Green-solvent processable semiconducting polymers applicable in additive-free perovskite and polymer solar cells: molecular weights, photovoltaic performance, and thermal stability

Junwoo Lee a, Tack Ho Lee b, Mahdi Malekshahi Byranvand a, Kyoungwon Choi a, Hong Il Kim a, Sang Ah Park a, Jin Young Kim *b and Taiho Park *a
aDepartment of Chemical Engineering, Pohang University of Science and Technology, San 31, Nam-gu, Pohang, Gyeongbuk 790-780, Republic of Korea. E-mail: taihopark@postech.ac.kr; Fax: +82-54-279-8298; Tel: +82-54-279-2394
bDepartment of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. E-mail: jykim@unist.ac.kr

Received 15th January 2018 , Accepted 6th February 2018

First published on 7th February 2018


In this study, we demonstrated the effects of the molecular weight (MW) of a green-solvent processable semiconducting polymer (asy-PBTBDT) on its photovoltaic performance and device thermal stability in green processed devices for the first time. The asy-PBTBDT with a high MW (132 kDa) had the highest μh values (4.91 × 10−3 cm2 V−1 s−1 without dopants and 5.77 × 10−3 cm2 V−1 s−1 with dopants) as a result of increase in the π–π stacking along with MW as compared to low-MW asy-PBTBDTs (27 and 8 kDa). The high-MW asy-PBTBDT with a high μh achieved the best power conversion efficiencies of 18.2% and 20.0% for the undoped and doped states in PerSCs, respectively, and 5.7% in PSCs in green processed devices. Furthermore, the glass transition temperature increased with an increase in MW; this indicated an effective decrease in heat-induced morphological degradation in the photovoltaic devices. In addition, an increase in the chain density along with MW led to good robustness against humidity and oxygen.


Over the past several decades, organic semiconductors (OSCs) have attracted significant attention because of their flexibility, low-temperature and low-cost solution fabrication, and applicability in photovoltaic devices including their application as hole-transporting materials (HTMs) in perovskite solar cells (PerSCs) and polymer solar cells (PSCs).1–5 However, it is necessary to improve the power conversion efficiency (PCE), which is currently lower than that of inorganic semiconductors, and stability of OSCs.6

Various small molecules and polymers have been reported as OSCs in photovoltaic applications.7–12 According to previous reports on HTMs and PSCs, small molecular and polymeric materials should have a high hole mobility (μh) of ∼10−3 cm2 V−1 s−1 when they are used in high efficiency photovoltaic applications. Many research groups have attempted to change the chemical structures of OSCs13–20 to increase μh in the case of parallel and vertical hole transport that requires significant efforts. However, unlike small molecular organic semiconductors, the μh values of semiconducting polymers (SCPs) can be easily enhanced by increasing the molecular weight (MW) of the polymer.21,22 Frechet et al. controlled the parallel field-effect mobility of highly crystalline regioregular poly(3-hexylthiophene) (RR-P3HT) by changing the MW.23,24 The parallel μh of RR-P3HT increased from 1.7 × 10−6 to 9.4 × 10−3 cm2 V−1 s−1 as the MW increased from 3.2 to 36.5 kDa. The low-MW, highly crystalline RR-P3HT showed a rod-like morphology with grain boundaries, thus leading to charge recombination. However, the high-MW RR-P3HT showed isotropic nodules, which led to a small crystalline domain connected by long tie molecules. This low crystalline phenomenon led to good charge-transport ability by decreasing the grain boundaries. Our group has demonstrated that localized π–π stacking (inter-junction) in low-crystalline RP33 results in a low activation energy for charge-carrier hopping in organic thin film transistors (OTFTs) that leads to superior mobility.25 However, to demonstrate the intrinsic effect of MW, studies on crystalline SCPs are undesirable because of the abovementioned morphological issues. Furthermore, since photovoltaic applications require a face-on orientation, investigation of the effect of MW on inter-junction and vertical hole charge transport is necessary unlike well-known edge-on orientation by OTFTs (Fig. 1a).


image file: c8ta00479j-f1.tif
Fig. 1 (a) Schematic of the microstructures of asy-PBTBDTs with different MWs. Increase in MW increases inter-junctions and chain density. (b) Configurations of photovoltaic devices using asy-PBTBDTs with a green solvent (2-methylanisole) and gel permeation chromatographs of the polymers.

