Journal of Physics and Chemistry of Solids 72 (2011) 220–226

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

Chemic

Beijing

E-m

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

Synthesis and application of the solar cell of poly(3-hexylthiophene)/Fe N/titanium dioxide nanocomposite material

Xin Zheng a, Jingchang Zhang a,b,n, Xiuying Yang b, Weiliang Cao a,b

a Institute of Modern Catalysis, Beijing University of Chemical Technology, State Key Laboratory of Chemical Resource Engineering, Beijing 100029, Chinab Institute of Science and Technology, Haikou 571126, China

a r t i c l e i n f o

Article history:

Received 23 June 2010

Received in revised form

23 December 2010

Accepted 23 December 2010Available online 29 December 2010

Keywords:

A. Nanostructures

A. Polymers

B. Chemical synthesis

C. X-ray diffraction

D. Optical properties

97/$ - see front matter & 2010 Elsevier Ltd. A

016/j.jpcs.2010.12.015

esponding author at: Institute of Modern Cat

al Technology, State Key Laboratory of Che

100029, China. Tel./fax: +86 01 64434904.

ail address: zhangjc1@mail.buct.edu.cn (J. Zha

a b s t r a c t

A series of nanocomposites of poly(3-hexylthiophene) with Fe N-doped TiO2 (P3HT/Fe N/TiO2) were

synthesized by the chemical method in situ. The structure of the prepared composites was

characterized using X-ray diffraction patterns (XRD), infrared spectroscopy (IR), transmission electron

microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Optical and electrochemical properties

were determined using UV–vis spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. These

tests indicated that P3HT/Fe N/TiO2 is a new p–n semiconductor. Two solar cells based on P3HT/Fe

N/TiO2 were manufactured and studied.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Polymers with highly extended p-electrons have attractedincreasing interest because of their good conductivity [1]. Addi-tionally, their composites with metal oxides have also been alsostudied for their improved electrical, optical, and mechanicalproperties [2], and are considered as remarkable and potentialmaterials [3]. Among various polymers, polythiophene (PT) waswidely studied as a donor material in organic solar cells [4,5].Sharma et al. [6] studied the photoelectrochemical cells (PECs)based on poly(phenyl-azomethane-thiophene) (PPAT) and a com-posite film of nano-crystalline titanium dioxide (nc-TiO2) withPPAT, and found that the composite film was better than PPAT byitself. Moreover, Visy et al. [7] prepared poly(3-octylthiophene)(P3OT) and ferrous oxalate composite, Sharma et al. [8] appliedpoly(3-phenyl-hydrazone-thiophene) (PPHT) and titanium diox-ide composite in solar cells and obtained some improved results.However, there is no report on poly(3-alkylthiophene) (P3AT) andits inorganic-doped nanocomposites. Because P3AT has goodsolubility, processability, stability, and electrical activity, and isconvenient for mass production; it was considered as one of thebest donor materials.

ll rights reserved.

alysis, Beijing University of

mical Resource Engineering,

ng).

In this paper, we successfully constructed P3HT/Fe N/TiO2 nano-composites using the chemical method in situ. By characterizationssuch as XRD, IR, and XPS, we found that chemical interaction occurredbetween P3HT and Fe N-doped TiO2. Ultraviolet–vis and lumines-cence measurements indicated that these P3HT/Fe N/TiO2 nanocom-posites have excellent photoelectric properties. Electrochemicalresults showed that the forbidden band gap of the nanocompositehybrid was lower than those of P3HT and Fe N-doped TiO2.

2. Experiment details

2.1. Synthesis of P3HT/Fe N/TiO2 nanocomposites

2.1.1. Synthesis of poly(3-hexylthiophene)

P3HT was synthesized with Fe (III) as a catalyst according toRef. [9]. 3-hexylthiophene monomer and chloroform were addedto a 100 mL flask and ultrasonically dispersed for 15 min. Thesystem temperature was controlled at 0 1C. FeCl3 was dissolved inchloroform, and added dropwise to the flask. The reaction mixturewas stirred under high-purity nitrogen for 8 h. The product wasslowly added to methanol, precipitated, filtered, and subjected toSoxhlet extraction with methanol for 20 h. P3HT was then driedunder vacuum at room temperature. P3HT (0.01 g) was dissolvedin chloroform (2 mL) and dropped evenly on a clean quartz glassand indium tin oxide (ITO) conductive glass. Red-brown P3HTfilms were obtained after removing chloroform in vacuum.

