Effect of aggregation behavior and phenolic hydroxyl group content on the performance of lignosulfonate doped PEDOT as a hole extraction layer in polymer solar cells

Abstract

Using lignosulfonate (LS) and alkyl chain-coupled lignosulfonate-based polymer (ALS) as the raw materials, the aggregation behavior of LS and ALS was investigated, and they both showed a unique aggregation behavior to form a block-like self-assembly for the first time. The aggregation behavior and mechanism of LS and ALS were investigated by SEM, TEM and DLS. The block-like aggregates prepared from ALS (micron size) were larger than that of LS (nano size). The unique aggregates were also further confirmed by XPS, meanwhile, SAXS was applied to explore the regular intrinsic characteristics of the block-like aggregates. Inspired by the aggregation behavior of LS and ALS, the electron transfer properties of LS and ALS were also studied including the electrochemical properties and hole mobility measurements. The oxidation peaks at 1.2 V and 1.4 V were observed at the LS and ALS modified electrode, respectively. We studied the hole transport properties of LS and ALS using the space-charge-limited current method (SCLC). Average hole mobilities of 2.95 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 3.18 × 10⁻⁷ cm² V⁻¹ s⁻¹ were estimated for LS and ALS, respectively. The above results indicated that LS and ALS are potential water soluble polymeric p-type semiconductors, and the hole transport property of LS is better than that of ALS. Based on the unique aggregation behavior and hole mobility property described above which will facilitate charge transport, water soluble PEDOT:LS and PEDOT:ALS were prepared and applied as the hole extraction layer (HEL) in polymer solar cells. The PCE decreased with a decrease of the phenolic hydroxyl group content (–OH), which suggested that –OH is important for the strength of the PCE. The application properties were consistent with the results of the aggregation behavior and electron transfer properties. The power conversion efficiency (PCE) of 5.19% from PEDOT:LS-1 : 1 as the HTL was achieved with a device structure of ITO/HEL/PTB7:PC71BM/Al in our study. Our results showed that the phenolic hydroxyl group content and conjugation structure of amorphous LS contribute to its promising potential as a dopant of semiconductors, such as PEDOT in organic electronics. Our results provide a novel perspective for the design of dopants for semiconductive polymers. In summary, the phenolic hydroxyl group of the polymer will provide hole transport capability due to its oxidation during device operation.

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Introduction

Self-assembled aggregates from polymers have attracted considerable attention owing to their unique ordered structure, including spheres,¹ wires,² sheets,³ rods,⁴ vesicles,⁵ and many other complex systems such as anisotropic, patterned or hierarchical materials.^6–8 The various structures are central to a range of advanced application nanotechnologies.^9–11 In previous work, variations of controllable self-assemblies could be realized with numerous synthetic polymers with known structures such as poly(styrene sulfonic acid) (PSS)¹² and poly(N-isopropylacrylamide) (PNI),¹³ however, it is relatively challenging to obtain a unique self-assembly with an amorphous polymer. Recently, we were surprised to find that lignosulfonate (LS) and alkyl chain-coupled lignosulfonate-based polymer (ALS) from our previous manuscript¹⁴ could form a unique block-like self-assembly in selective solvents without the help of a foreign interface. To the best of our knowledge, a similar self-assembly from water soluble amorphous polymer has been rarely reported, to date.

Based on our previous work,^14,15 it is now well known that amorphous lignosulfonate (LS) exists as a small aggregate of nano-size in solution. However the unique block-like aggregate of LS in selective solvents is rarely reported and very different to nanospheres of LS. The high value-added application of LS is of great importance. We noticed an odd chemistry where lignin and lignosulfonate will be converted into an oxidized state with radicals, which can be detected by electron spin resonance (ESR).^16–18 Inspired by the unique aggregation behaviour of LS and ALS via their self-assembly and the electron transfer process during the oxidation of LS, we proposed a study of the hole transport properties of LS and ALS. Based on the aggregation behaviour and electron transfer properties of LS and ALS, LS and ALS were used as dopants to disperse poly(3,4-ethylene dioxythiophene) (PEDOT) for preparing the conductive polymers PEDOT:LS and PEDOT:ALS.

