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Hexacyclic lactam building blocks for highly efficient polymer solar cells Received 00th January 20xx, Accepted 00th January 20xx
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a
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Jiamin Cao,‡ Chuantian Zuo,‡ Bin Du,* Xiaohui Qiu* and Liming Ding*
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DOI: 10.1039/x0xx00000x www.rsc.org/
Two hexacyclic lactam building blocks, TD1 and TD2, and four D-A copolymers have been developed. Compared with thiophene copolymers, selenophene analogues PSeTD1 and PSeTD2 possess medium optical bandgaps, better packing, higher hole mobilities, enhanced EQE, and higher Jsc. Inverted PSeTD2:PC71BM solar cells gave a decent PCE of 8.18%, which is the record for D-A copolymers using selenophene as the donor unit. Polymer solar cells (PSCs) have advantages of low cost, flexibility, lightweight, and easy fabrication.1 The power conversion efficiencies (PCEs) of PSCs are determined by three parameters: open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). Voc is proportional to the difference between the HOMO energy levels of the donor materials and the LUMO energy levels of the acceptor materials.2 Jsc correlates with sunlight harvesting capability, charge carrier mobility, and morphology of the active layers.3 Charge carrier mobility and lifetime, morphology affect FF significantly.3,4 Polycyclic D-A copolymers are promising donor materials because of extended conjugation length, reduced bandgap, enhanced π-electron delocalization, strong π-π stacking, and high hole mobility.5 As a result, polycyclic D-A copolymers can achieve high PCEs via increasing Jsc and FF and maintaining high Voc. However, many efforts focused on polycyclic donor units, and excellent polycyclic acceptor units are rare, such as naphthobisthiadiazole, naphthobistriazole, isoindigo, pyridophenanthridine, and dihydropyrroloindoledione.6 Recently, a pentacyclic lactam acceptor unit thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline5,11(4H,10H)-dione (TPTI) was first reported by our group.7 D-A copolymer PThTPTI possesses good light absorption, deep HOMO energy level, partial crystallinity, and good hole-transporting property.7a However, the photovoltaic performance was limited by the moderate optical bandgap of 1.86 eV with an absorption onset
of 666 nm. Further lowering the bandgap and widening the absorption spectra favour to harvest more photons and enhance the efficiency. Hence, two bigger hexacyclic lactam acceptor units TD1 and TD2 with extended conjugation length were developed (Scheme 1). The new units can be regarded as the derivatives of TPTI by replacing benzene ring with thieno[3,2-b]thiophene (TT). Fusing rigid and coplanar TT into conjugated building blocks can extend the effective conjugation length, leading to lower optical 8 bandgaps. Moreover, the ordered packing and strong interchain interactions caused by TT can improve charge carrier mobility, 9 which is beneficial to high Jsc. Another effective strategy to reduce bandgaps is replacing thiophene with selenophene. Compared with sulfur atom (2.58), selenium atom possesses smaller electronegativity (2.55), therefore selenophene exhibits lower aromatic resonance energy (1.25 eV) than that of thiophene (1.26 eV) and stronger electron-donating capability, leading to longer effective conjugation length and reduced optical bandgap.10 The larger, easily polarizable Se atom and the strong intermolecular Se···Se interaction can facilitate interchain charge transfer, thus enhancing hole mobility.11 In a word, replacing thiophene with selenophene can increase Jsc and FF, stabilize Voc, thus enhancing
Fig. 1 Structures for D-A copolymers.
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Scheme 1 Synthetic routes for monomers and copolymers.
PCEs. In this work, four D-A copolymers PThTD1, PSeTD1, PThTD2, and PSeTD2 using TD1 or TD2 as acceptor units and thiophene (Th) or selenophene (Se) as donor units were developed (Fig. 1). Solar cells based on selenophene polymers show higher PCEs than cells of thiophene polymers, and the inverted PSeTD2:PC71BM solar cells gave a decent PCE of 8.18%. The centrosymmetric TD1 and TD2 were constructed by six fused aromatic rings with a thieno[3,2-b]thiophene at the center, two thiophenes at the end, and two pyridones as the bridges. In order to endow monomers and polymers with good solubility, 2octyldodecyl (OD) or 2-hexyldecyl (HD) as side chains were introduced onto TD1 or TD2, respectively. As shown in Scheme 1, monomer synthesis started from bromination of diethyl thieno[3,2b]thiophene-3,6-dicarboxylate (1)12. Compound 2 was hydrolyzed to produce compound 3, which reacted with oxalyl chloride, and then with N-(2-octyldodecyl)thiophen-3-amine or N-(2hexyldecyl)thiophen-3-amine to produce 4a or 4b, respectively. TD1 or TD2 was obtained via intramolecular coupling reaction. In 1H NMR spectra, two double peaks at 7.51-7.12 ppm confirm the chemical structures of TD1 and TD2 (Fig. S1 and Fig. S2). Monomers TD1-Br and TD2-Br were synthesized via bromination of TD1 or TD2, respectively. Polymerization of TD1-Br or TD2-Br with 2,5bis(trimethylstannyl)thiophene or 2,5bis(trimethylstannyl)selenophene using Pd(PPh3)4 as the catalyst in toluene refluxing for 24 h gave four target copolymers. The crude products were purified by Soxhlet extraction sequentially with methanol and hexane to remove monomers and low MW polymers, followed by extraction with chloroform. The chloroform extract was concentrated and added dropwise into methanol to precipitate the polymers. All polymers possess good solubility in common organic solvents at room temperature, such as chloroform, chlorobenzene, ο-dichlorobenzene, and toluene. The number-average molecular weights (Mn) of PThTD1, PSeTD1, PThTD2, and PSeTD2 are 58.1, Table 1 Optical and electrochemical data for the polymers
polymers PThTD1 PSeTD1 PThTD2 PSeTD2
λsolution [nm] 617 632 619 637
λfilm [nm] 579,630 597,651 578,629 602,656
Egopt [eV]a 1.82 1.75 1.82 1.75
HOMO [eV] -5.36 -5.34 -5.30 -5.31
LUMO [eV] -2.56 -2.70 -2.63 -2.74
Egec [eV]b 2.80 2.64 2.67 2.57
Fig. 2 Absorption spectra for polymer films.
