Photothermally Activated Pyroelectric Polymer Films for Harvesting of

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Photothermally-Activated Pyroelectric Polymer Films for Harvesting of Solar Heat with a Hybrid Energy Cell Structure Teahoon Park, Jongbeom Na, Byeonggwan Kim, Younghoon Kim, Haijin Shin, and Eunkyoung Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04042 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on September 3, 2015

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Photothermally-Activated Pyroelectric Polymer Films for Harvesting of Solar Heat with a Hybrid Energy Cell Structure Teahoon Park, Jongbeom Na, Byeonggwan Kim, Younghoon Kim, Haijin Shin, and Eunkyoung Kim* Active Polymer Center for Pattern Integration, Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea. Fax: +82-2-312-6401; Tel: +82-2-2123-5752; E-mail: [email protected]

ABSTRACT

Photothermal effects in poly(3,4-ethylenedioxythiophene)s (PEDOTs) were explored for pyroelectric conversion. A poled ferroelectric film was coated on both sides with PEDOT via solution casting polymerization of EDOT, to give highly conductive and effective photothermal thin films of PEDOT. The PEDOT films not only provided heat source upon light exposure but worked as electrodes for the output energy from the pyroelectric layer in an energy harvester

ACS Paragon Plus Environment

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hybridized with a thermoelectric layer. Compared to a bare thermoelectric system under NIR irradiation, the photothermal-pyro-thermoelectric device showed more than 6 times higher thermoelectric output with the additional pyroelectric output. The photothermally driven pyroelectric harvesting film provided a very fast electric output with a high voltage output (Vout) of 15 V. The pyroelectric effect was significant due to the transparent and high photothermal PEDOT film, which could also work as an electrode. A hybrid energy harvester was assembled to enhance photo-conversion efficiency (PCE) of a solar cell with a thermoelectric device operated by the photothermally generated heat. The PCE was increased more than 20% under sunlight irradiation (AM 1.5G) utilizing the transmitted light through the photovoltaic cell as a heat source that was converted into pyroelectric and thermoelectric output simultaneously from the high photothermal PEDOT electrodes. Overall, this work provides a dynamic and static hybrid energy cell to harvest solar energy in full spectral range and thermal energy, to allow solar powered switching of an electrochromic display.

KEYWORDS: Photothermal, Pyroelectric, Thermoelectric, PEDOT, Hybrid Energy Harvesters

Photothermal effects in conducting polymers have great potentials in applications for therapy, photo catalysts, energy harvester, and actuators.1-3 In particular the heat generated by light could provide viable static energy sources from the living environment, through several energy conversion methods including thermoelectric (TE)

4-10

and pyroelectric (PE)

11-14

methods.

Although the energy harvesting efficiency from TE and PE conversion is generally low, those can be coupled with hybrid energy harvesters employing two or more energy sources. 15-30 For


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instance, a solar cell hybridized with a TE harvester showed enhanced PCE due to the additional TE conversion of solar heat. 25 Obviously, the PCE of a solar cell could be improved further when the efficiency of solar heat conversion become higher by employing multiple thermal energy harvesters. Since such a hybrid energy harvester could be designed by multiplying thermal energy harvesters according to different conversion mechanisms, neither of the energy harvesters should inhibit the other. Therefore, to harvest superfluous heat from solar energy, it is necessary to design photothermal (PT) materials with an appropriate absorption and high conversion efficiency in the wide solar spectrum. 31-33 To meet these requirements, conducting polymer thin films (CPs) are suitable materials for a hybrid energy system because of their transparency in the dye absorption range plus high PT conversion. 4-6, 35 In particular, PEDOTs and their derivatives seem promising for a multi energy harvester, as their doping states are easily controlled to optimize transparency and PT conversion. 5, 6 Therefore, the photothermallydriven heat from a solar environment could be converted into electricity through PE and TE combination at once. Furthermore, a photothermally-driven PE (PT/PE) system can be combined with a photovoltaic (PV) cell, to boost energy harvesting from a solar source. 25-27, 36 Because the typical dye in a dye-sensitized solar cell (DSSC) absorbs only part of the incident light, the transmitted and wasted sunlight can be reused for PT/PE harvesting and, eventually, for TE conversion. Therefore the major challenge in this hybrid harvester is to prepare PEDOT films with high 1) PT effect, 2) transparency in visible range to allow maximum dye absorption, 3) electrical conductivity to be able to use them as an electrode, and 4) surface energy matching with a pyroelectric film. Although several PEDOT films were reported for an energy harvester, it is rare to find photothermally activated PEDOT films for a hybrid energy harvester.


