Photovoltaic devices based on polythioph

Homopolymers and copolymers were investigated for their performance in photovoltaic devices, and the use of both solid polymer electrolyte and liquid electrolyte was examined.. The reduc

Photovoltaic devices based on polythiophenes and

substituted polythiophenes

C.O Tooa, G.G Wallacea,*, A.K Burrellb, G.E Collisb, D.L Of®cerb,

E.W Bogec, S.G Brodiec, E.J Evansc

a Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia

b IFS-Chemistry, Massey University, Private Bag 11222, Palmerston North, New Zealand

c BHP Steel Research Laboratories, The Broken Hill Proprietary Company Limited,

P.O Box 202, Port Kembla, NSW 2505, Australia Received 4 February 2000; accepted 15 September 2000

Abstract

In recent years there has been considerable interest in the fabrication of photovoltaic devices using polymeric and organic materials This paper presents work carried out using a range of polythiophenes, including some substituted with porphyrin moieties as light harvesters Homopolymers and copolymers were investigated for their performance in photovoltaic devices, and the use of both solid polymer electrolyte and liquid electrolyte was examined Both photoelectrochemical cells and Schottky devices were investigated The best photoelectrochemical cell was fabricated using polyterthiophene which had Vocˆ 139 mV, Iscˆ 123:4 mA cm 2, ®ll factor ˆ 0:38, and energy conversion efficiency ˆ 0:02% at a halogen lamp intensity of 317 W m 2 The Schottky device gave a Vocˆ 0:5 V and Isc of 0.98 mA cm 2at a halogen lamp intensity of 500 W m 2 # 2001 Elsevier Science B.V All rights reserved

Keywords: Photovoltaic devices; Polythiophenes; Conducting polymers; Porphyrins

1 Introduction

Commercial photoelectric conversion devices are made

mostly from inorganic semiconductors In the last two

decades, much work has focused on polymeric and organic

materials [1,2] since the structure and properties of these

photoactive materials can be readily controlled and they are

considerably cheaper than the inorganic equivalents These

studies include the incorporation of conducting polymers

into photovoltaic devices Early work involved devices

based on polyaniline [3], poly(p-phenylene vinylene) [4],

poly(p-phenylene vinylene)/perylene heterojunction [5],

poly(2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene

viny-lene composite [6,7], poly(3-methylthiophene) [8] and

polythiophene [9]

In the approach described previously for

poly(3-methylthiophene) (P3MTh) [8], the P3MTh functions as a

p-type semiconductor where holes or hole-polarons are the

dominant carriers that cause the measured photocurrent The holes generated by irradiation of light cause, via the external circuit, the counter electrode consisting of a Pt coating on indium tin oxide (ITO) coated glass to be positively charged, whilst the electrons move to the P3MTh/solid polymer electrolyte (front) junction The holes oxidise the electron donor iodide to generate triiodide at the counter electrode The electrons injected to the front contact reduce the triio-dide back to iotriio-dide Thus, the cell converts light to electricity

in a renewable process where there is no net chemical reaction

Polythiophenes can be electrosynthesised by oxidative polymerisation according to Eq (1)

(1)

The properties of the polymer can be controlled by the judicious selection of the substituents R1 and/or R2 In addition, the type of counter-anion (A ) can also influence

* Corresponding author Tel.: ‡61-2-4221-3127; fax: ‡61-2-4221-3114.

E-mail address: gordon_wallace@uow.edu.au (G.G Wallace).

0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V All rights reserved.

PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 7 5 - 0

the properties of the polymer Polythiophenes can undergo

electrochemical redox reactions such as

(2)

where A is a mobile counter-anion that can be exchanged

with other anions in solution If the counter-anion is large

and immobile, then the redox reaction is more likely to be

represented by Eq (3)

