Photodegradation of Polythiophene-Based Polymers: Excited State

Dec 24, 2008 - These polymers are susceptible to oxidative photodegradation in air. ... and singlet-state properties in the photodegradation of the po...
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J. Phys. Chem. C 2009, 113, 11507–11513

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Photodegradation of Polythiophene-Based Polymers: Excited State Properties and Radical Intermediates† Marius Koch,‡ Roxana Nicolaescu, and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame Indiana 46556 ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: NoVember 17, 2008

Polythiophene-based polymers are an important class of organic semiconductors that serve as the building blocks for polymer-based hybrid solar cells. These polymers are susceptible to oxidative photodegradation in air. A model water-soluble polymer, the sodium salt of poly[2-(3-thienyl)ethoxy-4-butylsulfonate], was employed to investigate the role of excited-state and singlet-state properties in the photodegradation of the polymer. The singlet excited state produces characteristic absorption at 770 nm with a lifetime of 22 ps. The triplet state exhibits a broad absorption in the 650-800 nm region and has a lifetime of 18.7 µs in deaerated water. The excited triplets are readily quenched by oxygen with a rate constant of 1.9 × 109 M-1 s-1 via both electron and energy transfer pathways. Pulse radiolysis experiments have been conducted to verify the identity of the cation radical and hydroxyl adduct of the polymer. It is evident from the photochemical experiments that the high photochemical reactivity of triplets with oxygen is responsible for the photodegradation. Introduction Organic semiconducting polymers have drawn a lot of attention in recent years because of the ease of solution processing for designing hybrid solar cells, organic light emitting diodes and optoelectronic devices.1-6 The heterojunctions of regioregular polymers, such as poly-3-hexylthiophene (P3HT) and a soluble fullerene derivative, [6,6]-phenyl-C-61-butyric acid methyl ester fullerene (PCBM), have been found to be quite effective in organic solar cells with power conversion efficiencies up to 5%.7-18 The morphology of the heterogeneous junction is very sensitive to the treatment procedure adopted during the film casting and annealing process. It has been proposed that an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites to form aggregates of PCBM, is responsible for morphology evolution in these cells.14,19,20 Most of the recent efforts have been focused on understanding the factors controlling the power conversion efficiency in such hybrid organic solar cells. Singlet and triplet excitons formed at the heterojunction undergo charge separation, and the electrons and holes are driven in the opposite direction for their collection at the electrode surface. The presence of PCBM or a semiconductor nanocrystal at the heterojunction facilitates separation and transport of charge carriers.21 Spectroscopic studies have focused on the elucidation of the charge separation and charge transport properties. Recently, local film structures with device performance have been mapped using photoconductive atomic force microscopy with 20 nm resolution22 and electrostatic force microscopy. An important problem that plagues the utilization of organic polymer solar cells is its susceptibility to photodegradation under ambient conditions.23 One way to overcome this problem is to exclude air from the system. In fact, most laboratories have adopted inert glovebox †

Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. E-mail: [email protected]. ‡ Visiting student from University of Bielefeld, Germany.

CHART 1: Sodium Poly[2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS)

conditions to cast polymer films on conducting electrode surfaces and seal them under inert atmosphere or vacuum. Oligothiophenes modified with electron donor or acceptor moieties exhibit superior hole-transport or electron-transport properties.24,25 Triplet state properties of oligothiophenes have also been studied in detail.26-28 Production of long-lived triplets opens up new avenues to undergo photochemical transformations in photoactive polymers.29 One such common excitedstate interaction results in the production of singlet oxygen. Holdcraft and co-workers30-32 have pointed out that singlet oxygen reaction can decrease the p-conjugation and result in the scission of the polymer chain. In addition, the photoproducts were also found to quench the singlet excited-state of the parent polythiophene polymer. In a recent study, we showed that ethylenevinylene-based oligomers exhibit relatively high yields of triplet (Φf ) 0.4).33 The formation of the triplet excited state was also observed for polythiophene derivatives using transient absorption spectroscopy. An absorption band observed below the π-π transition band (1.5 eV) was attributed to the triplet excited state. Similarly, polymer triplet formation was noted during LED operation.34 The obvious question is, what is the role of the long-lived excited state in dictating the stability of the polymer? In our quest to understand the photodegradation mechanism of polythiophene-based polymers, we used a water-soluble polymer, sodium poly[2-(3-thienyl)ethoxy-4-butylsulfonate], PTEBS (Chart 1), and investigated its singlet and excited-state properties