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Herein, we report a PT/PE conversion using PEDOT film, which is inserted in a hybrid energy harvester designed to increase the open-circuit voltage (VOC) and thus to improve the energy harvesting efficiency from a solar source. This hybrid energy harvester ensures a high VOC in a solar cell, which is crucial to enhance the conversion efficiency as well as to operate an organic electronic implement with a high working voltage (Vwk). Moreover, a combination of PT and PE films provides a compact PV-PT/PE-TE hybrid energy cell, similar to a tandem solar cell system, for the harvesting of the full solar spectrum.

Result and discussion A highly-conductive PEDOT film (1,240 ± 140 S cm-1) was used for the PT conversion and as an electrode for the PE conversion. Thus, the PEDOT was directly coated on both sides of a PVDF-TrFE film (β-phase) through the solution casting polymerization (SCP) of EDOT. 5, 37 The composition of the SCP was optimized by adding a polymeric surfactant, poly(ethylene glycol)block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer (PEPG) (Mw: 2,800), following previous research. 5, 37 It is abbreviated as PP-PEDOT hereafter. To optimize the PT effect of PP-PEDOT film, three types of films were examined using a NIR source (808 nm, power): 1) a bare PVDF-TrFE film (P0), 2) one side of a PVDF-TrFE filmcovered by the PP-PEDOT (P1), 3) both sides (top and bottom) of the PVDF-TrFE film-covered by PP-PEDOT (P2), as shown in Figure S1. The thickness of each PP-PEDOT layer was 120 nm. As expected from the transparency of the bare PVDF-TrFE film (P0) in Figure 1a and S1, most of the NIR was transmitted through the film. With the double-sided PP-PEDOT electrodes (P2), the NIR was less transmitted and the surface temperature was the highest among the three samples, as shown in the thermal image for the surface temperature rise (Figure 1a) after


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excitation with a NIR laser for 1 min. Under a dose of 30 mW cm-2 for the NIR irradiation, P2 showed the highest PT effect, with the greatest temperature change (∆T >10 °C). In the case of P0, no significant temperature increase was observed. Therefore, it can be concluded that the PT effect was driven by the PP-PEDOT layer. The NIR PT conversion for the PP-PEDOT film was determined as 50%, according to the previous methods.

35, 37

Compared to other conjugated

polymers, PP-PEDOT showed higher conversion efficiency due to its high electrical conductivity and dense film morphology.5 Considering that the level of heat generation could be dependent on the thickness of the electrodes, four different types of P2 were prepared by coating the PP-PEDOT onto the top (T) or bottom (B) surface, or both, keeping the thickness of each processed film almost same (Table S1): the structures were T1B1, T2B1, T1B2, and T2B2 (Figure 1c), where “T” represents the top surface facing directly towards the NIR source, and “B” represents the bottom phase of the PE film facing towards the TE module. The numbers “1” and “2” indicate the number of PP-PEDOT layers, which were fabricated through a single or double SCP process, respectively. The UVVIS-NIR spectra in Figure 1b show that all PP-PEDOT samples were in a doped state, regardless of the number of SCP processes. The P1 film showed a relatively low transmittance at 808 nm (Figure 1c), due to the doped PP-PEDOTs. The transmittance of the T1B1 film was 5.5% at 808 nm. In the cases of T2B1 and T2B2, the transmittance was extremely low (0.2%), which is favorable for high PT by the NIR irradiation. A photothermally-driven PE and TE device was prepared as schematically drawn in Figure 2a. A thin K-type thermocouple (TC) was inserted in between the PE and TE interface, to determine the temperature of the interface (Ti), as measured at the bottom surface of the PE. The polarized PE film was stabilized at ambient condition. When the NIR laser was irradiated to the PP-PEDOT electrodes, photothermally generated heat