(3)

where M‡is a cation inserted from the electrolyte solution to

preserve charge balance in the polymer matrix

In this work, tetrabutylammonium perchlorate was used as

the electrolyte and so the counter-anion, perchlorate, would

be expelled from the polymer on reduction, thus leaving the

polymer backbone uncharged (Eq (2)) The neutral polymer,

therefore, behaves like a p-type semiconductor

We report here investigations into polymers made from

commercially available 3-methylthiophene (3MTh),

bithio-phene (BiTh) and terthiobithio-phene (TTh); such monomers and

their derivatives have been successfully electropolymerised

by others [8±15] In addition, we have also investigated

the use of porphyrin substituents on polythiophenes in

order to enhance the light harvesting capabilities of the

conducting polymer materials The use of porphyrins in

photoelectric conversion is well-documented [16±20] Thus,

trans-1(20-(50,100,150,200-tetraphenylporphyrinyl))-2-(300

-thienyl)ethene (I) and trans-1-(20-(50,100,150,200

-tetraphe-nylporphyrinyl))-2-(3000-terthienyl)ethene (II) were prepared

and electropolymerised with and without other thiophene

monomers

In summary, the main objectives of this work were to

electrosynthesise photoactive coatings from

thiophene-based monomers and fabricate them into

photoelectro-chemical and Schottky devices The electropolymerisation

conditions for each polymer or copolymer were investigated

and optimised to produce the best photovoltaic response

2 Experimental 2.1 Reagents and materials The trans-1-(20-(50,100,150,200 -tetraphenylporphyrinyl))-2-(300-thienyl)ethene (TPP-Th) (I) and trans-1-(20-(50,100,

150,200-tetraphenylporphyrinyl))-2-(3000-terthienyl)ethene (TPP-TTh) (II) were synthesised by Burrell et al [21] at Massey University In addition 3-methylthiophene (3MTh, Aldrich), bithiophene (BiTh, Aldrich), terthiophene (TTh, Aldrich), tetrabutylammonium perchlorate (TBAP, Fluka), polyethylene oxide (PEO, Mv c.600 000, Aldrich), potas-sium iodide (Univar, Ajax), iodine (Univar, Ajax or Aldrich 99.8%), methanol (Univar, Ajax), acetonitrile (ACN, Univar, Ajax), dichloromethane (DCM, Univar, Ajax), isopropanol (Univar, Ajax), tetrapropylammonium iodide (Aldrich,

98%), ethylene carbonate (Aldrich 99%), and propylene carbonate (Aldrich 99%) were used as received

ITO coated glass (40 or 10 O sq 1) was purchased from Delta Technologies Limited (USA), cut into required sizes, washed with Teepol, rinsed thoroughly with Milli-RO water followed by isopropanol, and allowed

to dry

The solid polymer electrolyte (SPE) was formulated by dissolving polyethylene oxide (1 g) in 10 cm3 of I2/KI in methanol, which was made up by dissolving I2(0.15 g) and

KI (1.5 g) in 100 cm3 of methanol In addition, a liquid electrolyte was formulated by dissolving I2 (60 mM) and tetrapropylammonium iodide (500 mM) in ethylene carbo-nate/propylene carbonate (1:1 by weight)

A thin layer of platinum was sputter coated onto the ITO coated glass using a Dynavac Magnetron Sputter Coater Model SC100MS The sputtering was performed at a current

of 50 mA and Ar pressure of 2  10 3mbar Under these conditions the Pt thickness obtainable would be 2 AÊ s 1 A

Pt thickness of 10 AÊ was sputter coated In addition, Al was also sputter coated at 150 mA for 1±2 min when fabricating Schottky devices

2.2 Equipment Electrosynthesis and testing of polymers were achieved

by using an electrochemical hardware system comprising

of an EG&G PAR 363 Potentiostat/Galvanostat, a Bioana-lytical Systems CV27 Voltammograph, a MacLab 400 with Chart v 3.5.7/EChem v 1.3.2 software (AD Instruments), and a Macintosh computer A three-electrode electrochemi-cal cell was used which comprised of a working electrode (Pt disc or ITO coated glass or these substrates with polymer coatings on them), a Pt mesh auxiliary electrode and a Ag/

Ag‡reference electrode with salt bridge

Testing of devices was done at BHP Steel Research Laboratories (Port Kembla) using an EG&G PAR 263A Potentiostat/Galvanostat with associated software and

