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Cite this: Nanoscale, 2014, 6, 1329
Received 3rd October 2013 Accepted 19th November 2013
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Room-temperature chemical integration of ZnO nanoarchitectures on plastic substrates for flexible dye-sensitized solar cells† Geng-Jia Chang, Shou-Yen Lin and Jih-Jen Wu*
DOI: 10.1039/c3nr05267b www.rsc.org/nanoscale
ZnO nanoarchitectured anodes composed of the ZnO nanocactus array and the top ZnO particle layer are chemically integrated on ITO– PET substrates using a facile room-temperature chemical bath deposition method for dye-sensitized solar cells (DSSCs). In the absence of high-temperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the flexible ZnO DSSC.
Recently, there has been a great deal of interest in exible and light-weight dye-sensitized solar cells (DSSCs) since they are potentially suitable to be renewable and mobile power sources for portable electronic devices.1–8 The fabrication of DSSCs on indium tin oxide (ITO) coated plastic substrates, such as polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), is therefore an important undergoing research topic.1 Solar energy conversion efficiencies over 12.5% have been achieved in DSSCs fabricated on transparent conducting oxide (TCO) coated glasses.9 To attain such high efficiency, the anode of DSSC typically consists of a TiO2 nanoparticle (NP) lm with a thickness of 10–15 mm on the TCO electrode. A high-temperature treatment at 450–550 C is required for the removal of organic binders and the sintering of the TiO2 NPs on the TCO electrode.10 An efficient electron transport pathway through the NP lm to TCO is therefore ensured in the high-temperature treated DSSC photoanode. However, the formation of efficient TiO2 NP-lm anodes on plastic substrates is a critical challenge Department of Chemical Engineering, National Cheng Kung University, Tainan, 701, Taiwan. E-mail: [email protected]; Fax: +886 6234 4496; Tel: +886 6275 7575 ext. 62694 † Electronic supplementary information (ESI) available: Experimental details, SEM and TEM images of ZnO NP seed layer, XRD pattern of ZnO TP lm, photographs of the exible ZnO NC-TP anode and the corresponding DSSC, inuences of array length on density of primary NW array as well as Jsc and efficiency of the ZnO NC DSSCs, photovoltaic performances of exible D149-sensitized ZnO NC-TP DSSCs fabricated using 10 mm thick ZnO NC arrays and ZnO TP lms with various thicknesses, J–V curve of ZnO NC-TP-g DSSC, transmittance spectra of ITO–PET and ITO–glass substrates, and bending test results of the unsealed ZnO NC-TP DSSC cells. See DOI: 10.1039/c3nr05267b
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since most plastic substrates are only sustainable at a limited temperature of 150 C for thermal treatment. With the restriction of temperature treatments, relatively low conversion efficiencies are attained in the plastic-substrate DSSCs so far even though alternative techniques have been developed for improving the connection of TiO2 NPs and the adhesion of TiO2 lm with substrate. Connection (necking) of TiO2 NPs at a low temperature of 130 C has been demonstrated using a binder-free paste composed of TiO2 NPs and an interparticle binding agent.2 A conversion efficiency of 5.8% is yielded in the exible DSSC with a binder-free coated mesoporous TiO2 anode under 1 sun illumination. Mechanical compression techniques which provide an external pressure on the lowtemperature prepared TiO2 lms on plastic substrates have been widely employed to enhance the physical connection between TiO2 NPs.3,4 For an incident solar energy of 100 mW cm 2, conversion efficiencies of 8% and 6.3% are respectively reported for the plastic-substrate DSSCs using uniaxial compression3 and cold isostatic pressing techniques.4 With an energy band gap similar to that of the TiO2, ZnO is an alternative anode material for DSSC.11,12 However, the highest power conversion efficiency for ZnO-based DSSC is 7.5%,13 which is lower than that for TiO2-based DSSC.9 It may be ascribed to that DSSC dyes are designed for TiO2 NP lm anodes and no efficient dye is available for ZnO anodes.5,12,14 Nevertheless, due to its high electron mobility, low crystallization temperature, and anisotropic growth behavior,11 ZnO is a very promising candidate for exible DSSC anodes. However, only a few papers reported the fabrication of plastic-substrate ZnO DSSCs.5–8 Porous crystalline ZnO lms which were formed by electrochemical deposition of a ZnO–dye hybrid at 70 C have been reported for plastic ZnO DSSCs with a conversion efficiency of 4% for a miniature cell.5 Necking of acetic acid gelled ZnO NP lm by ammonia activation at room temperature has been developed for exible DSSC anodes.6 The corresponding exible ZnO DSSC demonstrated a conversion efficiency of 3.8%. A ZnO anode on an ITO–PEN substrate prepared using room-temperature electrophoretic deposition (EPD) followed by
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a heat treatment at 85 C has been reported to yield a DSSC efficiency of 3.76%.7 An efficiency of 4.17% is further achieved with a light scattering TiO2 layer added on the top of the EPD ZnO anode. An efficiency of 4.91% has been measured in the exible ZnO nanotrapod–SnO2 NP composite DSSC in which the anode is prepared on ITO–PEN substrate using a low-temperature fabrication technique of acetic acid gelation–mechanical press–ammonia activation.8 We have demonstrated a rapid room-temperature (RT) chemical bath deposition (CBD) method using supersaturated precursor solutions to construct hierarchical nanostructures on ZnO nanowire (NW) and nanoneedle arrays for solar cells.15–17 The RT-grown ZnO nanostructures, including ZnO nanocactus (NC) array,15 ZnO nanosheet (NS) framework16 and NC–NS framework,17 exhibit superior electron transport properties and an upward-shied conduction band edge compared to the primary one-dimensional ZnO nanostructure arrays. Signicant enhancements of the photovoltaic performances are therefore attained through the modications of the primary ZnO nanostructures via the RTCBD method.15–17 In this work, a compression-free route is developed to construct ZnO nanoarchitecture on the plastic substrate. The facile RTCBD method is employed to chemically integrate a top ZnO particle layer and the ZnO NC array on an ITO–PET substrate for the construction of a exible ZnO nanoarchitectured DSSC anode. In the absence of the hightemperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the plasticsubstrate ZnO DSSC by taking the ZnO advantages of low crystallization temperature and anisotropic growth behavior.