In addition to efficiency, thermal stability is an important factor for the state-of-the-art OSCs. As the temperature increases, increased chain motion induces morphological degradation. This reduces the long-term stability of the photovoltaic device as a result of change in the domain size and influx of external factors such as humidity and oxygen. Many research groups have addressed this issue through the cross-linking approach, which can improve the thermal stability by reducing chain dynamics.26 However, this method requires additional steps (heat, UV, and additives) and also leads to a relatively low efficiency that results from the by-product of the cross-linking agent. According to the Flory–Fox relationship, an increase in the MW leads to a high glass transition temperature (Tg) by decreasing the free volume caused by reduced number of chain ends.27 That is, the relationship increases the energy required for the onset of long-range segmental motion (Fig. 1a). The high Tg resulting from the increased MW can effectively protect photovoltaic devices from heat-induced morphological degradation.

In this study, we have evaluated the effects of MW on vertical hole transport and thermal stability using green processable asy-PBTBDT, an amorphous SCP, that exhibits a face-on orientation for vertical charge transport and amorphous properties resulting from the irregularity of the asymmetric alkyl chains.28 The amorphous properties can clearly characterize various properties, such as photoelectric and electronic properties, regardless of unflavored crystallite issues. In the aspect of large-area fabrication, the study of SCPs under green process must be accompanied to meet the environment protection requirement. Accordingly, we evaluated PerSCs and PSCs using asy-PBTBDT and 2-methylanisole (2-MA) as a green solvent to determine the effects of MW on photovoltaic performance and thermal stability in green processed devices for the first time.

Herein, three asy-PBTBDT samples with number-average MWs of 132, 27, and 8 kDa were prepared by varying the ratios of monomer balance during polymerization. The number-average MWs were determined through gel permeation chromatography using chlorobenzene (CB) as the eluent (Fig. 1b; also see Table S1). The calibration curve was generated using polystyrene standards.

The three asy-PBTBDTs showed similar oxidation in cyclic voltammograms (Fig. S1), thus indicating that the highest occupied molecular orbital did not change with the MW. Therefore, the compatible energy levels for photovoltaic performance can be achieved regardless of MW.

The two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) images of the asy-PBTBDTs are shown in Fig. 2. The images show a (010) peak representing out-of-plane π–π stacking, thus indicating a face-on orientation (Fig. 2a–c). In addition, long-range chain ordering was not observed because of the structural irregularity originating from the asymmetric structure. It is a novel study of the MW effect on hole transport regardless of crystallinity, which affects surface roughness.29 This can be observed by atomic force microscopy (inset in Fig. 2a–c and S2). The asy-PBTBDT films exhibited smooth surfaces of 0.28–0.36 nm, thus implying considerably reduced crystal domains. The degree of π–π stacking in the asy-PBTBDT films was analysed based on the out-of-plane line cuts extracted from the 2D-GIWAXS data. The (010) π–π stacking peak was gradually enhanced with an increase in the MW (Fig. 2d). This indicates that the long polymer chain influences π–π interaction as an inter-junction; thus, it has the potential to enhance the hole transport ability.


image file: c8ta00479j-f2.tif
Fig. 2 Two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) images of asy-PBTBDTs with MWs of (a) 132 kDa, (b) 27 kDa, and (c) 8 kDa. (d) Out-of-plane profiles of the asy-PBTBDTs. (e) SCLC of the asy-PBTBDTs. (f) Vertical μh values of the doped and undoped asy-PBTBDTs with different MWs and schematics of hole transport for different MWs.

