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Parameters in planar quantum dot-polymer solar cell: Tuned by QD Eg, ligand exchange and fabrication process
Organic Electronics 69 (2019) 1–6
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Parameters in planar quantum dot-polymer solar cell: Tuned by QD Eg, ligand exchange and fabrication process
T
Qiaomu Xiea,b, Shuaiqiang Mingb, Lijun Chena,b, Yulei Wub,c, Wenxiao Zhangb,c, Xiaohui Liub, Meng Caoa, Hai-Qiao Wangb,c,∗, Junfeng Fangb,c,∗∗ a
School of Materials Science and Engineering, Shanghai University, Nanchen Road 333, Shanghai, 200444, China Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China c University of Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Polymer Planar hybrid solar cell Device parameters
Planar hybrid heterojunction PbS-polymer solar cell is lagging behind compared with its counterparts due to limited studies. This work is aimed to understand the mechanisms of this technology and improve its performance, by tuning the quantum dot (QD) bandgap (Eg) and the modified ligand exchange and fabrication process. With planar structure of PbS QDs and PTB7-Th, relative high VOC (> 0.5 V) was obtained in device with QD Eg in the range of 1.3–2.0 eV. The highest VOC and JSC was recorded to be 0.645 V and of 15.5 mA/cm2 respectively. And the best device performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was demonstrated. Which is over double the performance of previously reported similar planar hybrid PbS-organic heterojunction solar cells, due to the improved and balanced parameters after optimization.
1. Introduction Owing to the unique optoelectronic properties, like size-tunable bandgap manipulation, strong and broad absorption from the ultraviolet to the near-infrared range [1],size and morphological stability of inorganic nanocrystals, and also facile electronic property design by ligand chemistry [2,3], semiconducting quantum dot (QD) materials (such as CdTe, CdSe and PbS QDs) have been widely applied in optoelectronic devices [4,5]. Especially, they are promising candidates for photovoltaic applications, due to their characteristics of solution-processability, feasibility for use in flexible and large-scale devices [6], high charge carrier mobility [7,8], and multiple-exciton generation [9]. Hence QDs based solar cells have attracted great attention from the community. Among them, lead sulfide QDs based solar cell [7,10–12] is one of the most widely studied system due to its facile synthesis, easy fabrication, highly tunable bandgap, abundance of material, and importantly the potential to achieve high performance. With Schottky junction architecture, the devices can be easily fabricated with less interfaces [13,14], but often limited in open-circuit voltage (VOC) [15,16] and absolute device performance [14,17]. To
overcome the limiting issues including charge transport and extraction problems, heterojunction structure18 were developed, combined with other strategies [18–20] like ligand treatment of PbS QDs, electrode surface engineering (such as ZnO nanowire array) and bandgap/energy level alignment engineering. The hybrid concept was first reported in 1996 based on a 300–500 nm thick CdSe/CdS QDs-MEH-PPV composite, with power conversion efficiency (PCE) of 0.49% [21]. Then with ligand (BDT/EDT etc.) treated PbS QDs and small molecule (C60/PCBM etc.) bilayer structure, the hybrid heterojunction solar cells achieved efficiency of 2.2% and 1.27% respectively [15,22], compared to 1.6% in the corresponding Schottky device [23], due to the VOC and FF improvement by reduced recombination and higher carrier collection efficiency. After that, hybrid bulk heterojunction solar cell was fabricated with P3HT and PbS QDs [24]. PCE up to 1% was demonstrated (JSC 7.1 mA/cm2, VOC 0.42 V, and FF 34%). Which was then greatly improved to 3.48% (JSC 10.82 mA/cm2, VOC 0.63 V, and FF 51%) by Lee et al. [25], due to the broadband absorption, properly aligned bandgap and improved interface contact, based on hybrid bulk heterojunction architecture (PbS:PSBTBT). It was also demonstrated that both components can contribute to the photocurrent for different polymer-PbS
∗ Corresponding author. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. ∗∗ Corresponding author. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. E-mail addresses: [email protected] (H.-Q. Wang), [email protected] (J. Fang).
