Effects of Seed Layer on Growth of ZnO Nanorod and Performance of

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Effects of Seed Layer on Growth of ZnO Nanorod and Performance of Perovskite Solar Cell Dae-Yong Son, Kyeong-Hui Bae, Hui Seon Kim, and Nam-Gyu Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03276 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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The Journal of Physical Chemistry

Effects of Seed Layer on Growth of ZnO Nanorod and Performance of Perovskite Solar Cell

Dae-Yong Son, Kyeong-Hui Bae, Hui-Seon Kim and Nam-Gyu Park*

School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea

* To whom all correspondence should be addressed Tel: +82-31-290-7241; Fax: +82-31-290-7272 E-mail address: [email protected]

1

ACS Paragon Plus Environment


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Abstract Effects of seed layer on growth of ZnO nanorod and photovoltaic performance of perovskite solar cell were investigated. Three different coating solutions (the clear solution, the colloidal solution and the nano-powder dispersed solution) were prepared for making seed layers. Vertically aligned ZnO nanorods were grown on the colloidal-based seed layer, while tilted nanorods were obtained on the solution- and powder-based seed layers. Open-circuit voltage (Voc) of the CH3NH3PbI3 perovskite solar cell was predominantly influenced by the seed layers, where highest Voc was obtained from the ZnO nanorod grown on the colloidal seed layer. Impedance spectroscopic study revealed that recombination resistances at the seed layer contact and the ZnO nanorod/perovskite interface were increased by the colloidal coating, which was responsible for the enhanced Voc. Surface modification of ZnO nanorods improved further Voc and fill factor, leading to power conversion efficiency of 14.35%. This work emphasizes that the seed layer in the ZnO nanorod-based perovskite solar cell play major role in determining photovoltage and the ZnO nanorod/perovskite interface get involved partly in this role.

Keywords: ZnO nanorod, seed layer, colloidal, photovoltage, perovskite solar cell, methylammonium lead iodide, impedance

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Introduction Since the report on long-term stable perovskite solar cell in 2012,1 following two reports on attempts of methylammonium lead halide perovskite as a sensitizer,2,3 intensive researches on perovskite solar cell have been performed. As a result, power conversion efficiencies (PCEs) of 18-20% were recently certified.4,5 Such high PCEs are likely to be related to the intriguing properties of organo lead halide perovskite light harvester with high absorption coefficient of >10-4 cm-1 in the visible light,2,6-8 suitable refractive index of ca. 2.6 for low reflectivity of 0.2-0.3,8-10 high static dielectric constant of about 70 at 20 Hz along with small exciton binding energy of around 2 meV,11 photo-induced giant dielectric constant of 106 at 1 Hz,12 long charge diffusion length exceeding 1 µm for the solution-processed perovskite films13,14 or more than 10 µm for the millimeter scale single crystal,15 high charge carrier mobility of 25 cm2/Vs16 and charge accumulation availability.17 Thanks to the excellent opto-electronic properties, organo lead halide perovskite is able to be applied to any kind of solar cell structures from mesoscopic structure employing mesoporous oxide layer to p-i-n or p-n junction planar structure.18-21 Regarding charge diffusion length, electron diffusion length of the solution processed CH3NH3PbI3 (MAPbI3) was about 130 nm that was slightly longer than hole diffusion length of about 100 nm.13,14 However, electron beam-induced current (EBIC) imaging study showed that the diffusion length for hole was longer than that for electron for the solution-processed MAPbI3, which was suggested to require an electron transport layer to compensate shorter electron diffusion length.22 This indicates that it is still arguable regarding diffusion length probably depending on fabrication procedure, precursor stoichiometry, and morphology of MAPbI3. Recently, mesoporous oxide layer can help to extend electron diffusion length, 3



