as an Energy Relay Dye in Dye-Sensitized Solar Cells - American

Letter pubs.acs.org/NanoLett

Time-Evolution of Poly(3-Hexylthiophene) as an Energy Relay Dye in Dye-Sensitized Solar Cells Nicola Humphry-Baker,† Kristina Driscoll,† Akshay Rao,† Tomas Torres,‡,§ Henry J. Snaith,*,∥ and Richard H. Friend*,† †

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, CB3 0HE, United Kingdom Departamento de Química Orgánica (C-I), Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain § IMDEA Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain ∥ Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom ‡

S Supporting Information *

ABSTRACT: Energy relay dyes (ERD) and Förster resonant energy transfer (FRET) are useful techniques for increasing absorption in dye-sensitized solar cells. We use femtosecond transient absorption spectroscopy to monitor charge generation processes in a solid-state DSC containing poly(3-hexylthiophene) (P3HT) as both the holetransporter and the ERD with a zinc phthalocyanine dye (TT1) as the sensitizer. We observe efficient FRET occurring on picosecond time scales and subsequent hole transfer from TT1 to P3HT occurring onward from 100 ps. KEYWORDS: Dye-sensitized solar cell, solid-state, FRET, transient absorption spectroscopy

D

a DSC without reducing the absorption coefficient of the dye. Energy relay dyes are fluorescent, light-absorbing dyes dispersed within the hole transporting material that transfer energy via Förster resonant energy transfer (FRET) to the lower energy gap (near-infrared) sensitizing dye. These broaden the spectral response of DSCs and improve the power conversion efficiency (PCE) of both liquid and solidstate DSCs.8,9 Nevertheless, the efficiency of energy transfer between the two dyes can be decreased by the photoluminescence quenching of the ERD within the hole transporter.9−11 The use of a light-absorbing, conjugated polymer as both the hole transporter and the energy relay dye could potentially remove this quenching issue. This type of architecture, a light-absorbing polymer hole transporter combined with a near-infrared (IR) sensitizer, has shown broad photocurrent spectra,12−14 but the actual mechanism behind the contribution from the conjugated polymer remains unclear. In order to develop and improve ERD systems, it is important to disentangle the different processes going on in these cells and know the time-scales over which they take place. In this paper, we study a solid-state DSC containing poly(3hexylthiophene) (P3HT) as both the hole transporter and the energy relay dye in conjunction with a near IR sensitizing dye, zinc 3,4-tert-butyl-phthalocyanine (TT1). Structures of these

ye-sensitized solar cells (DSC) show considerable potential as commercially viable photovoltaic cells due to their low fabrication costs and promising power conversion efficiencies.1−3 DSCs consist of a porous, wide bandgap semiconductor,3 such as titanium dioxide, sensitized with a monolayer of dye and filled with a hole transporting material. The dye absorbs the incident light and injects electrons into the semiconductor and the hole on the dye is transported out of the device via the hole transporter. This infiltrated hole transporting material (HTM) can be either a liquid electrolyte, creating a “liquid” DSC,4 or a small molecule or polymer material, resulting in a “solid-state” DSC.5 Liquid DSCs are constructed to absorb most of the incident light over the absorbing range of the dye2 and are typically 10 μm thick. Solid-state DSCs have optimum thickness typically in the range 1−2 μm and absorb as little as 20% of the incident light over a large part of the active region. This thickness limitation is set by the difficulty in achieving effective pore filling and inefficient charge collection due to shorter electron lifetimes.6,7 In the selection of the dye there is a trade-off between spectral absorption range and absorption coefficient. Broad absorption spectrum dyes extending into the infrared (IR), as far as 900 nm3, have been shown to produce efficient liquid DSCs but what they gain in breadth, they loose in their absorption coefficient, making them less useful for solid-state DSCs due to their low light-harvesting capabilites.2 To circumvent this issue of absorption depth, Hardin et al.8 first demonstrated the use of energy relay dyes (ERDs) in liquid DSCs, which allows the broadening of the spectral response of © 2011 American Chemical Society