The μh values of the asy-PBTBDT samples were calculated using the space-charge-limited current (SCLC) method (Fig. 2e). The μh values of the undoped asy-PBTBDTs with the MWs of 132, 27, and 8 kDa were calculated to be 4.91 × 10−3, 3.12 × 10−3, and 1.64 × 10−3 cm2 V−1 s−1, respectively. The low-MW asy-PBTBDT (8 kDa) showed relatively high mobility because of the reduction in crystal domains unlike highly crystalline RR-P3HT,23,24 and increase in MW corresponded to the tendency of enhanced π–π stacking as inter-junction among polymer backbones (Fig. 2f). The asy-PBTBDTs films including two dopants, such as 2 mM lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 0.07 mM t-butylpyridine (t-BP), exhibited remarkably increased μh values of 5.77 × 10−3, 3.98 × 10−3, and 2.27 × 10−3 cm2 V−1 s−1 for the MWs of 132, 27, and 8 kDa, respectively. These results demonstrate that the μh values of both doped and undoped polymeric materials can be easily enhanced by increasing the MW if sufficient solubility is guaranteed.

To demonstrate the effects of MW of asy-PBTBDT on the performances of solar cells under green process using 2-MA, PerSCs were first fabricated using the asy-PBTBDTs with identical thicknesses as the HTMs in the configuration FTO/TiO2/m-TiO2/perovskite/HTM/Au (Fig. S3). The current density–voltage (JV) characteristics were measured under simulated air mass 1.5 global (AM 1.5G). For the undoped asy-PBTBDTs, MW clearly played an important role in PerSC performances and reproducibility (Fig. 3a, b and Table 1). The PCEs increased from 14.5% to 18.2% with an increase in the MW (Fig. 3a and Table 1). The high vertical μh induced by the high MW enhanced the JSC and FF in the PerSCs; JSC was confirmed by the incident photon-to-current efficiency (IPCE) (inset in Fig. 3a). The good reproducibility (Fig. 3b and S4) may have originated from ductile films, thus leading to good coverage with rough perovskite by increasing the MW.12,30 The photovoltaic performances of the PerSCs with doped asy-PBTBDTs were also characterized, as shown in Fig. S5 and Table S2. The PCEs of the devices using asy-PBTBDTs doped with Li-TFSi and t-BP were enhanced from 16.8% to 20.0% along with MW, which corresponded to the increase in μh caused by doping.


image file: c8ta00479j-f3.tif
Fig. 3 (a) JV curves and IPCEs of PerSC devices containing undoped asy-PBTBDTs. (b) FF histogram of the 20 devices constructed using dopant-free asy-PBTBDTs. (c) Steady-state photoluminescence spectra of glass/perovskite/HTM samples.
Table 1 Summary of the photovoltaic parameters obtained from the best PerSC devices using the dopant-free asy-PBTBDTs. The average PCE (PCEavg) of 20 devices is shown. Measurements were performed under AM 1.5 solar illuminationa
M.W. (kDa) PerSC JV curves
J SC (mA cm−2) V OC (V) FF (%) PCE (PCEavg) (%)
a Cell size: 0.09 cm2.
132 22.4 1.14 73.2 18.2 (16.7)
27 22.0 1.07 70.0 16.3 (15.4)
8 21.9 1.06 62.9 14.5 (13.2)


Steady-state photoluminescence was used to investigate the ability of the polymers to extract holes. The photoluminescence peak at 760 nm corresponding to perovskite upon excitation at 464 nm was significantly decreased when the asy-PBTBDTs were cast on perovskite, thus indicating good hole extraction (Fig. 3c). The high-MW leads to better hole extraction than that of the others due to high μh and good coverage. The quenching rates were characterized using time-resolved photoluminescence, as shown in Fig. S6. The quenching rates become fast with an increase in MW; this may be attributed to the enhanced vertical μh with the increasing MW.31

