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Journal of Photochemistry & Photobiology A: Chemistry

journal homepage: www.elsevier.com/locate/jphotochem

Annealing effect of E-beam evaporated TiO2 films and their performance inperovskite solar cells

M.F. Hossaina,b,⁎, S. Nakaa, H. Okadaa,⁎

aGraduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama, 930-8555, JapanbDepartment of Electrical and Electronic Engineering, Rajshahi University of Engineering and Technology, Rajshahi, 6204, Bangladesh

A R T I C L E I N F O

Keywords:Perovskite solar cellAnnealing temperatureTitanium dioxideSeries resistancePerovskite

A B S T R A C T

Titanium dioxide (TiO2) films have been deposited on indium-doped tin oxide (ITO) glass substrate by electron-beam evaporation (EBE) system at room temperature (RT, 25 °C). After that, prepared TiO2 films are annealed atdifferent temperatures of 150, 250, 350, 400, 450 and 500 °C. The novelty of this work is to investigate the effectof annealing temperature on the photovoltaic performances of perovskite solar cell (PeSC) with these annealedTiO2/ITO structures. For the fabrication of PeSC, perovskite (PRV) layer has been fabricated on TiO2 films bytwo-steps method. The TiO2 film prepared at RT shows the amorphous structure with the compact surface andTi3+ state. Interestingly, crystallinity and porosity of the TiO2 films increase with the increase of annealingtemperature. The TiO2 films annealed at 500 °C have shown high crystallinity with Ti4+ state and mixed phases(anatase and rutile). The grain boundary becomes visible, the size and porosity of TiO2 films increase with theincrease of annealing temperature. The PeSC with TiO2 films prepared at RT shows the highest photoelectricconversion efficiency (PCE) of 6.15% with maximum photocurrent (14.37mA/cm2) than the other PeSCs withannealed TiO2 films. The PCE, photocurrent and fill factor of PeSC decrease but the photovoltage increases withincrease in annealing temperature, which may be caused by carrier leakage between PRV layer and ITO sub-strate, high series and low shunt resistances, and also the enhanced sheet resistance of ITO substrate.

1. Introduction

Perovskite solar cells (PeSCs) have considered as an alternativephotovoltaic technology due to their excellent photoelectric conversionefficiency (PCE) along with their low material costs [1]. Organic-in-organic lead-halide perovskites have emerged as the most encouragingmaterials for next-generation solar cells owing to their potential prop-erties for solar cell application such as broad and intense absorptionspectra, direct bandgap, high crystallinity, long carrier diffusion length,low fundamental energy loss, excellent carrier transport, ease fabrica-tion and low production cost [2–5]. Several groups have reported thePCEs from 10% to 15% for solution processed organo-metal halide PeSC[3,6,7]. A long-term durable solid-state PeSC has firstly reported withthe PCE of 9.7% in 2012 [8], a PCE as high as 19.3% has confirmed in2014, and a certified PCE of 17.9% has publicized in 2014 [9]. Anunprecedented certified PCE of 20.1% has been achieved for the PeSC[10,11], which makes it one of the most favorable thin film photo-voltaic technologies [12]. Recently, researchers are reported thehighest PCE of the PeSCs, 22.1% [11,13].

A compact titanium dioxide (TiO2) layer is the most promising

electron-transport layer for the PeSC [14]. For achieving the higherefficiency PeSCs with a suitable electronic property of TiO2, severalmethods such as sol−gel, spin-coating, slot-die coating, and bladecoating, electrodeposition, atomic layer deposition, and electron beamevaporation (EBE) have been used [15–17]. Among these methodolo-gies, EBE is one of the most attractive techniques because it is common,versatile and least expensive technology which can produce TiO2 filmswith good optical and mechanical properties [18,19].

