Electrochemical and morphological characterization of electrodeposited poly(2,2′:5′,2″-terthiophene) for photovoltaic applications

Synthetic Metals 158 (2008) 661–669

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Electrochemical and morphological characterization of electrodeposited poly(2,2 :5 ,2 -terthiophene) for photovoltaic applications Deborah L. Wakeham, Scott W. Donne, Warwick J. Belcher, Paul C. Dastoor ∗ Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, Australia

a r t i c l e

i n f o

Article history: Received 17 March 2008 Received in revised form 11 April 2008 Accepted 15 April 2008 Available online 27 June 2008 Keywords: Polyterthiophene DSSC Photoelectrochemical cell Photovoltaic Solar cell Morphology

a b s t r a c t The growth, structure and morphology of polyterthiophene films has been characterised using a range of electrochemical and structural techniques. The kinetics of the growth process, including the diffusion coefficient of the terthiophene monomer and the rate constant of the polymerization process has been measured. The electrochemical analysis is consistent with a Stranski–Krastanov growth mode involving the completion of a dense precursor layer prior to the growth of a more porous bulk overlayer. Fabricating polyterthiophene films under diffusion-controlled conditions appears to maximise the thickness of the dense precursor layer. The implication of these results on the photovoltaic energy conversion efficiency of photoelectrochemical cells fabricated from electropolymerized terthiophene is discussed. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Photoelectrochemical cells (PECs) have attracted considerable interest as a realistic low cost alternative to silicon-based photovoltaic devices [1]. In a conventional dye-sensitised solar cell (DSSC) a highly photoactive dye is incorporated into a mesoporous TiO2 substrate and an electrolyte containing an iodide/tri-iodide redox couple is held between the TiO2 substrate and a semi-transparent platinum electrode [2]. Despite achieving power conversion efficiencies in excess of 10% [3], conventional DSSC devices still suffer from a number of limitations. In particular, the need for a liquid electrolyte and consequent sealing does place limitations on the ultimate cost and flexibility of such devices [4]. A potentially attractive alternative to the traditional DSSC approach is to construct PECs based on conducting polymers, which can be electrochemically grown onto flexible substrates with relative ease [5] and incorporated into all solid-state devices [6–8]. However, the efficiencies of these devices are considerably lower than those obtained for traditional DSSC cells and thus in recent years there have been a number of studies focussed on improving the performance of polymer PECs. PECs based on polythiophenes and their derivatives have received much attention, driven in part by their high charge car-

∗ Corresponding author. E-mail address: [email protected] (P.C. Dastoor). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.04.022

rier mobility values in the regioregular form and stability of their undoped and doped states in air [9]. Previous work has shown that PECs based on polyterthiophene (PTTh) perform better than those from either bithiophene (BTh) or poly(3-methylthiophene) [10] as a consequence of an improved conjugation length [11]. Further improvements in efficiency have been achieved by copolymerizing PTTh with either bis-terthiophene [12] or porphyrins [13,14] to increase the lightharvesting ability of the active layer. The creation of efficient charge separation and transport networks within the active polymer layer is key to achieving high efficiencies. Recent advances in the performance of bulk heterojunction (BHJ) devices based on spin coated blends of conducting polymers and fullerene derivatives has demonstrated that optimising the structure and morphology is crucial to improving the energy conversion efficiencies of organic photovoltaic devices [15–19]. Previous work on this material system has focussed on optimising the performance of PTT-based PEC cells by varying the experimental conditions used for electrochemical deposition of the PTT layer [20]. In a related study, the optimum conditions for the electropolymerization of 3,4-disubstituted thiophene were investigated [21]. More recently, the optimum conditions for incorporating anionic light-harvesting dyes into the electrodeposited PTT film has been studied with a view to improving device performance [22]. Although the importance of thin film structure and morphology is identified as a key parameter in these papers, there