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226 221

2.1.2. Preparation of nano-TiO2

Nano-TiO2 was prepared with TiCl4 as the starting materialaccording to Ref. [10]. Surfactant polyethylene glycol (PEG 800)was added to the solution of 0.3 mol/L of TiCl4, followed byvigorous stirring at ambient temperature. The pH of the solutionwas adjusted to 5–6 by dripping ammonia. The sol was filteredand washed with deionized water until the absence of Cl� ,transferred into a PTFE autoclave (100 mL), and maintained at240 1C for 12 h. Then the sol was washed by ethanol untilcomplete removal of water. The obtained alcogel was dried atroom temperature, and calcined at 500 1C for 1 h to get nano-TiO2.TiO2 (0.01 g) was suspensed in isopropyl alcohol (2 mL) anddropped evenly on clean quartz glasses and ITO. White TiO2 filmswere obtained after drying in vacuum.

2.1.3. Preparation of nano-Fe N/TiO2

Surfactant polyethylene glycol (PEG 800) was added to thesolution of 0.3 mol/L of TiCl4 and 0.005 M Fe(NO3)3, followed bystirring vigorously at ambient temperature. The pH of the solutionwas adjusted to 5–6 by dripping ammonia. The sol was filteredand washed with ammonia until removal of Cl� , transferred intoa PTFE autoclave (100 mL), and maintained at 240 1C for 12 h.Then the sol was washed by ethanol until complete removal ofwater. The obtained alcogel was dried at room temperature, andcalcined at 500 1C for 1 h to get nano-Fe N/TiO2. Fe N/TiO2 (0.01 g)was suspensed in isopropyl alcohol (2 mL) and dropped evenlyon clean quartz glasses and ITOs. White Fe N/TiO2 films wereobtained after drying in vacuum.

2.1.4. Preparation of P3HT/Fe N/TiO2 nanocomposite

The prepared nano-Fe N/TiO2, 3-hexylthiophene monomer,and chloroform were added to a 100 mL flask and ultrasonicallydispersed for 15 min. The system temperature was controlled at0 1C. FeCl3 was dissolved in chloroform, and added dropwise tothe flask. The reaction mixture was stirred in high-purity nitrogenfor 8 h, and then slowly added to methanol to give a precipitate.The precipitate was filtered and subjected to Soxhlet extractionwith methanol for 20 h. The P3HT/Fe N/TiO2 nanocompositepowder was then dried in vacuum at room temperature.

P3HT/Fe N/TiO2 (0.01 g) was dissolved in chloroform (2 mL)and dropped evenly on clean quartz glass and ITO conductiveglass. Red–brown P3HT/Fe N/TiO2 nanocomposite films wereobtained after removing chloroform in vacuum.

2.2. Preparation of the dye-sensitized solar cell

The prepared TiCl4 sol–gel was dropped on the fluorine tinoxide (FTO) substrate and spun on a uniform plastic machine at arate of 3000 evolutions/min for 20 s. The prepared anatase TiO2

powder of particle size about 14 nm was dropped on the sol–gelcoated FTO substrate, and suffered spinning by the machine at arate of 3000 evolutions/min for 20 s. After sintering at 450 1C for30 min, a good film was produced. H2PtCl6 was evenly dropped onanother FTO substrate and burnt at 380 1C for 30 min and then aPt electrode was obtained. The anode was sensitized by P3HT andP3HT/Fe N/TiO2 chloroform solution, and packaged by the pre-pared Pt electrode. Finally, the electrolyte was added.

2.3. Preparation of organic thin-film solar cells [11]

2.3.1. Preparation of single-layer Schottky cell

A single layer Schottky cell was prepared by dissolving P3HT inchloroform, dropping on the FTO substrate, spinning on a uniformplastic machine, baking for 20 min at 110 1C, and packaging bythe prepared Pt electrode.