https://doi.org/10.1016/j.orgel.2019.02.026 Received 3 January 2019; Received in revised form 25 January 2019; Accepted 24 February 2019 Available online 25 February 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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excitonic absorption peak. In the devices, nanocrystal ZnO was deposited on the cathode as an electron-transporting and hole-blocking layer (Fig. 2a). The tetrabutylammonium iodide (TBAI) ligand treated PbS QDs film present ntype property as its Femi level is more close to the conduction band, deduced from the result of ultraviolet photoelectron spectroscopy (UPS) measurement (Fig. S1b, Fig. 2c). To explore the influence of QD Eg on performance of planar hybrid device, PbS QDs with different Eg were applied to fabricate the solar cells. As a p-type material, PTB7-Th plays the role of hole-transport. Besides which, it polishes the PbS film surface and benefits the PbS/anode contact as well. Although PTB7-Th absorbs photons passed through the PbS film and generates excitons in principle, the photocurrent in our device could be mainly attributed to the PbS in front but not the PTB7-Th at the back due to its limited thickness. The incorporation of MoO3 (15 nm) is to block the electron transport to anode (Fig. 2a). And the high work function of which pins the Fermi level of top contact and benefit the VOC. Firstly, three cycles of deposition of PbS with high concentration (30 mg/ml) was applied to form the PbS film (220 nm), to fabricate the planar hybrid heterojunction solar cells. For which, overall high VOC was obtained as expected. Linear increase of VOC is observed with Eg in the low value side (Fig. 3a). Which is consistent with the results reported by other groups [35–37]. While it decreases fast in the high value side, probably due to the unsuitable band/energy level alignment. The maximum VOC (0.645 V) was obtained by using the QDs with Eg of 1.79 eV. The trend of VOC loss, i.e. Vdeficit of this planar hybrid type solar cell according to the QD Eg is presented in Fig. 3a as well. Relative high JSC was obtained for devices with low QD Eg, but decreased fast with Eg over 1.6 eV (Fig. 3b). As a combination effect of the parameters, relative high PCE was obtained for the devices using QDs with Eg in the range of 1.2–1.8 eV (Fig. 3d). Which is consistent with the reported results [11]. To summarize, relative high VOC and modest performances were obtained for the planar hybrid heterojunction solar cells based on QDs with different Eg, due to the low FF and JSC. Which we consider could be mainly accounted for by the limited carrier transport in PbS film due to ligand issues. To overcome the problems, PbS QD sample synthesized at 120 °C was chosen as the model to fabricate devices due to its better performance. Thickness of each deposited single PbS layer was much reduced to promote the ligand exchange, by lowering the PbS QDs concentration and increasing the spin speed in deposition. And more cycles of deposition were applied to obtain the desired thickness. For each of the single PbS QD layer, ligand exchange was promoted via prolonged exchanging time and more times of rinse. On the other hand, after ligand treatment, thermal annealing was applied before the last deposition cycle of PbS to eliminate organic residuals. But for the last single layer, no thermal annealing was applied, to avoid film cracks by heating. Fig. 2b shows the TEM and HRTEM lattice fringe image of the adopted PbS QDs. For which average diameter of 3.5 nm is determined. The Eg is calculated to be 1.39 eV according to its first excitonic absorption peak (Fig. S3). The work function of PbS-TBAI is determined to be 4.62 eV according to the cut-off binding energy (16.6 eV) of the PbSTBAI film, as shown in Fig. 2c. Based on which and the energy difference (0.84 eV) between the Femi energy level (Ef) and valence band edge (Ev), the conduction band edge (Ec) and valance band edge (Ev) were determined to be −4.07 eV and −5.46 eV respectively, as illustrated in the energy level alignment diagram (Fig. 2a). Fourier transform infrared (FTIR) measurement was conducted to analyze the surface chemistry of PbS QDs in film after ligand treatment, as shown in Fig. 4. The strong peaks at 2852 cm−1, 2924 cm−1 and 1401 cm−1 and 1516 cm−1 are originated from C-H bending and the asymmetric and symmetric stretching vibrations of -COO- group on QDs. It is clear that decreased intensity of these peaks are observed for the PbS layer fabricated with low concentration and high spin speed, suggesting more effective removal of organics in QD film. Which is
hybrid systems [26]. By utilizing a clever strategy of vertical phase segregation of the active layer, professor Ma et al. demonstrated a record PCE of 5.5% for PbS-PDTPBT hybrid bulk heterojunction solar cell [27] due to improved charge transport and extraction in device, suggesting promising potential for PbS-polymer hybrid heterojunction solar cells. And recently, based on a homojunction-like architecture, where both the p- and n-materials of the junction are composed of PbS QDs treated by different chemical ligands, the state of the art performance of PbS QDs solar cells were hugely improved to over 10% [28–30]. Although huge progresses have been achieved for homojunction PbS QD solar cells, hybrid heterojunction solar cell especially the planar hybrid heterojunction solar cell is still lagging much behind, due to its limited study up to today. We consider that more efforts are needed to study and understand the mechanisms, and it is meaningfull to improve this technology. Since besides the aforementioned advantages, the planar hybrid heterojunction system has specific superiorities i.e. abundant polymer candidates with stable and easy designable optoelectronic properties, more architecture options owing to that both PbS and polymer can act as the p- or n-material in junction according to their energy levels, and opportunity to achieve complementary absorption for solar spectrum. In this study, based on PbS QDs and polymer poly [ [2,6′-4,8-di(5ethylhexylthienyl)benzo[1,2- b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th), parameters of planar hybrid heterojunction solar cell are studied by tuning the QD Eg and modified ligand exchange and fabrication process. Relative high VOC was obtained for the planar hybrid solar cells with QD Eg in the range of 1.3–2.0 eV, indicating a less VOC loss in our device architecture compared with the Schottky [14,17] and bilayer heterojunction [15,22] types. And the best device performance with PCE of 4.43%, JSC of 14.47 mA/cm2, VOC of 0.561 V and FF of 54.6% was demonstrated. Which is over double the performance of previously reported planar hybrid PbS-organic solar cells [15,31–33]. 2. Results and discussion Oleic acid capped PbS QDs were synthesized according to literature [34]. PbS QDs with different band gap (Eg) were obtained by tuning the synthetic temperature from 45 to 185 °C. High crystallinity is confirmed for all the PbS QDs synthesized at different temperature by the XRD measurement, as shown in Fig. S1. Well-defined diffraction peaks were assigned to the (111), (200), (220) and (311) etc. planes respectively. And all diffraction peaks match well with the standard XRD lines of face-centered cubic PbS (JCPDS Card No. 05–0592). Narrowing diffraction peak is recorded with increased synthesis temperature due to the increased particle size, confirmed by the TEM measurements (Fig. S2). Absorption spectra of the samples are presented in Fig. 1. Eg from 0.86 to 2.1 eV was determined according to their corresponding first
Fig. 1. Absorption spectra of PbS QDs synthesized with different temperature 45–185 °C. 2
Organic Electronics 69 (2019) 1–6
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Fig. 2. (a) The diagram of energy level alignment of the materials in devices; (b) TEM/HRTEM images of the PbS QDs synthesized at 120 °C; and (c) and (d) the UPS spectrum of the PbS QDs after ligand treatment.