10.1021/jp808141u CCC: $40.75  2009 American Chemical Society Published on Web 12/24/2008

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by transient absorption spectroscopy. Independent confirmation for the formation of radical intermediates was also obtained from the pulse radiolysis experiments by reacting with N3• and OH• radicals. The results that describe the excited-state behavior and electron transfer reactions of PTEBS are presented. Experimental Section Chemicals. PTEBS was purchased from American Dye Source, Inc. (Catalog no. ADS2000P) and used as received. All other chemicals were of highest purity and were used as obtained. Pulse Radiolysis. Pulse radiolysis experiments were carried out using the Notre Dame 8-MeV Titan Beta model TBS-8/ 16-1S linear accelerator, with a pulse width of 2.5 ns. Each experimental point is the average of at least four replicate shots using the continuous flow mode of the instrument. Dosimetry was carried out with N2O-saturated solutions of 10 mM KSCN, on the basis of the extinction coefficient for (SCN)2•- of ε472nm ) 7580 M-1 cm-1 and the radiation chemical yield G ) 6.13. (The G value is defined as the number of species formed per 100 eV, and G ) 1 corresponds to 0.1036 µM J-1 in SI units.) The reaction of the PTEBS polymer with two oxidative species was investigated. The •OH radical is a very strong oxidant (E0 ) 1.9 V vs NHE)35 and was generated by irradiation of the N2O-saturated aqueous solutions. The azide radical, •N3, is a mild oxidant (E0 ) 1.33 V vs NHE)36 and was formed in N2Osaturated aqueous solutions containing 10 mM of sodium azide. The azide radical is more selective than the hydroxyl radical and participates in one-electron oxidation via primary formation of radical cations. The azide radical has a sharp absorption band at 274 nm and very little absorption above 300 nm. The oxidative species concentration was in the micromolar range, and their formation during the pulse radiolysis is described by the following equations. • nH2O vvvvv f •OH (2.7) + eaq (2.7) + H (0.55) +

H2 (0.45) + H2O2 (0.71) + H+ (2.7) where the numbers in parentheses indicate the G values. • H2O + N2O + eaq (2.7) f N2(g) + OH + OH (2.7)

Under these conditions, G(•OH) is ∼5.4 and G(H•) is ∼0.55 molecules/100 eV. In other words, about 90% of the radicals formed in the N2O-saturated aqueous solutions are •OH radicals. •

• 10 OH + N3 f OH + N3k ) 1.2 × 10

-1 -1

M

s )

Analysis of optical absorption versus time was performed using Origin (Microcal) software. Spectroscopy Measurements. Transient absorption experiments in the nanosecond-microsecond region were carried out using 337 nm laser pulse (pulse width of 10 ns) from a nitrogen laser model UV 24 by Laser Photonics Inc. Each data point is an average of at least five shots. Femtosecond laser photolysis was carried out using a ClarkMXR 2010 laser system (pulse width 130 fs, repetition rate of 1 kHz, 775 nm, 1 mJ/pulse) and an excitation of 387 nm. Detection software was acquired from Ultrafast Systems (Helios). A CCD spectrograph (Ocean optic S2000-UV-vis) was used as detector, and 1000 excitation pulses were averaged to collect one data point. Emission spectra were collected using a Fluorolog-3 spectrofluorometer of Jobin Yvon, Inc. Electrochemical measurements were conducted using an ESA 400 Gamry electrochemical

Figure 1. Absorption and emission spectra of deaerated solution of PTEBS (0.005 wt %) in water. Excitation wavelength for recording emission was set at 348 nm.

