This invention relates to polymeric semiconductors and, in particular, although not exclusively, to polymeric semiconductors which are usable in photovoltaic or photoresponsive devices.
Semiconducting polymers make remarkably effective substitutes for conventional inorganic semiconductors in a range of optoelectronic devices including light emitting diodes (LEDs), photovoltaic (PV) diodes, field effect transistors (FETs), and lasers. Conjugated polymers offer considerable material advantages over inorganic semiconductors including chemically tunable optoelectronic properties and low-temperature, solution-based processing suitable for printed electronics. However, their additional functional potential has not been so widely recognized until recently. One functional advantage offered by conjugated polymers is their capacity to employ both electronic and ionic charge carriers in device operation. Whereas solid state inorganic semiconductors are typically impermeable and unstable towards extrinsic ions, ion transport is at the heart of energy conversion and signaling in the soft functional materials found in nature.
Benefits of using ionic charge carriers have been demonstrated in polymer light emitting devices (LEDs). In that case, efficient polymer LEDs have been fabricated by blending the active layer with electrolytes, or substituting it with single-component conjugated polyelectrolytes (CPEs) that have ion pairs tethered to the sidechains. The added ions were originally believed to facilitate electrochemical doping under applied bias, however mounting experimental evidence supports an electrodynamic model whereby the redistribution of mobile ions enhances the field locally at the electrodes, leading to facile and balanced electronic carrier injection. However, solid-state photoluminescence (PL) efficiencies of CPEs are found to be considerably lower than their neutral counterparts and dependent on the nature and size of counterions present. Accordingly, CPEs are deployed most effectively as thin injection layers between the electrodes and highly emissive neutral conjugated polymers. Extrinsic In3+ and Cl− ions have also been found to induce PL quenching in films of neutral polymers without evidence of any electrochemical doping.
It is desirous to utilize the properties of CPEs in photoresponsive devices.
Polymer solar cells may comprise a layer or film of active layer, the donor layer, and a layer or film of acceptor molecules sandwiched between a pair of contacts. The donor layer may comprise conjugated polymer species which possess delocalized π electrons which can be excited by light (usually visible light) from the highest occupied molecular orbital (HOMO) to the molecules lowest unoccupied molecular orbital (LUMO), a π-π* transition. The band gap between the HOMO and LUMO corresponds to the energy of the light which can be absorbed.
In polymers the exciton electron-hole pairs created by such light absorption are strongly bound. However, the exciton pair can be dissociated by providing an interface across which the chemical potential of the electrons decreases. After dissociation, the electron will pass to the donor layer and be collected as a contact, whereas the hole will be collected by its respective contact. Of course, if the charge carrier mobility of either donor or acceptor layer is too low or not sufficiently high the charge carriers will not reach the contacts. For instance, the charge carriers may recombine at trap sites or remain in the respective layer or remain in the device as undesirable space charges that oppose the drift of new carriers.
A prior art polymer solar cell comprises a polyethylene teraphthalate (PET) substrate, upon which is provided successive layers of indium tin oxide (ITO), Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS), an active layer which may be a polymer:fullerene blend, and an aluminium layer. In such a solar cell device architecture the polymer chain is the electron donor layer and fullerene is the electron acceptor layer.
It is a non-limiting object of the present invention to provide a new species for use in solar cells and a corresponding solar cell architecture which will, or may, lead to performance enhancements over prior art solar cell architectures.
Accordingly, a first aspect of the invention provides a photoresponsive device including a semiconducting polymer comprising redox inert ions.
The semiconducting polymer may be a copolymer.
A second aspect of the invention provides a solar cell having an electron donor region and electron acceptor region, the donor and acceptor regions comprising conjugated polymers, ion pairs being, preferably preferentially, located at, near or towards the interface between the donor and acceptor regions.
The cation and anion pairs may be located at either side of the interface or the cations one side and anions the other.
Further exploitation of ions in polymer optoelectronic devices will be enabled by better understanding the interactions between ions and electronic excitations, particularly the origin of the observed luminescence quenching. The difficulty of uncovering the inherent photophysical interactions arises, in part, because ions tethered to conjugated polymers introduce amphiphilic character which can induce rigid ordered backbone conformations and the formation of aggregates and interchain states. This prompted us to investigate the solid state photophysics of a derivative of poly(9,9′-dioctylfluorene-alt-benzothiadiazole) (F8BT) with a low density of ions that are tethered statistically. This arrangement was chosen to minimize the likelihood of ion-induced ordering while ensuring that ions are distributed with sufficient density to interact with electronic excitations in the film.
By time resolving emission and absorption spectra of excitons encountering ions in our CPE films, we show that, contrary to existing views, ions do not destroy optical excitations but rather induce the formation of long-lived, weakly emissive and immobile charge-transfer (CT) states via Coulombic interactions.
