- Email: [email protected]
Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells
Synthetic Metals 255 (2019) 116112
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells
T
Ram Datta,b, Swati Bishnoia,b, Ramashanker Guptaa,b, D. Haranathc, Shailesh N. Sharmaa, ⁎ Govind Guptaa, Sandeep Aryad, S. Kumare, Vinay Guptaa,e, a
CSIR-National Physical Laboratory, New Delhi, 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India c Department of Physics, National Institute of Technology, Warangal, 506004, Telangana, India d Department of Physics, University of Jammu, Jammu, 180006, J&K, India e Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar Campus, Masdar City, PO Box 54224, Abu Dhabi, UAE b
A R T I C LE I N FO
A B S T R A C T
Keywords: Power conversion efficiency Organic solar cell Cathode buffer layer Energy transfer PCDTBT
Here, we report a luminescent cathode buffer layer (CBL) for power conversion efficiency (PCE) enhancement of organic solar cells (OSCs). ZnO doped with Aluminum (Al) and Europium (Eu) was cast as CBL by a solutionprocessed method. CBL serves a dual purpose by acting both as a spectral conversion and an electron transporting layer. The luminescent ZnO:Al,Eu CBL layer has broad absorption spanning the ultraviolet (300–400 nm) spectrum, contributing to absorption enhancement. Moreover, the emission of CBL layer overlaps with the absorption of poly [N -9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) polymer thus elevating the overall absorption of the active layer and improving the photocurrent. With this ZnO:Al,Eu CBL in the inverted device configuration (ITO/CBL/active layer/MoOx/Al), an enhanced power conversion efficiency (PCE) of 6.9% was obtained while the device with pristine ZnO as CBL showed PCE of 5.9%. A blend of PCDTBT donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor was used as an active layer in both the cases. In ZnO:Al,Eu CBL layer, Al doping improves the conductivity, while Eu doping significantly enhances the emission in the visible region by down-shifting the incoming solar UV light to the visible range which overlaps with the absorption of PCDTBT polymer resulting in energy transfer and improved overall device efficiency. The findings of the study show the significance of luminescent ZnO:Al,Eu nanoparticle CBL in enhancing the performance of organic solar cells.
1. Introduction The overall energy demand of the world has increased tremendously in the last few years, resulting in the consumption of fossil fuels at a much faster rate leading to their depletion. They also contribute significantly to environmental pollution. Therefore, need for alternative energy resources has become the focus of research, one of such is harvesting the everlasting solar energy and had resulted in extensive research and development of various technologies including the organic and inorganic solar cells [1–3]. Among all possible alternatives, organic solar cells have been receiving remarkable research attention due to their low-cost, ease of fabrication by roll-to-roll coating technologies and have rapid energy payback time [4–11]. Many processes have been developed to advance the PCE of OSCs, includes the development of narrow bandgap donor conjugated materials having suitable energy
⁎
levels, solvent additives, interface engineering, thermal/solvent annealing, improved morphology using post-treatments and development of modified device architectures. As a result of this, the PCE of OSCs has crossed > 14% [12,13,22–25,14–21]. In OSCs, the inverted architecture with certain advantages over the conventional one is attaining more attention owing to long-term stability on-air exposure, also reduced ITO etching by the acidic PEDOT:PSS interfacial layer [26–31]. In inverted OSCs, the properties of the cathode buffer layer (CBL) like optical & electrical properties, the condition of interfacing between the photoactive layer & CBL, etc., has a strong influence on the overall device performance. Among all the commonly used CBL materials zinc oxide (ZnO) is one of the potential choice owing to its high electron mobility, suitable energy levels, transparency in the visible spectrum and cost-effective with excellent environmental stability [8,32–38]. To improve the PCE of inverted OSCs, various approaches have been used
Corresponding author at: Khalifa University of Science and Technology, Masdar Campus, Masdar City, PO Box 54224, Abu Dhabi, UAE. E-mail address: [email protected] (V. Gupta).
https://doi.org/10.1016/j.synthmet.2019.116112 Received 18 March 2019; Received in revised form 24 June 2019; Accepted 9 July 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
Fig. 1. (a) PL emission spectrum of Eu doped ZnO with varying Eu concentration. Inset shows the excitation corresponding to 611 nm emission. (b) PL emission corresponding to varying Al concentration in ZnO host. Inset shows the corresponding excitation spectra. (c) PL emission spectrum of optimized ZnO:Al (0.01),Eu(0.05) sample under 370 nm excitation and emission of pristine ZnO. (d) UV-VIS absorption spectra of ZnO:Al,Eu and pristine ZnO. Inset shows the transmission of bare ITO and ZnO;Al,Eu.
article, we report the enhanced efficiency by using a luminescent as well as conductive ZnO based CBL doped with III group and downshifting lanthanide ions such as Aluminium (Al) and Europium (Eu) respectively.
till date to modify the ZnO based CBL layer such as enhancing its conductivity by suitably doping various III group dopant such as Al, Ga, In, etc [39–43]. or various I and II group elements. Trivalent Al is a widely explored dopant for modification of ZnO layer in OSC, which improves the conductivity of the ZnO layer. Most of the reports on Aldoped ZnO as CBL in OSC are based on techniques like Pulsed Laser deposition (PLD), sputtering, chemical vapor depositions, spray pyrolysis, electron beam evaporation etc. [44–47], which requires vacuum processing and thus increase the overall fabrication cost. Low-cost alternatives for deposition of Al-doped ZnO layers are solution-based deposition techniques based on sol-gel techniques, wet chemical routes, and dispersed nanoparticle solutions. [48–50]. Besides, III group elements, rare earth ions are also one of the potential dopants that offer numerous advantages owing to their high conductivity, luminescent behavior, enhanced magnetic properties, etc. Additionally, rare earth ions possess 4f electrons and have high stability in the trivalent state. The excited states of rare earth ions exhibit longer lifetime thus provide sufficient time for electron transfer and thus can improve the efficiency of the solar cell. Recently people have used rare earth ions like Yb, Eu, sm as dopants in ZnO for CBL fabrication [49–52]. Rare earth doped ZnO layers serve dual purposes when employed as CBL firstly they act as an efficient electron transport layer, and secondly, they also act as spectral converting layers and that too with an easier fabrication process. The two advantages of rare-earth-doped ZnO as CBL are; (a) to improve the absorption by spectral conversion through energy transfer, (b) to ensure efficient electron transport properties. Although significant efforts have been made in the past in order to enhance the conductivity as well to improve the absorption properties of ZnO CBL by spectral conversion (using rare earth ions as dopants) [51–54], but there exists no reports on employing luminescent (spectral converting) and simultaneously conductive Al, Eu co-doped ZnO CBL in the inverted device configuration. Al doping contributes to improving the conductivity of ZnO CBL and hence increase charge transport due to lesser recombination centers, whereas rare-earth ion Eu doping significantly enhances the down-shifting luminescence property. In this
