Solid-State Electronics 53 (2009) 1116–1125

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Solid-State Electronics

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Preparation and characterization of solid n-TiO2/p-NiO hetrojunction electrodesfor all-solid-state dye-sensitized solar cells

Yi-Mu Lee *, Chun-Hung LaiDepartment of Electronic Engineering, National United University, MiaoLi 36003, Taiwan

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 February 2009Received in revised form 3 July 2009Accepted 9 July 2009Available online 3 August 2009

The review of this paper was arrangedby Dr. Y. Kuk

Keywords:Thin filmNiOSol–gel processp–n junctionHall effectDye-sensitized solar cell

0038-1101/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.sse.2009.07.004

* Corresponding author. Tel.: +886 37 381532; fax:E-mail address: ymlee@nuu.edu.tw (Y.-M. Lee).

Thin films of p-type nickel oxide (NiO) were prepared by a sol–gel spin deposition on ITO/TiO2 to fabri-cate the photoelectrodes and all-solid-state dye-sensitized solar cells. The Ni(OH)2 sol was formed fromnickel (II) acetate tetrahydrate in a mixture of alcohol solution and poly(ethylene glycol), and followed bydifferent heat treatments in air (350–800 �C). The formation and composition of NiO thin film was veri-fied by EDX and X-ray diffraction (XRD) analysis, which shows preferred orientation along the (1 1 1)plane. The thickness of the NiO film calcined at 450 �C for 1 h is 120.6 nm with average grain size of22 nm, and high UV transparency (�75%) in the visible region is observed. The electrical properties ofthe sol–gel NiO films such as hole carrier concentration, sheet resistance and carrier mobility were exam-ined using Hall measurement. Results show that the Hall mobility is dominated by the hole concentra-tion. Furthermore, all-solid-state dye-sensitized solar cells comprising n-TiO2/p-NiO compositeelectrode were fabricated and the performance was evaluated. The current–voltage (I–V) characteristicsof the composite TiO2/NiO electrode in dark demonstrate a good rectifying curve, verifying the p-typebehavior of NiO films. Solar cells when sensitized with Ru-dye (N719) demonstrate short-circuit photo-current (ISC) of 0.33 mA/cm2 and open-circuit photovoltage (VOC) of 210 mV; the overall energy conver-sion efficiency of the device is about 0.025%.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Among the metal oxide materials, Nickel oxide (NiO) is a trans-parent semiconductor oxide with p-type conductivity, and becomesan attractive material due to its superior chemical [1] and electricalstability [2] as well as optical, electrical and magnetic properties[3]. NiO is usually a non-stoichiometric crystal which reveals p-typecharacteristics by hole transport originated from nickel vacanciesand/or oxygen interstitials [4]. The NiO thin films have been appliedin diverse fields, including electrochromic display devices [5], elec-trochemical supercapacitors, gas sensors, fuel cell electrodes [6].Due to its p-type conducting properties and wide band-gap charac-teristic, NiO becomes a promising candidate in photoelectrodes anddye-sensitized solar cell devices [1,6]. So far, various methods suchas sputtering [7–9], evaporation [10], spray pyrolysis [11], electro-chemical deposition [12] and sol–gel techniques [13,14] have beenemployed to prepare high-quality NiO films.

NiO prepared by different deposition technologies provide filmsof different morphologies. Many researches have been focused onreactive sputtering technique because excellent optical and electri-cal properties of the sputtered-NiO films are obtained under proper