Chemical and thermal stability are required in the fabrication pro-cess of the PeSC [20]. Generally, most of the researcher were usedfluorine-doped tin oxide (FTO) substrates in PeSCs [10–14,21,22].However, an indium-doped tin oxide (ITO) is most widely used trans-parent conducting films because of its wide bandgap (> 3.5 eV) andexcellent merits [23,24]. It has the highly desired combination of bothbetter electrical conductivity and higher optical transparency [24,25].The ITO coatings are much smoother, and thinner than the FTO sub-strate [26]. It can be developed on large area substrates and requireslow processing temperature [24,27]. Moreover, ITO can only be coatedon the plastic substrate which is much popular in flexible electronic andoptoelectronic devices [28]. However, indium is a rare metal that ranks

https://doi.org/10.1016/j.jphotochem.2018.04.025Received 14 February 2018; Received in revised form 6 April 2018; Accepted 10 April 2018

⁎ Corresponding authors at: Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama, 930-8555, Japan.E-mail addresses: faruk94_ruet@yahoo.com (M.F. Hossain), okada@eng.u-toyama.ac.jp (H. Okada).

Journal of Photochemistry & Photobiology A: Chemistry 360 (2018) 109–116

Available online 19 April 20181010-6030/ © 2018 Elsevier B.V. All rights reserved.

T

68th in Clarke number (0.00001%) and has low thermal stability [29].New materials and structures with a barrier layer on ITO layer havebeen investigated to overcome the problems regarding lack of assetsand ITO properties [30]. For improving the thermal stability of the ITO,an effective technique is covering the ITO layer by a metal oxide layer,like TiO2 films [20,31]. Generally, researcher suggested that the var-iation in defects, like oxygen vacancies, affect structural, optical andsurface morphological properties of the ITO under annealing conditionhas greatly influenced [32,33]. Moreover, the key factor that affects thechanging properties of the ITO is not fully understood because the ITOis polycrystalline with grain boundaries and many defects [18]. Someresearch groups have tried to show the influence of annealing tem-perature on the properties of E-beam evaporated TiO2 films on glasssubstrate [20,34] and some others groups have to fabricate the PeSCswith E-beam evaporated TiO2 films on the flexible substrate [19,35,36].But, no researchers have found to investigate the effect of annealingtemperature on the photovoltaic performances of PeSCs with E-beamevaporated TiO2/ITO structure. Therefore, in this work, the TiO2 pho-toelectrode has been prepared on bare ITO substrate by the EBE systemat room temperature (RT, 25 °C). After that, prepared TiO2 films areannealed at different temperatures of 150, 250, 350, 400, 450, and500 °C. The effect of annealing temperature on the structural, optical,and surface morphological properties of TiO2/ITO and perovskite(PRV)/TiO2/ITO structures have been correlated and discussed withtheir photovoltaic performance of PeSCs.

2. Experimental section

At first, the ITO glass substrates were etched by photolithographyand were thoroughly cleaned using the process of ultra-sonication, andirradiation in an UV-ozone chamber. Fig. 1 shows the schematic re-presentation of EBE system with the source to substrate distance, 20 cm.Initially, the chamber was evacuated to a background pressure below4×10−6 Torr. The TiO2 films were evaporated on ITO substrate byEBE system at RT. The film thicknesses of all the TiO2 films were around300 nm which was measured by computer-controlled quartz crystalmicrobalance apparatus. The prepared TiO2 thin films were annealedwith different temperatures such as 150, 250, 350, 400, 450, and 500 °Cfor 1 h.

All chemical materials were purchased from commercial suppliersand were used as received unless stated otherwise. PRV layer was de-posited on TiO2 thin films by the two-step method in the glovebox withan N2 environment (−43.4 °C to −36.5 °C). Perovskite precursor so-lutions were prepared by dissolving PbI2 (Sigma-Aldrich Co., 99%purity) in anhydrous N, N-dimethylformamide (400mg/ml) and me-thylammonium iodide (MAI) (Sigma-Aldrich Co.) in ethanol (50mg/ml,19:1 ratio). At first, PbI2 was depositing on prepared TiO2/ITO films by

the spin-coating method at 3,000 rpm for the 60 s under N2 environ-ment. After that, a single-drop (∼80 μl) of MAI solution was added andagain spinning at 3,000 rpm for 60 s. Finally, the PRV-coated TiO2