a halogen lamp (317 W m 2) to obtain current±voltage (I±V) curves

Subsequently, testing was done at the University of

Wollongong using an halogen lamp (SoLux MR-16 from

Wiko Ltd.) and a set-up comprising of a Macintosh

com-puter/MacLab 400 with EChem v1.3.2 software (AD

Instru-ments)/CV27 Voltammograph (Bioanalytical Systems) In

general, a light intensity of 500 W m 2 was used unless

indicated otherwise

UV±VIS spectra were obtained using a Shimadzu

UV1601 spectrophotometer and scanning over the range

300±1100 nm

2.3 Photovoltaic device fabrication

2.3.1 Photoelectrochemical cell

The polymers and copolymers were electrodeposited on

ITO coated glass and rinsed with dichloromethane or

acet-onitrile and then allowed to dry In general, the polymer and

copolymer coatings were electroreduced at 0.4 or 0.8 V

in 0.1 M TBAP/DCM or 0.1 M TBAP/ACN before being

assembled as devices The device (Scheme 1) was assembled

by sandwiching up to 1 ml of SPE solution per cm2of area

between the polymer coated ITO coated glass electrode and

the Pt sputtered ITO coated glass electrode, and allowed to

dry for 24 h

In the case of the liquid electrolyte, the device was

assembled by sandwiching the liquid electrolyte between

the two respective electrodes This was done with and

without a border of plastic ®lm as spacer between the

electrodes

2.3.2 Schottky junction devices

Schottky devices were made from electropolymerised

conducting polymer, e.g polyterthiophene (PTTh) in the

reduced form, on ITO coated glass The polymer was then

coated with a thin layer of Al by sputter coating The device

is illustrated in Scheme 2

2.4 Photovoltaic testing The photovoltaic devices were tested by linear sweep voltammetry (LSV) The characteristics of an I±V curve are shown in Scheme 3 Thus, the open circuit voltage (Voc) is where the current is zero, and the short circuit current (Isc) is where the voltage is zero Other characteristics of a photo-voltaic device are given below

The ®ll factor (FF) is given by FFˆvoltage at peak power …Vopen circuit voltage …Vpp†current at peak power …Ipp†

oc†short circuit current …Isc† The energy conversion ef®ciency (ECE) is given by ECE ˆtotal power of light radiating on the cell areaVpp Ipp (5) or

ECE ˆtotal power of light radiating on the cell areaVoc Isc FF (6)

3 Results and discussion 3.1 Poly(3-methylthiophene) The poly(3-methylthiophene) photovoltaic system was chosen as the starting point for our investigations because

of published information already available [8] Poly(3-methylthiophene) was electrodeposited on ITO coated glass from a solution consisting of 3-methylthiophene (0.5 M) in TBAP (0.1 M)/ACN The constant potential (CP) method was used at a potential of ‡1.4 V versus Ag/Ag‡and growth time was 30 min The dried polymers were examined by UV±VIS spectrophotometry in their oxidised and reduced states Thus, the spectrum of poly(3-methylthiophene) in its

Scheme 1 Photovoltaic device.

Scheme 2 Schottky device.

oxidised state showed the free-carrier tail at longer

wave-length (1100 nm) which is characteristic of a conducting

polymer After reduction, this free-carrier tail was lost;

signifying a loss of conductivity The reduced

poly(3-methylthiophene) was fabricated into solid state devices

using the SPE Vocof 52 mV, Iscof 1.23 mA cm 2, ®ll factor

of 0.24 and ECE of 5  10 5% were obtained at a halogen

light intensity of 317 W m 2 This compares favourably

with the results reported by Yohannes et al [8] of

Vocˆ 140 mV, Iscˆ 0:35 mA cm 2 at a light intensity of

1000 W m 2

3.2 Polybithiophene

PolyBiTh was electrodeposited by cyclic voltammetry

(CV) and by constant potential (Table 1) From the growth

CV, it was observed that BiTh began to oxidise at 0.90 Vand

that the current increased with increasing number of cycles,

thus signifying that a conductive polymer was being formed

The UV±VIS spectra of the oxidised and reduced polymer

were obtained (Fig 1a and b) In the spectrum of

poly-bithiophene, reduced at 0.4 V, the loss of the bands above

600 nm was observed, thus demonstrating a loss of con-ductivity of the polymer

Table 1 summarises the results obtained from these reduced polymers when fabricated into photoelectrochem-ical cells These devices were tested with a 317 W m 2

halogen light source In general, thinner polymer ®lms gave better photovoltaic responses The better devices were obtained from polymers produced by CV growth rather than potentiostatic growth The best device incorporated a polymer grown for 2 cycles by CV at 100 mV s 1, with potential limits of 0.4 to ‡1.2 V (Vocˆ 247 mV, Iscˆ 13:4 mA cm 2, ®ll factor ˆ 0:33, ECE ˆ 0:0034%) The best device with potentiostatic-growth polymer was obtai-ned at 1.1 V and grown for 15 mC cm 2 (Vocˆ 234 mV,

Iscˆ 5:71 mA cm 2, ®ll factor ˆ 0:36, ECE ˆ 0:0015%)

Scheme 3 Current±voltage characteristics of a photovoltaic device The

maximum output of the cell is given by the product I pp V pp , where I pp is the

current at peak power and V pp is the voltage at peak power.