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changed the degree of dipole in the PE film, to allow electron flow through the circuit affording electric output by PE. As shown in Figure 2b, the temperature rise in P0 was lower than the TC itself under NIR irradiation, mainly because the low thermal conductivity of P0 (PVDF-TrFE). The T2B1 film, which had a thick PP-PEDOT layer as its top electrode, showed a lower Ti (37 °C) than T1B1 or T1B2, because a large amount of NIR was absorbed by the thick top electrode, yielding a less photon energy absorption at the bottom layer of the PE, which led to lower photothermal conversion. Among the devices, the T1B2 film showed the highest heat generation at the bottom of the PE film, with a Ti of 42.2 °C. The partially transmitted NIR source from the top electrode was almost absorbed at the bottom PP-PEDOT layers, and generated heat to create a temperature gradient useful for the operation of a TE device. Thermal NIR images were obtained to confirm the temperature at the top of the PT/PE film (Figure S2). For an efficient heat transfer from the PE film to the TE module, a thermal paste (TP) was coated (70 ± 5 µm) onto the interfaces between the PE and TE. As shown in Figure 2b and c, the temperature of the TP-coated PEs increased less than without TP. This result indicated that the photothermally-generated heat of the PP-PEDOT electrodes in the PE layer was almost transferred, through heat conduction from TP, to the TE device. Therefore, further experiments were performed with a T1B2 type PE with TP. Figure 2d shows the harvested electricity of the PE device through the photothermal conversion of the PP-PEDOT electrode by NIR irradiation (104 mW, 305.9 mW cm-2) under the optical setup described in the experimental part. An optical shutter was positioned between the sample and the NIR source to turn the NIR exposure on and off. When NIR was irradiated onto the PPPEDOT electrode in the PE layer, a PT/PE voltage immediately appeared as pyroelectricity is


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dependent on the temperature gradient (dT) over time (dt), and the temperature of the PPPEDOT film increased sharply in a very short time upon NIR exposure (Figure 2a). The photothermally-generated heat was then conducted and dissipated into the entire hybrid device, so that the temperature in the PE film could be balanced. Therefore the PE voltage output reduced accordingly. When the NIR light was switched off, the output voltage generation showed a negative signal in a symmetric shape. The power density (Pd) and integrated power (Pint) of the PT/PE device are presented in Figure 2e. With only one cycle of NIR switching (NIR on/off), the Pint was over 200 µJ m-2, which indicates that the PT/PE device itself could be potentially applied as an energy harvester and also as a photo-sensor. 39 To confirm the effects of PT/PE on the TE performance, an authentic TE module was examined on a heat sink with and without a PT/PE film (Figure 3a and b). In this experiment, the input NIR was focused through a lens to match the light exposed area (diameter of 6.5 mm) within the sample, so that the energy conversion efficiency of the heat from PT to PE, and eventually to TE, could be estimated more accurately. The output voltage (Vout) from the hybrid harvester is shown in Figure 3c. Without a PE film, the Vout (TE voltage) was maximized at 1.7 mV (black square, filled). However, in the presence of PE (PT/PE-TE), the Vout of the hybrid harvester (PT/PE-TE voltage, red circle, filled) reached 6 times higher (10.8 mV) than that without a PE device. This is more than 5 times higher than the Vout of the hybrid energy harvester reported in the literature. 31, 32

The maximum output current (Iout) of the TE module (TE current) in the hybrid system (13.6

mA) under NIR exposure was much higher than that of the TE module only (Iout =1.1 mA) under NIR, as compared in Figure 3c. Therefore, it can be assumed that the PT effect of the PE electrodes, PP-PEDOT, played a key role in operating the TE module effectively. With an increasing NIR intensity (INIR), the electric output (Vout and Iout) of the TE module in the hybrid