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conditions (sputtering pressure: 10�3–10�2 torr, O2 atmosphereand heated substrate) [2–4,15]. It has been found that an O-richNiO is formed by sputtering method with the stoichiometry of O/Ni ratio �1.21 [4]. The crystallite size was observed to increasefrom 4 to 18 nm depending on the film thickness, RF power andthe substrate temperature [2,15]. Also, the surface morphology re-vealed that the crystallite sizes of the NiO film deposited at 400 �Cwere smaller than that of unheated samples due to the formationof tiny crystallites [15]. The NiO sample prepared at 350 �C andRF power of 150 W shows higher optical transmittance of �65%,which decreases with increasing film thickness. The electricalproperties of the sputtered-NiO films have been evaluated byHall measurement. The highest carrier concentration of 3.13 �1018 cm�3 and mobility of 0.55 cm2/V s were observed [2], indicat-ing the p-type conductivity of the NiO films is very low. The lowestresistivity of the sputtered-NiO film was reported to be 16.7 X. It isalso interesting to note that the NiO thin films prepared by sputter-ing have low carrier conductivity due to the decrease in oxygenvacancies [15]. It has been shown that the resistivity can be re-duced by five orders of magnitude by depositing NiO in O2 as com-pared to Ar atmosphere, and Hall mobility can be increased to�2 cm2/V s in CuO2–NiO multicomponent p-type oxide usingCuO2–NiO mixed powder target [3]. In particular, two crystalorientations, (1 1 1) and (2 0 0), were observed in sputtered-NiO

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Fig. 1. A schematic illustration of the SS-DSSC test cell: (1) ITO glass; (2) 3.5 lmcompact TiO2 layer; (3) 3.5 lm dye-sensitized mesoporous TiO2 layer; (4) 120 nmdye-sensitized p-type NiO layer; (5) Pt counter electrode. The shadow regions showthe position of Ru-dye (N719).

Fig. 2. Surface SEM micrographs of the NiO films by calcination of Ni(OH)2 at various temsubstrate is ITO-glass.

Y.-M. Lee, C.-H. Lai / Solid-State Electronics 53 (2009) 1116–1125 1117

depending on the substrate temperature. The binding energy andbonding configuration of the sputtered-NiO have been studied byXPS [4]; the results suggested that the NiO film prepared by RFsputtering is thermodynamically unstable.

Sol–gel process consisting of hydrolysis and polymerization ofthe metal precursors is a controllable and low cost method, andhas been widely used in the preparation of new materials andnanostructures [14]. Most of previous studies focused on thin filmpreparation, since the preparation of the precursor is complicateddue to the problems of precursor precipitation and the hardnessand adhesion of the films [14]. NiO thin films and nanowires havebeen synthesized by a sol–gel process, in which acetate, chloride,and sulphate of Ni metal were used as precursors. It should benoted that nanoparticles need to be well dispersed in the solvent,which would have an influence on optical transparency of theNiO films. The NiO films prepared by sol–gel method exhibited uni-form size distribution [16] and compact and highly homogeneoussurface morphology [17] compared with the films deposited by

perature profiles. The average grain size is �22 nm for NiO calcined at 450 �C. The

Table 1BET surface area and grain size of the sol–gel derived NiO films at different calcinationtemperatures.

Calcination temperature (�C) Grain size (nm) Specific surface area (m2/g)

350 15.3 ± 1.2 37.97 ± 0.09450 22.0 ± 1.0 33.27 ± 0.19550 33.0 ± 5.7 8.88 ± 0.04650 43.0 ± 9.6 4.39 ± 0.06800 97.5 ± 3.3 2.04 ± 0.01

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sputtering technique. The crystallite sizes of the sol–gel NiO filmwere in the range of 13–120 nm depending on the nickel precur-sors, base types and calcination temperatures [16,18]. The BET sur-face areas of the NiO films calcined at 750 �C were ranged from 2.9to 5.5 m2/g [16]. Up to now, the NiO prepared by sol–gel methodhas been studied for the applications of electrochromic [13] andoptoelectronic devices [12]. However, the electrical properties ofthe sol–gel derived NiO films have yet been reported.

The concept of all-solid-state DSSC (SS-DSSC) with implementof hole conducting materials was proposed to solve some of theproblems encountered in the liquid electrolyte-DSSCs [6,19,20].The solid hole conducting layer should possess p-type, low-resis-tivity characteristics and great thermal stability. Recently, nano-structured p-type NiO film acting as a photocathode wassuccessfully employed in DSSC [6]. The photocurrent in this typeof device is attributed to hole injection from dye molecule to thevalence band of the p-type NiO electrode. In this way, the valuesof ISC, VOC and IPCE are very low because the photocurrent is catho-dic in contrast to the DSSC developed by Gratzel [21]. The short-circuit current and open-circuit voltage were reported to be0.079 mA/cm2 and 98.5 mV, respectively. The overall conversionefficiency is very low (g = 0.0033%); however, it is the first timethat p-NiO is presented to serve as a solid hole collector [6]. Thecombination of photoanode and photocathode or p–n junction