Table 1

Characteristics of photoelectrochemical cells from polybithiophene a

Polymer growth conditions V oc (mV) I sc (mA cm 2 ) Fill factor ECE (%)

CV at 100 mV s 1 , 0.4 to ‡1.1 V, 2 cycles 240 7.04 0.32 0.0017

CV at 100 mV s 1 , 0.4 to ‡1.1 V, 5 cycles 232 4.47 0.33 0.0011

CV at 100 mV s 1 , 0.4 to ‡1.2 V, 2 cycles 247 13.4 0.33 0.0034

CV at 100 mV s 1 , 0.4 to ‡1.2 V, 5 cycles 208 3.64 0.31 0.0008

CV at 100 mV s 1 , 0.4 to ‡1.4 V, 2 cycles 206 3.86 0.32 0.0008

CV at 100 mV s 1 , 0.4 to ‡1.4 V, 5 cycles 196 5.28 0.33 0.0011

a Monomer solution was BiTh (10 mM)/TBAP (0.1 M)/CH 3 CN Polymers grown by CP were pre-reduced at 0.4 V (vs Ag/Ag ‡ ) before assembly into photoelectrochemical cells Light source: halogen lamp (317 W m 2 ) CV ˆ cyclic voltammetry, and CP ˆ constant potential.

Fig 1 UV±VIS spectra of polybithiophene on ITO coated glass (a) Polymer in oxidised form; (b) polymer after electrochemical reduction at 0.4 V (vs Ag/Ag ‡ ).

3.3 Polyterthiophene

Commercial terthiophene was easily electropolymerised

onto ITO coated glass and fabricated into PV devices As

previously, the currents increased with increasing number of

cycles during growth of polyterthiophene by CV

Electro-polymerisation was also achieved by potentiostatic and

constant current (CI) methods Potentiostatic growth at

1.0 V afforded a slow increase in the current with time

Galvanostatic growth at 1 mA cm 2 occurred at 1.11 V

Examples of UV±VIS spectra of oxidised and reduced

polyterthiophene (PTTh) are given in Fig 2a and b,

respec-tively Once again, it can be seen from these spectra that, on

reduction of the polymer at 0.4 V, the bands above 600 nm are lost, signifying a loss of conductivity

The reduced polymers were fabricated into photoelectro-chemical cells with SPE and tested The results (Table 2) suggest that polymer growth by CV gives better devices than polymer growth by CI or CP I±V tests, however, also demonstrate that potentiostatic and galvanostatic growth give similar results Thus, the best device had Vocˆ

179 mV, Iscˆ 15:1 mA cm 2, ®ll factor ˆ 0:35, and ECE

ˆ 0:0030%

3.4 Copolymers with TPP-thiophene (TPP-Th) or TPP-terthiophene (TPP-TTh)

Homopolymers from TPP-thiophene could not be suc-cessfully grown, and although homopolymers from TPP-terthiophene could be grown, poor photovoltaic responses were obtained Therefore, copolymers of 3MTh, BiTh or TTh with TPP-Th or TPP-TTh, light harvesting molecules, were electrochemically synthesised on ITO coated glass and fabricated into photoelectrochemical cells in the expectation

of obtaining better photovoltaic characteristics

3.4.1 Copolymer of 3-methylthiophene with TPP-thiophene (TPP-Th)

3-Methylthiophene was copolymerised with TPP-Th on ITO coated glass at ‡1.4 V from a comonomer solution consisting of 3-methylthiophene (0.01, 0.06, 0.5, 2.0 M)/ TPP-Th (0.01 M)/TBAP (0.1 or 0.2 M)/DCM The UV±VIS spectra of the deposits (e.g Fig 3a) exhibit peaks (423 and