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has been no systematic study of the growth and morphology of electrodeposited PTT layers. Previous work by some of us (WJB and PCD) showed that the efficiency of PTT-based PEC devices is governed directly by the structure and morphology of the electrochemically deposited polymer layer. In particular, the electrodeposited PTT films appear to undergo a two-stage Stranski–Krastanov growth mode involving the formation of a thin compact precursor layer followed by the growth of a less dense thicker overlayer. Moreover, the photovoltaic performance of these devices appears to be almost entirely governed by the properties of the thin compact precursor layer. Indeed, the results indicated that the majority of the photocurrent generation occurred in the precursor layer and that the thicker overlayer served simply to limit the short-circuit current generated by the PEC device. We hypothesised that in order to produce more efficient PEC devices, we would need to enhance the growth of the thin compact layer and reduce the growth of the thicker overlayer [23]. Here we present a systematic study of the electrochemical deposition of PTT on ITO with a view to understanding the morphology and structure of the growing films under a range of electrochemical conditions. We show how the structure and morphology of the growing film depend critically upon the electrochemical deposition conditions and draw conclusions about the optimum conditions for the fabrication of future PEC devices. 2. Experimental 2.1. Reagents All reagents used in this work were supplied by Sigma–Aldrich and were of the highest purity possible. This included the monomer 2,2 :5 ,2 -terthiophene (TTh; >99%), the supporting electrolyte salt tetrabutylammonium perchlorate (TBAP; >99%), and the solvent used for electrochemical testing, acetonitrile (>99.93%). To avoid exposure to moisture the monomer and supporting electrolyte were stored in a vacuum dessicator, while dry molecular sieves were placed in the solvent bottle to absorb any water present. 2.2. Electrochemical cell A schematic of the electrochemical cell used in this work is shown in Fig. 1. It essentially consists of a cylindrical teflon sleeve pressed onto a conductive ITO-coated glass slide (working elec-

trode, 2.47 cm2 ), held in place by a Perspex cover and baseplate which were secured with three bolts. A constant pressure was maintained between the teflon sleeve and the glass slide by tightening the nuts on the securing bolts to a constant torque of 0.75 Nm. The cavity within the teflon sleeve above the working electrode was filled with ∼10 mL of electrolyte, which was composed of 5 mM TTh and 0.1 M TBAP in acetonitrile. A Pt coil counter electrode and a Ag/AgCl reference electrode were then inserted into the cell and held in place by a cap which also functioned to seal the cell. Before any electrochemical experiment was started the electrolyte was purged with a dry, acetonitrile-saturated N2 gas stream to remove any dissolved O2 . 2.3. Electrochemical protocols A number of electrochemical techniques were employed to study the electrodeposition of poly(2,2 :5 ,2 -terthiophene) (PTTh). These included the following controlled voltage techniques, which were conducted to examine the electrochemical behaviour of the deposition: (I) Cyclic voltammetry at 10 mV/s over various voltage ranges. (II) Chronoamperometry with the use of different voltage step sizes. These experiments employed a computer controlled PerkinElmer VMP multi-channel potentiostat/galvanostat for system control and data acquisition. (III) Rotating ring-disk voltammetry at 5 mV/s using rotation rates from 500–2000 rpm. For these experiments a Pine Instruments AFMT28TPTPT Pt disk and ring electrode was used, in conjunction with an AFCBP1 Bipotentiostat controlled by PineChem software, and an MSRX Analytical Rotator. A number of controlled current chronopotentiometry experiments were also conducted to prepare polymer samples for structural and morphological characterization. Anodic current densities were chosen so as to examine both activation and diffusion limited conditions. Irrespective of the electrochemical method used, when the experiment was complete, the PTTh coated ITO slide was removed from the electrochemical cell and rinsed with dry acetonitrile to remove any entrained electrolyte, and then allowed to dry in air. 2.4. X-ray diffraction X-ray diffraction patterns of PTTh were recorded using a Philips ˚ employing X’Pert Diffractometer fitted with a Cu source (1.5418 A) an accelerating voltage and current of 40 kV and 40 mA, respectively. Samples were examined in the 2 range 10–40◦ at a scan rate of 0.063◦ 2/s (∼8 h scan). 2.5. Scanning electron microscopy

Fig. 1. Schematic diagram of the electrochemical cell used.

Micrographs were recorded using a Philips XL30 Scanning Electron Microscope (SEM). Polymer deposits were examined in both plan and cross-sectional views. For cross-sectional analysis the deposit on the ITO slide was dipped into liquid N2 for 30 s to essentially remove any elasticity in the polymer film. The slide was then removed and fractured such that the polymer thin film was fractured as well, thus providing a cross-sectional view. The specimens were then sputter coated with Au prior to being placed into the SEM chamber for examination.

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3. Results and discussion 3.1. Cyclic voltammetry A typical series of cyclic voltammograms (CVs) for the electrodeposition of PTTh from TTh is shown in Fig. 2(a). Here the voltage scan was started at the cell open circuit voltage (−0.045 V) and then scanned anodically at 10 mV/s to initiate the polymerization; i.e., TTh → TTh•+ + e−

(1)

The CVs are consistent with the accepted mechanism of polymerization, which involves the formation of a TTh•+ radical cation followed by dihydrodication coupling via ␣ ␣ bonding [24,25]

