- Email: [email protected]
ZnO nanowalls grown at low-temperature for electron collection in high-efficiency perovskite solar cells
Solar Energy Materials & Solar Cells 154 (2016) 18–22
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
ZnO nanowalls grown at low-temperature for electron collection in high-efficiency perovskite solar cells Jian-Fu Tang a, Zong-Liang Tseng b,n, Lung-Chien Chen b,n, Sheng-Yuan Chu a,c,n a
Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei, Taiwan c Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 24 October 2015 Received in revised form 9 April 2016 Accepted 16 April 2016
This paper reports on the fabrication of ZnO nanowalls for use as an electron collecting layer (ECL) in CH3NH3PbI3 perovskite solar cells (PSCs). Two-dimension ZnO nanowalls were grown using a lowtemperature chemical bath method with a thin Al film as a seed layer. We compared electron collecting layers based on nanowalls and sol–gel derived ZnO thin film in PSCs and sought to identify the mechanism underlying the collection of electrons. The proposed ZnO nanowalls achieved a fill factor significantly higher than that of ZnO thin films, which translated into a remarkable improvement in power conversion efficiency, reaching 13.6% under AM 1.5G illumination. & 2016 Elsevier B.V. All rights reserved.
Keywords: ZnO CH3NH3PbI3 Perovskite Nanowall Nanostructure
1. Introduction Perovskite based solar cells (PSCs) have been attracting considerable attention [1,2], since Miyasaka et al. first incorporated perovskite semiconductors in photovoltaic devices in 2009 [3], By 2012, several groups had made remarkable progress in boosting the power conversion efficiency (PCE) of perovskite solar cells from 4%. to over 10% [4,10]. The highest PCE recorded from these devices exceeded 20% [11]. Typically, the device architecture of PSCs includes a mesoporous TiO2 film as an electron collection layer (ECL); however, these devices tend to have low electron mobility (0.1–4 cm2/V s) [12], and must be fabricated using complex methods at high temperatures. ZnO exhibits crystallization at low temperature process and enables electron mobility of up to several hundred (110–138 cm2/V s) [13–15]. ZnO thin films can be prepared using a simple solution process that does not require expensive equipment or extreme growth conditions. ZnO thin films have recently been introduced into planar perovskite solar cells to function as an efficient ECL in PSCs [16–22]. Many ZnO nanostructures with one-dimensional (1D) and twodimensional (2D) morphologies, such as nanorods, nanobelts, nanowalls and nanoflowers, have been synthesized [23–26]. ZnO nanowalls (NWs) provide a 2D network and surface area larger n
Corresponding authors. E-mail addresses: [email protected] (Z.-L. Tseng), [email protected] (L.-C. Chen), [email protected] (S.-Y. Chu). http://dx.doi.org/10.1016/j.solmat.2016.04.034 0927-0248/& 2016 Elsevier B.V. All rights reserved.
than that of ZnO nanorods [27]. NWs have considerable application potential in energy-storage devices [28], field emission devices [29], and biological sensors [30,31]. The application of ZnO nanostructures in PSCs has attracted considerable attention as a substitute for mesoporous TiO2 nanostructures in conventional PSCs [32–34]. However, PSCs based on ZnO nanostructures present a low fill factor (FF). Furthermore, no previous study has reported on the use of ZnO NWs as an electron collection layer in PSCs. In this study, we developed a Ch3NH3PbI3 PSCs using ZnO NWs as an electron collection layer to improve the FF and thereby enhance the PCE of the resulting PSCs. ZnO NWs were deposited on an indium tin oxide (ITO)/glass substrate using an inexpensive aqueous solution route at low temperature. Through the study of charge carrier dynamics in Ch3NH3PbI3 layers deposited on ZnO NWs, we observed rapid electron behavior highly conducive to the efficient collection of electrons. The performance of the proposed device and underlying electronic mechanism are detailed in the following.