Received: September 28, 2011 Revised: December 27, 2011 Published: December 30, 2011 634

dx.doi.org/10.1021/nl203377r | Nano Lett. 2012, 12, 634−639

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P3HT layers were carried out as previously. Finally, a 100 nm silver electrode was thermally evaporated onto the substrates. Device Characterization. The absorption of the devices and films was measured on a Hewlett-Packard UV−vis spectrometer before electrode deposition. The external quantum efficiency of the devices was measured at intensities of ∼1 mW cm−2 using a Keithley 2623 dual-channel source meter to monitor short-circuit current. The light was generated using a quartz-halogen lamp passed through a monochromator and continuously monitored with a photodiode (ThorLabs, SM05PD1A). All measurements were conducted in air on nonencapsulated devices. Transient Absorption Setup. A Ti/sapphire amplifier system (Spectra-Physics Solstice) was used to generate a 1 kHz train of 90 fs pulses. Portions of this beam were split off to pump a tunable optical parametric amplifier (Light Conversion TOPAS) and a home-built noncollinear parametric amplifier (NOPA) that allowed us to probe the sample from 530 to 800 nm in one shot. For short-time measurements (500 fs to 2 ns), the TOPAS was used to pump the samples and the beam from the NOPA was the probe, delayed with respect to the pump using a mechanical stage (Newport). For long-time measurements (1 ns to 1 ms), the probe remained the same and the pump beam was now generated using a frequency-doubled qswitched Nd/YVO4 laser (Advanced Optical Technologies AOT-YVO-25QSPX). The pump and probe beams were focused onto the same place on the samples. A third reference beam, identical to the probe, was used to correct for shot-toshot variation in the latter. After the sample, the probe and reference beam were sent into an imaging spectrograph (PI Acton Spectrapro 2150i) and dispersed onto two 256-pixel photodiode arrays (Hamamatsu S3901256Q). Every second pump pulse was omitted either with a mechanical chopper when using the TOPAS or electronically for long-time measurements. The transmission through the sample for every shot was detected and the differential transmission (ΔT/T) was calculated and averaged over 500 “pump on” and “pump off” measurements. The samples were measured under vacuum (∼10−6 mbar) and at room temperature. The P3HT was excited at 490 nm for short-time measurements and 532 nm for long-time measurements with the pump intensity set to 18 μJ cm−2 in both cases. Selective excitation of the TT1 dye was performed at 680 nm. Figure 2 shows the steady-state absorption spectra of the different materials used in this study. P3HT was chosen as the hole-conductor for its high hole mobility16 and its broad absorption spectrum from 400 to 650 nm, which complements that of the sensitizing dye. Evidence of vibronic progression in P3HT is present in the absorption spectrum indicating some degree of ordered crystalline lamellae in the P3HT film within the porous TiO2.17 As-spun P3HT films do not generally show much crystallinity and usually require thermal annealing before ordering is observed.16,18 We consider that the improved order here results from slower solvent evaporation from these nanoporous structures, allowing “solvent annealing” that enables improved ordering of the polymer chains without the requirement for thermal annealing.19,20 The amount of P3HT in the pores of the TiO2 corresponds to a solid P3HT film of 80−85 nm thick, as determined from the peak absorbance of P3HT in porous titania. The porosity of the TiO2 film is approximately 60%,8,11,21 so that the P3HT fills no more than 13% of the pore volume. Abrusci et al.21 reported recently that P3HT seems to only coat the internal surface of the pores with

materials are shown in Figure 1. P3HT is known to provide very efficient light harvesting and hole generation when

Figure 1. Structures of TT1 and P3HT and energy diagram of the DSC, highlighting the possible charge generation pathways: 1. Charge injection from the P3HT to the dye followed by charge injection into the TiO2. 2. Resonant energy transfer from the P3HT to the TT1 dye followed by charge separation at the TT1−TiO2 interface.