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resulting in a condensation polymerization to form the insoluble polymeric species, PTTh, which subsequently deposits onto the electrode [26]. The rate of TTh oxidation increases with increasing overpotential resulting in the monotonic increase in current observed with increasing applied voltage. After sweep reversal at +0.9 V a cathodic peak was observed at ∼+0.7 V due to the reduction of the PTTh deposit to PTTh− , rather than reversal of the polymerization process and is typical of the reduction of solid PTTh electrode material [21]. Although the cathodic scan was continued to −0.4 V, minimal cathodic current was flowing by +0.2 V. Electrochemical cycling between −0.4 and +0.9 V was continued for 5 cycles. The inset in Fig. 2(a) shows that the current progressively increases with increasing cycle number and suggests that there is systematic increase in electrode area as a result of the actual deposition of PTTh. This hypothesis is further supported by Fig. 2(b), which shows the cumulative and incremental (inset) charge passed during the cyclic voltammetry experiments depicted in Fig. 2(a). Fig. 2(b) shows that there is a progressively increasing amount of anodic charge being put into the system, suggesting that not every cationic radical (TTh•+ ) formed in the electrochemical initiation is used in the polymerization process, or that reduction of the PTTh film is not an efficient process. Based on the incremental charge passed, the anodic/cathodic efficiency is ∼30%, although it does tend to increase slightly with cycle number. From our experiments conducted using a different voltage window, changing the cathodic limit from −0.4 up to ∼+0.2 V has little impact on the voltametric behaviour. Furthermore, increasing the anodic limit does not lead to a diffusion limited wave, but instead the current continues to increase, which is also consistent with the idea that the electrode area is increasing. The diffusion coefficient for TTh in 0.1 M TBAP in acetonitrile was determined from the anodic voltametric data in Fig. 2(a) using the conventional approach described by Bard and Faulkner [27]. In this method the measured current (i) is related to a normalized reversible current function ((t); for which standard values are tabulated in [27] as a function of E − E1/2 ) using the expression:

i = nFAC

 DnFv 1/2 RT

(t)

(2)

where n is the stoichiometric number of electrons transferred, F is Faraday’s constant (96486.7 C/mol), A is the electrode area (m2 ), C the bulk concentration of TTh (mol/m3 ), D the diffusion coefficient (m2 /s), v the scan rate (V/s), R the gas constant (8.3143 J/K/mol) and T is the temperature (298 ± 1 K). E1/2 is the half-wave potential, estimated using the relationship: E1/2 = Ep/2 +

Fig. 2. (a) Cyclic voltammetry on 5 mM TTh + 0.1 M TBAP in acetonitrile at 10 mV/s and (b) cumulative and incremental charge passed during the cyclic voltammetry experiments.

0.028 mV at 298 K n

(3)

where Ep/2 is the voltage when the current is half its maximum value. Eq. (2) assumes a constant electrode area and thus the full data set up to +0.9 V was not used because of the apparent increase in electrode area upon PTTh formation. As such, only a small portion of the increasing current before the expected peak was used to calculate the diffusion coefficient, which for TTh in 0.1 M TBAP in acetonitrile was determined to be 0.65 × 10−10 m2 /s. This result is comparable to the diffusion constant for other similar sized molecules, such as naphthalene, measured in similar electrolytes; i.e., 2.74 × 10−10 m2 /s in 0.1 M tetraethylammonium perchlorate in acetonitrile.

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Fig. 3. Chronoamperometric data set showing current density versus time for voltage steps from 0.2 to 0.6–0.9 V. The inset shows an expanded view showing the steps to lower voltages. Fig. 4. Comparison between theoretical and experimental chronoamperometric data. The inset shows the calculated electrode area as a result of PTTh deposition for a step potential of 0.9 V.

3.2. Chronoamperometry Chronoamperometry was used to study the electrochemical initiation of PTTh formation and involved monitoring the current flowing as a function of time following the application of a stepwise voltage signal. The ITO working electrode potential was stepped from 0.2 V, where no oxidation was apparent, to a final potential in the range 0.6–0.9 V and the results are shown in Fig. 3. Clearly only step potentials of 0.90, 0.85 and 0.80 V were high enough to cause significant oxidation to occur and hence initiate polymerization. The inset of Fig. 3 shows an expanded view of the lower step potentials and reveals that although the 0.75 V step potential does indicate the occurrence of some oxidation, with an associated increase in current due to polymerization, the extent of polymerization is greatly reduced due to the lack of oxidized TTh•+ radicals. It is well known that polyterthiophene can only be electrodeposited when the applied potential exceed that required to oxidize the monomer [11]. √ Moreover, the current response differs from the expected i ∼ 1/ t behaviour predicted from the Cottrell equation assuming planar semi-infinite diffusion [27]: i=

nFAD1/2 C (t)1/2

(4)