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which was confirmed by employing mesoporous TiO2 layer.23 In addition to the diffusion length control for optimal design of perovskite solar cell, carrier mobilities should be taken into consideration. Electron and hole moblities were found to be almost balanced and remained high to microsecond time scale.16 However, the performance may be degraded in mesoscopic structure if the mobility of the injected electron in TiO2 layer is slower than that of perovskite. Thus, carful design of oxide layer with high electron mobility is important to achieve high photovoltaic performance. Among the studied metal oxide layers for electron injection in mesoscopic structured perovskite solar cells,24-27 ZnO nanorod was found to be one of effective charge collection systems presumably due to higher electron mobility compared to TiO2 nanorod.28 ZnO nanorods were grown based on the ZnO seed layer formed from the ethanol solution of zinc acetate, which led to PCE of 11.13%.27 However, we have found that photovoltaic performance of ZnO nanorod-based perovskite solar cells depend significantly on the extent of coverage of the seed layer on the high haze fluorine-doped tin oxide (FTO) conductive substrate, which is indicative of importance of seed layer. Here, we report on effects of the ZnO seed layer on growth of ZnO nanorod and photovoltaic performance of perovskite solar cell. Three different precursor solutions are used for preparing the ZnO seed layers. Morphology of ZnO nanorods and interface between ZnO nanorod and FTO substrate are found to be altered by coating solutions to form seed layer, which has predominantly influence on open-circuit voltage. In addition to the conformal coverage of seed layer, surface of ZnO nanorod being in contact with perovskite layer is found to be also important because it involves electron injection. UV-Vis and impedance spectroscopic studies are performed to elucidate the role of seed layer and interfaces. A PCE of 14.35% is achieved by controlling seed layer coverage and surface of ZnO nanorod. 4


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Experimental Preparation of seed layers for ZnO Nanorods. Three different seed layers were prepared for the growth of ZnO nanorods using a zinc acetate solution (hereafter ‘solution’), a ZnO colloidal solution (hereafter ‘colloidal’) and a powdered ZnO dispersed in alcoholic solvent (hereafter “powder’). Zinc acetate dehydrate (0.07 mg, Aldrich 98%) was dissolved in ethanol (40 mL) to make the ‘solution’ precursor for a seed layer. The ‘colloidal’ solution was prepared by acid-base reaction according to a method reported elsewhere.29 The ethanolic solution of zinc acetate dehydrate (2.95 g in 125 mL) was first prepared, to which an ethanolic solution of KOH (1.45 g in 65mL) was dropped at 65 oC for 15 min. The mixture was stirred for 20 min at 65 oC. The solution was kept in refrigerator prior to use. For the ‘powder’ seed layer coating solution, ZnO nanopowder with average particle size of 20 nm (Nanostructured & Amorphous Materials Inc.) was dispersed in ethanol with ratio of 0.04 g/L. Growth of ZnO nanorods. ZnO nanorods were prepared by a method reported elsewhere.27 The seed layer were first deposited using three different precursor solutions on a FTO substrate by spin coating at rate of 2000 rpm for 20 s and heating at 125 oC for 5 min, which was repeated 3 times. Finally the deposited seeded layers were annealed at 350 oC for 15 min. The solution for growing ZnO nanorod was prepared by dissolving equimolar zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Aldrich, 98%) and hexamethylenetetramine (HMTA, Aldrich, 99%) in deionized (DI) water.27 The solution concentration was adjusted to 25 mM. The FTO substrates with ZnO seed layers were immersed in the solution at 85 oC for 2 h, where FTO side was placed facedown. The ZnO nanorod film was washed with ethanol and DI water several times and then annealed at 450 oC for 30 min. For the surface modification of the ZnO nanorods with titanium oxide, the ZnO nanorods were immersed in an aqueous solution 5



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of H3BO3 (17.5 mM) and (NH4)2TiF6 (7 mM) at 90 oC for 10 min.30-32 The surface modified ZnO nanorods were washed with DI water and then annealed at 500 oC for 30 min. Fabrication of perovskite solar cells. Methylammonium lead iodide, CH3NH3PbI3 (MAPbI3), was deposited on the ZnO nanorod film by two-step spin-coating method33 using an N,Ndimethyl formamide (DMF) solution of PbI2 (1.165 g PbI2 in 2.5 mL DMF) and a 2-propanol solution of CH3NH3I (MAI) (0.04 g MAI in 5 mL 2-propanol). A 50 µL of the PbI2 solution was spun on the ZnO nanorod film at 3000 rpm for 20 s and dried at 100 oC for 10 min, which was followed by spin-coating 200 µL of MAI at 2000 rpm for 20 s after loading for 20 s and drying at 100 oC for 2 min. The hole transporting material (HTM) was formed on the perovskite/ZnO

