Enhanced Photoresponse in Solid-State Excitonic Solar Cells via

Nov 9, 2010 - Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response. Giulia Grancini , R. Sai Sa...
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Enhanced Photoresponse in Solid-State Excitonic Solar Cells via Resonant Energy Transfer and Cascaded Charge Transfer from a Secondary Absorber Kristina Driscoll,*,† Junfeng Fang,‡ Nicola Humphry-Baker,† Toma´s Torres,§,| Wilhelm T. S. Huck,*,‡,⊥ Henry J. Snaith,*,¶ and Richard H. Friend*,† †

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom, Melville Laboratory, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom, § Departamento de Quı´mica Organica, Universidad Auto´noma de Madrid, 28049 Madrid, Spain, | IMDEA-Nanociencia, Facultad de Ciencias, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain, ⊥ Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands, and ¶ Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom ‡

ABSTRACT We present a spiro-linked molecule 2,2′,7,7′-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4yl)thiophen-2-yl)-9,9′-spirobifluorene which acts as a secondary absorber in solid-state excitonic solar cells. Blending with a holetransporting material 2,2′7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene and used in conjunction with a near-infrared dye (termed TT1) results in an extended spectral response which yields a notable increase in short-circuit current and power conversion efficiency. This enhancement is due to both exciton energy transfer and also nanoscale charge generation in the blend via the formation of an excited state spiro-complex with charge transfer character. KEYWORDS Solar cell, solid-state, energy transfer, charge transfer, TiO2

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rom the widespread interest in the development of alternative energy sources, excitonic solar cells (XSCs) have emerged as one of the front running prospects due mainly to a favorable balance between cost and efficiency.1-4 Included among these devices are organic and dye-sensitized solar cells, both of which show promise toward large area solar energy conversion as a result of the inexpensive and scalable fabrication processes employed. In these solar cells, the photogenerated exciton is split into an electron-hole pair at the interface between an electrondonating (nominally p-type) and electron-withdrawing (ntype) material. Because of the small excitonic diffusion lengths of 3-15 nm for the materials typically used,5,6 XSCs require a nanoscale network of interpenetrating electron and hole transporters to harvest the carriers before the exciton undergoes recombination. In addition, a high surface area contact between the two materials is desirable to enhance the number of excitons that can be ionized and separated. In organic solar cells, these operational requirements are met using bulk heterojunction (BHJ) structures2,3 in which electron donating and electron accepting components, often comprised of solution processable conjugated polymers and

molecules, are blended at the nanoscale leading to maximum power conversion efficiencies of over 6%.7-9 However, the morphology of these blends is highly sensitive both to materials used and also to the processing conditions. The development of hybrid XSCs addresses the need for deterministic, well-ordered geometries. In these devices, the morphology is fixed by the replacement of the electronaccepting organic material with an inorganic semiconductor that is nanostructured prior to the addition of the organic electron-donating phase. Current challenges addressed in hybrid XSC research include increased control and optimization of the nanostructure as well as engineering of the inorganic-organic junction with interfacial modifiers to promote more efficient exciton dissociation and charge separation.10,11 Alternatively, in dye-sensitized solar cells (DSSCs) the exciton is created within a monolayer of dye located at the interface between a mesoporous metal oxide anode (typically TiO2) and a hole transporting material (HTM).1,4 The TiO2 film consists of nanometer scale sintered particles ensuring a large internal surface area. After photoexcitation of the dye, the exciton is immediately split at the nanoparticle surface with electrons injected into the conduction band of the TiO2 and the oxidized dye regenerated via hole transfer from the dye to the HTM. Liquid-electrolyte DSSCs, which utilize an iodide/triodide redox couple as the HTM, use thick TiO2 layers (∼10 µm) optimized for maximum light

* To whom correspondence should be addressed. E-mail: (K.D.) [email protected]; (W.T.S.H.) [email protected]; (H.J.S.) [email protected]; (R.H.F.) [email protected]. Received for review: 09/1/2010 Published on Web: 11/09/2010 © 2010 American Chemical Society