obtained due to the improved carrier transport in PbS film. FF approaching 55% was recorded for the device based on QDs with Eg = 1.39 eV, i.e. improved by 37.5% compared with the control device. Gradually enhanced JSC was obtained when the deposition cycles
further confirmed by the promoted device performance. For devices fabricated with the above adjusted procedure, 5 to 15 deposition cycles were applied to form the PbS film to approach the ideal thickness. As expected, overall improved FF and JSC (Fig. 5) were
Fig. 3. Influence of Eg of PbS QDs on the parameters of planar hybrid heterojunction solar cells, (a) VOC and Vdeficit, (b) JSC, (C) FF, and (d) PCE. 3
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decrease in parallel resistance Rsh (Table S1) due to increased QD surface traps when thicker PbS film applied. Although the performance lags behind, this still implies big improvement space for planar hybrid type solar cells, if a thicker PbS film can be applied and meanwhile the film is further promoted by ideal ligand treatment and good quality of synthesized PbS QDs. Fig. 6 shows the J-V curve of the optimized device, and the corresponding EQE curve. The maximum EQE value over 76% was recorded at certain wavelengths. The integrated photocurrent 13.3 mA/cm2 is consistent with the measured JSC. And the slight mismatch could be due to the unencapsulated situation of the device in air in the EQE test process. 3. Conclusions To summarize, planar hybrid heterojunction PbS-PTB7-Th solar cells were fabricated and studied. Overall relative high VOC (> 0.5 V) was obtained for the devices with QD Eg in the range of 1.3–2.0 eV, due to the suitable architecture and well aligned band/energy levels. The highest VOC of 0.645 V and highest JSC of 15.5 mA/cm2 were recorded. The best device performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was demonstrated, after ligand exchange and fabrication process optimization. Which is over double the performance of previously reported similar planar hybrid PbS-organic solar cells, due to the improved and balanced parameters. This work implies that planar hybrid heterojunction PbS-polymer solar cell has the potential to provide efficient performance, even similar as homojunction type PbS solar cell does, given the thickness and charge transport in PbS film are well optimized.
Fig. 4. FTIR absorption spectra of PbS QD layer before (full rectangle and circle) and after (half full rectangle and cirle) ligand treatment, formed by single deposition cycle with high concentration and low spin speed (black color), or low concentration and high spin speed (red color). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of PbS increase owing to the increased thickness of absorbing layer. Which reaches the maximum value (up to 15.5 mA/cm2) when 11 deposition cycles applied. While decreased JSC was observed when the deposition cycles further increased. Combined with improved VOC (from 0.51 to 0.56 V), the highest performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was obtained when 9 deposition cycles of PbS applied, due to the balanced parameters. Which is more than double the performance of previously reported planar hybrid PbS-organic solar cells. Considering the depletion width [38] of ∼200 nm in PbS at the heterojuntion interface and the charge carrier diffusion length [39] of ∼80 nm, it is believed that a PbS film not thicker than ∼300 nm is reasonable for device. However, compared to which, our optimal device has a much thinner PbS film (∼125 nm) according to the cross-sectional TEM image (Fig. S4). Further increase of the PbS film thickness resulted in lowered performance due to the decreased VOC and FF (Fig. 5). Which could be attributed to the fast
4. Experimental 4.1. Synthesis of PbS QDs PbS quantum dots synthesis was adapted from the literature previously documented34 with a modified recipe. All operations were performed under the standard air-free Schlenk line with nitrogen atmosphere. Lead oxide (PbO, 0.36 g) was dissolved in 15 ml of 1-octadecene (ODE) and 1 ml of oleic acid (OA) at 145 °C with stirring for 3 h under vacuum in a three-neck flask. The solution then was heated to
Fig. 5. Parameters optimization of the planar hybrid heterojunction solar cells. The PbS film has been fabricated with low concentration PbS, high spin speed, adjusted ligand treatment, and different deposition cycles. The 5, 7, 9, 11 and 15-layers corresponds to 72, 101, 125, 150 and 202 nm in thickness for PbS film. 4
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Fig. 6. (a) J-V curve of the optimal planar hybrid heterojunction PbS-PTB7-Th solar cell. (b) The EQE curve of the optimal device and the corresponding integrated photocurrent curve.
