Solar Energy 195 (2020) 1–5
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Temperature dependent photovoltaic performance of TiO2/PbS heterojunction quantum dot solar cells Meibo Xinga, Yaohong Zhangb, Qing Shenb, Ruixiang Wanga,
a Beijing Engineering Research Centre of Sustainable Energy and Buildings, School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b Department of Informatics and Engineering, The University of Electro Communications, Chofu, Tokyo 182-8585, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dot solar cells PbS Temperature J-V characteristics
A planner heterojunction quantum dot solar cells (QDSCs) structure of FTO/TiO2/PbS-EMII/PbS-EDT/Au is fabricated via layer-by-layer spin coating method, and then the temperature dependent photovoltaic performance of QDSCs is studied. The results indicate that the environment temperature has great inﬂuence on the current density-voltage (J-V) characteristics of quantum dot solar cell. The short-circuit photocurrent density (JSC), open-circuit voltage (VOC) and ﬁll factor (FF) are all increased when the temperature decreases from 353 K to 253 K. A top value of power conversation eﬃciency (PCE, 9.78%) is obtained for the QDSCs when the environment temperature is lowered to 253 K, with a VOC of 0.63 V, JSC of 33.1 mA/cm2 and FF of 0.47, which is 33% above the PCE at room temperature (7.34%). In conclusion, it is necessary to cool the device for keeping the high eﬃciency operation of solar cell.
1. Introduction Solar energy is widely used as the most important renewable energy sources in recent years. The most commonly ways to use solar energy are to convert sunlight directly into heat by solar collector and electricity by solar cells in our daily life (Kabir et al., 2018; Zarmai et al., 2015; Su et al., 2018). Solar cell is kind of photoelectric semiconductor sheets that directly convert light energy into electrical energy through photoelectric or photochemical eﬀects. Presently, quantum dot solar cells (QDSCs) have attracted much attention due to their low cost and high theoretical energy conversion eﬃciency as the new generation of solar cells (Sargent, 2012; Liu et al., 2014; Zhang et al., 2012). The extinction coeﬃcients and absorption spectrum of quantum dots (QDs) could be easily regulated by the material sizes. Moreover, the multiple exciton generation (MEG) phenomena in QDs provides the possibility of obtaining external quantum eﬃciency (EQE) higher than 100%, that is, more than one electron generated per absorbed photon at a broad wavelength range across the solar spectrum (Nozik, 2010; Hetsch et al., 2011; Davis et al., 2015). The bandgaps of QDs could conveniently tuned by adjusting their sizes due to the quantum conﬁnement eﬀect. Thus, among the candidates for next generation photovoltaic devices, QDSCs is a promising alternative to traditional silicon solar cells (Emin et al., 2011). Lead sulﬁde (PbS) QDs are considered as the inspiring material in ⁎
QDSCs due to their adjustable optical band gap from 0.4 to 1.5 eV improves infrared absorption (Hyun et al., 2008), which would signiﬁcantly enhance the power conversion eﬃciency (PCE) of solar cells. Currently, the certiﬁed record PCE of PbS QD based device has reached as high as 12% (Xu et al., 2018). However, it’s still lower than the theoretical value (40%). One of the main reasons is the low carrier concentration and mobility in the QDs layer. Wang et al. (2018) proposed a new and eﬃcient method to accomplish selective inorganic ligand exchange on Pb-rich surface of PbS colloidal QDs. The QDs passivation via bromide combined with sulfur exhibit a stronger electronic coupling between adjacent QDs and remarkably enhanced carrier mobility, contributed to the boost in the PCE. Beygi et al. (2018) introduced the QD solution preparation for the single-step deposition QD layers. The results indicated that QDSCs fabricated showed higher PCE compared to those fabricated by the traditional layer-by-layer (LBL) method, and the PCE of the single-step deposited solar cells ups to 3.22%. Hu et al. (2018) doped silver to improve the carrier concentration in the PbS layer, and found that the PCE of solar cell enhanced from 9.1% to 10.6%. Furthermore, Ag added PbS quantum dot ﬁlm could regulate the carrier concentration, mobility, band extrema and Fermi energy levels though controlling the additive amount of Ag. Semitransparent QDSCs using PbS quantum dot absorber and gold as the electrodes was reported by Zhang et al. (2016). The investigation on the semitransparent QDSCs and the regulation in the light absorption