2.3.2. Preparation of double-layer p–n heterojunction organic thin-

film cell

A double-layer p–n heterojunction organic thin-film cell wasprepared by dissolving P3HT in chloroform, dropping on theanode discussed in Section 2.2, spinning on a uniform plasticmachine, baking for 20 min at 110 1C, and packaging by theprepared Pt electrode.

2.3.3. Preparation of bulk heterojunction organic thin-film cell

A bulk heterojunction organic thin-film cell was prepared bydissolving P3HT/Fe N/TiO2 in chloroform, dropping on the FTOsubstrate, spinning on a uniform plastic machine, baking for20 min at 110 1C, and packaging by the prepared Pt electrode.

2.4. Structural characterization

2.4.1. X-ray diffraction (XRD)

XRD measurements were performed using a ShimadzuHR6000X (Cu target X tube, voltage 40.0 kV, current 30.0 mA,and scan angle 3–801).

2.4.2. Infrared spectroscopy (IR)

IR spectra were recorded in the range of 400–4000 cm�1 by aprestige-21IR spectrophotometer with KBr pellet.

2.4.3. Transmission electron microscope (TEM)

TEM measurements were performed using a HITACHI-800transmission electron microscope.

2.4.4. X-ray photoelectron spectroscopy (XPS)

XPS analyses were conducted by an ESCALAB 250, in which theelectronic binding energy of the samples were measured.

2.5. Performance characterization

2.5.1. UV

UV–vis spectra were measured using a Hitachi U-3010 spec-trophotometer (scan range 200–700 nm).

2.5.2. Fluorescence

The excitation and emission spectra measurements were per-formed on a Hitachi F-7000 spectrofluorophotometer equippedwith a 150 W xenon lamp as the excitation source. The spectrawere recorded with a monochromator slit of width 1.5 nm on bothexcitation and emission sides.

2.5.3. Cyclic voltammetry (CV)

CV measurements were carried out with a Potentiostiostat/Galvanostat (EG&G PAR Model 283). The counterelectrode wasplatinum and the reference electrode was non-aqueous Ag/Ag+.The CV curves were recorded in acetonitrile with 0.01 mol/LLiClO4 as an electrolyte at a scan rate of 50 mV/s. Before using,the solution was insufflated with N2 for 30 min.

2.5.4. Current–voltage (I–V)

I–V measurements were taken in air at room temperature(298 K) using a Keithley 236 high current source power meterunder white-light illumination from a 1000 W xenon lamp. Thelight intensity was about 100 mW/cm2 on the sample surfaces asmeasured by a photodetector. To measure the decay, we recordedthe short-circuit current and open-circuit voltage as functions ofillumination time.

Fig. 2. FT-IR spectra of Fe N/TiO2 (1), P3HT (2), and P3HT/Fe N/TiO2 (3).

Fig. 3. TEM pattern of nano-Fe N/TiO2.

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226222

3. Results and discussion

3.1. XRD characterization of TiO2, Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

The XRD patterns of TiO2, Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

are presented in Fig. 1. In XRD patterns of TiO2 (1) and Fe N/TiO2

(2), the peaks at 25.321, 37.881, 48.081, 53.801, 55.041, and 62.801are assigned to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and(2 1 5) lattice planes, respectively, which are attributed to thesignals of the anatase phase. There is little difference betweenTiO2 and Fe N/TiO2, indicating that doping elements are highlydispersed in TiO2. Using the Scherrer formula, it is calculated thatthe average TiO2 particle size is 14.16 nm and that of Fe N/TiO2 is12.96 nm. In the XRD pattern of P3HT (3), the broad peak at15–281 is associated with the amorphous phase, while peaks ataround 51, 101, 15.111, and 22.921 reflect the crystalline structure.The peak at 22.921 is attributed to the interchain distancebetween p-stacking oxidized chains. The Bragg equation gives0.38 nm for this distance. Peaks at 51, 101, and 15.111 confirm thelayered structure of P3HT, which has a dominant first-orderreflection at 2y¼51 and a lamellar interlayer spacing of1.76 nm [7]. The diffraction peak of P3HT is quite narrow,showing the high degree of crystallization, plane structure, andthe high regularity of P3HT segments. For the P3HT/Fe N//TiO2

(4) nanocomposite, the XRD pattern mainly shows the strong FeN/TiO2 peaks. The characteristic P3HT broad peak cannot be seenin the XRD pattern of P3HT/Fe N/TiO2 nanocomposite. The XRDpattern of the nanocomposite has a higher background intensity,lower diffraction peak intensity, decreased interplanar spacing,and reduced sharpness of the peak profiles than those of Fe N/TiO2, confirming the chemical interaction between P3HT andnano-Fe N/TiO2.

