Letter pubs.acs.org/JPCL
Photochromic Electret: A New Tool for Light Energy Harvesting Rossella Castagna,†,‡ Michele Garbugli,‡ Andrea Bianco,*,§ Stefano Perissinotto,‡ Giorgio Pariani,†,§ Chiara Bertarelli,†,‡ and Guglielmo Lanzani‡ †
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, piazza L. da Vinci 32, 20133 Milano, Italy Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, via G. Pascoli 70/3, 20133 Milano, Italy § Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy ‡
ABSTRACT: In this paper, a photochromic electret for light energy harvesting is proposed and discussed. Such electret directly converts the photon energy into electric energy thanks to a polarization modulation caused by the photochromic reaction, which leads to a change in dipole moment. Theoretical concepts on which the photochromic electret is based are considered with an estimation of the effectiveness as a function of material properties. Finally, an electret based on a photochromic diarylethene is shown with the photoelectric characterization as a proof of concept device.
SECTION: Energy Conversion and Storage current flows in the external circuit, as a consequence of charge balance, thus building work on a load resistance. We have been inspired by these devices in finding materials showing a polarization that can be directly switched using photons as stimulus. In other words, we looked at materials that show a reversible change of polarization upon photoirradiation. Photochromic materials are gifted with this property. When illuminated with a suitable light source, they undergo a photoisomerization that leads to a change in color.28,29 The reaction is reversible by using photons of different energy or simply through a thermal recovery mechanism.28 Luckily, color is not the only property that varies,30 but also polarizability,31 vibrational spectra,32−34 luminescence,35,36 and dipole moment31,37,38 change. Specifically, the change of dipole moment brings about a modulation in the polarization in a material with a preferential dipole orientation. Photoconversion of a photochromic molecule is usually triggered by UV photons, but, in principle, light can be harvested on a broad spectral range. It is worth noting that the key issue is that the process of energy storage is not dependent on charge transport properties, thus circumventing one major constraint of organic photovoltaics, as mentioned above. To our best knowledge, this is the first time that a photochromic electret is demonstrated for energy harvesting and that this mechanism is proposed for light energy conversion. Similar approaches have indeed been considered in the past, but just as a characterization technique,39 whereas
T
he energy demand of our society is constantly increasing. This makes the issue of energy supply a priority that requires a global approach, including new knowledge from scientific research for finding more efficient and renewable energy sources.1,2 It is apparent that the sun is a huge source of energy that is mandatory to exploit.3 In this framework, siliconbased solar cells play the most important role, and many efforts have been devoted to improve efficiency and reduce cost.4−6 Third-generation solar cells based on organic materials show interesting features, such as mechanical flexibility, spectral sensitivity, lightness, and low cost.7−9 However, their market exploitation is still hampered by a number of drawbacks, such as stability and performance. The latter seems significantly limited by poor transport in the active layer, which is a wellknown and hardly avoidable issue in organic semiconductors.7−10 In this context we propose an alternative mechanism to gather and convert light energy into electrical energy, which is still based on organic molecules, but it is free from the problem of charge transport. Many recent studies11−18 on energy harvesting exploit the modulation of polarization in a capacitor. Such kind of devices are often called electrets.19,20 Materials suitable for this application are piezoelectric and pyroelectric materials, which show a net polarization at no applied electric field.21−24 This polarization can be spontaneous (this is the case of inorganic compound)25 or induced by poling or other techniques (this is the case of polymers).24,26,27 By applying suitable stimuli, such as a change in temperature for pyroelectric systems or a deformation in the case of piezo, a change in the polarization occurs and, if a capacitor is built up around them, an electric © 2011 American Chemical Society
Received: November 1, 2011 Accepted: December 5, 2011 Published: December 5, 2011 51
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photochromic materials have been used as switchable interlayers in organic photovoltaic cells.40,41 On the other hand, the use of nanodipoles for energy storage has been recently reported in a theoretical paper.42 In order to investigate the feasibility of this new approach, we proceeded as follows: (i) we analyzed the material requirements, reporting the key factors that determine the effectiveness of the device, and (ii) we realized a proof of concept device based on diarylethene molecules. Material Requirement and Device Structure. Let us suppose to fill a plane capacitor with a dielectric material doped with a photochromic compound, which features different dipole moments in the colorless and colored form (μU and μC, respectively). The initial condition refers to photochromic molecules in the colorless form and randomly distributed in bulk. In this condition, the polarization (dipole moment per unit of volume, usually expressed in C/m2) is zero. By performing a process to align the dipoles (for example, a poling), the photochromic molecules rearrange with the dipole moment along the electric field direction, i.e., perpendicular to the capacitor plates. Accordingly, a polarization PU occurs, which can be calculated as follows:43 PU =
N ε∞ + 2 μ U⟨cos θ⟩ V 3
Figure 1. Scheme of the photochromic electret and its working principle (a poling process to align the dipoles is supposed).