2. Materials and methods 2.1. Materials The soluble salts of Zinc nitrate, Aluminum nitrate, and Europium nitrate were procured from Merck (Darmstadt, Germany) for nanoparticle synthesis. The PCDTBT polymer and PC71BM were purchased from 1-materials (Canada). 2.2. Synthesis of ZnO:Al, Eu by combined co-precipitation and hydrothermal method The stoichiometric ratios of Zinc acetate dihydrate (ZAD) [Zn (CH3COO)2H2O] (Aldrich, 99.9%), Aluminum Nitrate (Al(NO3)3 and Europium Nitrate (Eu(NO3)3 ·xH2O) were dissolved in the aqueous medium. Under continuous stirring and mild heating, an alkaline mixture of NaOH: KOH was added dropwise. At pH (˜ 8) a precipitate is formed, immediately 1 ml ethanolamine was added as sol stabilizer to restrict the particle growth. The resultant precipitate was filtered and transferred to Teflon lined stainless steel autoclave. The sealed autoclave was kept in a furnace, maintained at 100 °C for 12 h. The resultant product was allowed to cool naturally to room temperature (˜25 °C). The final product was washed with DI water, and ethanol sequentially to remove all traces of surface-bound impurities. Later on, the solid mixture was dried in an oven at 60 °C overnight, and then finally crushed to a fine powder. The resultant product was further annealed in the furnace to improve crystallinity. Afterward, the obtained ZnO:Al,Eu nanoparticles were ultrasonicated before further characterization and experimentation. For reference bulk ZnO:Al,Eu phosphor material was synthesized using convention solid-state reaction method using oxides 2
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
(20Ω/cm2) at 3000 rpm. After that, the casted film ZnO:Al,Eu and ZnO were annealed for 1 h at 200 °C in the air. The resultant thickness of ZnO films get ˜30 nm, determined by an Ambios XP-100 stylus profilometer. The blend of PCDTBT: PC71BM (1:4 w/w%, 35 mgml-1) prepared in Dichlorobenzene (DCB): Chlorobenzene (CB) (1:4 v/v %) with 0.4 v/v% 1,8-Diiodooctane (DIO), stirred overnight in glovebox. Before spin-casted, blend stirred for 30 min at 60 °C. The final photoactive layers (˜120 nm) were spin-casted in an N2 filled glove box from the prepared blend and annealed for 15 min at 80 °C to evaporate the residual solvent. Finally, the hole transport layer, MoOx (˜10 nm) and electrode Ag (˜100 nm) were evaporated sequentially under ˜10−6 Torr vacuum, by a thermal deposition method, using shadow aluminum mask to form an active area of ˜ 4.5 mm2. The Keithley 2600 source meter used to measure photovoltaic parameters (current density-voltage, J–V curve) and CEP-25ML spectral response measurement system used to record external quantum efficiency (EQE) spectra of fabricated devices. The solar simulator calibrated using a certified reference solar cell to an intensity of 100 mWcm-2 to ensure AM 1.5 G spectral emission during J–V measurements. 3. Results and discussion The photoluminescence (PL) emission spectra of Eu doped ZnO (with varying Eu concentration) under UV excitation (Eex = 395 nm) is shown in Fig. 1(a). The PL emission spectrum of Eu doped ZnO exhibits prominent peaks at 590 nm and 614 nm corresponding to 5 D0→7F1 and 5 D0→7F2 transitions of Eu3+. The emission peak at 590 nm originates from the magnetic dipole allowed 5D0 → 7F1 transition, and the peak at 614 nm is due to the electric-dipole allowed 5D0 → 7F2 transition. The emission peak intensity corresponding to the 5D0 → 7F2 transition is higher than that of 5D0 → 7F1 transition, suggesting that Eu3+ ions mainly occupy a site with inversion anti-symmetry in the ZnO host. The inset of Fig. 1(a) shows the PL excitation spectra of Eu doped ZnO registered at 614 nm emission, indicating major excitation peaks at 397 and 466 nm respectively. The excitation peaks at 397 nm and 466 nm corresponds to the 7F0 → 5L6 and 7F0 → 5D2 transitions of Eu3+ ions. The emission intensity increases with the increasing concentration of dopant Eu3+ up to 5 mol%; after that, concentration quenching occurs; hence, Eu concentration was fixed at 5 mol%. Fig. 1(b) shows the PL emission spectrum of Al-doped ZnO recorded at an excitation of 370 nm. According to the spectrum, as the concentration of Al increases, the defect-related broad luminescence of ZnO quenches (Fig. 1(b)). Al doping results in the formation of shallow donor levels below the conduction band of ZnO and electron recombination corresponding to Al shallow donor levels and ZnO based defect level leads to green emission [55,56]. The concentration of dopant Al was kept low (1%), as high Al doping quenches the defect emission in ZnO [57] and further leads to the formation of Zinc aluminate phase which is insulating. The inset of Fig. 1(b) shows the excitation graph of Al-doped ZnO recorded at an emission of 520 nm. The photoluminescence (PL) emission spectrum of optimized ZnO: Al3+,Eu3+ nanoparticles, as well as the pristine ZnO under 370 nm excitation is shown in Fig. 1(c). The emission graph reveals that the optimized ZnO: Al3+,Eu3+ sample exhibit a broad emission ranging from (450–700 nm) with a sharp peak at 590 and 614 nm, attributed to the f-f transition of Eu3+. Hence, the optimized concentration ZnO: Al (1 mol%), Eu (5 mol%) [abbreviated as ZnO:Al,Eu] was used for further experiments and named as ZnO:Al;Eu. Fig. 1(d) shows UV-VIS absorption spectra of ZnO:Al,Eu and pristine ZnO. The inset shows the transmission of bare ITO and ZnO:Al,Eu.Pristine ZnO shows negligible absorption in the visible region as compared to the ZnO:Al,Eu sample. The UV absorption in ZnO:Al, Eu sample is due to the UV absorption capability of Eu3+ ions. Hence Eu3+ ions in ZnO results in enhanced UV absorption in comparsion to pristine ZnO. The X-ray diffraction (XRD) spectra of the bulk and ZnO:Al,Eu nanoparticles are shown in Fig. 2(a). The XRD peaks match well with the
Fig. 2. Morphology of ZnO: Al, Eu nanoparticles (a) XRD spectra (b) TEM micrograph (c) EDAX spectra showing the elemental composition.
as precursors. For the synthesis of pristine ZnO (for reference device), the 0.1 M concentration of Zinc acetate dihydrate [Zn(CH3COO)2H2O] (Aldrich, 99.9%) prepared in anhydrous ethanol [CH3CH2OH] (99.5 + % Aldrich), and stirred at 80 °C for 2–3 h. Later on, the ethanolamine as sol stabilizer was added to the solution, followed by magnetic stirred at 60 °C for 12–15 h. 2.3. Device fabrication Before fabricating the devices, the ITO glass substrates need to be clean from dust, oil, and grease which get from environment and hands while handling, the process takes few steps to clean. Substrates rubbed in deionized (DI) water with soap solution, subsequently ultra-sonicate in acetone and isopropanol for 15 min each, followed by drying in an oven at 120 °C for 1 h. The UV-O3 treatment of substrate performed for 15 min before used to device application. The prepared colloidal solution of ZnO:Al,Eu and ZnO reference were spin-casted on ITO substrates 3
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
Fig. 3. (a) Resistivity vs. Temperature plot of ZnO and ZnO: Al, Eu nanoparticles (b) TRPL curve of ZnO:Al, Eu sample. Inset shows the decay of pristine ZnO. (c) UV–vis absorption spectra of PCDTBT film with ZnO and ZnO:Al,Eu. (b) PL emission of PCDTBT film with pristine ZnO and ZnO:Al,Eu. Fig. 4. (a) Chemical structures of PCDTBT and PC71BM, (b) Schematic diagram of fabricated device structure showing the mechanism of enhanced absorption by the active layer in presence of luminescent CBL, (c) PCE vs dopant concentration (d) J–V curve for PCDTBT/ PC71BM BHJ solar cell with ZnO and ZnO:Al,Eu as CBL, in dark and simulated AM 1.5 G irradiation (100 mWcm−2) (e) EQE spectra of PCDTBT/PC71BM BHJ solar cell under optimized conditions.
The effect of doping of Al and Eu on the electrical resistivity as a function of temperature is shown in Fig. 3a.The optimized ZnO:Al,Eu sample unveiled lower electrical resistivity as compared to pristine ZnO nanoparticles in the entire temperature range, because in ZnO:Al,Eu the trivalent Al3+ ions are substituted at Zn2+ sites, which liberates more free electrons, thus reducing the resultant electrical resistivity. The
standard (JCPDS-36-1451) of wurtzite ZnO, indicating that no secondary phase formation has occurred. High-resolution transmission electron microscopy (HRTEM) images revealed the size of ZnO:Al,Eu nanoparticles in the range < 20 nm (Fig. 2(b)).The EDAX spectrum furthermore confirmed the presence of dopants Al and Eu in the synthesized nanophosphor (Fig. 2(c)).