45–185 °C. After the solution was reached a desired temperature, the sulphur precursor was injected into the three-neck flask rapidly. Which was prepared by adding 168 μl of hexamethyldisilathiane ((TMS)2S) into 4 ml of ODE. Once the sulphur precursor were injected into the vigorously stirring lead oxide solution at the desired temperature, the heating mantle was turned off immediately to cool the solution to room temperature under the nitrogen atmosphere. Then the PbS QDs were purified by adding acetone/methanol to precipitate and followed by centrifugation twice, and finally dispersed in toluene (∼30 mg/ml).
Science and Technology Plan (No.2018C01047), Major Project of "Science and Technology Innovation 2025" of Ningbo, China.
4.2. Device fabrication
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.02.026. References
The patterned ITO substrates were cleaned with successive sonication in deionized water, acetone, isopropanol orderly and treated with oxygen plasma before spin-coating. A layer of ZnO was firstly deposited onto the indium tin oxide (ITO) substrate at 2000 rpm, and then thermally annealed at 200 °C for 30 min. The ZnO precursor was prepared by dissolving 1 mg of zinc acetate in mixture of 10 ml 2-methoxyethanol and 284 μl monoethanolamine. The PbS QDs film was fabricated via layer-by-layer spin-coating methods in air. For each single PbS layer, 50 μl of oleic-acid-capped PbS QDs in toluene (30 mg/ml or 15 mg/ml) was deposited onto the ZnO substrates with spin speed of 2000 rpm or 6000 rpm for 15s. For ligand-exchange of each single PbS QD layer, the TBAI solution in methanol (10 mg/ml) was applied to PbS QD layer and stayed for 20s or 30 s, and this was followed by twice rinses of methanol. The ligand-exchange process was repeated two times for each layer. For the device fabricated with high concentration PbS, thermal annealing was applied for the whole PbS film at 100 °C for 10 min. For the device fabricated with low concentration PbS, thermal annealing was applied before the last deposition cycle of PbS. The substrates were then stored in air over night and then transferred to a nitrogen-filled glovebox. Finally, a ∼15 nm MoO3 and 100 nm Al were thermally evaporated through a shadow mask. 4.3. Device characterization Current density−voltage (J−V) characteristics of the devices were measured with a computer-controlled Keithley 2400 source meter and Newport 6279 NS solar simulator (450 W) with 100 mW/cm2 illumination. The EQE curves were recorded through a Newport quantum efficiency measurement system (ORIEL IQE 200TM) with a lock-in amplifier in atmosphere. Each wavelength light intensity was calibrated with a standard Si/Ge solar cells. Acknowledgements This research was supported by National Natural Science Foundation of China (No. 51502313), Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-JSC047), and Zhejiang Province 5
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colloidal PbSe nanocrystals and thin ZnO films, ACS Nano 3 (2009) 3638–3648. [32] L. Wang, D. Zhao, Z. Su, D. Shen, Hybrid polymer/ZnO solar cells sensitized by PbS quantum dots, Nanoscale. Res. Lett. 7 (2017) 106. [33] G. Kim, H. Kim, B. Walker, et al., Effects of ionic liquid molecules in hybrid PbS quantum dot–organic solar cells, ACS Appl. Mater. Interfaces 5 (2013) 1757–1760. [34] M. Hines, G. Scholes, Colloidal PbS nanocrystals with sizetunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution, Adv. Mater. 15 (2003) 1844–1849. [35] C. Chuang, A. Maurano, R. Brandt, et al., Open-circuit voltage deficit, radiative subbandgap states, and prospects in quantum dot solar cells, Nano Lett. 15 (2015) 3286–3294. [36] S. Pradhan, A. Stavrinadis, S. Gupta, et al., Breaking the open-circuit voltage deficit floor in PbS quantum dot solar cells through synergistic ligand and architecture engineering, ACS. Energy Letters 2 (2017) 1444–1449. [37] W. Yoon, J. Boercker, M. Lumb, et al., Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells, Sci. Rep. 3 (2013). [38] P. Brown, R. Lunt, N. Zhao, et al., Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer, Nano Lett. 11 (2011) 2955–2961. [39] D. Zhitomirsky, O. Voznyy, S. Hoogland, E. Sargent, Measuring charge carrier diffusion in coupled colloidal quantum dot solids, ACS Nano 7 (2013) 5282–5290.