blended with fullerene electron-acceptors and thus fulfills the two roles required in the present devices, light harvesting and hole transport.15 These DSCs exhibit a broadening in their spectral response with significant contributions from both the dye and the P3HT. We use ultrafast transient absorption spectroscopy to monitor exciton and charge dynamics between P3HT and TT1. Our findings show that photoinduced excitations on P3HT undergo efficient energy transfer to the TT1 sensitizer, followed by the regeneration of the dye by the P3HT, clearly demonstrating the concept of multifunctional energy relay hole-transport materials in solid-state DSC. This is also the first report of the time scales involved in the Förster processes in these devices and the dominant loss mechanisms present in this architecture. Film Preparation. Quartz substrates were sonicated first in acetonitrile, then in isopropanol for 15 min, followed by oxygen-plasma etching for 3 min before a 10 mL of diisopropoxytitanium bis(acetylacetonate) solution (SigmaAldrich) at a concentration of 0.1 mL in ethanol was deposited onto the substrates via spray pyrolysis at 450 °C and annealed for 30 min forming a 100 nm compact TiO2 layer. A 0.67 mg mL−1 of TiO2 Dyesol paste (18 NR-T) in ethanol was subsequently deposited via spin coating at 1500 rpm for 60 s and further annealed at 350 °C for 30 min and then at 500 °C for 30 min. The films were then submerged in a 10 mM TiCl4 aqueous solution at 70 °C for 1 h and then sintered at 500 °C for 45 min. The final thickness of the nanoporous TiO2 was 1 μm. These were cooled to 70 °C and either deposited into a 10 mM chenodeoxycholic acid (Sigma-Aldrich) ethanol solution or a 500 μM TT1 and 10 mM chenodeoxycholic acid ethanol solution. Under an inert atmosphere, 70 μL of 5 mg mL−1 poly(3-hexylthiophene) (Merck, Mw = 52 000 g mol−1, regioregularity 95.6%) in anhydrous chlorobenzene was deposited onto the films for 30 s before spinning the substrate at 2000 rpm for 45 s. This step was repeated twice to improve pore filling with a 30 mg mL−1 P3HT solution. No thermal annealing was performed on the P3HT films. Device Fabrication. Fluorinated-SnO2 (FTO) coated glass (Solaronix, 15 Ω m−2) was patterned using zinc and 3 M hydrochloric acid and cleaned and plasma etched as above, before depositing a 100 nm compact layer of TiO2 via spray pyrolysis at 450 °C. Deposition of the porous titania, dye, and 635


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donor−acceptor separation at which FRET is 50% efficient ∞

RF6

⎛ 9000 ln(10)k Φ ⎞ ∫ FD(λ)εA (λ)λ4dλ ⎟⎟ 0 ∞ = ⎜⎜ 2 4 ⎝ 128π n NA ⎠ ∫0 FD(λ)dλ

(1)

where NA is Avogadro’s number, n is the refractive index, κ describes the relative orientation of the donor and acceptor molecules, Φ is the donor photoluminescence quantum efficiency (PLQE), FD is the donor emission spectrum and εA is the wavelength-dependent molar extinction coefficient of the acceptor.11,28 Assuming a refractive index of n = 1.4,29 a PLQE of 1% for P3HT,12 and a random orientation of the transition dipole moment (κ = 2/3),28 eq 1 gives a Förster radius of 2.2 nm. The average titania pore size is 30 nm in diameter,8,11,21 much larger than the Förster radius; however, excitons in the P3HT are able to diffuse through the manifold of available states bringing them closer to TT1 molecules increasing the likelihood of FRET between the two. The efficiency of energy transfer from a donor molecule to an acceptor is given by28

Figure 2. Steady-state absorption spectra of P3HT/TiO2 (red triangles), P3HT/TT1-cheno/TiO2 (green crosses), TT1-cheno/ TiO2 (blue circles), cheno/TiO2 (black curve), and porous TiO2 (black dashed curve). Normalized P3HT photoluminescence (red dotdash curve).