4
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
room temperature resistivity of pristine ZnO is 1.03 × 10−3 ohm-m, whereas it is 4.38 × 10-4 ohm-m for ZnO:Al,Eu nanoparticles. Fig. 3(b) shows the Time – resolve PL (TRPL) spectra of ZnO:Al, Eu and pristine ZnO samples. The decay-time of ZnO:Al,Eu is significantly higher as compared to pristine ZnO sample indicating the better luminescence behavior mediated by dopants. When the emission of the donor molecule overlap with the absorption of the acceptor molecule, FRET occurs [58], which is one of the fundamental principles of molecular spectroscopy. In our case the PL emission spectrum of ZnO:Al,Eu nanoparticles (Fig. 1c) (Donor) has significant overlapping with the UV–vis absorption spectrum of PCDTBT (acceptor), which enhances the charge transfer probability between the two and thus leads to a substantial increase in the absorption of PCDTBT after the addition of ZnO:Al,Eu nanoparticles (Fig. 3(c)). Consequently, the PL emission of the PCDTBT also enhances on the addition of ZnO:Al, Eu NPs as shown in Fig. 3(d) indicating that with the conjugation of ZnO:Al,Eu layer the overall absorption of the device enhances which results in the generation of more excitons and hence the resulting PL emission of PCDTBT also increases. This enhancement in the absorption and emission of PCDTBT is probably due to energy transfer from ZnO:Al,Eu layer (donor) to PCDTBT(acceptor). The energy transfer is a consequence of the overlap of emission spectrum of ZnO;Al, Eu CBL with the absorption spectrum of PCDTBT:PC71BM and is a well established phenomenon [58]. Fig. 4(a) shows the chemical structure of PCDTBT and PC71BM. Fig. 4(b) shows the schematic diagram of the device structure with the underlying spectral conversion mechanism between the CBL layer and the active layer. Fig. 4(c) shows the optimized PCE of inverted BHJ OSCs, of interfacial layers i.e. ZnO and ZnO:Al,Eu with different Al and Eu concentration under AM1.5 G irradiation. The device with reference ZnO CBL has shown the highest PCE of 5.9% (average 5.7%) addition of Al in ZnO CBL lead to a slight increase in the PCE to 6.1% (average 5.0%) for Al(0.01) and further doping of Al lead to a decrease in the PCE. Doping of Al increases the conductivity, and slightly improve the current density, but higher concentration may lead to current leakage and recombination. For the ZnO:Al(0.01), the optimized PCE is obtained for Eu (0.05) of 6.9% (average 6.8%). Fig. 4d shows the J–V characteristics of inverted BHJ OSCs, with different optimized interfacial layers i.e. ZnO and ZnO:Al,Eu. The reference ZnO CBL device shows open-circuit voltage (Voc) of 0.871 V, short-circuit current density (Jsc) of 11.0 mAcm-2, fill factor (FF) of 61.7%, and PCE of 5.9%. The series resistance (Rs) and shunt resistance (Rsh) shown the value of 3.92 Ω and 1.11 kΩ, respectively. Whereas, in the case of ZnO:Al,Eu CBL the photovoltaic parameters Jsc, Voc, FF, and PCE are enhanced simultaneously to 0.879 V, 66.7%, 11.85 mAcm-2, and 6.9%, respectively. The Rs decreased to 2.39 Ω and Rsh increased to 1.53 kΩ. The external quantum efficiency (EQE) of both reference ZnO and ZnO:Al,Eu CBL based devices are shown in Fig. 4e which shows a significant enhancement of EQE in the 600-700 nm wavelength. The ZnO based device shows a maximum EQE value of 66%, whereas the enhanced reaches 76% for our ZnO:Al,Eu based device. The enhancement of EQE attributed to the increase in current density, which is resulting due to enhanced absorption which generates more charge carriers. The value of Jsc calculated from J–V curve and integration of the EQE spectrum have nearly similar value (ca. 2% error).
functional CBL layer is easy to fabricate without changing the underlying geometry of the device. Dopant Al improves the conductivity of the layer, whereas Eu doping contributes to absorption enhancement by spectral conversion (UV to visible). The spectral overlapping between the emission of ZnO:Al,Eu CBL layer and PCDTBT (active layer) lead to an energy transfer from ZnO:Al,Eu to PCDTBT, resulting in an enhanced short circuit current with overall improved device efficiency. We observed an enhanced PCE of 6.9% for the device with ZnO:Al,Eu CBL layer as compared to 5.9% for the reference ZnO CBL based device. The findings of the study highlight the role of luminescent CBL in improving the device characteristics of OSC. Acknowledgments The authors (RD) thanks UGC India for UGC-SRF Ph.D. fellowship. The author (SB) sincerely acknowledge the Council of Scientific & Industrial Research (CSIR)- Research Associate fellowship (#31/ 1(0494)/2018-EMR-1, India), and V.G. thanks R. Sharma, KAIST, S. Korea for help in the device characterization. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595–6663, https://doi.org/10.1021/cr900356p. [2] A. Nozik, Quantum dot solar cells, Phys. E Low-Dimens. Syst. Nanostruct. 14 (2002) 115–120, https://doi.org/10.1016/S1386-9477(02)00374-0. [3] G.A. Chamberlain, Organic solar cells: a review G, Sol Cells 8 (1983) 47–83, https://doi.org/10.1016/0379-6787(83)90039-X. [4] F. He, L. Yu, How far can polymer solar cells go? In need of a synergistic approach, J. Phys. Chem. Lett. 2 (2011) 3102–3113, https://doi.org/10.1021/jz201479b. [5] Y. Cheng, S. Yang, C. Hsu, Synthesis of conjugated polymers for organic solar cell applications, Chem. Rev. 109 (2009) 5868–5923, https://doi.org/10.1021/ cr900182s. [6] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, et al., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868, https://doi.org/10.1038/nmat1500. [7] S.K. Hau, H.-L. Yip, Jen AK-Y, A review on the development of the inverted polymer solar cell architecture, Polym. Rev. 50 (2010) 474–510, https://doi.org/10.1080/ 15583724.2010.515764. [8] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, The role of buffer layers in polymer solar cells, Energy Environ. Sci. 4 (2011) 285–310, https://doi.org/10.1039/ c0ee00273a. [9] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photonics 6 (2012) 153–161, https://doi.org/10.1038/nphoton.2012.11. [10] R.F. Service, Outlook brightens for plastic solar cells, Science (80-) 332 (2011), https://doi.org/10.1126/science.332.6027.293 293–293. [11] O. Oklobia, T.S. Shafai, A quantitative study of the formation of PCBM clusters upon thermal annealing of P3HT/PCBM bulk heterojunction solar cell, Sol. Energy Mater. Sol. Cells 117 (2013) 1–8, https://doi.org/10.1016/j.solmat.2013.05.011. [12] M.P. de Jong, L.J. van IJzendoorn, M.J.A. de Voigt, Stability of the interface between indium-tin-oxide and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) in polymer light-emitting diodes, Appl. Phys. Lett. 77 (2000) 2255–2257, https://doi.org/10.1063/1.1315344. [13] H.J. Son, L. Lu, W. Chen, T. Xu, T. Zheng, B. Carsten, et al., Synthesis and photovoltaic effect in dithieno[2,3- d :2′,3′- d ′]Benzo[1,2- b :4,5- b ′]dithiophene-based conjugated polymers, Adv. Mater. 25 (2013) 838–843, https://doi.org/10.1002/ adma.201204238. [14] T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, S. Wakim, et al., Bulk heterojunction solar cells using thieno[3,4- c]pyrrole-4,6-dione and dithieno[3,2- b :2′,3′- d]silole copolymer with a power conversion efficiency of 7.3%, J. Am. Chem. Soc. 133 (2011) 4250–4253, https://doi.org/10.1021/ja200314m. [15] S.C. Price, A.C. Stuart, L. Yang, H. Zhou, W. You, Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer−fullerene solar cells, J. Am. Chem. Soc. 133 (2011) 4625–4631, https://doi.org/10.1021/ja1112595. [16] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, et al., Efficient tandem polymer solar cells fabricated by all-solution processing, Science (80-) 317 (2007) 222–225, https://doi.org/10.1126/science.1141711. [17] J.H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, et al., Improved highefficiency organic solar cells via incorporation of a conjugated polyelectrolyte interlayer, J. Am. Chem. Soc. 133 (2011) 8416–8419, https://doi.org/10.1021/ ja2037673. [18] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, et al., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Adv. Mater. 23 (2011) 4636–4643, https://doi.org/10.1002/ adma.201103006. [19] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, et al., Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 6 (2007) 497–500, https://doi.org/10.1038/nmat1928. [20] X. Liu, W. Wen, G.C. Bazan, Post-deposition treatment of an arylated-carbazole
4. Conclusion In this article, we have demonstrated the synthesis and characterization of an efficient ZnO:Al,Eu based cathode buffer layer with enhanced luminescence and conductivity, for the fabrication of an inverted BHJ solar cell. The ZnO:Al,Eu CBL with improved conductivity and luminescence plays a dual role in improving the device efficiency, firstly by acting as an electron transport layer and secondly as a spectral conversion layer. In comparison with the conventional spectral conversion layer deposited either above or below the device, this dual 5