(2009) 5. [23] S. Tsang, H. Wang, J. Lu, et al., Highly efficient cross-linked PbS nanocrystal/C60 hybrid heterojunction photovoltaic cells, Appl. Phys. Lett. 95 (2009) 183505. [24] Y. Firdaus, E. Vandenplas, Y. Justo, et al., Enhancement of the photovoltaic performance in P3HT:PbS hybrid solar cells using small size PbS quantum dots, J. Appl. Phys. 116 (2014) 094305. [25] M. Nam, J. Park, S. Kimb, K. Lee, Broadband-absorbing hybrid solar cells with efficiency greater than 3% based on a bulk heterojunction of PbS quantum dots and a low-bandgap polymer, J. Mater. Chem. A. 2 (2014) 3978–3985. [26] R. Mastria, A. Rizzo, C. Giansante, et al., On the role of polymer in hybrid polymer/ PbS quantum dot solar cells, J. Phys. Chem. C 119 (2015) 14972–14979. [27] Z. Liu, Y. Sun, J. Yuan, et al., High-efficiency hybrid solar cells based on polymer/ PbSx Se1-x nanocrystals benefiting from vertical phase segregation, Adv. Mater. 25 (2013) 5772–5778. [28] Y. Wang, K. Lu, L. Han, et al., Reconfigurable photonic platforms: a lithography‐free and field‐programmable photonic metacanvas, Adv. Mater. 30 (2018) 1704871. [29] R. Wang, X. Wu, K. Xu, et al., Highly efficient inverted structural quantum dot solar cells, Adv. Mater. 30 (2018) 1704882. [30] L. Hu, Z. Zhang, R. Patterson, et al., Achieving high-performance PbS quantum dot solar cells by improving hole extraction through Ag doping, Nanomater. Energy 46 (2018) 212–229. [31] K. Leschkies, T. Beatty, M. Kang, et al., Solar cells based on junctions between
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Parameters in planar quantum dot-polymer solar cell: Tuned by QD Eg, ligand exchange and fabrication process
T
Qiaomu Xiea,b, Shuaiqiang Mingb, Lijun Chena,b, Yulei Wub,c, Wenxiao Zhangb,c, Xiaohui Liub, Meng Caoa, Hai-Qiao Wangb,c,∗, Junfeng Fangb,c,∗∗ a
School of Materials Science and Engineering, Shanghai University, Nanchen Road 333, Shanghai, 200444, China Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China c University of Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Polymer Planar hybrid solar cell Device parameters
Planar hybrid heterojunction PbS-polymer solar cell is lagging behind compared with its counterparts due to limited studies. This work is aimed to understand the mechanisms of this technology and improve its performance, by tuning the quantum dot (QD) bandgap (Eg) and the modified ligand exchange and fabrication process. With planar structure of PbS QDs and PTB7-Th, relative high VOC (> 0.5 V) was obtained in device with QD Eg in the range of 1.3–2.0 eV. The highest VOC and JSC was recorded to be 0.645 V and of 15.5 mA/cm2 respectively. And the best device performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was demonstrated. Which is over double the performance of previously reported similar planar hybrid PbS-organic heterojunction solar cells, due to the improved and balanced parameters after optimization.
1. Introduction Owing to the unique optoelectronic properties, like size-tunable bandgap manipulation, strong and broad absorption from the ultraviolet to the near-infrared range [1],size and morphological stability of inorganic nanocrystals, and also facile electronic property design by ligand chemistry [2,3], semiconducting quantum dot (QD) materials (such as CdTe, CdSe and PbS QDs) have been widely applied in optoelectronic devices [4,5]. Especially, they are promising candidates for photovoltaic applications, due to their characteristics of solution-processability, feasibility for use in flexible and large-scale devices [6], high charge carrier mobility [7,8], and multiple-exciton generation [9]. Hence QDs based solar cells have attracted great attention from the community. Among them, lead sulfide QDs based solar cell [7,10–12] is one of the most widely studied system due to its facile synthesis, easy fabrication, highly tunable bandgap, abundance of material, and importantly the potential to achieve high performance. With Schottky junction architecture, the devices can be easily fabricated with less interfaces [13,14], but often limited in open-circuit voltage (VOC) [15,16] and absolute device performance [14,17]. To
overcome the limiting issues including charge transport and extraction problems, heterojunction structure18 were developed, combined with other strategies [18–20] like ligand treatment of PbS QDs, electrode surface engineering (such as ZnO nanowire array) and bandgap/energy level alignment engineering. The hybrid concept was first reported in 1996 based on a 300–500 nm thick CdSe/CdS QDs-MEH-PPV composite, with power conversion efficiency (PCE) of 0.49% [21]. Then with ligand (BDT/EDT etc.) treated PbS QDs and small molecule (C60/PCBM etc.) bilayer structure, the hybrid heterojunction solar cells achieved efficiency of 2.2% and 1.27% respectively [15,22], compared to 1.6% in the corresponding Schottky device [23], due to the VOC and FF improvement by reduced recombination and higher carrier collection efficiency. After that, hybrid bulk heterojunction solar cell was fabricated with P3HT and PbS QDs [24]. PCE up to 1% was demonstrated (JSC 7.1 mA/cm2, VOC 0.42 V, and FF 34%). Which was then greatly improved to 3.48% (JSC 10.82 mA/cm2, VOC 0.63 V, and FF 51%) by Lee et al. [25], due to the broadband absorption, properly aligned bandgap and improved interface contact, based on hybrid bulk heterojunction architecture (PbS:PSBTBT). It was also demonstrated that both components can contribute to the photocurrent for different polymer-PbS
∗ Corresponding author. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. ∗∗ Corresponding author. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. E-mail addresses: [email protected] (H.-Q. Wang), [email protected] (J. Fang).
