Panchromatic “Dye-Doped” Polymer Solar Cells:
From Femtosecond Energy Relays to Enhanced
Photo-Response
Giulia Grancini,1,2 Raavi 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.
5Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135,
6525 AJ Nijmegen, The Netherlands
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In Figure S1 the CW photoluminescence (PL) spectra of films of neat spiro-TBT, neat
PCPDTBT, and PCPDTBT:spiro-TBT blend on glass substrates are shown.
The spiro-TBT emission is entirely quenched when blended with PCPDTBT and the emission
of the blend closely follows that of the neat PCPDTBT film.
The PL excitation spectra of the neat PCPDTBT and the blend films, probing the emission at
850 nm, are shown in Fig. S1b. For the neat polymer film there is a dip in the PLE spectrum at
500 nm, corresponding to the dip in the absorption spectrum, while for the blend film the PLE
spectrum shows a clear contribution from the spiro-TBT absorption, indicating efficient
excitation energy transfer, via a FRET process.
Figure S1. a) Photoluminescence (PL) spectra of the neat spiroTBT thin film (blue squares),
excited at 540nm, and of the neat PCPDTBT thin film (red diamonds) excited at 700 nm and the
PCPDTBT:spiroTBT blend (1:0.4) thin film (black full dots) excited at 540nm. b) PL Excitation
spectra for the neat PCPDTBT and for the PCPDTBT:spiro-TBT blend upon fixing the emission
detection at 850 nm.
2
Figure S2 represents the AFM images of the samples. No phase separation is observed when
the spiroTBT is added in the polymer matrix. A large surface roughness within 100 nm height is
measured, mainly induced by the substrate rough topography.
Figure S2. AFM maps of a 1µmx1µm area of the PCPDTBT/ZnO a) and of the
PCPDTBT:SpiroTBT/ZnO b). Scalebar: 100 nm.
Figure S3 shows the Transient Absorption measurements upon exciting at 500 nm for the pure
SpiroTBT. It shows a PB and SE signals at shorter wavelength side and a huge negative band to
PA of singlets state in the IR part of the spectrum.
3
Figure S3. Sub-12 fs TA spectra for the spiro-TBT pristine on glass upon 500 nm excitation.
The pump pulse energy density used in the experiment is kept deliberately low (pump fluence<
30 µJ/cm2).
4
Figure S4. Comparison of Spectral response and Photocurrent-voltage traces for
ZnO/PCPDTBT/Ag and ZnO/spiroTBT:PCPDTBT/Ag device a) Photocurrent-voltage traces of
ZnO/PCPDTBT (red solid line) and ZnO/spiroTBT:PCPDTBT (black full dots) devices
measured under AM 1.5 G simulated sun light of 100 mWcm-2. In the inset the table showing
the short circuit current for the presented devices.b) External Quantum Efficiency measure for
ZnO/PCPDTBT (red solid line) and ZnO/spiroTBT:PCPDTBT (black full dots). In the inset is
present a schematic of the device.
The solar cell performances are evaluated under simulated A.M. 1.5G 100 mWcm-2 solar
illumination and are presented in Figure S4a. In presence of the spiro-TBT the short-circuit
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current Jsc increases by a factor 5 with respect to the neat PCPDTBT, going from 0.17 mA/cm2,
to 0.85 mA/cm2
Notably, the dye doping also doubles the device open-circuit voltage, possibly due to an
enhanced charge generation process across the hybrid interface thanks to better delocalization of
the charge wavefunctions and reduced charge recombination ,though that may simply be due to
the increased photocurrent competing better with the dark current onset.
Figure S5. Chemical structure of the SpiroTBT molecule.
Detailed Experimental Methods
Femtosecond transient absorption set-up: In a typical pump-probe experiment, the system
under study is photoexcited by a short pump pulse and its subsequent dynamical evolution is
detected by measuring the transmission changes ∆T of a delayed probe pulse as a function of
pump-probe delay τ and probe wavelength λ. The signal is given by the differential transmission
∆T/T = [(Tpump on-Tpump off)/Tpump off]. The pump probe set-up is driven by 1 kHz
repetition rate pulse train at λ= 780 nm centre wavelength with 150 fs duration produced by a
regeneratively amplified mode-locked Ti:Sapphire laser (Clark-MXR Model CPA-1).
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150fs –time resolution pump-probe set-up: A fraction of this beam is used to pump an optical
parametric amplifier (OPA) capable of delivering tunable pulses in the visible (500 – 700 nm)
with ≈10 nm bandwidth and 70-100 fs duration. Another small fraction of the Ti: sapphire
amplified output is focused into a 1-mm-thick sapphire plate to generate a stable single-filament
white-light supercontinuum which serves as a probe pulse. A short-pass filter with 760-nm cutoff
wavelength is used to filter out the residual 800 nm pump light thus limiting our probing window
to the (450-760 nm) spectral region. The pump and probe beams are spatially and temporally
overlapped on the sample, controlling the time delay by a motorized slit. The minimum
detectable signal is ∆T/T ~10-4. The system has a ~150 fs temporal resolution. Details of the
experimental set-up can be found elsewhere
[4]
. The pump beam energy density used in the
experiment is kept deliberately low (pump fluence= 10 µJ/cm2) to minimize bimolecular effects.
