Article pubs.acs.org/JPCC
Integrated Energy-Harvesting System by Combining the Advantages of Polymer Solar Cells and Thermoelectric Devices Yajie Zhang,† Jin Fang,† Chang He,‡ Han Yan,† Zhixiang Wei,*,† and Yongfang Li‡ †
National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, P. R. China National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, Zhongguancun, Beijing 100190, P. R. China
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S Supporting Information *
ABSTRACT: A polymer solar cell-thermoelectric (PSC−TE) hybrid energy-harvesting system was designed and fabricated, which realizes harvesting electricity from solar light and solar heat simultaneously. A series of measurements has been performed to study the relationship between the PSC−TE hybrid system and individual devices. The PSC−TE system improved the total power output compared with individual PSCs when a temperature gradient across TE module was introduced. The physical process that determines the overall power generation of the PSC−TE hybrid system was also studied and analyzed. The optimal power output of PSC−TE hybrid system is given, which can act as a guideline for further optimizing the hybrid energy-harvesting system. Interestedly, we demonstrate that the hybrid system can drive a commercial light-emitting diode by effectively utilizing solar energy, while it cannot be realized by an individual device. The hybrid system is proved to be a more efficient way for obtaining electricity by integrating multiple devices with different functions.
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INTRODUCTION Harvesting solar energy is one of the most attractive ways to solve the energy requirements. As one of the most promising candidates, photovoltaic (PV) technologies have been extensively studied in recent years. One key point to improving the conversion efficiency of a PV device is to utilize the solar energy over a broad wavelength range via material engineering and device design. For instance, tandem solar cells combining two subcells have reached higher power-conversion efficiency than that of a single cell by using a broader range of solar energy.1−4 However, in most of solar cells, high-wavelength infrared photons cannot be efficiently used. Thermoelectric (TE) device can harvest electricity from solar heat,5,6 waste heat, or heat source with temperature gradients relative to environmental temperature.7 Therefore, an integrated energy harvesting system that combines the advantages of PV cell and TE device is considered to be a promising way to improve the energy usage. Moreover, it is generally believed that hybrid photovoltaic−thermoelectric (PV−TE) systems also have better stability of output power compared with individual PV devices,8 which allows the system to compensate for the solar deficit in winter. The PV−TE hybrid devices have been reported by several groups by using different types of solar cells. For example, Park et al. studied a PV−TE hybrid device based on Si PV. The power output of PV device was improved by 30% comparing with individual solar cell.9 A hybrid system consisting of a dyesensitized solar cell (DSSC) and TE modules was also reported by Wang’s group.10 The power conversion efficiency (PCE) of the hybrid device was improved significantly. van Sark has simulated the performance of Si-based PV−TE hybrid devices © 2013 American Chemical Society
and demonstrated the energy-harvesting capacity of the hybrid system with different TE materials.11 Theoretical and experimental research about the PV−TE hybrid devices constructed with dye sensitized solar cells−TE10,12 and Si solar cell−TE13,11 has also been studied in recent years. Compared with PV technologies based on inorganic materials and organic dyes, polymer solar cells (PSCs) show superior features of low cost, flexibility, light weight, and largearea fabrication capacity.14 Rapid progress has been achieved by using new materials15−17 and device structures18−22 to improve the PCE of these devices. The highest PCE of PSC devices has been increased to 6−10%.7,8,14−27 At present, most PSC devices are based on bulk heterojuctions of conjugated− polymer donor and a fullerene−derivative acceptor (e.g., phenyl-C61-butyric acid methyl ester, PC60BM). Although various high−performance polymer donors have been developed, poly(3-hexylthiophene) (P3HT) is still the most representative donor because it is easy to access with high mobility and performance reproducibility in different laboratories. Because a new indene-C60 bisadduct (IC60BA) donor was synthesized, the open-circuit voltage (Voc) of the devices based on P3HT was improved to as high as ca. 0.85 V, and thus the PCE was improved to over 6%.23,28 Although PSCs have been rapidly developed in recent years, a hybrid system based on PSC and TE (PSC−TE hybrid system) has not been reported. Received: May 6, 2013 Revised: October 25, 2013 Published: October 30, 2013 24685
dx.doi.org/10.1021/jp4044573 | J. Phys. Chem. C 2013, 117, 24685−24691
The Journal of Physical Chemistry C
Article
Figure 1. (a) Schematic illustration of the PSC−TE hybrid device using PSC as the top cell and TE as bottom cell. (b) General mechanism for photoenergy conversion in excitonic polymer solar cells. (c) Cross-section of TE made of many pairs of p- and n-type thermoelectric elements, with the holes dissipating from hot side to cold side in P-leg and electrons in N-leg. (d) Ultraviolet−visible absorption spectra of the PSC based on P3HT/IC60BA and the possible utilized solar radiation spectrum.