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40] M. Koppe, H.-J. Egelhaaf, G. Dennler, M.C. Scharber, C.J. Brabec, P. Schilinsky, et al., Near IR sensitization of organic bulk heterojunction solar cells: towards optimization of the spectral response of organic solar cells, Adv. Funct. Mater. 20 (2010) 338–346, https://doi.org/10.1002/adfm.200901473. [41] Y. Yang, W. Chen, L. Dou, W.-H. Chang, H.-S. Duan, B. Bob, et al., High-performance multiple-donor bulk heterojunction solar cells, Nat. Photonics 9 (2015) 190–198, https://doi.org/10.1038/nphoton.2015.9. [42] L. Lu, T. Xu, W. Chen, E.S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency, Nat. Photonics 8 (2014) 716–722, https:// doi.org/10.1038/nphoton.2014.172. [43] Z. Yin, Q. Zheng, S.-C. Chen, D. Cai, L. Zhou, J. Zhang, Bandgap tunable Zn 1- x Mg x O thin films as highly transparent cathode buffer layers for high-performance inverted polymer solar cells, Adv. Energy Mater. 4 (2014) 1301404, , https://doi. org/10.1002/aenm.201301404. [44] B.J. Lokhande, P.S. Patil, M.D. Uplane, Deposition of highly oriented ZnO films by spray pyrolysis and their structural, optical and electrical characterization, Mater. Lett. 57 (2002) 573–579. [45] N. Wan, T. Wang, H. Sun, G. Chen, L. Geng, X. Gan, et al., Indium tin oxide thin films for silicon-based electro-luminescence devices prepared by electron beam evaporation method, J. Non. Solids 356 (2010) 911–916, https://doi.org/10.1016/ j.jnoncrysol.2009.12.026. [46] R. Ondo-Ndong, F. Pascal-Delannoy, A. Boyer, A. Giani, A. Foucaran, Structural properties of zinc oxide thin films prepared by r.f. Magnetron sputtering, Mater. Sci. Eng. B 97 (2003) 68–73, https://doi.org/10.1016/S0921-5107(02)00406-3. [47] C. Guillén, J. Herrero, Structure, optical and electrical properties of Al:ZnO thin films deposited by DC sputtering at room temperature on glass and plastic substrates, Phys. Status Solidi 206 (2009) 1531–1536, https://doi.org/10.1002/pssa. 200925061. [48] N. Al-Dahoudi, M.A. Aegerter, Comparative study of transparent conductive In2O3:Sn (ITO) coatings made using a sol and a nanoparticle suspension, Thin Solid Films 502 (2006) 193–197, https://doi.org/10.1016/j.tsf.2005.07.273. [49] J. Ederth, P. Heszler, A. Hultåker, G. Niklasson, C. Granqvist, Indium tin oxide films made from nanoparticles: models for the optical and electrical properties, Thin Solid Films 445 (2003) 199–206, https://doi.org/10.1016/S0040-6090(03) 01164-7. [50] I. Maksimenko, P.J. Wellmann, Low-temperature processing of transparent conductive indium tin oxide nanocomposites using polyvinyl derivatives, Thin Solid Films 520 (2011) 1341–1347, https://doi.org/10.1016/j.tsf.2011.04.142. [51] T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov, C.J. Brabec, Inverted organic solar cells using a solution processed aluminum-doped zinc oxide buffer layer, Org. Electron. 12 (2011) 1539–1543, https://doi.org/10.1016/j.orgel.2011.05.027. [52] H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, C.J. Brabec, Comparison of various sol–gel derived metal oxide layers for inverted organic solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 2194–2199, https://doi.org/10.1016/j.solmat.2011.03. 023. [53] J. Alstrup, M. Jørgensen, A.J. Medford, F.C. Krebs, Ultra fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating, ACS Appl. Mater. Interfaces 2 (2010) 2819–2827, https://doi.org/10. 1021/am100505e. [54] S. Trost, K. Zilberberg, A. Behrendt, A. Polywka, P. Görrn, P. Reckers, et al., Overcoming the “Light-Soaking” issue in inverted organic solar cells by the use of Al:ZnO electron extraction layers, Adv. Energy Mater. 3 (2013) 1437–1444, https:// doi.org/10.1002/aenm.201300402. [55] O. Pachoumi, C. Li, Y. Vaynzof, K.K. Banger, H. Sirringhaus, Improved performance and stability of inverted organic solar cells with sol-gel processed, amorphous mixed metal oxide electron extraction layers comprising alkaline earth metals, Adv. Energy Mater. 3 (2013) 1428–1436, https://doi.org/10.1002/aenm.201300308. [56] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films, Appl. Phys. Lett. 78 (2001) 2285–2287, https://doi.org/10.1063/1.1361288. [57] B.K. Sharma, N. Khare, Stress-dependent band gap shift and quenching of defects in Al-doped ZnO films, J. Phys. D Appl. Phys. 43 (2010) 465402, , https://doi.org/10. 1088/0022-3727/43/46/465402. [58] T. Forster, Naturwissenschaften, Energiewanderung und Fluoreszenz 33 (1946) 166, https://doi.org/10.1007/BF00585226.
conjugated polymer for solar cell fabrication, Adv. Mater. 24 (2012) 4505–4510, https://doi.org/10.1002/adma.201201567. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, et al., A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446, https://doi.org/10.1038/ncomms2411. L. Ye, S. Zhang, W. Zhao, H. Yao, J. Hou, Highly efficient 2D-conjugated benzodithiophene-based photovoltaic polymer with linear alkylthio side chain, Chem. Mater. 26 (2014) 3603–3605, https://doi.org/10.1021/cm501513n. Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, et al., Single-junction polymer solar cells with high efficiency and photovoltage, Nat. Photonics 9 (2015) 174–179, https://doi.org/10.1038/nphoton.2015.6. Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, et al., Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells, Nat. Commun. 5 (2014) 1–8, https://doi.org/10.1038/ncomms6293. A. Aghassi, C.D. Fay, Effects of IPA treatment on the photovoltaic performance of bulk heterojunction organic solar cells, J. Phys. Chem. Solids 130 (2019) 136–143, https://doi.org/10.1016/j.jpcs.2019.02.025. B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, et al., Solution-processed organic solar cells based on dialkylthiol-substituted benzodithiophene unit with efficiency near 10%, J. Am. Chem. Soc. 136 (2014) 15529–15532, https://doi.org/10.1021/ ja509703k. K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant, Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol. Cells 90 (2006) 3520–3530, https://doi.org/10.1016/j.solmat.2006.06.041. K.W. Wong, H.L. Yip, Y. Luo, K.Y. Wong, W.M. Lau, K.H. Low, et al., Blocking reactions between indium-tin oxide and poly (3,4-ethylene dioxythiophene):poly (styrene sulphonate) with a self-assembly monolayer, Appl. Phys. Lett. 80 (2002) 2788–2790, https://doi.org/10.1063/1.1469220. Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer, Adv. Mater. 23 (2011) 1679–1683, https://doi.org/10.1002/adma. 201004301. H. Cao, W. He, Y. Mao, X. Lin, K. Ishikawa, J.H. Dickerson, et al., Recent progress in degradation and stabilization of organic solar cells, J. Power Sources 264 (2014) 168–183, https://doi.org/10.1016/j.jpowsour.2014.04.080. T. Yang, D. Qin, L. Lan, W. Huang, X. Gong, J. Peng, et al., Inverted polymer solar cells with a solution-processed zinc oxide thin film as an electron collection layer, Sci. China Chem. 55 (2012) 755–759, https://doi.org/10.1007/s11426-0124512-2. M. Jørgensen, K. Norrman, S.A. Gevorgyan, T. Tromholt, B. Andreasen, F.C. Krebs, Stability of polymer solar cells, Adv. Mater. 24 (2012) 580–612, https://doi.org/10. 1002/adma.201104187. A.K.K. Kyaw, X.W. Sun, C.Y. Jiang, G.Q. Lo, D.W. Zhao, D.L. Kwong, An inverted organic solar cell employing a sol-gel derived ZnO electron selective layer and thermal evaporated MoO3 hole selective layer, Appl. Phys. Lett. 93 (2008) 221107, , https://doi.org/10.1063/1.3039076. Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, et al., ZnO nanowire growth and devices, Mater. Sci. Eng. R Rep. 47 (2004) 1–47, https://doi.org/10. 1016/j.mser.2004.09.001. K. Ellmer, Resistivity of polycrystalline zinc oxide films: current status and physical limit, J. Phys. D Appl. Phys. 34 (2001) 3097–3108, https://doi.org/10.1088/00223727/34/21/301. Z. Liang, Q. Zhang, L. Jiang, G. Cao, ZnO cathode buffer layers for inverted polymer solar cells, Energy Environ. Sci. 8 (2015) 3442–3476, https://doi.org/10.1039/ C5EE02510A. C.H. Luong, S. Kim, S. Surabhi, T.S. Vo, K.-M. Lee, S.-G. Yoon, et al., Fabrication of undoped ZnO thin film via photosensitive sol–gel method and its applications for an electron transport layer of organic solar cells, Appl. Surf. Sci. 351 (2015) 487–491, https://doi.org/10.1016/j.apsusc.2015.05.170. D. Lee, T. Kang, Y.-Y. Choi, S.-G. Oh, Preparation of electron buffer layer with crystalline ZnO nanoparticles in inverted organic photovoltaic cells, J. Phys. Chem. Solids 105 (2017) 66–71, https://doi.org/10.1016/j.jpcs.2017.02.008. Y.-C. Chen, C.-Y. Hsu, Lin RY-Y, K.-C. Ho, J.T. Lin, Materials for the active layer of organic photovoltaics: ternary solar cell approach, ChemSusChem 6 (2013) 20–35, https://doi.org/10.1002/cssc.201200609.
