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Received 00th January 2012, Accepted 00th January 2012
Juan Dong, Yanhong Zhao, Jiangjian Shi, Huiyun Wei, Junyan Xiao, Xin Xu, Jianheng Luo, Jing Xu, Dongmei Li, Yanhong Luo, Qingbo Meng *
DOI: 10.1039/x0xx00000x www.rsc.org/
Al-doped ZnO (AZO) modified ZnO nanorods have been applied in CH3NH3PbI3 perovskite solar cells, which can show a positive effect on open circuit voltage and power conversion efficiency. The average power conversion efficiency is improved from 8.5% to 10.07% and the maximum efficiency reaches 10.7%. Recently, thin-film solar cells with perovskite organic lead halide compounds (e.g., CH3NH3PbX3, X =Cl, Br, I) have attracted much attention due to their excellent photovoltaic performance and low cost.1-3 The first attempt of using perovskite as light harvester was made in liquid-type sensitized solar cell, leading to a power conversion efficiency (PCE) of 3.1-3.8%.4 Through years of efforts, solid-state thin film perovskite solar cells have already exhibited high PCE beyond 16.7% with organic hole transport materials (HTMs).5 Moreover, 20% of PCE can be prospected for the cells.6-7 For the inorganic n-type semiconductor nanostructure scaffold layer in the cell, mesoscopic TiO2 is usually employed.8-10 However, recent investigation revealed that, the lower intrinsic electron mobility of TiO2 will lead to unbalanced charge transport of perovskite solar cells.11 It is generally considered that ZnO could be a good candidate to replace TiO2, which is also an n-type semiconductor with a wide direct band gap (Eg=3.37 eV at 300 K) and specific fast electron transport mobility.12 Currently, photovoltaic performance of cells based on ZnO nanostructure can be comparable to the TiO2 based ones.13 However, it is found that serious charge recombination can occur at the ZnO nanostructure/perovskite interface, limiting the cell performance. Thus, a suitable and simple way is needed to effectively modify the ZnO/absorber hetero-interface and suppress charge recombination in order to further improve the cell performance of ZnO-based perovskite solar cells. As Al-doped ZnO (AZO) has a higher conduction band than ZnO, faster electron mobility and higher electron density,14 which is supposed to well restrain the recombination at the ZnO/perovskite interface. Herein, AZO is first employed to modify the ZnO nanorods surface in order to promote the performance of ZnO nanorods based perovskite cells. A series of efficient ZnO nanorods
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(NRs)/AZO shell/CH3NH3PbI3/spiro-OMeTAD/Au solid-state solar cells were fabricated with the structure depicted in Figure 1. In comparison with untreated devices, the average power conversion efficiency was improved from 8.5% to 10.07% and the maximum efficiency reached 10.7%.
Figure 1. Schematic diagram of the ZnO NRs/AZO/CH3NH3PbI3/ spiro-OMeTAD/Au solid-state solar cell and the corresponding energy diagram of ZnO, AZO and CH3NH3PbI3.
Figure 2. Cross-sectional SEM images of (a) ZnO NRs with 450nm length and (b) ZnO/AZO NRs filled with CH3NH3PbI3. ZnO NRs were grown on polycrystalline sol-gel ZnO seed layer according to literature methods,15-16 the cross sectional scanning electron microscope (SEM) image of which with a thickness of 450 nm is presented in Figure 2(a). The AZO thin layer was achieved by spin-coating a mixed ethanolic solution of Al(NO3)3 and Zn(Ac)2 onto the prepared ZnO NRs. For clarity, the Al doped proportion is labelled as x% AZO, where x = Al/(Al + Zn) (mol %): 0%; 2%; 5%; 10% and 15%. CH3NH3PbI3 layer was deposited onto the ZnO film
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Impressive Enhancement on the Cell Performance of ZnO Nanorods-Based Perovskite Solar Cells with Aldoped ZnO Interfacial Modification
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via a two step method in air.17 After modified with AZO, the morphology of ZnO nanorods show no obvious change in SEM images (not shown). Transmission electron microscopy (TEM) was also applied to characterize the detail morphology change of the ZnO nanorods without and with 5% AZO modification. As seen in Figure 3 (b), a different contrast of a thin layer about 1 nm can be observed coating on the ZnO nanorods, which can be attributed to the AZO modification layer. Figure 2(b) demonstrates that CH3NH3PbI3 fills the voids of ZnO NRs with a thin overlayer on the top. Our experimental results show that introducing this AZO layer onto the surface of ZnO NRs can significantly improve the VOC and PCE of the cell. A highest PCE of 10.7 % with short-circuit current density (JSC) of 19.77 mA/cm2, VOC of 900 mV and fill factor (FF) of 0.60 has been achieved after 5% AZO interfacial modification.