During the past decades of development of organic electronic devices including polymeric light emitting diodes (PLEDs), organic solar cells (OSCs) and organic photovoltaics (OPVs), interface modifier materials played indispensable roles for both the anode and cathode.^19–24 It is an exciting result that a power conversion efficiency (PCE) exceeding 10% has been demonstrated in a highly efficient single junction PSC with a poly[(9,9-bis(3,-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) modified cathode.^25,26

In contrast, for the modification of the anode, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) acts as a hole transport material (HTM) and plays an indispensable role as one of the most widely employed HTMs to modify indium-tin oxide (ITO) anodes resulting in good performances.²⁷ However, numerous alternative materials to PEDOT:PSS have been pursued because it is well known to corrode ITO at elevated temperatures due to its high acidity.²⁸ New solution-processable HTMs are still in demand and chemists have made great efforts to explore new alternative materials.^29–36 Recently, we reported that PEDOT:SL as a hole transporting material exhibited promising performance in polymer solar cells.³⁷ In order to study the effect of –OH and the aggregation behaviour of lignosulfonate for the performance of PSCs, we chose LS and ALSs as starting materials in our previous manuscript¹⁴ to study the effect in detail. The motivation for this is also based on the following considerations.

Firstly, lignin,^38–40 the second most abundant plant resource, has three types of phenol derivatives including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which can be oxidized into radicals or radical cations. The electron transfer process accompanied with oxidation provides a potential for lignin to act as the hole transport material. The traditional and classical hole transport/injection layer (HTL/HIL) poly-N-vinylcarbazole (PVK) contains an electron-rich building block carbazole based derivative. However, there are rarely reports of hole injection/transport layers based on electron-rich compounds such as phenol and its derivatives. It is well known that most HTMs will form radical cations during oxidation as shown in Scheme 1. PVK is one of the most widely studied HTMs in previous works. It is noteworthy finding that the highest occupied molecular orbital (HOMO) of PVK is −5.6 eV. We propose LS can act as the HTM/HIT like PVK. The HOMO energy level of LS is also −5.6 eV and it is a potentially good candidate for an anode modifier. Secondly, the high acidity of PEDOT:PSS is caused by the anionic dispersant PSS, however, the limited sulfonation degree of LS and ALS will provide a high pH for the dispersed conductive polymer, which will reduce its corrosion of ITO. Thirdly, as a water soluble polymer from biomass, lignosulfonate showed good dispersion properties in various industrial fields such as dimethomorph water-dispersible granule,⁴¹ cement–water systems,⁴² TiO₂ suspension⁴³ and coal water slurry as shown by our group.¹⁴

Scheme 1 The oxidation of PVK (a) and LS (b) as hole transport materials.

In the following report, we find that LS and ALS could form a unique block-like self-assembly in selective solvents. The aggregation behavior and formation mechanism were investigated in detail. The block-like aggregates from ALS were obviously larger than those from LS. It suggested that alkyl chain cross-linked polymerization of LS may lead to the intensive aggregation behavior of ALS. Furthermore, water soluble PEDOT:LS and PEDOT:ALS were prepared and applied as hole extraction layers in polymer solar cells (PSCs). The results indicated that the content of phenolic hydroxyl group (–OH) affects the power conversion efficiency (PCE) of PSCs, and a PCE of 5.19% from PEDOT:LS-1 : 1 was achieved with the device structure ITO/HEL/PTB7:PC₇₁BM/Al. Our results showed that LS might be promising as a potential dopant of semiconductors, such as PEDOT in organic electronics. The underlying mechanism was also studied and discussed in detail.