Fig. 1 (a) Schematic of flexible ZnO NC-TP DSSC anode. SEM images of (b) ZnO NW array, (c) ZnO NC array, and (d) ZnO NC-TP anode on ITO–PET substrates. The insets in (b) and (c) are high-magnification SEM images of NWs and NCs, respectively.
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Fig. 1a shows the schematic of the ZnO nanoarchitectured DSSC anode which consists of a ZnO NC array and ZnO particles sequentially constructed on ITO–PET substrate via a lowtemperature wet chemical route. A thin seed layer is rst prepared by the spin coating of a butanolic solution of ZnO NPs with a size of 8 nm on the ITO–PET substrate (Fig. S1 in the ESI†). The ZnO NW array with a thickness of 10 mm, as shown in Fig. 1b, is then grown on the seeded ITO–PET substrate by CBD at 95 C. The ZnO NC array, as shown in Fig. 1c, is subsequently derived from the NW array via the RTCBD method.15,17 As shown in our previous work,15 the ZnO hierarchical nanostructures composed of shells and spines are deposited on the primary ZnO NWs aer the RTCBD to form the NC array. A ZnO particle lm is additionally constructed on the top of the NC array by the drop casting method. The sizes of the ZnO particles and the thickness of the top ZnO particle lm (named ZnO TP lm hereaer) are 200–500 nm and 5 mm, respectively. Finally, a RTCBD is carried out for both the connecting of the particles with the underneath NCs and the inter-necking of the ZnO particles. For simplicity, the resultant ZnO nanoarchitecture on ITO–PET substrate, which is shown in Fig. 1d, is named ZnO NC-TP anode. X-ray diffraction (XRD) characterization reveals the polycrystalline structure of the ZnO TP lm (Fig. S2 in the ESI†). Inter-necking of the particles in the ZnO TP lms by RTCBD were further examined by transmission electron microscopy (TEM). Typical TEM images and the corresponding selective area electron diffraction (SAED) pattern in Fig. 2a show the single-crystalline structure and at surface of the as-prepared ZnO particle. The bumpy surface of the ZnO particle as a result of the nanostructure formation aer RTCBD is observed by TEM characterization as shown in Fig. 2b. High-resolution (HR) TEM image, as shown in Fig. 2c, reveals that the RT-grown nanostructure on the ZnO particle surface possesses the crystalline ZnO structure as well. A typical TEM image of the fragment scratched from the ZnO TP lm aer RTCBD is shown in Fig. 2d. It illustrates that the RT-grown nanostructures developing from individual particles ll the interstices between the particles. Fig. 2e shows the high-magnication TEM image of the region denoted in Fig. 2d, which evidently reveals the internecking of the particles in the ZnO TP lm. The ZnO NW array, NC array, and NC-TP nanoarchitecture on ITO–PET substrates are employed as anodes to fabricate DSSCs. The details for DSSC fabrication and characterization are described in the ESI.† Photographs of the exible ZnO NC-TP anode and the corresponding DSSC are shown in Fig. S3 in the ESI.† Photocurrent density (J)–voltage (V) characteristics of the three exible D149-sensitized ZnO DSSCs are shown in Fig. 3a. For comparison, the J–V curves of the DSSCs fabricated individually using the ZnO NC-TP nanoarchitecture on ITO–PET substrate without further RTCBD (named ZnO NC-TP-x DSSC hereaer) and ZnO TP lm on ITO–PET substrate with RTCBD (ZnO TP DSSC) are presented in Fig. 3b. The photovoltaic properties of these cells are listed in Table 1. It shows that the performance of the exible ZnO NC DSSC is superior to that of the exible ZnO NW DSSC, which is consistent with the evaluation of the ZnO NW and ZnO NC DSSCs fabricated using
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(a) High-magnification TEM image of the surface region of an as-prepared ZnO particle for ZnO TP film denoted in the right-handside inset. The scale bar in the inset is 50 nm. The left-hand-side inset is the corresponding SAED pattern. (b) High-magnification TEM image of the surface region of a ZnO particle after RTCBD denoted in the inset. The scale bar in the inset is 50 nm. (c) HRTEM image of the RT-grown ZnO nanostructure denoted in (b). (d) TEM image of fragment scratched from ZnO TP film after RTCBD. (e) High-magnification TEM image of the region denoted in (d).
Fig. 2
ITO–glass substrates in the previous work.15 Compared to the primary ZnO NWs, the RT-grown ZnO nanostructures exhibit more suitable surface for dye adsorption, superior electron transport properties and upward-shied conduction band edge, resulting in the enhancements of photovoltaic performances in the ZnO NC DSSC.15,17 In the absence of high-temperature posttreatment and mechanical compression, an efficiency (h) of 4.43% is achieved in the exible DSSC fabricated using a D149sensitized 10 mm thick ZnO NC array anode on the ITO–PET substrate. The result demonstrates that the ZnO NC array is a promising anode for the exible DSSCs with plastic substrates. However, we found that it is hard to increase the DSSC efficiency further by thickening the NC array (Fig. S4 in the ESI†). The density of the primary NW array is decreased and the surface area of the NC array is therefore not efficient increased when the array thickness is increased. To further increase the light harvesting efficiency of the ZnO NC array anode on the ITO–PET substrate, a layer of ZnO particles is constructed on the top of the NC array. As shown in Fig. 3b and Table 1, however, the performance of the ZnO
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Fig. 3 (a) J–V curves of flexible D149-sensitized ZnO NW, NC, and NC-TP DSSCs. (b) J–V curves of flexible ZnO NC-TP-x and TP DSSCs.
