Photoinduced Charge Transfer Rates in Soluble P3HT:PCBM Nano-Aggregates Predict the Solvent Dependent Film Morphology

Palas Roy^a, Ajay Jha^a, Jyotishman Dasgupta^a

^aDepartment of Chemical Sciences, Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai, PIN-400005

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Palas Roy, 328
Publication date: 5th February 2015

Solution-processable bulk heterojunction (BHJ) solar cells offer a cheaper alternative to the conventional crystalline Si-based photovoltaics. Power conversion efficiency of BHJ solar cells is critically dependent on the nano-morphology of the spin-casted polymer:fullerene blend in its active layer. In order to achieve synthetic control on blend morphologies, it is imperative to understand the evolution of polymer:fullerene solution structures during the film casting process. Herein we use photoinduced charge transfer rates to probe the effective length scale of the pre-formed solution structures arising from a mixture of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)in three different organic solvents. Our results show solvent-dependent bi-exponential rise of the polaronic states in solution which is analogous to the charge generation mechanism in film (1). The ultrafast sub-picosecond component was assigned to immediate generation of polarons at the polymer:fullerene interface while slower component of 5-18 ps results from exciton dissociation subsequent to its diffusion to the interface. Using the diffusive component of the CT rate, we deduce ~2 times smaller functional size of polymer:fullerene nano-domains in toluene than in chlorobenzene thus correctly predicting the relative domain sizes previously observed in spin-casted films (2). We therefore demonstrate the direct spectroscopic evidence of the postulated intermediate polymer:fullerene:solvent ternary phase that seeds the eventual nano-morphology in the films. The significance of such solvent-control in other classes of solution-processable solar cells including the perovskite class will be discussed.
Solvent dependence to the ultrafast biphasic polaron generation rates provide evidence for the existence of a polymer:fullerene:solvent ternary phase.
1. (a) Guo, J. M.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154-6164. (b) Herrmann, D.; Niesar, S.; Scharsich, C.; Kohler, A.; Stutzmann, M.; Riedle, E. Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption. J. Am. Chem. Soc. 2011, 133, 18220-18233. (c) Paraecattil, A. A.; Banerji, N. Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material. J. Am. Chem. Soc. 2014, 136, 1472-1482. 2. (a) Yang, X. N.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nanoscale morphology of high-performance polymer solar cells. Nano Lett. 2005, 5, 579-583. (b) Moon, J. S.; Lee, J. K.; Cho, S. N.; Byun, J. Y.; Heeger, A. J. "Columnlike" Structure of the Cross-Sectional Morphology of Bulk Heterojunction Materials. Nano Lett. 2009, 9, 230-234. (c) Heitzer, H. M.; Savoie, B. M.; Marks, T. J.; Ratner, M. A. Organic Photovoltaics: Elucidating the Ultra-Fast Exciton Dissociation Mechanism in Disordered Materials. Angew. Chem. Int. Ed. 2014, 53, 7456-7460. (d) Hoppe, H.; Sariciftci, N. S. Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 2006, 16, 45-61.

nanoGe is a prestigious brand of successful science conferences that are developed along the year in different areas of the world since 2009. Our worldwide conferences cover cutting-edge materials topics like perovskite solar cells, photovoltaics, optoelectronics, solar fuel conversion, surface science, catalysis and two-dimensional materials, among many others.

MATSUS

Previously nanoGe Spring Meeting (NSM) and nanoGe Fall Meeting (NFM), MATSUS is a multiple symposia conference focused on a broad set of topics of advanced materials preparation, their fundamental properties, and their applications, in fields such as renewable energy, photovoltaics, lighting, semiconductor quantum dots, 2-D materials synthesis, charge carriers dynamics, microscopy and spectroscopy semiconductors fundamentals, etc.

International Conference on Hybrid and Organic Photovoltaics

International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.

Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics

The main topics of the Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP) are discussed every year in Asia-Pacific for gathering the recent advances in the fields of material preparation, modeling and fabrication of perovskite and hybrid and organic materials. Photovoltaic devices are analyzed from fundamental physics and materials properties to a broad set of applications. The conference also covers the developments of perovskite optoelectronics, including light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.

International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics

The International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics (NIPHO) is the best place to hear the latest developments in perovskite solar cells as well as on recent advances in the fields of perovskite light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.