ηFRET =

an approximately 1 nm thick layer of P3HT. Despite this extremely sparse coating of polymer, charge transport is not impeded through the film due to the high charge carrier mobility of P3HT. The zinc phthalocyanine dye, TT1 (Figure 2 blue circles), has a large extinction coefficient at 680 nm (191 500 M−1 cm−1),8,22 which strongly overlaps the photoluminescence (PL) of P3HT, making these two materials a good FRET pair. Furthermore, its absorption barely overlaps with that of P3HT, maximizing the breadth of the solar spectrum absorbed by this complementary system. TT1 is a planar molecule, which can allow formation of face-to-face aggregates (H-aggregates) when in films and on the surface of the TiO2. This aggregation lowers the lowest unoccupied molecular orbital (LUMO)23 and reduces the efficiency of charge injection into the TiO2 conduction band.24,25 It has been found that diluting TT1 with a “dye spacer”, chenodeoxycholic acid (cheno) (Figure 2 black curve), reduces the number of H-aggregates by spatially separating the dye molecules.26 The addition of cheno has also been reported to slow down charge recombination27,26 and improve the photovoltage of devices via a negative shift of the TiO2 conduction band.26 Furthermore, cheno is not photoactive over the wavelengths studied, as can be observed from the comparison of the absorption spectra of cheno on TiO2 and plain TiO2 (Figure 2 dotted line). The absorption spectrum of TT1 on TiO2 shows the strong monomer absorption at 680 nm and a shoulder centered at 630 nm corresponding to the Haggregate dimer,24,26 indicating some aggregation on the TiO2, despite the presence of the spacer.8,22,26,27 The appearance of the dimer absorption band at shorter wavelengths than the monomer is becasue the transition from the ground state is not allowed due to symmetry considerations and thus the first optically allowed transition is to the second excited state.23 The materials in these DSCs were chosen with the aim to broaden the spectral response of DSCs via Förster resonant energy transfer where P3HT is acting as an energy relay dye and TT1 as the sensitizing dye. FRET is a nonradiative process that stems from the dipole−dipole interactions between donor and acceptor molecules.28 The strength of this interaction is strongly dependent on the overlap integral of the donor emission (P3HT) and the acceptor absorption (TT1) and can be qualitatively summarized by the Förster radius, RF, the

kF −1

τ

+ kF

(2)

where kF is the rate of Förster transfer and τ is the lifetime of the donor in the absence of the acceptor. In a DSC, the donor is surrounded by a spherical surface of acceptor molecules leading to the FRET rate being proportional to r−4,11,30 which enhances the decay rate and further improves the efficiency of FRET. The transfer rate for a donor a distance r from the center of the pore is11

kF(r) =

1 τ

CARF6 R p4

⎛ r2 ⎞ 4π⎜1 + 2 ⎟ Rp ⎠ ⎝ 4 ⎛ r2 ⎞ ⎜1 − 2 ⎟ Rp ⎠ ⎝

(3)

where CA is the concentration of acceptor molecules on the surface of the pores and Rp is the pore radius. 1. Photovoltaic characteristics. The external quantum efficiencies (EQE) of P3HT/TiO2, P3HT/cheno/TiO2, and P3HT/TT1-cheno/TiO2 devices are shown in Figure 3. The EQE of the P3HT/cheno/TiO2 device (red circles) is less than 2%. This is much lower than that of the P3HT/TiO2 device

Figure 3. External quantum efficiency of P3HT/cheno/TiO2 (red circles), P3HT/TiO2 (red triangles), and P3HT/TT1-cheno/TiO2 devices (green crosses). 636