nanorod

composite

by

spin-coating

the

2,2′,7,7′-tetrakis(N,N-

pdimethoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) solution at 3000 rpm for 30 s, where 72.3 mg of spiro-MeOTAD was mixed with 28.8 µL of 4-tert-butylpyridine and 17.5 µL of lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) solution (520 mg of LiTFSI in 1 mL of acetonitrile) in 1 mL of chlorobenzene. Finally ca. 60 nm of Au was deposited using a thermal evaporator at a deposition rate of 1.0 Å/s. Characterizations. Photocurrent density-voltage curves were obtained under AM 1.5G one sun (100 mW/cm2) illumination by using a solar simulator (Oriel Sol 3A class AAA) equipped with a 450 W xenon lamp (Newport 6279NS) and a Keithley 2400 source meter. The light intensity was adjusted using a NREL-calibrated Si solar cell with a KG-2 filter. The cells were covered with a metal mask having an aperture area of 0.12 cm2. Incident photonto-electron conversion efficiency (IPCE) was measured using an IPCE system (PV Measurements Inc.). The source light for generating the monochromatic beam was a 75 W Xenon lamp (USHIO, Japan). IPCE data was collected at DC mode.34 Surface and crosssectional morphologies were investigated using scanning electron microscopy (SEM, JSM6


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7600F, JEOL). The focused ion beam assisted cross-sectional SEM image was measured by using high-resolution scanning electron microscope (SEM, HITACHI S-48000, Japan) at 5 kV. A 20 nm-thick Pt layer was deposited on the surfaces to avoid charging effect. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a potentiostat/galvanostat (PGSTAT 128N, Autolab, Eco-Chemie). EIS data were recorded under dark and illumination (100 mW/cm2) conditions at applied voltages ranging from 0 V to 1 V, where the small perturbation with AC 20 mV was put on the DC bias voltage with frequency ranging from 0.1 Hz to 1 MHz and 5 s of equilibrium time. The measured data were fit with an equivalent circuit comprising a series resistance (Rs) and two series connected R-C (resistance and capacitance in parallel) components.35

Results and discussion Three different seed layers were prepared for the growth of ZnO nanorods using a clear zinc acetate solution (hereafter ‘solution’), a ZnO colloidal solution (hereafter ‘colloidal’) and a powdered ZnO dispersed in ethanol (hereafter “powder’). In Figure 1, each precursor solution in the glass vial is shown. The ‘solution’ sample is colorless because zinc acetate is dissolved in ethanol (Figure 1a), while the ‘colloidal’ sample is bluish due to the presence of ZnO colloids (Figure 1b). ZnO colloids are formed by addition of alkaline solution in acidic zinc acetate solution. The ‘powder’ sample shows white opaque solution although 20 nm-sized ZnO powders are dispersed in ethanol (Figure 1c), which is probably due to aggregation of the primary nanoparticles. Plane-view scanning electron microscopy (SEM) images show that the coverage of the deposited seed layer is better using the colloidal coating solution than the solution or powder coating solutions. It is clearer from the cross7



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sectional SEM images that FTO surface is almost fully covered with ZnO seed layer by the colloidal coating solution, whereas FTO is partly and locally covered by the solution and the powder coating solution, respectively. The deposited seed layers are seen to be composed of ZnO nanoparticles regardless of coating solutions.

Fig. 1 Plane-view and cross-sectional SEM images of the seed layers for growth of ZnO nanorods formed by (a) a ethanolic solution of zinc acetate (solution), (b) a colloidal solution of zinc acetate in the presence of KOH (colloidal) and (c) a ZnO nanopowders dispersed in ethanol (powder).