2. Experimental ZnO nanostructures were grown on an ITO/glass substrate using a chemical bath method [35] using an evaporating layer of Al (c.a. 20 nm) with a precursor solution of zinc acetate dehydrate (0.02 M) and hexamethylenetetramine (0.02 M). Following the growth of NWs at 80 °C for 6 h, the samples were removed from the solution and cleaned with DI water and dried at 60 °C. A sol–
J.-F. Tang et al. / Solar Energy Materials & Solar Cells 154 (2016) 18–22
gel ZnO precursor solution (0.5 M) was prepared by combining monoethanolamine (MEA) and zinc acetate [Zn(CH3COO)2 2H2O] at a molar ratio of 1:1 in 2-ethoxyethanol followed under stirring over a period of 24 h. ZnO thin films were spin-coated (5000 rpm) onto an ITO/glass substrate for 30 s and then annealed at 150 °C for 20 min. The CH3NH3PbI3 precursor solution (1.25 M) was prepared by combining PbI2 (Aldrich) and CH3NH3I (Ruilong Inc., Taiwan) in a mixed solvent of DMSO and GBL (with volume ratio of 1:1) under stirring for 3 h at 60 °C. A layer of perovskite CH3NH3PbI3 was spin-coated (5000 rpm) onto the samples over a period of 30 sec and subsequently treated with toluene dropcasting [36] over ITO substrates coated with ZnO NWs and a ZnO thin film before undergoing annealing at 100 °C for 5 min SpiroOMeTAD was then deposited by spin-coating (4000 rpm) for 30 s for use as a hole transport layer. The spiroOMeTAD (2,20 ,7,70 -tetrakis[N,N-di(4-methoxyphenyl)amino]-9,90 -spirobifluorene) precursor solution was prepared by dissolving 80 mg spiroOMeTAD, 28.5 μL 4-tert-butylpyridine, and 17.5 μL lithium bis(trifluoromethyl-sulphonyl)imide solution (520 mg in 1.0 mL acetonitrile) in 1 mL chlorobenzene. Finally, the sample was transferred to a vacuum chamber for the deposition of an Ag electrode (100 nm). XRD data were collected using a Rigaku RINT 2100 diffractometer and the surface morphology was studied using a fieldemission scanning electron microscope (FE-SEM, Hitachi S4100 and SU8000). The J–V characteristics of the cells were obtained using a Keithley 2400 source measuring unit under a simulated AM1.5G light (Wacom solar simulator) at 100 mW cm 2. All the J– V curves, including the forward and reverse scan, were measured using a voltage step of 0.01 V and a delay time of 200 ms (i.e. a scanning speed of 0.05 V/s). The incident photo-to-current
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conversion efficiency (IPCE) was measured using the QE-R3011 measurement system in air (Enlitech Inc., Taiwan).
3. Results and discussion 3.1. Characterization results Fig. 1(a) presets photographic images of bare ITO, ZnO thin film produced using the sol–gel method, and transparent NW films deposited on patterned ITO substrates. Fig. 1(b) presents an SEM image of the ZnO NWs with the inset showing an SEM image in cross-section. As shown in the SEM images, the large number of porous (sponge-like) ZnO nanostructures resulted in a very large surface area. The thickness of the film and sidewalls was measured at c.a. 320 and 30–80 nm, respectively. Fig. 1(c) presents a transmission electron microscopy (TEM) image of the proposed ZnO NWs, the sidewall of which is composed of uniformly sized ZnO nanocrystals with grain size of less than 10 nm. Selected area electron diffraction (SAED) patterns (Fig. 1(d)) of the ZnO NWs reveals seven diffraction rings, which perfectly match the index positions associated with bulk ZnO ceramics, which indicate the crystallinity of the ZnO NWs with a hexagonal wurtzite structure. Fig. 2 presents the dense, uniform surface morphology and large grain size of CH3NH3PbI3 deposited over the ZnO thin film and NWs. The smaller number of pinholes was reduced (in the cross-sectional view) in the CH3NH3PbI3 deposited on the ZnO NWs which can provided a scaffold, similar to a mesoscopic TiO2 configuration, for CH3NH3PbI3 growth and carrier-collection [37].