6
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells
T
Ram Datta,b, Swati Bishnoia,b, Ramashanker Guptaa,b, D. Haranathc, Shailesh N. Sharmaa, ⁎ Govind Guptaa, Sandeep Aryad, S. Kumare, Vinay Guptaa,e, a
CSIR-National Physical Laboratory, New Delhi, 110012, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India c Department of Physics, National Institute of Technology, Warangal, 506004, Telangana, India d Department of Physics, University of Jammu, Jammu, 180006, J&K, India e Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar Campus, Masdar City, PO Box 54224, Abu Dhabi, UAE b
A R T I C LE I N FO
A B S T R A C T
Keywords: Power conversion efficiency Organic solar cell Cathode buffer layer Energy transfer PCDTBT
Here, we report a luminescent cathode buffer layer (CBL) for power conversion efficiency (PCE) enhancement of organic solar cells (OSCs). ZnO doped with Aluminum (Al) and Europium (Eu) was cast as CBL by a solutionprocessed method. CBL serves a dual purpose by acting both as a spectral conversion and an electron transporting layer. The luminescent ZnO:Al,Eu CBL layer has broad absorption spanning the ultraviolet (300–400 nm) spectrum, contributing to absorption enhancement. Moreover, the emission of CBL layer overlaps with the absorption of poly [N -9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) polymer thus elevating the overall absorption of the active layer and improving the photocurrent. With this ZnO:Al,Eu CBL in the inverted device configuration (ITO/CBL/active layer/MoOx/Al), an enhanced power conversion efficiency (PCE) of 6.9% was obtained while the device with pristine ZnO as CBL showed PCE of 5.9%. A blend of PCDTBT donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor was used as an active layer in both the cases. In ZnO:Al,Eu CBL layer, Al doping improves the conductivity, while Eu doping significantly enhances the emission in the visible region by down-shifting the incoming solar UV light to the visible range which overlaps with the absorption of PCDTBT polymer resulting in energy transfer and improved overall device efficiency. The findings of the study show the significance of luminescent ZnO:Al,Eu nanoparticle CBL in enhancing the performance of organic solar cells.
1. Introduction The overall energy demand of the world has increased tremendously in the last few years, resulting in the consumption of fossil fuels at a much faster rate leading to their depletion. They also contribute significantly to environmental pollution. Therefore, need for alternative energy resources has become the focus of research, one of such is harvesting the everlasting solar energy and had resulted in extensive research and development of various technologies including the organic and inorganic solar cells [1–3]. Among all possible alternatives, organic solar cells have been receiving remarkable research attention due to their low-cost, ease of fabrication by roll-to-roll coating technologies and have rapid energy payback time [4–11]. Many processes have been developed to advance the PCE of OSCs, includes the development of narrow bandgap donor conjugated materials having suitable energy
⁎
levels, solvent additives, interface engineering, thermal/solvent annealing, improved morphology using post-treatments and development of modified device architectures. As a result of this, the PCE of OSCs has crossed > 14% [12,13,22–25,14–21]. In OSCs, the inverted architecture with certain advantages over the conventional one is attaining more attention owing to long-term stability on-air exposure, also reduced ITO etching by the acidic PEDOT:PSS interfacial layer [26–31]. In inverted OSCs, the properties of the cathode buffer layer (CBL) like optical & electrical properties, the condition of interfacing between the photoactive layer & CBL, etc., has a strong influence on the overall device performance. Among all the commonly used CBL materials zinc oxide (ZnO) is one of the potential choice owing to its high electron mobility, suitable energy levels, transparency in the visible spectrum and cost-effective with excellent environmental stability [8,32–38]. To improve the PCE of inverted OSCs, various approaches have been used
Corresponding author at: Khalifa University of Science and Technology, Masdar Campus, Masdar City, PO Box 54224, Abu Dhabi, UAE. E-mail address: [email protected] (V. Gupta).
https://doi.org/10.1016/j.synthmet.2019.116112 Received 18 March 2019; Received in revised form 24 June 2019; Accepted 9 July 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
Fig. 1. (a) PL emission spectrum of Eu doped ZnO with varying Eu concentration. Inset shows the excitation corresponding to 611 nm emission. (b) PL emission corresponding to varying Al concentration in ZnO host. Inset shows the corresponding excitation spectra. (c) PL emission spectrum of optimized ZnO:Al (0.01),Eu(0.05) sample under 370 nm excitation and emission of pristine ZnO. (d) UV-VIS absorption spectra of ZnO:Al,Eu and pristine ZnO. Inset shows the transmission of bare ITO and ZnO;Al,Eu.
article, we report the enhanced efficiency by using a luminescent as well as conductive ZnO based CBL doped with III group and downshifting lanthanide ions such as Aluminium (Al) and Europium (Eu) respectively.
till date to modify the ZnO based CBL layer such as enhancing its conductivity by suitably doping various III group dopant such as Al, Ga, In, etc [39–43]. or various I and II group elements. Trivalent Al is a widely explored dopant for modification of ZnO layer in OSC, which improves the conductivity of the ZnO layer. Most of the reports on Aldoped ZnO as CBL in OSC are based on techniques like Pulsed Laser deposition (PLD), sputtering, chemical vapor depositions, spray pyrolysis, electron beam evaporation etc. [44–47], which requires vacuum processing and thus increase the overall fabrication cost. Low-cost alternatives for deposition of Al-doped ZnO layers are solution-based deposition techniques based on sol-gel techniques, wet chemical routes, and dispersed nanoparticle solutions. [48–50]. Besides, III group elements, rare earth ions are also one of the potential dopants that offer numerous advantages owing to their high conductivity, luminescent behavior, enhanced magnetic properties, etc. Additionally, rare earth ions possess 4f electrons and have high stability in the trivalent state. The excited states of rare earth ions exhibit longer lifetime thus provide sufficient time for electron transfer and thus can improve the efficiency of the solar cell. Recently people have used rare earth ions like Yb, Eu, sm as dopants in ZnO for CBL fabrication [49–52]. Rare earth doped ZnO layers serve dual purposes when employed as CBL firstly they act as an efficient electron transport layer, and secondly, they also act as spectral converting layers and that too with an easier fabrication process. The two advantages of rare-earth-doped ZnO as CBL are; (a) to improve the absorption by spectral conversion through energy transfer, (b) to ensure efficient electron transport properties. Although significant efforts have been made in the past in order to enhance the conductivity as well to improve the absorption properties of ZnO CBL by spectral conversion (using rare earth ions as dopants) [51–54], but there exists no reports on employing luminescent (spectral converting) and simultaneously conductive Al, Eu co-doped ZnO CBL in the inverted device configuration. Al doping contributes to improving the conductivity of ZnO CBL and hence increase charge transport due to lesser recombination centers, whereas rare-earth ion Eu doping significantly enhances the down-shifting luminescence property. In this