https://doi.org/10.1016/j.orgel.2019.02.026 Received 3 January 2019; Received in revised form 25 January 2019; Accepted 24 February 2019 Available online 25 February 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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excitonic absorption peak. In the devices, nanocrystal ZnO was deposited on the cathode as an electron-transporting and hole-blocking layer (Fig. 2a). The tetrabutylammonium iodide (TBAI) ligand treated PbS QDs film present ntype property as its Femi level is more close to the conduction band, deduced from the result of ultraviolet photoelectron spectroscopy (UPS) measurement (Fig. S1b, Fig. 2c). To explore the influence of QD Eg on performance of planar hybrid device, PbS QDs with different Eg were applied to fabricate the solar cells. As a p-type material, PTB7-Th plays the role of hole-transport. Besides which, it polishes the PbS film surface and benefits the PbS/anode contact as well. Although PTB7-Th absorbs photons passed through the PbS film and generates excitons in principle, the photocurrent in our device could be mainly attributed to the PbS in front but not the PTB7-Th at the back due to its limited thickness. The incorporation of MoO3 (15 nm) is to block the electron transport to anode (Fig. 2a). And the high work function of which pins the Fermi level of top contact and benefit the VOC. Firstly, three cycles of deposition of PbS with high concentration (30 mg/ml) was applied to form the PbS film (220 nm), to fabricate the planar hybrid heterojunction solar cells. For which, overall high VOC was obtained as expected. Linear increase of VOC is observed with Eg in the low value side (Fig. 3a). Which is consistent with the results reported by other groups [35–37]. While it decreases fast in the high value side, probably due to the unsuitable band/energy level alignment. The maximum VOC (0.645 V) was obtained by using the QDs with Eg of 1.79 eV. The trend of VOC loss, i.e. Vdeficit of this planar hybrid type solar cell according to the QD Eg is presented in Fig. 3a as well. Relative high JSC was obtained for devices with low QD Eg, but decreased fast with Eg over 1.6 eV (Fig. 3b). As a combination effect of the parameters, relative high PCE was obtained for the devices using QDs with Eg in the range of 1.2–1.8 eV (Fig. 3d). Which is consistent with the reported results [11]. To summarize, relative high VOC and modest performances were obtained for the planar hybrid heterojunction solar cells based on QDs with different Eg, due to the low FF and JSC. Which we consider could be mainly accounted for by the limited carrier transport in PbS film due to ligand issues. To overcome the problems, PbS QD sample synthesized at 120 °C was chosen as the model to fabricate devices due to its better performance. Thickness of each deposited single PbS layer was much reduced to promote the ligand exchange, by lowering the PbS QDs concentration and increasing the spin speed in deposition. And more cycles of deposition were applied to obtain the desired thickness. For each of the single PbS QD layer, ligand exchange was promoted via prolonged exchanging time and more times of rinse. On the other hand, after ligand treatment, thermal annealing was applied before the last deposition cycle of PbS to eliminate organic residuals. But for the last single layer, no thermal annealing was applied, to avoid film cracks by heating. Fig. 2b shows the TEM and HRTEM lattice fringe image of the adopted PbS QDs. For which average diameter of 3.5 nm is determined. The Eg is calculated to be 1.39 eV according to its first excitonic absorption peak (Fig. S3). The work function of PbS-TBAI is determined to be 4.62 eV according to the cut-off binding energy (16.6 eV) of the PbSTBAI film, as shown in Fig. 2c. Based on which and the energy difference (0.84 eV) between the Femi energy level (Ef) and valence band edge (Ev), the conduction band edge (Ec) and valance band edge (Ev) were determined to be −4.07 eV and −5.46 eV respectively, as illustrated in the energy level alignment diagram (Fig. 2a). Fourier transform infrared (FTIR) measurement was conducted to analyze the surface chemistry of PbS QDs in film after ligand treatment, as shown in Fig. 4. The strong peaks at 2852 cm−1, 2924 cm−1 and 1401 cm−1 and 1516 cm−1 are originated from C-H bending and the asymmetric and symmetric stretching vibrations of -COO- group on QDs. It is clear that decreased intensity of these peaks are observed for the PbS layer fabricated with low concentration and high spin speed, suggesting more effective removal of organics in QD film. Which is