Fig. 1. (a) Photographic image of ITO, ZnO thin film, and ZnO NWs on a glass substrate; (b) SEM image of ZnO NWs deposited on ITO/glass with inset showing an SEM image in cross-section; (c) TEM Image; and (d) selected area electron diffraction (SAED) of ZnO NWs.
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J.-F. Tang et al. / Solar Energy Materials & Solar Cells 154 (2016) 18–22
Fig. 2. SEM images of CH3NH3PbI3 deposited on (a) ZnO NWs in top-view and (b) cross-section; (c) ZnO thin films in top view and (d) cross-section.
3.2. Photovoltaic device performance Fig. 3(a) and (b) presents a schematic illustration and energy band diagram depicting the structure of the device fabricated in this study. The J–V curves of PSCs produced using ZnO NWs and ZnO thin films as ECLs are shown in Fig. 3(c). The best of the cells produced using ZnO NWs presented a Jsc of 18.9 mA cm 2, Voc of 1.0 V, and fill factor (FF) of 72.1%, which combined to achieve power conversion efficiency (PCE) of 13.6% (average efficiency ¼12.6%). The PSC device featuring an ECL of planar ZnO films resulted in a PCE of 11.3% (average efficiency ¼10.4%), with a Jsc of 18.6 mA cm 2, Voc of 0.98 V, and FF of 62.0%. The IPCE curves in Fig. 3(d) reveal high external quantum efficiency in the range of 350 nm to 750 nm, with a peak of 82.7% appearing at 520 nm in the of PSC based on ZnO NWs, and a peak of 81.9% appearing at 520 nm in the PSC based on a thin films. The profiles obtained from the two IPCE curves were not entirely consistent, due perhaps to differences in surface roughness between the ZnO NWs and the thin films. The IPCE curves were used to calculate the integration current of PSCs based on ZnO NWs (18.4 mA/cm2) and PSCs based on thin films (18.2 mA/cm2). These values are very close to the Jsc values (18.9 and 18.6 mA/cm2) obtained from I–V curves. Fig. S1 presents the J–V curves obtained by forward and reverse scanning the device with the best performance. Hysteresis effects were observed in the function of the metal oxide acting as an electron transport layer in the PSCs [18,36]. This may be due to the fact that the generation of ferroelectricity in perovskites requires time for the charge redistribution to reach a new state of equilibrium. It should be noted that the FF of the cell based on ZnO NWs far exceeded that of the cell based on ZnO thin films. It is widely
believed that FF is the key parameter associated with the efficiency of perovskite solar cells, which may be associated with carrier recombination at the interface between perovskite and ECL [38]. Thus, the crystallinity and electron transfer dynamics in the CH3NH3PbI3 layers must be taken into account when seeking to elucidate the reasons for the improvement in fill factor. Fig. 4 (a) presents the XRD patterns of CH3NH3PbI3 layers deposited on both of the ZnO structures featured in this study. The diffraction peaks in the CH3NH3PbI3 perovskite were identified as (110), (200), (211), (202), (220), (310), (312) and (224), which was consist with post-reported data from a powder sample [2]. Pure CH3NH3PbI3 phase can be clearly differentiated in the film deposited on ZnO thin films; however, a PbI2 (004) peak can be observed in the CH3NH3PbI3 deposited on the ZnO NWs. The alkaline behavior of the ZnO surface leads to deprotonation of the methylammonium cation on the CH3NH3PbI3/ZnO interface, leading to loss of methylamine and the formation of PbI2 [39]. Fig. 4(b) presents the results of time-resolved photoluminescence (TRPL) of CH3NH3PbI3/ZnO NWs/ITO and CH3NH3PbI3/ZnO thin film/ITO samples. As fitted from TRPL data, the carrier lifetime of CH3NH3PbI3 films deposited on ZnO NWs and thin films were 24.3 and 67.1 ns, respectively. Fig. 4(c) illustrates the pronounced PL quenching behavior caused by fast charge transfer at the interface [40]. The perovskite film based on ZnO NWs resulted in faster carrier transport, which is an indication of a decrease in recombination and good electron collection. The improvement in electron collection in CH3NH3PbI3 deposited on the ZnO NWs can be attributed to the presence of PbI2 phase reducing recombination at the ECL/perovskite interface [41]. Thus, we surmised that increasing the interfacial contact of perovskite films based on ZnO NWs in the presence of PbI2 species could further
J.-F. Tang et al. / Solar Energy Materials & Solar Cells 154 (2016) 18–22
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Fig. 3. (a) Schematic illustration showing the device and direction of carrier transport; (b) energy band diagram of the devices; (c) J–V curves of PSCs based on ZnO NWs and ZnO thin films under AM 1.5G Illumination; (d) corresponding IPCE spectrum of both devices in (b).