Journal Name DOI: 10.1039/C4CC04908J for a higher Al doping concentration. Impressively, the VOC of the cell improves significantly by 100 mV after AZO treatment, shown in Figure 4(b). Meanwhile, the FF in Figure 4(c) also increases slightly. Thus, the average PCE of the cells has been improved from 8.5% to 10.07% with the AZO interfacial modification, as in Figure 4(d). For comparison, pure ZnO or Al2O3 are also used to modify the ZnO NRs by the same method, no obvious enhancement in the cell performance is observed, as shown in Table S2, which further confirms that the enhancement in cell performance is indeed caused by the AZO.
Figure 5. (a) I-V curves, (b) IPCE and UV-vis absorption spectra of the cells based on ZnO NRs modified without and with 5% AZO.
Figure 3. TEM images of (a) ZnO NRs and (b) ZnO NRs modified with 5% AZO
Figure 4. Box charts of (a) JSC, (b) VOC, (c) FF and (d) PCE for perovskite solar cells based on ZnO NRs with AZO modification on different Al doping levels (molar ratio Al/(Zn+Al): 0%, 2%, 5%, 10% and 15%). When a few Al(NO3)3 is introduced into the pure Zn(Ac)2 in ethanol, Al can substitute the Zn site during the formation of ZnO in the annealing process, which can slightly rise the conduction band minimum (CBM) of ZnO.14 EDX analysis confirms the existence of Al in the AZO-ZnO NRs film, as shown in Figure S1 (Seen in ESI). Table S1 shows the typical current-voltage (I-V) characteristics of the cells treated with AZO on different Al doping levels measured under AM 1.5 illumination (light intensity of 100 mW/cm2). Statistic results of the cell performance are shown in Figure 4 as box charts. As in Figure 4a, the JSC basically has no change when the Al doping concentration in the AZO is less than 10%, but decreases obviously
2 | J. Name., 2012, 00, 1-3
Figure 6. (a) Nyquist plots of the device treated with AZO on different Al doping level at applied bias (600 mV) under dark conditions. (b) Equivalent circuit employed to fit the IS data. Plots of (c) the series resistance (RS) and (d) the charge transport resistance (Rrec) obtained from IS results as a function of the applied bias. I-V characteristics of the cells with highest PCE before and after the AZO treatment are shown in Figure 5(a). The cell modified with 5% AZO has almost the same JSC, but a much higher VOC, compared to the untreated cell. The dark current of the treated and untreated cells keep almost zero at voltage below 400 mV, indicating that both cells have large shunt resistances and good diode properties. With AZO interfacial modification, the onset of the dark current is increased to a higher voltage of 600 mV, implying a significant suppression of charge recombination in the device. According to the Ref. 17, the reverse saturated current density (J0) of the cells with and without AZO interfacial modification is calculated to be 7.5×10-5 and 6.5×10-4 mA/cm2, respectively, indicating that the charge recombination in the ZnO/CH3NH3PbI3/HTM interface has been
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To have a deeper insight into the influence of the AZO modification on the cell performance, especially on VOC, impedance spectroscopy (IS) has been systematically analysed, which was obtained under different bias voltage in the dark.19-21 From IS, we can extract the series resistance (RS) and the charge recombination resistance (Rrec) in the cell. Nyquist plots of the cells modified by AZO with different Al doping concentrations at 600 mV is shown in Figure 6(a). The impedance spectra are dominated by a large semicircle. No obvious arc related with HTM has been observed, which is also reported by Ivan Mora-Sero and his colleagues.22 The specific doping characteristics of HTM, thickness of the HTM and charge properties in perovskite may be the reasons. In this case, an equivalent circuit with one R-C element (Figure 6b) is employed to fit the IS data.22-23 The RS fitting results are shown in Figure 6c. As can be seen, the RS of the cells is not obviously changed with the AZO modification. In the perovskite solar cell, charge transport resistances of FTO, ZnO, CH3NH3PbI3 and HTM constitute most part of the series resistance. Thus, the introduction of a highly conductive interfacial layer can hardly change the RS of the cell. Moreover, the un-increased RS also indicates that no insulating Al2O3 appears during this interface modification. The Rrec explicitly relates to the electron and hole recombination within the solar cells under forward bias. Variation of Rrec against