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Side Chain Influence of Wide Bandgap Copolymers Based on Naphtho[1,2-b:5,6-b]bispyrazine and Benzo[1,2-b:4,5b’]dithiophene for Efficient Photovoltaic Applications Ting Yu, Xiaopeng Xu, Ying Li, Zuojia Li, and Qiang Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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ACS Applied Materials & Interfaces

Side Chain Influence of Wide Bandgap Copolymers Based on Naphtho[1,2-b:5,6-b]bispyrazine and Benzo[1,2-b:4,5-b’]dithiophene for Efficient Photovoltaic Applications

Ting Yu, Xiaopeng Xu, Ying Li,* Zuojia Li, Qiang Peng*

Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, P. R. China

*Corresponding authors: e-mail: Ying Li: [email protected]; Qiang Peng: [email protected] Tel: +86-28-86510868; fax: +86-28-86510868;

1

ACS Paragon Plus Environment


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Abstract: In this work, we reported new types of wide bandgap copolymers, PBDTA-NPz, PBDTT-NPz,

PBDTF-NPz

and

PBDTP-NPz,

based

on

naphtho[1,2-b:5,6-b]bispyrazine (NPz) acceptor building block for efficient photovoltaic applications. The influencing factors of the introduced side chains were investigated in detail, including alkoxyl, alkylthienyl, alkylfuryl and alkoxylphenyl. These copolymers possessed wide bandgaps from 1.79 eV to 1.88 eV with different non-conjugated or conjugated side chains. They also possessed deep HOMO levels less than -5.25 eV, which was positive to achieve high Vocs from their polymer solar cells (PSCs). The XRD results indicated their excellent crystallinity and molecular stacking features, especially for PBDTF-NPz containing alkylfuryl side chains. Their photovoltaic performances were measured by using bulk-heterojunction (BHJ) single-junction

PSCs

with

a

configuration

of

ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al under the same processing conditions. Different side chains of NPz-based copolymers induced largely different device performances. Without the additive of DIO, the primary PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz devices showed PCEs of 4.53%, 6.09%, 7.06% and 3.49%, respectively. When adding 3 vol% DIO, the device performances were elevated to a higher level. The PBDTF-NPz devices exhibited the highest PCE of 8.63%, which would thank for their improved Voc, Jsc and FF values caused by its inherent properties. Our results indicated that NPz is a potential acceptor unit to construct high-powered wide bandgap copolymers for efficient PSCs in the future. Keywords:

Polymer

solar

cells;

Side

chain

influence;

Naphtho[1,2-b:5,6-b]bispyrazine; Benzo[1,2-b:4,5-b’]dithiophene

2


Wide

bandgap;

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Introduction Solution-processed organic solar cells have received wide attention during the past decade, which are regarded as one of the cheap photovoltaic technologies in the near future.1 Employing conjugated copolymers as donor materials as well as fullerene compounds as acceptors, polymer solar cells (PSCs) could be applied in fabricating flexible and large-area solar modules by employing the simple wet-processing methods.2-5 In searching for highly efficient BHJ PSCs, tremendous efforts have been performed in the past decades, which includes developing the new electron-donating and electron-accepting materials, changing different additives to control the active layer morphology, optimizing the device fabrication conditions as well as adopting novel device architectures.6-11 So far, the power conversion efficiencies (PCEs) of PSCs were elevated more than 10% both in single-junction and tandem device architectures.10-18 Further increasing PCE becomes more and more challenging in single-junction PSCs because of the insufficient absorption of solar radiation and the thermalization losses of hot charge carriers generated by high energy photons.19 To address the limitations of single-junction PSCs, a new device architecture of tandem cells was presented and developed, in which individual cells were stacked together with complementary absorption characteristics.20,21 Dennler et al. had predicted that tandem PSCs could be realized high PCEs of 15%, exhibiting a bright future for the commercialization of organic solar cells.22 Typically, a double-junction tandem PSC is composed of a bottom device based on a wide bandgap (Eg) polymer (Eg > 1.7 eV) as well as a top device with a low bandgap polymer (Eg < 1.5 eV), which are linked by an interconnecting layer.23Actually, the previously reported high-performance PSCs are based on medium bandgap (Eg ≈ 1.6 eV) or low bandgap polymers.6-15 The lacking of efficient wide bandgap copolymers limited the progress of tandem PSCs. So it is very urgent to design and develop high-performance wide bandgap copolymers for usage in tandem cells. As mentioned above, seldom successful wide bandgap copolymers have been developed for constructing high-performance single-junction and tandem PSCs. In the 3