hybrid systems [26]. By utilizing a clever strategy of vertical phase segregation of the active layer, professor Ma et al. demonstrated a record PCE of 5.5% for PbS-PDTPBT hybrid bulk heterojunction solar cell [27] due to improved charge transport and extraction in device, suggesting promising potential for PbS-polymer hybrid heterojunction solar cells. And recently, based on a homojunction-like architecture, where both the p- and n-materials of the junction are composed of PbS QDs treated by different chemical ligands, the state of the art performance of PbS QDs solar cells were hugely improved to over 10% [28–30]. Although huge progresses have been achieved for homojunction PbS QD solar cells, hybrid heterojunction solar cell especially the planar hybrid heterojunction solar cell is still lagging much behind, due to its limited study up to today. We consider that more efforts are needed to study and understand the mechanisms, and it is meaningfull to improve this technology. Since besides the aforementioned advantages, the planar hybrid heterojunction system has specific superiorities i.e. abundant polymer candidates with stable and easy designable optoelectronic properties, more architecture options owing to that both PbS and polymer can act as the p- or n-material in junction according to their energy levels, and opportunity to achieve complementary absorption for solar spectrum. In this study, based on PbS QDs and polymer poly [ [2,6′-4,8-di(5ethylhexylthienyl)benzo[1,2- b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th), parameters of planar hybrid heterojunction solar cell are studied by tuning the QD Eg and modified ligand exchange and fabrication process. Relative high VOC was obtained for the planar hybrid solar cells with QD Eg in the range of 1.3–2.0 eV, indicating a less VOC loss in our device architecture compared with the Schottky [14,17] and bilayer heterojunction [15,22] types. And the best device performance with PCE of 4.43%, JSC of 14.47 mA/cm2, VOC of 0.561 V and FF of 54.6% was demonstrated. Which is over double the performance of previously reported planar hybrid PbS-organic solar cells [15,31–33]. 2. Results and discussion Oleic acid capped PbS QDs were synthesized according to literature [34]. PbS QDs with different band gap (Eg) were obtained by tuning the synthetic temperature from 45 to 185 °C. High crystallinity is confirmed for all the PbS QDs synthesized at different temperature by the XRD measurement, as shown in Fig. S1. Well-defined diffraction peaks were assigned to the (111), (200), (220) and (311) etc. planes respectively. And all diffraction peaks match well with the standard XRD lines of face-centered cubic PbS (JCPDS Card No. 05–0592). Narrowing diffraction peak is recorded with increased synthesis temperature due to the increased particle size, confirmed by the TEM measurements (Fig. S2). Absorption spectra of the samples are presented in Fig. 1. Eg from 0.86 to 2.1 eV was determined according to their corresponding first
Fig. 1. Absorption spectra of PbS QDs synthesized with different temperature 45–185 °C. 2
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Fig. 2. (a) The diagram of energy level alignment of the materials in devices; (b) TEM/HRTEM images of the PbS QDs synthesized at 120 °C; and (c) and (d) the UPS spectrum of the PbS QDs after ligand treatment.
obtained due to the improved carrier transport in PbS film. FF approaching 55% was recorded for the device based on QDs with Eg = 1.39 eV, i.e. improved by 37.5% compared with the control device. Gradually enhanced JSC was obtained when the deposition cycles
further confirmed by the promoted device performance. For devices fabricated with the above adjusted procedure, 5 to 15 deposition cycles were applied to form the PbS film to approach the ideal thickness. As expected, overall improved FF and JSC (Fig. 5) were
Fig. 3. Influence of Eg of PbS QDs on the parameters of planar hybrid heterojunction solar cells, (a) VOC and Vdeficit, (b) JSC, (C) FF, and (d) PCE. 3
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decrease in parallel resistance Rsh (Table S1) due to increased QD surface traps when thicker PbS film applied. Although the performance lags behind, this still implies big improvement space for planar hybrid type solar cells, if a thicker PbS film can be applied and meanwhile the film is further promoted by ideal ligand treatment and good quality of synthesized PbS QDs. Fig. 6 shows the J-V curve of the optimized device, and the corresponding EQE curve. The maximum EQE value over 76% was recorded at certain wavelengths. The integrated photocurrent 13.3 mA/cm2 is consistent with the measured JSC. And the slight mismatch could be due to the unencapsulated situation of the device in air in the EQE test process. 3. Conclusions To summarize, planar hybrid heterojunction PbS-PTB7-Th solar cells were fabricated and studied. Overall relative high VOC (> 0.5 V) was obtained for the devices with QD Eg in the range of 1.3–2.0 eV, due to the suitable architecture and well aligned band/energy levels. The highest VOC of 0.645 V and highest JSC of 15.5 mA/cm2 were recorded. The best device performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was demonstrated, after ligand exchange and fabrication process optimization. Which is over double the performance of previously reported similar planar hybrid PbS-organic solar cells, due to the improved and balanced parameters. This work implies that planar hybrid heterojunction PbS-polymer solar cell has the potential to provide efficient performance, even similar as homojunction type PbS solar cell does, given the thickness and charge transport in PbS film are well optimized.