the doping concentration is given in Figure 6(d). Interestingly, when AZO with a low Al concentration (2% and 5%) is introduced, the Rrec of the cell increases significantly at applied biases in the range of 400 mV and 950 mV. However, when the concentration of Al is further increased, the Rrec decreases, which agrees with variation tendency of the VOC in Figure 4(b). In other words, the charge recombination current density (J0) of the cell modified with 5% AZO significantly decreases, agreeing with the above calculation result. But for a higher Al concentration (10% and 15%), J0 of the cell increases according to the Rrec results. In the solar cell, the VOC can be 17 calculated as
VOC =
AK BT J SC , ln e J0
where A is the ideality factor of a cell. It has been pointed out that the JSC is not obviously influenced by the AZO modification for low Al concentration. Thus, the significant decrease in J0 can increase the VOC of the cell. According to the IS results, the J0 in the cell indeed significantly decreases, which can be the main reason for the enhancement in VOC and PCE for AZO modified cell. For a higher Al concentration, the JSC decreases whereas the J0 increases simultaneously, leading to a drop in VOC and PCE of the cell. As the CBM of AZO is higher than that of ZnO, an interfacial bridge energy band is introduced between the ZnO electron transport layer and the CH3NH3PbI3 absorber layer, as shown in Figure 1. Obviously, the conduction band offset between ZnO and CH3NH3PbI3 can be buffered, and the charge recombination and charge transport loss coming from band offset are thus suppressed. However, when more Al is added in the AZO precursor solution, a heavy doping of ZnO surface can be achieved in the AZO layer, leading to the electron density in the ZnO NRs increases
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significantly. Due to this heavy doping, CBM of AZO will drop and thus the buffering effect mentioned above begins to weaken. At the same time, increased electron density in ZnO can also significantly decrease the carrier depletion region width of built-in electric field in the ZnO layer. It is thus suggested that free carriers can accumulate at the interface of ZnO NRs/CH3NH3PbI3 for the heavy doping of AZO, resulting in enhanced charge recombination and decreased cell performance. In conclusion, we fabricate highly efficient AZO modified ZnO nanorods-based perovskite solar cells. Interfacial modification with the AZO will lead to significant improvement in VOC and PCE of the cell. IS measurement reveals that introduction of this interfacial layer can effectively suppress charge recombination at ZnO/CH3NH3PbI3 heterointerface. The band position and electron density in AZO are suggested being the origin for this positive effect of interfacial modification. This work is supported by Beijing Science and Technology Committee (No. Z131100006013003), National Key Basic Research Program (No. 2012CB932903), and Natural Science Foundation of China (Nos. 21173260 and 91233202). The authors also thank Huijue Wu for measurements of solar cells.
Notes and references Key Laboratory for Renewable Energy, Chinese Academy of Sciences (CAS), Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100190, P. R. China * Corresponding author [email protected], +86-10-8264-9242 (Qingbo Meng) Electronic Supplementary Information (ESI) available: Experimental procedures, fabrication details and device characterization. See DOI: 10.1039/c000000x/ 1. Di Giacomo, F.; Razza, S.; Matteocci, F.; D'Epifanio, A.; Licoccia, S.; Brown, T. M.; Di Carlo, A., J. POWER SOURCES 2014, 251, 152-156. 2. Carnie, M. J.; Charbonneau, C.; Davies, M. L.; Troughton, J.; Watson, T. M.; Wojciechowski, K.; Snaith, H.; Worsley, D. A., Chem. Commun. 2013, 49, 7893-7895. 3. Cai, B.; Xing, Y.; Yang, Z.; Zhang, W.-H.; Qiu, J., Energy Environ. Sci. 2013, 6, 1480-1485. 4. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., J. Am. Chem. Soc. 2009, 131, 6050-6051. 5. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., J. Am. Chem. Soc. 2014,136, 7837–7840. 6. Snaith, H. J., J. Phys. Chem. Lett. 2013, 4, 3623-3630. 7. Park, N.-G., J. Phys. Chem. Lett. 2013, 4, 2423-2429. 8. Bi, D.; Moon, S.-J.; Häggman, L.; Boschloo, G.; Yang, L.; Johansson, E. M.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A., Rsc Adv. 2013, 3, 18762-18766. 9. Lv, S.; Han, L.; Xiao, J.; Zhu, L.; Shi, J.; Wei, H.; Xu, Y.; Dong, J.; Xu, X.; Li, D.; Wang, S.; Luo, Y.; Meng, Q.; Li, X., Chem. Commun. 2014, 50, 6931-6934.