Figure 2 shows cross-sectional SEM images of ZnO nanorods grown on different seed layers. ZnO nanorods with diameter of about 60 nm and length of about 600 nm are grown on the seed layers formed from the solution (Figure 2a) and colloidal (Figure 2b) coating solutions, whereas larger diameter of around 120 nm is observed from the seed layer deposited by the powder coating solution (Figure 2c). Such a larger diameter is attributed to the aggregated ZnO nanoparticles in seed layer as can be seen in Figure 1c. The same and 8


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smaller diameter of ZnO nanorods for both the solution and colloidal coating solution are due to the similar ZnO particle size in the seed layer. However, the ZnO nanorods are relatively tilted on the solution-based seed layer compared to the colloidal-based one, which is due to the fact that there is partially uncovered FTO surface. ZnO nanorods are more tilted when they are growing on the powder-based seed layer because most of FTO surface is not covered with ZnO seed layer. Vertically aligned ZnO nanorods on the colloidal-based seed layer are thus attributed to conformal coverage of seed layer on FTO substrate.

Fig. 2 Cross-sectional SEM images of ZnO nanorods grown on the seed layers prepared by (a) solution, (b) colloidal and (c) powder precursor solutions.

Transmittances of seed layers are hardly changed by seed layers in Figure 3a, which indicates that incident light passes through the ZnO seed layers because of thin film characteristics. On the other hand, ZnO nanorods exhibit strong band-edge absorption near 370 nm,36 which is indicative of crystalline ZnO nanorod with band gap of about 3.3 eV, which is consistent with the previous observation with n-ZnO nanorod vertically grown on pGaN.37 The ZnO nanorod grown on the colloidal-based seed layer shows enhanced transmittance, where maximum transmittance at 600 nm is enhanced from 82.9% for the bare FTO substrate to 87.6% for the ZnO nanorod deposited on FTO (Figure 3b). Transmittance of 83.9% at 600 nm for the ZnO nanorod grown on the solution-based seed layer is almost the same as that of the bare FTO, but transmittance of the ZnO nanorod on the powder-based 9



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seed layer is decreased to 74.3%. The enhanced transmittance of the ZnO nanorod grown on the colloidal-based seed layer is related to vertical alignment,38 while the reduced transmittance and the pronounced reduction in transmittance at blue light for the ZnO nanorod grown on the powder-based one are related to the Rayleigh scattering of light because of the titled structure.38,39 The Rayleigh scattering has been known to be inversely proportional to the wavelength to the fourth power (~λ-4), which can lead to scattering of blue light (shorter wavelength light) is relatively stronger than that of red light (longer wavelength light). Therefore, the much lowered transmittance at shorter wavelength than at longer wavelength shown in the ZnO nanorod on the powder-based seed layer results from Rayleigh scattering.

Fig. 3 Transmittance spectra of (a) three different seed layers (SLs) formed from solution, colloidal and powder precursor solutions and (b) ZnO nanorods (NRs) grown on the three different seed layers. MAPbI3 was deposited on the ZnO nanorods using two-step spin-coating methods.33 Focused ion beam (FIB) assisted cross-sectional SEM images for the full cell structure are investigated in Figure 4. The ZnO nanorods are well attached on the colloidal-based seed

10


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layer even after completion of deposition of perovskite, spiro-MeOTAD hole transporting material (HTM) and Au layers (Figure 4b), while some of ZnO nanorods are detached from the substrate for both the solution- and powder-based seed layers resulting in FTO surface uncovered with seed layer (Figures 4a and 4c). This separation of ZnO nanorod from the substrate is attributable to weak adhesion between ZnO nanorod and FTO surface due to the reduced surface area for adhesion because of tilting. Most of nanorods deposited on the powder-based seed layer are detached because tilting angle is greater than the solution-based seed layer case. The detachment phenomena may be preceded while depositing MAPbI3 since the surface morphology of the MAPbI3 capping layer is quite different. It can be also seen that some of the detached ZnO nanorods are directly contacted with HTM.

Fig. 4 FIB-assisted cross-sectional SEM images of the perovskite solar cells based on ZnO nanorods grown on the three different seed layers formed from (a) solution, (b) colloidal and (c) powder precursor solutions.