Fig. 4. (a) XRD, (b) PL And (c) TRPL of MAPbI3 films deposited on ZnO NWs and ZnO thin films. Two exponential decay curves were fitted from the TRPL data.
improve electron collection and recombination and thereby improve the FF.
4. Conclusions This paper reports a ZnO NW ECL for PSCs, prepared using a low-temperature solution-based chemical bath method. We achieved power conversion efficiency of 13.6%, which is comparable to that of PSCs based on other types of ZnO nanostructures. TRPL studies indicate that the transfer of electrons from the CH3NH3PbI3 layer to the ZnO NW is faster than the transfer of electrons to ZnO thin films, which results in the superior electron
collection ability of ZnO NW ECLs. This, in turn, leads to an improvement in the FF of PSCs based on ZnO NW ECLs, due perhaps to an increase in the surface area of the ZnO NWs in the presence of PbI2 species. Further study will be required to elucidate the mechanism responsible for the increased contact interface between ZnO NW and CH3NH3PbI3.
Author contributions J. F. Tang and Z. L. Tseng made equal contributions to this paper.
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Acknowledgments We gratefully acknowledge Financial support from the Ministry of Science and Technology (MOST), Taiwan, ROC (Grant number: NSC103-2221-E-027-029-MY2, 102-2221-E006-186-MY3).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2016.04. 034.
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
ZnO nanowalls grown at low-temperature for electron collection in high-efficiency perovskite solar cells Jian-Fu Tang a, Zong-Liang Tseng b,n, Lung-Chien Chen b,n, Sheng-Yuan Chu a,c,n a
Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei, Taiwan c Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 24 October 2015 Received in revised form 9 April 2016 Accepted 16 April 2016
This paper reports on the fabrication of ZnO nanowalls for use as an electron collecting layer (ECL) in CH3NH3PbI3 perovskite solar cells (PSCs). Two-dimension ZnO nanowalls were grown using a lowtemperature chemical bath method with a thin Al film as a seed layer. We compared electron collecting layers based on nanowalls and sol–gel derived ZnO thin film in PSCs and sought to identify the mechanism underlying the collection of electrons. The proposed ZnO nanowalls achieved a fill factor significantly higher than that of ZnO thin films, which translated into a remarkable improvement in power conversion efficiency, reaching 13.6% under AM 1.5G illumination. & 2016 Elsevier B.V. All rights reserved.