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system was increased (Figure 3d and Figure S3), with a simultaneous increase in the energy conversion efficiency (Figure 3e). With an INIR of 104 mW, the conversion efficiency reached 0.15%, and the power density of the PT/PE-TE harvester was > 4 W m-2. The stability of the PPPEDOT electrodes and PVDF-TrFE film after NIR irradiation were confirmed by FE-SEM images and X-ray diffraction (XRD) (Figure S4). The morphology of the PP-PEDOT and the XRD peaks for the β-phase PVDF-TrFE remained the same as the unexposed samples when INIR was lower than 105 mW. As shown in Figure S5, the temperature on the PEDOT surface was 140 °C with an ipw of 198 mW (582.3 mW cm-2), to damage PE film. Importantly, when the two energy harvesters are combined (PE/TE), the Vout (hybrid) was maximized to ~ 200 mV, due to the PE via PT effect. Furthermore, the Vout was always larger than that from TE only, showing a unique complementary energy generation curve. As shown in Figure 3f, most pyro-electricity was generated right after the NIR irradiation, while the thermoelectricity generation was low in this initial period of NIR exposure. As time passed, the PE output gradually decreased (black) and the TE output increased simultaneously (red). Considering the total amount of energy generated by the PT effect of PP-PEDOT, the PE and TE outputs complemented one another and produced an enhanced Vout in the PT/PE-TE system (blue) compared to that produced by a single harvester (PE or TE). This is the first example of a photothermally driven PE for energy harvesting and a hybrid system, to provide a very fast output generation with high Vout value. The PE effect become significant due to the transparent and high photothermal PP-PEDOT film, which can also work as electrodes to harvest the PE output through the direct connection into a circuitry. Most of the sensitizers in organic PVs only absorb part of the UV and visible range of sunlight. Therefore, much effort has focused on harvesting the solar energy in transmitted NIR light in


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order to increase the efficiency of PV devices. The PT/PE-TE hybrid energy harvester based on PP-PEDOT is interesting in itself, but can be combined with a PV cell to harvest solar energy in the full spectral range, from UV to NIR light. As shown in Figure 4a, the schematic diagram for the hybrid energy cell in this study is a combination of PV, photothermal, pyroelectric, and thermoelectric conversion. Under sunlight irradiation, the dye sensitized solar cell (DSSC) harvests solar energy through the PV effect while heat is generated by the PT effect of the PPPEDOT film from the transmitted light (mainly in the near IR region) through the DSSC, to increase the temperature of the PE device and to generate electricity through PE conversion. The unused heat is then transferred to the TE device to generate a temperature gradient and harvest additional energy. Because the PE and TE devices are dependent on the heat originally generated from the light source, the PT conversion of the electrodes is important. As shown in Figure 4b, more than 50% of 700–1,000 nm range of light was transmitted through a DSSC (filled area). The DSSC consisting of the organic N719 dye absorbed most of the UV light and part of the visible region. The transmitted light in the range of 400–1,600 nm including NIR region was then absorbed by the PT/PE film (PP-PEDOT electrodes). Consequently, a PT effect could be generated, and more energy could thus be harvested in the hybrid energy cell of Figure 4a than in a single PV cell. To examine the dependence of the light absorption in the DSSC on the output of the hybrid energy harvester, the PE and TE output were obtained under a solar simulator (AM 1.5G, 100 mW cm-2) with and without a DSSC based on the N719 dye. Without the DSSC, the output of the TE module sharply increased upon exposure to sunlight (Figure 4c, filled, black) but decreased as soon as the sunlight was off. The Vout of TE part was smaller and increased slowly upon exposure to light in the presence of the DSSC (filled, red), however, interestingly, the Vout


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was higher and slowly decreased after the sunlight was turned off. The PT effect was reduced due to the smaller light transmittance by the DSSC absorption in the presence of the DSSC (~ 60%, Figure S6 and Table S2). However, the PE output (Figure 4c, open) was fast and almost the same in both cells with (black) and without (red) the DSSC. In addition, the heat generation of the DSSC under sunlight was considered (Figure S7). The light absorbed by the DSSC from the overall light intensity was well matched to the slow saturation rate and lower maximum output in the hybrid energy cell. Finally, the photocurrent density-voltage (J-V) curve of the PV and PT/PE-TE hybrid device was obtained for a small area (PV = 0.16 × 4 cm2, PT/PE and TE = 2 × 2 cm2) and a large one (PV = 2 × 2 cm2, PT/PE and TE = 4 × 4 cm2) connected in series (Figure 4d). Above all, in the hybrid device, the VOC was increased to produce higher photo conversion efficiency (PCE). The overall yields increased from 9.72% (only DSSC) to 11.7% as a result of an important VOC increase thanks to the PT/PE-TE part of the device. In addition, under a higher light intensity (11.8 times of AM 1.5G), the hybrid energy harvester showed a much enhanced PCE of 41.3%. As the light intensity increased, more temperature gradients were generated. The harvested energy from the PT/PE-TE was then increased and contributed to increase total energy harvesting. The increase in the total conversion efficiency is summarized in Table 1 and Figure S8. As the hybrid energy harvester showed a high VOC, which is crucial to the operation of organic electronics working at a high voltage, we further explored the possibility of a solarpowered ECD. A simple circuit was prepared for a PV-PT/PE-TE hybrid energy harvester composed of capacitors and a diode bridge rectifier (Figure 5a). As shown in Figure 5b, the PE output was well generated by the rectifier showing a positive output signal while the light was on/off. Under