Fig. 3. The SEM cross-section micrographs of (a) one-layer NiO calcined at 450 �C (b) twoNiO calcined at 650 �C. The average thickness of one NiO layer is 120.6 nm and 130.9 nm

exhibiting rectification behavior was further suggested to generatea potential gradient to enhance charge separation, which is neededfor functioning as a photovoltaic devices and solid-state DSSCs[22]. The composite p–n electrodes constructed with TiO2/NiOand SnO2/NiO have been prepared by dipping TiO2 or SnO2 elec-trode into NiSO4 solution by Bandara’s group [22]. The electro-lyte-DSSC consisting of TiO2/NiO demonstrates fill factor of 49.5%and conversion efficiency of 1.6% [22]. For SS-DSSCs with n-TiO2/p-NiO electrode, the fill factor and conversion efficiency were re-ported to be 47.6% and 0.032%, respectively [23], where the thick-ness of the NiO thin film was roughly measured to be �1.2 nm.Although the conversion efficiency of SS-DSSCs is low as comparedto liquid-DSSC, SS-DSSCs have received increasing attention sincethey possess environment-benign characteristics, high stabilityand also make the manufacture of tandem structures simple whichcould be used as a mobile power in the flexible electronics.

In this study, we report a simple sol–gel method combined withcalcinations to prepare p-type NiO thin films. The structural andoptical properties of the NiO are investigated, and its electricalproperties are evaluated by Hall measurement. In addition, I–Vcharacteristics of all-solid-state DSSCs constructed with n-TiO2/p-NiO composite electrode in the dark and under 1-sun illumination(air mass, AM 1.5) are investigated, and the conversion efficiency isevaluated.

2. Experimental

2.1. Preparation of NiO thin film and cell fabrication

Indium tin oxide (ITO) coated glass (AUO Co, Ltd.) was used astransparent conducting substrate with the sheet resistance of7 X/h. The ITO glasses received wet chemical cleaning before useby sonication in methanol and de-ionized water for 10 min, respec-tively. It was then rinsed with distilled water and dried with N2.The details of TiO2 (p-25, Degussa) film preparation by sol–gel

-layer NiO calcined at 450 �C, and (c) one-layer NiO calcined at 650 �C (b) two-layercalcined at 450 �C and 650 �C, respectively. The substrate is ITO-glass.

Y.-M. Lee, C.-H. Lai / Solid-State Electronics 53 (2009) 1116–1125 1119

method are similar to those reported elsewhere [21,24]. After thecoating of two TiO2 layers, the thickness of the TiO2 film was about7 lm determined by a stylus surface roughness detector (Alphastep 200).

The nickel hydroxide sol was formed from 0.5 M nickel (II) ace-tate tetrahydrate, Ni(CH3COO)2�4H2O, in a mixture of alcohol solu-tion and poly(ethylene glycol). The poly(ethylene glycol) is aadditive material to improved the adhesion and hardness of thefilms during heating, which provides a suitable mixture of sols.Subsequently, ammonia aqueous solution NH3OH (4–9 wt.%) wasadded drop-wise into the mixture until a clear and blue mixedsolution is formed. Received Ni(OH)2 sol was further aged at roomtemperature for 24 h, then was deposited in air at two steps withdifferent spinning speed, which are 800 rpm and 4800 rpm bothfor 30 s. Finally, the coated films were dried at 200 �C in an ovento remove H2O and chemical residue followed by calcinations inair at varied temperatures of 350, 450, 550, 650 and 800 �C for 1 h.

Then the composite p–n electrode was fabricated by spin coat-ing p-type NiO oxides onto n-type TiO2 layer. The composite p–nelectrodes were immersed in a solution of Ru 719 dye (2,20bipyri-dine-4-COOH,40-COO� ruthenium (II)) [25] in ethanol for 24 h.Counter electrode was prepared by the sputtering of Pt on the

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Fig. 4. (a) The EDX spectrum and (b) XRD spectrum of the NiO on ITO substrate calcinedglass.