setup. For spectroelectrochemical measurements, the electrochemical setup was coupled with a Cary 50 Bio UV-visible spectrophotometer. Results and Discussion Absorption and Emission Properties. Stronger absorption in the visible and relatively higher fluorescence yields are crucial in maximizing the photoconversion efficiency of solar cells. The polythiophene-based polymers exhibit strong absorption in the visible region. Figure 1 shows the absorption and emission spectra of 0.005 w % PTEBS polymer in water. The concentration of the polymer employed in the present study corresponds to 0.176 mM of monomer units. The polymer exhibits absorption and emission maxima at 453 and 585 nm, respectively. In chlorobenzene solutions, the emission yield for regioregular polythiophene, P3HT, is as high as 0.33.37,38 The polymer PTEBS exhibits an excited singlet quantum yield of 0.03 in water, as estimated using rhodamine 6G as reference (φf ) 0.95 in ethanol).The quantum yield for the water-soluble derivative is expected to be lower than this value because of the polar medium. The role of structural variation, conjugate length, and medium effects in tuning the polymer emission was elucidated in earlier studies. 37,39

PTEBS + hν f 1PTEBS* f PTEBS + hν′

(1)

PTEBS* f PTEBS* f PTEBS + heat

(2)

1

3

The singlet excited state was also probed by femtosecond transient absorption spectroscopy. The time-resolved transient absorption spectra recorded following 387 nm laser pulse excitation of deaerated solution of polythiophene derivative (PTEBS) are shown in Figure 2. The solution was passed through a flow cell to avoid accumulation of photoproducts. The time-resolved spectra in Figure 2 show a difference absorption maximum at 770 nm corresponding to the S1-Sn transition. The singlet monitored at 770 nm exhibits a single exponential decay with a lifetime of 22.3 ps, which is in good agreement with the initial bleaching recovery lifetime of 24.6 ps. These observed lifetimes are more than an order of magnitude smaller than the one observed for P3HT in chlorobenzene solutions.38,40 The residual absorption and bleaching in the IR and visible region indicate long-lived triplet formation as a result of intersystem crossing. As discussed earlier, the intersystem crossing rate in polythiophene-based polymers is

Photodegradation of Polythiophene-Based Polymers

Figure 2. Transient absorption spectra recorded following the 387 nm laser pulse excitation (pulse width, 130 fs) of deaerated PTEBS (0.005 wt %) solution in water. The time-resolved spectra were recorded using a flow cell. The absorption time profiles at 475 and 770 nm and corresponding first-order kinetic fits (solid line) are shown in the inset.

determined primarily by the relatively large spin-orbit interaction due to the sulfur heteroatom.40,41 Other effects, such as chain defects, chain ends, or effects due to the side groups, have minimal effect on the intersystem crossing. Triplet Excited State. Nanosecond laser flash photolysis experiments were conducted to follow the long-lived transients.

J. Phys. Chem. C, Vol. 113, No. 27, 2009 11509 Figure 3A and B shows the transient absorption spectra recorded following 337 nm laser pulse excitation of polythiophene polymer in deaerated and air equilibrated solutions. The transient spectrum recorded immediately after the laser pulse excitation exhibits absorption in the infrared with a maximum at 750 nm and a bleaching in the 350-500 nm region. The absorption arising from the formation of the triplet excited state during laser pulse excitation decays with a lifetime of 18.7 µs. The relatively long triplet life facilitates excited-state interaction with quenchers that may coexist in the system. For example, in an air-equilibrated solution, the triplet excited state exhibits a fast decay. Most of the excited state disappears within 1.8 µs as the dissolved oxygen deactivates the triplet excited state (Figure 3B) via an energy and electron transfer process. Oxygen Quenching of Triplet Excited State. We further probed the oxygen quenching reaction by varying the dissolved oxygen concentration. The bimolecular quenching rate constant for the oxygen quenching reaction was measured from the dependence of the pseudo-first-order decay rate constant on the oxygen concentration (Figure 4A). A bimolecular rate constant of 1.9 × 109 M-1 s-1 indicates a relatively high reactivity of the triplet excited polymer toward oxygen. Although earlier studies have pointed out the formation of singlet oxygen (reaction 3), there is also evidence in the present study for the participation of oxygen in the electron transfer process (reaction 4). Comparison of the transient absorption spectra recorded in deaerated and air-equilibrated solution at longer times (e.g.,