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temperature fluctuation, the PE output was steadily generated and charged the capacitor (Figure 5d). In the cases of PV and TE, two different energy harvesters were combined and generated electricity using photons and heat energy respectively. Interestingly, as PV works under sunlight, the output voltage of PV decreased steadily due to the fast recombination phenomenon during the PV operation (Figure 5c). However, when PV was combined with a TE device, the total output voltage increased steadily, indicating TE power generation. Consequently, the reduced PV output performance was complemented by the output of the TE in the hybrid device. This result was also confirmed during the charging of the capacitor. The plot (Figure 5e) of the accumulative charges and energies in the capacitors demonstrated the enhanced performance of the hybrid system. In other words, the PV and thermoelectric devices connected in series showed a more stable and enhanced performance from their complementing of one another. Taking advantage of the enhanced output energy from the hybrid energy harvester, a homemade circuitry was prepared to operate an LED lamp and electrochromic display (ECD) device (Figure S9) by light. Although small, the harvested energy from the PE film was enough to turn an LED lamp on (Figure 5f). However, it was not possible to turn on an ECD, which required a Vwk over 2 V in two electrodes system, along with a current density of tens of µA cm-2. 40, 41 As the Vwk should be > 2 V, the DSSC could not operate the ECD. Interestingly, after only several minutes of sunlight exposure, the ECD was operated and showed a color switching from blue to colorless and reversely (Figure 5g, video clip in the Supporting Information). The solar powering of ECD had never been achieved through hybrid energy cells with PT/PE before.

Conclusions


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In summary, highly conductive and photothermally-efficient PP-PEDOT films were successfully used as electrodes for a PE film device, acting as the heat source and driving force of a TE module. The TE generator converged to use the heat generated by the PT effect. Under NIR and sunlight irradiation, the photothermally-generated heat in the electrode of the pyroelectric device operated the thermoelectric device with more than 6 times higher output voltage and current. In addition, the simultaneously generated output of PE was also obtained through the PP-PEDOT film electrodes. This represents the first conception of a hybrid system for PE and TE energy harvesting using a PT effect with PP-PEDOT under NIR and sunlight exposure. Furthermore, the PV was combined with the PT/PE-TE system for a large energy output utilizing the full solar spectrum. The series connected PV-TE showed a stable and 20% enhanced power conversion efficiency by complementing each other, while the output of the PE film was generated additionally charging a capacitor through a rectifier. By utilizing either a sunlight focus lens or a highly efficient TE module, or both simultaneously, this new hybrid energy harvester based on a highly conductive polymer electrode and its PT effect can find applications in many fields.


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Experimental method

Materials and reagent: Iron(III) chloride hexahydrate (purity 97 %), p-toluenesulfonic acid monohydrate (purity > 98.5 %), poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (PEPG, weight average molecular weight 2800), 3,4ethylenedioxythiophene (EDOT) (purity 97 %), pyridine (purity 99.8 %), titanium bis(ethyl

acetoacetate),

propylimidazolium

iodide,

chloroplatinic iodine,

acid

hexahydrate,

tert-butylpyridine,

lithium

1,

2-dimethyl-3-

iodide,

1-butyl-3-

methylimidazolium bis(trifluoromethylsulfonyl)imide, anhydrous methnol, ethanol, isopropyl alcohol, n-butanol, and acetonitrile were purchased from Aldrich Chemicals. The anhydrous n-butanol was used after molecular sieve treatment to remove water. Other materials were used without further purification.