Measurements. The current density/voltage curves were recorded using a Keithley 2400 source-measure unit under ambient conditions. Photocurrent was measured under AM 1.5G illumination at 100 mW/cm2 using a Newport Thermal Oriel 91159A solar simulator. Light intensity was calibrated with a Newport oriel PN 91150 V Si-based solar cell. IPCEs were measured using a Newport Merlin digital lock-in amplifier 70140. The devices were illuminated using a chopped light from a xenon lamp (Newport 66902) and were sent to a CS260 Corner stone monochromator to generate scanning monochromatic light. The beam path was focused on a test cell/ calibrated Si detector (Newport oriel PN 91150 V) with a welldefined area, and a ratiomentric measurement was made. Total optical power incident on the detector was compared with the current generated by the cell under test. The temperature was measured by an infrared thermometer DT8530 (Tianjin Cheerman Technology).
A PSC−TE hybrid energy system by the integration of PSC devices based on P3HT/IC60B and a TE module consisting of Bi2Te3 based compounds was studied. As PSC utilizes visible light and TE module converts heat to electricity, the total electrical energy produced by PSC−TE hybrid device could be improved efficiently. It is proved that such PSC−TE hybrid system was promising to harvest electricity from solar light and solar heat simultaneously.
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EXPERIMENTAL SECTION Materials. P3HT was purchased from Aldrich. Indene-C60 bisadduct (IC60BA) was synthesized according to the published procedure.29 Fabrication and Characterization of the PSC−TE Hybrid Device. The PSC devices were fabricated by the reported procedure.23 Patterned indium tin oxide (ITO) glass with a sheet resistance of 10 Ω sq−1 was purchased from CSG HOLDING (China). The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol and then treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company) for 15 min. The blend of P3HT:IC 60 BA (1:1 w/w, 34 mg/mL for P3HT:IC 60BA) was dissolved in ortho-dichlorobenzene (DCB) and spincoated on PEDOT:PSS-modified ITO glass at 800 rpm for 30 s. The prepared samples were annealed at 150 °C for 10 min before vacuum deposition of metal negative electrode. The active area of PSC was 0.12 cm2. The finished PSC was encapsulated in a nitrogen-filled glovebox using UV epoxy and covered glass. Then, the TE module (TGP-715 from Micropelt, Germany) was attached onto the backside of packaged PSC with thermal silicone paste. The active area of TGP-715 was ∼0.12 cm2 with 540 pairs of P−N junctions. The Net SeebeckVoltage at 23 °C was 140 mV/k.
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RESULTS AND DISCUSSION
A schematic illustration of the PSC−TE devices is illustrated in Figure 1a. A TE module was attached to the backside of PSC to get a hybrid system in which the PSC acted as top cell and the TE module acted as “bottom cell”. The electrical connection of the PSC and TE module is in series, with the cathode of PSC and the anode of TE module performing as the cathode and anode of the hybrid device, respectively, while the anode of PSC and cathode of TE were directly connected. For the PSC devices, P3HT and IC60BA were blended and used as an active layer due to the high stability and reproducibility of the PSC based on P3HT.30−32 As shown in Figure 1b, in bulk heterojunction solar cell the absorption of incident light is followed by ultrafast photoinduced exciton formation, con24686
dx.doi.org/10.1021/jp4044573 | J. Phys. Chem. C 2013, 117, 24685−24691
The Journal of Physical Chemistry C
Article
Figure 2. (a) IPCE and absorption spectra of the PSC. (b) Output voltage at various Thot and ΔT. (c) Voltage versus current of TE module in the hybrid cell. (d) Photocurrent density−voltage (J−V) characteristic of PSC and PSC−TE hybrid devices.
for the degradation under outdoor conditions.34 Therefore, to put a heat-extracting TE device under the heat-originating PSC device can increase the electricity production, while temperature of PSC needs to be maintained at a suitable level for prolonging its lifetime. To ensure that TE modules generate efficient power, an effective temperature difference between the PSC and heat extractor is very important due to the low efficiency of the most TE modules at present. Without any auxiliary measures, only 1−4 °C temperature difference could be obtained in our TE modules corresponding to AM1.5 condition because of the heat loss through air convection and the poor heat sink. Only 1 are expected to be competitive against other methods of electric power generation. Bismuth telluride alloys have so far been the best TE materials with the concerned efficiency.7 The working mechanism is represented by the temperature difference over the TE which induces the charge carriers to diffuse (see Figure 1 c) from one end to another. During the photoelectric conversion process in PSC, the output power is lower than that of the incident light. Referring to the solar spectra analysis, only a minor part of high-energy photons can be absorbed by the active layer of PSC, which is determined by the band gap of the adopted materials. Figure 1d indicates the utilization of solar energy by the active layer of PSC based on P3HT from the possibly utilized solar spectrum. Calculating from the spectrum, we concluded that