All the measurements were taken with the samples in a vacuum chamber, to prevent any
influence from oxygen or sample degradation. The pump-probe measurements were taken on
PCPDTBT and spiroTBT film spin cast on the ZnO substrates.
Sub 12 fs –time resolution pump-probe set-up: The laser source is used to pump two noncollinear OPAs (NOPAs), which have the capability of generating broadband pulses exploiting
non-collinear phase matching, in β-barium borate (BBO). The first NOPA generates 15-20 fs
visible pulses centered at 510 nm and is used to resonantly pump the first excited singlet state of
spiro-TBT. The second NOPA can be adjusted to generate either 7 fs pulses in the visible [550750 nm] region or 12 fs pulses in the near-IR [830-1050 nm] spectral range. Pulse compression
is achieved by chirped mirrors for the visible pulses and by a fused-silica prism pair for near-IR
pulses [1, 2]. The pulses derived from the NOPAs are synchronized by a delay line and focused on
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the sample using only reflective optics, in a standard non-collinear pump-probe configuration.
Broadband probe spectra are dispersed on a optical multichannel analyzer with single shot
detection capability at 1 kHz, allowing the acquisition of 2D differential transmission (∆T /T
(λ,τ)) maps as a function of probe wavelength λ and delay τ.
Time
Resolved
Photoluminescence:
Time
Resolved
Photoluminescence
(TRPL)
Measurements: TRPL measurements were performed using a femtosecond laser source and
streak camera detection system. A Ti:Sapphire laser oscillator (Coherent Chameleon Ultra II)
operating at 80MHz was tuned to provide pulses with central wavelengths of 780 nm, energies of
~50 nJ, and temporal duration and spectral bandwidth of ~140 fs and ~ 5 nm respectively. These
were launched into a photonic crystal fiber (NKT Photonics NL-PM-750) to generate a supercontinuum from which the desired pump wavelength of 540nm was selected using an
interference filter. The pump pulses were focused onto the sample and collected emission was
analyzed by a spectrograph (Princeton Instruments Acton SP2300) coupled to a streak camera
(Hamamatsu C5680) equipped with an 80 MHz synchroscan voltage sweep module (Hamamatsu
M5675). In this way, measurements of photoluminescence intensity as a function of both
wavelength and time were obtained with spectral and temporal resolutions of ~1 nm and ~3 ps
respectively. Temporal broadening of the pump pulses caused by transmission through the fiber
was confirmed to be well below the response time of the detection system. The pump fluence
was kept very low (<30 µJ/cm2, ~100 µm spot diameter) in order to avoid any saturation effects
in the sample. The samples were kept in a vacuum chamber.
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Device fabrication: Fluorine doped tin oxide (FTO) coated glass sheets were etched with zinc
powder and HCl (2 Molar) to obtain the required electrode pattern. The sheets were then washed
with soap (Hellmanex at 2% in water), de-ionized water, acetone, methanol and finally treated
under an oxygen plasma for 10 minutes to remove the last traces of organic residues. The FTO
sheets were subsequently coated with a compact layer of ZnO (100 nm) by aerosol spray
pyrolysis deposition at 400º C on a Zn powder solution diluted in Methanol, using air as the
carrier gas. PCPDTBT was dissolved in chlorobenzene at 30 mg/mL concentration, heated at 70º
C for 1 hour and stirred overnight and spin coated on the substrate at 1000 rpm for 45 seconds.
Optionally, Spiro-TBT was added to the PCPDTBT solution. A different range of concentrations
have been tested from 1% to 50%. The optimized device consists of 25% of SpiroTBT (w%w)
with respect to the PCPDTBT. The films were then placed in a thermal evaporator where 150 nm
thick silver electrodes were deposited through a shadow mask under high vacuum (10-6 mbar), to
give rectangular cells with an active area of ~ 0.12 cm2.
Device testing: The active areas of the devices were defined by single aperture metal optical
masks with a round aperture of 2.5 mm giving an area of 0.0491 cm2. The EQE measurements
were taken employing a monochromated Xenon light source calibrated with a silicon reference
diode and measured on a PC interfaced Keithley 2400 sourcemeter [3].
REFERENCES
1) Cerullo, G.; Manzoni, C.; Lüer L.; Polli, D. Time-resolved methods in biophysics. 4.
Broadband pump–probe spectroscopy system with sub-20 fs temporal resolution for the study of
energy transfer processes in photosynthesis. Photochem. Photobiol. Sci. 2007, 6, 135–144.
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2) Polli, D.; Lüer, L.; Cerullo, G. High-Time-Resolution pump-probe System with Broadband
Detection for the Study of Time-domain Vibrational Dynamics Rev. Sci. Instrum. 2007, 78,
103108.
3) Abrusci, A.; Ding, I. K.; Al-Hashimi, M.; Segal-Peretz, T.; McGehee, M .D.; Heeney, M.;
Frey, G. L.; Snaith, H. J. Facile Infiltration of Semiconducting Polymer into Mesoporous
Electrodes for Hybrid Solar Cells. Energy Environ. Sci., 2011, 4, 3051–3058.
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