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early time, regioregular poly(3-hexylthiophene) (P3HT) was extensively researched as a wide bandgap polymer for PSCs.11,24-27 Despite of the great advance, P3HT-based device commonly suffers from a low open circuit voltage (Voc) of about 0.6 V because of its high HOMO level.24-26 Furthermore, the fabrication of P3HT-based PSCs requires a time-consuming process (thermal annealing, solvent annealing and organic additive treatment) to obtain optimized performance, which is not suited to high yield manufacturing by roll-to-roll technology.11,24-27 Therefore, design and development of new wide bandgap polymers with high photovoltaic efficiency would be highly desirable for usage in PSCs. In this field, developing novel electron-donating and electron-accepting moieties as well as sophisticated side chain engineering are the key design strategies to construct highly efficient D-A photovoltaic copolymers.28-31 Hwang et al. prepared another wide bandgap copolymer (PTIPSBDT-DFDTQX) using triisopropylsilylethynyl (TIPS)-substituted BDT and fluorinated quinoxaline (Qx) as donor and acceptor blocks.32 The fabricated single-junction and tandem devices achieved PCEs around 6% and 7.40%, respectively. Jang et al. used thiazolothiazole-based wide bandgap polymer (PSEHTT) and IC60BA as the front cell to fabricate the tandem cells. A PCE of 8.91% was achieved without any loss in the summation of Vocs by incorporating PEDOT:PSS/PEIE as the interconnecting layer.33 Our group also reported a new wide bandgap copolymer (PBDTFBZS) based on fluorinated benzotrizole (BTz) and dialkylthiol-substituted benzodithiophene (BDT), which showed high PCE of 7.74% and 9.40% in single-junction and tandem devices.17 Recently, Hou and Sun et al. reported some successful cases of wide bandgap copolymers consisting of benzo[1,2-c:4,5-c′]dithiophene-4,8-dione with high PCEs over 10% in PSCs, which included PM6, PDBT-T1, PBT1-MP, PBT1-EH, and PBT1-BO.34-36 However, the wide bandgap copolymers have still received much less attention, compared to the medium bandgap and low bandgap counterparts. Herein,

we

presented

a

series

of

new

copolymers

based

on

a

naphtho[1,2-b:5,6-b]bispyrazine (NPz) unit for efficient photovoltaic applications. The chemical structures are provided in Scheme 1. As we know, quinoxaline (Qx) was commonly employed as an electron-accepting block in D-A copolymers without 4


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significantly reducing the bandgap, but showing great potential in achieving high device performance.37-39 Chou et al. described a medium bandgap copolymer using a fluorinated Qx derivative, and the fabricated single-junction PSCs exhibited a significantly high PCE of 8.0%.39 Compared with Qx, NPz possesses the extended π-conjugated area and the enlarged aromatic plane, which is expected to promote interchain packing of the designed copolymer and thus elevate its carrier mobility as well as photovoltaic performance.40 On the other hand, to guarantee the wide bandgap, a weak electron-donating unit of BDT was employed here as a donor block for its rigid and symmetric fused aromatic structure. The properties of the resulting copolymers, PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz, were tuned finely and investigated in detail by changing with alkoxyl, alkylthienyl, alkylfuryl and alkoxylphenyl side chains attached on the BDT skeleton. The newly designed copolymers showed good efficiencies with PCEs more than 8% in single-junction PSCs.

