Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response

Letter pubs.acs.org/JPCL

Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response Giulia Grancini,†,‡ R. Sai Santosh Kumar,‡ Margherita Maiuri,§ Junfeng Fang,∥ Wilhelm T. S. Huck,∥,⊥ Marcelo J. P. Alcocer,‡,§ Guglielmo Lanzani,‡ Giulio Cerullo,§ Annamaria Petrozza,*,‡ and Henry J. Snaith*,† †

Clarendon Laboratory, Department of Physics, Oxford University, Parks Road, Oxford, OX13PU, U.K. Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133 Milano, Italy § IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy ∥ Melvile Laboratory of Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge, CB2 1TN, U.K. ⊥ Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands ‡

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ABSTRACT: There has been phenomenal effort synthesizing new low-band gap polymer hole-conductors which absorb into the near-infrared (NIR), leading to >10% efficient all-organic solar cells. However, organic light absorbers have relatively narrow bandwidths, making it challenging to obtain panchromatic absorption in a single organic semiconductor. Here, we demonstrate that (poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) can be “photo-sensitized” across the whole visible spectrum by “doping” with a visible absorbing dye, the (2,2,7,7-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9-spirobifluorene) (spiro-TBT). Through a comprehensive sub-12 femtosecond−nanosecond spectroscopic study, we demonstrate that extremely efficient and fast energy transfer occurs from the photoexcited spiro-TBT to the PCPDTBT, and ultrafast charge injection happens when the system is interfaced with ZnO as a prototypal electron-acceptor compound. The visible photosensitization can be effectively exploited and gives panchromatic photoresponse in prototype polymer/oxide bilayer photovoltaic diodes. This concept can be successfully adopted for tuning and optimizing the light absorption and photoresponse in a broad range of polymeric and hybrid solar cells. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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to isolate and purify and produced at a relatively low yield, and in addition, C70PCBM is limited in its own spectral width. Nanostructured hybrid architectures, where the polymer is infiltrated into a metal oxide scaffold, employ a transparent ntype oxide as the electron acceptor.12−14 For this system, the light harvesting capacity of the polymer can be enhanced by a surface adsorbed dye, as a fusion between dye-sensitized solar cells (DSSCs) and organic photovoltaics (OPV).12,23 However, extremely careful engineering of the interface is required to ensure good charge generation from both the dye and the polymer phases.15−20 Alternatively, in order to achieve intense panchromatic absorption from an organic system, additional dyes can be employed as “light harvesting antennas”, and transfer their captured photon energy to the organic component responsible for charge generation in the solar cell.

emiconducting polymers are attracting a growing interest as active materials for clean power generation: they offer excellent light harvesting capabilities and good charge carrier mobility.1−4 However, in contrast to inorganic absorbers, the energy bands are relatively narrow, and the low band gap polymers tend to incompletely absorb light in the visible region of the spectrum. Although this opens aesthetic possibilities for applications such as building integrated photovoltaics, it limits the overall solar light absorbed and hence the efficiency of a polymer-based solar cell.3 In addition, in contrast to inorganic absorbers, a heterojunction is required between the light absorbing polymer and an electron acceptor in order to ionize the photoinduced excitons.4−6 For all organic solar cells, panchromatic absorption is achieved by employing an electron acceptor that also absorbs visible light.7−9 However, despite significant effort on developing n-type light absorbing polymers and molecules, solar cells incorporating the (6,6)-phenyl-C70butyric acid methyl ester (C70-PCBM),10,11 or derivatives thereof remain twice as efficient as those incorporating the next best electron acceptor. Fullerene derivatives, especially the larger molecules such as C70-PCBM, are reportedly challenging © XXXX American Chemical Society

Received: December 23, 2012 Accepted: January 15, 2013

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dx.doi.org/10.1021/jz302150q | J. Phys. Chem. Lett. 2013, 4, 442−447

The Journal of Physical Chemistry Letters

Letter

Figure 1. (a) Absorption spectra of a thin film of PCPDTBT (red dashed line), spiro-TBT (blue open squares), and of the PCPDTBT:spiro-TBT blend (black dotted line), exhibiting good spectral coverage of all the visible and near IR spectral region. The photoluminescence spectrum of the spiro-TBT is also presented (black dot line), showing a good overlap with the PCPDTBT absorption spectrum. (b) Schematic energy level diagram for a ZnO/spiro-TBT: PCPDTBT solar cell highlighting charge generation and energy transfer pathways. (c) TRPL observed for the blend upon excitation at 540 nm at various temporal delays after excitation (see legend); note that the detector cutoff wavelength is ∼850 nm. (d) PL kinetics of blend and pristine polymer probed at 630 and 850 nm. Fit of the signal rise in the blend as red solid line.

In this respect, the additional dye can function as an “energy relay dye” (ERD).12,21−23 A highly efficient mechanism for funneling energy is the Fö rster Resonant Energy Transfer (FRET). 12,24,25 The mechanism is a nonradiative energy transfer process induced by the dipole−dipole interaction of a donor−acceptor pair through an electric-dipole field. Due to the ability to channel excitation energy from one location to another, or from one component to another, FRET has been adopted in many different configurations in excitonic solar cells.12,21−23,26,27 For organic photovoltaics, energy transfer has been used in planar multilayer device structures to obtain longer range exciton quenching. In this case, a thin film (