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.solener.2019.11.010 Received 29 July 2019; Received in revised form 18 September 2019; Accepted 4 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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spectrum by changing the QDs size exhibit the potential applications into buildings or automobiles. The results of above researches indicated that the performance of PbS QDSCs has remarkable temperature dependence due to the carrier mobility, diﬀusion lengths, and doping concentrations of PbS QDs ﬁlms with ligands as a function of temperature (Speirs et al., 2016; Ju et al., 2010). More importantly, a part of solar energy would convert into heat when the solar cell working, the temperature of device would inevitably arise. The undesired temperature increasing will reduce the PCE and stability of QDSCs device. In this work, the PbS based planner heterojunction QDSCs were fabricated with TiO2 compact layer and PbS QD absorbing layer using 1-ethyl-3-methylimidazolium iodide (EMII) as ligand source, using FTO and gold as the electrodes. The temperature dependent J-V characteristics of the QDSCs device was investigated in the temperature range of 253–353 K. Fig. 1. UV–Vis–NIR Spectroscopy of synthesized PbS QD.
2.4. Fabrication of PbS QDs solar cells
To fabricate PbS QDSCs, PbS QDs layer are deposited on TiO2 compact layer via layer-by-layer spin coating method. As the introduced in Section 2.2, after synthesis, the surface of PbS QDs is coated with original ligand OA which does not conduct electricity. Thus, OA must be replaced by short ligand in order to enhance the coupling eﬀect between QDs during the fabrication process of the QDSCs device. In this paper, EMII was used as the source of short ligand I- to exchange OA from the surface of PbS QDs. Firstly, 7 layer of PbS-EMII were deposited on TiO2 substrate by using EMII as the ligand source. After that, two layers EDT capped PbS were coating on the PbS-EMII layer. At last a 100 nm Au contact is thermally evaporated onto the surface of solar cells though a mask to create four identical cells on each substrate. The area deﬁned by the overlap of the electrodes equal to 0.375 cm2, a mask of 0.16 cm2 was used when measuring J-V characteristics.
Lead(II) oxide (PbO, Wako, 99.5%), oleic acid (OA, Aldrich, 90%), 1-octadecene (ODE, Aldrich, 90%), Hexamethyldiilathiane (TMS, Sigma Aldrich, synthesis grade), cadmium chloride (CdCl2, Wako, 95%), tetradecylphosphonic acid (TDPA, Aldrich, 97%), oleylamine (OLA, Aldrich, 70%), Titanium (IV) butoxide (Aldrich, 97%), 1-Ethyl-3methylimidazolium iodide (EMII, Aldrich, 97%), 1,2-ethanedithiol (EDT, Aldrich, 98%).
2.2. Synthesis of QDs The hot injection method is used to synthesize colloidal PbS QDs. Speciﬁcally, blend 6 mmol of PbO, 15 mmol of OA and 50 ml of ODE in a three-necked ﬂask. The mixed solution is stirred and degassed under the room temperature for 30 min, and then kept at 100 °C for 2 h until a clearness transparent Pb-OA/ODE solution is obtained. Furthermore, 3 mmol TMS is injected into a 10 ml ODE at 80 °C. The next step is natural cooling of the Pb-OA solution in nitrogen atmosphere, and the TMS/ODE mixture is injected quickly into above Pb-OA solution at 85 °C. 1 mmol CdCl2, 0.1 mmol TDPA and 3 mmol OLA is mixed and then injected into the colloidal PbS as the solution is cooled to 75 °C. After the reaction solution is cooling down to room temperature, 150 ml acetone is added into the reaction solution as cooling down to room temperature. Further, the mixture centrifuged at 4000 rpm for 5 min to get the PbS QDs sediment. The obtain precipitate is dispersed in 10 ml toluene, and add 30 ml acetone, 20 ml ethyl alcohol and 40 ml methyl alcohol for washing by centrifugation. For removing the uncoupled OA ligands on the PbS QDs, the puriﬁcation process is repeated thrice. After that, nitrogen ﬂowed the PbS QDs solution to dry the QDs, and then dispersed into 15 ml octane. Finally, the Oleic acid capped PbS QDs solution ﬁltered from 0.1um PTFE ﬁlter head. The obtained colloidal PbS concentration is about 100 mg/mL.