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effectively suppressed with 5% AZO modification. The UV-vis absorption spectra and incident photon to current efficiency (IPCE)18 spectra in Figure 5(b) demonstrate that the AZO modification has no influence on the light harvester, charge injection and collection capacity of the cell in short circuit condition, agreeing with the I-V results.
ChemComm
Journal Name DOI: 10.1039/C4CC04908J
10. Qiu, J.; Qiu, Y.; Yan, K.; Zhong, M.; Mu, C.; Yan, H.; Yang, S., Nanoscale 2013, 5, 3245-3248. 11. Ponseca, C. S., Jr.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; Wolf, J. P.; Sundstrom, V., J. Am. Chem. Soc. 2014, 136, 5189-5192. 12. Kim, K. D.; Lim, D. C.; Hu, J.; Kwon, J. D.; Jeong, M. G.; Seo, H. O.; Lee, J. Y.; Jang, K. Y.; Lim, J. H.; Lee, K. H.; Jeong, Y.; Kim, Y. D.; Cho, S., ACS Appl. Mater. Inter. 2013, 5, 8718-8723. 13. Son, D.-Y.; Im, J.-H.; Kim, H.-S.; Park, N.-G., J. Phys. Chem. C 2014, 118, 16567-16573. 14. Deng, J.; Wang, M.; Liu, J.; Song, X.; Yang, Z., J. Colloid Interf. Sci. 2014, 418, 277-282. 15. Xu, C.; Shin, P.; Cao, L.; Gao, D., J. Phys. Chem. C 2009, 114, 125129. 16. Wang, H.; Kubo, T.; Nakazaki, J.; Kinoshita, T.; Segawa, H., J. Phys. Chem. Lett. 2013, 4, 2455-2460. 17. Shi, J.; Dong, J.; Lv, S.; Xu, Y.; Zhu, L.; Xiao, J.; Xu, X.; Wu, H.; Li, D.; Luo, Y.; Meng, Q., Appl. Phys. Lett. 2014, 104, 063901. 18. Guo, X.-Z.; Luo, Y.-H.; Li, C.-H.; Qin, D.; Li, D.-M.; Meng, Q.-B., Curr. Appl. Phys. 2012, 12, 54-58. 19. Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J., Phys. Chem. Chem. Phys. 2011, 13, 9083-9118. 20. Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W. S.; Barea, E. M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J., Nano lett. 2014, 14, 888-893. 21. Juarez-Perez, E. J.; Wuβler, M.; Fabregat-Santiago, F.; Lakus-Wollny, K.; Mankel, E.; Mayer, T.; Jaegermann, W.; Mora-Sero, I., J. Phys. Chem. Lett. 2014, 5, 680-685. 22. Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Azaceta, E.; TenaZaera, R.; Bisquert, J., Phys. Chem. Chem. Phys. 2011, 13, 7162-7169. 23. Suganya, L.; Sundaresan, B.; Sankareswari, G.; Ravichandran, K.; Sakthivel, B., J. Mater. Sci.-Mater. El. 2013, 25, 361-368.
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Al-doped ZnO modified ZnO nanorods applied in CH3NH3PbI3 perovskite solar cells can show a positive effect on VOC and PCE.