Results and discussion The synthetic routes of the NPz-based copolymers are illustrated in Scheme 2. 3,7-Dibromo-naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (1) was used as a raw material for synthesis of NPz.41,42 A tetraamine intermediate was prepared without separation, which was condensed directly with 1,2-bis[3-(octyloxy)phenyl]ethane-1,2-dione to afford bispyrazine derivative 2. Intermediate 3 was yielded by reacting dibromide 2 with 2-(tributylstannyl)thiophene via Stille cross coupling reaction. The monomer 4 was prepared by bromination of compound 3 using NBS in THF. The copolymers PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz were finally synthesized the via typical Stille-coupling polymerization with reasonable yields using the catalyst of Pd2(dba)3/P(o-tolyl)3. The crude copolymers were purified by precipitating into methanol for several times, and then treated with hexane and chloroform by sequential Soxhlet extraction to get rid of the oligomers. The target copolymers were finally obtained from the chloroform solution by precipitating into methanol again. All the copolymers showed excellent solubility in organic solvents, such as 5



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tetrahydrofuran, chloroform, chlorobenzene and so on, originating from the alkyl side chains attached on the copolymer backbone. The number-average molecular weights (Mns) of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz are determined to be 41, 25, 27 and 27 kDa, respectively, with polydispersity indices (PDIs) between 2.5 to 3.1. The thermogravimetric analysis (TGA) (Figure S1a and Table 1) indicated that the degradation temperatures (Tds, 5% weight loss) of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz were about 325, 424, 460 and 387 °C, respectively. It is obviously that the 2D conjugated side chains substituted BDTs showed much better thermal stability than the alkoxyl-substituted analogue, especially for alkylthienyl-substituted BDT (over 400 oC). As shown in Figure S1b, no endo- or exothermal signals were found from the differential scanning calorimetry (DSC) curves between 0 oC and 250 oC, which indicated that the obtained copolymers are thermally robust in this temperature range. The absorption spectra of the resulting copolymers both in chloroform solution (1×10-5 M) and as thin solid films are given in Figure 1. The corresponding data are summarized in Table 2. These copolymers exhibit similar behavior both in solution and in thin film, with relatively broad absorption spectra spanning from the UV region to above 700 nm. In diluted chloroform solution, a first absorption band centered around 300 and 400 nm comes from the π-π* and n-π transition of the conjugated backbones. Another low energy band peaked at 580 nm for PBDTA-NPz, 582 nm for PBDTT-NPz, 586 nm for PBDTF-NPz and 583 nm for PBDTP-NPz, can be assigned to the intramolecular charge transfer (ICT) interaction between the D and A segments. It is noted that all these polymers show a shoulder peak at low energy band of about 620 nm, which are attributed to aggregation resulting from strong polymer chain packing even in dilute solution.9,43 Compared to their absorption in solution, the absorption of these copolymer films is broadened and red-shifted slightly, suggesting again a strong intermolecular interaction between the copolymer backbones in film state. The results were explained by the planar conformations induced by the BDT and NPz building blocks with largely π-extended planar heteroarene structures. In particular, the strongest shoulder peak was found in the longer wavelength range of 6


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the PBDTF-NPz film, indicating that there existed the best ordered structures at this state, which should be positive for enhancing the charge mobility and solar cell performance of PBDTF-NPz.9,43 PBDTA-NPz showed the weakest absorption intensity among these copolymers, indicating the positive effect on the optical absorption ability by incorporation of 2D conjugated side chains (thienyl, furyl and phenyl) onto BDT core. It is interesting that PBDTT-NPz and PBDTF-NPz films possessed the close absorption coefficients, but larger than that of PBDTP-NPz film. The results implied that PBDTT-NPz- and PBDTF-NPz-based PSCs fabricated later could be expected to harvest solar photons more effectively, giving rise to more photo-induced carriers and higher Jsc values. The calculated optical bandgaps (Egs) of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz through the UV-vis onset values of their solid films were 1.88, 1.79, 1.83 and 1.85 eV, respectively. Because the thienyl group exhibited a better electron-donating property than the other side chain groups on BDT, PBDTT-NPz showed a slightly lower bandgap. The results would afford useful information for design of different bandgap copolymers by using different side chains. The energy levels (HOMO and LUMO) of the copolymers were measured by cyclic voltammetry (CV).44-46 The related electrochemical data are provided in Table 2. The onset oxidation potential (Eoxonset) (Figure 2a) of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz were measured to be 0.88, 0.95, 0.99 and 0.91 eV, corresponding to their HOMOs of -5.25, -5.32, -5.36 and -5.28eV, respectively. When using the 2D conjugated side chains instead of the alkoxy side chain, the HOMOs of these copolymers were lowered. This change is attributed to the mesomeric effect of the alkoxy chains,47 which readily alters the form from aromatic to quinoidal structure by introduction of the extended 2D conjugated side chains.3 It is known that the furan ring possesses a lower electron density than thiophene and benzene moieties because of the higher ionization potential.25,48 Therefore, PBDTF-NPz with alkylfuryl side chains exhibited the lowest-lying HOMO level, which was helpful for achieving a satisfied Voc value from the corresponding PSCs because this parameter was limited by the gap of HOMO (donor material) and LUMO (acceptor material).49 The LUMOs 7