3. Results and discussion The Ultraviolet–Visible–Near Infrared (UV–Vis–NIR) Spectroscopy of PbS is shown in Fig. 1. It indicated that the ﬁrst exciton peak of PbS QDs is 933 nm. According to the empirical formula between the sizes and band gap of PbS, the sizes of PbS QDs can be estimated (Moreels et al., 2009). The calculated result shows that the average diameter of PbS QDs is 3.3 nm. This is further conﬁrmed by the Transmission Electron Microscope (TEM) measurements. As shown in Fig. 2, the size
2.3. Preparation of TiO2 compact layer The thin TiO2 compact layer with a smaller number of defects is coated onto the FTO surface for electron transport. The precursor solution of TiO2 was prepared by a reported method (Zhang et al., 2017). Brieﬂy, 15 drops of saturated HCl are added gradually into a 25 ml ethyl alcohol, and then 875 µl water is added, and then 3 ml Titanium (IV) butoxide is added. The mixture is stirred at room temperature of 20 °C for 24 h under nitrogen atmosphere. The solution is dropped onto FTO surface and spin cast at 3000 rpm for 30 s and heated at 120 °C for 30 min. Finally, the substrate is annealed at 500 °C for 30 min.
Fig. 2. TEM of colloidal PbS QDs. 2
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Fig. 3. FT-IR spectra of PbS with OA and EMII.
Fig. 5. Curves of a solar cell under dark and under one sun illumination.
of PbS QDs is approximately 3 nm. As it well known, ligand exchange plays a critical role as PbS QDs solar cells fabrication, which shortens distance between QDs and provides charge transport. The EMII solution is selected for ligand exchange with the OA that was capping the PbS surface when synthesis of QDs. Fig. 3 shows the Fourier transform infrared spectroscopy (FT-IR) spectra of colloidal Pbs QDs just synthesized and after ligand exchange by EMII. It can be seen from the FT-IR spectra measurements that the CeH absorption stretching peaks at 2961, 2929, and 2854 cm−1 are greatly decreased after EMII ligand treatments. Moreover, the ]CeH stretching peak of OA at 3006 cm−1 disappeared that is implying the EMII has been connected onto PbS surface instead of long-chain OA during ligand exchange process. Fig. 4 shows the Scanning Electron Microscopy (SME) image of a cross section of the fabricated PbS QDs solar cell, and the thickness of each layer can be observed. It can be seen that the thickness of TiO2 compact layer is about 50 nm. PbS QDs were deposited onto TiO2 compact layer substrates using a layer-by-layer spin coating method employing 1-Ethyl-3-methylimidazolium iodide (EMII) ligand exchange to increase carrier mobility and insolubilize the QDs. Hence, the thickness of the PbS QD layer was determined by the number of spin coating cycles. It is can be seen from SEM image that the thickness of PbS QDs layer is about 250 nm for seven cycles in this study. Recent results by Speirs et al. showed that PbS ﬁlms on TiO2 consisting of 200 nm ligand capped PbS (Speirs et al., 2016), which support this conclusion. Finally, the back contact of 100 nm Au is thermally evaporated. Herein, the contact type between PbS and Au is Ohmic contact. 100 nm Au layer is just the charge carrier collection electrode of the device. The HOMO level of PbS QDs is about −5.2 eV and the work
function of Au is about −5.0 eV, so the contact between PbS and Au is a typical metal/semiconductor Ohmic contact. During the photovoltaic eﬃciency measurement, the photocurrent data of the solar cells is record through a Keithley 2400 source meter, and all parameters of the device are calculated by the measurement software. The PCE of solar cell can be calculated by following equation:
Jsc × Voc × FF × 100% Pin