3.2. IR results of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

The FT-IR spectra of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2 areshown in Fig. 2. The spectrum of Fe N/TiO2 (1) exhibits thecharacteristic absorption band of Ti–O–Ti at about 497 cm�1

[12]. In the spectrum of P3HT (2), there is a low-intensity peakat 3055 cm�1, which can be attributed to the thiophene ring C–Hstretching vibration. The band at 721 cm�1 corresponds to methy-lene vibration. The bands at 826 and 1029 cm�1 are assigned toout-of-plane bending of C–H in the thiophene ring. The bands at

102θ (°)

(4)

(1)

(2)

(3)

20 30 40 50 60 70 80

Fig. 1. XRD patterns of TiO2 (1), Fe N/TiO2 (2), P3HT (3), and P3HT/Fe N/TiO2 (4).

2922 and 2853 cm�1 belong to the stretching vibration of C–H.The asymmetric and symmetric deformation vibrations occur at1373 and 1457 cm�1, respectively. The bands at 1513 and649 cm�1 correspond to the symmetric vibration of the C¼Cand the asymmetric vibration of the C–S, respectively. The bandat 1190 cm�1 corresponds to the Ca–Ca single resonance absorp-tion. Bands belonging to both Fe N/TiO2 and P3HT are observed inthe spectrum of P3HT/Fe N/TiO2 nanocomposite (3), whereas thebands originating from Fe N/TiO2 and P3HT are located at lowwavenumbers. In addition, several new peaks occur at 3896, 3740,576, 505, and 460 cm�1, which indicate the chemical interactionbetween P3HT and nano-Fe N/TiO2.

3.3. TEM measurements of nano-Fe N/TiO2

Fig. 3 demonstrates the TEM results of nano-Fe N/TiO2. It canbe seen that the particle size is 13–15 nm, which is basically thesame as obtained from the Scherrer formula. The particles havegood dispersibility and clear mesh structure. This is attributed tothe use of alcohol exchange in the preparation of powder, which

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226 223

eliminated the liquid surface tension and reduced the agglomera-tion of particles during the burning process.

Fig. 5. UV–vis spectra of Fe N/TiO2 (1), P3HT (2), and P3HT/Fe N/TiO2 (3).

3.4. XPS characterization of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

The high-resolution XPS spectra of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2 are shown in Fig. 4a–d. From the XPS spectrum of P3HT/Fe N/TiO2 sample (Fig. 4a), it is easy to see C, O, S, Ti, Fe, and Npeaks. Fig. 4b shows the sulfur peaks in P3HT. Two sulfur peaks(S2p3/2) are found. One with a binding energy of 164.3 eVbelongs to neutral thiophene units in the polymer chain, andthe other with a binding energy of 165.3 eV belongs to oxidizedthiophene units [13,14]. Fig. 4c shows the sulfur peaks in P3HT/FeN/TiO2. There are also two sulfur peaks (S2p3/2). One with abinding energy of 164.1 eV belongs to neutral thiophene units inthe polymer chain, and the other with a binding energy of165.1 eV belongs to oxidized thiophene units, indicating that Satom of P3HT interacts with some other atom except the C atomof P3HT in the composite. Fig. 4d shows the displacement of Ti2ppeaks in the composite. The binding energy became larger duringthe complex process, indicating that there were electrons transferand Fe N/TiO2 was combined with P3HT at the Ti position. Oncomparing Fig. 4b and c, we found that the sulfur peaks aredisplaced after composite reaction, indicating that P3HT com-bined with Fe N/TiO2 at the sulfur position.