If we consider the 0.8, 0.85 and 0.9 V current–time plots, Fig. 3 shows that immediately after the applied potential step, the current spikes for all three applied potentials due to the almost instantaneous oxidation of the TTh in the vicinity of the electrode. The small drop in current following this spike is actually in accordance with the Cottrell equation since all of the TTh near the electrode has been oxidized and thus diffusion must begin to supply more TTh to the electrode. At this stage, with an abundance of the cationic radical TTh•+ at the electrode surface, polymerization begins leading to the deposition of PTTh. While this polymerization is apparently quite fast, it is not instantaneous. Following the current spike and subsequent rapid decay, the current then starts to increase steadily. This increasing current is not consistent with the behaviour expected from the Cottrell equation, which predicts a monotonic decrease in current with time. This increasing current is consistent, however, with an increasing electrode area arising from the electrodeposition of PTTH. Moreover, coupled with the PTTh being conductive,

this increasing electrode area leads to more TTh being oxidized and consequently more polymerization occurring leading to increased current flow. Thus, it appears that the chronoamperometry data arises from two competing processes: diffusion limited oxidation and polymerization of TTh which predicts a monotonically decreasing current, and an increasing electrode area that results in an increasing current. Ultimately, the current reaches a maximum value and, with increasing time, decreases again. This decrease, which is observed to varying extents for the three applied potentials, is consistent with the continued depletion of TTh in the electrolyte similar to that defined by the Cottrell equation, with the irregularity compared to the expected t−1/2 response most likely arising from the continued polymerization producing a rough surface and hence a changing electrode area. In order to quantify the increase in electrode area caused by PTTh deposition, Fig. 4 shows a comparison between the theoretical diffusion limited i–t relation based on the Cottrell equation (Eq. (4)) and the experimental data (Fig. 3) for the 0.9 V step profile. The theoretical curve (dashed line) in Fig. 4 assumes a fixed electrode area (2.47 cm2 ) and a diffusion coefficient of 0.65 × 10−10 m2 /s, as determined previously. Assuming planar diffusion, the difference between the theoretical and experimental current densities shown in Fig. 4 is due only to the increase in area of the electrode; i.e., i ∝ A, where i is the change in current resulting from the change in electrode area (A). Although this assumption is unlikely to be strictly valid given the increasing surface roughness, it does allow an approximate measure of the magnitude of area increase during PTTh deposition. The inset to Fig. 4 shows the calculated change in electrode area as a function of time during PTTh deposition for a step potential of 0.9 V. Interestingly, the calculated area does not increase monotonically for the duration of the deposition but instead exhibits a distinct maximum at around 200 s deposition time, which indicates that the area of the PTTh deposit actually decreases as deposition continues before subsequently increasing again. Calculating the total charge density at this point reveals that the maximum in the electrode area occurs at a charge density of 52 mC/cm2 .

D.L. Wakeham et al. / Synthetic Metals 158 (2008) 661–669

The presence of a maximum in the electrode area as a function of time is consistent with the completion of layer of deposited PTTh. Initially, there is a relatively large concentration of TTh•+ radicals which very quickly polymerizes to form a large number of PTTh “seeds” on the electrode surface. With a large number of nucleation sites formed, the electrode area increases dramatically but as time and the diffusion limited supply of TTh continues, these initial nucleation sites begin to grow, eventually merging together necessarily resulting in a decrease in the overall electrode area. Our previous work has shown that the growth transition between the formation of the dense precursor layer and the more porous bulk layer occurs at a critical charge density of around 50 mC/cm2 , which is in excellent agreement with the charge density observed at the maximum in the electrode area [23]. Once this layer is completed the deposition process reaches a steady state where the electrode area increases progressively as a result of a balance established between PTTh nucleation and growth, reflecting essentially the intrinsic surface texture formed as a result of the conditions used. As such, the chronoamperometry data is entirely consistent with the growth of a dense precursor layer followed by a less dense bulk layer. 3.3. Rotating ring-disk voltammetry Rotating ring-disk electrode (RRDE) voltammetry is a powerful technique for determining the kinetics of electrochemical processes. In particular, the exchange current density (i0 ) and rate constant (k) of the electrochemical reaction can be determined, as can the diffusion coefficient of the electrochemically active species, provided of course the redox reaction itself is well behaved in the sense that the product of the redox reaction does not undergo additional chemical reactions. Unfortunately, the TTh redox system is not so well behaved since once TTh is oxidized it undergoes subsequent polymerization to deposit PTTh and thus it is not possible to extract quantitative information about the diffusion coefficient. Nevertheless, quantitative kinetic information concerning the polymerization process can be extracted including the rate constant of the polymerization reaction, kp , which is of interest here. A typical example of the RRDE data collected in this part of the study is shown in Fig. 5, which shows a comparison between the disk (iD ) and ring (iR ) currents. The graph indicates that some of the cationic TTh•+ radical formed on the disk is spun away and collected (reduced) at the ring, suggesting that the TTh•+ does have a finite lifetime before being polymerized. The conditions used to collect the data in Fig. 5 are relatively extreme; i.e., 100 mV/s disk voltage scan rate, meaning that the TTh is oxidized to TTh•+ quickly, and a 2000 rpm rotation rate, meaning that the transport of TTh•+ from the disk to the ring is fast. Furthermore, the potential of the ring electrode (0.1 V) is sufficiently low to reduce the TTh•+ back to TTh. To quantify the loss of TTh•+ from the disk electrode, Table 1(a) lists the collection efficiencies (NK = −iR /iD ) for all the experiments conducted. Also included in Table 1(b) is the collection efficiency (N) of a well behaved system, in this case 10 mM ferrocence in an electrolyte of 0.1 TBAP in acetonitrile. The ferrocence collection efficiencies represent the maximum amount of disk-generated species that can be detected at the ring electrode. Clearly in the TTh case only a small fraction of the TTh•+ is lost to the electrolyte and detected at the ring. Table 1(c) lists the percentage of the cationic radical lost to the bulk electrolyte. Using the RRDE data, it is possible to relate the differences between NK and N to the presumed first order rate constant for the polymerization reaction (kp ) [27]:  2/3