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This article can be cited before page numbers have been issued, to do this please use: J. Dong, Y. Zhao, J. Shi, H. Wei, J. Xiao, X. Xu, J. Luo, J. Xu, D. Li, Y. Luo and Q. Meng, Chem. Commun., 2014, DOI: 10.1039/C4CC04908J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 2012, Accepted 00th January 2012
Juan Dong, Yanhong Zhao, Jiangjian Shi, Huiyun Wei, Junyan Xiao, Xin Xu, Jianheng Luo, Jing Xu, Dongmei Li, Yanhong Luo, Qingbo Meng *
DOI: 10.1039/x0xx00000x www.rsc.org/
Al-doped ZnO (AZO) modified ZnO nanorods have been applied in CH3NH3PbI3 perovskite solar cells, which can show a positive effect on open circuit voltage and power conversion efficiency. The average power conversion efficiency is improved from 8.5% to 10.07% and the maximum efficiency reaches 10.7%. Recently, thin-film solar cells with perovskite organic lead halide compounds (e.g., CH3NH3PbX3, X =Cl, Br, I) have attracted much attention due to their excellent photovoltaic performance and low cost.1-3 The first attempt of using perovskite as light harvester was made in liquid-type sensitized solar cell, leading to a power conversion efficiency (PCE) of 3.1-3.8%.4 Through years of efforts, solid-state thin film perovskite solar cells have already exhibited high PCE beyond 16.7% with organic hole transport materials (HTMs).5 Moreover, 20% of PCE can be prospected for the cells.6-7 For the inorganic n-type semiconductor nanostructure scaffold layer in the cell, mesoscopic TiO2 is usually employed.8-10 However, recent investigation revealed that, the lower intrinsic electron mobility of TiO2 will lead to unbalanced charge transport of perovskite solar cells.11 It is generally considered that ZnO could be a good candidate to replace TiO2, which is also an n-type semiconductor with a wide direct band gap (Eg=3.37 eV at 300 K) and specific fast electron transport mobility.12 Currently, photovoltaic performance of cells based on ZnO nanostructure can be comparable to the TiO2 based ones.13 However, it is found that serious charge recombination can occur at the ZnO nanostructure/perovskite interface, limiting the cell performance. Thus, a suitable and simple way is needed to effectively modify the ZnO/absorber hetero-interface and suppress charge recombination in order to further improve the cell performance of ZnO-based perovskite solar cells. As Al-doped ZnO (AZO) has a higher conduction band than ZnO, faster electron mobility and higher electron density,14 which is supposed to well restrain the recombination at the ZnO/perovskite interface. Herein, AZO is first employed to modify the ZnO nanorods surface in order to promote the performance of ZnO nanorods based perovskite cells. A series of efficient ZnO nanorods
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(NRs)/AZO shell/CH3NH3PbI3/spiro-OMeTAD/Au solid-state solar cells were fabricated with the structure depicted in Figure 1. In comparison with untreated devices, the average power conversion efficiency was improved from 8.5% to 10.07% and the maximum efficiency reached 10.7%.
Figure 1. Schematic diagram of the ZnO NRs/AZO/CH3NH3PbI3/ spiro-OMeTAD/Au solid-state solar cell and the corresponding energy diagram of ZnO, AZO and CH3NH3PbI3.
Figure 2. Cross-sectional SEM images of (a) ZnO NRs with 450nm length and (b) ZnO/AZO NRs filled with CH3NH3PbI3. ZnO NRs were grown on polycrystalline sol-gel ZnO seed layer according to literature methods,15-16 the cross sectional scanning electron microscope (SEM) image of which with a thickness of 450 nm is presented in Figure 2(a). The AZO thin layer was achieved by spin-coating a mixed ethanolic solution of Al(NO3)3 and Zn(Ac)2 onto the prepared ZnO NRs. For clarity, the Al doped proportion is labelled as x% AZO, where x = Al/(Al + Zn) (mol %): 0%; 2%; 5%; 10% and 15%. CH3NH3PbI3 layer was deposited onto the ZnO film
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Impressive Enhancement on the Cell Performance of ZnO Nanorods-Based Perovskite Solar Cells with Aldoped ZnO Interfacial Modification
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via a two step method in air.17 After modified with AZO, the morphology of ZnO nanorods show no obvious change in SEM images (not shown). Transmission electron microscopy (TEM) was also applied to characterize the detail morphology change of the ZnO nanorods without and with 5% AZO modification. As seen in Figure 3 (b), a different contrast of a thin layer about 1 nm can be observed coating on the ZnO nanorods, which can be attributed to the AZO modification layer. Figure 2(b) demonstrates that CH3NH3PbI3 fills the voids of ZnO NRs with a thin overlayer on the top. Our experimental results show that introducing this AZO layer onto the surface of ZnO NRs can significantly improve the VOC and PCE of the cell. A highest PCE of 10.7 % with short-circuit current density (JSC) of 19.77 mA/cm2, VOC of 900 mV and fill factor (FF) of 0.60 has been achieved after 5% AZO interfacial modification.