Fig. 4. XPS spectra of P3HT/Fe N/TiO2 (a), the sulfur peaks in P3HT

3.5. UV–vis spectra of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

The UV–vis spectra of the Fe N Cl/TiO2 film, P3HT film, andP3HT/Fe N Cl/TiO2 nanocomposite film are presented in Fig. 5.

(b), the sulfur peaks in P3HT/Fe N/TiO2 (c), and Ti 2p peaks (d).

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226224

Fe N/TiO2 (1) shows maximum absorption at a wavelength ofabout 313 nm and a weak absorption at about 500 nm. The UV–vis spectrum of P3HT (2) with maximum absorption at a wave-length of about 510 nm for the p–p* transition indicates theformation of the big p conjugative structure along the main chain[15,16]. From the spectrum of P3HT/Fe N/TiO2 (3) nanocompositefilm it can be seen that there are two absorption bands withmaximum wavelengths of 263 and 519 nm in the ultraviolet andvisible light regions, respectively. On comparing the spectra of FeN/TiO2 and P3HT, we found that the absorption scope of P3HT/FeN/TiO2 nanocomposite film becomes obviously broader. It isreasonable to believe that the P3HT/Fe N/TiO2 nanocompositewill perform better than P3HT and Fe N/TiO2 as a semiconductor.

3.6. Fluorescence spectra of Fe N/TiO2, P3HT, and P3HT/Fe N/TiO2

The fluorescence spectra of the Fe N/TiO2 film, P3HT film, andP3HT/Fe N/TiO2 nanocomposite films are shown in Fig. 6. Theemission peaks of Fe N/TiO2 (1) appear at 387 and 450 nm. Themaximum emission of P3HT (2) appears at 479 nm. The fluores-cence spectrum of the P3HT/Fe N/TiO2 (3) nanocomposite filmwith a maximum emission at 502 nm is totally different fromthose of either P3HT or Fe N/TiO2, which indicates that there is anew optical property for the new chemical action between P3HTand Fe N/TiO2.

3.7. Cyclic voltammetry

Fig. 7a–c shows the cyclic voltammetry results of the P3HTfilm and P3HT/Fe N/TiO2 nanocomposite film. The oxidationpotentials and redox of electrode are listed in Tables 1–3. EHOMO

and ELUMO were obtained using the following formulas [17,18]:

EHOMO ðeVÞ ¼ E0�eFox ðVÞ

ELUMO ðeVÞ ¼ E0�eFred ðVÞ

where EHOMO is the electrode potential of the highest occupiedmolecular orbital, E0 is the standard electrode potential of Ag/Ag+

electrodes and its value is �4.4 eV, Fox is the oxidation potentialof electrode (as opposed to Ag/Ag+ electrodes), ELUMO is theelectrode potential of the lowest unoccupied molecular orbital,and Fred is the redox potentials of electrode (as opposed toAg/Ag+ electrodes). Fig. 7a shows the cyclic voltammetry curves

Fig. 6. Fluorescence spectra of Fe N/TiO2 (1), P3HT (2), and P3HT/Fe N/TiO2 (3).

Fig. 7. Cyclic voltammetry waves of P3HT synthesized under different conditions

(a), P3HT/Fe N/TiO2 with the same ratio of P3HT and Fe N/TiO2 synthesizes under

different conditions (b), and P3HT/Fe N/TiO2 with different mol ratios of P3HT and

Fe N/TiO2 at 0 1C for 8 h (c).

of P3HT synthesized under different conditions. It was found thatoxidation area of P3HT was larger than reduction area, and thatthe initial oxidation potential was fairly low. Table 1 gives the

Table 1Band gap Eg and p/n (the ratio of p to n area) of P3HT synthesized under different

conditions.