electrode was heated with 100 °C for 10min. The Spiro-MeOTAD so-lution was prepared by dissolving 72.3 mg of Spiro-MeOTAD (Sigma-Aldrich Co., 99% purity), 17.5 μl of lithium bis (tri-fluoromethanesulfonyl) imide (LITFSI, Wako, 99.95%) solution(520mg of LITSFI in 1ml acetonitrile (Sigma-Aldrich, 99.8%)) and28.8 μl of 4-tert-butylpyridine (Sigma-Aldrich Co., 96%) in 1ml ofchlorobenzene (Sigma-Aldrich Co., 99.8%). The hole-transport layerwas deposited on top of the PRV-coated TiO2/ITO substrate by spincoating at 4,000 rpm for 30 s outside of the glovebox. Finally, a 50 nmthick gold metal-electrode was deposited on the Spiro-MeOTAD layerby thermal evaporation with a constant deposition rate of 1.0 Å/s.

The surface morphologies were investigated using field emissionscanning electron microscope (FE-SEM, JEOL 6700F). The electricalproperty of ITO substrate was measured by Hall-effect measurementsystem with the help of regulated voltage with a constant current source(Kikusui PMC35-3A), electromagnet: (Magnet: IB-291, Gauss meter:GM-432, and regulated DC power supply: Kikusui PAD110-30L), andKeithley 2100 digital multimeter. The crystal structure of the TiO2 andPRV/TiO2 were examined by X-ray diffractometer (XRD, BrukerDiscover 8) analysis with Cu-Kα line. The data were recorded from 2θvalues 10° to 80° with a step of 0.04°. The optical properties of the filmswere measured by UV/VIS spectrophotometer (Hitachi U-1900) at RTwithin the wavelength range 300–900 nm. The chemical composition inweight-percentage (wt%) and compositional morphology was also es-timated by wavelength dispersive spectroscopy (WDS) using Electronprobe microanalyzer (EPMA, JXA-8230, JEOL). The X-ray photoelec-tron spectroscopy (XPS) measurement of TiO2 photoelectrode wascarried out by Thermo Fisher Scientific XPS (EXCALAB 250Xi) spec-troscopy using MgKα radiation. The photovoltaic performances of thePeSCs were measured using a semiconductor parameter analyzer(KEYSIGHT Technology, B1500A) and solar simulator AM 1.5(Yamashita Denso, YSS E40). The IPCE spectra were measured using amotor-controlled monochromator (Shimadzu, SPG-100ST, AT-100PCC,and AT- 100PL), a halogen lamp (Scott, MegaLight 100), and a digitalelectrometer (Advantest, TR8652). The IPCE was calibrated using a Siphotodiode (Hamamatsu Photonics, S1223).

3. Results and discussion

Fig. 2(a)–(g) show the surface morphology of ITO substrate at dif-ferent annealing temperatures of RT, 150, 250, 350, 400, 450, and500 °C, respectively. The annealing temperature has changed the sur-face of ITO. In Fig. 2(a), the surface of ITO without annealing tem-perature is very compact with less open surface and grain boundariesare invisible. If the annealing temperature increases, the compactness ofITO films becomes poor, and the defect of the ITO thin film increasesand the fine grains are clearly exhibited and become more in quantities,as shown in Fig. 2(d)–(g). It is clear that the size of nano-grain increaseswith the increase of annealing temperature because the annealingtemperature is sufficient to move grain boundaries, which leads to in-crease the crystallinity of the material or it may reduce the deformationor defects present in the crystal [37]. From the FE-SEM images, theaverage grain size of ITO films is 4.85, 5.6, 8.2, 22.8, 27.4, 34.3, and45.7 nm with the annealing temperature of RT, 150, 250, 350, 400,450, and 500 °C respectively.

Fig. 3(a) shows the transmittance spectra of ITO substrate withdifferent annealing temperatures ranges from RT–500 °C. The averagetransmittance of ITO films is changed averagely from 90 to 83% withthe annealing temperature of RT∼500 °C, respectively. The band-edgeof ITO film is little bit red-shifted from 2.80 to 2.54 eV with the increaseof the annealing temperature, which may be due to the Burstein–Mossshift [38] and/or it could also be related to the quantum-size effect[39]. Inset of Fig. 3(a) shows the sheet resistance (Ω/□) [40] of ITOFig. 1. The schematic diagram of E-beam evaporation system.