Journal Name DOI: 10.1039/C4CC04908J for a higher Al doping concentration. Impressively, the VOC of the cell improves significantly by 100 mV after AZO treatment, shown in Figure 4(b). Meanwhile, the FF in Figure 4(c) also increases slightly. Thus, the average PCE of the cells has been improved from 8.5% to 10.07% with the AZO interfacial modification, as in Figure 4(d). For comparison, pure ZnO or Al2O3 are also used to modify the ZnO NRs by the same method, no obvious enhancement in the cell performance is observed, as shown in Table S2, which further confirms that the enhancement in cell performance is indeed caused by the AZO.
Figure 5. (a) I-V curves, (b) IPCE and UV-vis absorption spectra of the cells based on ZnO NRs modified without and with 5% AZO.
Figure 3. TEM images of (a) ZnO NRs and (b) ZnO NRs modified with 5% AZO
Figure 4. Box charts of (a) JSC, (b) VOC, (c) FF and (d) PCE for perovskite solar cells based on ZnO NRs with AZO modification on different Al doping levels (molar ratio Al/(Zn+Al): 0%, 2%, 5%, 10% and 15%). When a few Al(NO3)3 is introduced into the pure Zn(Ac)2 in ethanol, Al can substitute the Zn site during the formation of ZnO in the annealing process, which can slightly rise the conduction band minimum (CBM) of ZnO.14 EDX analysis confirms the existence of Al in the AZO-ZnO NRs film, as shown in Figure S1 (Seen in ESI). Table S1 shows the typical current-voltage (I-V) characteristics of the cells treated with AZO on different Al doping levels measured under AM 1.5 illumination (light intensity of 100 mW/cm2). Statistic results of the cell performance are shown in Figure 4 as box charts. As in Figure 4a, the JSC basically has no change when the Al doping concentration in the AZO is less than 10%, but decreases obviously
2 | J. Name., 2012, 00, 1-3
Figure 6. (a) Nyquist plots of the device treated with AZO on different Al doping level at applied bias (600 mV) under dark conditions. (b) Equivalent circuit employed to fit the IS data. Plots of (c) the series resistance (RS) and (d) the charge transport resistance (Rrec) obtained from IS results as a function of the applied bias. I-V characteristics of the cells with highest PCE before and after the AZO treatment are shown in Figure 5(a). The cell modified with 5% AZO has almost the same JSC, but a much higher VOC, compared to the untreated cell. The dark current of the treated and untreated cells keep almost zero at voltage below 400 mV, indicating that both cells have large shunt resistances and good diode properties. With AZO interfacial modification, the onset of the dark current is increased to a higher voltage of 600 mV, implying a significant suppression of charge recombination in the device. According to the Ref. 17, the reverse saturated current density (J0) of the cells with and without AZO interfacial modification is calculated to be 7.5×10-5 and 6.5×10-4 mA/cm2, respectively, indicating that the charge recombination in the ZnO/CH3NH3PbI3/HTM interface has been
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To have a deeper insight into the influence of the AZO modification on the cell performance, especially on VOC, impedance spectroscopy (IS) has been systematically analysed, which was obtained under different bias voltage in the dark.19-21 From IS, we can extract the series resistance (RS) and the charge recombination resistance (Rrec) in the cell. Nyquist plots of the cells modified by AZO with different Al doping concentrations at 600 mV is shown in Figure 6(a). The impedance spectra are dominated by a large semicircle. No obvious arc related with HTM has been observed, which is also reported by Ivan Mora-Sero and his colleagues.22 The specific doping characteristics of HTM, thickness of the HTM and charge properties in perovskite may be the reasons. In this case, an