Sample Fox (eV) Fred (eV) EHOMO (eV) ELUMO (eV) Eg (eV) p/n

25 1C, 6 h 0.524 �0.675 �4.964 �3.765 1.199 6.12

0 1C, 3 h 0.584 �0.604 �5.024 �3.836 1.188 2.56

0 1C, 5 h 0.575 �0.543 �5.015 �3.897 1.118 6.33

0 1C, 8 h 0.501 �0.610 �4.945 �3.830 1.111 14.4

0 1C, 12 h 0.525 �0.627 �4.969 �3.813 1.152 9.49

Table 2Band gap Eg and p/n (the ratio of p to n area) of P3HT/Fe N/TiO2 (the mol ratio of

P3HT and Fe N/TiO2 is 1:1) synthesized at 0 1C for different duration, TiO2, and Fe

N/TiO2.

Sample Fox

(eV)

Fred

(eV)

EHOMO

(eV)

ELUMO

(eV)

Eg (eV) p/n

P3HT/Fe N/TiO2; 3 h 0.306 �0.523 �4.746 �3.917 0.829 2.79

P3HT/Fe N/TiO2; 5 h 0.303 �0.534 �4.745 �3.906 0.837 6.87

P3HT/Fe N/TiO2; 8 h 0.3367 �0.488 �4.777 �3.952 0.824 2.03

P3HT/Fe N/TiO2; 12 h 0.395 �0.532 �4.835 �3.908 0.927 3.45

Fe N/TiO2 1.885 �0.648 �6.325 �3.792 2.534 0.076

TiO2 2.140 �1.012 �6.580 �3.428 3.152 0.079

Table 3Band gap Eg and p/n (the ratio of p to n area) of P3HT/Fe N/TiO2 synthesized at 0 1C

for 8 h with different compositions.

Sample Fox (eV) Fred (eV) EHOMO (eV) ELUMO (eV) Eg (eV) p/n

1:0.5 0.355 �0.558 �4.799 �3.882 0.913 3.85

1:1 0.337 �0.488 �4.777 �3.952 0.824 2.03

1:1.5 0.320 �0.567 �4.764 �3.873 0.886 7.97

Fig. 8. Photovoltaic performance of solar cells sensitized by P3HT (1) and P3HT/Fe

N/TiO2 (2).

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226 225

forbidden band gap Eg and p/n (the ratio of p to n area) of P3HT.P3HT has the smallest Eg (1.111 eV). P3HT synthesized at 0 1C for8 h has the largest p/n (14.4). Fig. 7b shows the cyclic voltam-metry curves of P3HT/Fe N/TiO2 composites, which were synthe-sized under different conditions with the same composition, andtheir electrical data are listed in Table 2. Eg and p/n of P3HT/Fe N/TiO2 are smaller than those of P3HT. The composite synthesizedat 0 1C for 8 h has the smallest Eg (0.824 eV) and the smallest p/n(2.03). The result indicates that P3HT/Fe N/TiO2 is a new kind ofp–n semiconductor with improved performance. Fig. 7c andTable 3 show the CV results of P3HT/Fe N/TiO2 with differentmol ratios of P3HT and Fe N/TiO2. When the mol ratio is 1:1, thecomposite has the smallest Eg and p/n.

3.8. Results of solar cells performance test

3.8.1. Dye-sensitized solar cells

Fig. 8(1) and (2) displays the photovoltaic performance of solarcells sensitized, respectively, by P3HT and P3HT/Fe N/TiO2, andTable 4 lists the parameters of dye-sensitized solar cells. Understandard global AM1.5G (100 mW/cm2) solar condition, theP3HT-sensitized solar cell gave a short-circuit current (JSC) of0.938 mA/cm2 and an open-circuit voltage (VOC) of 0.420 V,corresponding to an overall conversion efficiency (Z) of 0.175%.The P3HT/Fe N/TiO2 (with mol ratio of 1:1)-sensitized solar cellgave a short-circuit current (JSC) of 1.440 mA/cm2 and an open-circuit voltage (VOC) of 0.414 V, corresponding to an overallconversion efficiency (Z) of 0.245%. The enhanced effeciency of

P3HT/Fe N/TiO2 can be related to the chemical interactionbetween P3HT and Fe N/TiO2.

