Temperature Dependence of a Perovskite-Sensitized Solar Cell: A

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Temperature Dependence of PerovskiteSensitized Solar Cell: A Sensitized “Thermal” Cell Sachiko Matsushita, Seiya Sugawara, Toshihiro Isobe, and Akira Nakajima ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01522 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Energy Materials

Temperature Dependence of Perovskite-Sensitized Solar Cell: A Sensitized “Thermal” Cell Sachiko Matsushita*, Seiya Sugawara, Toshihiro Isobe, and Akira Nakajima Department of Materials Science and Engineering, Tokyo Institute of Technology (Tokyo Tech.), 2-12-1 S7-8, Ookayama, Meguro-ku, Tokyo 152-8552, Japan KEYWORDS: sensitized solar cell, thermal excitation, semiconductor, redox reaction, CH3NH3PbI3, energy problem ABSTRACT: Efficient and low-cost thermal energy-harvesting systems are needed to solve the global energy problem. Here, we suggest a new energy conversion system for generating electric power from heat without a cooling element. This device is a sensitized thermal cell (STC) based on a dye-sensitized solar cell. A semiconductor is used instead of the dye, and as a result, this system can work by using heat instead of by light. To prove the efficacy of this system, we focused on an organic perovskite. This material has been used in a sensitized solar cell, and recent calculations show that it can generate several thermally excited charges at temperatures over 60 °C. Thus, if this perovskite cell can generate electric power with both light irradiation and thermal excitation, the STC concept would be perfectly proven, and the device succeeds thanks to the careful analysis of the perovskite’s unstableness. This is the dawn of the three-dimensional utilization of heat.

Introduction Efficient and low-cost thermal energy-harvesting systems are needed to solve the global energy problem. Steam turbines, the Seebeck effect1 and alkali metal thermal power generation systems have been developed. However, the use of steam turbines conflicts with water resources, and the Seebeck effect and alkali metal thermal power generation systems require a temperature difference, i.e., one side of the generator must be maintained as the cooling section.2 Here, we suggest a new energy conversion system for turning heat into electric power that does not require a cooling part. This is a sensitized thermal cell 3 (STC) based on a dye-sensitized solar cell (DSSC).4-9 In DSSCs, the light absorption function is separated from the charge carrier transport. The current is generated when a photon is absorbed by a dye molecule, which results in the injection of an electron into the conduction band of an n-type semiconductor on which the dye is adsorbed.4 To complete the circuit, the dye is regenerated by the transfer of an electron from a redox species in the electrolyte, and the oxidized species is then reduced at the counter electrode58. As a result, the maximum electron voltage is the difference between the Fermi level of the n-type semiconductor and the redox potential of the redox species. An STC uses the thermally excited charge carriers in a semiconductor, instead of the photoexcited charge carriers generated in a dye molecule. In short, the STC is a novel system for converting heat into electrical power without utilizing a temperature difference (Figure 1). Sometimes, the power of heat is underestimated and is said to be “poor quality energy” or “low-density energy”. For example, a 50 K temperature difference corresponds to only ~0.004 eV of energy. However, in a semiconductor, the heat is changed to a powerful energy source and generates excited electrons and holes. This is because of the large electron density in the va-

Figure 1. Schematic representation of the sensitized solar and thermal cells, indicating the electron energy levels in the different phases. The observed cell voltage under light irradiation or at high temperature corresponds to the potential difference, ∆V, between the quasi-Fermi level of the organic perovskite and the electrochemical potential of the electrolyte. The latter is equal to the Nernst potential of the redox couple, which is used to mediate the charge transfer between the electrodes.

lence band of the semiconductor. After the thermal excitation, the excited electron has the energy of the conduction band, regardless of the temperature. The advantage of STCs is their large output power per volume compared with those of other existing technologies for the conversion of heat because STCs have a small, thin, layer-by-layer design and can be three-dimensionally applied to heat sources. The previous paper3 that first proposed this STC concept showed that a thermally excited charge carrier has the same redox ability as a photoexcited charge carrier; however, the STC concept was not perfectly achieved because a solid electrolyte, which has a low ionic transfer number, was used in the detection of the redox ability by elemental analysis. To clearly demonstrate the STC principle, a liquid electrolyte with the