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films as a function of annealing temperature. The sheet resistance ofITO films is increased slowly from 11.1 to 20.4Ω/□ with the increaseof annealing temperature from RT∼250 °C, respectively. However,these resistances have become high (58.2–78.9Ω/□) due to increasingthe annealing temperature of 350–500 °C. The resistance of ITO be-comes high which may be happened by the decrease of electron con-centration [20,30], as shown in Fig. 3(b). From Fig. 3(b), it is clear thatcarrier concentration decreases, however, mobility increases with theincrease of annealing temperature [30]. This is caused by the oxygenvacancies in ITO films that oxidized by annealing in the air [20,30,41].

The XRD pattern of the bare-ITO substrate, TiO2/ITO structure atdifferent annealing temperatures, PRV/TiO2 layer at RT and 500 °C, areshown in Fig. 4(a). Four strong prominent peaks of I(211), I(222), I(400), I(440), and I(622) of ITO substrate at RT conditions are ex-hibited, which is indicating that the ITO has high crystallinity. The TiO2

at RT shows the amorphous structure. The crystallinity of TiO2

Fig. 2. FE-SEM images for the surface of ITO substrate annealed at different temperatures of (a) RT, (b) 150 °C, (c) 250 °C (d) 350 °C (e) 400 °C (f) 450 °C, and (g)500 °C.

Fig. 3. (a) Transmittance spectra of ITO substrate at different annealing tem-peratures of RT, 150, 250 °C, 350, 400, 450, and 500 °C; (Inset) Sheet resistanceof ITO substrate versus annealing temperature, and (b) carrier concentrationand mobility curves of ITO substrate versus annealing temperature.

10 20 30 40 50 60 70 80

P

I(21

1)

I(62

2)

I(44

0)

I(40

0)

A(2

20)

A(1

05)

R(1

11)

A(2

00)

A(0

04)

I(22

2)

PPPP P P

P

PRV/500oCPRV/RT

500oC450oC400oC350oC250oC150oC

RT

Diffraction angle 2 (deg.)

Bare-ITO

R: RutileP: PerovskiteI: ITO

A(1

01)

0 2 4 6 8

InIn

RT 500oC

O

Si

Ti

Ti

Ti

Ti Sn Sn

Fig. 4. XRD pattern of bare-ITO substrate; TiO2/ITO thin films annealed withdifferent temperatures; and PRV/TiO2 with RT and 500 °C annealing tempera-ture; and (b) WDS spectra of TiO2 films annealed at RT and 500 °C temperature,(Inset) Table I: chemical compositional data in wt.% of TiO2 films annealed atdifferent temperatures.

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increases with the increase of annealing temperature. All the diffractionpeaks are indexed to A(101), A(004), A(200), A(105), and A(220) at 2θvalues: 25.35°, 37.78°, 48.07°, 53.92°, and 70.36°, respectively, whichindicates a pure anatase, i.e., body-centered tetragonal structure of TiO2

(JCPDS card of 00-00477). The crystal peaks of TiO2 films increaseswith the increase of annealing temperature. The TiO2 annealed at500 °C shows very strong and highest number of anatase peaks. More-over, a single rutile peak R(111) is observed at 41.19°. The enhance-ment of crystallinity of TiO2 films may be caused by the decreaseddefects like oxygen vacancies [35]; meanwhile mobility of TiO2/ITO isincreased. The crystallite size of the particles has been estimated fromthe Debye–Scherer’s equation as follows [42]:

=dβ θ0.94λ

cos (1)

Where d is the crystallite size, λ is the wavelength of the X-ray radiation(Cu-Kα=0.15406 nm), θ is the diffraction angle and β is the full widthhalf maximum (FWHM). The diffraction peaks of A(101) has beenchosen for the calculation of the crystallite size. The derived crystallitesizes are 3.47, 3.50, 3.53, 6.34, 6.43, 10.1, and 11.42 nm for the TiO2

thin films prepared at the different temperature of RT, 150, 250, 350,400, 450, and 500 °C respectively. The FWHM decreases with increasein annealing temperature, which means that the increased crystallinity.The PRV layer on the TiO2 thin films at RT (PRV/RT) and 500 °C (PRV/500 °C) has high crystallinity with seven peaks. The PRV/500 °C showsalmost the same crystallinity as compared to the crystallinity of PRV/RT.