The long-term stabilities of uncapsulated PerSCs were tested at room temperature (25 °C) and a high temperature (75 °C) with 50–70% relative humidity (Fig. 4). At room temperature, the efficiency of the device with the high-MW (132 kDa), undoped asy-PBTBDT decreased slightly by ∼3% after 30 days, whereas those of the devices with the asy-PBTBDT MWs of 27 and 8 kDa decreased by ∼6% and ∼10%, respectively. For doped asy-PBTBDT, the devices with asy-PBTBDT MWs of 132, 27, and 8 kDa retained 81%, 70%, and 57%, of their initial efficiencies, respectively. The remarkable reduction in efficiency as compared to that of the undoped HTLs is attributed to the polar dopants, which destroy the perovskite.12,32 The trends observed at room temperature were consistent with those found at high temperatures. These results demonstrate that a high MW leads to great robustness against humidity, oxygen, and heat resulting from the increased chain density and Tg (Fig. S7).


image file: c8ta00479j-f4.tif
Fig. 4 Long-term stabilities of PerSCs with undoped and doped asy-PBTBDTs under 50–70% relative humidity at (a) 25 °C and (b) 75 °C.

In the UV-vis absorption spectra of the asy-PBTBDT films (Fig. 5a and Table S1), the pristine asy-PBTBDT film exhibits two distinct absorption bands at ∼410 and ∼600 nm; these peaks are assigned to π–π* transitions on donor BDT moieties and intramolecular charge transfer from the donor to acceptor moieties. The shoulder peak associated with the red-shift phenomenon appears as a result of polymer chain aggregation.33 Among all samples, the shoulder peak had the greatest intensity for the high-MW (132 kDa) asy-PBTBDT. This indicates that a high MW leads to more aggregation regardless of film formation (inset in Fig. 5a). The absorption spectra in the solution state indicated a red shift with an increase in MW (Fig. 5b) that led to a change in the apparent colour (inset in Fig. 5b). This may have resulted from an increase in the effective conjugation length of the backbone related to backbone planarity in the solution state. This hypothesis can be confirmed by investigating the affinity, such as solubility capacity, between the polymers and 2-MA. The absolute solubilities of the asy-PBTBDTs (132, 27, and 8 kDa) in 2-MA were calculated to be 18.2, 21.7, and 30.5 mg ml−1, respectively; the differences were attributed to differences in the mixing entropy in statistical thermodynamics.34 The abovementioned relationship between UV-vis absorption in the solution state and solubility indicates that poor solubility leads to good chain planarity in the solution state because of a reduction in the degrees of freedom of the chain conformation.35 Interestingly, the absorption ability increased with an increase in MW in the film and solution states. This may be attributed to the increased chain density, which is the primary reason for the high Tg, robustness and inter-junction. Furthermore, the enhanced absorption ability of the active materials helps to increase JSC in PSC applications.


image file: c8ta00479j-f5.tif
Fig. 5 UV-vis spectra of asy-PBTBDTs in the (a) film and (b) solution states. (c) JV curves and (d) IPCE spectra of PSC devices employing asy-PBTBDTs without dopants. Charge-recombination characteristics: dependencies of (e) JSC and (f) VOC on light intensity.

PSC devices using undoped asy-PBTBDTs as active layers and 2-MA as a green solvent were fabricated with the device structure of indium tin oxide (ITO)/PEDOT:PSS/asy-PBTBDTs:PC71BM/Al to demonstrate the effects of MW on PSC devices. The JSC of the PSC device increased drastically from 8.8 to 10.4 mA cm−2 with an increase in the MW (Fig. 5c and Table 2); this was confirmed by the IPCE measurements (Fig. 5d). This increase in JSC might have resulted from the enhanced light absorption that accompanied the enhanced vertical μh (Fig. 5a and 2f).