In order that the invention may be more fully understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:
Referring first to
A neutral, bound, exciton 5 generated in the donor phase 2 will migrate towards the ionic region at the interface 3 and is aligned with the ion pairs to generate a stable charge-transfer (CT) state 6.
It is considered that because CT states are stabilized by the Coulomb field of ions means that the strength of interaction with a neutral photoexcitation can be extrinsically tuned by varying the nature (i.e., size, valency) of the added ions. The screening of the electron-hole electrostatic attraction will facilitate separation of the electron hole pair.
Referring now to
Under the influence of an external bias, ions are displaced to some degree and thus redistribute the electric field across the active layer of the polymer solar cell. Again, ions are localized at the interface between donor 1′ and acceptor 2′ phases so that the electric field is enhanced at charge-separating interfaces where it can affect the dissociation of geminate charge pairs.
Ions 4′ at donor-acceptor interfaces 3′ are available to screen the mutual Coulomb attraction between photogenerated geminate electron-hole pairs 7, thus enhancing the likelihood of electronic charge-pairs escaping their binding radius and increasing the yield of free charges.
There are several ways to realize the intended ionic polymer solar cell architecture that localizes ion pairs 4, 4′ near the interfaces 3, 3′ between nanostructured donor 1, 1′ and acceptor 2, 2′ phases.
For example, and referring to
Alternatively, and referring to
Additionally, and referring here to
In order to further exemplify the invention, reference is made to the following non-limiting Example.
Copolymerization of the bis(6-bromohexyl)-fluorenyl boronic ester M1 and the 9,9-dioctylfluorene boronic ester M2 with 4,7-dibromo-2,1,3-benzothiadiazol was achieved using palladium mediated Suzuki cross-coupling copolymerization (see Stork et al; Adv. Mater. 14 (2002) pp 361-366). The reaction route is outlined in
NMR analysis of FNBr-7% revealed that a 1:9 M1:M2 feed ratio gave a copolymer containing 7% of the bis(6-bromohexyl)fluorine and 93% of F8BT repeats. The bromohexyl tails were treated with trimethylamine in THF to give the corresponding trimethylammonium derivatives with bromide counterions (FNBr-7%). The counter ions were then exchanged to tetrafluoroborate by dissolving the polymer in a THF and water solution containing an excess of NaBF4. The solvent was then removed under reduced pressure, and the solid washed several times with deionized water to give the resulting polymer (FN-BF4-7%) in 45% yield. The relatively low density of ionic sidechains in FN-BF4-7% ensures that the polymer is soluble in most of the same solvents used to process F8BT.
The bottom panel of
The observation of PL quenching and subtle spectral shifts prompted us to undertake time-resolved measurements by the time-correlated single photon counting (TCSPC) method in order to better understand the perturbations induced by the ions in thin films.
We turned to TA spectroscopy as a more direct probe of charge transfer (including non-emissive states), as shown in
In the case of FN-BF4-7% (
For comparison, the TA spectrum of charge pairs in pristine F8BT was independently obtained by exciting the sample with a significantly higher (>25-fold) fluence, known to produce polaron pairs via exciton-exciton annihilation. Indeed, the resulting high-fluence F8BT polaron pair TA spectrum shown in
Interchain and intrachain CT states derived from F8BT excitons coupled to electron-donating units have also been shown to give rise to weakened and red-shifted PL, longer radiative lifetimes, loss of SE, and broadened photoinduced absorption across the visible region. The TA and time-resolved PL spectra of FN-BF4-7%, provide strong evidence that the added ions induce the photogeneration of CT states that have increased electron-hole separation compared with the emissive bound exciton. For brevity, our subsequent references to CT states will include both weakly emissive CT states and non-emissive polaron pairs.
Ionic charges have the potential to stabilize CT states in conjugated polymers by establishing local Coulomb fields that perturb the HOMO and LUMO orbital energies. For example, an anion will raise the energy levels of HOMO and LUMO orbitals of neighboring chains, thus attracting holes and repelling electrons, while a cation will have the reverse effect. The distribution of both anions and cations in the conjugated polymer film is thus expected to lead to local configurations where electron-hole pairs are separated under the influence of ions. The electronic structure of F8BT enhances the interaction with the Coulomb field of ions. The alternating fluorene (donor) and benzothiadiazole (acceptor) units give rise to CT character in the lowest energy excitonic states of F8BT, and consequently solvatochromism in the absorption and emission spectra.
In solid films of FN-BF4-7%, CT excitons are stabilized when BF4− counter anions interact with a partially positive fluorene donor unit, whereas destabilization will occur if the BF4− ions interact with the partially negative benzothiadiazole (BT) units. Likewise, theoretical calculations show that quarternary amine cations attached to the polymer sidechains are poised to undergo electrostatic interaction with electronegative BT units.