The chemical (wt%) compositional analysis for titanium (Ti) andoxygen (O) has estimated from WDS data. Fig. 4(b) exhibits the WDSspectra of TiO2 films annealed at RT and 500 °C. From the WDS spectra,the peaks of Ti, O, Sn, In, and Si have found. The peaks of Sn, In, and Siexhibit due to the ITO glass substrate. The peaks of Ti and O demon-strate that the composition of these products has the chemically highpurity [43]. All peaks of TiO2 annealed at 500 °C have chemicallyshifted to higher energy which may be due to increasing the oxidationstate as well as decreasing oxygen vacancies. Inset Table 1 of Fig. 4(b)shows the chemical composition weight in percentage (wt.%) of Ti, O,Si, Sn, and In for the TiO2/ITO films annealed at different temperatures.The wt% of Ti increases and O2 decreases with the increase of annealingtemperature which happens due to change amorphous to the crystallinestructure. These data have a good agreement with the XRD results, asshown in Fig. 4(a).

The XPS measurements have been carried out to investigate thechemical states of Ti4+ in TiO2 films. Fig. 5(a) and (b) shows the Ti 2pand O 1s spectra of TiO2 films prepared without annealing and with500 °C annealing temperature. The XPS signal of Ti 2p is recordedranging from 452 to 472 eV and the signal O 1s is recorded rangingfrom 527 to 534 eV. From Fig. 5(a), the binding energies (BEs) of twospin-orbit components, Ti 2p1/2 and Ti 2p3/2 for TiO2 prepared at RTare showed at 456.58, and 462.46 eV, respectively. The Ti 2p3/2 peak islocated at the values of 456.58 eV, which means that the TiO2 preparedat RT has the existence of Ti3+ (456.7 eV) ions of the Ti2O3 films [44],which may cause amorphous in nature. The large amounts of O− ra-dicals present on the surface of Ti3+ materials which results in more

photo-effective [45]. It has been agreed that the holes are located atoxygen vacancy which helps to increase the adhesive strength of TiO2

films for attaching perovskite materials. On the other hand, both thepeaks of Ti 2p3/2 and Ti 2p1/2 of TiO2 annealed at 500 °C have beenpositively shifted to 459.2 eV and to 464.98 eV, respectively, whichmay be caused due to form high crystalline TiO2 films [as shown inFig. 4(a)]. It has good agreement with the existence of Ti4+ (458.7 eV)ions in the surface of TiO2 films [46], which has high crystallinity.Moreover, the energy difference between XPS Ti 2p3/2 and Ti 2p1/2peaks for the sample TiO2 annealed at 500 °C (ca. 6.5 eV) which isslightly higher than that of TiO2 prepared at RT is ca. 5.9 eV [47]. Theslight increase in energy difference of the Ti 2p peaks can be attributedto the formation of a mixed phase of rutile and anatase in the TiO2

sample with 500 °C annealing temperature [48].In Fig. 5(b), it is presented the O 1s spectrum, which is asymmetric.

The TieO peak shifts from 529.55 eV for TiO2 at RT to 530.4 eV for theTiO2 films annealed at 500 °C. Also, it has been accepted that deviationof the core-level O 1s from that of Ti 2p3/2 (ΔBE) is a sensitive indicatorof the oxidation state of Ti. In our case, ΔBE has been 72.92 eV for TiO2

at RT and 71.2 eV for TiO2 at 500 °C annealing temperature and con-sistent with the value for Ti2O3 and TiO2 films [43]. In O 1s XPSspectrum, a new peak located at 529.98 and 530.48 eV is attributed toTi-OH. The higher BE component of 531.6 eV for TiO2 at RT and532.5 eV for TiO2 at 500 °C annealing temperature is attributed to thedefective oxides [49].

The surface morphology of TiO2 annealed at different temperaturesof RT, 150, 250, 350, 400, 450, and 500 °C, cross-section of TiO2 thin

Table 1Compositional data (wt.%).