Figure 3. Transient absorption spectra recorded following the 337 nm laser pulse excitation (pulse width 10 ns) of (A) deaerated and (B) airequilibrated solutions of 0.005 wt % PTEBS polymer in water.

Figure 4. (A) Triplet quenching of PTEBS by dissolved oxygen. The absorption-time profiles were recorded at 750 nm at different oxygen concentrations and were fit with first-order kinetics. Inset shows the dependence of pseudo-first-order decay rate constant on the oxygen concentration based on two sets of data (triangles and squares). (B) Transient spectra recorded 20 µs after 337 nm laser pulse excitation of PTEBS in deaerated solutions: (a) deaerated with argon and (b) air-equilibrated.

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Figure 5. (A) Cyclic voltammogram showing the oxidation of a solid film of PTEBS polymer cast on a conducting glass electrode in deaerated acetonitrile containing 0.1 M TBAP (Ag/AgCl reference). (B) Schematic diagram illustrating the configuration of spectroelectrochemical experiment. (C) The absorption spectra of PTEBS polymer film cast on a conducting glass electrode at different applied potentials between 0.5 and 1.2 V using a spectroelectrochemical cell. (D) Change in the absorbance during the oxidation and regeneration of the polymer by successive application of 0.9 and 0 V vs Ag/AgCl showing the reversibility of the absorption change following the electrochemical oxidation.

spectrum 20 µs in Figure 4B) shows the difference in absorption between the T-T absorption and the cation radical absorption. Note that the triplet is totally quenched within 20 µs in airequilibrated solution and a residual absorption of the triplet persists in deaerated solutions. Both the bleaching maximum and isosbestic point show a red shift (30-60 nm) as the triplet interacts with oxygen to produce long-lived product (see, for example, the spectral difference in Figure 4B). The insensitivity of the long-lived product toward dissolved oxygen suggests it to be an oxidation product arising from reaction 4. The residual absorption at 750 nm and the bleaching maximum at 460 nm seen in the long-lived transient in air equilibrated solution thus arise from the electron transfer process.

PTEBS* + O2 f PTEBS + 1O*2

(3)

+ O∼• 2

(4)

3

+•

PTEBS* + O2 f PTEBS

3

The triplet energy for polythiophene-based polymers is ∼1.5 eV or ∼34.5 kcal/mol.42 This energy is significantly greater than the energy of singlet oxygen energy (23 kcal/mol) and readily facilitates energy transfer (reaction 3). On the basis of the relatively low cation radical yield as visualized from the tail absorption and residual bleaching (Figure 3B), we consider the electron transfer pathway (reaction 4) to be a minor pathway. To further characterize the cation radical of PTEBS polymer, we further conducted spectroelectrochemical and pulse radiolysis experiments. Electrochemical Oxidation and Spectroelectrochemistry of PTEBS. The polymer, PTEBS, undergoes reversible oxidation (∼0.73 V vs Ag/AgCl reference). The cyclic voltammogram