NK = N − (ˇ )



U∗ 1− A1



+

A22 2 U ∗ (ˇ ) 2A1

Fig. 5. Example of RRDE data collected in this work with the upper trace showing the variation of disk current and the lower trace showing the variation of ring current as a function of disk voltage. Scan rate = 100 mV/s, rotation rate 2000 rpm, and Ering = 0.1 V.

Table 1 Collection efficiencies for (a) TTh oxidation (NK ), and (b) ferrocene oxidation (N) Rotation rate (rpm) 0 (a) NK Scan rate (mV/s)

2

− 2A2  T2

(5)

500

1000

1500

2000

5 20 100

0.04 0.01 0.01

1.07 0.56 1.46

0.73 0.66 1.73

1.43 0.74 1.75

0.60 0.75 1.92

5 20 100

27.12 23.73 27.81

21.19 24.35 25.39

22.95 24.71 25.23

22.86 24.55 25.16

22.53 24.47 24.35

(c) Percentage lost to the bulk electrolyte 5 0.14 Scan rate (mV/s) 20 0.06 100 0.05

5.04 2.30 5.76

3.18 2.66 6.87

6.24 3.03 6.94

2.66 3.07 7.91

(b) N Scan rate (mV/s)

In (c) the percentage of TTh• + lost to the bulk electrolyte is reported.

where the various individual parameters are defined as A1 = 1.288 



r3 = 3 ln r2 tanh(A1 ) ∗ U =  ˇ

1/3 A2 = 0.6431/6 Dr  2 T2 = 0.718 ln r1

=

(6)

1/2 kp ω−1/2 D−1/6 1/6 (0.51)−1/3

and where  is the kinematic viscosity (= / where is the electrolyte viscosity (3.45 × 10−4 Pa s), and  is the electrolyte density (786 kg/m3 )), ω the angular frequency, r1 the radius of the disk (2.29 × 10−3 m), and r2 and r3 are the inner (2.47 × 10−3 m) and outer (2.69 × 10−3 m) radii of the ring, respectively. Table 2 com-

Table 2 Polymerization rate constant (k, s−1 ) Rotation rate (rpm)

Scan rate (mV/s)

4/3

665

5 20 100

500

1000

1500

2000

190 343 369

568 700 664

704 387 992

1100 1531 1003

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pares the polymerization rate constants determined using this approach. As expected, the magnitudes of the tabulated rate constants indicate that the polymerization process is indeed a fast reaction. In addition, the polymerization rate constant is dependent on the experimental conditions used; it increases as both the rotation rate and voltage scan rate increase, although the latter effect is not as significant. Given that the rate constant is an intrinsic value and should remain constant irrespective of how much TTh•+ is formed, the dependence of kp upon ω must be due to some other artefact of the experiment. As indicated in Fig. 3, the electrode area increases dramatically following initiation of the polymerization reaction. As such, the disk current increases proportionately, effectively increasing the amount of TTh oxidation. Thus, we postulate that the generation of larger disk currents at fast scan and rotation rates is actually due to the increasing electrode area as a result of PTTh deposition. Under these conditions, there is considerable TTh•+ being formed and hence increased deposition of PTTh and a consequent increased electrode area. It is the rapidly increasing electrode area that leads to the apparent increase in polymerization rate constant at the faster scan and rotation rates. 3.4. Chronopotentiometry To determine the boundary between diffusion and activation control we have made use of the Sand equation for constant current electrolysis: nFAD1/2 1/2 i 1/2 = C 2