From eqs 1, 3, and 4, it follows that ⎛ε + 2 ε +2 ⎞ ΔQ N = ⟨cos θ⟩⎜ ∞ C μC − ∞ U μ U⎟ ⎝ ⎠ A V 3 3 ε +2 N (μC − μ U) ≅ ⟨cos θ⟩ ∞ V 3
Actually, the dielectric constant at optical frequency changes upon photoisomerization, but the difference is usually small30 especially compared to the change in dipole moment. Therefore, it is assumed likely constant in our analysis. It is worth making some calculations to quantify the capability of a photochromic electret. Let us consider first ε∞ = 2.6, a perfect orientation of the dipoles and a complete conversion of the photochromic molecules: the change in surface charge at the capacitor plates can be plotted as a function of the molecular concentration and of the change in dipole moment (Δμ). The results are reported in Figure 2.
(1)
where N/V is the density of dipoles, ε∞ is the dielectric constant at optical frequency, θ is the angle between the molecular dipole moment and the electric field, and hence ⟨cos θ⟩ is the average orientation of the dipoles. Here we are considering the permanent dipole contribution and the internal local field contribution to the polarization according to the Onsager approach.44 Such polarization generates an electric field equal to EU = −
PU ε0
(2)
If the capacitor is connected to an external load resistance, a standard RC circuit is obtained, where the electric field EU inside the dielectric is balanced by the field produced by the free charges recalled on the plates; therefore, the total electric field in the device is zero at equilibrium. By impinging the capacitor with UV light, the photochromic molecules convert into their colored form, which has a different dipole moment (μC). As a consequence, both PU and EU change into PC =
N ε∞ + 2 μC⟨cos θ⟩ V 3
(5)
EC = −
PC ε0
Figure 2. Charge per unit of surface recalled on the capacitor plates upon photoisomerization as function of the dipole modulation for different concentrations of molecules.