Table 1 Photovoltaic properties of flexible D149-sensitized ZnO NW, NC, NC-TP, NC-TP-x, and TP DSSCs
Flexible DSSC
Voc (V)
Jsc (mA cm 2)
FF
h (%)
NW NC NC-TP NC-TP-x TP
0.54 0.70 0.73 0.68 0.62
5.95 9.87 11.19 8.34 7.66
0.41 0.64 0.64 0.46 0.39
1.33 4.43 5.24 2.65 1.87
NC-TP-x DSSC which is fabricated using the exible ZnO NC-TP anode without further RTCBD is even worse than that of exible ZnO NC DSSC. It is mainly ascribed to the poor connection of ZnO particles on the top of the NC array. On the other hand, as shown in Fig. 3a and Table 1, the performance of the exible DSSC are signicantly improved once the RTCBD is conducted for the inter-necking of the nanostructures in the ZnO NC-TP anode. Compared to the ZnO NC DSSC, a 13% enrichment of Jsc is achieved by the addition of ZnO TP lm. A notable efficiency of 5.24% is attained in the exible DSSC fabricated using the D149-sensitized ZnO NC-TP anode on the ITO–PET substrate without high-temperature post-treatment and mechanical compression. The thickness of the ZnO particle lm has been optimized to be 5 mm for the photocurrent and efficiency of the exible ZnO NC-TP DSSC (Fig. S5 and Table S1 in the ESI†). As Nanoscale, 2014, 6, 1329–1334 | 1331
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shown in Fig. 3b and Table 1, the exible DSSC with a 5 mm thick ZnO particle anode (ZnO TP lm) exhibits an efficiency of 1.87%, conrming that the inter-necking of the particles are well-conducted via the RTCBD for photoelectron transport in the ZnO TP anode. Moreover, the performance of the exible ZnO NC-TP DSSC is comparable to that of the DSSC fabricated using the ZnO NC-TP nanoarchitecture on ITO–glass substrate (named ZnO NC-TP-g DSSC hereaer). Jsc and FF of the exible ZnO NC-TP DSSC are slightly less than those of the ZnO NC-TP-g DSSC, which are respectively attributed to the lower optical transmittance and larger electrical resistance of the ITO–PET substrate than those of the ITO–glass substrate (Fig. S6 in the ESI†). Fig. 4a shows the diffuse reectance spectra of the ZnO NW array, ZnO NC array, and ZnO NC-TP nanoarchitecture on the ITO–PET substrates. The reectance values of the ZnO NC-TP nanoarchitecture are larger than those of the ZnO NC array in the wavelength range of 450–800 nm and the enhancement is getting signicant at longer wavelength, revealing the light scattering ability of the ZnO particle lm on the top of the NC array. The incident-photon-to-current efficiency (IPCE) spectra of the exible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs are shown in Fig. 4b Considerable enrichments of the IPCE values
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are obtained in the ZnO NC DSSC compared to the ZnO NW DSSC, which conrms the signicant improvement of Jsc aer the formation of RT hierarchical nanostructures on the ZnO NW anode. Moreover, the IPCE values of the exible ZnO DSSC are further improved, which is consistent with the increased Jsc, as adding the ZnO TP lm followed by RTCBD. Dynamics of recombination and electron transport in the exible ZnO DSSCs were investigated using intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS), respectively. Light intensity dependence of electron lifetimes and transit times in the photoanodes of the exible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs are shown in Fig. 5. It reveals that the electron lifetimes in the exible ZnO NW, ZnO NC, and ZnO NC-TP anodes are quite similar. They are on the time scale of 0.01–0.02 s although the surface areas of the ZnO NC array and ZnO NC-TP nanoachitetcure are signicantly larger than that of the ZnO NW array. We have reported that the semipolar surface of the RT hierarchical nanostructures constructed on the NW-array template is more suitable for D149 and possesses higher dye adsorption coverage than the m-plane surface of the ZnO NW anode.15 In this work, the J–V curves of the exible ZnO DSSCs in Fig. 3a also show signicantly larger shunt resistance in the ZnO NC and ZnO NC-TP DSSCs (1500 and 1600 U cm2, respectively) compared to that in the ZnO NW cell (170 U cm2). The back reaction of photoelectrons in the ZnO NC and ZnO NC-TP anodes with I3 in the electrolyte may be reduced due to the high dye adsorption coverage. The FFs of the ZnO NC and ZnO NC-TP DSSCs are therefore enhanced because of the formation of RT hierarchical nanostructures on the NW array. On the other hand, the dependence of dynamics of the electron transport on light intensity is observed in the three exible ZnO DSSCs, as shown in Fig. 5, indicating that traps exist in the three ZnO anodes.18 In our previous works, the dynamics of electron transport in the ZnO NC anode which is grown on the annealed seed layer/ITO–glass are insensitive to light intensity.15 Moreover, aer taking the thicknesses of the arrays into account, the electron transit times in the ZnO arrays grown on the annealed seed layer/ITO–glass substrates are
Fig. 4 (a) Diffuse reflectance spectra of the ZnO NW array, ZnO NC
array, and ZnO NC-TP nanoarchitecture on ITO–PET substrates. (b) IPCE spectra of flexible D149-sensitized ZnO NW, NC, and NC-TP DSSCs.
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Light intensity dependence of electron transit times and lifetimes in photoanodes of flexible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs.