equivalent circuit with one R-C element (Figure 6b) is employed to fit the IS data.22-23 The RS fitting results are shown in Figure 6c. As can be seen, the RS of the cells is not obviously changed with the AZO modification. In the perovskite solar cell, charge transport resistances of FTO, ZnO, CH3NH3PbI3 and HTM constitute most part of the series resistance. Thus, the introduction of a highly conductive interfacial layer can hardly change the RS of the cell. Moreover, the un-increased RS also indicates that no insulating Al2O3 appears during this interface modification. The Rrec explicitly relates to the electron and hole recombination within the solar cells under forward bias. Variation of Rrec against the doping concentration is given in Figure 6(d). Interestingly, when AZO with a low Al concentration (2% and 5%) is introduced, the Rrec of the cell increases significantly at applied biases in the range of 400 mV and 950 mV. However, when the concentration of Al is further increased, the Rrec decreases, which agrees with variation tendency of the VOC in Figure 4(b). In other words, the charge recombination current density (J0) of the cell modified with 5% AZO significantly decreases, agreeing with the above calculation result. But for a higher Al concentration (10% and 15%), J0 of the cell increases according to the Rrec results. In the solar cell, the VOC can be 17 calculated as
VOC =
AK BT J SC , ln e J0
where A is the ideality factor of a cell. It has been pointed out that the JSC is not obviously influenced by the AZO modification for low Al concentration. Thus, the significant decrease in J0 can increase the VOC of the cell. According to the IS results, the J0 in the cell indeed significantly decreases, which can be the main reason for the enhancement in VOC and PCE for AZO modified cell. For a higher Al concentration, the JSC decreases whereas the J0 increases simultaneously, leading to a drop in VOC and PCE of the cell. As the CBM of AZO is higher than that of ZnO, an interfacial bridge energy band is introduced between the ZnO electron transport layer and the CH3NH3PbI3 absorber layer, as shown in Figure 1. Obviously, the conduction band offset between ZnO and CH3NH3PbI3 can be buffered, and the charge recombination and charge transport loss coming from band offset are thus suppressed. However, when more Al is added in the AZO precursor solution, a heavy doping of ZnO surface can be achieved in the AZO layer, leading to the electron density in the ZnO NRs increases
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significantly. Due to this heavy doping, CBM of AZO will drop and thus the buffering effect mentioned above begins to weaken. At the same time, increased electron density in ZnO can also significantly decrease the carrier depletion region width of built-in electric field in the ZnO layer. It is thus suggested that free carriers can accumulate at the interface of ZnO NRs/CH3NH3PbI3 for the heavy doping of AZO, resulting in enhanced charge recombination and decreased cell performance. In conclusion, we fabricate highly efficient AZO modified ZnO nanorods-based perovskite solar cells. Interfacial modification with the AZO will lead to significant improvement in VOC and PCE of the cell. IS measurement reveals that introduction of this interfacial layer can effectively suppress charge recombination at ZnO/CH3NH3PbI3 heterointerface. The band position and electron density in AZO are suggested being the origin for this positive effect of interfacial modification. This work is supported by Beijing Science and Technology Committee (No. Z131100006013003), National Key Basic Research Program (No. 2012CB932903), and Natural Science Foundation of China (Nos. 21173260 and 91233202). The authors also thank Huijue Wu for measurements of solar cells.