Dipole concentration (N/V) is a key parameter to define our electret performance, which is limited by the material density and by the content of photochromic unit. Moreover, the oriented dipoles create an electric field that tends to separate themselves preventing a good orientation during the poling. Both issues have been deeply investigated in the case of push− pull molecules for second order nonlinear optics (NLO).45 It has been shown that the nonlinear optical response as function of the molecular concentration reaches a maximum followed by a decay.46 The best concentration depends on the molecular dipole moment and on the molecular form factor. Therefore, in the case herein described, it would be better to have a large Δμ, with small μU or μC. Both experiments and theory suggest that values between 1020 and 1021 molecules/cm3 are reasonable. A second key factor is the change in dipole moment (Δμ), which is strongly related to the molecular structure of the photochromic unit. Unfortunately, only few experimental
(3)
This change of polarization induces a current in the external circuit that restores the equilibrium (see scheme in Figure 1), i.e., the total electric field in the capacitor is still zero. The amount of electric charge recalled on the plates is ΔQ = Q C − Q U = (PC − PU)A
(4)
where A is the area of the plates. It is apparent that the trend of the displacement current depends on the conversion kinetics in the photochromic dielectric and on the characteristics of the RC circuit. When photochromic material is switched back to the colorless form, a charge −ΔQ flows through the circuit to restore the starting condition. This means that a current of opposite sign is produced. 52
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values47−49 and some calculated values using different approaches have been reported in the literature. In the case of spiro compounds, a Δμ of about 8D is found both theoretically37 and experimentally,48 when the photochromic skeleton bears a nitro group, which is a strong acceptor. Regarding diarylethenes, changes are strongly dependent on the chemical structure and on the presence of specific polar groups.31,50,51 Very large values have been calculated in donor− acceptor-substituted dihydropyrene-[2,2]metacyclophanediene photochromic systems, showing a Δμ on the order of 20−30 D.52 According to the discussion above, it is important to keep in mind that a sort of upper limit of (N/V)μ exists that corresponds to the best orientation of dipoles perpendicular to the capacitor plates. Using eqs 1, 2, and 3 the change in electric field due to the photochromic reaction can be worked out and, in turn, the energy produced by the reaction can be estimated. Supposing to switch instantaneously all the dipoles, the polarization changes from PU to PC and the unbalanced field (ΔE) is equal to ΔE =
PU − PC ε0
Figure 3. Calculated conversion efficiency as function of molecular orientation degree and modulation of dipole moment.
the latter usually being considered in photovoltaic cells. Considerations on power would require the study of the conversion rate in the bulk. At the molecular level, the photochromic conversion is a fast process on the order of 1− 100 ps,59−62 but at the solid state the full conversion is slower. The issue of power conversion is thus premature at this level of discussion, and left to further developments. The concept of the “photochromic electret” has been demonstrated using a diarylethene and the contact poling technique to achieve the dipole orientation. Regarding the photochromic material, we considered 1,2-bis[2-methyl-5-(p-cyanophenyl)-3-thienyl]perfluorocyclopentene (DTE-CN) (reported in Figure 4 together with UV−vis absorption spectra).
(6) 3
The energy density U (J/cm ) related to the unbalanced electric field is 1 ε0ΔE2 2 1 = (PU − PC)2 2ε0
U=
=
⎞2 1 ⎛ N ε∞ + 2 ⎜ ⟨cos θ⟩⎟ (μ U − μC)2 ⎠ 2ε0 ⎝ V 3
(7)
In order to assess the “internal” energy conversion efficiency, we evaluated the ratio between the stored energy (U) and the absorbed photon energy (u), which is calculated considering a density of photons, with energy hν, equal to that of the photochromic molecules in the electret and assuming a quantum yield of the photochromic process (ϕ): ⎞2 N ϕ ⎛ ε∞ + 2 U ⎜ ⟨cos θ⟩⎟ (μ U − μC)2 = ⎠ u 2ε0Vhν ⎝ 3
(8)
−3
Considering (i) a concentration of 10 cm , μC = 15 D, μU = 5 D, ⟨cos θ⟩ = 0.9, ε∞ = 2.6 as above, (ii) illuminating the sample with light at 500 nm (2.56 eV), and (iii) a quantum yield of the photochromic conversion of 0.5, we obtain a value of 16%. A plot of the efficiency as a function of orientation degree and dipole modulation is reported in Figure 3. In order to boost the performance of the device, it is necessary (i) to have a dielectric material bearing a large content of dipolar photochromic moieties, (ii) to maximize the modulation of the dipole moment, and (iii) to achieve a good and stable orientation of the dipoles. This last issue is critical, and different approaches borrowed from the organic NLO field can be followed; in particular, we mention the poling process53−55 and self-assembly.56−58 The main conceptual difference relies in the fact that the former produces a metastable state, and the latter produces a thermodynamic stable state. We note that the discussion above on ideal electret performance is expressed in terms of energy and not power, 21
Figure 4. Above: Photochromic reaction for DTE-CN and corresponding values of the dipole moment (calculated31). Below: absorption spectra of the open (dashed line) and closed (solid line) forms of DTE-CN in n-hexane.
