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Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response Giulia Grancini, Raavi 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 J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz302150q • Publication Date (Web): 15 Jan 2013 Downloaded from http://pubs.acs.org on January 19, 2013

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The Journal of Physical Chemistry Letters

Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response Giulia Grancini,1,2 R. Sai Santosh Kumar,2 Margherita Maiuri,3 Junfeng Fang,4 Wilhelm T. S. Huck,4,5 Marcelo J. P. Alcocer,2, 3 Guglielmo Lanzani, 2 Giulio Cerullo, 3 Annamaria Petrozza 2*, and Henry J. Snaith 1* 1

Oxford University, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX13PU, UK.

2

Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133 Milano, Italy.

3

IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy.

4

University of Cambridge, Department of Chemistry, Melvile Laboratory of Polymer Synthesis, UK. 5

Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands

ACS Paragon Plus Environment

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ABSTRACT There has been phenomenal recent effort synthesizing new low-band-gap polymer hole-conductors absorbing into the 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)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3benzothiadia-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-2yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9-spirobifluorene) (Spiro-TBT). Through a comprehensive sub-12 fs -ns 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 electronacceptor compound. The visible photo-sensitization can be effectively exploited and gives panchromatic photo-response 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.

TOC GRAPHIC


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KEYWORDS Hybrid polymer/oxide interface, low-band-gap polymer, energy transfer, spectral response, ultrafast spectroscopy

Semiconducting polymers are attracting a growing interest as active materials for green power generation: they offer excellent light harvesting capabilities and good charge carrier mobility (14). However, in contrast to inorganic absorbers, the energy bands are relatively narrow, and the low band gap polymers, commonly used as light antenna, 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 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 which 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-C70-butyric acid methyl ester (C70-PCBM) (10-11), or derivatives thereof remain twice as efficient as those incorporating the next best electron acceptor. Fullerene derivatives and especially the larger molecules such as C70-PCBM are reportedly challenging to isolate and purify and produced at a relatively low yield, and in addition C70PCBM is limited in its own spectral width. Nano-structured hybrid architectures, where the polymer is infiltrated into a metal oxide scaffold, employ a transparent n-type 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 (DSSC) and organic photovoltaics (OPV) (12). However,


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extremely careful engineering of the interface is required to assure good charge generation from both the dye and the polymer phases (11-12). 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. 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 Förster Resonant Energy Transfer (FRET) (12, 24-25). The mechanism is a non-radiative 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, 2123,26-27). 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 ( 10 ps) a broad PA band appears which is assigned to charged species formed after electron injection into the ZnO. This proves that charge injection occurs at the spiro-TBT/ZnO interface. Finally, Figure 3.c shows the TA spectrum of the spiroTBT:PCPDTBT blend on ZnO. Though the spiro-TBT is being predominantly photoexcited, at only 200 fs delay we can already observe a PA band which is assigned to charge absorption in the PCPDTBT phase (see also the dynamics in Fig. 3.d). Notably, similarly fast charge generation dynamics have been found in the efficient PCPDTBT: fullerene blend based photovoltaic system (30,33). This is the first evidence, to the authors’ knowledge, of such a fast charge transfer process at a polymer/metal oxide hybrid interface. It has been recently found that, at the PCPDTBT: PCBM interface, a higher degree of delocalization of the hot interfacial


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excited states with respect to the relaxed ones enhances the probability of dissociation in the first 200 fs (33). On the other hand, R.H Friend et al. have shown that photogenerated charges upon band-gap excitation stay bound and later recombine at the ZnO/polymer interface (34). Based on this, we can speculate that the FRET from the spiro-TBT to the PCPDTBT, which, importantly, involves high-energy electronic states of the polymer, can boost an extremely efficient charge separation, even if a further investigation would be needed. It is worth noticing that the PCPDTBT PB signal in the blend keeps growing over a few ps, together with the charge absorption band. The rise time of the PCPDTBT PB matches the value derived from the PL measurements (shown in Figure 1d), consistent with FRET from the photoexcited spiro-TBT to the PCPDTBT. Thus, the measurements indicate that the FRET mechanism will occur very efficiently in the whole photovoltaic system, and ensure that the exciton formed on the spiro-TBT are effectively transferred from the very initial timescale up to around 20 ps to the PCPDTBT. The broad spread of times over which energy transfer occurs (10 fs to 20 ps) is consistent with a FRET process being the dominant mechanism, with the spread resulting from a distribution of average distances between the energy donor and the acceptor molecules. To demonstrate that cooperative light absorption leads to panchromatic photoresponse, we fabricated a simple photovoltaic diode made of a hybrid ZnO/polymer blend bilayer, as a proof of concept of the mechanism discovered. Figure 4 shows the comparison of the EQE spectra (normalized at 700 nm to highlight the contribution of the SpiroTBT, mostly at 500 nm) of the PCPDTBT/ZnO bilayer with the SpiroTBT:PCPDTBT/ZnO one. The EQE illustrates the cooperative contribution of the two compounds to the photocurrent generation in the blend device, with a massive increase in the 500 nm region where spiro-TBT absorbs strongly and PCPDTBT absorbs weakly. The measured EQE and the current/voltage