Results and discussion

Aggregation behaviour of LS and ALS

To investigate the aggregation behaviour of LS and ALS, we were surprised to find that the unique block-like self-assembly was fabricated from LS and ALS in selective solvents in this study. The preparation preceded as follows. LS was first dissolved in water to prepare a 0.2 mg mL⁻¹ aqueous solution, followed by the addition of ethanol to achieve a 0.05 mg mL⁻¹ solution. The ratio of H₂O/ethanol was 1/3. The mixture solution was stirred for 3 h and left at room temperature to obtain the unique block-like self-assembly. The detailed formation of the block-like aggregate structures at different stages was analysed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS) and small-angle X-ray scattering (SAXS).

The novelty of the block-like aggregate formation mechanism emerges by comparison to other lignin nano- and micro-structures, which was investigated by SEM as shown in Fig. 1. It is well known that lignosulfonate exists as nano-sized oval-shaped aggregates in a dilute water solution.^14,15 As shown in Fig. 1a and d, a lot of nano-aggregates were observed by SEM in the first stage. The diameter was approximately 20 nm. When ethanol was added into the aqueous solution, the degree of aggregation from LS and ALS was obviously different from each other. Structures consisted of many nano-aggregates yielding a block-like self-assembly as shown in the second stage (Fig. 1b and e). Finally, the homogeneous and unique block-like self-assembly was formed both from LS and ALS as shown in Fig. 1c and f. However, block-like self-assemblies of nano-size were observed from LS, and that of micro-size were observed from ALS. Similar SEM images are also shown in Fig. S1.† It is suggested that alkyl chain cross-linked polymerization of LS may lead to the intensive aggregation behavior of ALS. Lignosulfonate, as an amorphous polymer, is an amphiphilic polymer with many aromatic benzene rings, which are easy to aggregate in selective solvents via CH–π and π–π interactions to yield the unique self-assembly.⁴⁴

Fig. 1 (a–c) The nano-sized block-like aggregates from LS via self-assembly in the first stage (a), second stage (b) and third stage (c). (d–f) The micro-sized block-like aggregates from ALS via self-assembly in the first stage (d), second stage (e) and third stage (f).

In order to investigate the formation mechanism of block-like self-assembly in more detail, TEM and DLS were conducted in this study. It was confirmed that LS could self-assemble into nano-sized block-like aggregates in a H₂O/ethanol solution and ALS could form a micro-sized block-like self-assembly as shown in Fig. 2. The block-like structure containing many compacted nano-aggregates from LS and ALS seemed to be homogeneous and well-defined. A dynamic light scattering study of block-like aggregates can give important information about the solution self-assembly process. As shown in Fig. S2,† the average diameter of the block-like aggregates from the LS solution was about 300 nm and that from the ALS solution was 9 μm, which was consistent with the electron microscopy results above. These results indicated that the block-like structure was formed via self-assembly in selective mixed solvents of water and ethanol and the aggregation degree of ALS is obviously different compared with LS.

Fig. 2 Typical TEM images of the homogeneous nano-sized block-like self-assembly from LS (a) and the micro-sized block-like self-assembly from ALS (b).

X-ray photoelectron spectroscopy (XPS) measurements were conducted to test the element distribution of the block-like aggregates. As shown in Fig. 3a, high proportions of carbon and oxygen elements were detected from the block-like structures, which further supported the above result that the block-like self-assembly was formed from LS and ALS.

Fig. 3 (a) X-ray photoelectron spectroscopy (XPS) of block-like self-assemblies from LS and ALS on the same aluminum stubs with SEM. (b) The small-angle X-ray scattering (SAXS) pattern of the block-like structure from LS and ALS.

To determine the regular intrinsic characteristics of the block-like structure, small-angle X-ray scattering (SAXS) measurements were conducted by dropping a sample solution of the block-like self-assembly on a silicon wafer and drying at room temperature. As shown in Fig. 3b, at low angles, strong reflections (8.1° and 16.2°) were detected by SAXS. All the above results indicated that the block-like structure had significant reflections. It is also further supported that the block-like self-assembly was fabricated from LS and ALS.

Electron transfer properties of LS and ALS

Based on the unique aggregation behaviour of ALS as described above, the electron transfer properties of ALS were also studied in this work, including electrochemical properties and hole mobility measurements.