Fig. 5
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much shorter than those on exible ITO–PET substrates. Therefore, the traps which inuence the electron transport properties may mainly exist in the seed layer of the ZnO NC anode on exible ITO–PET substrate. While the electron transport properties in the exible ZnO NC DSSC is inferior to those in the previously reported ZnO NC DSSC due to the unannealed seed layer, the electron transports in the exible ZnO NC and ZnO NC-TP anodes are signicantly faster than those in the ZnO NW anode by showing shorter electron transit times, as revealed in Fig. 5. It is consistent with the reported results that the RT-grown hierarchical nanostructures on NW-array templates outperform the primary NW arrays in terms of electron transport properties.15,17 Moreover, as shown in Fig. 5, the electron transit times are one order of magnitude shorter than the electron lifetimes in the exible ZnO NC and ZnO NC-TP DSSCs, indicating the efficient electron collection of both exible ZnO NC and ZnO NC-TP anodes on the ITO–PET substrates. As shown in Fig. 3a and 4b, the sensitized NC array in the nanoarchitectured ZnO NC-TP photoanode contributes to the majority of the photocurrent. Nevertheless, a 13% enrichment of Jsc is achieved by the addition of ZnO TP lm. Fig. 5 reveals that the electron transit times in the ZnO NC-TP anode are slightly longer than those in the ZnO NC array anode in low light intensity region. The results reect signicant characteristics of the ZnO NC-TP anode. First, the longer electron transit times in the NC-TP anode indicate that photoelectrons injected into the ZnO TP lm contribute to the photocurrent in the exible ZnO NC-TP DSSC.19 Furthermore, the successful collection of the photoelectrons from the ZnO TP lm region reveals that the inter-necking of the ZnO TP lm and the connection of the particle lm with underneath NC array are well conducted by the RTCBD. Consequently, the higher Jsc in the exible ZnO NC-TP DSSC compared to the ZnO NC DSSC results from that the ZnO TP lm increases the light harvesting efficiency of the exible DSSC not only through scattering light back to the ZnO NC array but also by enlarging the surface area for dye adsorption without sacricing the fast electron transport of the ZnO NC array. Compared to the high-temperature posttreatment and mechanical-compression methods, this facile RTCBD internecking method offers a very attractive strategy for scaling up and mass production of low-cost exible DSSC photoanodes. It is well known that the leakage of the electrolyte due to the poor adhesion of sealing material with plastic substrate surface is one of the major issues for the poor stability of the exible DSSC fabricated using plastic substrates.1 We found that the liquid electrolyte leaks from the ZnO NC-TP DSSC during bending even though the cell is sealed by glue. The performance of the cell is therefore degraded during bending testing, which is mainly due to the electrolyte deciency. In this work, an expedient method for examining the exible nature of the ZnO TP-NC photoanode is to conduct the bending test on an unsealed liquid-electrolyte ZnO NC-TP DSSC cell. While the liquid electrolyte leaks from the unsealed cell during bending, it is rellable into the attened cell aer bending. The bending test results of the unsealed ZnO NC-TP DSSC cells (Fig. S7 and Table S2 in the ESI†) demonstrate the good bending
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stability of the ZnO TP-NC photoanode. Fabrication of quasisolid-state or solid-state exible ZnO NC-TP DSSCs is suggested to solve the issue of electrolyte leakage for the improvement of the bending stability of the cell.
Conclusions Using the facile RTCBD method, the ZnO nanoarchitectured DSSC anodes composed of the ZnO NC array and the top ZnO particle layer have been successfully chemically integrated on ITO–PET substrates. With the outperformed sensitized NC array contributing to the majority of the photocurrent, the ZnO particle lm on the top of NC array further increases the light harvesting efficiency of the exible DSSC. Dynamics of recombination and electron transport measurements indicate the efficient electron collection in the ZnO NC-TP anodes on the ITO–PET substrates. The inter-necking of the ZnO TP lm and the connection of the particle lm with underneath NC array are well conducted by the RTCBD. In the absence of high-temperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the plastic-substrate ZnO DSSC by taking the ZnO advantages of low crystallization temperature and anisotropic growth behavior.
Acknowledgements We thank Professor Jen-Sue Chen for the technique support. Financial support from the National Science Council in Taiwan under Contract no. NSC 99-2221-E-006-198-MY3 and NSC 1002628-E006-032-MY2 are gratefully acknowledged.
Notes and references 1 H. C. Weerasinghe, F. Huang and Y.-B. Cheng, Nano Energy, 2013, 2, 174–189. 2 T. Miyasaka, M. Ikegami and Y. Kijitori, J. Electrochem. Soc., 2007, 154, A455. 3 T. Yamaguchi, N. Tobe, D. Matsumoto, T. Nagai and H. Arakawa, Sol. Energy Mater. Sol. Cells, 2010, 94, 812–816. 4 F. Huang, D. Chen, Q. Li, R. A. Caruso and Y.-B. Cheng, Appl. Phys. Lett., 2012, 100, 123102. 5 T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauport´ e, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. W¨ ohrle, K. Funabiki, M. Matsui, H. Miura and H. Yanagi, Adv. Funct. Mater., 2009, 19, 17–43. 6 X. Liu, Y. Luo, H. Li, Y. Fan, Z. Yu, Y. Lin, L. Chen and Q. Meng, Chem. Commun., 2007, 2847–2849. 7 X. Yin, X. Liu, L. Wang and B. Liu, Electrochem. Commun., 2010, 12, 1241–1244. 8 W. Chen, Y. Qiu and S. Yang, Phys. Chem. Chem. Phys., 2010, 12, 9494–9501. 9 A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. Diau, C. Y. Yeh, S. M. Zakeeruddin and M. Gratzel, Science, 2011, 334, 629–634. 10 B. O'regan and M. Grtzeli, Nature, 1991, 353, 24. 11 Q. Zhang, C. S. Dandeneau, X. Zhou and G. Cao, Adv. Mater., 2009, 21, 4087–4108. Nanoscale, 2014, 6, 1329–1334 | 1333
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16 Y.-H. Sung, W.-P. Liao, D.-W. Chen, C.-T. Wu, G.-J. Chang and J.-J. Wu, Adv. Funct. Mater., 2012, 22, 3808–3814. 17 W.-T. Jiang, C.-T. Wu, Y.-H. Sung and J.-J. Wu, ACS Appl. Mater. Interfaces, 2013, 5, 911–917. 18 T. Oekermann, T. Yoshida, H. Minoura, K. Wijayantha and L. Peter, J. Phys. Chem. B, 2004, 108, 8364–8370. 19 J.-J. Wu, Y.-R. Chen, W.-P. Liao, C.-T. Wu and C.-Y. Chen, ACS Nano, 2010, 4, 5679–5684.