(7)

where is the transition time indicating the crossover from activation to diffusion control. Using the diffusion coefficient determined previously and a desired current density, can be calculated, as can the total amount of charge passed in the electrodeposition process. Using this relationship the position of the boundary between diffusion and activation control was calculated for an array of PTTh films deposited using different current densities (5, 50 and 100 mA/cm2 ) and total electrodeposition charge densities (50, 100 and 250 mC/cm2 ). Within these values only the use of a current density of 100 mA/cm2 with a total charge density of 250 mC/cm2 lead to diffusion limited conditions, with all others being activation controlled. Fig. 6 shows the typical chronopotentiometric response as a function of varying current density. The nomenclature uses two numbers separated by a dash to define the conditions used; e.g., 5–100. The first number indicates the current density used, while the second indicates the total charge passed. Fig. 6 shows that an increase in current density leads to an increase in the voltage at which polymerization occurs and is essentially a consequence of the electrode surface being more oxidizing due to the presence of more TTh•+ compared to TTh. In addition, the fact that the voltage increases also indicates that the polymerization process is not under diffusion limited conditions since otherwise a constant voltage would be expected. As expected from the preceding sections, the voltage is very irregular compared to the expected behaviour, which is again due to the deposition of PTTh causing an increase in effective electrode area.

Fig. 6. Chronopotentiometric data collected in this work showing examples of the effects of current density. Note the sample naming system used; i.e., the first number corresponds to the current density (mA/cm2 ), while the second corresponds to the specific capacity (mC/cm2 ).

each sample. Unfortunately a full structural analysis is beyond the scope of this present publication and we limit ourselves here to a semi-quantitative description only. The XRD data allow the relative crystallinity of the PTTh films to be compared as a function of the synthesis conditions used and a number of observations can be made. First, the intensity of the diffracted X-rays increases with the amount of charge used in the electrodeposition since, as expected, the increased charge density results in thicker PTTh films. Second, when different rates of deposition were used the crystal growth was more pronounced along different growth axes. For example, the diffraction peaks

3.5. X-ray diffraction analysis The array of PTTh samples prepared using chronopotentiometry was then examined using XRD, as shown in Fig. 7. Also shown in this figure is the XRD pattern of the substrate, an ITO-coated glass slide. As is apparent, the incident X-rays in all cases penetrated the PTTh deposit giving rise to a background pattern for

Fig. 7. XRD data for PTTh deposited under a range of different conditions. Note the sample naming system used; i.e., the first number corresponds to the current density (mA/cm2 ), while the second corresponds to the specific capacity (mC/cm2 ).

D.L. Wakeham et al. / Synthetic Metals 158 (2008) 661–669 Table 3 Calculated crystal size for the XRD peaks at 19.1◦ and 20.0◦ 2

Sample

Peak position (◦ 2)

˚ Crystallite size (A) 19.1

20.0

100–250 50–250 5–250

349 350 781

174 211 321

at 12.0◦ , 15.9◦ and 26.6◦ are quite pronounced at low current densities (5 mA/cm2 ) but have a minimal presence when higher current densities are used. Another more subtle example of this is evident in the 2 range 17.6–20.7◦ . For 100–250 and 50–250 samples, there are three obvious peaks present in this range at 18.4◦ , 19.1◦ and 20.0◦ . However, in both cases the intensity of these peaks diminishes with decreasing charge density. However, when a current density of 5 mA/cm2 was used (particularly sample 5–100), only the peak at 20.0◦ was obvious, with a very small shoulder at 19.1◦ while the peak at 18.4◦ was not present at all. Thus, it would appear the orientation and nature of the crystalline PTTh phase is strongly dependent on the electrodeposition conditions. In order to better quantify the extent of the crystallinity of the PTTh films, a line broadening analysis was carried out using the peaks at 19.1◦ and 20.0◦ 2, since these were common samples prepared using different current densities. A Lorentzian line-shape plus a linear background was fitted to each of these peaks using linear least squares regression; i.e., Lorentzian line − shape : I =

Imax W 2 2

2

(8)

4[(W/2) + (2 − 2) ]

where I is the calculated intensity, Imax the intensity maximum, W the peak width at half height, and 2 the peak position. W was then corrected for instrument broadening using the XRD pattern of crystalline Y2 O3 , coupled with the Warren equation: B2 = W 2 − BS2

(9)

where B and BS represent peak broadening due to the PTTh and the instrument (Y2 O3 ), respectively. Table 3 shows the results of this analysis and reveals that the crystallinity increases as the current density used for deposition decreases. This observation demonstrates that under slower rates of PTTh production, such as occurs with lower current densities, the equilibrium between crystal nucleation and growth is shifted slightly towards growth. Crystal size in this case is dependent on the relative efficiencies of how well TTh•+ initiates polymerization compared to how well it can add on to an already existing polymer backbone. At low current densities the rate of TTh•+ generation is relatively slow, implying that the rate of nucleation is suppressed. As such, the growth of PTTh is enhanced, thus leading to larger crystals. Conversely, with the use of a high current density the rate of TTh•+ formation is fast, effectively leading to an enhancement of PTTh nucleation. 3.6. Morphological characterization The preceding electrochemical and structural characterization of the electrodeposited PTTh films indicates that there are complex changes in surface morphology and are consistent with our earlier work on PEC devices based on PTTh, which showed that a dense thin (∼100 nm) layer of PTTh is formed initially followed by a more porous ‘bulk’ polymer layer [23]. These earlier experiments, however, were conducted using only a limited range of experimental deposition conditions and, given that surface morphology has