ITO plate as the back contact, and two binder clips were used tohold the electrodes together. Fig. 1 illustrates a completed solid-state DSSC (SS-DSSC) structure, where the Ru-dye was penetratedinto the NiO film (layer 4), the TiO2/NiO interface and the TiO2 film(layer 3). This interpenetrated network structure is similar to themonolithic structure in SS-DSSC proposed by [26]. In these cases,the dye molecule exists in the TiO2/NiO interface and also in theTiO2 layer, which facilitates the released electrons transportingfrom the dye toward the ITO-glass after light soaking [27,28].

2.2. Materials characterization and photovoltaic propertymeasurement

The surface morphology, crystallization, film thickness andaverage grain size of the deposited film were studied by field emis-sion scanning electron microscopy (SEM, JEOL JSM-6700F). The for-mation and composition of the NiO film were examined by XRDand energy dispersive X-ray spectroscopy (EDX). Optical transmis-sion spectra of the films were analyzed by using a UV–visible spec-trophotometer (Perkin Elmer Lambda 20). The surface area ofprepared nickel oxides were determined by nitrogen physisorptiondata using a Micrometritics ASAP 2020. The carrier concentration

50 60 70 80

a (deg.)

(102)

at 450 �C for 1 h in air. The sharp peaks in squares (h) are originated from the ITO-

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and mobility of NiO deposited on a square glass were measuredwith the Hall equipment using van der Pauw’s method. Current–voltage (I–V) characteristics and rectification effects of the deviceswere measured at room temperature using an electrochemicalworkstation (Jiehan 5000) under potentiostat/galvanostat mode.The scan rate was 50 mV/s. Cell performance was measured underAM 1.5 and 100 mW/cm2 using a solar simulator (Peccell, Japan).The power of the simulated light was adjusted with a silicon pho-todiode (Peccell, Japan). Here, the active area of the devices was ad-justed to 1.0 � 1.0 cm2.

3. Results and discussion

3.1. Characterization of NiO film

The direct calcination of Ni(OH)2 will proceed according to thefollowing reaction [12]:

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450C350C550C50C200C800C

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Fig. 5. (a) Transmittance spectra of the NiO films prepared at various calcination tempcalcination temperatures, where the value of transmittance is measured at wavelength

NiðOHÞ2 ! NiOþH2O

According to the above reaction, the NiO was the direct product ofthe calcination. Fig. 2 shows SEM image of NiO film calcined at var-ious temperatures for 1 h. It can be seen that surface of the film cal-cined at 200 and 350 �C are loose and smooth, indicating that thecrystal degree is not well. Even for the calcination of Ni(OH)2 upto 2 h, the crystal degree of the film is not well [29]. In contrast,we could clearly observe the porous structures with crystal grainsand grain boundaries from Fig. 2c–f. It is clearly found that theNiO film (450 �C) has grain size of 22.0 ± 1.0 nm distributed acrossthe film, and becomes larger with increasing calcination tempera-ture. SEM images reveal that the NiO thin film crystallized for heattreatment above 450 �C, which agrees with the results that the crys-tallization temperature of nickel oxide should be above 400 �C [30].The film calcined at 800 �C exhibits a more dense and granularstructure as seen in Fig. 2f. The reason for this phenomenon maybe attributed to the enhancement of the thermal excitation of the

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eratures. (b) The relation between optical transmittance and grain size at differentof 550 nm.

Table 2Comparison of material properties of NiO films deposited by various methods.