Upon exposure to UV light, the photochromic molecule in the open (colorless) form converts into the closed (blue colored) form, and a strong absorption occurs in the visible (peaked at 590 nm). This molecule shows a larger dipole moment in the colorless form and a dipole close to zero in the colored one. The direction of the dipole moment is along “the axis of symmetry of” the perfluorocyclopentene. It is apparent that the dipole moments of both forms are rather small, as is their difference 53
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(Δμ). The choice of DTE-CN stems from the fact that we aimed to detect the signal coming from the switching dipoles to avoid the effects of homo charges. The device consists of a thin film (0.8 μm) of polystyrene (PS) doped with DTE-CN molecules, placed between two gold electrodes. The semitransparent, 20 nm thick, top gold electrode was evaporated in a ultravacuum chamber on top of a glass substrate. The active material layer was deposited by spin coating. The bottom electrode, about 80 nm thick, was again gold, which was grown by ultrahigh vacuum evaporation. The spin coated film is an isotropic material, with the photochromic molecules randomly oriented inside the PS matrix. The alignment of the molecules has been performed in the organic layer by means of the contact poling process. Contact poling has been preferred to corona poling, since it allows the photochromic material to be poled directly in the fabricated device, thus avoiding device fabrication post processes, such as electrode deposition and contacting, after poling. On the other hand, it is known that the contact poling technique usually gets a lower ratio of molecules oriented with respect to corona poling. The quantitative estimation of the ratio of molecules that are actually oriented in the layer is not trivial. Nevertheless, by considering that the strength of the dipole moment of the diarylethene under study is low and contact poling is less effective in orienting the dipoles if compared to corona poling, we expected a very small amount of aligned molecule; notwithstanding this, the obtained orientation was large enough to demonstrate the working principle of the device. Second harmonic generation (SHG) was monitored during the poling process, as a real time diagnostic of the alignment.63 Before the poling, the SHG signal was zero, and it grew up with time applying the electric field and increasing the temperature; when the electric field was turned off, the SHG signal showed a partial drop. As a result, the DTE-CN molecules had an average orientation of the dipole perpendicular to the capacitor plates (see Figure 1) and a net polarization was generated in the same direction. By shining the sample with UV light at 375 nm, the photochromic molecules were converted to the closed form, which is characterized by a dipole moment smaller than the open form (according to calculations31). The conversion causes a decrease of the polarization that produces a transient current between the electrodes, detected by the amperometer. As already pointed out, the current signal depends on the electric circuit coupled with the photochromic capacitor, on the conversion efficiency, and, obviously, on the change in polarization. In Figure 5, the time-resolved current density measurement after the poling process is shown for a series of light pulses. Upon the first exposure to UV light, a transient current peak is generated, with a decay of a few seconds. The following UV exposures produce much smaller signals. This is probably due to the effect of detrapping of homo charges that have been introduced into the electret during the poling. The effect of homo and hetero charges (dipole moments) is well-known in the case of polymer poling,19,64,65 and the amount of homo charges into the dielectric depends mainly on the strength of the poling field; afterwards, the situation is stabilized by the oriented dipoles. Since the photochromic conversion leads to a molecule with smaller dipole moment, the sign of the current is equal for the two contributions. Another clue that leads to the hypothesis of the proposed mechanism concerns the weak peaks that also occurred after the conversion of the photochromic molecules, since the UV light is still absorbed
Figure 5. Displacement current density generation from a poled photochromic film of DTE-CN irradiated by UV light at 375 nm (blue shaded area).
by the layer and the energy provided enables the detrapping of some charges. By inducing the backward reaction of the photochromic molecules with visible light at 532 nm, a displacement current density peak was observed in the opposite direction, due to the increase of the polarization, as shown in Figure 6. Certainly this signal is related to the increase of molecular dipole moment, whereas there is no effect of the homo charges that would produce a negative current.
Figure 6. Displacement current density generation from a poled photochromic film of DTE-CN irradiated by visible light at 532 nm (green shaded area).