Synthesis of oxidant: Iron(III) chloride hexahydrate (FeCl3•6H2O) (30 mmol) was reacted with excess NaOH (100 mmol) in aqueous solution. The precipitate was then filtered through filter paper and washed twice with water. It was then transferred to a 250 mL flask containing 70 mL of methanol. p-Toluenesulfonic acid monohydrate (100 mmol) was added to this solution. After a 3 hour reaction at 45 ˚C, the solution was filtered and evaporated by a rotary evaporator. The product was finally obtained after further drying in a vacuum oven at 70 ˚C. The solid powder was stored in a desiccator.


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Preparation of PVDF-TrFE film coated with PP-PEDOT: Chromel etchant was used to remove the metal electrode (Ni/Cu). The film was immersed into the etchant solution and was then sonicated for 30s, before being washed with ethanol. Prior to the fabrication of the electrodes, the metal electrodes on poly[(vinylidenefluoride-co-trifluoroethylene] (PVDF-TrFE) (Measurement Specialties Inc.) were removed with a chemical etchant. The bare film was analyzed with Fourier transform infrared spectroscopy (FT-IR) to check the presence of any damage from the etching process (Figure S10). There was no damage to the β-phase of PVDF-TrFE. The oxygen plasma was treated before the PEDOT fabrication. PEDOT films were obtained directly by the SCP method using a solution containing iron(III) tris-p-toluenesulphonate (Fe-Tos), EDOT, pyridine, and triblock copolymers (Mw: 2,800). In this solution, the pyridine acts as a base and reduces the high reactivity of the oxidant. Poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) triblock copolymer (PEPG) inhibits crystallization of the oxidant molecules and slows the polymerization rate of PEDOT. More specifically, pyridine (13.54 mg) and PEPG triblock co-polymer (200 mg) were added into 1 g of the oxidative solution containing 40 wt% of Fe-Tos in n-butanol. The molar ratio of pyridine: iron(III) tosylate: EDOT was fixed as 0.55: 2.25: 1.

5, 37

The above EDOT solution containing

oxidant, pyridine, and PEPG was spincoated onto the PVDF-TrFE films. All the samples were polymerized at 60 °C and 33 % humidity for 1 hour. The samples were then washed with ethanol and dried under an N2 flow and annealed on a hot-plate at 60 °C for 10 min. For the multi-layer electrodes, this SCP process was repeated.


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Fabrication of DSSC: The Conventional compact TiO2 layer with a 200 nm thickness was prepared by spin coating a titanium bis(ethyl acetoacetate) solution (2 wt% in butanol) onto FTO (Pilkington, 8 Ω−1 ) glass at 2000 rpm for 30 s, followed by calcination at 450 °C for 15 min and 500 °C for 15 min. The commercial TiO2 paste (Dyesol 18NRT) was cast onto the interfacial layer (CC-TiO2 film) coated photoelectrode using a doctor-blade technique and dried at 70 °C for 1 h, followed by successive sintering at 500 °C for 15 min and cooling to 30 °C for 8 h. Nanocrystalline TiO2 films of 7 µm thickness were immersed in the N719 dye (Solaronix) solution (0.3 mM in ethanol) for 24 h at room temperature. The platinum counter electrodes were prepared by drop-casting the H2PtCl6 solution (2 mg of Pt in 1 mL isopropyl alcohol) onto the conductive FTO and then heating at 400 °C, maintaining the temperature for 20 min, and cooling to 30 °C for 8 hr. The dye adsorbed TiO2 electrodes and the Pt counter were fabricated to sandwich-type cell using a hot-melt film (Surlyn, 25 µm) as spacer and heating at 90 °C. The acetonitrile solution of electrolyte consisting of 0.6 M 1, 2-dimethyl-3-propylimidazolium iodide, 0.1 M iodine, 0.5 M tert-butylpyridine, and 0.1 M lithium iodide was injected into the cell through a hole in the counter electrode.

Preparation of electrochromic device (ECD) and operation circuitry: EC polymer films were directly obtained through the SCP method following a previous study.