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absorption and yield power conversion efficiencies exceeding 11%.12-14 The main drawback of liquid-electrolyte DSSCs is the absence of long-term stability due to solvent leakage, evaporation, and corrosion, posing great difficulties in up scaling. In moving toward a liquid-free arrangement the organic HTM 2,2′7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) has been established as the preferred hole transporting material in fully solid-state DSSCs (ss-DSSCs).4 However, difficulty in achieving infiltration of the organic HTM into the mesoporous layer limits the thickness of the TiO2 films in ss-DSSCs to only about 2 µm, which decreases the light-harvesting ability and results in marked lower power conversion efficiencies.15-17 Recently DSSC architectures based on energy relay dyes, highly emissive secondary dyes placed in the HTM, have been shown to increase overall light absorption and improve the collection of high energy photons using Fo¨rster resonant energy transfer (FRET).18-24 In this design, visible light is absorbed by the dye dispersed within the HTM matrix. The energy from the excited dye is transferred to a lower band gap sensitizing dye anchored to the TiO2 surface via FRET. A similar concept had been demonstrated in hybrid XSCs where the use of the p-type conjugated polymer poly(3-hexylthiophene) (P3HT) to both absorb visible light and transport holes in dye-sensitized TiO2 nanostructures has been shown to provide an increased and broadened photoresponse.25-27 Here, we report on a novel materials platform for use in XSCs that combines the advantages of a FRET-based energy relay system together with the charge-generating properties of a conjugated molecular blend. Specifically, we present a secondary absorber that when blended with the hole transporting material, spiro-OMeTAD, and used in conjunction with a near-infrared dye extends the spectral response of solid-state excitonic solar cells (ss-XSCs) and leads to improved power conversion efficiencies in comparison to the standard dye only devices. The increased collection of visible photons is found to be due to a unique combination of both exciton energy transfer to the dye and also nanoscale charge generation in the blend, facilitated by the compatible optical and electrical properties of the dye, this new “n-type” absorber and spiro-OMeTAD. The requirements of the secondary absorber are that it should be reasonably emissive and with the appropriate electron energy levels and also be highly soluble and thus easily infiltrated in to the nanoporous layer. Since conjugated small molecules are promising candidates for such applications, we have introduced an oligomer of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that is extensively used in light-emitting field effect transitors28 and polymer blend photovoltaics29,30 as the basic unit. Although bypassing the difficulties of infiltrating a bulky polymer into the nanosize pores, these small F8TBT molecules suffer from poor thermal stability with a tendency to crystallize even at room temperature. To prevent possible pore clogging, we have © 2010 American Chemical Society