Annealing Temp. (C) Ti O Si Sn In

RT 29.03 51.75 14.51 3.87 0.85150 29.22 51.26 14.46 4.14 0.92250 29.56 49.81 15.72 3.87 1.04350 29.81 49.35 15.49 4.23 1.12400 30.27 48.21 16.2 4.17 1.15450 30.52 47.53 17.56 3.18 1.21500 30.8 46.81 18.43 2.81 1.16

Fig. 5. XPS curve of (a) Ti 2p and (b) O 1s spectra of TiO2 thin films annealed atRT and 500 °C annealing temperature.

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films at RT, and PRV/RT and PRV/500 °C are shown in Fig. 6(a)–(g),Fig. 6(h), and Fig. 6(i)–(j), respectively. In Fig. 6(a), the TiO2 preparedat RT shows very smooth and compact with the less open surface. Thegrain boundaries of TiO2 films are become visible after annealing withthe temperature of 150–500 °C. From Fig. 6(b)–(g), the surface of TiO2

films has become porous with the more open surface with the increaseof annealing temperature. The grain-cluster size is increased from 17.2to 37.14 nm with the temperature of 150–500 °C, respectively. This isbecause increasing the annealing temperature leads to increase the

crystallinity of the material and hence increases the number of crys-tallites, which exhibits similar behavior as discussed in XRD analysis, asshown in Fig. 4(a). Fig. 6(h) displays the cross-sectional view of TiO2 atRT. It is cleared that the measured thickness of TiO2 is 300 nm, which isthe same thickness as measured by the quartz crystal resonator duringthe TiO2 evaporation. From Fig. 6(i)–(j), it is clearly observed that thesurface of PRV/RT and PRV/500 °C has almost same structure andcompact with closely bounded grains.

It is well known that the optical property of a TiO2 film is the sig-nificant feature and related to the transmittance of film. Therefore,Fig. 7(a) shows the transmittance spectra of TiO2 thin films annealed atthe different temperature as a function of wavelength within the rangeof 300–900 nm. All films display the good transmittance (up to 83.65%)in the visible region and a sharp fall in the UV region. The usual in-terference pattern is observed in the range of low absorption. It hasbeen observed that the transmittance edge shows a little red shift withthe increase of annealing temperature, which may be caused due to thegood crystallinity of the TiO2 films, as mentioned in Fig. 4(a). Theaverage transmittance varies from 77.97% to 83.65% with the increaseof annealing temperature.

The refractive index of the film (nλ) has calculated from the opticaltransmittance data using the Swanepoel's envelope method [42] em-ploying the relation,

= + −n N N n n[ ( ) ]λ2

02

12 1/2 1/2

(2)

Where = − + +N n n T T T T n n2 [(( )/ ) ( )/2].M m M m0 1 (λ) (λ) (λ) (λ) 02

12

The TM(λ) and Tm(λ) are the maximum and minimum transmit-tance of the TiO2 thin film at a wavelength λ, and n0 and n1 are therefractive indices of air and glass substrates respectively. The porosity

Fig. 6. FE-SEM images for TiO2 thin films annealed at (a) RT, (b) 150 °C, (c)250 °C (d) 350 °C (e) 400 °C (f) 450 °C, and (g) 500 °C, (h) cross-section of TiO2

films at RT, (i) PRV/TiO2 at RT, and (j) PRV/TiO2 at 500 °C annealing tem-perature.

Fig. 7. (a) Transmittance spectra, (b) (αhν)1/2 versus photon energy curves ofTiO2 thin films at different annealing temperatures.

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of the films is calculated using the following equation [50].

⎜ ⎟= ⎡

⎣⎢ − ⎛

⎝

−−

⎞⎠

⎤

⎦⎥ ×Porosity

nn

1( 1)( 1)

100%.d

λ2

2(3)

Where nd is the refractive index (2.52) of the pore-free (bulk) TiO2

films. It is seen that the porosity of the prepared TiO2 thin film is quitehigher than that of the bulk TiO2 films [51]. Inset of Fig. 7(a), the re-fractive index (at λ=435 nm) decreases from 1.640 to 1.475 with theincrease of annealing temperature from RT to 500 °C, respectively. Thevalues of the refractive index are lower than that of the bulk TiO2

probably due to the narrow size of the grains [51,52]. From inset ofFig. 7(a), it is observed that the porosity is inversely proportional to therefractive index of TiO2 films. The porosity increases with the increaseof annealing temperature which has good agreement with FE-SEM re-sults [as shown in Fig. 6(a)–(g)].