Fig. 4. FTIR absorption spectra of PbS QD layer before (full rectangle and circle) and after (half full rectangle and cirle) ligand treatment, formed by single deposition cycle with high concentration and low spin speed (black color), or low concentration and high spin speed (red color). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of PbS increase owing to the increased thickness of absorbing layer. Which reaches the maximum value (up to 15.5 mA/cm2) when 11 deposition cycles applied. While decreased JSC was observed when the deposition cycles further increased. Combined with improved VOC (from 0.51 to 0.56 V), the highest performance with PCE of 4.43% (VOC 0.561 V, JSC 14.47 mA/cm2 and FF 54.6%) was obtained when 9 deposition cycles of PbS applied, due to the balanced parameters. Which is more than double the performance of previously reported planar hybrid PbS-organic solar cells. Considering the depletion width [38] of ∼200 nm in PbS at the heterojuntion interface and the charge carrier diffusion length [39] of ∼80 nm, it is believed that a PbS film not thicker than ∼300 nm is reasonable for device. However, compared to which, our optimal device has a much thinner PbS film (∼125 nm) according to the cross-sectional TEM image (Fig. S4). Further increase of the PbS film thickness resulted in lowered performance due to the decreased VOC and FF (Fig. 5). Which could be attributed to the fast
4. Experimental 4.1. Synthesis of PbS QDs PbS quantum dots synthesis was adapted from the literature previously documented34 with a modified recipe. All operations were performed under the standard air-free Schlenk line with nitrogen atmosphere. Lead oxide (PbO, 0.36 g) was dissolved in 15 ml of 1-octadecene (ODE) and 1 ml of oleic acid (OA) at 145 °C with stirring for 3 h under vacuum in a three-neck flask. The solution then was heated to
Fig. 5. Parameters optimization of the planar hybrid heterojunction solar cells. The PbS film has been fabricated with low concentration PbS, high spin speed, adjusted ligand treatment, and different deposition cycles. The 5, 7, 9, 11 and 15-layers corresponds to 72, 101, 125, 150 and 202 nm in thickness for PbS film. 4
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Fig. 6. (a) J-V curve of the optimal planar hybrid heterojunction PbS-PTB7-Th solar cell. (b) The EQE curve of the optimal device and the corresponding integrated photocurrent curve.
45–185 °C. After the solution was reached a desired temperature, the sulphur precursor was injected into the three-neck flask rapidly. Which was prepared by adding 168 μl of hexamethyldisilathiane ((TMS)2S) into 4 ml of ODE. Once the sulphur precursor were injected into the vigorously stirring lead oxide solution at the desired temperature, the heating mantle was turned off immediately to cool the solution to room temperature under the nitrogen atmosphere. Then the PbS QDs were purified by adding acetone/methanol to precipitate and followed by centrifugation twice, and finally dispersed in toluene (∼30 mg/ml).
Science and Technology Plan (No.2018C01047), Major Project of "Science and Technology Innovation 2025" of Ningbo, China.
4.2. Device fabrication
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.02.026. References
The patterned ITO substrates were cleaned with successive sonication in deionized water, acetone, isopropanol orderly and treated with oxygen plasma before spin-coating. A layer of ZnO was firstly deposited onto the indium tin oxide (ITO) substrate at 2000 rpm, and then thermally annealed at 200 °C for 30 min. The ZnO precursor was prepared by dissolving 1 mg of zinc acetate in mixture of 10 ml 2-methoxyethanol and 284 μl monoethanolamine. The PbS QDs film was fabricated via layer-by-layer spin-coating methods in air. For each single PbS layer, 50 μl of oleic-acid-capped PbS QDs in toluene (30 mg/ml or 15 mg/ml) was deposited onto the ZnO substrates with spin speed of 2000 rpm or 6000 rpm for 15s. For ligand-exchange of each single PbS QD layer, the TBAI solution in methanol (10 mg/ml) was applied to PbS QD layer and stayed for 20s or 30 s, and this was followed by twice rinses of methanol. The ligand-exchange process was repeated two times for each layer. For the device fabricated with high concentration PbS, thermal annealing was applied for the whole PbS film at 100 °C for 10 min. For the device fabricated with low concentration PbS, thermal annealing was applied before the last deposition cycle of PbS. The substrates were then stored in air over night and then transferred to a nitrogen-filled glovebox. Finally, a ∼15 nm MoO3 and 100 nm Al were thermally evaporated through a shadow mask. 4.3. Device characterization Current density−voltage (J−V) characteristics of the devices were measured with a computer-controlled Keithley 2400 source meter and Newport 6279 NS solar simulator (450 W) with 100 mW/cm2 illumination. The EQE curves were recorded through a Newport quantum efficiency measurement system (ORIEL IQE 200TM) with a lock-in amplifier in atmosphere. Each wavelength light intensity was calibrated with a standard Si/Ge solar cells. Acknowledgements This research was supported by National Natural Science Foundation of China (No. 51502313), Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-JSC047), and Zhejiang Province 5
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