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12 J. A. Anta, E. Guill´ en and R. Tena-Zaera, J. Phys. Chem. C, 2012, 116, 11413–11425. 13 N. Memarian, I. Concina, A. Braga, S. M. Rozati, A. Vomiero and G. Sberveglieri, Angew. Chem., Int. Ed., 2011, 123, 12529– 12533. 14 J.-J. Wu, G.-R. Chen, H.-H. Yang, C.-H. Ku and J.-Y. Lai, Appl. Phys. Lett., 2007, 90, 213109. 15 C.-T. Wu and J.-J. Wu, J. Mater. Chem., 2011, 21, 13605.
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Cite this: Nanoscale, 2014, 6, 1329
Received 3rd October 2013 Accepted 19th November 2013
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Room-temperature chemical integration of ZnO nanoarchitectures on plastic substrates for flexible dye-sensitized solar cells† Geng-Jia Chang, Shou-Yen Lin and Jih-Jen Wu*
DOI: 10.1039/c3nr05267b www.rsc.org/nanoscale
ZnO nanoarchitectured anodes composed of the ZnO nanocactus array and the top ZnO particle layer are chemically integrated on ITO– PET substrates using a facile room-temperature chemical bath deposition method for dye-sensitized solar cells (DSSCs). In the absence of high-temperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the flexible ZnO DSSC.
Recently, there has been a great deal of interest in exible and light-weight dye-sensitized solar cells (DSSCs) since they are potentially suitable to be renewable and mobile power sources for portable electronic devices.1–8 The fabrication of DSSCs on indium tin oxide (ITO) coated plastic substrates, such as polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), is therefore an important undergoing research topic.1 Solar energy conversion efficiencies over 12.5% have been achieved in DSSCs fabricated on transparent conducting oxide (TCO) coated glasses.9 To attain such high efficiency, the anode of DSSC typically consists of a TiO2 nanoparticle (NP) lm with a thickness of 10–15 mm on the TCO electrode. A high-temperature treatment at 450–550 C is required for the removal of organic binders and the sintering of the TiO2 NPs on the TCO electrode.10 An efficient electron transport pathway through the NP lm to TCO is therefore ensured in the high-temperature treated DSSC photoanode. However, the formation of efficient TiO2 NP-lm anodes on plastic substrates is a critical challenge Department of Chemical Engineering, National Cheng Kung University, Tainan, 701, Taiwan. E-mail: [email protected]; Fax: +886 6234 4496; Tel: +886 6275 7575 ext. 62694 † Electronic supplementary information (ESI) available: Experimental details, SEM and TEM images of ZnO NP seed layer, XRD pattern of ZnO TP lm, photographs of the exible ZnO NC-TP anode and the corresponding DSSC, inuences of array length on density of primary NW array as well as Jsc and efficiency of the ZnO NC DSSCs, photovoltaic performances of exible D149-sensitized ZnO NC-TP DSSCs fabricated using 10 mm thick ZnO NC arrays and ZnO TP lms with various thicknesses, J–V curve of ZnO NC-TP-g DSSC, transmittance spectra of ITO–PET and ITO–glass substrates, and bending test results of the unsealed ZnO NC-TP DSSC cells. See DOI: 10.1039/c3nr05267b
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since most plastic substrates are only sustainable at a limited temperature of 150 C for thermal treatment. With the restriction of temperature treatments, relatively low conversion efficiencies are attained in the plastic-substrate DSSCs so far even though alternative techniques have been developed for improving the connection of TiO2 NPs and the adhesion of TiO2 lm with substrate. Connection (necking) of TiO2 NPs at a low temperature of 130 C has been demonstrated using a binder-free paste composed of TiO2 NPs and an interparticle binding agent.2 A conversion efficiency of 5.8% is yielded in the exible DSSC with a binder-free coated mesoporous TiO2 anode under 1 sun illumination. Mechanical compression techniques which provide an external pressure on the lowtemperature prepared TiO2 lms on plastic substrates have been widely employed to enhance the physical connection between TiO2 NPs.3,4 For an incident solar energy of 100 mW cm 2, conversion efficiencies of 8% and 6.3% are respectively reported for the plastic-substrate DSSCs using uniaxial compression3 and cold isostatic pressing techniques.4 With an energy band gap similar to that of the TiO2, ZnO is an alternative anode material for DSSC.11,12 However, the highest power conversion efficiency for ZnO-based DSSC is 7.5%,13 which is lower than that for TiO2-based DSSC.9 It may be ascribed to that DSSC dyes are designed for TiO2 NP lm anodes and no efficient dye is available for ZnO anodes.5,12,14 Nevertheless, due to its high electron mobility, low crystallization temperature, and anisotropic growth behavior,11 ZnO is a very promising candidate for exible DSSC anodes. However, only a few papers reported the fabrication of plastic-substrate ZnO DSSCs.5–8 Porous crystalline ZnO lms which were formed by electrochemical deposition of a ZnO–dye hybrid at 70 C have been reported for plastic ZnO DSSCs with a conversion efficiency of 4% for a miniature cell.5 Necking of acetic acid gelled ZnO NP lm by ammonia activation at room temperature has been developed for exible DSSC anodes.6 The corresponding exible ZnO DSSC demonstrated a conversion efficiency of 3.8%. A ZnO anode on an ITO–PEN substrate prepared using room-temperature electrophoretic deposition (EPD) followed by