where VOC is open circuit photovoltage, JSC is the short circuit current density, FF is the ﬁll factor (the maximum power divided by the product of JSC and VOC), and Pin is the incident light intensity. For one sun illumination, Pin is 100 mW/cm−2. Fig. 5 shows the J-V curves of a solar cell. In order to study temperature dependent on the photovoltaic performance of TiO2/PbS heterojunction quantum dot solar cells, the J-V characteristics of the device was studied as a function of temperature 253–353 K (−20 to 80 °C) in this work. The J-V curves were taken under AM1.5G 1 Sun intensity using a calibrated solar simulator. The JV curves of PbS QDSCs which measured under diﬀerent temperature are shown in Fig. 6. Corresponding, Fig. 7 displays the solar cell parameters that are extracted from the J-V curves in Fig. 6. The solar cell has a VOC of 0.58 V, JSC of 29.1 mA/cm2, FF of 0.43, and an overall PCE of 7.34% at room temperature. As the temperature decreases to 253 K, the solar cell exhibits a VOC of 0.63 V, a JSC of 33.1 mA/cm2, a FF of 0.47, and a PCE of 9.78%, which is 33% above the PCE measured at room temperature. Meanwhile, the solar cell shows a VOC of 0.49 V, a JSC of 19.1 mA/cm2, a FF of 0.31, and PCE of 2.86% when the temperature increases to 353 K, which is 61% decline PCE compare to that value at
Fig. 4. Across-sectional SEM image and energy band diagram of the solar cell. 3
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comparing to those of thin ﬁlm solar cells which are increasing with the increased temperature. The TiO2/PbS quantum dot solar cell shows a bigger short circuit current at the lower temperature. It may explain as follow the photoluminescence measurements show that the bandgap in fact narrows as the temperature is decreased (Speirs et al., 2016) this will simultaneously increase the amount of photons absorbed and generated carrier as a result. Moreover, it is indicated that the great temperature dependence could be caused by thermal contraction of the organic ligand, which decreases the width of the tunneling barrier and thereby increases the coupling between quantum dots (Ju et al., 2010; Loef et al., 2009). Besides, the ligand has a freezing/melting temperature, near where the peak change in performance is observed (Ju et al., 2010). The research indicated that the freezing of the ligand would further decrease the spacing between particles as discussed elsewhere for PbSe quantum dots (Liu et al., 2010). Following the eﬀect of the decreased spacing would result in increased carrier mobility. Taken together, a bigger Jsc would be expected with the decreased temperature. In brief, a large increase in device performance as the quantum dot solar cell at lower temperature, which is contribute to narrow bandgap and enhanced coupling between quantum dots due to thermal contraction and freezing of the organic ligand. The short circuit current density shows the opposite temperature characteristics with conventional thin ﬁlm solar cells for the TiO2/PbS heterojunction quantum dot solar cells. Therefore, the volt-ampere characteristic curve of the non-ideal diode would be proceeded to analysis the temperature dependent characteristics of TiO2/PbS quantum dot solar cells. From the temperature characteristics of semiconductors, it is known that the intrinsic carrier concentration increases in the form of exponent with the increase of temperature. Furthermore, the dark current will also increase. Dark current includes diﬀusion current and reverse saturated current in space charge region. The diffusion current and reverse saturated current are directly proportional to the quadratic and the primary power of the carrier concentration. The relationship between open-circuit voltage and reverse saturated current is as follows:
Fig. 6. J-V curves under 1 Sun illumination with various temperatures.