The absorption edge and optical band gap, Eg, of TiO2 thin filmsdepend on the phase structure and the crystallite size of the particle[53] has affected by the annealing temperature. The optical band gapbetween the top of the valence band and the bottom of the conductionband is calculated using the method of Tauc and Menth [54]:

= −αhν A hν E( ) ( )g1/2 (4)

Where α is absorption coefficient, A is the edge width parameter, and hνis the photon energy [h is the Plank’s constant (4.136× 10 −15 eV-s)and υ is the frequency of light (s−1)]. Fig. 7(b) shows the plots ofαhν( )1/2 versus the photon energy of the films grown at the differentannealing temperature. The optical band gap of the films has de-termined from the extrapolation of the linear plots of αhν( )1/2 versus hνat α=0. The band gap of the TiO2 film decreases from 3.31 to 3.10 eVwith the increase of annealing temperature from RT to 500 °C, respec-tively. The decreasing the band gap indicates that the photoactivityTiO2 is also increased.

The entire device fabrication procedure was carried out at a lowtemperature of below 150 °C. A cross-sectional SEM image of PeSC cellwith TiO2 films prepared at RT is shown in Fig. 8(a). In this figure, ITO,

TiO2, perovskite, Spiro-MeOTAD and Au layer are clearly observed. Theaverage thickness of perovskite and Spiro-MeOTAD are approximately400 and 350 nm, respectively.

The important parameter for a solar cell is its photoelectric con-version efficiency, i.e. the ratio of the output power to the incidentpower. The IPCE of the PeSC can be calculated from the expression[55]:

= ×J V FFP

η 100%SC OC

in (5)

Where JSC is the integral photocurrent density in mA/cm2, VOC is theopen-circuit voltage, and FF is the fill factor, = ×FF I V I V( ) /max SC OC.Fig. 8(b) shows the photocurrent-photovoltage characteristics of PeSCunder the 100mW/cm2 light illumination. The performance of the PeSCis summarized in Table 2. It is important to notice that the PeSC withTiO2 prepared at RT shows the maximum η of 6.15% and JSC of14.37mA/cm2, however, it has lowest VOC of 674mV. The efficiency ofPeSCs is still low because of thickness and amorphous structure of TiO2

films [19]. The efficiency and photocurrent density decreases with theincrease of annealing temperature from 150 to 500 °C, respectively. ThePeSC TiO2 annealed at 500 °C shows the lowest efficiency, η of 1.38%,JSC of 3.75mA/cm2 and VOC of 830mV. The fill factor decreases from63.3% to 44.3% with the increase of annealing temperature, which may

Fig. 8. (a) FE-SEM image of a cross-sectional view of PeSC cell, (b) photocurrent versus photovoltage curve, (c) IPCE curve, and (d) V-t rise and fall curve of PeSCwith TiO2 films annealed at RT, 150, 250 °C, 350, 400, 450, and 500 °C.

Table 2Photovoltaic data of PeSCs with TiO2 thin films at different annealing tem-peratures.

AnnealingTemperature

Voc (V) Jsc (mA/cm2)

FF (%) η (%) Rs (Ωcm2) Rsh (Ω-cm2)

RT 0.674 14.37 63.3 6.15 8.63 2784.82150 °C 0.701 10.46 62.6 4.59 8.90 1675.66250 °C 0.712 7.98 62.6 3.56 12.23 1518.60350 °C 0.730 6.51 62.1 2.95 20.96 651.16400 °C 0.770 5.91 60.5 2.74 29.70 305.74450 °C 0.840 5.00 52.4 2.20 44.80 249.92500 °C 0.830 3.75 44.3 1.38 66.33 246.51

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be due to increasing the series resistance and decreasing the shunt re-sistance [56] in the PeSC. The resistance of the TiO2 photoelectrode,perovskite layer, Spiro-MeOTAD, Au and the resistance, resulted fromthe interface: PRV/TiO2, Spiro-MeOTAD/PRV, and Au/Spiro-MeOTADare the main contributors to the total series resistance of PeSC. It ismainly due to the fact that the charge transfer between TiO2 and ITOsubstrate become very poor due to increase porosity and surfaceroughness which enhance the series resistance. From Table 2, the seriesresistance increases and the shunt resistance decreases of PeSC cell withthe increase of annealing temperature for TiO2 films.