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a heat treatment at 85 C has been reported to yield a DSSC efficiency of 3.76%.7 An efficiency of 4.17% is further achieved with a light scattering TiO2 layer added on the top of the EPD ZnO anode. An efficiency of 4.91% has been measured in the exible ZnO nanotrapod–SnO2 NP composite DSSC in which the anode is prepared on ITO–PEN substrate using a low-temperature fabrication technique of acetic acid gelation–mechanical press–ammonia activation.8 We have demonstrated a rapid room-temperature (RT) chemical bath deposition (CBD) method using supersaturated precursor solutions to construct hierarchical nanostructures on ZnO nanowire (NW) and nanoneedle arrays for solar cells.15–17 The RT-grown ZnO nanostructures, including ZnO nanocactus (NC) array,15 ZnO nanosheet (NS) framework16 and NC–NS framework,17 exhibit superior electron transport properties and an upward-shied conduction band edge compared to the primary one-dimensional ZnO nanostructure arrays. Signicant enhancements of the photovoltaic performances are therefore attained through the modications of the primary ZnO nanostructures via the RTCBD method.15–17 In this work, a compression-free route is developed to construct ZnO nanoarchitecture on the plastic substrate. The facile RTCBD method is employed to chemically integrate a top ZnO particle layer and the ZnO NC array on an ITO–PET substrate for the construction of a exible ZnO nanoarchitectured DSSC anode. In the absence of the hightemperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the plasticsubstrate ZnO DSSC by taking the ZnO advantages of low crystallization temperature and anisotropic growth behavior.
Fig. 1 (a) Schematic of flexible ZnO NC-TP DSSC anode. SEM images of (b) ZnO NW array, (c) ZnO NC array, and (d) ZnO NC-TP anode on ITO–PET substrates. The insets in (b) and (c) are high-magnification SEM images of NWs and NCs, respectively.
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Fig. 1a shows the schematic of the ZnO nanoarchitectured DSSC anode which consists of a ZnO NC array and ZnO particles sequentially constructed on ITO–PET substrate via a lowtemperature wet chemical route. A thin seed layer is rst prepared by the spin coating of a butanolic solution of ZnO NPs with a size of 8 nm on the ITO–PET substrate (Fig. S1 in the ESI†). The ZnO NW array with a thickness of 10 mm, as shown in Fig. 1b, is then grown on the seeded ITO–PET substrate by CBD at 95 C. The ZnO NC array, as shown in Fig. 1c, is subsequently derived from the NW array via the RTCBD method.15,17 As shown in our previous work,15 the ZnO hierarchical nanostructures composed of shells and spines are deposited on the primary ZnO NWs aer the RTCBD to form the NC array. A ZnO particle lm is additionally constructed on the top of the NC array by the drop casting method. The sizes of the ZnO particles and the thickness of the top ZnO particle lm (named ZnO TP lm hereaer) are 200–500 nm and 5 mm, respectively. Finally, a RTCBD is carried out for both the connecting of the particles with the underneath NCs and the inter-necking of the ZnO particles. For simplicity, the resultant ZnO nanoarchitecture on ITO–PET substrate, which is shown in Fig. 1d, is named ZnO NC-TP anode. X-ray diffraction (XRD) characterization reveals the polycrystalline structure of the ZnO TP lm (Fig. S2 in the ESI†). Inter-necking of the particles in the ZnO TP lms by RTCBD were further examined by transmission electron microscopy (TEM). Typical TEM images and the corresponding selective area electron diffraction (SAED) pattern in Fig. 2a show the single-crystalline structure and at surface of the as-prepared ZnO particle. The bumpy surface of the ZnO particle as a result of the nanostructure formation aer RTCBD is observed by TEM characterization as shown in Fig. 2b. High-resolution (HR) TEM image, as shown in Fig. 2c, reveals that the RT-grown nanostructure on the ZnO particle surface possesses the crystalline ZnO structure as well. A typical TEM image of the fragment scratched from the ZnO TP lm aer RTCBD is shown in Fig. 2d. It illustrates that the RT-grown nanostructures developing from individual particles ll the interstices between the particles. Fig. 2e shows the high-magnication TEM image of the region denoted in Fig. 2d, which evidently reveals the internecking of the particles in the ZnO TP lm. The ZnO NW array, NC array, and NC-TP nanoarchitecture on ITO–PET substrates are employed as anodes to fabricate DSSCs. The details for DSSC fabrication and characterization are described in the ESI.† Photographs of the exible ZnO NC-TP anode and the corresponding DSSC are shown in Fig. S3 in the ESI.† Photocurrent density (J)–voltage (V) characteristics of the three exible D149-sensitized ZnO DSSCs are shown in Fig. 3a. For comparison, the J–V curves of the DSSCs fabricated individually using the ZnO NC-TP nanoarchitecture on ITO–PET substrate without further RTCBD (named ZnO NC-TP-x DSSC hereaer) and ZnO TP lm on ITO–PET substrate with RTCBD (ZnO TP DSSC) are presented in Fig. 3b. The photovoltaic properties of these cells are listed in Table 1. It shows that the performance of the exible ZnO NC DSSC is superior to that of the exible ZnO NW DSSC, which is consistent with the evaluation of the ZnO NW and ZnO NC DSSCs fabricated using
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(a) High-magnification TEM image of the surface region of an as-prepared ZnO particle for ZnO TP film denoted in the right-handside inset. The scale bar in the inset is 50 nm. The left-hand-side inset is the corresponding SAED pattern. (b) High-magnification TEM image of the surface region of a ZnO particle after RTCBD denoted in the inset. The scale bar in the inset is 50 nm. (c) HRTEM image of the RT-grown ZnO nanostructure denoted in (b). (d) TEM image of fragment scratched from ZnO TP film after RTCBD. (e) High-magnification TEM image of the region denoted in (d).