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of the copolymers are located from -3.08 to -3.38 eV, which are higher than that of PC71BM (-4.00 eV). The electrochemical bandgaps (Egcvs) of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz were then determined to be around 2.17, 1.94, 1.98 and 2.00 eV, respectively. The larger Egcvs than the corresponding optical bandgaps might be induced by the different exciton binding energy.50 Figure 2b showed the energy level diagram of the resulting copolymers and PC71BM. The HOMO gaps of 0.64-0.75 eV and LUMO gaps of 0.92-0.62 eV between the copolymers and PC71BM could provide an enough driving force to make sufficient exciton dissociation and enhance electron transfer in the related devices.51 X-ray diffraction (XRD) measurement has been employed to investigate the crystallinity and molecular packing features of these conjugated copolymers in solid films. The XRD patterns are provided in Figure 4a. A strong peak can be found at the region from 1 to 10 degrees, indicating that good ordered lamellar crystalline packing can be formed in these four polymers. The distinct peaks (100) at ca. 2θ = 3.55°, 3.49°, 3.44° and 3.32° corresponded to d-spacings of 24.86, 25.29, 25.65 and 26.58 Å for PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz, respectively. Although the acceptor skeleton and its side chains are the same, the lamellar spacing can also be affected by the different side chains attached on the BDT skeletons. After introducing the 2D conjugated side chain, this spacing value was increased. Therefore, PBDTP-NPz exhibited the longest lamellar spacing due to its longest side chain length of alkoxyl-subsituted phenyl moiety. These copolymers also exhibited good π-π stacking characteristics, which caused from the strong inter- or intramolecular interactions from π-π and/or dipolar interactions. As shown in Figure 3a, the (010) diffraction peaks have a broad pattern located at ca. 2θ = 22.9°, 23.1°, 23.6° and 22.2°, directing a face to face stacking distance of 3.88, 3.85, 3.77 and 4.00 Å for PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz, respectively. Clearly, the attached 2D conjugated side chains would extend the delocalization of BDT unit and induce stronger π-π and/or dipolar interactions, decreasing the face to face packing distance. PBDTF-NPz with furyl side chains exhibited the best π-π stacking properties. The thiophene or benzene ring has a larger steric bulk than furan,43 thus 8


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the torsion angle between the alkylfuryl and the polymer main chain in PBDTF-NPz become smaller than those in PBDTT-NPz and PBDTP-NPz. The smaller torsion angles in PBDTF-NPz allow extended electron delocalization, thereby providing the best π-π stacking properties. Through comprehensive consideration of the XRD results, PBDTF-NPz is expected to possess the best carrier mobility than other three copolymers, which is positive for elevation of Jsc value as well as PCE in the fabricated PSCs later. Higher hole mobility would be expected for promoting better hole transport without sacrificing the generating photocurrent.52-54 The SCLC method was employed here to test the hole mobilities of above copolymers. The linear fits for the J-V curves are prvided in Figure 3b. The mobilities are estimated to be 5.6×10-5, 8.5×10-5, 2.1×10-4 and 4.2×10-5 cm2 V-1 s-1 for neat PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz, respectively. The PBDTF-NPz film had the highest hole mobility, which was in accordance with its high crystallization with ordered molecular packing and closed π-π stacking. The XRD results also indicated that π-π stacking might play a more important role for improving hole mobility than lamellar stacking in these copolymers. To evaluate the solar cell performance of these NPz-based copolymers in PSCs, solar cell devices were fabricated from the blend mixture of the resulting copolymers and PC71BM, with a configuration of ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al. PEDOT:PSS was used here to promote the hole extraction on the ITO anode. PC71BM was used as the acceptor due to its complementary absorption with the resulting NPz-based copolymers.55 The polymeric blend layers were formed by spin-coating an o-dichlorobenzene solution of the copolymers and PC71BM with the optimized weight ratio of 1:1.5 (w/w). To achieve the ideal nanoscale phase separation of the photoactive blends, 1,8-diiodooctane (DIO, 3 vol%) was selected and added as an additive. The J-V characteristics and photovoltaic data are provided in Figure 4 and Table 3. Without adding the DIO additive (Figure 4a), PBDTA-NPz, PBDTT-NPz, PBDTF-NPzand PBDTP-NPz devices exhibited PCEs of 4.35%, 6.09%, 7.06% and 9