Cyclic voltammetry (CV) was used to study the oxidative activity of LS and ALS. The cyclic voltammograms of the LS and ALS film in a 0.1 M Bu₄NPF₆ (in dichloromethane) solution were used to investigate the electrochemical behaviour of LS and ALS as shown in Fig. 4. The oxidation peaks at 1.2 V and 1.4 V were observed at the LS and ALS modified electrode, respectively. It suggested that the oxidative activity of LS is better than that of ALS. The reaction mechanism for the reaction of LS and ALS at the surface of the electrode was proposed. LS was proposed to be oxidized to form radical cations. Meanwhile, the phenolic hydroxyl group (–OH) of LS might be also oxidized into phenol radicals. The –OH content of LS is greater than that of ALS, which could lead to the better oxidative activity of LS. The electron transfer process provides a new concept for the design of HTM molecules.

Fig. 4 Cyclic voltammograms of the LS and ALS film in a 0.1 M Bu₄NPF₆ DCM solution.

In order to make a quantitative evaluation of the hole transport properties of LS and ALS materials, the hole mobility was measured by testing J–V characteristics of the hole-only devices with the device structure ITO/MoO₃/sample/MoO₃/Al as shown in the structure diagram inserted in Fig. 5. As a wide band gap semiconductor, MoO₃ is used to assist the hole transport to the ITO anode and block the electrons injected from the Al cathode at the same time. The observed dark current–voltage curve was then fitted by using the space-charge-limited current (SCLC) model, which is described by the equation as follows:

J = (9/8)ε_rε₀μ(V²/d³)

Fig. 5 The J–V curves of hole-only devices with a SCLC fitting from LS and ALS (the structure of hole-only devices is inserted as a picture in the top-left corner).

In the equation above, ε₀ is the dielectric permittivity of free space, ε_r is the relative permittivity of the sample, μ is the mobility, and d is the thickness of the sample. The hole mobility value was obtained by the SCLC equation with the detailed data listed in Fig. 5 and Table 1.

Table 1 The hole mobilities of samples from LS and ALS

Samples	Thickness (L) (nm)	Slope (ρ)	Hole-mobility (cm^2 V^−1 s^−1)
LS	35	13.86	2.95 × 10^−6
ALS	50	2.45	3.18 × 10^−7

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As can be seen from Fig. 5 and Table 1,the hole mobility 2.95 × 10⁻⁶ cm² V⁻¹ s⁻¹, estimated for LS, is nearly one order of magnitude higher than that of ALS (3.18 × 10⁻⁷ cm² V⁻¹ s⁻¹). The result indicates that the LS and ALS are potential water soluble organic semiconductors with hole transporting properties. Moreover, LS with a greater content of phenolic groups, has a higher hole mobility than ALS with a smaller content of phenolic groups. This suggested that the phenolic group contributes to the hole transporting property, which is in good agreement with the electrochemical property results described above.

Dispersion of PEDOT and its application in polymer solar cells

Based on the unique aggregation behaviour and hole mobility of LS and ALS, LS and ALS was applied to the conductive polymer PEDOT to prepare PEDOT:LS and PEDOT:ALS aqueous dispersions, which were applied as hole extraction layers in polymer solar cell devices. The schematic for the polymerization of EDOT in the presence of LS and ALS templates was proposed as shown in Fig. 7.

UV-vis absorption spectra of PEDOT:LS and PEDOT:ALS aqueous dispersions are shown in Fig. S3.† Because of the higher phenolic hydroxyl group content of LS, the absorption intensity of PEDOT:LS from 250 to 300 nm is obviously stronger than that of PEDOT:ALS. The UV spectra of PEDOT:LS and PEDOT:ALS aqueous dispersions showed the presence of a bipolaron absorption band at 800 nm, which was ascribed to the π–π transition in the PEDOT polymer chain.⁴⁵ The broad absorption from 600 to 900 nm was also detected in both PEDOT:LS and PEDOT:ALS aqueous dispersions. It further confirmed that PEDOT:LS and PEDOT:ALS were successfully prepared in our study.