2. Materials and methods 2.1. Materials The soluble salts of Zinc nitrate, Aluminum nitrate, and Europium nitrate were procured from Merck (Darmstadt, Germany) for nanoparticle synthesis. The PCDTBT polymer and PC71BM were purchased from 1-materials (Canada). 2.2. Synthesis of ZnO:Al, Eu by combined co-precipitation and hydrothermal method The stoichiometric ratios of Zinc acetate dihydrate (ZAD) [Zn (CH3COO)2H2O] (Aldrich, 99.9%), Aluminum Nitrate (Al(NO3)3 and Europium Nitrate (Eu(NO3)3 ·xH2O) were dissolved in the aqueous medium. Under continuous stirring and mild heating, an alkaline mixture of NaOH: KOH was added dropwise. At pH (˜ 8) a precipitate is formed, immediately 1 ml ethanolamine was added as sol stabilizer to restrict the particle growth. The resultant precipitate was filtered and transferred to Teflon lined stainless steel autoclave. The sealed autoclave was kept in a furnace, maintained at 100 °C for 12 h. The resultant product was allowed to cool naturally to room temperature (˜25 °C). The final product was washed with DI water, and ethanol sequentially to remove all traces of surface-bound impurities. Later on, the solid mixture was dried in an oven at 60 °C overnight, and then finally crushed to a fine powder. The resultant product was further annealed in the furnace to improve crystallinity. Afterward, the obtained ZnO:Al,Eu nanoparticles were ultrasonicated before further characterization and experimentation. For reference bulk ZnO:Al,Eu phosphor material was synthesized using convention solid-state reaction method using oxides 2
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
(20Ω/cm2) at 3000 rpm. After that, the casted film ZnO:Al,Eu and ZnO were annealed for 1 h at 200 °C in the air. The resultant thickness of ZnO films get ˜30 nm, determined by an Ambios XP-100 stylus profilometer. The blend of PCDTBT: PC71BM (1:4 w/w%, 35 mgml-1) prepared in Dichlorobenzene (DCB): Chlorobenzene (CB) (1:4 v/v %) with 0.4 v/v% 1,8-Diiodooctane (DIO), stirred overnight in glovebox. Before spin-casted, blend stirred for 30 min at 60 °C. The final photoactive layers (˜120 nm) were spin-casted in an N2 filled glove box from the prepared blend and annealed for 15 min at 80 °C to evaporate the residual solvent. Finally, the hole transport layer, MoOx (˜10 nm) and electrode Ag (˜100 nm) were evaporated sequentially under ˜10−6 Torr vacuum, by a thermal deposition method, using shadow aluminum mask to form an active area of ˜ 4.5 mm2. The Keithley 2600 source meter used to measure photovoltaic parameters (current density-voltage, J–V curve) and CEP-25ML spectral response measurement system used to record external quantum efficiency (EQE) spectra of fabricated devices. The solar simulator calibrated using a certified reference solar cell to an intensity of 100 mWcm-2 to ensure AM 1.5 G spectral emission during J–V measurements. 3. Results and discussion The photoluminescence (PL) emission spectra of Eu doped ZnO (with varying Eu concentration) under UV excitation (Eex = 395 nm) is shown in Fig. 1(a). The PL emission spectrum of Eu doped ZnO exhibits prominent peaks at 590 nm and 614 nm corresponding to 5 D0→7F1 and 5 D0→7F2 transitions of Eu3+. The emission peak at 590 nm originates from the magnetic dipole allowed 5D0 → 7F1 transition, and the peak at 614 nm is due to the electric-dipole allowed 5D0 → 7F2 transition. The emission peak intensity corresponding to the 5D0 → 7F2 transition is higher than that of 5D0 → 7F1 transition, suggesting that Eu3+ ions mainly occupy a site with inversion anti-symmetry in the ZnO host. The inset of Fig. 1(a) shows the PL excitation spectra of Eu doped ZnO registered at 614 nm emission, indicating major excitation peaks at 397 and 466 nm respectively. The excitation peaks at 397 nm and 466 nm corresponds to the 7F0 → 5L6 and 7F0 → 5D2 transitions of Eu3+ ions. The emission intensity increases with the increasing concentration of dopant Eu3+ up to 5 mol%; after that, concentration quenching occurs; hence, Eu concentration was fixed at 5 mol%. Fig. 1(b) shows the PL emission spectrum of Al-doped ZnO recorded at an excitation of 370 nm. According to the spectrum, as the concentration of Al increases, the defect-related broad luminescence of ZnO quenches (Fig. 1(b)). Al doping results in the formation of shallow donor levels below the conduction band of ZnO and electron recombination corresponding to Al shallow donor levels and ZnO based defect level leads to green emission [55,56]. The concentration of dopant Al was kept low (1%), as high Al doping quenches the defect emission in ZnO [57] and further leads to the formation of Zinc aluminate phase which is insulating. The inset of Fig. 1(b) shows the excitation graph of Al-doped ZnO recorded at an emission of 520 nm. The photoluminescence (PL) emission spectrum of optimized ZnO: Al3+,Eu3+ nanoparticles, as well as the pristine ZnO under 370 nm excitation is shown in Fig. 1(c). The emission graph reveals that the optimized ZnO: Al3+,Eu3+ sample exhibit a broad emission ranging from (450–700 nm) with a sharp peak at 590 and 614 nm, attributed to the f-f transition of Eu3+. Hence, the optimized concentration ZnO: Al (1 mol%), Eu (5 mol%) [abbreviated as ZnO:Al,Eu] was used for further experiments and named as ZnO:Al;Eu. Fig. 1(d) shows UV-VIS absorption spectra of ZnO:Al,Eu and pristine ZnO. The inset shows the transmission of bare ITO and ZnO:Al,Eu.Pristine ZnO shows negligible absorption in the visible region as compared to the ZnO:Al,Eu sample. The UV absorption in ZnO:Al, Eu sample is due to the UV absorption capability of Eu3+ ions. Hence Eu3+ ions in ZnO results in enhanced UV absorption in comparsion to pristine ZnO. The X-ray diffraction (XRD) spectra of the bulk and ZnO:Al,Eu nanoparticles are shown in Fig. 2(a). The XRD peaks match well with the
Fig. 2. Morphology of ZnO: Al, Eu nanoparticles (a) XRD spectra (b) TEM micrograph (c) EDAX spectra showing the elemental composition.
as precursors. For the synthesis of pristine ZnO (for reference device), the 0.1 M concentration of Zinc acetate dihydrate [Zn(CH3COO)2H2O] (Aldrich, 99.9%) prepared in anhydrous ethanol [CH3CH2OH] (99.5 + % Aldrich), and stirred at 80 °C for 2–3 h. Later on, the ethanolamine as sol stabilizer was added to the solution, followed by magnetic stirred at 60 °C for 12–15 h. 2.3. Device fabrication Before fabricating the devices, the ITO glass substrates need to be clean from dust, oil, and grease which get from environment and hands while handling, the process takes few steps to clean. Substrates rubbed in deionized (DI) water with soap solution, subsequently ultra-sonicate in acetone and isopropanol for 15 min each, followed by drying in an oven at 120 °C for 1 h. The UV-O3 treatment of substrate performed for 15 min before used to device application. The prepared colloidal solution of ZnO:Al,Eu and ZnO reference were spin-casted on ITO substrates 3
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
Fig. 3. (a) Resistivity vs. Temperature plot of ZnO and ZnO: Al, Eu nanoparticles (b) TRPL curve of ZnO:Al, Eu sample. Inset shows the decay of pristine ZnO. (c) UV–vis absorption spectra of PCDTBT film with ZnO and ZnO:Al,Eu. (b) PL emission of PCDTBT film with pristine ZnO and ZnO:Al,Eu. Fig. 4. (a) Chemical structures of PCDTBT and PC71BM, (b) Schematic diagram of fabricated device structure showing the mechanism of enhanced absorption by the active layer in presence of luminescent CBL, (c) PCE vs dopant concentration (d) J–V curve for PCDTBT/ PC71BM BHJ solar cell with ZnO and ZnO:Al,Eu as CBL, in dark and simulated AM 1.5 G irradiation (100 mWcm−2) (e) EQE spectra of PCDTBT/PC71BM BHJ solar cell under optimized conditions.
The effect of doping of Al and Eu on the electrical resistivity as a function of temperature is shown in Fig. 3a.The optimized ZnO:Al,Eu sample unveiled lower electrical resistivity as compared to pristine ZnO nanoparticles in the entire temperature range, because in ZnO:Al,Eu the trivalent Al3+ ions are substituted at Zn2+ sites, which liberates more free electrons, thus reducing the resultant electrical resistivity. The
standard (JCPDS-36-1451) of wurtzite ZnO, indicating that no secondary phase formation has occurred. High-resolution transmission electron microscopy (HRTEM) images revealed the size of ZnO:Al,Eu nanoparticles in the range < 20 nm (Fig. 2(b)).The EDAX spectrum furthermore confirmed the presence of dopants Al and Eu in the synthesized nanophosphor (Fig. 2(c)).