Ref. no. Crystalline size (nm) XRD Process

[16] 80–120 (1 1 1) (2 0 0) (2 2 0) Sol–gel/spin[2] 11–18 (1 1 1) (2 0 0) Sputtering[15] 10–16 (1 1 1) (2 2 0) Sputtering[43] 12–23 (1 1 1) (1 0 2) Doctor blade[44] �28 (1 1 1) Spray pyrolysis[44] �20 (1 1 1) (2 0 0) CBDThis work �22 (1 1 1) (2 0 0) Sol–gel/spin

Y.-M. Lee, C.-H. Lai / Solid-State Electronics 53 (2009) 1116–1125 1121

mobility on the surface [11]. Table 1 lists the BET surface area andgrain size of the NiO films calcined from 350 to 800 �C. The highmagnification SEM cross-section micrograph of the NiO film cal-cined at 450 �C and 650 �C can be seen in Fig. 3a and c. It is observedthat the thickness of the NiO film (450 �C) is 120.6 ± 2.8 nm and isincreased to 130.9 ± 1.7 nm calcined at 650 �C as compared inFig. 3b and d. The results show the NiO thin films have good unifor-mity by the layer-by-layer spin coating method. There is an emptyspace between ITO substrate and NiO film due to the volume con-traction during the drying procedure in the sol–gel method [31];however, the contraction was not observed for the case of the sput-tered-NiO film [15]. This volume contraction can be relieved byultrasonication procedure during the sol preparation as discussedin Section 3.3. The EDX spectrum of NiO is illustrated in Fig. 4a, inwhich the peaks of O and Ni are pronounced. The formation andcomposition of a crystalline NiO layer are justified from quantita-

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Fig. 6. Hall measurement of the sol–gel NiO thin films: (a) sh

tive analysis which reveals that the components of Ni and O arestoichiometric for NiO. The peaks in the high energy range are orig-inated from the ITO-substrate. Fig. 4b shows XRD spectrum of theNiO obtained in the present work. The XRD patterns further confirm

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eet resistance and (b) carrier concentration and mobility.

Table 3Comparison of electrical properties of NiO thin films prepared by sputtering (power:150 W [2] and power: 50 W/substrate temperature: 200 �C [38]) and sol–gel process.

Grain size(nm)

Carrier concentration(cm�3)

Hall mobility (cm2/V s)

Process

11.5a 1.2 � 1017 0.52 Sputtering[2]

18.0b 5.0 � 1017 0.38 Sputtering[2]

No data 1.3 � 1019 �2.0 Sputtering[38]

22.0c 5.7 � 1020 24.1 Sol–gel33.0d 6.4 � 1020 23.9 Sol–gel

a Substrate temperature: 25 �C.b Substrate temperature: 150 �C.c Calcination temperature: 450 �C.d Calcination temperature: 550 �C.

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that the NiO film after 450 �C calcinations is crystal orientation(1 1 1), which is single crystalline in nature with the hexagonalstructure as seen in [32,33].

The transmittance spectra of the NiO films received differentheat treatment is shown in Fig. 5a. The results indicate that theNiO film (450 �C) shows highest transparency (�75% at 550 nmwavelength) in the visible region. In contrast, UV transparency inthe NiO films (50 and 200 �C) is mush lower due to the amorphouscharacteristics and low porosity of these films [34,35]. Fig. 5bshows the relation between optical transmittance and grain sizeat different calcinations temperatures. It appears that the NiO filmcalcined at 800 �C becomes condensed with larger grain size(97.5 ± 3.3 nm), which causes the high reflection of the light andthen results in extremely low transparency (<10%). The resultsconfirm that the size of nanoparticles would influence light scat-tering and transmittance of the coated films [36]. The decreasingtrend of transmittance with increasing temperatures was also ob-served in the Al-doped ZnO transparent films [37]. It is worthnoticing that the sol–gel NiO film exhibits much higher transmit-tance as compared to the value (40–65%) observed in sputtered-NiO films [2,38].

Fig. 7. (a) Surface SEM morphology and (b) cross-section SEM morphology of TiO2/NiO hetrojunction electrodes on the ITO substrate.