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(red triangles), reaching a maximum of 5.5% at 550 nm, despite these two devices having comparable absorption. This suggests that in the absence of cheno, excitons in the P3HT are being dissociated to a small extent at the P3HT−TiO2 interface, whereas the presence of the cheno layer acts as an electronblocking layer at the P3HT−TiO2 interface. The addition of the dye to the surface of the TiO2 increases the EQE to almost twice its value in the P3HT region as compared to the P3HT− TiO2 sample (green crosses). Furthermore, the spectral response of the cell is extended by 100 nm into the near IR due to the photoaction of the TT1. Nevertheless, an EQE of 12% at 680 nm is lower than EQEs observed in a standard Spiro-MeOTAD-tBP-LiTSFI/TT1-cheno/TiO2 device that reaches ∼20% at 680 nm,29 suggesting an increased loss of charges in the device architecture studied here. The small peak at 630 nm in the EQE spectrum of the P3HT/TT1-cheno/TiO2 device is the contribution from the TT1 dimer. By taking the ratio of the monomer peak (680 nm) to the dimer peak (630 nm) in both the EQE and the absorption spectra (for device absorption spectrum see Supporting Information Figure S1), it is possible to get an estimate of the relative efficiencies of charge injection into the titania from the monomer and the dimer; however, there is some overlap of the EQE from P3HT and the TT1 dimer, which makes this analysis only qualitative. The absorption ratio is 1.03 and the EQE ratio is 1.57, suggesting charge injection from the monomer is 50% more efficient than from the dimer. This reduction in charge injection efficiency is likely due to a more negative LUMO, reducing the driving force behind charge injection.23,31 Comparison of the ratios of the P3HT and TT1 peaks at 500 and 680 nm, respectively, in both the absorption spectrum, 2.08, and the EQE spectrum, 0.77, shows that only 40% of the P3HT is photoactively contributing to the photocurrent of the cell. From eq 1, we calculated that P3HT and TT1 form a FRET pair with a Förster radius of 2.2 nm. Excitons generated within the P3HT are also known to diffuse over a range of >5 nm.32,33 Given that the pore filling fraction of P3HT in the mesoporous TiO2 is low, such that, if the P3HT uniformly wets the internal surface of the dye-sensitized film, the P3HT thickness will be on the order of 1 nm,21 energy transfer within the P3HT and to the TT1 could be very effective. 2. Transient Absorption Spectroscopy. Transient absorption spectroscopy (TA) measures the differential change in the transmitted spectrum (ΔT/T) through the sample following photoexcitation. We measure here ground-state bleach (GSB) and new absorption features due to neutral (exciton) and charged (polaron) electronic excitations, termed photoinduced absorption (PIA). Stimulated emission (SE) may also be present as a positive signal, occurring at the same energy as the photoluminescence of the material. We performed transient absorption spectroscopy on P3HT/TiO2, P3HT/cheno/TiO2, and P3HT/TT1-cheno/TiO2 films. The spectra and lifetimes of the first two films were similar (see Supporting Information Figure S2), thus only the P3HT/cheno/TiO2 is studied in detail here. Figure 4 shows the TA spectra of the P3HT/ cheno/TiO2 and P3HT/TT1-cheno/TiO2 films excited at 490 nm (selectively exciting the P3HT). The laser fluence was set to 18 μJ cm−2, which is within the linear response of the TA signal of P3HT. The TA spectrum of the P3HT/cheno/TiO2 (Figure 4A) shows a steady decay of the P3HT GSB between 500 to 630 nm (Figure 4A). The two peaks in the GSB correspond to the vibrational transitions from S0→S1. This GSB

Figure 4. The temporal evolution of the transmission spectra of (A) P3HT/cheno/TiO2 and (B) P3HT/TT1-cheno/TiO2 films at 0.6 ps (red crosses), 50 ps (green circles), and at 1200 ps (blue triangles). Films excited at 490 nm with fluence set to 18 μJ cm−2.

