Efficient All-Polymer Solar Cells Based on Conjugated Polymer

Oct 5, 2017 - We developed a novel wide-bandgap conjugated polymer PTzBI-O based on an alkoxylated electron-deficient monomer 4 ...
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Efficient All-Polymer Solar Cells Based on Conjugated Polymer Containing an Alkoxylated Imide-Functionalized Benzotriazole Unit Wenkai Zhong,† Kang Li,† Jing Cui,‡ Tianyi Gu,† Lei Ying,*,† Fei Huang,*,† and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China S Supporting Information *

ABSTRACT: We developed a novel wide-bandgap conjugated polymer PTzBI-O based on an alkoxylated electron-deficient monomer 4,8-di(thiophen-2-yl)-[1,2,3]triazolo[4,5-f ]isoindole-5,7(2H,6H)-dione (TzBI-O). Regarding that of alkyl-substituted imide-functionalized benzotriazole (TzBI) unit, the incorporation of oxygen atom into the substitution of TzBI-O increased the electronegativity. The resulting polymer PTzBI-O exhibited an absorption onset of 708 nm, corresponding to a bandgap of 1.75 eV. The PTzBI-O:N2200 blend exhibited strong aggregation in toluene solution, resulting in the enhanced absorptivity in thin film compared to those of equivalent films processed with chlorinated solvents. The fabricated all-polymer solar cell based on PTzBI-O:N2200 blend film processed with toluene exhibited an impressive power conversion efficiency of 7.91%. The higher efficiency of the toluene-processed device than those based on films processed with chlorinated solvents can be attributed to more effective charge dissociation, trivial bimolecular recombination, greater charge transportation, and more favorable thin film morphology of the toluene-cast blend film. These findings indicated that the resulting copolymer has great potential for the construction of high-performance all-polymer solar cells.

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polymers based on fluorinated benzotriazole or imide functionalized 4,8-di(thiophen-2-yl)-[1,2,3]triazolo[4,5-f ]isoindole-5,7(2H,6H)-dione (TzBI) units, all-PSCs with PCEs greater than 8% can be constructed.28,29 Moreover, two solubilizing groups can be incorporated into the imide functionalized electrondeficient TzBI units, either at the nitrogen (N) atom in the triazole unit or the N atom in the imide unit.30,31 These groups increase the solubility of the resulting copolymer (PTzBI) in non-chlorinated solvents, such as 2-methyltetrahydrofuran. To achieve close-packing of the polymer chains and favorable morphology in the interpenetrating network of bulk-heterojunction (BHJ) films, it is necessary to incorporate appropriate side chains into the TzBI groups. Minor modifications to the molecular structures of the side chains can affect the optoelectronic properties of the conjugated polymers.32−35 Hence, it hypothesized that it would be beneficial to replace the alkyl side chain of the TzBI unit with a polar substituent. The incorporation of an oxygen atom between the N atom of the imide unit and the alkyl side chain would reduce the electron density and the highest occupied molecular orbital (HOMO) energy level of the monomer, without affecting the solubility of the target polymer.36,37 Therefore, in the present study, we developed a novel electron-deficient TzBI derivative (TzBI-O)

INTRODUCTION In recent years, significant advancements have been made in polymer solar cell (PSC) research.1−3 A wide range of highly efficient single-junction and multijunction PSCs consisting of fullerene or non-fullerene small-molecule electron acceptors have been developed.4−10 Although all-polymer solar cells (allPSCs), which composed of both polymer donors and polymer acceptors, exhibit various advantages such as wide-absorption bands, highly stable film morphologies, and a wide range of donor and acceptor components, the development of all-PSCs lags behind that of non-fullerene PSCs due to the lower power conversion efficiencies (PCEs).11−25 Typically, high-performance all-PSCs are fabricated from a wide-bandgap polymer donor and an n-type narrow-bandgap polymer acceptor. The complementary absorption and energy levels of these two components can effectively overcome spectral limitations and reduce the energy loss of the photoactive layers. By virtue of these merits, complementary donor and acceptor polymers can enhance short-circuit current densities (JSC) and open-circuit voltages (VOC), which can significantly increase overall performances of the resulting devices. The narrow-bandgap n-type copolymer poly{[N,N′-bis(2octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]alt-5,5′-(2,2′-bithiophene)} (sold commercially as N2200, Figure 1) has been extensively used for the fabrication of allPSCs because of its high electron mobility and suitable energy levels.26,27 By integrating N2200 with wide-bandgap conjugated © XXXX American Chemical Society

Received: July 5, 2017 Revised: September 20, 2017

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DOI: 10.1021/acs.macromol.7b01432 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Specific structures of PTzBI, PTzBI-O, and N2200.