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a considerable impact on the performance of organic photovoltaic devices, the goal of this study was to more fully explore the effect of deposition conditions on the morphology of the as-deposited film. In particular, the PTTh films deposited here using chronopotentiometry were prepared under a wide range of conditions and encompassed both activation and diffusion limited conditions with a view to differentiating the effects of these conditions on morphology. Fig. 8 presents SEM images taken from selected PTTh samples in both plan (a) and cross-sectional (b) views. When 50 mC/cm2 of charge was used it would appear from Fig. 8(a) that a relatively dense layer of PTTh was formed. This observation agrees well with our previous work, which showed that the transition between the dense precursor layer and the more porous ‘bulk’ layer occurs at a charge density of around 50 mC/cm2 . Furthermore, Fig. 8 also reveals that the current density has an effect on the morphology of the precursor layer, with the deposit formed at 5 mA/cm2 being relatively flat, while the film produced at 100 mA/cm2 has significant surface texture. Based on our previous conclusions this observation is indicative of an increased rate of PTTh nucleation. When a much higher charge density of 250 mC/cm2 of charge was used there are significant morphological differences apparent in Fig. 8(a) that are also dependent upon current density. For a current density of 5 mA/cm2 the deposit appears as an irregular assembly of rather large crystals. However, when a higher current density (100 mA/cm2 ) was used, the deposit appeared quite similar to that produced when only 50 mC/cm2 of charge was used, just with a greater surface texture. Again, these morphological observations support our diffraction analysis and demonstrate that at low current densities more crystal growth occurs, albeit leading to an irregular array of crystals, while at higher current densities more nucleation occurs. Interestingly, the sample deposited using a current density of 100 mA/cm2 to a total charge density of 250 mC/cm2 (sample 100–250) was the only sample produced under diffusion limited conditions, and produces a compact film with a smooth morphology. The cross-sectional appearances of the PTTh deposits (Fig. 8(b)) also support our conclusions about the relationship between current density and crystallinity. It is clear in the image of sample 5–250 that there are substantial crystals present compared to 100–250, which in turn has a substantial impact on the thickness of the deposit. As can be seen in Fig. 8(b), when a lower current density is used, the resultant PTTh deposit is much thicker compared to the corresponding deposit made with a higher current density. The difference is substantial; for example, using a charge density of 50 mC/cm2 , the deposit thickness is ∼2 ␮m with a current density of 5 mA/cm2 and ∼0.5 ␮m with current density of 100 mA/cm2 . The cause of this increased deposit thickness is the apparent stacking of PTTh crystals. With a low current density, the relatively large crystals that form appear to have a quite random stacking implying that there is significant void volume within the deposit. On the other hand, when a high current density is used, the resultant smaller crystals apparently stack much better leading to a more compact deposit. Our previous work on this material system showed that the photovoltaic efficiency of these cells appears to be governed by the precursor film, which is characterised by a dense, uniform morphology of polymer of increasing conjugation length. On the other hand, the ‘bulk’ overlayer, which manifests as a nodular and highly cracked surface layer, appears to be primarily responsible for limiting device efficiency. The results of this study suggest that it should be possible to maximise the thickness of the dense uniform layer by ensuring that the electropolymerization occurs under diffusion limited conditions, which in turn should increase the energy conversion efficiency of these devices. Future experiments are planned to test this hypothesis.

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Fig. 8. SEM images of selected PTTh deposits prepared using chronopotentiometry. (a) Plan view and (b) cross-sectional view. Note the sample naming system used; i.e., the first number corresponds to the current density (mA/cm2 ), while the second corresponds to the specific capacity (mC/cm2 ).