of the polymer was recorded by casting a thin film on a conducting glass electrode and immersing it in an electrochemical cell containing a Pt counter electrode and a Ag/AgCl reference electrode (Figure 5A). The cation radical formed during oxidation of the polymer essentially represents a trapped hole. The efficient migration of the hole along the polymer chain is an important criterion for charge transport in solar cells. The ability of the polythiophene polymer to undergo reversible oxidation shows its ability to promote hole transport without inducing chemical oxidation. We further characterized the electrochemical oxidation process using spectroelectrochemistry. A thin layer electrochemical cell (Figure 5B) consisting of an optically transparent conducting glass as the working electrode, Pt gauze as the counter electrode, and a Ag/AgCl reference electrode was placed in the spectrophotometer. The absorption changes of the PTEBS film were recorded as we swept the anodic potential in the range of 0.5-1.2 V at 0.1 V steps. The different absorption spectra (Figure 5C) show the formation of a new absorption band in the red-IR region with increasing anodic potential. The decrease in the ground-state absorption parallels the red-IR band growth. The isosbestic point seen at 540 nm confirms the existence of two species; namely, the ground state and cation radical of PTEBS. The absorption maximum seen at 750 nm thus can be attributed to the formation of the polymer cation radical. At potentials greater than 0.8 V vs Ag/AgCl, we see a blue shift in absorption peaks, indicating the possibility of forming more complex or multiple oxidation states of the polymer at these extreme oxidative conditions.

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PTEBS + N3• f PTEBS+•+N3-

Figure 6. Reaction of PTEBS with hydroxyl radicals. Transient absorption spectra recorded following the pulse radiolysis of N2Osaturated PTEBS solution in water. Inset shows the absorption-time profile at selected monitoring wavelengths.

The cation radical generated in the polymer film remains stable under anodic bias conditions. The absorption of the polymer can be restored by reversing the applied bias, thus confirming the reversibility of the electrochemical oxidation process. Figure 5D shows the absorbance change recorded at potentials 0.9 and 0 V vs Ag/AgCl corresponding to the oxidation and regeneration of the polymer, respectively. The reversibility of the absorption change further reflects the ability of PTEBS to capture holes and facilitate hole transport without undergoing any major chemical transformations. Reaction with Hydroxyl and Azide Radicals. Pulse radiolysis can be conveniently used to investigate the polymer susceptibility to different oxidative conditions. Using an 8 MeV electron pulse, we generated hydroxyl radicals in N2O-saturated aqueous solution. The reaction of PTEBS with the hydroxyl radical was followed by recording absorption changes at different wavelengths immediately following the electron pulse irradiation. The hydroxyl radical is a powerful oxidant, although it is commonly accepted that it can add to aromatic rings with nearly diffusion-controlled rates to form hydroxycyclohexadienyl radicals or OH adducts.43,44 Usually, hydroxyl adducts have lifetimes on the order of tens of microseconds. The spectral changes associated with the addition of hydroxyl radicals (reaction 5) are shown in Figure 6.

PTEBS + •OH f (PTEBS-OH)•

(6)

Figure 7 shows the transient absorption spectra recorded following the pulse radiolysis of PTEBS in aqueous solution (N2O-saturated) containing 0.01 M NaN3. The spectrum shows the growth of the cation radical with absorbance maximum at ∼750 nm. Inset shows the representative traces of absorption-time profiles recorded at 460 and 750 nm. The cation radical is relatively long-lived and exhibits little decay during the monitoring period of a few milliseconds. The radiolysis and spectroelectrochemical measurements thus give credence to our argument that the long-lived transient in the laser flash photolysis of PTEBS in air-equilibrated solution (Figure 3B) is due to the formation of electron transfer product; namely, the cation radical. Role of Oxygen in the Photodegradation of PTEBS. The results presented here aim to obtain independent confirmation on the nature of transients formed during photoexcitation of the PTEBS. Of particular interest is the role of oxygen in deactivating the excited states of the polymer. Figure 8 shows the absorption changes associated with steady state photoirradiation (>400 nm) of the PTEBS polymer solution (deaerated versus aerated). Whereas the polymer degradation

Figure 7. Reaction of PTEBS with azide radicals. Transient absorption spectra recorded following the pulse radiolysis of N2Osaturated PTEBS solution containing 0.01 M NaN3 in water. Inset shows the absorption-time profile at selected monitoring wavelengths.

(5)

The hydroxyl adduct has a characteristic absorption at 550 nm and is unusually long-lived (lifetimes greater than 1 ms). It is interesting to note that the absorption features of the hydroxyl adduct are distinctively different from the infrared absorption of the radical cation. The addition of hydroxyl radical is usually a primary step in the oxidation of aromatic molecules. Typically, such hydroxyl adducts are unstable as they undergo transformation to generate cation radical or products in the microsecond-millisecond time scale. In the present study, we found the hydroxyl adduct of the polymer to be stable during the duration of several milliseconds, as shown in the kinetic trace (inset of Figure 6). If the radiolysis of N2O-saturated aqueous solution is carried out in the presence of NaN3 (0.01 M), the hydroxyl radicals are quickly scavenged by the azide ions to produce azide radicals. These azide radicals can then undergo electron transfer with PTEBS molecules on a relatively longer time scale (reaction 6).

Figure 8. Long-term photoirradiation of 0.005 wt % PTEBS solution in air-equilibrated water. The inset shows the absorption spectra in the deaerated solution without illumination and after 60 min of irradiation. For both measurements, a long bandpass filter of 400 nm was used. The cuvette was located 50 cm from the lamp.

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during the 1 h visible irradiation was negligible in the deaerated solution, the change in the absorbance was significant in aerated solutions. The absorbance steadily decreased with increasing time of visible irradiation; in addition, the formation of cation radical at 750 nm is observed. The shift in the absorption maximum to lower wavelengths is an indication that the polymer is losing its extended geometry of conjugation.31,32 As shown earlier, the singlet oxygen generated during the quenching of the excited triplet induces both reduced π-conjugation and scission of the polymer chain to produce smaller chain-length units. In addition, we also expect formation of other oxidative products. However, formation of cation radicals during the quenching of triplet PTEBS by oxygen suggests that electron transfer to oxygen is also an important pathway in the degradation of the polythiophene polymer. The electron transfer reactions conducted in γ-radiolysis experiments (reaction with azide radicals) also confirm a similar blue shift in the absorption peak (see Figure S1 in the Supporting Information) with increasing duration of radiolysis. Efforts are underway to probe the fragmentation products and establish the difference in the polymer scission mechanism. Conclusions The excited-state behavior of PTEBS as probed by transient absorption spectroscopy reveals the deactivation of the singlet excited state with a lifetime of 22 ps to generate a longlived triplet excited state. Both singlet and triplet excited states, as well as a cation radical of PTEBS, exhibit absorption in the IR region with a maximum around 750 nm. The major difference between these species is their lifetimes. Hence, caution must be exercised while assigning the photogenerated transient in a spectroscopy measurement. In the presence of oxygen, the polymer undergoes degradation. The quenching of the triplet excited state by oxygen via energy and electron transfer is the primary event leading to the photodegradation. Future efforts will be directed toward suppressing the triplet formation so that the photostability of the polythiophene-based polymers will be increased. Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. M.K. acknowledges the financial support of the German Academic Exchange Service in the International Study and Training Partnerships (ISAP). We also thank Kevin Tvrdy for his assistance in spectroscopy measurements. This is contribution NDRL 4778 from the Notre Dame Radiation Laboratory. Supporting Information Available: Spectral changes associated with radiolysis experiments. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J. Conducting polymer composites of soluble polythiophenes in polystyrene. Synth. Met. 1987, 22, 79–87. (2) Pei, J.; Yu, W. L.; Huang, W.; Heeger, A. J. A novel series of efficient thiophene-based light-emitting conjugated polymers and application in polymer light-emitting diodes. Macromolecules 2000, 33, 2462–2471. (3) Fichou, D. Structural order in conjugated oligothiophenes and its implications on opto-electronic devices. J. Mater. Chem. 2000, 10, 571– 588. (4) Perepichka, I. F.; Perepichka, D. F.; Meng, H.; Wudl, F. Lightemitting polythiophenes. AdV. Mater. 2005, 17, 2281–2305. (5) Spano, F. C. Excitons in conjugated oligomer aggregates, films, and crystals. Annu. ReV. Phys. Chem. 2006, 57, 217–243.

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