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3.49%, respectively. High PCE over 7% was achieved without using DIO and any post fabrication treatments, which indicated NPz-based copolymers would be one type of efficient polymer donor candidates in solar cell applications. After adding 3% DIO additive, the PCEs of all the devices were improved greatly (5.87% for PBDTA-NPz, 7.02% for PBDTT-NPz, 8.63% for PBDTF-NPz and 4.91% for PBDTP-NPz), which can be attributed to the largely increased Jsc and FF values (Figure 4b). However, the addition of 3% DIO would lower the charge-separated as well as charge-transfer-state energies, and then drop the Voc values of the related PSCs.56 From Table 3, the attached two-dimensional side chains would elevate the Voc of the devices for its positive effect on the lowering HOMOs of the relevant copolymers. Obviously, the furan moiety seemed to contribute more to this hybridization of HOMO levels, which enabled PBDTF-NPz devices to possess higher Voc than PBDTT-NPz and PBDTP-NPz even with 2D thienyl and phenyl conjugated side chains. Thus, the PSCs based on PBDTF-NPz showed the highest Voc, which was also consistent with the CV measurements, as described above. The high Jsc is caused by the strong optical absorption, large carrier mobility, and excellent film morphology. This enhancement of Jsc (15.33 mA cm-2) was induced by the improved optical absorption and carrier mobility of PBDTF-NPz. Above increase was identified further by the EQE measurements. The EQE obtained without or with DIO are depicted in Figure 4c and 4d. The profiles of the EQE curves are similar to the corresponding absorption spectra of the used copolymers, indicating that most of the photocurrent output is generated by the copolymers. Without the DIO additive, the EQE response ranges are broad from 300 nm to 700 nm with the average values of 61.5%, 66.8%, 68.9% and 50.0% for PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz devices, respectively. After adding 3 vol% DIO, the EQEs were then largely improved to 76.3%, 75.6%, 79.2% and 67.4% for the PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz devices. The calculated Jsc values are within 5% error, compared to those obtained from the J-V measurements. The highest EQE response (79.2%) of PBDTF-NPz device gave rise to the highest Jsc of 15.33 mA cm-2, which was in accordance with the calculated Jsc value of 14.60 mA cm-2. 10


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Atomic force microscopy (AFM) was also used to evaluate the morphology of the active layers for better understanding the performance enhancement discussed above. As shown in Figure 5, the blend films possessed different surface morphologies with various side chains incorporated on the BDT. The root-mean-square (RMS) roughnesses were 3.36, 3.23, 2.71 and 4.30 nm for PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz blend films. The active films exhibit uniform donor-acceptor interpenetrating networks featured with suitable domain sizes. The PBDTF-NPz:PC71BM active layer showed the best morphology even with some fibril features. This favorable morphology is desirable for promoting charge separation and transport in PSCs. Obviously, the smallest domain size of PBDTF-NPz:PC71BM blend films is favorable for excitons dissociation at the interface of donor and acceptor. Thus, the highest Jsc and FF values of the PBDTF-NPz device would be thank for this best phase separation. The AFM results agreed well with the device performances as mentioned above.