3.2. Electrical properties of p-NiO films—Hall effect measurement

Fig. 6 shows the results of Hall effect measurement of NiO thinfilms at various calcination temperatures. It is clearly seen that theresistivity, Hall coefficient, carrier concentration and mobility var-iation are dependent on calcinations temperatures. The resistivityof the NiO films decreases with increase of temperatures in therange of 350–650 �C, indicating the negative temperature coeffi-cient resulting from the semiconductor properties of NiO [39].The large increase of resistivity for NiO calcined at 800 �C may bedue to decrease in carrier mobility as shown in Fig. 6b. For theNiO calcined at 450 �C, the carrier concentration of and Hall mobil-ity of are measured to be 5.7 � 1020 cm�3 and 24.1 cm2/V s, respec-tively. The highest carrier concentration is 8.86 � 1020 cm�3

(650 �C) corresponding to the highest mobility of 28.7 cm2/V s(Fig. 6b). Hall mobility depends strongly on either the grain size,their texture, the crystallinity and the fluctuation of grain orienta-tion as well. Thus, the electrical properties of the sol–gel derivedNiO are improved by 1–2 orders of magnitude compared to sput-tered-NiO with low conductivity [2,38]. The explanation is thatthe structures of the sputtered-NiO films have many tiny crystal-lites in each grain, which leads to the nonuniform microstructureand smaller grain size [2,40]. The structural and electrical proper-ties of the sputtered-NiO and sol–gel NiO are compared in Tables 2and 3, respectively.

3.3. Electrical characterization of TiO2/NiO electrode and DSSC

Fig. 7 shows the surface and cross-section SEM morphology ofthe TiO2/NiO hetrostructure calcined at 450 �C. Comparing Figs.2c and 7a, clear aggregates of NiO crystallites are observed in bothSEM images but does not reveal any distinguished difference be-tween TiO2 film and TiO2/NiO stacked film due to the uniform cov-erage of NiO overlayer. For the result presented in Fig. 7b, thevolume contraction at the ITO/TiO2 interface is disappeared. Thisimprovement is attributed to the fact that the TiO2 sol receivedultrasonication for 30 min before deposited on the ITO-glass [31].The dark I–V characteristics of the devices constructed with singleTiO2 layer, TiO2/dye and TiO2/NiO in the forward and reverse biasare shown in Fig. 8. It is found that single TiO2 layer and TiO2/dye reveal a high-resistance junction behavior by demonstratingpoor rectification slopes. The absence of the diode characteristicsin TiO2/dye electrode has been explained from the viewpoint ofelectron transfer [1,23]. Enhanced rectification of the solid TiO2/NiO electrode was observed as compared to the structures of singleTiO2 layer and TiO2/dye. The p-type behavior of NiO film evidencedby the significant rectifying curve is noticed.

The effect of calcination temperature on the dark I–V character-istics of TiO2/NiO hetrojunction (without dye soaking) is alsoinvestigated. As seen in Fig. 9, the NiO film calcined at 350 �Cshows onset voltage of 4 V and poor rectification behavior; how-ever, the films received heat treatment at 450 �C and 550 �C exhibitsignificant junction behavior with greater rectification slope, andthe onset voltage is reduced to the range of 2–2.5 V which is muchlower than the value (2.5–4.5 V) of homojunction (TiO2/Fe3+–TiO2)[41] and hetrojunction (n-ZnO/p-ZnO:N) reported in [42]. The rec-tification slope of TiO2/NiO junction electrode is as high as1.37 mA/V and the rectifying current can reach 1.18 mA measured

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Fig. 8. Dark I–V characteristics of cells made from (a) TiO2, (b) TiO2/dye and (c) TiO2/NiO. The TiO2/dye was prepared by immersing bare TiO2 electrode in N719 at roomtemperature for 24 h.

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Fig. 9. I–V curves of the TiO2/NiO electrode calcined at different temperatures.

Y.-M. Lee, C.-H. Lai / Solid-State Electronics 53 (2009) 1116–1125 1123

at 4 V. This rectification slope is significantly increased by 1–2 or-ders of magnitude as compared to the values in TiO2/NiO-coatedTiO2 [22,23] and TiO2/Fe3+–TiO2 electrodes [41]. It is worthwhileto point out that the TiO2/NiO calcined 550 �C demonstratesslightly high rectifying current at higher applied voltages. This re-sult is believed to be due to the fact that the resistance was de-creased in this condensed NiO film as illustrated in Fig. 6a.However, the composite electrode calcined at 450 �C shows high-est optical transmittance and excellent junction properties, thusthe p-type NiO layer in place of the liquid electrolyte in DSSCs isfeasibility.