Fig. 2
ITO–glass substrates in the previous work.15 Compared to the primary ZnO NWs, the RT-grown ZnO nanostructures exhibit more suitable surface for dye adsorption, superior electron transport properties and upward-shied conduction band edge, resulting in the enhancements of photovoltaic performances in the ZnO NC DSSC.15,17 In the absence of high-temperature posttreatment and mechanical compression, an efficiency (h) of 4.43% is achieved in the exible DSSC fabricated using a D149sensitized 10 mm thick ZnO NC array anode on the ITO–PET substrate. The result demonstrates that the ZnO NC array is a promising anode for the exible DSSCs with plastic substrates. However, we found that it is hard to increase the DSSC efficiency further by thickening the NC array (Fig. S4 in the ESI†). The density of the primary NW array is decreased and the surface area of the NC array is therefore not efficient increased when the array thickness is increased. To further increase the light harvesting efficiency of the ZnO NC array anode on the ITO–PET substrate, a layer of ZnO particles is constructed on the top of the NC array. As shown in Fig. 3b and Table 1, however, the performance of the ZnO
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Fig. 3 (a) J–V curves of flexible D149-sensitized ZnO NW, NC, and NC-TP DSSCs. (b) J–V curves of flexible ZnO NC-TP-x and TP DSSCs.
Table 1 Photovoltaic properties of flexible D149-sensitized ZnO NW, NC, NC-TP, NC-TP-x, and TP DSSCs
Flexible DSSC
Voc (V)
Jsc (mA cm 2)
FF
h (%)
NW NC NC-TP NC-TP-x TP
0.54 0.70 0.73 0.68 0.62
5.95 9.87 11.19 8.34 7.66
0.41 0.64 0.64 0.46 0.39
1.33 4.43 5.24 2.65 1.87
NC-TP-x DSSC which is fabricated using the exible ZnO NC-TP anode without further RTCBD is even worse than that of exible ZnO NC DSSC. It is mainly ascribed to the poor connection of ZnO particles on the top of the NC array. On the other hand, as shown in Fig. 3a and Table 1, the performance of the exible DSSC are signicantly improved once the RTCBD is conducted for the inter-necking of the nanostructures in the ZnO NC-TP anode. Compared to the ZnO NC DSSC, a 13% enrichment of Jsc is achieved by the addition of ZnO TP lm. A notable efficiency of 5.24% is attained in the exible DSSC fabricated using the D149-sensitized ZnO NC-TP anode on the ITO–PET substrate without high-temperature post-treatment and mechanical compression. The thickness of the ZnO particle lm has been optimized to be 5 mm for the photocurrent and efficiency of the exible ZnO NC-TP DSSC (Fig. S5 and Table S1 in the ESI†). As Nanoscale, 2014, 6, 1329–1334 | 1331
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shown in Fig. 3b and Table 1, the exible DSSC with a 5 mm thick ZnO particle anode (ZnO TP lm) exhibits an efficiency of 1.87%, conrming that the inter-necking of the particles are well-conducted via the RTCBD for photoelectron transport in the ZnO TP anode. Moreover, the performance of the exible ZnO NC-TP DSSC is comparable to that of the DSSC fabricated using the ZnO NC-TP nanoarchitecture on ITO–glass substrate (named ZnO NC-TP-g DSSC hereaer). Jsc and FF of the exible ZnO NC-TP DSSC are slightly less than those of the ZnO NC-TP-g DSSC, which are respectively attributed to the lower optical transmittance and larger electrical resistance of the ITO–PET substrate than those of the ITO–glass substrate (Fig. S6 in the ESI†). Fig. 4a shows the diffuse reectance spectra of the ZnO NW array, ZnO NC array, and ZnO NC-TP nanoarchitecture on the ITO–PET substrates. The reectance values of the ZnO NC-TP nanoarchitecture are larger than those of the ZnO NC array in the wavelength range of 450–800 nm and the enhancement is getting signicant at longer wavelength, revealing the light scattering ability of the ZnO particle lm on the top of the NC array. The incident-photon-to-current efficiency (IPCE) spectra of the exible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs are shown in Fig. 4b Considerable enrichments of the IPCE values
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are obtained in the ZnO NC DSSC compared to the ZnO NW DSSC, which conrms the signicant improvement of Jsc aer the formation of RT hierarchical nanostructures on the ZnO NW anode. Moreover, the IPCE values of the exible ZnO DSSC are further improved, which is consistent with the increased Jsc, as adding the ZnO TP lm followed by RTCBD. Dynamics of recombination and electron transport in the exible ZnO DSSCs were investigated using intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS), respectively. Light intensity dependence of electron lifetimes and transit times in the photoanodes of the exible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs are shown in Fig. 5. It reveals that the electron lifetimes in the exible ZnO NW, ZnO NC, and ZnO NC-TP anodes are quite similar. They are on the time scale of 0.01–0.02 s although the surface areas of the ZnO NC array and ZnO NC-TP nanoachitetcure are signicantly larger than that of the ZnO NW array. We have reported that the semipolar surface of the RT hierarchical nanostructures constructed on the NW-array template is more suitable for D149 and possesses higher dye adsorption coverage than the m-plane surface of the ZnO NW anode.15 In this work, the J–V curves of the exible ZnO DSSCs in Fig. 3a also show signicantly larger shunt resistance in the ZnO NC and ZnO NC-TP DSSCs (1500 and 1600 U cm2, respectively) compared to that in the ZnO NW cell (170 U cm2). The back reaction of photoelectrons in the ZnO NC and ZnO NC-TP anodes with I3 in the electrolyte may be reduced due to the high dye adsorption coverage. The FFs of the ZnO NC and ZnO NC-TP DSSCs are therefore enhanced because of the formation of RT hierarchical nanostructures on the NW array. On the other hand, the dependence of dynamics of the electron transport on light intensity is observed in the three exible ZnO DSSCs, as shown in Fig. 5, indicating that traps exist in the three ZnO anodes.18 In our previous works, the dynamics of electron transport in the ZnO NC anode which is grown on the annealed seed layer/ITO–glass are insensitive to light intensity.15 Moreover, aer taking the thicknesses of the arrays into account, the electron transit times in the ZnO arrays grown on the annealed seed layer/ITO–glass substrates are
Fig. 4 (a) Diffuse reflectance spectra of the ZnO NW array, ZnO NC
array, and ZnO NC-TP nanoarchitecture on ITO–PET substrates. (b) IPCE spectra of flexible D149-sensitized ZnO NW, NC, and NC-TP DSSCs.