Scheme 1. Synthesis of PTzBI-O

extraction with methanol, acetone, and hexane. The polymer was finally dissolved in dichloromethane and precipitated in methanol. The number-average molecular weight (Mn) of PTzBI-O was estimated to be 19.6 kDa (dispersity, Đ = 2.11) via high-temperature gel permeation chromatography (GPC) at 150 °C with 1,2,4-trichlorobenzene as the eluent (Figure S1, Supporting Information). The decomposition temperature with 5% weight loss of PTzBI-O was 370 °C, as determined by thermogravimetric analysis (TGA) (Figure S5). No phase transitions were observed in differential scanning calorimetry (DSC) data obtained over a scanning range from 30 to 300 °C for the resulting copolymer PTzBI-O (Figure S6a), while the copolymer N2200 exhibited a relatively high glass transition temperature of about 210 °C (Figure S6b). To evaluate the effects of incorporating a polar substituent into TzBI, we simulated the electronic structures of TzBI and TzBI-O. First, we generated electrostatic potential (ESP) maps of the optimized lowest energy geometries of these species based on density function theory (DFT) calculations at the B3LYP/6-31G* level (Figure 2). The oxygen atoms of the carbonyl groups are the most negative regions in the ESP map

containing an alkoxyl substituent on the imide group. It is interesting to note that both of the pristine PTzBI-O and PTzBI-O:N2200 blends exhibited a high tendency of aggregations in toluene solution, leading to the improved light harvesting properties and more favorable film morphology than those cast from chloroform and chlorobenzene. The resulting all-PSCs based on the PTzBI-O:N2200 processed with toluene exhibited an impressive PCE up to 7.9%.

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RESULTS AND DISCUSSION Synthesis and Theoretical Calculations. The synthetic route of the target polymer PTzBI-O is depicted in Scheme 1. The alkoxylated monomer 4,8-bis(5-bromothiophen-2-yl)-2octyl-6-(octyloxy)-[1,2,3]triazolo[4,5-f ]isoindole-5,7(2H,6H)dione (6) was synthesized in a multistep reaction, using 4,8di(thiophen-2-yl)-5H,7H-isobenzofuro[5,6-c][1,2,5]thiadiazole-5,7-dione (1) as the starting material. Palladiumcatalyzed Stille polymerization of monomers 6 and (4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene2,6-diyl)bis(trimethylstannane) (7) afforded PTzBI-O in 92% yield. Purification of the polymer was carried out by Soxhlet B

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“push−pull” effects; the stronger electronegativity may lead to the red-shift of the absorption profile of PTzBI-O regarding that of PTzBI.38,39 This assumption agrees with the observed red-shifted absorption profile shown in Figure S9b (see the Supporting Information). Since the only difference in molecular structures of PTzBI-O and PTzBI is the oxygen atom of the substituents, it is reasonable to envisage that the enhanced intramolecular charge transfer effects stem from the incorporation of electronegative oxygen atom. Optical and Electrochemical Properties. As shown in Figure 3a, solutions of PTzBI-O in either chloroform (CF) or chlorobenzene (CB) at a concentration of about 0.01 mg mL−1 exhibited similar UV−vis absorption profiles, for which these spectra exhibit a broad absorption band from 380 to 680 nm, corresponding to intramolecular charge transfer between the electron-rich BDT units and the electron-deficient TzBI-O units. However, absorption profile of solution of PTzBI-O in toluene is significantly red-shifted of 30 nm. This finding is consistent with the variation of its photoluminescence (PL) spectra in different solvents, where the observed PL spectra from the chlorinated solution (CF and CB) are nearly identical, and the PL profile in toluene solution obviously broadened and red-shifted (Figure S10). This observation agrees with the nearly identical absorption profiles of PTzBI-O in chlorinated solvents and in toluene at 80 °C (Figure S9a), which suggests the possible formation of preaggregation of PTzBI-O in the toluene solution. To identify the effects of solvents on the absorption profiles of the PTzBI-O and N2200 pure films, we measured the UV−vis absorption spectra of their pure films cast from CF, CB, and toluene (see Figure S11). It is noted that the UV−vis profiles of PTzBI-O film slightly red-shift from the films cast from the solvent of CF, to CB, and further to that