3. Conclusions In summary, new types of wide bandgap copolymers, PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz, based on naphtho[1,2-b:5,6-b]bispyrazine (NPz) acceptor block, have been synthesized for efficient single-junction PSCs. The strategy of side chain engineering was also employed to fine-tune the properties of these NPz-based

copolymers,

including

the

absorption

response,

energy

levels,

crystallinities, carrier mobilities as well as the photovoltaic performances. These copolymers possessed wide bandgaps from 1.79 eV to 1.88 eV with different non-conjugated or conjugated side chains, such as alkoxyl, alkylthienyl, alkylfuryl and alkoxylphenyl side chains. On the other side, they also exhibited deep HOMOs less than -5.25 eV, indicating that a high Voc would be expected from their PSCs. The XRD results indicated that PBDTF-NPz containing alkylfuryl side chains showed the best crystallinity and molecular packing, which resulted in highest carrier mobility and thus highest Jsc than other copolymers. As expected from the photovoltaic tests, different side chains of NPz-based copolymers largely affected different device 11



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performances. Preliminarily, the PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz devices showed PCEs of 4.53%, 6.09%, 7.06% and 3.49%, respectively. After adding 3 vol% additive of DIO, the PCEs of these devices were enhanced significantly. The PBDTF-NPz devices exhibited the highest PCE of 8.63%, which would be attributed to their improved Voc, Jsc and FF values because of the intrinsic superior properties of PBDTF-NPz as mentioned above. Our results indicated that NPz is a potential acceptor unit to construct efficient wide bandgap copolymers for PSC applications with required device structure in the future, such as single-junction PSCs, tandem PSCs, non-fullerene PSCs and other type of PSCs.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TGA and DSC spectra, measurements and characterization, PSC device fabrication and characterization, Material synthesis and Characterization (PDF).

Acknowledgements The authors are grateful to NSFC (51573107, 91633301 and 21432005) and Foundation of SKLPME (sklpme 2014-3-05) for the financial support.

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of a Naphthobisthiadiazole-Based Semiconducting Polymer. J. Am. Chem. Soc. 2012, 134, 3498-3507. (43) Huo, L. J.; Ye, L.; Wu, Y.; Li, Z. J.; Guo, X.; Zhang, M. J.; Zhang, S. Q.; Hou, J. H. Conjugated and Nonconjugated Substitution Effect on Photovoltaic Properties of Benzodifuran-Based Photovoltaic Polymers. Macromolecules 2012, 45, 6923-6929. (44) Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J. Electrochemical Properties of Luminescent Polymers and Polymer Light-emitting Electrochemical Cells. Synth. Met. 1999, 99, 243-248. (45) Peng, Q.; Lu, Z. Y.; Huang, Y.; Xie, M. G.; Han, S. H.; Peng, J. B.; Cao, Y. Synthesis and Characterization of New Red-Emitting Polyfluorene Derivatives Containing

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2-Pyran-4-ylidene-Malononitrile

Moieties.

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Quinoxaline

Low-Band-Gap Co-polymers for High-Performance Polymer Solar Cells. Chem. Eur. J. 2014, 20, 13259-13271. (53) Peng, Q.; Huang, Q.; Hou, X. B.; Chang, P. P.; Xu, J.; Deng, S. J. Enhanced Solar Cell Performance by Replacing Benzodithiophene with Naphthodithiophene in Diketopyrrolopyrrole-Based Copolymers. Chem. Commun. 2012, 48, 11452-11454. (54) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce, O. R.; Butler, M. R.; Boudreault, P. L.; Strzalka, J.; Morin, P. O.; Leclerc, M.; Lopez, N. J. T.; Ratner, M. A.;

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Facchetti,

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Marks,

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J.