3.8.2. Organic thin-film solar cells

Table 5 lists the parameters of organic thin-film solar cells.Three kinds of organic thin-film solar cells were prepared, includ-ing single-layer Schottky cell, double-layer p–n heterojunctionorganic thin-film cell, and bulk heterojunction organic thin-filmcell. Under standard global AM1.5G (100 mW/cm2) solar condition,the P3HT single-layer Schottky cell gave a short-circuit current (JSC)of 0.000284 mA/cm2 and an open-circuit voltage (VOC) of 0.192 V.The P3HT, TiO2 double-layer p–n heterojunction organic thin-filmcell gave a short-circuit current (JSC) of 0.000289 mA/cm2 and anopen-circuit voltage (VOC) of 0.184 V. The P3HT/Fe N/TiO2 (withmol ratio of 1:0.5) bulk heterojunction organic thin-film cell gave ashort-circuit current (JSC) of 0.000696 mA/cm2 and an open-circuitvoltage (VOC) of 0.302 V. The P3HT/Fe N/TiO2 (with mol ratio of1:1.5) bulk heterojunction organic thin-film cell gave a short-circuit current (JSC) of 0.000437 mA/cm2 and an open-circuitvoltage (VOC) of 0.466 V. The P3HT/Fe N/TiO2 (with mol ratio of1:1) bulk heterojunction organic thin-film cell gave a short-circuitcurrent (JSC) of 0.0309 mA/cm2 and an open-circuit voltage (VOC) of0.469 V, corresponding to an overall conversion efficiency (Z) of0.0065%, showing the best result.

4. Conclusion

A new series of P3HT/Fe N/TiO2 composites were successfullysynthesized in situ. In the composite, the Ti–S bond was formed,leading to some new optical properties. CV tests show that P3HT/Fe N/TiO2 (with mol ratio of 1:1) synthesized at 0 1C for 8 h hasthe smallest forbidden band gap (0.824 eV) and p/n (2.03). Solarcell performance measurements indicate that the P3HT/Fe N/TiO2

(with the mol ratio of 1:1)-sensitized solar cell is better than theP3HT-sensitized solar cell, while the bulk heterojunction organicthin-film cell is the best organic thin-film solar cell. The double-layer p–n heterojunction organic thin-film cell is better thansingle-layer Schottky cell. All these results show that P3HT/FeN/TiO2 is a new kind of p–n material with excellent photoelectricperformance.

Table 4Parameters of dye-sensitized solar cells.

Effective

area (cm2)

Voc (V) Jsc

(mA/cm2)

Pmax

(mW)

Z (%) FF

P3HT 0.16 0.420 0.938 0.0280 0.175 0.444

P3HT/Fe N/TiO2 (with mol ratio of 1:1) 0.22 0.414 1.44 0.0539 0.245 0.412

Table 5Parameters of organic thin-film solar cells.

Effective

area (cm2)

Voc (V) Jsc (mA/cm2) Pmax(mW) Z (%) FF

P3HT single-layer Schottky cell 1.11 0.192 0.000284 1.80�10�5 1.62�10�5 0.298

P3HT/TiO2 double-layer p–n heterojunction organic thin-film cell 1.09 0.184 0.000289 1.94�10�5 1.78�10�5 0.335

P3HT/Fe N/TiO2 (with mol ratio of 1:0.5) bulk heterojunction organic thin-film cell 1.05 0.302 0.000696 5.44�10�5 5.18�10�5 0.246

P3HT/Fe N/TiO2 (with mol ratio of 1:1) bulk heterojunction organic thin-film cell 1.02 0.469 0.0309 6.63�10�3 0.0065 0.449

P3HT/Fe N/TiO2 (with mol ratio of 1:1.5) bulk heterojunction organic thin-film cell 1.13 0.466 0.000437 7.36�10�5 6.51�10�5 0.320

X. Zheng et al. / Journal of Physics and Chemistry of Solids 72 (2011) 220–226226

Acknowledgments

The work was supported by the Key Planned Science andTechnology Project of Hainan Province (ZDXM 20100062), theNational High Technology Research and Development Program ofHainan under Grant no. 509013, the National High TechnologyResearch and Development Program of China (863 Program)under Grant no. 2006AA03z412, the Scientific Research Projectof Hainan Education Department under Grant no. Hj 2010-52, andthe Scientific Research Foundation of Graduate School of BeijingUniversity of Chemical and Technology (no. 09Si005).

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