35, 42

The

ECD was assembled by placing the electrolyte medium between the two electrodes. The space between the two electrodes was controlled by placing Kapton tapes (DuPont) and Surlyn (Thermoplastic resins, DuPont) as a spacer and adhesive, respectively. An electrolyte solution was injected into the sandwiched electrodes with double holes as


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inlets, and the holes were sealed with a Silicone sealant (Shin Etsu). A circuitry consisting of two switches and capacitors was prepared to operate the ECD switching.

Output measurement and connection of hybrid device: A TE module was placed on a Al heat sink treated with a thermal compound (Nano Aluminum, prolimatech) consisting of Al (70~88 wt%), ZnO (18~34%), oil (8~12 wt%), and an antioxidant (0.5~2 wt%) between them. The PE film device was then put onto the TE module. The wired line using a silver paste (H20E, EPO-TEK) on the PE device was connected to a capacitor. Finally, a DSSC was placed on the PE module. The wires were attached to the DSSC by Sn soldering (CERASOLZER, Kuroda Techno Co., LTD.). These wires were connected to the TE module to achieve the hybrid system. Photoelectrochemical performance characteristics were measured using an electrochemical workstation (Keithley Model 2400) and a solar simulator (1000 W xenon lamp, Oriel, 91193). The light was homogeneous across an 8 × 8 cm2 area and was calibrated with a Si solar cell (Fraunhofer Institute for Solar Energy System, Mono-Si+KG filter, Certificate No. C-ISE269) to a sunlight intensity of AM 1.5 G (100 mW cm-2). This calibration was confirmed with a NREL-calibrated Si solar cell (PV Measurements Inc.). The incident photon-to-current efficiency (IPCE) measurement was performed using a 300 W Xe light source and a monochromator McScience (Polaronix K3100 IPCE Measurement System, McScience).

Characterization:

UV-Vis-NIR

absorption

spectroscopy

was

performed

using

PerkinElmer Lambda 750. Fourier Transform Infrared (FT-IR) spectra were obtained with a TENSOR 37 (Bruker). The temperature was measured through the K-type


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thermocouples (Omega) connected Agilent 34970A. A Keithley 6485 picoammeter and a Keithley 2842A nanovoltmeter were used for low-noise and precise current/voltage measurements to detect currents and voltages generated by the hybrid energy harvester. The NIR coherent diode laser (808 nm, B&W Tek, Inc.) was used as an NIR source. Heat generation images (Figure S2) were taken with an IR camera (ThermaCAM P25, FLIR, Sweden). The thickness of the films was determined with an Alpha step profilometer (Tencor Instruments, Alpha-step IQ). Field-emission scanning electron microscopy (FESEM) was performed using JEOL-JSM-6700F with a thin platinum coating to image the surface morphology of PP-PEDOT electrodes. The stability of PE film was investigated via High-resolution X-ray diffraction (HR-XRD, RigaKu, SmartLab).


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FIGURES

Figure 1. (a) Schematic, photographic, and thermal NIR images of PVDF-TrFE (P0), PEDOT/PVDF-TrFE (P1), and PEDOT/PVDF-TrFE/PEDOT (P2) under NIR irradiation. (beam diameter (red circle) = 1 cm, intensity = 132 mW cm-2). (b) UV-Vis-NIR spectra of the P0, P1, and all organic PE devices with double sided PEDOT electrodes. (c) Transmittance at 808 nm of all samples and schematic illustration of samples with different thickness of PP-PEDOT electrode (T: top, B: bottom), (the inset).


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Figure 2. (a) Illustration of photothermal pyro-thermoelectric device. The NIR source (56 mW, 164 mW cm-2) was irradiated from the top of the PE device placed on a TE module. The K-type thermocouple (TC) was placed between the PE film and TE module to measure the temperature of the interface. (P: degree of dipole, red dot: fluorine atom) (b) Temperature of the interface for different electrode samples under NIR exposure. (c) Temperature measured for all samples, including samples covered with thermal paste (w/TP). (d) Output voltage of photothermally activated PE film device with PP-PEDOT under NIR on/off. Depending on the NIR laser


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exposure, it showed positive and negative voltage generation respectively. (e) Output power density and integrated power (blue line) of PE energy harvester.