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18


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Legends for Schemes, Figures and Tables Scheme 1. Molecular structures of Qx, PQx, NPz acceptor skeletons and NPz-based copolymers. Scheme 2. Synthesis of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz. Figure 1. UV-vis absorption spectra of the polymers in solution (a) and thin films (b). Figure 2. (a) Cyclic voltammograms of the polymers. (b) HOMO and LUMO energy levels of the polymers and PC71BM. Figure 3. (a) X-ray diffraction patterns of the polymer films on ITO glass substrates. (b) J1/2-V characteristics of polymer based hole-only devices measured at ambient temperature. Figure 4. J-V curves of copolymer:PC71BM-based regular single solar cells without (a) or with (b) 3 vol% additive of DIO under AM 1.5 G illumination, 100 mW/cm2. (b) EQE curves of copolymer:PC71BM-based regular single solar cells without (c) or with (d) 3 vol% additive of DIO. Figure 5. AFM topographic height (a-d) and phase (e-h) images of the film blends (polymer:PC71BM=1:1.5, w/w): PBDTA-NPz (a, e), PBDTT-NPz (b, f), PBDTF-NPz (c, g), and PBDTP-NPz (d, h). Image size: 5×5 µm2. Table1. Molecular weights and thermal properties of the copolymers. Table 2. Optical and electrochemical data of the copolymers. Table 3. Photovoltaic properties of the PSCs based on copolymer/PC71BM (1:1.5, w/w) under AM 1.5G, 100 mW/cm2.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

R

N

R

N

N

R

R

N

R

N

N

R

Pyrazino[2,3-g]quinoxaline (PQx)

Quinoxaline (Qx)

C8H17O

R

N

R

N

N

R

N

R

naphtho[1,2-b:5,6-b]bisquinoxaline (NPz)

PBDTA-NPz: A =

OC8H17

Page 20 of 30

O

C6H13 C2H5

A

N

S

N

A

C2H5

S

S

S

C6H13

PBDTT-NPz: A =

S

O N

N

C6H13

PBDTF-NPz: A =

n

C2H5 C6H13

C8H17O

OC8H17

PBDTP-NPz: A =

O

C2H5

Scheme 1. Molecular structures of Qx, PQx, NPz acceptor skeletons and NPz-based copolymers.

20


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Scheme 2. Synthesis of PBDTA-NPz, PBDTT-NPz, PBDTF-NPz and PBDTP-NPz.

21



(b)

1.0

Normalized Absorbance (a. u.)

(a) Normalized Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8 0.6 0.4 0.2 0.0 300

PBDTA-NPz PBDTT-NPz PBDTF-NPz PBDTP-NPz 400

500

600

700

1.0 0.8 0.6 0.4 0.2 0.0 300

800

Page 22 of 30

PBDTA-NQx PBDTT-NQx PBDTF-NQx PBDTP-NQx 400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

Figure 1. UV-vis absorption spectra of the polymers in solution (a) and thin films (b).

22


-2.5 -3.0

0.05 mA

Energy Level (eV)

-3.5

-1.5

-1.0

-0.5

0.0

0.5

1.0

-3.38 eV -3.38 eV -3.28 eV

-5.0 -5.5

-5.25 eV -5.32 eV -5.36 eV

-6.0

HOMO levels

-4.0 -4.5

-6.5 -2.0

-3.08 eV

-5.28 eV

LUMO levels -4.0 eV

PC71BM BM PC 71

(b)

PBDTP-NPz PBDTP-NPz

PBDTT-NPz PBDTP-NPz

PBDTF-NPz PBDTF-NPz

PBDTA-NPz PBDTF-NPz

PBDTT-NPz PBDT T-NPz

(a)

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PBDTA-NPz PBD TA-NPz

Page 23 of 30

-6.0 eV

1.5

Potential (V)

Figure 2. (a) Cyclic voltammograms of the polymers. (b) HOMO and LUMO energy levels of the polymers and PC71BM.

23


100

PBDTA-NPz PBDTT-NPz PBDTF-NPz PBDTP-NPz

10

0.1

0

5

10

15

20 25 2θ(deg)

30

35

Page 24 of 30

(b) -2

PBDTP-NPz PBDTF-NPz PBDTT-NPz PBDTA-NPz

(a)

Current Density (A m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a. u.)


40

1

10

Vappl-Vbi-Va (v)

Figure 3. (a) X-ray diffraction patterns of the polymer films on ITO glass substrates. (b) J1/2-V characteristics of polymer based hole-only devices measured at ambient temperature.