mkB T ⎛ Jsc ln ⎜ 0 + 1⎞⎟ q ⎠ ⎝ Jm
where m is Ideal factor, kB is Boltzmann constant, Jsc is short circuit current, Jm0 is reverse saturated current of non-ideal diode. The reverse saturated current increases and the short circuit current decreases with the increased temperature, which lead to the decrease of open circuit voltage. This is consistent with the experimental results, the open circuit voltage of the solar cell decreases with the increase of temperature under the measured range. In addition, the recombination rate of internal charge increases and meanwhile the shunt resistance decreases as the temperature increases, this leads to the decrease of the ﬁlling factor of solar cell. The phenomenon meaning that the temperature has critical inﬂuence on the solar cell characteristics, especially at the higher temperature. Based on this, the QDSCs are properly employed in some cold area or environment, such as the north and south poles of the earth, or a high-altitude area with plenty of sunshine, or a factory with cold environment and good light. In addition, the solar cell would get a part of solar energy to generate heat when the sun shines on it for application of photovoltaic eﬀect. Hence, the temperature would increase when the solar cell working. In conclusion, it is necessary to cool the cell for keeping the high eﬃciency operation of solar cell.
Fig. 7. Solar cell parameters extracted from the J-V curves.
room temperature. Furthermore, it can also be found from Fig. 7 that as the temperature is decreased from 353 K to 253 K, the JSC, VOC and FF show declined tendency over the measured range. Usually, the carrier mobility and diﬀusion length in semiconductors increase with the rising temperature which may beneﬁt to improve the eﬃciency of solar cells devices. But for QDSCs, due to there are many surﬁcial and interfacial charge recombination centers in the devices, most of them will be exposed at high temperature. Those recombination centers play a heavy role than the enhanced carrier mobility and diﬀusion length to reduce the performance of the device. The Jsc is the largest value of photocurrent that can be supplied by a solar cell per unit area. It mainly depends on light power, the absorption spectrometry, absorbance of light absorbing layer, and the charge recombination in the solar cells. Fig. 7 reveals that for the TiO2/PbS QDSCs, the Jsc shows the opposite temperature characteristics
4. Conclusions The temperature dependent characteristics of TiO2/PbS heterojunction solar cell were investigated in this work. Our results demonstrated that the temperature has great inﬂuence on the J-V characteristics of solar cell, especially at the higher temperature. The results 4
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shown that the JSC, VOC and FF are all increased when the temperature decreased from 353 K to 253 K. As the temperature decreases to 253 K the solar cell exhibits 33% improvement of PCE compared to the room temperature (from 7.34% improved to 9.78%). Moreover, the solar cell shows 61% decline PCE than the room temperature as the temperature increases to 353 K (from 7.34% reduced to 2.86%). It is concluded that a large increase in device performance as the quantum dot solar cell is cooled. Therefore, further study concerning the heat management of the solar cell as well as experimental work will be necessary in the next step.
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Acknowledgements This study is ﬁnancially supported by the Beijing Advanced Innovation Center for Future Urban Design of Beijing University of Civil Engineering and Architecture under the Grant UDC2018031121, China and the Science and Technology Project of Beijing Education Commission under the Grant KM201910016010, China. The authors would like to thank for the sponsorships. References Beygi, Hossein, Sajjadi, Seyed Abdolkarim, Babakhani, Abolfazl, et al., 2018. Solution phase surface functionalization of PbS nanoparticles with organic ligands for singlestep deposition of p-type layer of quantum dot solar cells. Appl. Surf. Sci. 459, 562–571. Davis, Nathaniel J.L.K., Böhm, Marcus L., Tabachnyk, Maxim, et al., 2015. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum eﬃciencies exceeding 120%. Nature Commun. 6, 8259. Emin, Saim, Singh, Surya P., Han, Liyuan, et al., 2011. Colloidal quantum dot solar cells. Solar Energy 85, 1264–1282. Hetsch, Frederik, Xu, Xueqing, Wang, Hongkang, et al., 2011. Semiconductor nanocrystal quantum dots as solar cell components and photosensitizers: material, charge transfer, and separation aspects of some device topologies. J. Phys. Chem. Lett. 2, 1879–1887. Hu, Long, Zhang, Zhilong, Patterson, Robert J., et al., 2018. Achieving high-performance PbS quantum dot solar cells by improving hole extraction through Ag doping. Nano Energy 46, 212–219. Hyun, Byung-Ryool, Zhong, Yu-Wu, Bartnik, Adam C., et al., 2008. Electron injection