The cyclic voltammograms of the PEDOT:LS film and PEDOT:ALS film in a 0.1 M Bu₄NPF₆ (in dichloromethane) solution were also obtained as shown in Fig. 6. Compared with that of PEDOT:ALS, an obvious oxidation peak at 0.65 V was observed at the PEDOT:LS modified electrode. Two oxidation potentials at 0.65 V and 0.75 V were observed at the PEDOT:LS and PEDOT:ALS modified electrode, respectively. The HOMO value of PEDOT:LS and PEDOT:ALS were calculated as −5.05 eV and −5.15 eV, respectively. It indicated that LS has a positive effect on the conductive polymer PEDOT and LS with a higher –OH content facilitates the hole transfer of PEDOT:LS. The corresponding oxidation mechanism was proposed as shown in Fig. 7. It encouraged us to study its potential application in organic electronics.

Fig. 6 Cyclic voltammograms of the PEDOT:LS film and PEDOT:ALS film in a 0.1 M Bu₄NPF₆ DCM solution.

Fig. 7 Proposed electron transfer process of PEDOT:LS (up) and PEDOT:ALS (down).

To evaluate the hole transport performance of PEDOT:LS and PEDOT:ALS, PSCs with PEDOT:LS and PEDOT:ALS as the anode modifier were fabricated. The detailed device architecture of the PSCs and current density (J)–voltage (V) curves of the PSCs with different HTMs are shown in Fig. 8. The power conversion efficiency (PCE), short-circuit current density (J_SC), open-circuit voltage (V_OC) and fill factor (FF) of the PSCs are given in Table 2. PEDOT only showed a poor PCE of 2.91% with a FF of 48.04%. The same devices (ITO/HEL/PTB7:PC71BM/Al) with the PEDOT:PSS modified anode showed a J_SC of 12.77, V_OC of 0.72 and PCE of 4.28% with a FF of 46.40% as shown in ref. 46. The PCE of the PSCs in a conventional device structure ITO/PEDOT:PSS/PTB7:PC₇₁BM/Al was 4.50%, as reported previously,⁴⁷ and are shown in Table 2. The PCEs of the device with PEDOT:LS and PEDOT:ALSs with different mass ratios in the device structure ITO/PEDOT:PSS/PTB7:PC₇₁BM/Al were all investigated in this study as shown in Table 2. In our previous manuscript,¹⁴ the content of –OH decreased gradually from LS to ALS3. As can be seen from Table 2, the PCEs of the device with PEDOT:LS-1 : 1, PEDOT:ALS1-1 : 1, PEDOT:ALS2-1 : 1 and PEDOT:ALS3-1 : 1 were 5.19%, 4.75%, 3.49% and 3.11%, respectively. The PCEs decreased with a decrease of –OH content, which indicated that –OH is important for the electron transfer process. The results were consistent with the results of CV and hole mobility. In addition, when the mass ratio of PEDOT:LS and PEDOT:ALS1 was 1 : 2, the PCEs of the device with PEDOT:LS-1 : 2 and PEDOT:ALS-1 : 2 were 5.02% and 3.73% which were lower than that of the device using PEDOT:LS-1 : 1 and PEDOT:ALS-1 : 1. It is noteworthy that when PEDOT:LS-1 : 1 was applied, a PCE of 5.19% with a J_SC of 15.06 mA cm⁻¹, V_OC of 0.58 V and FF of 58.87% was obtained. The PCE of PEDOT:LS-1 : 1 (5.19%) was higher than that of PEDOT:PSS⁴⁶ (4.50%). The results indicate that LS might be promising as a potential dopant in semiconductors. It encourages us to study its potential further in the future. We proposed the possible mechanism based the following three aspects.

Fig. 8 (a) Device architecture of polymer solar cell (PSC). (b) J–V curves of PSCs with PEDOT only, PEDOT:LS and PEDOT:ALSs with different mass ratios of ITO/HTM/PTB7:PC₇₁BM/Al in the devices.