523 nm) not present in the spectrum of poly(3-methylthio-phene), indicative of the incorporation of the TPP-Th monomer (see Fig 3b for UV±VIS spectrum of TPP-Th monomer) This was regardless of whether the copolymer was in its oxidised or reduced state

The TPP-Th/3MTh copolymerisation was further investi-gated in order to optimise copolymer growth Maximum

TPP-Th in the copolymer was sought TPP-The following comonomer

Fig 2 UV±VIS spectra of polyterthiophene on ITO coated glass (a)

Polymer in oxidised form; (b) polymer after electrochemical reduction at

0.4 V (vs Ag/Ag ‡ ).

Table 2

Characteristics of photoelectrochemical cells from polyterthiophene a

Polymer growth conditions V oc (mV) I sc (mA cm 2 ) Fill factor ECE (%)

CV at 100 mV s 1 , 0.4 to ‡1.1 V, 2 cycles 172 8.86 0.31 0.0016

CV at 100 mV s 1 , 0.4 to ‡1.1 V, 10 cycles 179 15.1 0.35 0.0030

CV at 100 mV s 1 , 0.4 to ‡1.2 V, 2 cycles 172 4.80 0.30 0.0008

CV at 100 mV s 1 , 0.4 to ‡2.0 V, 4 cycles 187 13.1 0.35 0.0027

CP at 1.0 V, 15 mC cm 2 , reduced at 0 V for 60 s 104 3.02 0.19 0.0002

CP at 1.1 V, 15 mC cm 2 , reduced at 0 V for 60 s 118 1.86 0.21 0.0002

CP at 1.0 V, 10 mC cm 2 , reduced at 0 V for 60 s 164 6.81 0.29 0.0011

CP at 1.1 V, 10 mC cm 2 , reduced at 0 V for 60 s 150 3.83 0.25 0.0005

a Monomer solution was terthiophene (10 mM)/TBAP (0.1 M)/CH 2 Cl 2 Polymers grown by CP or CI were pre-reduced at 0.4 V (vs Ag/Ag ‡ ) unless stated otherwise Light source: halogen lamp (317 W m 2 ) CV ˆ cyclic voltammetry, CP ˆ constant potential, and CI ˆ constant current.

ratios of TPP-Th/3MTh were investigated: 10/10, 10/20, 10/

30 mM In all cases, polymer deposition on ITO coated glass

was possible at ‡1.8 V and above but the deposits were

brittle and tended to ¯ake off Devices were fabricated from

the deposits after pre-reduction It was found that these

devices were not as good as the device made previously

from electrodeposition of copolymer from TPP-Th

(10 mM)/3MTh (2 M) at ‡1.4 V This is probably due to

the higher potential required to form the copolymer when

higher ratios of TPP-Th were used

Photoelectrochemical cells were assembled from these

reduced copolymers with SPE and tested The best result

was obtained from the copolymer grown from a comonomer

solution of TPP-Th (0.01 M)/3MTh (2 M)/TBAP (0.1 M)/

DCM This device had Vocˆ 83 mV, Iscˆ 1:67 mA cm 2,

®ll factor ˆ 0:23 and ECE ˆ 0:0001% at a halogen lamp

intensity of 317 W m 2 These results are better than those

obtained from the poly(3-methylthiophene) homopolymer

3.4.2 Copolymers of TPP-Th and BiTh

Copolymers of TPP-Th and bithiophene (BiTh) were

electrosynthesised on ITO coated glass 10 mM TPP-Th

and 10 mM BiTh were used and cyclic voltammetry was

performed from 0.8 or 0.0 V to ‡1.2, ‡1.4 or ‡1.6 V CV

during growth indicated that currents increased with each

cycle, thus con®rming conducting polymer growth

Poten-tiostatic growth also produces electroactive material With

all deposition methods, the deposits tended to be brittle and

¯aked off the electrodes The best results were obtained

when the limit was ‡1.4 Vor the polymers were grown at a constant potential of ‡1.4 V For comparison, deposits grown at 1.2 and those at 1.4 V were fabricated into devices after pre-reduction UV±VIS spectra were run on the reduced deposits Typically a major peak at 438 nm, ascribed to the porphyrin substituent, is apparent in the spectra (e.g Fig 4); reduced polyBiTh itself has a major peak at 474 nm (Fig 1b)