synthesized a compound consisting of two F8TBT oligomers connected via a spiro-link at the central carbon-9 position. This spiro-center has tetrahedral symmetry and increased thermal stability by introducing a 90° angle between the two units.31 Exploiting this bonding geometry also enables the use of high concentrations in the spiro-OMeTAD matrix, which is a similar spiro-centered molecule. Spiro-TBT (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) was synthesized via a palladiumcatalyzed Suzuki-coupling between the tetra-2,2′,7,7′tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′spirobi[fluorene] and a mono bromo TBT building block, which is obtained via monobromination of 4,7-bis(5′bromo-3,4′-dihexyl-[2,2′-bithiophen]-5-yl)benzo [1,2,5]thiadiazole (TBT) in chloroform solution at 0 °C. Chloroform instead of tetrahydrofuran (THF) was used to improve the yield of the reaction since THF lead to uncontrollable bromination and larger amounts of dibromination product. Spiro-bi[fluorene] was synthesized from 2,2′,7,7′tetrabromo-9,9′-spiro-bi[9H-fluorene] and bis(pinacolato)diboron using a palladium catalyst under CH3COOK conditions. The use of Li+ bases was not successful due to poor solubility of especially the tetra-Li+ intermediate. The solubility and stability of the intermediate boronic ester compounds is better and increases with the number of boronic esters in the palladium-catalyzed reaction system. An alternative method to synthesize spiro-TBT would be a direct palladium-catalyzed coupling between 2,2′,7,7′-tetrabromo-9,9′-spirobi[9H-fluorene] and mono boronic ester TBT or mono stannyl TBT, but the synthesis of the latter two is relatively difficult due to the unstable property of benzo[c][1,2,5]thiadiazole unit under the strongly reducing butyl lithium. Figure 1 shows the molecular structures and optical properties of the materials used in this study. The absorption spectrum of spiro-TBT is similar to that of F8TBT with the characteristic camel-back profile and extracted absorption coefficient R of 38 000 cm-1 at 510 nm. Additionally, the photoluminescence is peaked around 640 nm. The dye used in this study is an asymmetric near-infrared zinc-phthalocyanine (TT1) designed for sensitization of TiO2 in DSSCs.32,33 This dye is known for its high molar extinction coefficient of 190 000 M-1 cm-1 peaked at 680 nm. As demonstrated in Figure 1c, the optical properties of spiro-TBT and TT1 are near ideal for a FRET donor-acceptor pair in ss-DSSCs. Their cooperative absorption covers a significant portion of the spectrum and with negligible overlap, ensuring that the addition of this secondary absorber to standard devices will not disturb light harvesting by the primary absorber. Even more importantly, there is an extensive overlap between the donor emission and acceptor absorption, a necessity for efficient FRET. The energy landscape of the device, as depicted in Figure 1d, was determined using both empirical and theoretical 4982

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FIGURE 1. Molecular structures of (a) TT1 and (b) spiro-TBT. (c) TT1 absorption (red solid) adsorbed on surface of TiO2 nanoparticles; spiroTBT absorption (blue solid) and spiro-TBT emission (blue dash) from films. (d) Energy level alignment of a spiro-TBT XSC highlighting pathways for charge generation.

data based on cyclic voltammetry and recent literature reports. From the electrochemical ionization potentials measured on thin films and using ferrocene as a reference, the highest occupied molecular orbital (HOMO) level for spiroTBT was estimated at -5.45 eV with respect to vacuum level. In comparison, the HOMO of spiro-OMeTAD was measured under the same conditions to be -4.95 eV. Using an approach based on density functional theory, the HOMO of TT1 has been calculated to be as deep as -5.33 eV and as shallow as -5.15 eV.34 The lowest unoccupied molecular orbitals (LUMOs) for spiro-TBT and TT1 were approximated using the bandgaps obtained from the onset of absorption, 2.06 and 1.75 eV, respectively. This schematic also outlines the possible pathways to photovoltaic action after excitation of spiro-TBT. On the basis of the alignment of the HOMO and LUMO levels, charge generation is possible via hole transfer from spiro-TBT to spiro-OMeTAD and electron injection into the conduction band of TiO2. This electron transfer from spiro-TBT to TiO2 may occur through one of two routes: either direct electron transfer to the TiO2 or cascaded transport mediated by TT1 where the interfacial dipole induced by the dye molecule promotes exciton splitting and charge separation. Lastly, resonant energy transfer of the spiro-TBT excitation to TT1 and subsequent charge dissociation may lead to a photoresponse through the standard ss-DSSC mechanism. We note that as measured the spiro-TBT HOMO level is too deep to regenerate TT1. © 2010 American Chemical Society

FRET is a nonradiative energy transfer process facilitated through dipole-dipole coupling of a donor-acceptor pair through an electric-dipole field. The extent of this coupling can be quantified with the Fo¨rster radius Ro, which describes the donor-acceptor separation at which this process is 50% probable.35 The Fo¨rster Radius is calculated from the following equation, which depends strongly on the spectral overlap of the donor emission and acceptor absorption profiles

Ro6 )

9000 ln(10)k2QD 128π5n4NA

∫ FD(λ)εA(λ)λ4 dλ

(1)