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Light intensity dependence of electron transit times and lifetimes in photoanodes of flexible ZnO NW, ZnO NC, and ZnO NC-TP DSSCs.
Fig. 5
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much shorter than those on exible ITO–PET substrates. Therefore, the traps which inuence the electron transport properties may mainly exist in the seed layer of the ZnO NC anode on exible ITO–PET substrate. While the electron transport properties in the exible ZnO NC DSSC is inferior to those in the previously reported ZnO NC DSSC due to the unannealed seed layer, the electron transports in the exible ZnO NC and ZnO NC-TP anodes are signicantly faster than those in the ZnO NW anode by showing shorter electron transit times, as revealed in Fig. 5. It is consistent with the reported results that the RT-grown hierarchical nanostructures on NW-array templates outperform the primary NW arrays in terms of electron transport properties.15,17 Moreover, as shown in Fig. 5, the electron transit times are one order of magnitude shorter than the electron lifetimes in the exible ZnO NC and ZnO NC-TP DSSCs, indicating the efficient electron collection of both exible ZnO NC and ZnO NC-TP anodes on the ITO–PET substrates. As shown in Fig. 3a and 4b, the sensitized NC array in the nanoarchitectured ZnO NC-TP photoanode contributes to the majority of the photocurrent. Nevertheless, a 13% enrichment of Jsc is achieved by the addition of ZnO TP lm. Fig. 5 reveals that the electron transit times in the ZnO NC-TP anode are slightly longer than those in the ZnO NC array anode in low light intensity region. The results reect signicant characteristics of the ZnO NC-TP anode. First, the longer electron transit times in the NC-TP anode indicate that photoelectrons injected into the ZnO TP lm contribute to the photocurrent in the exible ZnO NC-TP DSSC.19 Furthermore, the successful collection of the photoelectrons from the ZnO TP lm region reveals that the inter-necking of the ZnO TP lm and the connection of the particle lm with underneath NC array are well conducted by the RTCBD. Consequently, the higher Jsc in the exible ZnO NC-TP DSSC compared to the ZnO NC DSSC results from that the ZnO TP lm increases the light harvesting efficiency of the exible DSSC not only through scattering light back to the ZnO NC array but also by enlarging the surface area for dye adsorption without sacricing the fast electron transport of the ZnO NC array. Compared to the high-temperature posttreatment and mechanical-compression methods, this facile RTCBD internecking method offers a very attractive strategy for scaling up and mass production of low-cost exible DSSC photoanodes. It is well known that the leakage of the electrolyte due to the poor adhesion of sealing material with plastic substrate surface is one of the major issues for the poor stability of the exible DSSC fabricated using plastic substrates.1 We found that the liquid electrolyte leaks from the ZnO NC-TP DSSC during bending even though the cell is sealed by glue. The performance of the cell is therefore degraded during bending testing, which is mainly due to the electrolyte deciency. In this work, an expedient method for examining the exible nature of the ZnO TP-NC photoanode is to conduct the bending test on an unsealed liquid-electrolyte ZnO NC-TP DSSC cell. While the liquid electrolyte leaks from the unsealed cell during bending, it is rellable into the attened cell aer bending. The bending test results of the unsealed ZnO NC-TP DSSC cells (Fig. S7 and Table S2 in the ESI†) demonstrate the good bending
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stability of the ZnO TP-NC photoanode. Fabrication of quasisolid-state or solid-state exible ZnO NC-TP DSSCs is suggested to solve the issue of electrolyte leakage for the improvement of the bending stability of the cell.
Conclusions Using the facile RTCBD method, the ZnO nanoarchitectured DSSC anodes composed of the ZnO NC array and the top ZnO particle layer have been successfully chemically integrated on ITO–PET substrates. With the outperformed sensitized NC array contributing to the majority of the photocurrent, the ZnO particle lm on the top of NC array further increases the light harvesting efficiency of the exible DSSC. Dynamics of recombination and electron transport measurements indicate the efficient electron collection in the ZnO NC-TP anodes on the ITO–PET substrates. The inter-necking of the ZnO TP lm and the connection of the particle lm with underneath NC array are well conducted by the RTCBD. In the absence of high-temperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the plastic-substrate ZnO DSSC by taking the ZnO advantages of low crystallization temperature and anisotropic growth behavior.
Acknowledgements We thank Professor Jen-Sue Chen for the technique support. Financial support from the National Science Council in Taiwan under Contract no. NSC 99-2221-E-006-198-MY3 and NSC 1002628-E006-032-MY2 are gratefully acknowledged.
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