24


Page 25 of 30

4

4

(b)

without DIO

0

-2

Current Density (mA cm )

-2

Current Density (mA cm )

(a)


-4

-8 -12

-16 -0.2

0.0

0.2

0.4

0.6

0.8

with 3% DIO

0

-8 -12

-16 -0.2

1.0


-4

0.0

Voltage (V)

(c)

0.2

0.4

0.6

0.8

1.0

Voltage (V)

80

(d) 80

without DIO

60

40 PBDTA-NPz PBDTT-NPz PBDTF-NPz PBDTP-NPz

20

0 300

with 3% DIO

60

EQE (%)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

500

40


20

600

700

0 300

800

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 4. J-V curves of copolymer:PC71BM-based regular single solar cells without (a) or with (b) 3 vol% additive of DIO under AM 1.5 G illumination, 100 mW/cm2. (b) EQE curves of copolymer:PC71BM-based regular single solar cells without (c) or with (d) 3 vol% additive of DIO.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. AFM topographic height (a-d) and phase (e-h) images of the film blends (polymer:PC71BM=1:1.5, w/w): PBDTA-NPz (a,e), PBDTT-NPz (b,f), PBDTF-NPz (c,g), and PBDTP-NPz (d,h). Image size: 5×5 µm2.

26


Page 26 of 30

Page 27 of 30

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Table1. Molecular weights and thermal properties of the copolymers. Yield

Mw[a]

Mn[a]

Td[b]

Tg[c]

(%)

(kDa)

(kDa)

(oC)

(oC)

PBDTA-NPz

73

129

41

3.1

325

153

PBDTT-NPz

67

67

25

2.5

424

118

PBDTF-NPz

62

74

27

2.7

360

127

PBDTP-NPz

60

77

27

2.9

387

130

copolymers

PDI

[a]

[a] Molecular weights and polydispersity indices were determined by GPC in THF using polystyrene as standards. [b] Onset decomposition temperature measured by TGA a heating rate of 10 oC min-1 under N2. [c] Glass transition temperature measured by DSC at a heating/cooling rate of 10 oC min-1 under N2.

27



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Page 28 of 30

Table 2. Optical and electrochemical data of the copolymers.

Abs. (nm)

Abs. (nm)

Eg[a]

HOMO

LUMO

Eg[b]

λSol max

film λmax

(eV)

(eV)

(eV)

(eV)

PBDTA-NPz

310,400,580，618

319,400,582,621

1.88

-5.25

-3.08

2.17

PBDTT-NPz

330,401,582,624

336,402,593,633

1.79

-5.32

-3.38

1.94

PBDTF-NPz

329,393,586,619

330,393,597,626

1.83

-5.36

-3.38

1.98

PBDTP-NPz

329,406,583,619

330,408,592,625

1.85

-5.28

-3.28

2.00

copolymers

[a] Optical band gap was estimated from the wavelength of the optical absorption edge of the copolymer film. [b] Electrochemical band gap was calculated from the LUMO and HOMO energy levels.

28


Page 29 of 30

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Table 3. Photovoltaic properties of the PSCs based on copolymer/PC71BM (1:1.5, w/w) under AM 1.5G, 100 mW/cm2.

DIO

Thickness

Voc

Jsc

FF

PCEmax(ave) [a]

(%)

(nm)

(V)

(mA/cm2)

(%)

(%)

PBDTA-NPz

no

110

0.80

11.07

51.2

4.53(4.01±0.52)

PBDTA-NPz

3

100

0.78

13.44

56.0

5.87(5.45±0.42)

PBDTT-NPz

no

113

0.83

13.57

54.0

6.09(5.53±0.56)

PBDTT-NPz

3

108

0.82

14.24

60.1

7.02(6.67±0.35)

PBDTF-NPz

no

116

0.86

13.89

59.1

7.06(6.66±0.40)

PBDTF-NPz

3

102

0.85

15.33

66.2

8.63(8.42±0.21)

PBDTP-NPz

no

120

0.82

8.91

47.8

3.49(2.91±0.58)

PBDTP-NPz

3

105

0.79

11.68

53.2

4.91(4.45±0.46)

copolymer

[a]

The average PCE value was calculated from 20 devices.

29



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Table of Contents (TOC)

30


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Side-Chain Influence of Wide-Bandgap ... - ACS Publications

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