These copolymers were fabricated into devices with SPE, and I±V test results indicate that CV growth produces the best devices In addition, for CP growth, thinner ®lms give better results The best device had Vocˆ 204 mV, Iscˆ 1:79 mA cm 2, ®ll factor ˆ 0:30, and ECE ˆ 0:0003% These results are not as good as those obtained from poly-BiTh alone

3.4.3 Copolymers of TPP-terthiophene (TPP-TTh) and BiTh

Electrocopolymerisation was performed by potentiody-namic and potentiostatic methods It was apparent from the

CV during growth that the current increased with subsequent cycles, thus indicating the growth of a conductive polymer

In addition, the cathodic peak potential shifted cathodically with increasing number of cycles The UV±VIS spectrum of the copolymer (reduced form) is shown as Fig 5 and is substantially different from the spectra of polyBiTh or poly(TPP-TTh) Increasing the BiTh content in the como-nomer solution from 0.1 to 0.2 M gave better copolymers Once again, CV growth of the copolymers produced the best devices with SPE The best solid state device had

Vocˆ 200 mV, Iscˆ 5:77 mA cm 2, ®ll factor ˆ 0:31,

Fig 3 (a) UV±VIS spectrum of copolymer of 3MTh/TPP-Th on ITO

coated glass Comonomer solution: 3MTh (1 M)/TPP-Th (0.01 M)/TBAP

(0.2 M)/DCM; copolymerisation potential: ‡1.4 V (vs Ag/Ag ‡ )

Copo-lymerisation time: 30 min (b) UV±VIS spectrum of TPP-Th dip coated

onto a glass slide from a 5 mM TPP-Th/DCM solution.

Fig 4 UV±VIS spectrum of TPP-Th/BiTh copolymer grown potentiody-namically and pre-reduced at 0.4 V (vs Ag/Ag ‡ ) in 0.1 M TBAP/ACN for 60 s.

Fig 5 UV±VIS spectrum of copolymer of BiTh and TPP-TTh in its reduced form.

and ECE ˆ 0:0011% Once again, the results obtained from

the copolymers were not as good as those obtained from the

polyBiTh homopolymer

3.4.4 Copolymer of TPP-terthiophene (TPP-TTh)

and terthiophene

Copolymers of terthiophene with TPP-terthiophene were

electrosynthesised on ITO coated glass and fabricated into

photoelectrochemical cells Potentiodynamic and

potentio-static methods were used The UV±VIS spectrum of the

reduced copolymer is given in Fig 6 and is different from the

spectra of the respective reduced homopolymers

The I±V test results (Table 3) indicate that all the

copo-lymers are comparable to poly-TTh as a material for solid

state photoelectrochemical cells Generally, devices made

from copolymer grown potentiodynamically were better than copolymer generated in other ways The performance

of the device also depended upon the thickness of the copo-lymer grown, with poor performance obtained from very thin

or very thick ®lms In addition, reducing the copolymer at 0.8 V (versus Ag/Ag‡) gave better results than reduction

at 0.4 or 0 V The best device gave Vocˆ 185 mV,

Iscˆ 15:9 mA cm 2, ®ll factor ˆ 0:28 and ECE ˆ 0:0026% 3.5 Photoelectrochemical cells assembled using

liquid electrolyte The best performing polymers and copolymers in solid state photoelectrochemical cells were utilised in fabricating new cells that incorporated a liquid electrolyte These cells were tested and a summary of their characteristics is given in Table 4 When liquid electrolyte was used, much higher currents were obtained, and the gain in ef®ciency was 2±7.6 times The best liquid electrolyte device was fabricated using potentiodynamically-grown polyterthiophene, which had Vocˆ 139 mV, Iscˆ 123:4 mA cm 2, ®ll factor ˆ 0:38 and ECE ˆ 0:0205%

3.6 Schottky devices Schottky devices (Scheme 2) were fabricated, and tested

in the same way as for the photoelectrochemical devices

Fig 6 UV±VIS spectrum of copolymer of terthiophene and TPP-TTh in

its reduced form Reduction potential: 0.8 V (vs Ag/Ag ‡ ).