Table 2 Summary of photovoltaic parameters obtained from the best PSC devices with a green processa
M.W. (kDa) PSC JV curves
J SC (mA cm−2) V OC (V) FF (%) PCE (%)
a Device configuration: ITO/PEDOT:PSS/asy-BTBDTs:PC71BM/Al.
132 10.4 0.80 69.7 5.7
27 9.3 0.78 66.4 4.8
8 8.8 0.80 60.1 4.3


To clarify the increase in JSC and FF, the charge-recombination characteristics were examined. Fig. 5e shows a log–log plot of JSCvs. light intensity. The curve slopes (α) are close to unity, thus suggesting suppressed bimolecular recombination at the interfaces between the polymers and PC71BM under short-circuit conditions.36 Among the asy-PBTBDTs, those with a high MW (132 kDa) showed the highest α (0.93), which indicated that less bimolecular recombination occurred in this asy-PBTBDT as compared to that in those with lower MW (27 and 8 kDa). In Fig. 5f, a slope greater than kT/q implies that the Shockley–Read–Hall (SRH) recombination is involved.37,38 Among the asy-PBTBDTs, the asy-PBTBDT with a high MW (132 kDa) had the smallest value (1.35kT/q), thus indicating that this asy-PBTBDT had the lowest amount of SRH recombination.

The long-term and thermal stabilities of uncapsulated PSC devices were tested at 25 °C and 75 °C inside a N2-filled glove box (Fig. 6). At 25 °C, the device with the high-MW (132 kDa) asy-PBTBDT retained ∼95% of its PCE after 270 h, whereas the retentions for the devices with the 27 kDa and 8 kDa asy-PBTBDTs were 91% and 84%, respectively. In the stability test at 75 °C, the devices retained 91%, 86%, and 81% of their initial PCEs after 220 h for asy-PBTBDTs with the MWs of 132, 27, and 8 kDa, respectively; this was similar to the results obtained at room temperature. The robustness of the devices containing the high-MW asy-PBTBDT is attributed to the increased chain density and Tg, which is consistent with the stability results for the PerSCs.


image file: c8ta00479j-f6.tif
Fig. 6 Long-term stability of PSC devices inside a N2-filled glove box at (a) 25 °C and (b) 75 °C.

In turn, the effect of MW on the vertical charge mobility was confirmed without the previous grain boundary issue using a green-solvent processable semiconducting polymer (asy-PBTBDT). The amorphous feature showed a higher value of vertical μh in low MW than that observed in highly crystalline SCPs. With an increase in MW, the vertical μh improved because of enhanced π–π stacking (inter-junction), thus leading to good performance in photovoltaic applications in green processed devices. Finally, the increase in chain density and Tg with an increase in MW led to a high resistance against humidity, oxygen, and morphological degradation.

Conclusions

We demonstrate the effects of MW on vertical hole transport using green-solvent processable asy-PBTBDT in green processed devices with 2-MA for the first time. The asy-PBTBDT exhibited an amorphous SCP with face-on orientation and without large crystal domains. The amorphous character led to quite a high μh (1.64 × 10−3 cm2 V−1 s−1) at a low MW (8 kDa). The high μh was attributed to a reduction in grain boundaries as compared to that in highly crystalline SCPs. With an increase in MW, the μh increased as a result of π–π stacking and inter-junctions between polymer chains. This tendency was reflected in PCEs in photovoltaic applications with green process. The high-MW asy-PBTBDT with the highest μh showed the best PCEs of 18.2% and 20.0% for the undoped and doped states in PerSCs, respectively, and 5.7% in PSCs. Furthermore, the good coverage from a high MW led to good reproducibility, and the increase in chain density and Tg revealed good robustness against humidity, oxygen, and heat in photovoltaic applications. The control of the MWs of amorphous SCPs represents a promising strategy to obtain high-performance and stable photovoltaic devices.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

2D-GIWAXS measurements were performed on beam line 3C at the Pohang Accelerator Laboratory, Korea. This work was supported by the Technology Development Program to Solve Climate Changes of NRF (2015M1A2A2056216 and 2016M1A2A2940914) and the National Research Foundation of Korea (NRF) grant (Code No. 2015R1A2A1A10054230) funded by the Korean government (MSIP). T. P. thanks the DONGJIN SEMICHEM Scholarship Foundation for providing partial financial support. This research was also supported by New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20123010010140).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta00479j
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018