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characteristic of the fabricated device is presented in Figure S4 of the Supplementary Information together with the solar cell figures of merit. Note that in the simple bilayer device we observed that doping PCPDTBT with the Spiro-TBT results in both a 5-fold increase in the photocurrent and a larger open circuit voltage with respect to the control device where neat PCPDTBT was used as the sole light harvesting compound (see Supplementary Information). As a proof of concept, it illustrates that this is a feasible route forward to improve the performances of both hybrid and all organic solar cells employing low band gap polymer or molecular holeconductors. However, further work is still needed on device optimization. In summary, we have demonstrated a unique approach for panchromatic photosensitization of low band gap polymers: by blending an emissive molecular ERD into a semiconducting polymer host, ultrafast and very efficient transfer of the photoexcitations from the donor to the host occurs. We have demonstrated that this process can be employed to achieve panchromatic photoresponse, with equal contribution from both the energy donor and acceptor, in a simple planar junction hybrid solar cell. This concept should be extended in the class of excitonic solar cells. In particular, for all organic solar cells, it relaxes the strict demands upon the electron acceptor (typically fullerene) to function as both an electron acceptor and light absorber, and opens up new design possibilities for optimizing energy donor-acceptor polymers for organic solar cells. This could lead to a separation of the primary functions of light harvesting and charge generation, similarly to what as is done in the DSSC, though the charge generation is still happening at only one interface. In this instance, a well operating but pooly absorbing low bandgap-polymer electron-acceptor pair, could be photosensitized to harvest completely accross the spectrum.


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FIGURES

Figure 1


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Figure 2


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Figure 3


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Figure 4


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FIGURE CAPTIONS

Figure 1. a) absorption spectra of a thin film of PCPDTBT (red dashed line), SpiroTBT (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 cut-off wavelength is ~850 nm; d) PL kinetics of blend and pristine polymer probed at 630 nm and 850 nm. Fit of the rise signal in the blend as red solid line.

Figure 2. Sub-12 fs TA spectra (a) and dynamics at selected probe wavelengths (b) for the PCPDTBT:spiro-TBT blend on glass upon 500 nm excitation. The dashed lines correspond to exponential fits. The pump pulse energy density used in the experiment is kept deliberately low (pump fluence < 30µJ/cm2). c) TA spectra integrated in the first 10 ps time window of the PCPDTBT:spiro-TBT blend on glass upon 780 nm excitation.

Figure

3.

TA

spectra

for

ZnO/PCPDTBT:spiro-TBT blend.

a)

the

ZnO/PCPDTBT,

b)

ZnO/spiro-TBT

and

c)

d) Time traces following the PB (740 nm) and charge

absorption PA (520 nm) dynamics in ZnO/PCPDTBT:spiro-TBT blend. The pump wavelength has been tuned to 780 nm to excite the ZnO/PCPDTBT sample and to 500 nm (resonant with the spiro-TBT main peak) for the ZnO/spiro-TBT and ZnO/PCPDTBT:spiro-TBT blend samples.


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The pump pulse energy density used in the experiment is kept deliberately low (pump fluence= 10 µJ/cm2). Figure 4. External Quantum Efficiency measure for ZnO/PCPDTBT (red solid line) and ZnO/spiroTBT:PCPDTBT (black full dots) normalized to the PCPDTBT peak at 700 nm. In the inset a schematic of the prototypal planar device fabricated.

The authors thank Dr M. Caironi for fruitful discussions. GC acknowledges financial support from the PRIN programme 2008JKBBK4. Supporting Information. Detailed experimental methods used and additional data on spectroscopy, morphological and device characterization are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *[email protected]; *[email protected]

Author Contributions Data were taken by G.G., R.S.S.K., M.M. and M.J.P.A. The material has been provided by J.F. and W.T.S.H. The data have been analyzed by G.G., A.P. and H.S. The experiment was conceived by G.G., A.P., H.S., G.L. and G.G. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources


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This project was part funded by EPSRC and the ERC through the ERC-2011-StG 279881HYPER project. AP and HJS thanks “The Royal Society International Exchanges Scheme 2012/R2”.

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