Here, QD is the photoluminescence quantum efficiency (PLQE) of the donor in the absence of the acceptor, n is the refractive index, k is the orientation factor for the dipole moment, εA is the molar extinction spectrum of the acceptor, and FD is the donor emission profile. Using eq 1 and assuming a refractive index of n ) 1.4 and random orientation of the dipoles, a Fo¨rster radius of 4.9 nm is calculated for a PLQE of 8%. This represents a fairly conservative estimate of Ro since the value of 8% for the quantum efficiency of spiro-TBT was obtained in as-spun films where intermolecular quenching is expected to contribute to a reduced PLQE. 4983

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FIGURE 2. (a) Emission spectra (for a pump wavelength of 540 nm) of spiro-TBT(12.5 µM)-only (black), TT1(10 µM)-only (black-dot), and various spiro-TBT(12.5 µM)/TT1 cosolutions in chlorobenzene. (b) Excitation spectra spiro-TBT(20 µM)-only (black), TT1(10 µM)-only (blackdot), and a spiro-TBT(20 µM)/TT1(10µM) cosolution (red). The excitation spectrum of spiro-TBT-only was obtained at an emission wavelength of 610 nm, while those for the TT1-only and blend solutions were measured for 700 nm emission.

To experimentally verify FRET action between these two molecules, we examined the fluorescence spectra of spiro-TBTonly, TT1-only, and several spiro-TBT/TT1 cosolutions of varying dye content in chlorobenzene. Figure 2a shows the relative PL of all samples for a pump wavelength of 540 nm. The PL in solution is slightly blue shifted with respect to the film emission with luminescence peaked at 610 and 700 nm for the spiroTBT-only and TT1-only solutions, respectively. It should be noted that very weak emission is detected for the TT1-only solution since there is minimal absorption by the dye in the region of the pump wavelength. As shown in Figure 2b, the excitation spectra of the single molecule solutions collected at the respective peak emission wavelengths roughly follow that of their individual absorption profiles. However, when exciting the cosolutions at 540 nm, we observe an increase in the longer wavelength TT1 emission with a simultaneous quenching and spectral filtering of the spiro-TBT PL due to the presence of the dye. This behavior is observed over a wide range of TT1 concentrations varying from 8 to 64 µM and has also been noted in a similar study of the DCM-pyran/SQ-1 FRET pair.23 In addition, the excitation profile for 700 nm emission in a cosolution is a superposition of the excitation spectra of spiroTBT and TT1 alone, indicating multiple routes toward TT1 emission. As expected, direct pumping of the dye generates 700 nm emission, but similarly indirect excitation from excited spiro-TBT units also results in the radiative relaxation of TT1, suggestive of energy transfer between the two molecules. To further validate energy transfer, we probed the spiro-TBT PL dynamics as a function of TT1 concentration (Supporting Information Figure S1). Fo¨rster theory predicts a decrease in the excited state lifetime of the donor with increasing acceptor concentrations as FRET introduces an additional nonradiative decay path for the donor into the system.36 In the absence of any acceptor, the fluorescence decay time of a dilute solution of spiro-TBT in chlorobenzene is 5.9 ns and decreases monotonically down to 4.1 ns with the addition of up to 1.5 mM TT1. From this time-dependent data, the Fo¨rster radius is calculated © 2010 American Chemical Society