Photocurrent (J)-voltage (V) curves (Figure 5a) and incident photon-to-electron conversion efficiency (IPCE) (Figure 5b) are compared for the three different seed layers. The photovoltaic parameters are listed in Table 1. Open-circuit voltage (Voc) is remarkably altered by the seed layers, where Voc is highest for the colloidal-based seed layer (Voc = 0.956 V), while the solution-based one (Voc = 0.808 V) exhibits intermediate value and the powderbased one shows lowest Voc of 0.526 V. It is noted that similar short-circuit current density (Jsc) ranging between 20.4 and 20.9 mA/cm2 is observed, which indicates that Jsc is hardly 11



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altered by the seed layers. Series resistance (Rs) and shunt resistance (RSH) are estimated from the J-V curves, where low Rs and high RSH are observed for the colloidal-based seed layer leading to better fill factor (FF). On the other hand, relatively high Rs and low RSH for the solution- and powder-based ones are related to the direct exposure of FTO to HTM due to the detachment of ZnO nanorods.40 As a result, PCE of 11.68% is obtained for the colloidalbased seed layer, whereas lower PCEs of 8.03% and 4.66% are observed for the solution- and powder-based ones, respectively. Little difference in IPCE over the entire wavelength, except for the wavelength below 370 nm, is consistent with the similar Jsc. The seed layer independent Jsc will be discussed by means of the detailed spectroscopic analysis.

Fig. 5 (a) J-V curves and (b) IPCE spectra for the perovskite solar cells based on ZnO nanorods grown on the three different seed layers. Active area was 0.12 cm2.

Table 1 Photovoltaic parameters of short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE), together with shunt (RSH) and series (Rs) resistances. Seed layer

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

RSH (Ω·cm2)

Rs (Ω·cm2)

Solution

20.76

0.808

0.479

8.03

1.1×103

8.95

12


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Colloidal

20.40

0.956

0.599

11.68

2.5×103

7.37

Powder

20.88

0.526

0.424

4.66

1.4×103

8.46

Average photovoltaic parameters are depicted in Figure 6. Average PCEs with standard deviation obtained from 10 cells are 7.26±1.02, 10.14±0.81 and 5.01±1.09% for the solution-, colloidal- and powder-based seed layers (Figure 6d), respectively, based on average Jscs of 20.13±0.78, 20.44±0.87 and 20.17±0.71 mA/cm2 (Figure 6a), average Vocs of 0.758±0.059, 0.896±0.035 and 0.528±0.052 V (Figure 6b) and average FFs of 0.476±0.040, 0.553±0.032 and 0.467±0.068 (Figure 6c). The highest PCE for the colloidal-based seed layer is mainly due to highest Voc, which is related to compact ZnO seed layer. Despite different transmittance as observed in Figure 3b, Jsc is confirmed to be invariant with seed layer. To explain this, light harvesting efficiency (LHE) and absorbed photon-to- current conversion efficiency (APCE) are investigated.

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Fig. 6 Statistical photovoltaic parameters of the perovskite solar cells depending on seed layers formed from solution, colloidal and powder precursor solutions for growth of ZnO nanorods. The data were obtained from 10 cells. LHE is calculated based on the relation LHE = (1-R)(1-10-A),41 where R and A represent reflectance and absorbance (Figure 7a), respectively. In the LHE spectra (Figure 7b), the MAPbI3 film based on the powder-based seed layer shows higher LHE at longer wavelength compared to those based on the solution- and colloidal-based ones. For the powder case, this higher LHE at long wavelength is due to the less dense ZnO nanorod layer and the similar LHE at short wavelength is related to the strong blue light scattering effect by the tilted ZnO nanorod, as mentioned previously. Despite higher LHE, lowest APCE (APCE = IPCE/LHE) is observed for the powder-based seed layer (Figure 7c), which indicates that photoexcited electrons by red light are not effectively collected. The APCE for the solution14


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based seed layer case is higher than the powder case, but still lower than the colloidal case. The ineffective APCE at red light is presumably associated with recombination because the detached ZnO nanorods are partly in contact with HTM as observed by SEM in Figure 4. Eventually, similar Jsc can be explained by LHE and APCE results.

Fig. 7 (a) absorbance and reflectance, (b) light harvesting efficiency (LHE) and (c) absorbed photon-to-current conversion efficiency (APCE) spectra for the MAPbI3 film deposited on ZnO nanorod films formed on three different seed layers using solution, colloidal and powder precursor solutions. LHE was obtained using LHE = (1-R)(1-10-A), where R and A represent reflectance and absorbance, respectively.