Table 3

Characteristics of photoelectrochemical cells from copolymer of terthiophene and TPP-TTh a

Polymer growth conditions V oc (mV) I sc (mA cm 2 ) Fill factor ECE (%)

CV at 100 mV s 1 , 0.8 to ‡1.2 V, 10 cycles 26 0.42 0.04 7  10 6

CV at 100 mV s 1 , 0.8 to ‡1.4 V, 10 cycles 158 4.59 0.25 0.0007

CV at 100 mV s 1 , 0.8 to ‡1.6 V, 10 cycles 186 8.48 0.23 0.0021

CV at 100 mV s 1 , 0.8 to ‡1.8 V, 2 cycles 173 12.2 0.30 0.0020

CV at 100 mV s 1 , 0.8 to ‡2 V, 4 cycles 185 15.9 0.28 0.0026

CV at 100 mV s 1 , 0.4 to ‡2 V, 4 cycles 160 2.01 0.20 0.0002

CP at 1.1 V, 30 mC cm 2 , reduced at 0 V for 60 s 46 1.29 0.14 7  10 6

a Monomer solution was terthiophene (10 mM)/TPP-TTh (10 mM)/TBAP (0.1 M)/CH 2 Cl 2 Unless stated otherwise, copolymers grown by CP were pre-reduced at 0.8 V (vs Ag/Ag ‡ ) before fabrication into devices Light source: halogen lamp (317 W m 2 ) CV ˆ cyclic voltammetry, and CP ˆ constant potential.

Table 4

Comparison of characteristics of photoelectrochemical cells assembled with SPE or liquid electrolyte a

Type of cell Growth conditions V oc (mV) I sc (mA cm 2 ) Fill factor ECE (%)

 Poly-BiTh CV, 0.4 to ‡1.2 V, 2 cycles 247 13.4 0.33 0.0034

 Poly-TTh CV, 0.4 to ‡2.0 V, 4 cycles 187 13.1 0.35 0.0027

 Copol TPP-TTh/TTh CV, 0.8 to ‡2.0 V, 4 cycles 185 15.9 0.28 0.0026

 Copol TPP-TTh/BiTh CV, 0.8 to ‡1.8 V, 2 cycles 218 4.74 0.30 0.0010

a Light source ˆ halogen lamp (317 W m 2 ) CV ˆ cyclic voltammetry Photoelectrochemical cells with: (  ) SPE or (  ) liquid electrolyte.

The best Schottky device was obtained using the reduced

form of polyterthiophene grown by CV at 100 mV s 1

between the potential limits of 0.8 to ‡1.1 V for 10 cycles

This device had a Vocof 0.5 Vand Iscof 0.98 mA cm 2under

a halogen lamp intensity of 500 W m 2 The variation of Voc

with light intensity showed that the open circuit voltage

increased linearly (R2ˆ 0:9943, 3 degrees of freedom) with

increasing light intensity within the range of 200±

1000 W m 2 This compares favourably with results

reported by Kaneko and Yamada [9] for Schottky devices

incorporating polythiophene tested at a xenon lamp intensity

of 640 W m 2 Their devices had Vocof 0.13 V and Isc of

0.25 mA cm 2 for the reduced form of polythiophene

whereas, for the oxidised form of polythiophene, Voc was

1.07 V and Isc was 1.35 mA cm 2

4 Conclusions

A summary of the best results for photoelectrochemical

devices is given in Table 4 Signi®cant improvement in Voc

and Iscas compared to the devices described by Yohannes

et al [8] from poly-3MTh have been obtained The best

devices provided Isc values which are at least 43±45 times

higher than that published by Yohannes et al., given that the

light source used had an intensity only one third of that used

by the previous workers In general, polymers containing

BiTh or TTh produced the best photoelectrochemical

devices

The use of liquid electrolyte greatly enhances the

ef®-ciency in comparison to SPE devices The best device made

was from poly-TTh This device had Vocˆ 139 mV,

Iscˆ 123:4 mA cm 2, ®ll factor ˆ 0:38, and efficiency ˆ

0:0205%

A Schottky device was successfully made from

poly-terthiophene from which a Voc of 0.5 V and Isc of

0.98 mA cm 2 was obtained under a light intensity of

500 W m 2

Acknowledgements

We wish to thank the Australian Research Council and

BHP Limited for ®nancial support of this project We are

also grateful to the New Zealand Public Good Science Fund

(MAU809) and the Massey University Research Fund

(GEC)

References

[1] M Kaneko, Photoelectric conversion by polymeric and organic

materials, in: H.S Nalwa (Ed.), Handbook of Organic Conductive

Molecules and Polymers, Vol 4, Wiley, New York, 1997, (Chapter 13).

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