed metal oxides as interface layers for organic solar cells. To study the interface properties, intrinsic and Al-doped ZnO _x were chosen as reference systems. From the class of n-type metal oxides, ZnO _x was chosen because it can be doped when it is solution processed. Furthermore, the influence of thin modification layers applied on top of the metal oxides is investigated.

T. Stubhan, N. Wolf, J. Manara, V. Dyakonov, C. J. Brabec

Near-Infrared Sensitization of Polymer/Fullerene Solar Cells: Controlling the Morphology and Transport in Ternary Blends

Abstract

The concept of near-infrared (NIR) sensitization can be used as an alternative strategy to extend the spectral sensitivity of wide-bandgap polymers in polymer/fullerene solar cells. In ternary systems consisting of a conjugated polymer donor, a fullerene acceptor, and a sensitizer, the fullerene needs to act as an electron acceptor as well as an electron-transport matrix, the polymeric donor should provide a sufficiently high hole mobility, and the sensitizers should sensitize the bulk heterojunction solar cell in the red/NIR region. So far we have used various optoelectrical and structural techniques to investigate the possible mechanisms of the charge transfer and charge transport among the three components and microstructure of the ternary blends. In this review-like chapter, we present our recent achievements on developing the concept of NIR sensitization for polymer/fullerene solar cells by mainly addressing the important aspect of the relationship between morphology and transport.

Tayebeh Ameri, Michael Forster, Ullrich Scherf, Christoph J. Brabec

Critical Dimensions in Small-Molecule Plasmonic Particle Solar Cells

Abstract

In this review, we summarize design principles of organic solar cells with plasmonic nanostructures. A process for scalable vacuum processing of silver nanoparticles is presented. Approximations for losses inside plasmonic structures are derived with respect to particle size and absorber material. We evaluate the characteristic length scales of plasmonic near-field enhancement and backscattering. The thickness of the absorber layers can be significantly reduced in plasmonic devices showing increased power conversion efficiency. The strongest plasmonic effects are observed in coupling particle structures.

Till Jägeler-Hoheisel, Johannes Benduhn, Christian Körner, Karl Leo

Synthesis of Conjugated Polymers with Complex Architecture for Photovoltaic Applications

Abstract

A common approach to bulk heterojunction solar cells involves a “trial-and-error” approach in finding optimal kinetically unstable morphologies. An alternative approach assumes the utilization of complex polymer architectures, such as donor–acceptor block copolymers. Because of a covalent preorganization of the donor and acceptor components, these materials may form desirable morphologies at thermodynamic equilibrium. This chapter reviews synthetic approaches to such architectures and shows the first photovoltaic results.

Anton Kiriy, Frederik C. Krebs

Electronic Properties of Interfaces with Oligo- and Polythiophenes

Abstract

Using energy- and femtosecond time-resolved two-photon photoemission spectroscopy and second harmonic generation to investigate interfaces with oligo- and polythiophenes provides important parameters such as energetic positions of transport levels, lifetimes of excitonic states, and charge-transfer times across donor–acceptor interfaces. They are essential for designing organic material–based optoelectronic devices.

Petra Tegeder

Impact of Charge Carrier Mobility and Electrode Selectivity on the Performance of Organic Solar Cells

Abstract

Low charge carrier mobilities as often observed for photoactive materials of organic solar cells have significant impact on their performance. They cause accumulation of charge carriers which can be described quantitatively by a nonohmic transport resistance in the framework of an analytical model. Further addressed in this work is surface recombination stemming from insufficient electrode selectivity which is another factor limiting the performance of organic solar cells.

Annika Spies, Jeneke Reinhardt, Mathias List, Birger Zimmermann, Uli Würfel

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