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same material composition as that of the DSSC should be used. In other words, the operating temperature should drop significantly from the previous value of 600 °C to a value of 100 °C or less. This reduction in operating temperature widely extends the possibilities of STC. Today, two thirds of the energy consumed in industrialized countries is released to surroundings as heat, and more than 80% of the exhaust heat is below 200 ℃. Consequently, there is a big demand for a heat-utilizing power generation device operated at low temperature. If this STC can operate below 200 ℃, such desired devices will be realized by taking advantage of the knowledge of conventional sensitized solar cells. With this purpose, we focused on an organic perovskite, the methylammonium lead halide perovskite CH3NH3PbI3 system.10-14. CH3NH3PbI3, which is a costeffective and high conversion efficiency photovoltaic material10 used as a photon absorber in DSSCs,11-12 has been computationally demonstrated to be a promising thermoelectric material at room temperature.15 According to He's calculation results, the phase transition from the tetragonal to the cubic crystal phase occurs at 60 °C or higher, and a several thermally excited electric charge carriers are generated.15-16 Therefore, a sensitized solar cell fabricated using CH 3NH3PbI3 with confirmed battery characteristics upon heating to 60 °C or more in the dark would perfectly prove the STC concept.

RESULTS AND DISCUSSION For long-time stable measurement using CH3NH3PbI3. Organic perovskite-sensitized solar cells

Figure 2. (a) UV-Vis absorbance and (b) Tauc plots displaying the extrapolated optical bandgaps of the CH3NH3PbI3 powder and compact.

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generally contain three elements: an organic perovskite supported on titanium oxide, an electrolytic solution, and a counter electrode. Initially, we used a spin-coating method, which is often used to create organic perovskite films (Supporting Information). An excitation current was confirmed for the fabricated solar cell in the presence of both light and heat (Figure S1). However, the deterioration was remarkable within the 5 minute period because of the liquid electrolyte. Therefore, comparing the photoexcitation and thermal excitations of "the same single cell" was impossible, and a detailed examination could not be performed. A different loading method was then devised. The severe deterioration occurs because the organic perovskite thin film, with a thickness of several nm that is produced by the spin-coating method, is dissolved by the electrolyte solution, and the film is removed from the titanium oxide.13 Therefore, if a compact body of titanium oxide and an organic perovskite is prepared, the battery characteristics of the same cell can be measured over a long time without delamination. First, an organic perovskite powder was prepared. The produced powder was black with a particle size of several micrometres, and the phase transition from tetragonal to cubic was confirmed at 60 °C or higher. The details were reported in our previous paper.14 The powder absorbed light with a wavelength of approximately 800 nm or less, and the bandgap was 1.55 eV, which was consistent with the previous report.10 Uniaxial pressure moulding was performed on this powder to obtain an organic perovskite compact with

Figure 3. (a) Schematic of the resistivity measurement set up and (b) an Arrhenius plot of the electrical resistivity of a CH3NH3PbI3 compact coated in Au. The average of the 4 measurements is plotted and the maximum and minimum values were represented by error bars.


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Figure 5. Arrhenius plot of a CH3NH3PbI3/TiO2 duallayer cell at each temperature.

temporal change immediately after electrolyte injection was linearly approximated, and the value at the injection time was used.

Figure 4. (a) Device performance of a CH3NH3PbI3/TiO2 dual-layer cell at room temperature with and without light at 89 oC. The scan rates were 30 mV/s. The digital camera image of the assembled cell is shown in the inset. (b) The temperature dependence of the open circuit voltage (circles) and the power (squares).

a relative density of 91%, an outer diameter of 20 mm, and a height of 3.53 mm. Although after the moulding the absorption wavelength onset slightly shifted to a shorter wavelength, the wavelength was nearly the same as that of the powder (Figure 2). According to the measurements from the 3-terminal method, this compact body exhibited semiconductive properties, i.e., the electrical resistivity decreased with increasing temperature (Figure 3). After the phase transition above 60 °C, the decrease in the electrical conductivity was more prominent. That is, as a result of the phase transition, several thermally excited charge carriers were generated.16 Cell characteristics. The above results show that an organic perovskite compact can be produced by pressure moulding, and the compact generates charges by both photons and thermal excitation. A titanium oxide/organic perovskite compact was subsequently produced by pressure moulding (Figure S2). The composite compact, an iodine-based electrolytic solution, and a platinum-coated counter electrode were used to construct a sensitized solar cell. To suppress the influence of electrolyte volatilization on long-term measurements of this system, a small hole was made in the counter electrode so that electrolyte could be re-injected, even after the deterioration (Figure S3). Once a cell was assembled, it could be measured more than once with a single battery. The error to the short-circuit current caused by the re-injection was small. To increase the reproducibility, the