Table 2 Photovoltaic performances of PSCs with PEDOT:PSS, PEDOT:LS and PEDOT:ALS with different proportions of ITO/HTM/PTB7:PC₇₁BM/Al in the devices

Anode	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE (%)
PEDOT:PSS^46	0.72	12.77	46.40	4.28
PEDOT:PSS^47	0.62	14.58	50.00	4.50
None	0.43	14.12	48.04	2.91
PEDOT:LS-1 : 1	0.58	15.06	58.87	5.19
PEDOT:LS-1 : 2	0.65	13.43	57.55	5.02
PEDOT:ALS1-1 : 1	0.57	14.69	56.40	4.75
PEDOT:ALS1-1 : 2	0.56	12.20	54.55	3.73
PEDOT:ALS2-1 : 1	0.68	11.17	45.88	3.49
PEDOT:ALS3-1 : 1	0.68	10.96	41.74	3.11

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On one hand, from the electrochemical properties and hole mobilities of LS and ALS, both LS and ALS have potential as semiconductors for hole collection in PSCs. However, the hole mobility of ALS was much lower than that of LS which suggested that the phenolic group content could affect the hole transporting property. Combined with the unique aggregation behaviour, the aggregation degree of ALS is obviously different compared with LS. PEDOT:LS and PEDOT:ALS were applied as novel and potential hole extraction layers (HELs) in polymer solar cells.

On the other hand, in our work, an alkyl chain was introduced in the phenolic group of LS to obtain ALSs; the results above indicated that the phenolic group content could affect the hole transporting property and electron transfer property as shown in Fig. 4 and 5. The oxidation process of PEDOT:LS and PEDOT:ALS was also investigated in our study as shown in Fig. 6. As can be seen in Fig. 9, the HOMO energy level values were 5.0 eV, 5.05 eV and 5.15 eV for PEDOT:PSS, PEDOT:LS and PEDOT:ALS, respectively, which suggested that there are smaller energy gaps in thin films for PEDOT:LS compared to PEDOT:ALS. This is very important for hole transport. Therefore, PEDOT:LS exhibited a better power conversion efficiency than PEDOT:ALS acting as the HEL in polymer solar cells.

Fig. 9 HOMO energy levels of ITO, PEDOT, PEDOT:LS, PEDOT:ALS and PTB7 (HOMO of PEDOT:LS and PEDOT:ALS were calculated as −5.05 eV and −5.15 eV, respectively, as shown in Fig. 6).

Finally, as studied previously, ALS is easier than LS to aggregate, especially in order to fabricate unique block-like aggregates with micro-sizes in selective solvents. The aggregation behavior also affects the hole transporting property of conductive PEDOT. Therefore, we studied the surface morphologies of the PEDOT:LS film and PEDOT:ALS by AFM as shown in Fig. 10a–f. There is a significant difference between them. Overall, the AFM images of PEDOT:ALS films were rough, which was ascribed to the intensive aggregation capability of ALS. The AFM images of PEDOT:LS films were more homogeneous and compacted than that of PEDOT:ALS films. The particle size of PEDOT:LS was also relatively smaller than that of PEDOT:ALS. In the above results, the more homogeneous, smaller sized and compact nano-aggregates might facilitate charge transport of PEDOT:LS polymers in a film. It is an interesting result that LS can act as a dopant of PEDOT even though it has such a complex, disordered and amorphous structure. Compared to PSS, which has a regular structure, the stronger crystallization character and oxidation behaviors of LS might have contributed to the unexpected hole transport of the PEDOT:LS film. Moreover, the pH value of the PEDOT:ALS aqueous dispersion (pH 3.9) was higher than that of PEDOT:PSS (pH 1.9), which is conducive to reduce the corrosion of ITO during device operation.

Fig. 10 AFM images of PEDOT:LS films with different mass ratios (a, b), PEDOT:ALS films with different mass ratios (c, d), PEDOT:ALS2 film (e), and PEDOT:ALS3 film (f). The size of the images is 3 μm × 3 μm.