Keywords: ZnO CH3NH3PbI3 Perovskite Nanowall Nanostructure
1. Introduction Perovskite based solar cells (PSCs) have been attracting considerable attention [1,2], since Miyasaka et al. first incorporated perovskite semiconductors in photovoltaic devices in 2009 [3], By 2012, several groups had made remarkable progress in boosting the power conversion efficiency (PCE) of perovskite solar cells from 4%. to over 10% [4,10]. The highest PCE recorded from these devices exceeded 20% [11]. Typically, the device architecture of PSCs includes a mesoporous TiO2 film as an electron collection layer (ECL); however, these devices tend to have low electron mobility (0.1–4 cm2/V s) [12], and must be fabricated using complex methods at high temperatures. ZnO exhibits crystallization at low temperature process and enables electron mobility of up to several hundred (110–138 cm2/V s) [13–15]. ZnO thin films can be prepared using a simple solution process that does not require expensive equipment or extreme growth conditions. ZnO thin films have recently been introduced into planar perovskite solar cells to function as an efficient ECL in PSCs [16–22]. Many ZnO nanostructures with one-dimensional (1D) and twodimensional (2D) morphologies, such as nanorods, nanobelts, nanowalls and nanoflowers, have been synthesized [23–26]. ZnO nanowalls (NWs) provide a 2D network and surface area larger n
Corresponding authors. E-mail addresses: [email protected] (Z.-L. Tseng), [email protected] (L.-C. Chen), [email protected] (S.-Y. Chu). http://dx.doi.org/10.1016/j.solmat.2016.04.034 0927-0248/& 2016 Elsevier B.V. All rights reserved.
than that of ZnO nanorods [27]. NWs have considerable application potential in energy-storage devices [28], field emission devices [29], and biological sensors [30,31]. The application of ZnO nanostructures in PSCs has attracted considerable attention as a substitute for mesoporous TiO2 nanostructures in conventional PSCs [32–34]. However, PSCs based on ZnO nanostructures present a low fill factor (FF). Furthermore, no previous study has reported on the use of ZnO NWs as an electron collection layer in PSCs. In this study, we developed a Ch3NH3PbI3 PSCs using ZnO NWs as an electron collection layer to improve the FF and thereby enhance the PCE of the resulting PSCs. ZnO NWs were deposited on an indium tin oxide (ITO)/glass substrate using an inexpensive aqueous solution route at low temperature. Through the study of charge carrier dynamics in Ch3NH3PbI3 layers deposited on ZnO NWs, we observed rapid electron behavior highly conducive to the efficient collection of electrons. The performance of the proposed device and underlying electronic mechanism are detailed in the following.
2. Experimental ZnO nanostructures were grown on an ITO/glass substrate using a chemical bath method [35] using an evaporating layer of Al (c.a. 20 nm) with a precursor solution of zinc acetate dehydrate (0.02 M) and hexamethylenetetramine (0.02 M). Following the growth of NWs at 80 °C for 6 h, the samples were removed from the solution and cleaned with DI water and dried at 60 °C. A sol–
J.-F. Tang et al. / Solar Energy Materials & Solar Cells 154 (2016) 18–22
gel ZnO precursor solution (0.5 M) was prepared by combining monoethanolamine (MEA) and zinc acetate [Zn(CH3COO)2 2H2O] at a molar ratio of 1:1 in 2-ethoxyethanol followed under stirring over a period of 24 h. ZnO thin films were spin-coated (5000 rpm) onto an ITO/glass substrate for 30 s and then annealed at 150 °C for 20 min. The CH3NH3PbI3 precursor solution (1.25 M) was prepared by combining PbI2 (Aldrich) and CH3NH3I (Ruilong Inc., Taiwan) in a mixed solvent of DMSO and GBL (with volume ratio of 1:1) under stirring for 3 h at 60 °C. A layer of perovskite CH3NH3PbI3 was spin-coated (5000 rpm) onto the samples over a period of 30 sec and subsequently treated with toluene dropcasting [36] over ITO substrates coated with ZnO NWs and a ZnO thin film before undergoing annealing at 100 °C for 5 min SpiroOMeTAD was then deposited by spin-coating (4000 rpm) for 30 s for use as a hole transport layer. The spiroOMeTAD (2,20 ,7,70 -tetrakis[N,N-di(4-methoxyphenyl)amino]-9,90 -spirobifluorene) precursor solution was prepared by dissolving 80 mg spiroOMeTAD, 28.5 μL 4-tert-butylpyridine, and 17.5 μL lithium bis(trifluoromethyl-sulphonyl)imide solution (520 mg in 1.0 mL acetonitrile) in 1 mL chlorobenzene. Finally, the sample was transferred to a vacuum chamber for the deposition of an Ag electrode (100 nm). XRD data were collected using a Rigaku RINT 2100 diffractometer and the surface morphology was studied using a fieldemission scanning electron microscope (FE-SEM, Hitachi S4100 and SU8000). The J–V characteristics of the cells were obtained using a Keithley 2400 source measuring unit under a simulated AM1.5G light (Wacom solar simulator) at 100 mW cm 2. All the J– V curves, including the forward and reverse scan, were measured using a voltage step of 0.01 V and a delay time of 200 ms (i.e. a scanning speed of 0.05 V/s). The incident photo-to-current
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conversion efficiency (IPCE) was measured using the QE-R3011 measurement system in air (Enlitech Inc., Taiwan).