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Electrochemical impedance spectroscopy (EIS) is studied to understand the strong dependency of Voc on the seed layer. Figure 8 shows dart current, recombination resistance and high frequency resistance as a function of applied bias potential. In Figure 8d, Nyquist plots measured at 0.1 V are presented along with fit results based on series connection of one component with single resistor (R) and two components with parallel connection of resistor (R) and capacitor (C) as can be seen in the inset of Figure 8d where the R1(Rhf)-CPE1 component in high frequency region reflects the carrier transport in selective electrodes and the following R2(Rrec)-CPE2 component in intermediate frequency region represents recombination process closely related to the perovskite material. In Figure 8a, dark current starts to increase from 0.4 V for the colloidal-based seed layer, which is higher than the solution- and powder-based ones. Dark current will be dominated as the conductance in photoanode increases due to the rise of Fermi level with bias potential.1 At high bias potential, dark current increases in the order of powder>solution>colloidal. When considering the similar Jsc, large dark current is expected to result in lowering Voc. The recombination resistance (Rrec) in the intermediate frequency region under light in Figure 8b is mirrored to the dark current behavior. However, Rrec at low bias potential (approaching short-circuit), related to perovskite interface or perovskite bulk,42 increases in the order of colloidal>solution>powder. Higher resistance leads to lower recombination and consequently higher Voc. Rrec at high bias potential is probably related to the ZnO nanorod/perovskite interface or perovskite/HTM interface, which decreases steeply with bias potential because of increase in conductance by the rise of Fermi level.42,43 In this region, Rrec is higher in the order

of

colloidal>solution>powder,

which

means

that

recombination

at

ZnO

nanorod/perovskite interface is suppressed for the colloidal-based seed layer case. The Rrec behavior is well correlated with interface and morphology of ZnO nanorod. The resistance at 16


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high frequency (Rhf) in Figure 8c is related to selective contacts or their interfaces.43,44 The colloidal-based seed layer shows higher Rhf in the bias potential raging between 0.2 and 0.7 V than the solution- and the powder-based ones. Such a high Rhf is due to the full coverage of seed layer by the colloidal method, leading to high Voc due to suppression of recombination at seed layer interface.

Fig. 8 Bias potential dependent EIS data represented by (a) dark current measured under dark, (b) recombination resistance obtained under light from low frequency semicircles and (c) high frequency resistance under light for the perovskite solar cells based on ZnO nanorods grown on three different seed layers. (d) Raw EIS data (circles) and fit results (lines) measured at bias potential of 0.1 V. Inset in (d) represents an equivalent circuit for fitting.

Based on the impedance spectroscopic studies, in addition to the role of seed layer 17



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ZnO nanorod/perovskite is found to be also important in determining photovoltaic performance. Thus, we have investigated effect of surface modification of ZnO nanorod on photovoltaic performance. For this purpose, surface of the ZnO nanorods grown on the colloidal-based seed layer was modified with (NH4)2TiF6.30-32 Due to acidic nature of the solution with (NH4)2TiF6 and H3BO3 (pH 4~5), the reaction time is limited to 2 min. The SEM images are compared before (Figure 9a) and after (Figure 9b) surface treatment. It is clearly seen that surface morphology is changed and the average diameter of ZnO nanorod increases from 24.1 nm to 37.3 nm after surface modification. Thus the increased wall thickness of about 6 nm is due to formation of TiO2 on the core ZnO nanorod.

Fig. 9 Plane-view SEM images of ZnO nanorod (a) before and (b) after surface treatment with (NH4)2TiF6. ZnO nanorods were grown on the colloidal-based seed layer. Elemental analysis using EDX showed that atomic ratio of Ti:Zn = 1:3.53 after surface treatment.

As expected, surface modification improves photovoltaic performance. J-V curves and IPCE are compared before and after surface treatment in Figure 10a and Figure 10b, respectively, and photovoltaic parameters are listed in Table 2. Voc and FF are substantially improved from 0.956 V to 1.055 V and 0.599 to 0.698, respectively, after surface modification, corresponding to 10.4% and 16.5% increments. Due to the significant increases in Voc and FF, PCE is enhanced from 11.68% to 14.35%. Jsc is slightly decreased by about 18


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4.7%, which is due to red shift of IPCE at short wavelength because of formation of smaller band gap TiO2 on the surface of ZnO nanorod.