Figure 2. Optimized molecular structures of TzBI and TzBI-O in their ground states, with ESP surface mapping from −0.025 to +0.025 hartree.

of TzBI. After the introduction of oxygen into the alkyl side chains, a significant negative ESP region is formed at the edge of the cyclic imide moiety of TzBI-O, as a result of the oxygen atoms present in the alkoxy chain and the carbonyl groups, which indicates the stronger electronegativity of TzBI-O. These results are consistent with the simulations of dimeric (BDTTzBI-O)2 repeating units of the polymer backbone (Figure S8). In addition, consider that the donor−acceptor type of πconjugated backbones of PTzBI-O would exhibit the electron

Figure 3. UV−vis absorption spectra of (a) PTzBI-O in different solvents, (b) pristine PTzBI-O and N2200 thin films cast from toluene solutions, (c) PTzBI-O:N2200 (1:0.5, wt:wt) in different solvents, and (d) PTzBI-O:N2200 thin films cast from different solvents. C

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Figure 4. J−V characteristics (a) and EQE curves and integrated JSC values (b) of all-PSCs with active layers consisting of PTzBI-O:N2200 (1:0.5, wt:wt) cast from different solvents.

Table 1. Photovoltaic Parameters of All-Polymer Solar Cells

a

processing solvent

VOC (V)

JSCa (mA cm−2)

Jcalb (mA cm−2)

FFa (%)

PCEa (%)

PCEbest (%)

CF CB toluene

0.88 0.87 0.86

12.07 ± 0.07 13.11 ± 0.26 14.43 ± 0.27

11.75 12.68 14.42

60.29 ± 1.04 62.89 ± 0.76 62.96 ± 1.02

6.43 ± 0.14 7.17 ± 0.11 7.78 ± 0.07

6.58 7.39 7.91

The statistic parameters were obtained from 10 individual devices. bIntegrated from the EQE curves.

alt-2,7-(9,9-dioctylfluorene)] dibromide (PFN-Br) was used as the cathode interfacial layer to improve electron transport.40,41 The PTzBI-O:N2200 (1:0.5, wt:wt) active layers were spin-cast from different solutions with thickness of 110 nm and thermal annealing conditions of 100 °C for 10 min prior to spin-casting of PFN-Br and the deposition of the cathode. The current density−voltage (J−V) characteristics of the fabricated all-PSCs were recorded under AM 1.5G illumination (100 mW cm−2); the external quantum efficiency (EQE) spectra are shown in Figure 4, and the relevant photovoltaic parameters are summarized in Table 1. Devices based on PTzBI-O:N2200 blend films cast from CF solutions exhibited PCEs of 6.43 ± 0.14% (VOC = 0.88 V, JSC = 12.07 ± 0.07 mA cm−2, FF = 60.29 ± 1.04%). The PCEs of devices based on blend films cast from CB solutions were 7.17 ± 0.11% (VOC = 0.87 V, JSC = 13.11 ± 0.26 mA cm−2, FF = 62.89 ± 0.76%). Devices based on blend films cast from toluene solutions exhibited the highest average PCEs of 7.78 ± 0.07% (VOC = 0.86 V, JSC = 14.43 ± 0.27 mA cm−2, FF = 62.96 ± 1.02%). The highest observed PCE was 7.91%, which to the best of our knowledge is among the highest values yet reported for an all-PSC.23,28,29,42 It is worth noting that these PCEs were obtained from N2200 with high molecular weight (Mn = 92 kDa; Đ = 2.11, Figure S2). In order to investigate the influence of polymer molecular weight on the photovoltaic performances of all-PSCs, toluene-processed devices using the low-Mn N2200 (Mn = 62 kDa; Đ = 1.92, Figure S3) were also fabricated, which showed a much lower PCEs of 6.63 ± 0.09% (VOC = 0.86 V, JSC = 12.57 ± 0.18 mA cm−2, FF = 61.46 ± 1.05%) (see the relevant J−V curve in Figure S12). Further optimization of the solar cells carried out by screening the blend ratio of PTzBI:N2200 (from 1:0.5 to 1:0.8 and 1:1) processed with toluene did not show the enhanced photovoltaic performances (Table S2 and Figure S13). Despite the use of solvent additive 1,8-diiodooctane (DIO, 0.5 vol %) can lead to the enhanced FF up to 70%, the JSC significantly dropped, leading the decreased