Performance of the PeSC can be quantified on a macroscopic level interms of the IPCE, which gives the ratio between the number of gen-erated charge carriers contributing to the photocurrent and the numberof incident photons, as given by [57]:

⎜ ⎟= ⎛⎝

× ⎞⎠

×IPCE JP

(%) 1240λ

100%SC

in (6)

Where, λ is the wavelength of the incident light. Fig. 8(c) shows theIPCE spectrum as a function of the wavelength of monochromatic ir-radiation for a PeSC containing a TiO2 annealed at different tempera-tures. The average IPCE value of PeSC with TiO2 at RT is about 47.1%around the 400–650 nm. The IPCE value decreases with the increase ofannealing temperature, which may be due to decrease the generatedcharge carrier by short-circuiting in solar cells [53]. The IPCE responsesfrom 380 to 750 nm suggest that the PRV layer plays a vital role in bothsolar-light absorption and photo-carrier generation in the PeSC cell.Overall IPCE values are not so high because this may be caused byenlarged carrier leakage [58]inside the PeSC cell.

Fig. 8(d) shows the photovoltage transient response to rising andfall conditions. When the light is ON, the photovoltage of all PeSC cellsreaches instantly the maximum value. The PeSC with TiO2 at annealingtemperature up to 250 °C shows very stable photovoltage, i.e., slightlyincrease from the previous value. However, photovoltage has becomemore unstable with increasing temperature from 350 to 500 °C, whichmay be due to enhancing short-circuiting (direct contact) between PRVlayers to ITO substrate as well increase the porosity of TiO2 layer, andthermally unstable ITO substrate. Moreover, the sheet resistance of ITOhas increased with the increase of annealing temperature, as shown inthe inset of Fig. 3. The photovoltage of PeSC with TiO2 at 500 °C de-creases from 830 to 674mV under light illumination within 30 s timeperiod. When the light is OFF, the photovoltage decreases very slowlyfor PeSC with annealing temperature up to 250 °C, which means thatthe PeSC solar cells with TiO2 films annealed at these temperatureshave long lifetime [49]. Whereas voltage decreases very fast due to theincrease of annealing temperature from 350 to 500 °C, this may becaused by direct-contacting between PRV layer and ITO substrate [58]as well as increasing series resistance of the PeSC cell.

4. Conclusions

The PeSCs cells were successfully fabricated with E-beam evapo-rated TiO2 films on ITO substrate. The cell performance was varied withthe annealing temperature of TiO2 films. The sheet resistance of ITOsubstrate was increased and the grain boundary of ITO films was be-come visible with the increase of annealing temperature. The grain,grain-cluster size, and porosity of TiO2 films had increased due to theincrease of annealing temperature. The photovoltaic performance ofPeSC had greatly affected by the properties of TiO2/ITO structure an-nealed at different temperatures. The PeSC with TiO2 at RT exhibitedthe maximum efficiency of 6.15% with photocurrent of 14.37mA/cm2

whereas, the PeSC with TiO2 at 500 °C showed the lowest efficiency of1.38%. The PeSCs with TiO2 annealed at 450 °C had the highest pho-tovoltage of 840mV. Moreover, FF and IPCE value decreased with theincrease of annealing temperature, which happened due to thermallyunstable of ITO substrate, porous TiO2 films as well as direct carrier

leakage in the PeSC cell.

Acknowledgment

One of the authors M. F. Hossain would like to thank the JapaneseSociety for Promotion of Science (JSPS) for the fellowship and the fi-nancial assistance (MEXT/JSPS KAKENHI Grant Number:JP16F16372).

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