In summary, based on the unique aggregation behavior, electron transfer property and hole mobility of LS compared with ALS, LS showed a novel application potential as a dopant for water soluble polymeric semiconductors. PEDOT:LS showed a promising performance as a hole-transport material in polymer solar cells (PSCs), which is comparable with that of conventional PEDOT:PSS. The results have rarely been reported, to date. LS, as a novel hole transport material from a renewable bioresource, might be a potential water soluble polymeric semiconductor. The dopant design concept of PEDOT provides a promising strategy to develop stable water soluble PEDOT semiconductive polymers, which will accelerate the commercialization of solution processable organic electronics in the future.

Conclusions

To summarize, the aggregation behaviour of LS and ALS was investigated and block-like structures were obtained via self-assembly with a complex three-dimensional amorphous structure for the first time. The block-like self-assemblies from LS and ALS may lead to the creation of a novel class of multifunctional materials. The size of block-like aggregates from ALS (micro-size) is obviously bigger than that from LS (nano-size), which was ascribed to the high molecular weight of ALS. We studied the hole transport property of LS and ALS using the space-charge-limited current method (SCLC). Average hole mobilities of 2.95 × 10⁻⁶ and 3.18 × 10⁻⁷ cm² V⁻¹ s⁻¹ were estimated for LS and ALS, which indicated that LS and ALS are potential water soluble polymeric p-type semiconductors and the phenolic group content could affect the hole transporting property. All of the results of CV and hole mobility indicate that LS could be applied as a novel hole transport material for organic electronics. On the basis of the unique aggregation behaviour and hole mobility of LS and ALS, water soluble PEDOT:LS and PEDOT:ALS were both prepared and applied as hole extraction layers in polymer solar cells. The PCEs decreased with a decrease of –OH content, which indicated that –OH is important for the electron transfer process. The results were consistent with the results of CV and hole mobility. Moreover, a power conversion efficiency (PCE) of 5.19% was achieved in devices composed of ITO/HTM/PTB7:PC₇₁BM/Al, which was higher than that of PEDOT:PSS^46,47 (PCE 4.28% and 4.50%). Moreover, PEDOT:LS might have potential applications in various fields including perovskite-based solar cells^48–53 and hybrid solar cells.^54–59 Our results showed that LS might be promising as a potential dopant of semi-conductive polymers such as PEDOT for organic electronics, as an alternative candidate to PSS which is non-conjugated. We believe the device performance will be further improved when better HTMs based on phenol derivatives are found. More importantly, this work provides a concept for the design of hole transport materials containing polyaromatic hydrocarbons^60–66 and also a novel perspective for the high value-added application of lignin, a cheap and renewable biomass.

Experimental section

Materials

LS was supplied by the Quanlin paper mill (Shandong province, China). ALS was prepared by the alky chain-coupled polymerization of LS in our lab. All other chemicals were of analytical grade.

Aggregation behaviour and hole mobility

Scanning electron microscopy (SEM) was performed using a Nova Nano SEM instrument (Zeiss, Netherlands). Samples were mounted on aluminum stubs by means of a double-sided conductive adhesive and sputtered with Au/Pd to reduce charging effects.

Atomic force microscopy (AFM) images were observed using a Park XE-100 instrument in tapping mode. The AFM samples were prepared by dropping the sample solution on the substrate as slow as possible and drying under room temperature for 24 hours.

Transmission electron microscopy (TEM) images were obtained using a HITACHI H-7650 electron microscope with an accelerating voltage of 200 kV. The TEM samples were prepared by dropping diluted sample solution onto copper grids coated with a thin carbon film.

Dynamic light scattering (DLS) experiments were performed on a Zeta PALS instrument (Brookhaver, America). The concentration of the samples was 0.05 mg mL⁻¹ in H₂O/ethanol (v/v, 1 : 3) at around pH 7.0.

The block-like self-assemblies for X-ray photoelectron spectroscopy (XPS) were prepared by dropping the sample solutions on tinfoil and drying naturally at room temperature.

The block-like self-assemblies for small-angle X-ray scattering (SAXS) were prepared by dropping the sample solutions on a Si substrate and drying naturally at room temperature.