We can also eliminate several other possible causes of exciton quenching in FN-BF4-7%. Firstly, the quarternary amine and BF4− ions are not redox active towards F8BT in either the ground state or the singlet exciton state, thus limiting their role to a physical perturbation upon the polymer photophysics. Secondly, the absence of heavy atoms precludes the participation of triplet excited states because intersystem crossing operates on a timescale of ˜40 ns in F8BT. Thirdly, the photophysics observed in FN-BF4-7% is not consistent with the presence of chemical keto defects—fluorenones that are found to appear as photo-oxidation products in some polyfluorenes. Fluorenone defects emit at significantly higher energy (λmax˜540 nm) than the secondary emissive state we observe in FN-BF4-7%, and are not considered to be important in fluorene copolymers such as F8BT with lower energy excited states. Additionally, spectroscopic measurements on solid films were carried out under vacuum (<10−5 Torr) to avoid the possibility of photo-oxidation.
Next we consider the possibility that tethered ions could induce conformational changes to the conjugated backbone, perhaps forming interchain aggregate states that act as recombination sites. Schwartz and coworkers (see Annu. Rev. Phys. Chem.; 54; pp 141-172 (2003)) have demonstrated that aggregates form when chains adopt extended conformations that permit close interchain contact, thus their formation depends strongly upon the nature of polymer sidechains and the solvent used for casting films. Aggregates display red-shifted absorption and emission spectra. As previously noted, the low density of tethered ions in FN-BF4-7% and their statistical incorporation is not expected to cause significant ordering as it can in amphiphilic block copolymers or CPEs with ions attached to all sidechains. The invariance of GS absorption spectra and transient absorption polarization anisotropy decay strongly suggest that FN-BF4-7% retains the same film morphology as F8BT. Additionally, previous studies that directly create and interrogate interchain interactions in F8BT provide little evidence that it could account for the photophysics observed in FN-BF4-7%. It has been shown that thermal annealing increased the planarity of F8BT and changed the interchain packing from an eclipsed to an alternating packing structure with respect to F8 and BT units of adjacent chains (e.g. Donley et. al.; J. Am. Chem. Soc.; 127; pp 12890-12899 (2005))
Thermal annealing induced clear shifts in absorption and PL spectra, yet the corresponding ˜10% variation in PL efficiencies shows that such changes in conformation and chain packing are insufficient to explain the strong quenching we observe in FN-BF4-7%. Schmidtke et. al. (Phys. Rev. Lett.; 99; pp 167401 (2007)) probed interchain interactions by carrying out photophysical studies on F8BT films under high pressures. Red-shifted absorption and PL spectra at pressures up to 5 GPa were actually explained mostly by the intramolecular planarization of F8BT chains, showing that even highly compressed chain conformations do not represent the photophysics of FN-BF4-7%.
A low density of ions could not induce such a pronounced perturbation of exciton decay without invoking exciton migration towards ionic sites. This mechanism is exploited for the amplification of exciton quenching via charge- or energy transfer in sensors, since excitons diffuse in three dimensions to sample a relatively large volume for the presence of an analyte. In FN-BF4-7%, two ion pairs are attached to the alkyl sidechains of 7% of monomer units in a statistical fashion, and the volume occupied by each monomer unit in a film is estimated to be ˜1.3 nm3 (based on the unit cell reported for F8BT films using x-ray diffraction techniques). Therefore, one would expect the mean distance between neighboring ion pairs to be ˜2.7 nm if the ion pairs are randomly dispersed throughout the film, which is certainly within the exciton diffusion radius of F8BT (>10 nm). Phase segregation between ionic and non-ionic regions could increase the size of pristine non-ionic regions, however the scope for phase segregation is expected to be rather constrained for a statistical copolymer with 7% ionic sidechains.
We undertook low temperature PL measurements in order to investigate the photophysical influence of dispersed ionic sites when exciton diffusion lengths are constrained due to insufficient energy for thermally activated exciton hopping.
Polarization anisotropy dynamics are a powerful probe of exciton motion in disordered chromophore materials.
Finally, we sought to quantify the fraction of excitations that form long-lived CT states (rather than decaying to the GS) by measuring the GS recovery kinetics by at a wavelength (λprobe=490 nm) that is resonant with the GS absorption band and too high in energy to induce excited state absorption or stimulated emission.