follows biexponential kinetics with lifetimes of 160 ps (71% of the excited population) and 1.4 ps (29%), comparable to a neat P3HT film of similar crystallinity (see Supporting Information Figure S3). The decay of the GSB is faster than the expected fluorescence lifetime (∼500 ps),34−36 suggesting some exciton annihilation occurring at this fluence34 or that there is some quenching at the TiO2 interface. The strong negative PIA signal beyond 650 nm arises from the thermal shift of the P3HT absorption spectrum.37 This PIA is reduced around 720 nm due to stimulated emission (SE) overlapping with the PIA.18,34,38 After 1 ns, most of the GSB has decayed to zero, indicating negligible charge generation. Figure 4B shows the evolution of the excited states within the P3HT/TT1-cheno/TiO2 film, excited at 490 nm. GSB of the TT1 at 680 nm is observed almost immediately after excitation (Figure 4B, red crosses) along with the characteristic P3HT GSB between 550 and 650 nm. We note that this is at a higher energy than the P3HT SE, generally observed around 720 nm.34,38 The TT1 GSB subsequently grows and broadens while the P3HT GSB decays to zero. This is followed by a decrease in the GSB of the TT1 and a slight rise in the P3HT GSB. The broadening of the TT1 GSB follows the two features observed in the TT1 absorption spectrum (Figure 2), leading us to attribute the GSB at 630 nm to excitation of the TT1 dimers. The overlap between the P3HT PL and the TT1 dimer absorption is not as good as the overlap between the P3HT PL and the monomer absorption, leading to a slower energy transfer to the dimer. Moreover, if the dye is selectively excited at 680 nm, the monomer GSB is seen at early times with the dimer GSB growing in soon after on a similar time scale to the decay of the monomer GSB (see Supporting Information Figure S4), which suggests energy or charge transfer occurs between the monomer and dimer sites.24,25 The excited dimer is longer-lived than the monomer, consistent with phthalocyanine dimers evolving to form triplets.25,39,40 Furthermore, the energy of the first excited state of the dimer is lower than the singlet excited state, thus reducing the electron transfer rate and efficiency from this species, increasing the TT1 dimer lifetime. 637


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This is consistent with the P3HT only forming a “wetting layer” on the internal pore wall of the TiO2.21 Once the dye has been excited, charges should be separated and extracted. Indeed, once the dye has been excited, the GSB of the TT1 monomer, followed by that of the TT1 dimer (see Supporting Information Figures S4 and S5), reduces as the P3HT GSB rises, whereas the P3HT GSB in the absence of any TT1 molecules continues to decrease and reaches close to zero at 55 ns. This implies that the hole on the dye is transferred to the P3HT, inducing a rise in the P3HT GSB. That it matches the loss of TT1 monomer GSB, indicates that this follows earlier electron transfer from TT1 to TiO2, since this returns the TT1 back to the ground state. This demonstrates that P3HT can regenerate the dye, fulfilling its role as a hole conductor. The dual action of the P3HT is in good agreement with experiments performed by Lee et al.13 They observed a substantial increase in the P3HT polaron band at 980 nm in the presence of TT1, indicating improved hole generation, but were unable to temporally resolve this signal to below 600 ns nanoseconds to determine the cause of this increase.13 An estimation of the charge generation efficiency can be obtained from Figure 5, which shows that of the initial amount of P3HT GSB only an eighth is recovered after TT1 regeneration. Supposing a positive polaron on the P3HT has a similar cross-section to a P3HT exciton in the GSB, this would imply an internal quantum efficiency of 13% arising from energy transfer from P3HT to TT1. In comparison, with an estimate of ∼95% of incident light being absorbed at 550 nm and an EQE of 8.9% at the same wavelength then the IQE in the P3HT/TT1-cheno/TiO2 device is approximately 9.5%. This estimate is on the lower side, as it does not take into account all the light losses at the different interfaces. Furthermore, it is difficult to quantify the percentage of charges lost between charge generation at the interface and charge collection at the electrodes by comparing these two IQEs, as the transient absorption measurements were performed on films, which is similar to open-circuit conditions, whereas the EQE measurements are performed under shortcircuit conditions. Nevertheless, this small difference between the two values indicates that charge extraction and charge recombination between holes on the P3HT and electrons in the TiO2 is not the dominant loss mechanism. As FRET is very efficient, the most important loss mechanism in these devices occurs at the dye interface and is likely due to dye aggregation.26,42 More work is needed to properly understand the interaction between TT1 monomer and dimer species at the surface of the TiO2, which lead to loss of charge. These results show that very efficient FRET occurs from P3HT to TT1, improving the spectral response of P3HT/TT1cheno/TiO2 DSCs. Furthermore, the appearance of a hole on the P3HT at later times demonstrates the feasibility of combining a hole-conductor and energy relay dye into one by using a light-absorbing conjugated polymer, bypassing also the issue of the hole transporter quenching the ERD. Nevertheless, charge generation in this system still needs improving. The IQE of these devices at 550 nm is 13% from transient absorption measurements and 9.5% from the EQE spectrum and the EQE at 550 nm is 8.9%. The main loss of charges is due to TT1 molecules forming H-aggregates at the surface of the TiO2 decreasing charge injection rates into the TiO2 or P3HT, rather than charge recombination and extraction losses.