Fig. 10 (a) J-V curves and (b) IPCE spectra of the perovskite solar cells based on ZnO nanorods grown on the colloidal seed layer. Filled circles and empty circles represent surface modified ZnO nanorods (treated with TiF62-) and bare ZnO nanorods (untreated), respectively.

Table 2. Photovoltaic parameters of Jsc, Voc, FF and PCE for the ZnO nanorod with and without (NH4)2TiF6 treatment. Surface treatment

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

untreated

20.40

0.956

0.599

11.68

with (NH4)2TiF6

19.44

1.055

0.698

14.35

Comparison of Rrec can explain the effect of surface modification. As can be seen in Figure 11, little change in Rrec at low bias potential (0.4V). It is thus evident that the increased Rrec by the surface modification is indicative of suppression of recombination at the ZnO nanorod/perovskite interface, which further improves Voc and in part influences FF. 19



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Fig. 11 Light induced recombination resistances for the perovskite solar cells based on the surface modified ZnO nanorods (treated) and the bare ZnO nanorods (untreated). ZnO nanorods were grown on the colloidal seed layer.

Since the post-treatment with (NH4)2TiF6 may have influence not only on the surface of ZnO nanorod but also on the seed layer, it is necessary to investigate its effect on the other two ZnO morphologies. To clarify this point, we compare the effect of the same treatment on the other two ZnO morphologies prepared from solution- and powder-derived seed layers. Figure 12 and Table 3 show the J-V curves and photovoltaic parameters, respectively, for solution and powder seed layers. As can be seen in Figure 12a and Figure 12b, Voc for the solution seed layer is improved by 17% (0.748 to 0.848 V) but Voc for the powder seed layer is improved significantly from 0.498 V to 0.717 V, corresponding to 44% increment. Such a difference in the Voc increment depending on seed layer indicates that TiO2 is also coated not only on the ZnO nanorod surface but also on the seed layer by the post-treatment, leading to s reduced leaking at the FTO substrate and thereby an improved Voc. Nevertheless, the 20


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improved Voc for both solution- and powder-derived seed layers could not reach that for the colloidal seed layer, which implies that the ca. 10% increment in Voc by the post-treatment for the colloidal seed layer (Figure 10) is due to the modification of ZnO nanorod and the interface between perovskite and ZnO nanorod plays different role in determining photovoltaic performance from the seed layer role.

Fig. 12 J-V curves of the perovskite solar cells based on ZnO nanorods grown on (a) the solution-derived and (b) the powder-derived seed layers. Filled circles and empty circles represent surface modified ZnO nanorods (treated with TiF62-) and bare ZnO nanorods (untreated)

Table 3. Photovoltaic parameters of Jsc, Voc, FF and PCE for the ZnO nanorods with and without (NH4)2TiF6 treatment. Seed layer Solution

Powder

Surface treatment

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

untreated

20.45

0.748

0.571

8.74

with (NH4)2TiF6

19.52

0.878

0.591

10.12

untreated

20.87

0.498

0.440

4.57

with (NH4)2TiF6

19.82

0.717

0.452

6.42

21



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Conclusion In conclusion, the seed layer was found to play important role in aligning ZnO nanorods and in determining photovoltage for the ZnO nanorod-based perovskite solar cell. We found that the colloidal solution produced a compact seed layer fully covering FTO surface and thereby vertically aligned ZnO nanorods. Consequently, high photovoltage was obtained due to the reduced recombination by this method. On the other hand, locally deposited seed layer using a clear solution or a nanopowder dispersed solution resulted in tilted nanorods that were easily detached from the substrate, which led to recombination sites. We also confirmed from impedance spectroscopy that the interface between ZnO nanorod and perovskite was involved partly in recombination, which could be overcome by surface modification with (NH4)2TiF6. Eventually PCE of 14.35% was achieved by controlling the seed layer and the nanorod/perovskite interface.

AUTHOR INFORMATION Corresponding Author *Tel: 82-31-290-7241. Fax: 82-31-290-7272. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. NRF-2012M1A2A2671721,

NRF-2012M3A7B4049986

(Nano

22


Material

Technology

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Development Program), NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System).

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Effects of Seed Layer on Growth of ZnO Nanorod and Performance of

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