to be 4.3-4.5 nm, consistent with initial estimates based solely on spectral overlap and PL efficiency. The possibility of charge generation and separation in the spiro-TBT/spiro-OMeTAD blend was studied using comparative PL measurements. In Figure 3, the PL of identical weight ratio blends of spiro-TBT in an inactive polystyrene environment and the spiro-OMeTAD matrix are plotted. The peak PL is quenched by a factor of about 60 in spiro-OMeTAD, consistent with efficient hole transfer to the spiro-OMeTAD. Further to the PL quenching, the remaining PL is significantly red shifted by 50 nm in comparison to the films of spiroTBT dispersed in polystyrene. However, time-dependent analysis of the red shifted PL peak reveals this emission is more than twice as long-lived as that from the spiro-TBT diluted in the polystyrene inert matrix with respective lifetimes of 10.8 and 5.1 ns and with no change in the ground state absorption. The red shift in the emission, combined with the long-lived species and absence in change to the ground state absorption, indicate the formation of an excited state complex (exciplex) between spiro-TBT and spiro-OMeTAD with a charge transfer character.36 The adverse affect of the quenching of the donor PL by the HTM on device performance is highly cited in FRET-enhanced DSSC literature as this becomes a competing process with energy transfer. In the liquid-state DSSCs, this quenching is due to collisions with iodide ions that instantly transfer the donor molecule to the ground state through a nonradiative transition and in solid-state cells the quenching is speculated to be due to unbalanced charge transfer to the hole transporter. In both cases the excitation is lost and does not contribute to the photocurrent. However, in this system, the charge transfer nature of the resulting complex may provide an alternative mechanism for photovoltaic action. Figure 4a shows the external quantum efficiencies (EQE) versus wavelength of fully processed devices (FTO conductive glass/compact TiO2-mesoporous TiO2/TT1 sensitizer/spiroTBT-spiro-OMeTAD blend/Ag electrode) with varying spiro4984

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FIGURE 3. (a) Emission spectra for spiro-TBT (for a pump wavelength of 470 nm) in a polystyrene matrix (blue) and spiro-OMeTAD matrix (red). (b) Time-resolved decay traces (measured for a fixed peak count) of 610 nm spiro-TBT emission (blue) and 660 nm spiro-TBT/ spiro-OMeTAD emission (red).

FIGURE 4. (a) External quantum efficiency versus wavelength of a standard TT1 device, devices incorporating varying amounts of spiroTBT and a control device with 50% spiro-TBT, but no TT1 sensitization. (b) External quantum efficiency versus wavelength of PCBA sensitized devices with varying spiro-TBT content (with inset depicting chemical structure of PCBA).

TBT/spiroMeOTAD blend ratios by weight. It should be noted here that thickness of the nanoporous films were around 1.2 µm, which accounts for the slight discrepancy between the measured EQEs and those previously achieved in TT1 solidstate DSSCs.37 Since we are investigating a newly synthesized molecule of limited batch size, the use of thinner films was employed to preserve materials and not due to a fundamental limitation of the device operation. While the peak EQE at 680 nm due to TT1 remains relatively constant near 18-19% for all devices, we note the emergence of a broad response in the 450-550 nm visible region of the spectrum with increasing spiro-TBT fractions, corresponding to the absorption band of spiro-TBT and exceeding 6% in the higher ratio blends. The excitation transfer efficiency (ETE) quantifies the degree of energy transfer from the excited donors to the acceptors via FRET though the following relation19

ETE )

∆EQEdonor IQEacceptorηabs,donor

tor and is the ηabs,donor. The ETEs calculated for the devices in Figure 4a with 10, 20, and 40% spiro-TBT are 14.9, 18.6, and 21.8%, respectively, with the relevant values of ∆EQEdonor, IQEacceptor, and ηabs,donor listed in Supporting Information Table S1. Although this figure of merit is somewhat ill-defined in this architecture, which has the promise for both energy transfer and cascaded charge transport, these values do provide insight into the potential of this material system. In devices incorporating larger proportions of spiro-TBT (>40%), we note a decrease in TT1 EQE with increasing spiro-TBT concentrations up to 100% (Supporting Information Figure S2) at which point the device exhibits no photoresponse. This trend is due to the aforementioned HOMO level misalignment of spiro-TBT and TT1, which prevents dye regeneration and ultimately imposes the requirement for a critical density of available spiro-OMeTAD molecules to achieve uninhibited hole transfer. To quantify the degree of direct electron transfer from spiro-TBT to TiO2, a 50:50 blend control device was tested without TT1 sensitization. This device produced a peak EQE of just 0.61% at 515 nm, confirming the enhanced photocurrent in the visible region of the dye-sensitized devices

(2)

Here, ∆EQEdonor is the contribution of the donors to the device EQE, IQEacceptor is the internal quantum efficiency of the accep© 2010 American Chemical Society