In this titanium oxide/organic perovskite twolayer compact cell, the open-circuit voltage and shortcircuit current were improved by light irradiation and showed solar cell characteristics (Figure 4a). The value of the open-circuit voltage was approximately 0.15 V. When half of the illuminated area was covered with black felt paper, a decrease in the current was confirmed (Figure S4). This result shows that the current was caused by photoirradiation. There was no unusual behaviour observed during the characterization. In this compact cell, the short-circuit current and opencircuit voltage were both reduced compared with those of the cell created using the spin-coating method. This was probably due to an increase in the internal resistance in the compact. In order to estimate the internal resistance, I-V curves of the cell at room temperature without light were examined (Figure S5). These clearly show pn junction behaviour caused at the semiconductor/electrolyte interface. The internal resistance of the cell can be calculated from the reciprocal of the slope in the high voltage area. The calculated internal resistance was about 3 kΩ for the spin-coating method and about 9 kΩ for the compact. It can be said that the internal resistance of the “whole cell” is greater for the compact as expected. The same cell with a confirmed solar cell performance was warmed, and the battery characteristics were measured (Figure 4a). The power was the maximum value calculated by multiplying the current by the voltage. The battery output was confirmed at temperatures over 60 °C, which is the phase transition temperature of the organic perovskite (Figure 4b). This result suggests that this power generation was due to thermally excited charges in the organic perovskite. In other words, even if our organic perovskite is deteriorated, it can be said that the electricity generated by heat exceeded the electricity generated by deterioration. These I-V values were obtained for other 5 samples (Figure S6). In any case, the battery output was confirmed at temperatures over 60 ºC. In some samples, the open circuit voltage was different at room temperature and high temperature. This is thought to



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be due to the difference in contact between the titanium oxide layer and the perovskite layer, but it is necessary to study the details in near future. The temperature dependence of the short-circuit current was also measured (Figure 5). The current increased with the temperature, and the increase was more rapid at higher temperatures. This behaviour is consistent with the trend observed in the electrical properties of the moulded body: The battery performance improved after the phase transition. We conducted similar experiments and confirmed power generation by thermal excitation in many other batteries. Especially, to obtain quantitatively investigation about temperature dependence (Figure 4), we examined which hole size to be opened in the counter electrode, how to inject the electrolyte, spacers, etc. In any case, the battery output was confirmed at temperatures over 60 ºC. However, depending on the cell, the open circuit voltage and the current value were different in the range of several tens of mV and of several A, respectively. This is because the internal resistance values cannot be kept constant by the current fabrication method. The cell shown in the paper was the one with the largest open circuit voltage among these experiments. Since the concept of a sensitized thermal cell was confirmed, here, we would like to consider the difference in open circuit voltage between photoexcitation and thermal excitation, as shown in Figure 6. Although the open circuit voltage was approximately 0.15 V under light irradiation, the open circuit voltage with heat was saturated at approximately 0.06 V. We speculate that this difference was caused by the Fermi level. In the photoexcitation of semiconductors, a quasi-Fermi level is introduced to represent the electrons and holes generated more than the population at thermal equilibrium. Therefore, it is considered that the difference between this quasi-Fermi level and the oxidation-reduction level of ions is obtained as the open circuit voltage (Figure 6a). On the other hand, in the case of thermal excitation, it is considered that the difference between the Fermi level, which is a thermal equilibrium state, and the oxidation-reduction potential of ions is obtained as the open circuit voltage (Figure 6b). That is, in principle, the open circuit voltage obtained by the sensitized thermal cell appears to be smaller than that obtained by the sensitized solar cell. How to consider the efficiency? The above results confirm perovskite-sensitized power generation by both photons and thermal excitation, i.e., the STC concept was confirmed. The “energy conversion efficiency”, 𝜼𝒆 𝜼𝒙 , can be determined using: ・ the theoretical quantum efficiency, 𝜼𝒙 : (charge carriers in the electrical current) / (total number of thermally excited charge carriers), such as in equation (1) 𝜼𝒙 = exp(−

𝐸𝑎 2𝑘𝐵 𝑇

) / exp(−

𝐸𝑔 2𝑘𝐵 𝑇

)

(1)

Figure 6. Schematic image of the output potential by (a) photoexcitation and (b) thermal excitation. ETM means an “electron transport material”.