Notes and references Key Laboratory for Renewable Energy, Chinese Academy of Sciences (CAS), Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100190, P. R. China * Corresponding author [email protected], +86-10-8264-9242 (Qingbo Meng) Electronic Supplementary Information (ESI) available: Experimental procedures, fabrication details and device characterization. See DOI: 10.1039/c000000x/ 1. Di Giacomo, F.; Razza, S.; Matteocci, F.; D'Epifanio, A.; Licoccia, S.; Brown, T. M.; Di Carlo, A., J. POWER SOURCES 2014, 251, 152-156. 2. Carnie, M. J.; Charbonneau, C.; Davies, M. L.; Troughton, J.; Watson, T. M.; Wojciechowski, K.; Snaith, H.; Worsley, D. A., Chem. Commun. 2013, 49, 7893-7895. 3. Cai, B.; Xing, Y.; Yang, Z.; Zhang, W.-H.; Qiu, J., Energy Environ. Sci. 2013, 6, 1480-1485. 4. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., J. Am. Chem. Soc. 2009, 131, 6050-6051. 5. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., J. Am. Chem. Soc. 2014,136, 7837–7840. 6. Snaith, H. J., J. Phys. Chem. Lett. 2013, 4, 3623-3630. 7. Park, N.-G., J. Phys. Chem. Lett. 2013, 4, 2423-2429. 8. Bi, D.; Moon, S.-J.; Häggman, L.; Boschloo, G.; Yang, L.; Johansson, E. M.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A., Rsc Adv. 2013, 3, 18762-18766. 9. Lv, S.; Han, L.; Xiao, J.; Zhu, L.; Shi, J.; Wei, H.; Xu, Y.; Dong, J.; Xu, X.; Li, D.; Wang, S.; Luo, Y.; Meng, Q.; Li, X., Chem. Commun. 2014, 50, 6931-6934.
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effectively suppressed with 5% AZO modification. The UV-vis absorption spectra and incident photon to current efficiency (IPCE)18 spectra in Figure 5(b) demonstrate that the AZO modification has no influence on the light harvester, charge injection and collection capacity of the cell in short circuit condition, agreeing with the I-V results.
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10. Qiu, J.; Qiu, Y.; Yan, K.; Zhong, M.; Mu, C.; Yan, H.; Yang, S., Nanoscale 2013, 5, 3245-3248. 11. Ponseca, C. S., Jr.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; Wolf, J. P.; Sundstrom, V., J. Am. Chem. Soc. 2014, 136, 5189-5192. 12. Kim, K. D.; Lim, D. C.; Hu, J.; Kwon, J. D.; Jeong, M. G.; Seo, H. O.; Lee, J. Y.; Jang, K. Y.; Lim, J. H.; Lee, K. H.; Jeong, Y.; Kim, Y. D.; Cho, S., ACS Appl. Mater. Inter. 2013, 5, 8718-8723. 13. Son, D.-Y.; Im, J.-H.; Kim, H.-S.; Park, N.-G., J. Phys. Chem. C 2014, 118, 16567-16573. 14. Deng, J.; Wang, M.; Liu, J.; Song, X.; Yang, Z., J. Colloid Interf. Sci. 2014, 418, 277-282. 15. Xu, C.; Shin, P.; Cao, L.; Gao, D., J. Phys. Chem. C 2009, 114, 125129. 16. Wang, H.; Kubo, T.; Nakazaki, J.; Kinoshita, T.; Segawa, H., J. Phys. Chem. Lett. 2013, 4, 2455-2460. 17. Shi, J.; Dong, J.; Lv, S.; Xu, Y.; Zhu, L.; Xiao, J.; Xu, X.; Wu, H.; Li, D.; Luo, Y.; Meng, Q., Appl. Phys. Lett. 2014, 104, 063901. 18. Guo, X.-Z.; Luo, Y.-H.; Li, C.-H.; Qin, D.; Li, D.-M.; Meng, Q.-B., Curr. Appl. Phys. 2012, 12, 54-58. 19. Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J., Phys. Chem. Chem. Phys. 2011, 13, 9083-9118. 20. Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W. S.; Barea, E. M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J., Nano lett. 2014, 14, 888-893. 21. Juarez-Perez, E. J.; Wuβler, M.; Fabregat-Santiago, F.; Lakus-Wollny, K.; Mankel, E.; Mayer, T.; Jaegermann, W.; Mora-Sero, I., J. Phys. Chem. Lett. 2014, 5, 680-685. 22. Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Azaceta, E.; TenaZaera, R.; Bisquert, J., Phys. Chem. Chem. Phys. 2011, 13, 7162-7169. 23. Suganya, L.; Sundaresan, B.; Sankareswari, G.; Ravichandran, K.; Sakthivel, B., J. Mater. Sci.-Mater. El. 2013, 25, 361-368.
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DOI: 10.1039/C4CC04908J
Al-doped ZnO modified ZnO nanorods applied in CH3NH3PbI3 perovskite solar cells can show a positive effect on VOC and PCE.
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TOC:
