Jin Wang*a,
Chenchen Guoa,
Yongqiang Yub,
Huabing Yinc,
Xueting Liua and
Yang Jiangd
aSchool of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 23009, P. R. China. E-mail: jinwang@hfut.edu.cn
bSchool of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 23009, P. R. China
cDepartment of Electronics and Electrical Engineering, University of Glasgow, G12 8QQ, UK
dSchool of Materials Science and Engineering, Hefei University of Technology, Hefei 23009, P. R. China
First published on 22nd January 2015
A new self-doped 3-hexylthiophene-b-sodium styrene sulfonate block copolymer (P3HT-b-PSSNa) was obtained via solution copolymerization of sodium styrene sulfonate (SSNa) with vinyl end-group poly(3-hexylthiophene) (v-P3HT), which was synthesized under Grignard metathesis (GRIM) reaction conditions. The self-doping structure can be controlled by altering the feed amounts of SSNa monomer, and it exhibited tunable properties. In comparison with v-P3HT, P3HT-b-PSSNa shows good solubility, broad absorption, higher electrical conductivity and good donor/acceptor (D/A) interface binding with CdSe quantum dots (QDs). Composites of P3HT-b-PSSNa and CdSe exhibit uniform dispersion, good compatibility and highly efficient fluorescence quenching, and surface chemical state analysis confirmed the chemical bonding between copolymer and QDs. This is helpful for rapid charge transfer of different semiconductors in the two-phase D/A interface. Here, we developed a strategy for producing a conjugated polymer with both doped stability during repeated electric cycles and compatibility with inorganic semiconductor materials.
Conjugated polymers with an extended π-electron system exhibit a pseudo-band gap in the range 1.5–2.5 eV, and have low conductivity and mobility.6 Generally, the conductivity of undoped conjugated polymers is 10−7–10−11 S cm−1. However, for the application of conjugated polymers instead of inorganic or traditional semiconductors, higher conductivity is required. Doping is an effective method to produce conducting polymers. In contrast to inorganic semiconductor systems, doping in the case of conjugated polymers refers to oxidation or reduction of the π-electron system, p-doping and n-doping, respectively. Polymers having conjugated double bonds can be oxidized or reduced using charge transfer agents (dopants) more easily for better electrochemical activity.
The issue with doped conductive polymers at the present time is their relative instability under atmospheric conditions. The use of external dopants has always been a problem in many practical applications due to their poor stability, especially under thermal and electric cycling. Self-doping in conducting polymers has been proved to improve their stability upon cycling in neutral and alkaline conditions.7 Self-doped polyanilines (PAns) have been studied extensively for this reason. This involves the use of a small-molecule acidic group for internal doping such as fuming sulfuric acid treatment of PAn,8 and copolymers of an acid and aniline, such as poly(aniline-co-N-benzoylsulfonic aniline), which is obtained by post-synthesis treatment of PAn in the presence of o-sulfobenzoic anhydride.9 Self-doping of polythiophene and its derivatives has also been explored. Ratna et al.10 synthesized poly(thiophene-3-oligoethylene glycol sulfonate), which was obtained by incorporating long sulfonate-terminated tethers into the conducting polymer polythiophene, and had a reversible doping and dedoping effect. The sulfonated polythiophene derivative poly[2-(3-sulfinopropyl)-2,3-dihydro-thieno[3,4-b][1,4]-dioxin] was synthesized from (3,4-ethylenedioxythiophene)sulfonate monomer in the presence of FeCl3.11 Due to the immobilization of a covalently linked sulfonate group, the thin polymer films were shown to be highly stable in air in their doped state, in contrast to the thiophene derivatives. In brief, people have recognized that the nature of the interaction between a dopant and a conjugated polymer has a strong influence on the stability in the doped state. Chemical bonding, such as hydrogen bonding, covalent bonding, and coordination bonding, is beneficial for stability, although it may not further extend the electroactivity.
Another issue with conducting polymers of wide concern is their solubilities from the real applications point of view. Grafting of pendant side groups onto a polymer backbone, such as a flexible alkyl chain and alkoxy groups, has also been investigated in order to enhance its solubility.12–14 However, researchers have observed that whilst the presence of alkyl constituent groups on the conducting polymer unit increases its solubility but decreases its conductivity. Polythiophene derivatives, such as P3HT, only dissolve in a limited number of organic solvents, such as chloroform. This is not good for blending with inorganic semiconductors, for example when used for hybrid heterojunctions. It has been reported that when sulfonic acid groups (–SO3−) were added to a conjugated polymeric backbone to form a self-doped copolymer it had better solubility and its photoelectric performance was improved after film-forming.15,16 Polythiophene derivatives also incorporate inorganic semiconductors to form active materials for photoelectronics applications.17–19 In inorganic–organic hybrid photoelectron conversion devices, one limitation to their efficiency is electron transfer from an excited state of a donor polymer to an inorganic electron-acceptor material. Possible nanoparticle aggregation often limits the energy transfer pathway and leads to uncontrollable phase separation of both components on a micrometer scale. It is obvious that ideal nanostructures will facilitate efficient charge photogeneration and device performance for organic semiconductors with short diffusion lengths. The relationship between the nanomorphology and photophysical properties of in situ grown CdS:P3HT blends has been researched,20 showing that charge generation from P3HT excitons is heavily nanomorphology-dependent. In our previous research,21 the interrelationship between the D/A type 3-hexylthiophene/pyridine copolymer (P3HT/PY) and CdSe QDs was studied, and it was shown that good compatibility between P3HT/PY copolymer and CdSe QDs is advantageous for electron transfer.
In this paper, we synthesized self-doped poly(3-hexylthiophene)-b-poly(sodium styrene sulfonate) (P3HT-b-PSSNa), in which sodium styrene sulfonate (SSNa) doping segments were bonded chemically with the P3HT molecular chain. On the one hand, the dopant sulfonic acid groups can improve the conductive properties of conjugated polymers greatly and form a stable and reversible redox system; on the other hand, the hydrophilic styrene sulfonate segments can improve the compatibility between thiophene polymers and inorganic semiconductor materials effectively. The good dispersion of inorganic QDs into the conjugated polymer promotes charge transfer at their D/A interface, accordingly increasing the photoelectric conversion efficiency of the heterojunction structure.
The composition of the block copolymer can be determined from the integral ratio of the signals of P3HT and PSSNa in the 1H NMR spectrum. According to the 1H NMR spectrum of P3HT-b-PSSNa, the signals at 8.5–7.5 ppm correspond to the protons of PSSNa. Thus, the ratio of P3HT to PSSNa segments in P3HT-b-PSSNa was determined to be 1:
0.2 (named as P3HT-b-PSSNa-0.2). P3HT-b-PSSNa block copolymers with different ratios can be copolymerized by adding different amounts of SSNa monomer; 1H NMR spectra can be seen in ESI Fig. S1 and S2.† The ratios of P3HT to PSSNa segments were 1
:
0.4 and 1
:
1.7 (named as P3HT-b-PSSNa-0.4 and P3HT-b-PSSNa-1.7), respectively. The composition ratio of P3HT to PSSNa in the block copolymers can also be calculated from the GPC test and this is consistent with the test result of the 1H NMR spectrum.
A UV-vis absorption shift and a variation in intensity are induced by electronic energy migration along the backbone between adjacent segments (i.e., intra-chain) or energy hopping between segments (i.e., inter-chain) in close proximity.24 As shown in Fig. 2(a) and (b), v-P3HT in non-selective good solvents, such as chloroform, exhibits broad absorption from 300 to 550 nm. The peak of the absorption spectrum of v-P3HT is at ca. 430 nm, which is due to intra-chain π–π* transition in P3HT. The characteristic absorption peaks of P3HT-b-PSSNa in chloroform are at ca. 265 nm and ca. 450 nm, respectively, which correspond to π–π* transitions of benzene and thiophene rings, respectively. In comparison with v-P3HT, the absorption spectra of P3HT-b-PSSNa block copolymers show a red shift by ca. 20 nm, which can be ascribed to the incorporation of the doping sulfonic acid group. The molecular chain of the polymer tends to be stretched in a good solvent and tends to be curled in a poor solvent. P3HT, with sulfonic acid groups, has typical amphiphilic properties which increase its solubility in different solutions. This favors macromolecular chain stretching in solution and a better coplanar molecular structure. Especially, P3HT-b-PSSNa-0.4 exhibits a broad extended absorption edge from 550 nm to 650 nm in chloroform and THF; this indicates that P3HT-b-PSSNa-0.4 has stronger electronic coupling and π–π* interactions in chloroform and THF solutions.
The UV-vis spectra of v-P3HT and P3HT-b-PSSNa solutions and films are shown in Fig. 4. The UV-vis spectra of v-P3HT and P3HT-b-PSSNa in the film phase have both become broader and undergone a red shift compared with their solutions. This could be ascribed to conformational changes increasing the degree of conjugation in the crystalline polymer backbone in the condensed state. P3HT-b-PSSNa film exhibits a broader absorption than that of v-P3HT, and the maximum-absorption wavelength is red-shifted by about 42 nm. This can be ascribed to an increase in the effective conjugation length of the polymer chain. Moreover, alignment of P3HT-b-PSSNa block copolymer segments occurs more easily and leads to a more regular structure.
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Fig. 5 Cyclic voltammograms of v-P3HT (a) and the block copolymers P3HT-b-PSSNa-0.2 (b), P3HT-b-PSSNa-0.4 (c), and P3HT-b-PSSNa-1.7 (d). |
The band gap of the copolymer could be estimated from the difference in onset potential between the reduction and oxidation processes. The highest occupied molecular orbital (HOMO) and LUMO are calculated according to the equations reported by de Leeuw et al.:25 EHOMO = −(Eonset,ox + 4.68 eV) and ELUMO = −(Eonset,red + 4.68 eV), where Eonset,ox and Eonset,red are the onset potentials for oxidation and reduction versus Ag/AgCl. The cyclic voltammograms of v-P3HT and P3HT-b-PSSNa are summarized in Table 1. In comparison to v-P3HT, P3HT-b-PSSNa self-doping copolymers have lower band gaps and lower HOMO energies. The band gap of v-P3HT is 1.92 eV, and the band gaps of the P3HT-b-PSSNa block copolymers are in the range 1.61–1.75 eV. This is due to the introduction of the self-doping styrene sulfonate segments, which improve the efficiency of charge transfer at the D/A interface, according to the above results. The beneficial point associated with the lower HOMO energies of the self-doping P3HT-b-PSSNa copolymers is the advantage of a higher open-circuit voltage (Voc) in photovoltaic devices.26
Polymer | Eoxonset | EHOMO (eV) | Ip (eV) | Eredonset | ELUMO (eV) | Ea (eV) | Eecg (eV) |
---|---|---|---|---|---|---|---|
v-P3HT | 0.95 | −5.63 | 5.63 | −0.97 | −3.71 | 3.71 | 1.92 |
P3HT-b-PSSNa-0.2 | 0.76 | −5.44 | 5.44 | −0.95 | −3.73 | 3.73 | 1.71 |
P3HT-b-PSSNa-0.4 | 0.75 | −5.43 | 5.43 | −0.86 | −3.82 | 3.82 | 1.61 |
P3HT-b-PSSNa-1.7 | 0.92 | −5.60 | 5.60 | −0.83 | −3.85 | 3.85 | 1.75 |
Conducting polymers undergo major physical and chemical changes when oxidized (doped) or reduced (dedoped). These changes are accompanied by charge transference and neutralization processes in electrolytes, involving ion movements to and from the polymer main chains. The proposed mechanism of charge balancing is shown in Scheme 2. The “self-doping” in P3HT conducting polymers, in which the anionic group sulfonate is attached to the polymer main chain, provides rapid charge neutralization and transference when the polymer is electrochemically switched between its oxidation states.27,28 In the oxidation reaction, thienyl groups with positive charges attract negatively charged sulfonic acid groups to form intermolecular ion pairs, whereas in the reduction reaction, due to the thienyl groups not being charged, this makes the negatively charged sulfonic acid groups leave, with a reversible doping and dedoping effect.
For further understanding, the electrochemical impedance of v-P3HT and P3HT-b-PSSNa block copolymers was measured in the frequency range 100 kHz–1 Hz. The impedance spectra are shown as Nyquist plots in Fig. 6. A single semicircle in the high-frequency region and a straight line in the low-frequency region for all spectra can be observed. Based on the Nyquist plots, the equivalent series resistance (ESR) of v-P3HT and P3HT-b-PSSNa block copolymers can be obtained from the intersection point of the curves with the real impedance axis. A difference in the ESR of electrodes can be attributed to the different conductance of electrode materials. The relatively low conductivity of v-P3HT resulted in its higher charge transfer resistance. In comparison, P3HT-b-PSSNa block copolymers have a lower ESR because of their better conductivity. The semicircle in the figure represents the solution resistance and the resistance of charge transfer (RΩ + Rα); since the RΩ of the system can be regarded as a fixed value, therefore the diameter of the semicircular figure could be directly compared with the value of the semicircle-based Rα. It can be seen from the Nyquist plots that P3HT-b-PSSNa block copolymers have a smaller diameter in comparison to that of v-P3HT and P3HT-b-PSSNa-0.4 has the smallest diameter among all the materials, which indicates that the P3HT-b-PSSNa-0.4 block copolymer has higher conductivity and faster charge transfer rates than the other samples. It is known that doping–dedoping of polythiophene involves protons, the diffusion process of which often limits the reaction rate. PSSNa is a proton-rich compound with higher proton conductivity. Its abundant protons available on the surface should remove the diffusion limit imposed by the solution phase to allow a fast reaction rate.29
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Fig. 7 TEM image of CdSe QDs (a), inset: magnified HRTEM image of an individual dot; TEM image and EDX pattern of P3HT-b-PSSNa/CdSe blend (b and c), inset in (b): SAED pattern. |
The charge transfer of different semiconductors at the two-phase D/A interface was studied by means of blending a self-doped P3HT-b-PSSNa block copolymer with CdSe QDs. The UV-vis and PL spectra of P3HT-b-PSSNa/CdSe blends are shown in Fig. 8. As shown in Fig. 8(a), the UV-vis spectrum of P3HT-b-PSSNa in chloroform solution presents a strong absorption peak in the range 430–450 nm, while the absorption band became broader and was red-shifted after blending incorporation of CdSe QDs, and the absorption intensity increased by increasing the amount of CdSe. This also indicates that P3HT-b-PSSNa/CdSe blends have highly efficient light absorption capability in the visible spectrum, which is conducive to better matching the solar spectrum. The efficiency of charge generation was further investigated by photoluminescence measurement. As shown in Fig. 8(b), 35% quenching of photoluminescence occurred at a 1:
0.2 mass ratio of P3HT-b-PSSNa
:
CdSe. The quenching of photoluminescence reached 65% at a 1
:
0.5 mass ratio, and the P3HT-b-PSSNa/CdSe blend film exhibits nearly complete quenching of more than 95% at a 1
:
1 mass ratio. Photoluminescence quenching is expected, due to attachment of the sulfonic acid dopant to the surface of CdSe, because the aromatic π-electrons can act as efficient acceptors for photogenerated holes, thus hindering the radiative recombination process.30 The high efficiency of quenching can be attributed to the presence of self-doped sulfonic acid groups in P3HT-b-PSSNa improving the dispersibility of CdSe QDs in the polymer matrix, which is helpful for effective charge separation and transfer at the two-phase D/A interface. The highly efficient photoluminescence quenching of P3HT-b-PSSNa/CdSe blends suggests ultrafast photoinduced charge transfer from P3HT-b-PSSNa to CdSe QDs.
Combination between CdSe QDs and P3HT-b-PSSNa could be further detected by the XPS technique. XPS patterns of P3HT, P3HT-b-PSSNa block copolymer and P3HT-b-PSSNa/CdSe blends are presented in Fig. 9(a–f), respectively. The energy scales were calibrated by the C 1s resonance peak, which was fixed at 284.8 eV. Fig. 9(a) shows the binding energy peaks of S 2p in the thiophene ring at 164.2 eV and 165.2 eV in bulk P3HT. In Fig. 9(b), the peaks at 168.9 eV and 164.4 eV correspond to the binding energy of S 2p in the –SO3− group and the thiophene ring,31 respectively. They also confirmed the existence of a doping –SO3− group, and that a self-doped P3HT-b-PSSNa block copolymer was obtained. The appearance at 532.4 eV in Fig. 9(c) is ascribed to the binding energy of O 1s in the –SO3− group. In Fig. 9(d), the peaks at 405.2 eV and 412.0 eV are ascribed to the binding energy of Cd 3d5/2 and Cd 3d3/2, respectively.32 The difference between these two peak positions is 6.8 eV, which is in agreement with the ref. 33. The surface chemical state of Cd is not easy to change in different chemical environments. The chemical bonding between CdSe QDs and P3HT-b-PSSNa can be detected from the change in the binding energy of S 2p. As can be seen in Fig. 9(e), in comparison with that of S 2p in the –SO3− group, the absence of the peak centered at 168.9 eV in P3HT-b-PSSNa illustrates that the S atom in the –SO3− group has been chemically shifted and may have undergone combination with CdSe QDs. Further information can be obtained in Fig. 9(f); the binding energy value of O 1s in P3HT-b-PSSNa/CdSe blends shows a peak at 531.9 eV. There is a chemical shift of about 0.5 eV in comparison with that in P3HT-b-PSSNa, which means that the surface electronic states of –SO3− groups in the P3HT-b-PSSNa/CdSe blends have been redistributed. That is, –SO3− groups can be doped in the P3HT conjugated polymer on one hand, on the other hand the doped –SO3− groups favor compatibility with CdSe QDs. Measurement of the binding energy provides evidence for the chemical bonding between CdSe QDs and P3HT-b-PSSNa. The XPS results coincide with the morphology observations in TEM images and the photoluminescence quenching between P3HT-b-PSSNa and CdSe QDs. The chemical bonding and good dispersion of CdSe QDs in the polymer matrix would help photogenerated excitons reach the D/A interface within their lifetime.
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Fig. 9 XPS spectra of S 2p peak in v-P3HT (a), S 2p and O 1s peaks in P3HT-b-PSSNa block copolymer (b and c), Cd 3d, S 2p and O 1s peaks in P3HT-b-PSSNa/CdSe blends (d–f). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15195j |
This journal is © The Royal Society of Chemistry 2015 |