・the theoretical efficiency of the voltage, 𝜼𝒆 : the difference between the Fermi level of the semiconductor and the redox potential of the redox species, such as in equation (2) 𝜼𝒆 = (1 −

𝐸𝐿 𝐸𝑔

)

(2)

where Ea is the activation energy of the total system (experimentally measurable), Eg is the bandgap of the semiconductor, and EL is the energy loss obtained from the band diagram (Supporting Information17). The Ea of this system was 78 kJ/mol based on the Arrhenius plot. As a result, 𝜼𝒙 = 𝟎. 𝟐𝟐𝟔, 𝜼𝒆 = 𝟎. 𝟒𝟏, and the maximum efficiency was 9.2%. The output of this system at approximately 90 ºC was 3.10 × 10-8 W/s. If the flow rate of thermally excited electrons in the perovskite is approximately equal to the chemical reaction rate constant of a known DSSC, i.e., 107-8/s, the efficiency would be approximately 0.38 to 3.8%. The output can be enhanced by improving the cell assembly.

CONCLUSIONS In this study, a new energy conversion system for turning heat into electrical power without a cooling part, called an STC, was attested. A compact of the organic perovskite was prepared to confirm this concept, and a stable cell was constructed for extended-time measurements. The battery characteristics of the cell were convinced both light irradiation and thermal excitation above the phase transition temperature of the organic perovskite. These results clearly prove the concept of an STC. The operating temperature in this manuscript is 60 ℃ to 90 ℃, which is the temperature range of exhaust heat in developed countries. From the results of this paper, it is also a matter of turning not only the ground but also the heat of surrounding into electric energy by STC. Based on the results in this paper, it was found that not only geothermal heat but surrounding heat can be converted to electric energy by the STC. When actually using this STC, the definition of energy efficiency becomes important. The energy conversion efficiency of the STC, 𝜼𝒆 𝜼𝒙 , was defined using


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ACS Applied Energy Materials the theoretical quantum efficiency, 𝜼𝒙 , and the theoretical acquired voltage, 𝜼𝒆 . According to this definition, the maximum efficiency of the cell reported in this paper was 0.31%. This concept can be further developed for materials that can generate thermally excited electrons. Here we should point out that the open circuit voltage of STC is in principle smaller than that of SSC due to the difference between the quasi Fermi energy and the Fermi energy. In the future, while aware of this difference, we aim to convert thermal energy into electrical energy and solve the energy problem in the world.

EXPERIMENTAL SECTION Fabrication and characterization CH3NH3PbI3 compact.

of

the

(1) Fabrication CH3NH3I (Wako Pure Chemical Industries, Ltd., Japan) and PbI2 (Kanto Chemical Co., Inc., Japan) powders were weighed for the target composition of 1:1 (mol/mol) and mixed with a small amount of N,N-dimethylformamide (DMF, Wako Pure Chemical Industries, Ltd., Japan) using an agate mortar. The obtained mixed dark-green powder was heated for 12 h at 130 °C (heating rate: 5 °C/min), which resulted in the formation of a black powder. The powder (4.1879 g) was consolidated with a residual pressure of approximately 10 MPa for 1 min using a high-pressure cylinder (J1, AS-ONE, Japan) equipped with a uniaxial press. A black compact (20 mm φ, 3.53 mm thickness) of CH3NH3PbI3 was successfully fabricated. (2) X-ray diffraction measurements An X-ray diffractometer (XRD, D8 Discover HR/Bruker AXS/50 kV–22 mA, USA) equipped with a graphite monochromator using the Cu Kα line (λ = 1.54 Å), a hot stage and PDXL (HighScore, Rigaku, Japan) were used to measure the temperature dependence of the lattice constants of the black powder. The temperature was maintained at 30, 50, 70, 100, and 127 °C, and a stable spectrum at each temperature was analysed to observe the phase transition. (3) Electrical conductivity measurements of the CH3NH3PbI3 compact The compact (20 mm φ, 3.53 mm thickness) sputtered with Au (One side was a dual ring with a thickness of approximately 22 nm.) was placed on a hotplate (SR350, Advantec, Japan), and the temperature dependence of the electrical resistivity was measured via the three-terminal method (concentric ring probe) using a source metre (2400, Keithley Instruments Inc., USA). The applied voltage was 10 V. The temperaturedependent electrical resistance, ρ(T), was measured at RT during the first hour, 4 points were plotted every 30 seconds for the last 2 min, and other points were plotted at approximately 40, 60, 80, 105, 135 and 155 °C for 2 min. The sample temperature was measured by