processed with toluene. A similar trend was also observed from the pure N2200-based films cast from these solvents. It is also worth noting that both PTzBI-O and N2200 pure films processed with different solvents exhibited quite similar absorption coefficient. Based on the onset of UV−vis absorption in PTzBI-O thin film, its optical bandgap was calculated to be 1.75 eV (Figure 3b), which is slightly red-shifted compared with that of PTzBI (Figure S9b) due to the stronger electronegativity of TzBI-O unit. Furthermore, the absorption profile of PTzBI-O is complementary with that of N2200, indicating that the blending of these copolymers in the BHJ layer will increase UV−vis absorption. The maximum absorption of the PTzBIO:N2200 blend is red-shifted to about 594 nm in toluene solution compared to that observed in chlorinated solutions (about 554 nm) (Figure 3c). Moreover, the blend film cast from toluene solution was found to have a slightly higher absorption coefficient than those of equivalent films cast from chlorinated solutions. All of the PTzBI-O:N2200 blend films exhibited nearly identical peak absorption at about 593 nm, indicating that aggregates were formed during the film deposition process. The energy levels of frontier molecular orbitals of PTzBI-O were evaluated by cyclic voltammetry (CV) measurement, with relevant characteristics shown in Figure S7 (see the Supporting Information). The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels were estimated to be −5.44 and −3.44 eV, respectively, which can pair with the n-type copolymer N2200. Additionally, the relatively deep HOMO level implies that PSC based on PTzBI-O devices will exhibit high open-circuit voltages. Photovoltaic Performance. To evaluate the photovoltaic performance of PzBI-O:N2200 blend films, we fabricated allPSCs with the following architecture: ITO/PEDOT:PSS/ photoactive layer/PFN-Br/Al. Alcohol-soluble poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)D

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Figure 5. Jph−Veff curves (a) and JSC under different light intensities (b) for solar cells based on PTzBI-O:N2200 (1:0.5, wt:wt) active layers processed with different solvents. J1/2−V curves of hole-only (c) and electron-only (d) devices constructed from the same films.

collection were evaluated using P(E,T) values, which were obtained by normalizing the photocurrent density (Jph) with the saturation current density (Jsat). Jph = JL − JD, where JL and JD represent the current densities under illumination and in dark conditions, respectively. It was assumed that photogenerated excitons were nearly completely dissociated into free charge carriers at high effective voltages (Veff = V0 − Va, where V0 is the voltage when Jph = 0 and Va is the applied bias; in this case, Veff = 3 V). It was also assumed that the saturation current density (Jsat) was only limited by the total amount of absorbed photons.43 Thus, under short circuit conditions, the devices processed with toluene, CB, and CF were found to have P(E,T) values of 95.1, 94.1, and 90.4%, respectively. Relevant Jph−Veff curves are shown in Figure 5a, and the parameters are summarized in Table S6. Overall, these observations indicate that the exciton generation and dissociation processes in the toluene-processed devices were more efficient than those in the CB- and CF-processed devices, which correspond with the higher JSC values observed from the toluene-processed devices. Charge carrier recombination in the fabricated all-PSCs was investigated by determining JSC values under various incident light intensities (Pin, 3−100 mW cm−2). The relation between JSC and Pin can be described as follows: JSC ∝ PinS, where S is the slope of a logarithmic plot. It has been established that bimolecular recombination of charge carriers is negligible when S is close to 1.44,45 As shown in Figure 5b, the S values of the fabricated all-PSCs range from 0.97 to 0.98, indicating that bimolecular recombination was negligible in all of these devices. Moreover, we measured the charge mobility of PTzBI-O pristine films cast from different solutions using the spacecharge limited current (SCLC) method. The measurements are carried out by hole-only devices with architecture of ITO/ PEDOT:PSS/polymer/MoO3/Al (see the relevant J1/2−V