4
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
room temperature resistivity of pristine ZnO is 1.03 × 10−3 ohm-m, whereas it is 4.38 × 10-4 ohm-m for ZnO:Al,Eu nanoparticles. Fig. 3(b) shows the Time – resolve PL (TRPL) spectra of ZnO:Al, Eu and pristine ZnO samples. The decay-time of ZnO:Al,Eu is significantly higher as compared to pristine ZnO sample indicating the better luminescence behavior mediated by dopants. When the emission of the donor molecule overlap with the absorption of the acceptor molecule, FRET occurs [58], which is one of the fundamental principles of molecular spectroscopy. In our case the PL emission spectrum of ZnO:Al,Eu nanoparticles (Fig. 1c) (Donor) has significant overlapping with the UV–vis absorption spectrum of PCDTBT (acceptor), which enhances the charge transfer probability between the two and thus leads to a substantial increase in the absorption of PCDTBT after the addition of ZnO:Al,Eu nanoparticles (Fig. 3(c)). Consequently, the PL emission of the PCDTBT also enhances on the addition of ZnO:Al, Eu NPs as shown in Fig. 3(d) indicating that with the conjugation of ZnO:Al,Eu layer the overall absorption of the device enhances which results in the generation of more excitons and hence the resulting PL emission of PCDTBT also increases. This enhancement in the absorption and emission of PCDTBT is probably due to energy transfer from ZnO:Al,Eu layer (donor) to PCDTBT(acceptor). The energy transfer is a consequence of the overlap of emission spectrum of ZnO;Al, Eu CBL with the absorption spectrum of PCDTBT:PC71BM and is a well established phenomenon [58]. Fig. 4(a) shows the chemical structure of PCDTBT and PC71BM. Fig. 4(b) shows the schematic diagram of the device structure with the underlying spectral conversion mechanism between the CBL layer and the active layer. Fig. 4(c) shows the optimized PCE of inverted BHJ OSCs, of interfacial layers i.e. ZnO and ZnO:Al,Eu with different Al and Eu concentration under AM1.5 G irradiation. The device with reference ZnO CBL has shown the highest PCE of 5.9% (average 5.7%) addition of Al in ZnO CBL lead to a slight increase in the PCE to 6.1% (average 5.0%) for Al(0.01) and further doping of Al lead to a decrease in the PCE. Doping of Al increases the conductivity, and slightly improve the current density, but higher concentration may lead to current leakage and recombination. For the ZnO:Al(0.01), the optimized PCE is obtained for Eu (0.05) of 6.9% (average 6.8%). Fig. 4d shows the J–V characteristics of inverted BHJ OSCs, with different optimized interfacial layers i.e. ZnO and ZnO:Al,Eu. The reference ZnO CBL device shows open-circuit voltage (Voc) of 0.871 V, short-circuit current density (Jsc) of 11.0 mAcm-2, fill factor (FF) of 61.7%, and PCE of 5.9%. The series resistance (Rs) and shunt resistance (Rsh) shown the value of 3.92 Ω and 1.11 kΩ, respectively. Whereas, in the case of ZnO:Al,Eu CBL the photovoltaic parameters Jsc, Voc, FF, and PCE are enhanced simultaneously to 0.879 V, 66.7%, 11.85 mAcm-2, and 6.9%, respectively. The Rs decreased to 2.39 Ω and Rsh increased to 1.53 kΩ. The external quantum efficiency (EQE) of both reference ZnO and ZnO:Al,Eu CBL based devices are shown in Fig. 4e which shows a significant enhancement of EQE in the 600-700 nm wavelength. The ZnO based device shows a maximum EQE value of 66%, whereas the enhanced reaches 76% for our ZnO:Al,Eu based device. The enhancement of EQE attributed to the increase in current density, which is resulting due to enhanced absorption which generates more charge carriers. The value of Jsc calculated from J–V curve and integration of the EQE spectrum have nearly similar value (ca. 2% error).
functional CBL layer is easy to fabricate without changing the underlying geometry of the device. Dopant Al improves the conductivity of the layer, whereas Eu doping contributes to absorption enhancement by spectral conversion (UV to visible). The spectral overlapping between the emission of ZnO:Al,Eu CBL layer and PCDTBT (active layer) lead to an energy transfer from ZnO:Al,Eu to PCDTBT, resulting in an enhanced short circuit current with overall improved device efficiency. We observed an enhanced PCE of 6.9% for the device with ZnO:Al,Eu CBL layer as compared to 5.9% for the reference ZnO CBL based device. The findings of the study highlight the role of luminescent CBL in improving the device characteristics of OSC. Acknowledgments The authors (RD) thanks UGC India for UGC-SRF Ph.D. fellowship. The author (SB) sincerely acknowledge the Council of Scientific & Industrial Research (CSIR)- Research Associate fellowship (#31/ 1(0494)/2018-EMR-1, India), and V.G. thanks R. Sharma, KAIST, S. Korea for help in the device characterization. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595–6663, https://doi.org/10.1021/cr900356p. [2] A. Nozik, Quantum dot solar cells, Phys. E Low-Dimens. Syst. Nanostruct. 14 (2002) 115–120, https://doi.org/10.1016/S1386-9477(02)00374-0. [3] G.A. Chamberlain, Organic solar cells: a review G, Sol Cells 8 (1983) 47–83, https://doi.org/10.1016/0379-6787(83)90039-X. [4] F. He, L. Yu, How far can polymer solar cells go? In need of a synergistic approach, J. Phys. Chem. Lett. 2 (2011) 3102–3113, https://doi.org/10.1021/jz201479b. [5] Y. Cheng, S. Yang, C. Hsu, Synthesis of conjugated polymers for organic solar cell applications, Chem. Rev. 109 (2009) 5868–5923, https://doi.org/10.1021/ cr900182s. [6] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, et al., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868, https://doi.org/10.1038/nmat1500. [7] S.K. Hau, H.-L. Yip, Jen AK-Y, A review on the development of the inverted polymer solar cell architecture, Polym. Rev. 50 (2010) 474–510, https://doi.org/10.1080/ 15583724.2010.515764. [8] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, The role of buffer layers in polymer solar cells, Energy Environ. Sci. 4 (2011) 285–310, https://doi.org/10.1039/ c0ee00273a. [9] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photonics 6 (2012) 153–161, https://doi.org/10.1038/nphoton.2012.11. [10] R.F. Service, Outlook brightens for plastic solar cells, Science (80-) 332 (2011), https://doi.org/10.1126/science.332.6027.293 293–293. [11] O. Oklobia, T.S. Shafai, A quantitative study of the formation of PCBM clusters upon thermal annealing of P3HT/PCBM bulk heterojunction solar cell, Sol. Energy Mater. Sol. Cells 117 (2013) 1–8, https://doi.org/10.1016/j.solmat.2013.05.011. [12] M.P. de Jong, L.J. van IJzendoorn, M.J.A. de Voigt, Stability of the interface between indium-tin-oxide and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) in polymer light-emitting diodes, Appl. Phys. Lett. 77 (2000) 2255–2257, https://doi.org/10.1063/1.1315344. [13] H.J. Son, L. Lu, W. Chen, T. Xu, T. Zheng, B. Carsten, et al., Synthesis and photovoltaic effect in dithieno[2,3- d :2′,3′- d ′]Benzo[1,2- b :4,5- b ′]dithiophene-based conjugated polymers, Adv. Mater. 25 (2013) 838–843, https://doi.org/10.1002/ adma.201204238. [14] T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, S. Wakim, et al., Bulk heterojunction solar cells using thieno[3,4- c]pyrrole-4,6-dione and dithieno[3,2- b :2′,3′- d]silole copolymer with a power conversion efficiency of 7.3%, J. Am. Chem. Soc. 133 (2011) 4250–4253, https://doi.org/10.1021/ja200314m. [15] S.C. Price, A.C. Stuart, L. Yang, H. Zhou, W. You, Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer−fullerene solar cells, J. Am. Chem. Soc. 133 (2011) 4625–4631, https://doi.org/10.1021/ja1112595. [16] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, et al., Efficient tandem polymer solar cells fabricated by all-solution processing, Science (80-) 317 (2007) 222–225, https://doi.org/10.1126/science.1141711. [17] J.H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, et al., Improved highefficiency organic solar cells via incorporation of a conjugated polyelectrolyte interlayer, J. Am. Chem. Soc. 133 (2011) 8416–8419, https://doi.org/10.1021/ ja2037673. [18] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, et al., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Adv. Mater. 23 (2011) 4636–4643, https://doi.org/10.1002/ adma.201103006. [19] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, et al., Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 6 (2007) 497–500, https://doi.org/10.1038/nmat1928. [20] X. Liu, W. Wen, G.C. Bazan, Post-deposition treatment of an arylated-carbazole
4. Conclusion In this article, we have demonstrated the synthesis and characterization of an efficient ZnO:Al,Eu based cathode buffer layer with enhanced luminescence and conductivity, for the fabrication of an inverted BHJ solar cell. The ZnO:Al,Eu CBL with improved conductivity and luminescence plays a dual role in improving the device efficiency, firstly by acting as an electron transport layer and secondly as a spectral conversion layer. In comparison with the conventional spectral conversion layer deposited either above or below the device, this dual 5