Fig. 10 shows the I–V characteristic of SS-DSSC constructed ofTiO2/NiO composited electrode (with N719 dye soaking) undersame 1-sun simulated illumination. The device has a short-circuitcurrent (JSC) of 0.33 mA/cm2 and an open-circuit voltage (VOC) of210 mV. The maximum conversion efficiency of the cell is evalu-ated to be 0.025%. The JSC is two times greater than the highest va-lue (JSC of 0.14 mA/cm2) of a SS-DSSC reported in [23]; the lowphotocurrent is attributed from poor dye loading, where the dyeis only adsorbed on the ultrathin NiO layer. Table 4 compares thecell performance of SS-DSSC with TiO2/NiO(dye), TiO2/NiO andTiO2/dye. It is seen that only the SS-DSSC with TiO2/NiO(dye) dem-

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Fig. 10. I–V characteristics of the SS-DSSC under illumination intensity of 100 mW/cm2 AM. 1.5. The active area of the test cell is 1.0 � 1.0 cm2.

Table 4Electrical performance of SS-DSSC with different structures at illumination intensityof 100 mW/cm2 AM 1.5.

Structure JSC (mA/cm2) VOC (V) g (%)

TiO2/NiO(dye) 0.33 0.21 0.025TiO2/NiO 5.9 � 10�5 0.26 <0.001TiO2/dye 1.2 � 10�4 0.41 <0.001

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onstrates significant device characteristics. In other words, theTiO2/NiO(dye) structure provides a good contact between nanopor-ous NiO and dye and at the TiO2/NiO interface, which facilitates thecell photocurrent. However, the value of open-circuit voltage islower, which could be due to the increase of series resistance bythe thicker NiO layer. Also, the detached and discontinuous holehopping movement may lead to the decrease of the shunt resis-tance and the efficiency of SS-DSSCs as well. Efforts are in progressto reduce series resistance by optimizing the NiO thickness andelectrode modifications [36], while the presented efficiency num-ber can be regarded as a lower limit.

4. Conclusions

P-type NiO thin films were prepared by combinatorial sol–gelmethod with spin coating technique. The formation and composi-tion of the coated film were justified by EDX and XRD results. Thestructural and optical properties of the NiO films calcined at vari-ous temperatures were investigated. The NiO film becomes crystal-lized at calcination temperature of 450 �C with orientation (1 1 1),and its thickness is 120.6 ± 2.8 nm with average grain size of22 ± 1.0 nm. This film shows the highest transparency of �75% inthe visible region, and the carrier concentration of 5.7 � 1020

cm�3 and Hall mobility of 24.1 cm2/V-s are obtained. It is con-cluded that sol–gel derived NiO is more suitable than sputtered-NiO for the fields of electrochromic and optoelectronic devices.

The electrical performance of composite TiO2/NiO electrode hasbeen studied. The rectification characteristics of the single TiO2 andTiO2/dye structures disappear, revealing a high-resistance junctionbehavior. However, the electrodes calcined at 450 and 550 �C dem-

onstrate significant junction behavior with onset voltage of 2–2.5 Vand great rectification slope. It is also observed that the photocur-rent and the rectification slope are slightly increased under 1-sunsimulated illumination. Hence the high-quality composite p–nelectrode calcined at 450 �C is proposed as a most promising struc-ture for the efficient solid-state DSSCs. The SS-DSSC made of a solidTiO2/NiO electrode has a short-circuit current (JSC) of 0.33 mA/cm2

and an open-circuit voltage (VOC) of 210 mV, yielding 0.025% con-version efficiency (1-sun illumination). Efforts in the optimizationof the devices such as NiO thickness and electrode modificationsare in progress, while the presented efficiency number can be re-garded as a lower limit for SS-DSSCs.

Acknowledgments

This work was supported by the National Science Council, Tai-wan, ROC under Grant NSC96-2221-E-239-015 and NSC97-2221-E-239-027. The financial support is gratefully acknowledged.

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