Cyclic voltammetry measurement was conducted as follows: a glassy carbon electrode was first polished carefully with alumina powder to a mirror finished surface and rinsed with distilled water repetitively. LS and ALS solutions were prepared by dissolving a 10 mg sample in 1 mL distilled water. Then the sample film was deposited at the surface of the clean glassy carbon electrode. PEDOT:LS and PEDOT:ALS films were directly deposited at the surface of the clean glassy carbon electrode. The resulting electrode was immersed in 0.1 M Bu₄NPF₆ DCM solutions, and was stabilized in 0.1 M Bu₄NPF₆ DCM solutions by scanning the potential between −0.6 and +2.0 V at a scan rate of 100 mV s⁻¹.

The hole mobilities of LS and ALS were measured using the space-charge-limited current (SCLC) method by testing J–V characteristics of the hole-only device structure: ITO/MoO₃ (10 nm)/sample/MoO₃ (10 nm)/Al (100 nm). The film of the sample was spin-coated with a concentration of 80 mg mL⁻¹ and the solvent of the sample is deionized water.

Dispersion of PEDOT and its application in polymer solar cells

Preparation of PEDOT:LS and PEDOT:ALS: a proposed schematic for the polymerization of EDOT in the presence of LS and ALS dispersants is shown in Fig. 7. The preparation was as follows. The sample (LS or ALS) (1 g and 2 g, respectively) was dissolved in 200 mL of distilled water, then a 1 g EDOT monomer was added with a slow stirring speed for 10 min and the pH value of the solution was adjusted to 2. To the mixed solution, 1.93 g of an oxidant ammonium persulfate (APS) solution was added slowly at a high stirring speed. The reaction was kept at room temperature for 24 hours, and dialyzed to remove inorganic salt. The dialysis product was used for detection through ultrasonication for 10 min. The UV-vis absorption spectra of PEDOT:LS and PEDOT:ALS aqueous dispersions were measured using a Shimadzu UV-3600 spectrophotometer (Japan).

For the fabrication and characterization of OPVs:ITO-coated glass substrates, they were first cleaned by sonication in acetone, detergent, deionized water, and isopropyl alcohol and dried in a nitrogen stream, followed by an oxygen plasma treatment. In order to fabricate photovoltaic devices, a thin hole-transportation layer (ca. 40 nm) of PEDOT:SL or PEDOT:ALS (filtered at 0.45 μm) was spin-cast on the pre-cleaned ITO-coated glass substrates and baked at 120 °C for 20 min under ambient conditions. The active layer PTB7:PC₇₁BM (10 : 15 mg mL⁻¹) was prepared by spin-casting a chlorobenzene solution with the addition of a small amount of DIO (CB : DIO = 100 : 3, V/V) at 1500 rpm for 30 s in a dry box. The thickness of the PTB7:PC₇₁BM layer was about 100 nm. 4 h later, methanol was spin-coated on the active layer at 2000 rpm for 30 s. The Al electrodes were thermally deposited for 100 nm through a mask in a vacuum (<5 × 10⁻⁴ Pa). All steps except for the processing of HTMs were performed in a glove box. The effective device area was about 0.16 cm². The current density–voltage (J–V) characteristics were measured using a Keithley 2400 source meter. The photovoltaic devices were characterized using a calibrated AM1.5 G solar simulator (Oriel model 91192), under a light intensity of 100 mW cm⁻².

Acknowledgements

The authors would like to acknowledge the financial support of National Natural Science Foundation of China (21402054, 21436004), National Basic Research Program of China 973 (2012CB215302), and International S&T Cooperation Program of China (2013DFA41670).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The results including SEM images of various block-like self-assemblies from LS and ALS; DLS measurement of the LS solution and ALS solution in H₂O/EtOH (v/v, 1/3); proposed schematic for the polymerization of EDOT in the presence of LS and ALS; UV absorption spectra of PEDOT:LS and PEDOT:ALS aqueous dispersion. See DOI: 10.1039/c5ra19676k

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