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FIGURE 5. (a) External quantum efficiency spectra and (b) photocurrent-voltage traces (under AM 1.5G conditions) for a standard TT1 device (black) and an optimized 20:80 spiro-TBT/spiro-OMeTAD enhanced device (red).

character of TT1.33 This novel charge generation mechanism exploits a double-tiered nanoscale morphology set first coarsely by the TiO2 porous film (with typical pore diameters of 20-30 nm) and then fine-tuned on the scale of the exciton diffusion length with the infiltration of the molecular blend. Though energy transfer is likely to dominate for excitons neighboring TT1 sites, this alternative route provides a second chance to harvest photons absorbed outside the critical FRET distance using concepts adapted from the bulk heterojunction model. The solar cell characteristics of the devices with EQEs shown in Figure 4a were evaluated at AM 1.5G solar conditions and are summarized in Figure S4 and Table S1 of the Supporting Information. While we observe a general increase in shortcircuit current due to the enhanced collection of visible photons with increasing spiro-TBT content, we also note a marked decrease in the fill factor, which limits the loading capacity to around 20%. This loss in fill factor is likely due to the more complex transport associated with the addition of the secondary absorber. Spiro-TBT is not expected to possess a high hole mobility and as previously mentioned will actually block hole transfer from the dye due to it is deep HOMO level. Replacement of the spiro-OMeTAD molecules with spiro-TBT reduces the effective conductivity of the interpore matrix and results in an increase in the parasitic resistance of the device, ultimately reducing the fill factor and limiting the attainable shortcircuit current. This behavior is similar to the degradation in device EQE observed at spiro-TBT fractions greater than 40% but occurs at lower loading ratios under solar conditions due to the greater charge densities generated. However, despite this drawback, devices incorporating 20% spiro-TBT still yield a significant improvement in overall device performance as the increase in short-circuit current outweighs the associated loss in fill factor. The EQE spectra and photocurrent density-voltage curves of a standard device (spiro-OMeTAD only), as well a device utilizing a 20:80 spiro-TBT/spiro-OMeTAD blend, are shown in Figure 5 with the relevant device characteristics presented in Table 1. Under AM 1.5G solar conditions, the standard TT1 sensitized DSSC performed reasonably well with

requires the presence of TT1 and that direct electron transfer from the spiro-TBT to TiO2 is ineffective. FRET has already been established as an enhancing mechanism in DSSCs incorporating a suitable donor-acceptor pair,18-24 leaving cascaded electron transport mediated by interfacial TT1 as a final means. Since it is experimentally difficult to separate out these contributions in a single device, a control series was tested that lacked FRET ability. In these devices, the TiO2 was sensitized with a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA). PCBA is considered a strong electron acceptor with little optical absorption in the visible spectrum and which when deposited on a metal oxide, provides a very effective electron-accepting electrode.38 The possibility of appreciable energy transfer from organic materials to fullerene-based molecules has been extensively studied.39,40 However, based on the absorption and emission spectra of PCBA and spiro-TBT (Supporting Information Figure S3), the Fo¨rster radius Ro is calculated to be 1.8 nm, a fraction of that of the TT1/spiro-TBT system (4.9 nm). This reduction translates into a considerable suppression of the FRET pathway since the rate of energy transfer from a single donor to a two-dimensional sheet of acceptor molecules is proportional to (Ro6/r4).41,42 Thus, in comparison to using TT1, energy transfer from the donor spiro-TBT is 400 times less likely to occur when the FRET acceptor is replaced with PCBA. In addition, the LUMO levels of spiro-TBT, PCBA and TiO2 are such that an electron cascade is established. Figure 4b plots EQE versus wavelength for devices with varying spiro-TBT/spiro-OMeTAD blend ratios with self-assembled PCBA upon the TiO2 surface. In contrast to the unsensitized control, we observe an extended photoresponse in the visible region with increasing spiro-TBT ratio, indicating the addition of the dipole at the interface enhances exciton dissociation and electron injection. Although the molecular structure and LUMOs of PCBA and TT1 differ significantly, the PCBA system still serves as a suitable model for cascaded electron extraction from the spiro-TBT/spiro-OMeTAD bulk blend as a directionally similar dipole is expected at the interface due to the “push-pull” © 2010 American Chemical Society