an infrared thermometer. The average of the 4 measurements was plotted and the maximum and minimum values were represented by error bars in Figure 3. Fabrication of the TiO2/CH3NH3PbI3 compact. The 0.090 g of CH3NH3Pb13 powder and 0.045 g of titanium oxide powder (ST - 01) were packed in layers in a mould and uniaxially pressed under 20 MPa for 3 min to obtain a 10 mm diameter, 0.6 mm thick, two-layer compact of CH3NH3Pb13/ TiO2 with a density of 70%. Cell assembly. Platinum, with a thickness of approximately 100 nm, was sputtered onto an ITO substrate (A110U80 / 1.1 × 20 × 25 mm) by an AUTO FINE COATER (JFC-1300, JEOL Ltd.) and was used as the counter electrode. An electrolyte containing 0.2 M lithium iodide (Wako Pure Chemical Industries, Ltd.) and 0.1 M iodine (Wako Pure Chemical Industries, Ltd.) in methoxyacetonitrile (Tokyo Chemical Industry Co., Ltd.) was used. A hole 6 mm in diameter was drilled in Kapton insulating tape (650 S #50 Teraoka Seisakusho, Jpn.) and attached to the counter electrode. The compact was placed so that it overlapped with the hole and so that its titanium oxide surface faced upward. The compact was sandwiched by the ITO to form a cell. The substrates were sequentially rinsed by ultrasonic treatment with acetone, distilled water and ethanol for 1 min. A hole for injecting the electrolytic solution was created in the counter ITO electrode, and 3-4 l of the electrolyte was injected into the hole, which was subsequently sealed by the Kapton tape. Measurements of the cell characteristics. (1) Solar cell The cell was connected to a potentiostat (HSV-100, Hokuto Denko Corporation) for the I–V curve measurements. These measurements were obtained using light (AM 1.5) from a solar simulator (JASCO) and irradiating the ITO substrate side of the cell. The light source was calibrated by a Si photodiode detector (0.0534 cm2, Bunkoukeiki, Co., Ltd.). The cell position was adjusted so that the standard current flow, 0.627 mA, was obtained using the reference cell. The I–V curve measurement was conducted from -0.50 to 0 V (30 mV/sec) at room temperature in air and was controlled by a potentiostat. The electrode area was 0.28 cm2, which was the same area as the hole 6 mm in diameter. These solar cell characteristics were determined by 2 cells. To measure the solar cell characteristics, we followed the Solar cell reporting summary, Nature Research. (2) Thermal cell The assembled cell was placed onto a hot plate. Cyclic voltammetry (CV) cycles were recorded between 0.30 V and 0 V at 30 mV sec-1. No redox peak was observed in the measurement range, even during the first cycle. No redox peak was observed. Chronoamperometry was used to measure the temperature dependence of the short-circuit current. The CH3NH3Pb13 powder and titanium oxide powder were



weighed (0.145 g and 0.008 g, respectively) and used to prepare a two-layer compact of CH3NH3Pb13/TiO2 with a diameter of 10 mm, a thickness of 0.72 mm and a relative density of 65%. A cell was constructed using this compact, and the plate temperature was increased from room temperature to 100 °C. Then, the temperature was decreased from 100 to 40 °C, and the shortcircuit current was measured at each temperature immediately with the voltage set to 0 V. These STC characteristics were determined by 4 cells.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Spin coating method, XRD and UV-Vis spectra of organic perovskite, SEM and digital camera images of dual layer compact, Evaluation of electrolyte re-injection, Efficiency calculations.

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Sachiko Matsushita: 0000-0001-8699-295X Toshihiro Isobe: 0000-0002-2726-6728 Akira Nakajima: 0000-0003-0056-9283 Author Contributions S.M. organized the research project. S.S. performed the experiments and analysed the results. S.M. and S.S. wrote the manuscript. T.I. and A.N. supported the experiments.

Funding Sources This work was financially supported by The Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number JP16K14054) and The Japanese New Energy and Industrial Technology Development Organization (NEDO) fund (16101200-0).

Notes The authors declare no competing interests

ACKNOWLEDGMENT This work was supported by Ookayama Analysis Division, Tech. Dept., Tokyo Tech.

ABBREVIATIONS STC, sensitized thermal cell; DSSC, a dye-sensitized solar cell

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Temperature Dependence of a Perovskite-Sensitized Solar Cell: A

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