Figure 5 shows the dynamics of the P3HT GSB with and without TT1 and shows the rise and decay of the dye monomer

Figure 5. Kinetics of the P3HT GSB at 550−570 nm with (blue crosses) and without the dye (red triangles) and of the TT1 monomer GSB at 680−700 nm (green circles). For dimer kinetics and cation kinetics see Supporting Information Figure S6. The negative PIA signal of the dye cation at 550 nm has been subtracted from the P3HT GSB with dye (blue crosses). The unmodified P3HT GSB is depicted in Supporting Information Figure S6.

GSB in the complete system (P3HT/TT1-cheno/TiO2). The cation of TT1 shows induced absorption at 550 nm41 (see Supporting Information Figure S5). This results in a negative PIA signal that lies underneath the P3HT GSB. The ratio of TT1 dimer GSB (at 630 nm) to TT1 cation (at 550 nm) is −3.4 at times beyond 10 ps, indicating that the TT1 cation is primarily located on the dimers (see Supporting Information Figure S4). This provides a procedure to remove the TT1 cation absorption in the region of the P3HT GSB signal by subtracting −1/3.4 of the transient dimer signal from the P3HT GSB kinetics (550 to 590 nm). Hence, we can determine the true kinetics of the P3HT GSB. In the presence of the dye the P3HT, GSB decays with decay times of 1.5 ps (50% of the excited population) and 9.1 ps (50%), approaching zero near 80 ps before rising to approximately 13% of its original value by 50 ns. The shorter decay time is comparable to the shorter decay mechanism in the P3HT/cheno/TiO2 film (1.4 ps), whereas the second decay time is 2 orders of magnitude shorter than the longer-lived excitation (160 ps) in the same film. Furthermore, as the P3HT GSB decays, the TT1 GSB rises with the monomer reaching a maximum just before the P3HT GSB minimum. This is consistent with a rapid energy transfer of excitons on the P3HT to the TT1 sensitizer, which occurs within 80 ps. We note that the alternative route for charge generation, electron transfer between P3HT and TT1, would leave a hole polaron on the P3HT, thus maintaining the P3HT GSB; however, this is not consistent with its rapid decay to zero within 80 ps followed by its rise as holes transfer back from TT1. Therefore, the very rapid decay of the P3HT GSB in the presence of TT1 and the complete loss of excitons on the P3HT at 80 ps lead to the conclusion that Förster resonant energy transfer occurs from photoinduced excitations on the conjugated polymer to the dye. The absence of a 160 ps lifetime component in the P3HT GSB decay implies that this process is very efficient. Using eq 3 and a FRET rate of 103 × 109 s−1, we determine that the P3HT excitons that undergo FRET are less than 1 nm away from the TT1 sensitized surface. 638


Nano Letters

Letter

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These experiments have enabled us to follow the whole progressions of P3HT excitons through to charges, giving us a good picture of the efficiency of each step. The lifetimes obtained in this paper demonstrate that the use of Förster transfer as a tool to broaden the spectral response is very efficient; however, better performing near IR sensitizers are required in order to improve charge separation and fulfill the potential of this type of cell architecture.

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ASSOCIATED CONTENT

S Supporting Information *

Kinetics of pure P3HT, P3HT/TiO2, P3HT/cheno/TiO2, and TT1 monomer and dimer species. Absorption spectrum of devices studied in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (R.H.F.) [email protected]; (H.J.S.) h.snaith1@ physics.ox.ac.uk.

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ACKNOWLEDGMENTS This work was funded by the EPSRC. T.T. gratefully acknowledges financial support by the MICINN (Spain) (CTQ2011-24187/BQU, PLE2009-0070, and ConsoliderIngenio Nanociencia Molecular CSD2007-00010), and Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533).

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REFERENCES

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