Figure 3. Photographic images of (a) TE and (b) PT/PE and TE hybrid systems. (c) Output voltage (filled) and output current (open) of TE and photothermally-activated TE by PP-PEDOT electrodes under NIR irradiation (beam diameter = 6.5 mm, intensity = 305 mW cm-2). (d) TE output voltage (dashed line) and current (line) generation dependence on NIR intensity (from 12 to 104 mW). (e) Calculated energy conversion efficiency and power density of TE versus NIR intensity. (f) Output voltage of hybrid energy harvester. Under NIR irradiation, the PE voltage sharply appeared and decreased as the output generated from the TE increased.


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Figure 4. (a) Illustration of photovoltaic and photothermal pyro-thermoelectric device. (b) UVVis-NIR transmittance spectra of DSSC and sunlight intensity (AM 1.5G, 100mW cm-2) (c) Output voltage of TE (filled) and PE (open) under sunlight on/off conditions without (black square) and with PV (red circle). (d) Comparison of photocurrent density-voltage (J-V) characteristic curves of PV (black square) and PV-PT/PE-TE hybrid (red circle) devices with small (filled) (PV = 0.16 cm2, PT/PE and TE = 1 × 1 cm2) and large size (open) (PV = 2 × 2 cm2,


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PT/PE and TE = 4 × 4 cm2), (inset: photographic images of hybrid devices under light on condition).

Figure 5. (a) Circuit diagram of photothermal hybrid energy harvester (PV = 2 × 2 cm2, PT/PE film and TE module = 4 × 4 cm2) comprised of four diodes and two capacitors for energy storage. (b) Rectified output voltage of PE film. (c) Output voltage of PV and PVTE series connected device. (d) Output of a capacitor (47 µF, 25 V) charged by PE film in hybrid system under sunlight on/off conditions, with enlarged image showing every charging step of PE (inset). (e) Output and accumulated energy of a capacitor (10 F, 5.4 V) charged by PV and PV-TE series connected device in hybrid system (filled circle and square: charge, open circle and square: energy). (f) LED lit up from the capacitor charged by PE. (g) Switching of ECD by the hybrid energy harvesting system.


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Table 1. Photovoltaic performance of PV and PV-PT/PE-TE hybrid devices under sunlight irradiation. (AM 1.5 G, 100 mW cm-2) Sample Area 2

η

VOC

JSC

Pmax -2

FF -2

[cm ]

[%]

[V]

[mA cm ]

[mW cm ]

PV: 0.16

9.72

0.69

20.7

9.72

0.68

PV-PT/PE-TE

PT/PE & TE: 1

11.7 (20%)

0.80

21.2

11.7

0.69

PV

PV: 1 x 4 (ea)

3.74

2.55

2.41

15.0

0.61

PT/PE & TE: 16

4.29 (15%)

2.75

2.49

17.3

0.63

PV

PV-PT/PE-TE

a)

a)

a)

Enhanced (%): relative increase of energy conversion efficiency of hybrid devices compared to the only PV system.


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ASSOCIATED CONTENT Supporting Information. Table summarizing energy calculation under sunlight irradiation, FTIR spectra, thermal NIR images, absorbed energy calculation of films from UV-VIS-NIR spectra, interfacial temperature measurement results. The additional movie file including lighting up of an LED and ECD switching. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Active Polymer Center for Pattern Integration, Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea. Fax: +82-2-312-6401; Tel: +82-2-2123-5752; E-mail: [email protected]

Author Contributions E. Kim formulated the project and wrote the manuscript. T. Park prepared the polymer films and collected major data. J. Na prepared DSSC cells. B. Kim helped the measurement on the photothermal properties. Y. Kim helped integration of the harvesters and circuitry. H. Shin prepared the electrochromic cells. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT We acknowledge the financial support of the National Research Foundation (NRF) grant funded by the Korean government (Ministry of Science, ICT & Future Planning, MSIP) through the Pioneer Research Center Program (2010-0019313) and the Active Polymer Center for Pattern Integration (APCPI) (2007–0056091)

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Table of Contents Graphic and Synopsis

Highly conductive and photothermally-efficient PEDOT films are fabricated and controlled to be used as electrodes for a pyroelectric film, acting as the heat source and driving force of a thermoelectric module. A photovoltaic cell is combined with this hybrid system for a large energy output utilizing the full solar spectrum.


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Photothermally Activated Pyroelectric Polymer Films for Harvesting of

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