Instead, we consider whether excitons could be quenched by prior photogenerated charges at the excitation intensities used in the TA measurements. Singlet excitons are known to be efficiently quenched by polarons in conjugated organic materials, typically via Förster resonant energy transfer from an emissive exciton to an absorbing polaron. The enhanced visible absorption of charges relative to excitons means that this bimolecular decay channel can still be operative at excitation densities below the threshold of exciton-exciton annhiliation. We calculated the Förster radius for resonant energy transfer from an F8BT exciton to an absorbing CT state. According to the application of Förster theory to conjugated polymers, the Förster radius (R0) is given by;
In equation 1, κ2 is the orientational factor (taken to be 0.655 for the case where the donor and acceptor dipoles lie in the plane of the film) η is the refractive index of the medium (previously measured to be 1.8 by ellipsometry), NAv is Avagadro's number, fD(λ) is the emission spectrum of the donor (in this case an F8BT singlet exciton) normalized such that ∫fD(λ) d v=1, and εA(λ) is the absorption spectrum of the acceptor (the CT state in this case) in units of molar extinction coefficient (M−1 cm−1). The absorption spectrum is obtained in the appropriate units using;
where σ(λ) is the absorption cross-section calculated from the transient transmission spectrum using;
In equation 3, the excitation density (N=4.7×1017 cm−3) is determined from the product of the incident fluence (6×1013 ph/cm2), the absorption efficiency at the excitation wavelength (η490 nm=0.78), the fraction of excitions that survive beyond 1 ns where the spectrum is measured (Φ=0.15, based in the red curve in
Rather than attempting to quantify these corrections, we measured GS recovery kinetics under identical conditions for F8BT blended with PFB (poly(9,9-dioctylfluorene-co-bis-N,N″-(4,butylphenyl)-bis-N,N″-phenyl-1,4-phenylene-diamine), a combination that is known to readily facilitate charge photogeneration and photovoltaic behavior. We were able to prepare a blended film morphology that exhibited F8BT exciton quenching on a comparable timescale to FN-BF4-7% by casting the film (1:1 F8BT:PFB by weight) from a mixture of low- and high-boiling-point solvents, in this case chloroform:chlorobenzene (90:10 by volume). The conversion of excitons to long-lived charge-pairs is known to be very efficient in this blend under steady state excitation (Φexcition→charge>60%).
Therefore, if no additional quenching channels were operational, one would expect the blend to exhibit GS recovery kinetics similar to unblended F8BT, but with the decay arrested at >0.6 of the initial level corresponding to the charge population. However, we observe that the GS recovery kinetics are markedly accelerated in the blend and only <10% of excitations survive 2 ns after the excitation pulse (
We have synthesized a CPE with a low density ionic sidechains and applied time-resolved spectroscopy techniques in order to isolate the inherent interactions between excitons and ionic charges in CPE films. Without wishing to be bound by any particular theory, we believe that time-resolved emission and absorption spectroscopy show that in films of FN-BF4-7%, the primary exciton resembles that of the non-ionic counterpart (F8BT). Excitons then undergo activated hopping (Eact=28 meV) until the majority encounter a region where ions interact with the polymer backbone in FN-BF4-7%. Beyond this timescale, the excited state population exhibits longer-lived emission that is stabilized by 0.3 eV, loss of stimulated emission, and a broadened photo-induced absorption signal. These spectral features provide strong evidence for photogenerated CT states induced by the interaction of ions with bound excitons in CPEs.
These findings have significant implications for the design of conjugated polymer devices that incorporate ionic charge carriers. Unless the interaction between ions and conjugated polymer backbone is well-controlled, morphologies must be optimized to exclude ions from exciton transporting domains. In the case of LEDs, it is notable that ions are most effectively deployed as a layer that is well-separated from the recombination zone where emissive excitons are generated.
Producing a favorable morphology that exploits mobile ionic charges to assist electronic charge separation in bulk heterojunction polymer PV devices is more challenging. Here, ions must be distributed on the nanometer lengthscale at donor-acceptor interfaces throughout the active layer in order to assist charge separation, whilst being excluded from within exciton transporting domains. Our data demonstrates that excitons are effectively funneled towards ionic regions, thereby allowing us to direct the motion of excitons by controlling the spatial distribution of ions and their interaction strengths. We note that counter ions could be readily substituted in order to tune the energetic balance between excitons and separated charges in CPEs.
Accordingly, the presented data demonstrates that CPE with low density ionic sidechains are eminently usable in PV devices and that the presence of ions at the interface between donor and acceptor layers can lead to improved performance of such devices.
It will be readily appreciated by the skilled person, that other ionic species may be incorporated into the polymeric structure. It will be further appreciated that other CPEs and molecular semiconductors (e.g. fullerenes) may be used in the invention outlined herein.
It will be further appreciated that one or both of the species may have tethered ions as discussed above.
Number | Date | Country | Kind |
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0907445.1 | Apr 2009 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2010/050726 | 4/30/2010 | WO | 00 | 12/14/2011 |