3. Results and discussion 3.1. Characterization results Fig. 1(a) presets photographic images of bare ITO, ZnO thin film produced using the sol–gel method, and transparent NW films deposited on patterned ITO substrates. Fig. 1(b) presents an SEM image of the ZnO NWs with the inset showing an SEM image in cross-section. As shown in the SEM images, the large number of porous (sponge-like) ZnO nanostructures resulted in a very large surface area. The thickness of the film and sidewalls was measured at c.a. 320 and 30–80 nm, respectively. Fig. 1(c) presents a transmission electron microscopy (TEM) image of the proposed ZnO NWs, the sidewall of which is composed of uniformly sized ZnO nanocrystals with grain size of less than 10 nm. Selected area electron diffraction (SAED) patterns (Fig. 1(d)) of the ZnO NWs reveals seven diffraction rings, which perfectly match the index positions associated with bulk ZnO ceramics, which indicate the crystallinity of the ZnO NWs with a hexagonal wurtzite structure. Fig. 2 presents the dense, uniform surface morphology and large grain size of CH3NH3PbI3 deposited over the ZnO thin film and NWs. The smaller number of pinholes was reduced (in the cross-sectional view) in the CH3NH3PbI3 deposited on the ZnO NWs which can provided a scaffold, similar to a mesoscopic TiO2 configuration, for CH3NH3PbI3 growth and carrier-collection [37].
Fig. 1. (a) Photographic image of ITO, ZnO thin film, and ZnO NWs on a glass substrate; (b) SEM image of ZnO NWs deposited on ITO/glass with inset showing an SEM image in cross-section; (c) TEM Image; and (d) selected area electron diffraction (SAED) of ZnO NWs.
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Fig. 2. SEM images of CH3NH3PbI3 deposited on (a) ZnO NWs in top-view and (b) cross-section; (c) ZnO thin films in top view and (d) cross-section.
3.2. Photovoltaic device performance Fig. 3(a) and (b) presents a schematic illustration and energy band diagram depicting the structure of the device fabricated in this study. The J–V curves of PSCs produced using ZnO NWs and ZnO thin films as ECLs are shown in Fig. 3(c). The best of the cells produced using ZnO NWs presented a Jsc of 18.9 mA cm 2, Voc of 1.0 V, and fill factor (FF) of 72.1%, which combined to achieve power conversion efficiency (PCE) of 13.6% (average efficiency ¼12.6%). The PSC device featuring an ECL of planar ZnO films resulted in a PCE of 11.3% (average efficiency ¼10.4%), with a Jsc of 18.6 mA cm 2, Voc of 0.98 V, and FF of 62.0%. The IPCE curves in Fig. 3(d) reveal high external quantum efficiency in the range of 350 nm to 750 nm, with a peak of 82.7% appearing at 520 nm in the of PSC based on ZnO NWs, and a peak of 81.9% appearing at 520 nm in the PSC based on a thin films. The profiles obtained from the two IPCE curves were not entirely consistent, due perhaps to differences in surface roughness between the ZnO NWs and the thin films. The IPCE curves were used to calculate the integration current of PSCs based on ZnO NWs (18.4 mA/cm2) and PSCs based on thin films (18.2 mA/cm2). These values are very close to the Jsc values (18.9 and 18.6 mA/cm2) obtained from I–V curves. Fig. S1 presents the J–V curves obtained by forward and reverse scanning the device with the best performance. Hysteresis effects were observed in the function of the metal oxide acting as an electron transport layer in the PSCs [18,36]. This may be due to the fact that the generation of ferroelectricity in perovskites requires time for the charge redistribution to reach a new state of equilibrium. It should be noted that the FF of the cell based on ZnO NWs far exceeded that of the cell based on ZnO thin films. It is widely