As the photochromic conversion was completed, a subsequent visible light exposure did not produce any current as expected, since the converted material is transparent at this wavelength. Cycles of illumination with UV and visible light produced subsequent current signals. A partial deorientation of the chromophores, which it is known to depend on the molecular structure and on the polymeric matrix,66 took place due to the metastable nature of the poled system. The same procedures were performed on analogous poled devices of bare PS matrix to verify whether the current peaks could arise from spurious effects. Actually, any current signal and no short circuits or current modulations were generated upon light exposure, thus confirming that the changes in current are a result of the photochromic isomerization. In conclusion, we proposed a new mechanism of light energy conversion based on a molecular photochromic electret. The device exploits the reversible light-induced switching of dipole moment in oriented photochromic molecules, and this approach achieves unprecedented energy conversion efficiency and opens up a new paradigm in solar energy conversion. We estimated the performances of an ideal device and its dependence on the orientation and molecular properties. Finally, a proof of concept device of the photochromic electrets 54
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based on a photochromic dithienylethene dispersed in PS was provided. Nevertheless, it is worth noting that a photochromic electret with high efficiency can actually be built if a number of technological challenges are overcome. A proper molecular design is required to find the best candidate to this aim: the photochromic molecule should absorb UV−visible light, have a large change in static dipole moment, and allow a simple and no power consuming process of space orientation. Among all the techniques to obtain a thermodynamically stable system, self-assembly is certainly one of the most appealing approaches to develop photochromic electrets. Although any photochemical reaction that produces a change in dipole moment enables the conversion of incident photons into electric current, the reversibility of the photochromic process offers the opportunity to reset the original state, thus obtaining a selfrestoring device. In this context, a thermally reversible photochromic system, able to regenerate the initial state spontaneously, would be more convenient than diarylethenes.
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EXPERIMENTAL METHODS Thin Film Preparation. The photochromic diarylethene DTECN was synthetized according to ref 67. A solution of PS (36 mg/mL) and DTE-CN (4 mg/mL) in CHCl3 was spin coated (Laurell WS-400B-6NPP/LITE) at 1000 rpm. The thickness was measured using a Filmetrics F20-EXR reflectometer. Poling Process. In a vacuum chamber at 10−2 mbar, a voltage of 110 V was applied to the gold electrodes generating a strong electric field inside the dielectric layer that forces the DTE-CN molecules to be aligned along the field axis. Keeping the voltage on, a thermal annealing process was carried out by raising the temperature until 75 °C (close to the glass transition temperature of PS) in order to favor the alignment of the DTE-CN molecules. Cooling the sample to room temperature, the molecules were frozen into their new orientation. SHG Measurements. The fundamental beam source is a Tisapphire laser with chirped pulse amplification, centered at 800 nm, with a pulse duration of approximately 60 fs, repetition rate of 1 kHz, and maximum pulse energy of 2 mJ. The pulse was attenuated to 100 μJ and polarized in the incident plane (ppolarization). Then the beam was sent to the sample with an angle of 60° in order to optimize the SHG signal. The SHG beam (at 400 nm), reflected by the gold electrode, was detected with a UV−vis photodiode after rejection of fundamental by a glass cutoff filter. In order to increase the sensitivity of the technique, we modulated the exciting fundamental beam by a chopper, at 213 Hz, and the output photodiode signal was set to a lock-in amplifier. Electric Measurements. The transient current peak was detected by connecting the capacitor electrodes directly with a Keithley 236 Source-Measure Unit. UV light: 2.5 mW at 375 nm; visible light: 30 mW at 532 nm.
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AUTHOR INFORMATION
Corresponding Author
*Mailing address: Osservatorio Astronomico di Brera, via Bianchi 46, 23807, Merate (LC), Italy; tel.: +390395971060; fax: +390395971001; e-mail:
[email protected].
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ACKNOWLEDGMENTS The authors acknowledge Fondazione Cariplo, who partly supported this work through the project MATHYS (Grant No. Ref.2009/2527). 55
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