4. Conclusions A systematic study of the electrodeposition of PTTh under a range of electrochemical conditions has been presented. Cyclic voltammetry showed that the electrode area increases systematically during electrodeposition and provided an estimated diffusion constant for TTh in 0.1 M TBAP in acetonitrile of 0.65 × 10−10 m2 /s, which is comparable to that measured for similar sized molecules. Chronoamperometry was used to provide a semi-quantitative measurement of the change in electrode area during deposition and revealed the presence of an initial layerwise growth mode followed by a monotonic increase in electrode area. The observation of a Stranski–Krastanov type growth mode is consistent with previous studies on this system and the charge density at which layerwise completion occurred (50 mC/cm2 ) was in excellent agreement with earlier results. The polymerization rate constants were obtained

using rotating ring-disk electrode voltammetry and although the quantification was limited by the changing surface morphology, the measurements indicated that the polymerization is a fast process. Chronopotentiometry showed that the electrodeposition was only diffusion controlled under the high current and charge density conditions. Consequently, this technique was used to prepare a variety of electropolymerized films across a wide range of deposition conditions for structural and morphological analysis. X-ray diffraction of the samples revealed that the crystallinity of the samples depends strongly on the deposition conditions and increases as the current density used for deposition decreases. Scanning electron microscopy confirmed the existence of a Stranski–Krastanov growth mode, but also showed that by moving into a diffusioncontrolled growth regime it should be possible to increase the thickness of the compact precursor layer that is primarily responsible for photovoltaic activity in these devices.

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References [1] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Miller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382–6390. [2] G.G. Wallace, P.C. Dastoor, D.L. Officer, C.O. Too, Chem. Innovation 30 (2000) 14. [3] Md.K. Nazeeruddin, T. Bessho, Le Cevey, S. Ito, C. Klein, F. De Angelis, S. Fantacci, P. Comte, P. Liska, H. Imai, M. Graetzel, J. Photochem. Photobiol. A: Chem. 185 (2007) 331. [4] B. Li, L. Wang, B. Kang, P. Wang, Y. Qiu, Sol. Energy Mater. Solar Cells 90 (2006) 549. [5] W.A. Gazotti, A.F. Nogueira, E.M. Girotto, M.C. Gallazzi, M.A. De Paoli, Synth. Met. 108 (2000) 151–157. [6] T. Yohannes, T. Solomon, O. Inganas, Synth. Met. 82 (1996) 215–220. [7] T. Yohannes, O. Inganas, Synth. Met. 107 (1999) 97–105. [8] M. Adi, T. Yohannes, T. Solomon, Sol. Energy Mater. Sol. Cells 83 (2004) 301–310. [9] S.N. Hoier, S.-M. Park, J. Phys. Chem. 96 (1992) 5188–5193. [10] C.O. Too, G.G. Wallace, A.K. Burrell, G.E. Collis, D.L. Officer, E.W. Boge, S.G. Brodie, E.J. Evans, Synth. Met. 123 (2001) 53–60. [11] M. Vignali, R. Edwards, V.J. Cunnane, J. Electroanal. Chem. 592 (2006) 37. [12] J. Chen, C.O. Too, A.K. Burrell, G.E. Collis, D. Officer, G.G. Wallace, Synth. Met. 137 (2003) 1373–1374. [13] J. Chen, A.K. Burrell, W.M. Campbell, D.L. Officer, C.O. Too, G.G. Wallace, Electrochim. Acta 49 (2003) 329–337.

669

[14] J. Chen, D.L. Officer, J.M. Pringle, D.R. MacFarlane, C.O. Too, G.G. Wallace, Electrochem. Solid-State Lett. 8 (2005) A528–A530. [15] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841–843. [16] D. Chirvase, J. Parisi, J.C. Hummelen, V. Dyakonov, Nanotechnology 15 (2004) 1317–1323. [17] H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner, N.S. Sariciftci, Adv. Funct. Mater. 14 (2004) 1005–1011. [18] J.K.L. van Duren, X. Yang, J. Loos, C.W.T. Bullie-Lieuwma, A.B. Sieval, J.C. Hummelen, R.A.J. Janssen, Adv. Funct. Mater. 14 (2004) 425–434. [19] C.R. McNeill, H. Frohne, J.L. Holdsworth, P.C. Dastoor, Synth. Met. 147 (2004) 101–104. [20] G. Tsekouras, C.O. Too, G.G. Wallace, Electrochim. Acta 50 (2005) 3224– 3230. ´ ` [21] J.H. Velez, F.R. D´ıaz, M.A. del Valle, J.C. Bernede, G.A. East, J. Appl. Polym. Sci. 102 (2006) 5314. [22] G. Tsekouras, C.O. Too, G.G. Wallace, Synth. Met. 157 (2007) 441. [23] C.J. Harris, W.J. Belcher, P.C. Dastoor, Sol. Energy Mater. Sol. Cells 91 (2007) 1127. [24] G. Marino, Adv. Heterocycl. Chem. 13 (1971) 235. [25] Y. Wei, C.-C. Chan, J. Tian, G.-W. Jang, K.F. Hsueh, Chem. Mater. 3 (1991) 888–897. [26] S.N. Hoier, S.-M. Park, J. Electrochem. Soc. 140 (9) (1993) 2454–2463. [27] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley and Sons, 1980.