believed that FF is the key parameter associated with the efficiency of perovskite solar cells, which may be associated with carrier recombination at the interface between perovskite and ECL [38]. Thus, the crystallinity and electron transfer dynamics in the CH3NH3PbI3 layers must be taken into account when seeking to elucidate the reasons for the improvement in fill factor. Fig. 4 (a) presents the XRD patterns of CH3NH3PbI3 layers deposited on both of the ZnO structures featured in this study. The diffraction peaks in the CH3NH3PbI3 perovskite were identified as (110), (200), (211), (202), (220), (310), (312) and (224), which was consist with post-reported data from a powder sample [2]. Pure CH3NH3PbI3 phase can be clearly differentiated in the film deposited on ZnO thin films; however, a PbI2 (004) peak can be observed in the CH3NH3PbI3 deposited on the ZnO NWs. The alkaline behavior of the ZnO surface leads to deprotonation of the methylammonium cation on the CH3NH3PbI3/ZnO interface, leading to loss of methylamine and the formation of PbI2 [39]. Fig. 4(b) presents the results of time-resolved photoluminescence (TRPL) of CH3NH3PbI3/ZnO NWs/ITO and CH3NH3PbI3/ZnO thin film/ITO samples. As fitted from TRPL data, the carrier lifetime of CH3NH3PbI3 films deposited on ZnO NWs and thin films were 24.3 and 67.1 ns, respectively. Fig. 4(c) illustrates the pronounced PL quenching behavior caused by fast charge transfer at the interface [40]. The perovskite film based on ZnO NWs resulted in faster carrier transport, which is an indication of a decrease in recombination and good electron collection. The improvement in electron collection in CH3NH3PbI3 deposited on the ZnO NWs can be attributed to the presence of PbI2 phase reducing recombination at the ECL/perovskite interface [41]. Thus, we surmised that increasing the interfacial contact of perovskite films based on ZnO NWs in the presence of PbI2 species could further
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Fig. 3. (a) Schematic illustration showing the device and direction of carrier transport; (b) energy band diagram of the devices; (c) J–V curves of PSCs based on ZnO NWs and ZnO thin films under AM 1.5G Illumination; (d) corresponding IPCE spectrum of both devices in (b).
Fig. 4. (a) XRD, (b) PL And (c) TRPL of MAPbI3 films deposited on ZnO NWs and ZnO thin films. Two exponential decay curves were fitted from the TRPL data.
improve electron collection and recombination and thereby improve the FF.
4. Conclusions This paper reports a ZnO NW ECL for PSCs, prepared using a low-temperature solution-based chemical bath method. We achieved power conversion efficiency of 13.6%, which is comparable to that of PSCs based on other types of ZnO nanostructures. TRPL studies indicate that the transfer of electrons from the CH3NH3PbI3 layer to the ZnO NW is faster than the transfer of electrons to ZnO thin films, which results in the superior electron
collection ability of ZnO NW ECLs. This, in turn, leads to an improvement in the FF of PSCs based on ZnO NW ECLs, due perhaps to an increase in the surface area of the ZnO NWs in the presence of PbI2 species. Further study will be required to elucidate the mechanism responsible for the increased contact interface between ZnO NW and CH3NH3PbI3.
Author contributions J. F. Tang and Z. L. Tseng made equal contributions to this paper.
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Acknowledgments We gratefully acknowledge Financial support from the Ministry of Science and Technology (MOST), Taiwan, ROC (Grant number: NSC103-2221-E-027-029-MY2, 102-2221-E006-186-MY3).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2016.04. 034.
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