PCEs (Table S3 and Figure S14). It is worth noting that the devices processed with toluene exhibited higher PCEs by virtue of the simultaneously enhanced JSC and FF, which may be attributed to the formation of favorable morphology as a result of preaggregation in the toluene solution. This observation is very interesting when considering that the toluene is less detrimental to the environment and to human health than chlorinated solvents, which is advantageous to the high throughput manufacturing such as high-throughput processing. Additionally, the photovoltaic performances of the reference polymer PTzBI (Mn = 20.4 kDa; Đ = 2.34, Figure S4) processed with different solutions were also evaluated (Figure S16 and Table S5). Similar to what we have observed for the all-PSCs based on PTzBI-O:N2200, the device processed with toluene exhibited the highest PCE of 6.48 ± 0.16% (VOC = 0.85 V, JSC = 12.24 ± 0.13 mA cm−2, FF = 62.49 ± 1.20%), which was much higher than those obtained from the CF (PCE = 4.23 ± 0.12%) and CB (PCE = 2.67 ± 0.14%). Considering that the difference in PTzBI-O and PTzBI comes from the oxygen atoms that incorporated into the side chains of TzBI units, the difference in the PCEs of these two polymers is likely correlated to their different intrinsic molecular properties. The average PCE of devices based on blend films processed with toluene decreased from 7.78 ± 0.07% to 6.88 ± 0.15% after continuous thermal annealing at 100 °C for 6 h (Table S4 and Figure S15). The JSC values obtained from the J−V characteristics are consistent with those obtained from the integration of the EQE spectra ( 1 V. Chem. Mater. 2014, 26, 2829−2835. (37) Zhu, D. Q.; Zhu, Q. Q.; Gu, C. T.; Ouyang, D.; Qiu, M.; Bao, X. C.; Yang, R. Q. Alkoxyl Side Chain Substituted Thieno[3,4-c]pyrrole4,6-dione To Enhance Photovoltaic Performance with Low Steric Hindrance and High Dipole Moment. Macromolecules 2016, 49, 5788−5795. (38) Liu, X. F.; Sun, Y. M.; Hsu, B. B. Y.; Lorbach, A.; Qi, L.; Heeger, A. J.; Bazan, G. C. Design and Properties of Intermediate-Sized Narrow Band-Gap Conjugated Molecules Relevant to SolutionProcessed Organic Solar Cells. J. Am. Chem. Soc. 2014, 136, 5697− 5708. (39) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Spectral Engineering in π-Conjugated Polymers with Intramolecular DonorAcceptor Interactions. Acc. Chem. Res. 2010, 43, 1396−1407. (40) Huang, F.; Wu, H. B.; Cao, Y. Water/Alcohol Soluble Conjugated Polymers as Highly Efficient Electron Transporting/ Injection Layer in Optoelectronic Devices. Chem. Soc. Rev. 2010, 39, 2500−2521. (41) Yang, T. B.; Wang, M.; Duan, C. H.; Hu, X. W.; Huang, L.; Peng, J. B.; Huang, F.; Gong, X. Inverted Polymer Solar Cells with 8.4% Efficiency by Conjugated Polyelectrolyte. Energy Environ. Sci. 2012, 5, 8208−8214. (42) Kang, H.; Lee, W.; Oh, J.; Kim, T.; Lee, C.; Kim, B. J. From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control. Acc. Chem. Res. 2016, 49, 2424−2434. H

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