Synthetic Metals 255 (2019) 116112
R. Datt, et al.
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40] M. Koppe, H.-J. Egelhaaf, G. Dennler, M.C. Scharber, C.J. Brabec, P. Schilinsky, et al., Near IR sensitization of organic bulk heterojunction solar cells: towards optimization of the spectral response of organic solar cells, Adv. Funct. Mater. 20 (2010) 338–346, https://doi.org/10.1002/adfm.200901473. [41] Y. Yang, W. Chen, L. Dou, W.-H. Chang, H.-S. Duan, B. Bob, et al., High-performance multiple-donor bulk heterojunction solar cells, Nat. Photonics 9 (2015) 190–198, https://doi.org/10.1038/nphoton.2015.9. [42] L. Lu, T. Xu, W. Chen, E.S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency, Nat. Photonics 8 (2014) 716–722, https:// doi.org/10.1038/nphoton.2014.172. [43] Z. Yin, Q. Zheng, S.-C. Chen, D. Cai, L. Zhou, J. Zhang, Bandgap tunable Zn 1- x Mg x O thin films as highly transparent cathode buffer layers for high-performance inverted polymer solar cells, Adv. Energy Mater. 4 (2014) 1301404, , https://doi. org/10.1002/aenm.201301404. [44] B.J. Lokhande, P.S. Patil, M.D. Uplane, Deposition of highly oriented ZnO films by spray pyrolysis and their structural, optical and electrical characterization, Mater. Lett. 57 (2002) 573–579. [45] N. Wan, T. Wang, H. Sun, G. Chen, L. Geng, X. Gan, et al., Indium tin oxide thin films for silicon-based electro-luminescence devices prepared by electron beam evaporation method, J. Non. Solids 356 (2010) 911–916, https://doi.org/10.1016/ j.jnoncrysol.2009.12.026. [46] R. Ondo-Ndong, F. Pascal-Delannoy, A. Boyer, A. Giani, A. Foucaran, Structural properties of zinc oxide thin films prepared by r.f. Magnetron sputtering, Mater. Sci. Eng. B 97 (2003) 68–73, https://doi.org/10.1016/S0921-5107(02)00406-3. [47] C. Guillén, J. Herrero, Structure, optical and electrical properties of Al:ZnO thin films deposited by DC sputtering at room temperature on glass and plastic substrates, Phys. Status Solidi 206 (2009) 1531–1536, https://doi.org/10.1002/pssa. 200925061. [48] N. Al-Dahoudi, M.A. Aegerter, Comparative study of transparent conductive In2O3:Sn (ITO) coatings made using a sol and a nanoparticle suspension, Thin Solid Films 502 (2006) 193–197, https://doi.org/10.1016/j.tsf.2005.07.273. [49] J. Ederth, P. Heszler, A. Hultåker, G. Niklasson, C. Granqvist, Indium tin oxide films made from nanoparticles: models for the optical and electrical properties, Thin Solid Films 445 (2003) 199–206, https://doi.org/10.1016/S0040-6090(03) 01164-7. [50] I. Maksimenko, P.J. Wellmann, Low-temperature processing of transparent conductive indium tin oxide nanocomposites using polyvinyl derivatives, Thin Solid Films 520 (2011) 1341–1347, https://doi.org/10.1016/j.tsf.2011.04.142. [51] T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov, C.J. Brabec, Inverted organic solar cells using a solution processed aluminum-doped zinc oxide buffer layer, Org. Electron. 12 (2011) 1539–1543, https://doi.org/10.1016/j.orgel.2011.05.027. [52] H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, C.J. Brabec, Comparison of various sol–gel derived metal oxide layers for inverted organic solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 2194–2199, https://doi.org/10.1016/j.solmat.2011.03. 023. [53] J. Alstrup, M. Jørgensen, A.J. Medford, F.C. Krebs, Ultra fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating, ACS Appl. Mater. Interfaces 2 (2010) 2819–2827, https://doi.org/10. 1021/am100505e. [54] S. Trost, K. Zilberberg, A. Behrendt, A. Polywka, P. Görrn, P. Reckers, et al., Overcoming the “Light-Soaking” issue in inverted organic solar cells by the use of Al:ZnO electron extraction layers, Adv. Energy Mater. 3 (2013) 1437–1444, https:// doi.org/10.1002/aenm.201300402. [55] O. Pachoumi, C. Li, Y. Vaynzof, K.K. Banger, H. Sirringhaus, Improved performance and stability of inverted organic solar cells with sol-gel processed, amorphous mixed metal oxide electron extraction layers comprising alkaline earth metals, Adv. Energy Mater. 3 (2013) 1428–1436, https://doi.org/10.1002/aenm.201300308. [56] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films, Appl. Phys. Lett. 78 (2001) 2285–2287, https://doi.org/10.1063/1.1361288. [57] B.K. Sharma, N. Khare, Stress-dependent band gap shift and quenching of defects in Al-doped ZnO films, J. Phys. D Appl. Phys. 43 (2010) 465402, , https://doi.org/10. 1088/0022-3727/43/46/465402. [58] T. Forster, Naturwissenschaften, Energiewanderung und Fluoreszenz 33 (1946) 166, https://doi.org/10.1007/BF00585226.
conjugated polymer for solar cell fabrication, Adv. Mater. 24 (2012) 4505–4510, https://doi.org/10.1002/adma.201201567. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, et al., A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446, https://doi.org/10.1038/ncomms2411. L. Ye, S. Zhang, W. Zhao, H. Yao, J. Hou, Highly efficient 2D-conjugated benzodithiophene-based photovoltaic polymer with linear alkylthio side chain, Chem. Mater. 26 (2014) 3603–3605, https://doi.org/10.1021/cm501513n. Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, et al., Single-junction polymer solar cells with high efficiency and photovoltage, Nat. Photonics 9 (2015) 174–179, https://doi.org/10.1038/nphoton.2015.6. Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, et al., Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells, Nat. Commun. 5 (2014) 1–8, https://doi.org/10.1038/ncomms6293. A. Aghassi, C.D. Fay, Effects of IPA treatment on the photovoltaic performance of bulk heterojunction organic solar cells, J. Phys. Chem. Solids 130 (2019) 136–143, https://doi.org/10.1016/j.jpcs.2019.02.025. B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, et al., Solution-processed organic solar cells based on dialkylthiol-substituted benzodithiophene unit with efficiency near 10%, J. Am. Chem. Soc. 136 (2014) 15529–15532, https://doi.org/10.1021/ ja509703k. K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant, Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol. Cells 90 (2006) 3520–3530, https://doi.org/10.1016/j.solmat.2006.06.041. K.W. Wong, H.L. Yip, Y. Luo, K.Y. Wong, W.M. Lau, K.H. Low, et al., Blocking reactions between indium-tin oxide and poly (3,4-ethylene dioxythiophene):poly (styrene sulphonate) with a self-assembly monolayer, Appl. Phys. Lett. 80 (2002) 2788–2790, https://doi.org/10.1063/1.1469220. Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer, Adv. Mater. 23 (2011) 1679–1683, https://doi.org/10.1002/adma. 201004301. H. Cao, W. He, Y. Mao, X. Lin, K. Ishikawa, J.H. Dickerson, et al., Recent progress in degradation and stabilization of organic solar cells, J. Power Sources 264 (2014) 168–183, https://doi.org/10.1016/j.jpowsour.2014.04.080. T. Yang, D. Qin, L. Lan, W. Huang, X. Gong, J. Peng, et al., Inverted polymer solar cells with a solution-processed zinc oxide thin film as an electron collection layer, Sci. China Chem. 55 (2012) 755–759, https://doi.org/10.1007/s11426-0124512-2. M. Jørgensen, K. Norrman, S.A. Gevorgyan, T. Tromholt, B. Andreasen, F.C. Krebs, Stability of polymer solar cells, Adv. Mater. 24 (2012) 580–612, https://doi.org/10. 1002/adma.201104187. A.K.K. Kyaw, X.W. Sun, C.Y. Jiang, G.Q. Lo, D.W. Zhao, D.L. Kwong, An inverted organic solar cell employing a sol-gel derived ZnO electron selective layer and thermal evaporated MoO3 hole selective layer, Appl. Phys. Lett. 93 (2008) 221107, , https://doi.org/10.1063/1.3039076. Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, et al., ZnO nanowire growth and devices, Mater. Sci. Eng. R Rep. 47 (2004) 1–47, https://doi.org/10. 1016/j.mser.2004.09.001. K. Ellmer, Resistivity of polycrystalline zinc oxide films: current status and physical limit, J. Phys. D Appl. Phys. 34 (2001) 3097–3108, https://doi.org/10.1088/00223727/34/21/301. Z. Liang, Q. Zhang, L. Jiang, G. Cao, ZnO cathode buffer layers for inverted polymer solar cells, Energy Environ. Sci. 8 (2015) 3442–3476, https://doi.org/10.1039/ C5EE02510A. C.H. Luong, S. Kim, S. Surabhi, T.S. Vo, K.-M. Lee, S.-G. Yoon, et al., Fabrication of undoped ZnO thin film via photosensitive sol–gel method and its applications for an electron transport layer of organic solar cells, Appl. Surf. Sci. 351 (2015) 487–491, https://doi.org/10.1016/j.apsusc.2015.05.170. D. Lee, T. Kang, Y.-Y. Choi, S.-G. Oh, Preparation of electron buffer layer with crystalline ZnO nanoparticles in inverted organic photovoltaic cells, J. Phys. Chem. Solids 105 (2017) 66–71, https://doi.org/10.1016/j.jpcs.2017.02.008. Y.-C. Chen, C.-Y. Hsu, Lin RY-Y, K.-C. Ho, J.T. Lin, Materials for the active layer of organic photovoltaics: ternary solar cell approach, ChemSusChem 6 (2013) 20–35, https://doi.org/10.1002/cssc.201200609.
6