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TABLE 1. Device Characteristics of TT1 Solar Cell with and without the Addition of Spiro-TBT VOC [mV] JSC [mA/cm2] TT1 only TT1 + 20% spiro-TBT ∆

743 734 -1.2%

1.87 2.62 +40.1%

FF [%]

ment offered in this device architecture results in an increased and extended photoresponse with respect to the individual materials employed and with further engineering of suitable donor-acceptor pairs and optimization of charge separation in conjugated molecular blends has the potential to become a continuing avenue toward enhancing hybrid XSCs efficiencies.

PCE [%]

0.66 0.91 0.55 1.06 -16.7% +16.5%

a power conversion efficiency (PCE) of 0.91% and corresponding short circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF) of 1.87 mA/cm2, 743 mV, and 0.66. Upon addition of spiro-TBT to the device, the Jsc increases by 40% to 2.62 mA/ cm2 with an increase in the PCE up to 1.06%. With the Voc relatively unaffected, the 16.5% increase in device efficiency is attributed to the increase in the Jsc due to the harvesting of visible photons in the 450-550 nm region resulting from both energy transfer and cascaded charger transport. The integration of secondary absorbers into ss-XSCs that have the ability to participate in both energy transfer and charge transfer is a unique approach toward improving the efficiencies of this class of solar cells. However, there is still much room for optimization of the optical and electrical properties of this additional component. Although the spectral compatibility of spiro-TBT and TT1 is near optimal and the relatively low PLQE of the donor (8%) is masked by the (PLQE)1/6 dependence of Ro, a PLQE of at least 50% is generally desirable for molecules acting as energy relay sites. In addition, we have observed that the inability of spiro-TBT to mediate hole transfer between TT1 and spiro-OMeTAD has been a primary limitation of this system. Adjustment of the LUMO and HOMO levels through the modification of the conjugation length or side chain groups is one possible step to facilitate both electron and hole transport. Finally, to take full advantage of the charge transfer process and avoid creating charge trapping sites, the load ratio must be tailored to ensure balanced percolating pathways exist between the electrodes and will depend strongly on the collective properties of the primary absorber, secondary absorber, and holetransporting material. To conclude, we have synthesized a spiro-linked conjugated small molecule to act as a secondary absorber in ssXSCs. The spiro-bonding ensures a high degree of miscibility with the conventional solid state hole transporting material spiro-OMeTAD and enables the use of considerable loading concentrations in devices. Fo¨rster resonant energy transfer from spiro-TBT to the near-infrared sensitizing dye TT1 was verified through a survey of the photoluminescence properties of the FRET pair including emission and excitation profiles and decay dynamics. In addition, the formation of a spiro-TBT/spiro-OMeTAD excited state complex was shown to exhibit charge transfer character, enabling directly enhanced photocurrent due to charge generation in this molecular blend. Incorporation of the optimal amount of spiro-TBT into TT1 solar cells results in a 40% increase in short-circuit current and 16.5% increase in power conversion efficiency due to the collection of high energy photons via FRET and cascaded electron transfer. As demonstrated, the multipath enhance© 2010 American Chemical Society

Acknowledgment. This work was partly funded by the EPSRC Nanotechnology Grand Challenges: Energy programme (EP/F056702/1) and Basic Technology Research Grant Initiative COSMOS (EP/D04894X/1). T.T. acknowledges support through the MEC, Spain (CTQ2008-00418/BQU, CONSOLIDER-INGENIO 2010 CDS 2007-00010, and PLE2009-0070) and CAM (MADRISOLAR-2, S2009/PPQ/1533). Supporting Information Available. Materials synthesis/ characterization, device fabrication/testing and referenced figures/tables. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)

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DOI: 10.1021/nl103087s | Nano Lett. 2010, 10, 4981-–4988