70.8, 33.2, and 35.5 kDa, with PDI of 5.78, 3.58, 4.41, and 2.63, respectively (Table S1). The decomposition temperatures (Td, 5% wt loss) of the four polymers are 439, 429, 442, and 443 °C, respectively, suggesting excellent thermal stability for solar cell application (Fig. S3). The absorption spectra for polymers PThTD1, PSeTD1, PThTD2, and PSeTD2 in chloroform and as thin films are presented in Fig. S4 and Fig. 2, and the corresponding optical data are listed in Table 1. The polymers show two absorption peaks both in solution and in solid state. In solution, four polymers showed the long-wavelength absorption peaks at 617, 632, 619, and 637 nm, while in solid state the long-wavelength peaks locate at 630, 651, 629, and 656 nm, respectively, displaying red shifts of 13, 19, 10, and 19 nm, respectively. Selenophene-based polymers exhibited remarkable 11 red-shifts, which might be due to stronger π-π stacking. Selenophene-based polymers PSeTD1 and PSeTD2 possess narrower optical bandgaps of 1.75 eV than thiophene analogues (1.82 eV). The electrochemical properties for the four polymers were investigated by cyclic voltammetry (CV) and all potentials were + corrected against Fc/Fc (Fig. S5). The energy levels of polymers were calculated according to empirical formulas: HOMO or LUMO = onset onset 13 or Ered + 4.8) eV. All polymers exhibited similar HOMO −(Eox energy levels of -5.36~-5.30 eV. PSeTD1 and PSeTD2 showed deeper LUMO energy levels (-2.70 and -2.74 eV) than PThTD1 and PThTD2 (-2.56 and -2.63 eV) (Table 1). The bandgap reduction for PSeTD1 and PSeTD2 results from the lowering of their LUMO energy levels. Solar cells with a conventional structure of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al were firstly investigated. The best devices for the four polymers had a D/A ratio of 1:0.8 (w/w), an active layer thickness of 110~125 nm, 3 vol% 1,8diiodooctane (DIO) additive in chlorobenzene, and took no thermal annealing. PThTD1, PSeTD1, PThTD2 and PSeTD2 solar cells gave PCEs of 5.95%, 6.03%, 6.86% and 7.73%, respectively (Fig. S6 and Table S14). Inverted solar cells with a structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag were also fabricated using the optimized conditions for conventional cells. The J-V curves and EQE spectra for the best cells are shown in Fig. 3 and the performance
Egopt = 1240/λonset; bEgec = LUMO - HOMO.
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Fig. 3 J-V curves (a) and EQE spectra (b) for inverted polymer:PC71BM solar cells.
data are listed in Table 2. High Voc of 0.83~0.87 V were obtained for all solar cells due to the deep HOMO energy levels. Inverted cells gave higher PCEs than conventional cells because of enhanced Jsc, 14 which results from higher transmittance of ZnO. Polymers with shorter side chains exhibited higher efficiencies (6.90% for PThTD2 and 8.18% for PSeTD2) than polymers with longer side chains (6.02% for PThTD1 and 6.22% for PSeTD1). Shorter side chains favour polymer backbone packing, leading to higher hole mobility and photocurrent.15 Compared with thiophene analogues, selenophenebased polymers show enhanced EQE (Fig. 3b). The best inverted PSeTD2:PC71BM solar cells gave a PCE of 8.18%, with a Jsc of 13.55 mA/cm2, a Voc of 0.85 V, a FF of 71.06%, and over 65% EQE at 471668 nm. 8.18% is the highest PCE for D-A copolymers using selenophene as the donor unit.16 The integrated photocurrent from EQE spectra are 9.33, 10.73, 11.43 and 12.90 mA/cm2 for PThTD1, PSeTD1, PThTD2 and PSeTD2, respectively, which are consistent with those from J-V measurements. Morphologies of the blend films were studied by atomic force microscope (AFM). As shown in Fig. 4 and Fig. S7-S8, DIO addition can effectively reduce the surface roughness for all blend films. The RMS roughnesses were reduced from 5.89 nm to 2.08 nm for PThTD2:PC71BM blend film, from 6.28 nm to 2.57 nm for PSeTD2:PC71BM blend film. PThTD2:PC71BM and PSeTD2:PC71BM blend films without DIO additive show large domains (Fig. 4a and 4c), which significantly reduce D/A interface, are unfavorable for exciton dissociation, leading to low Jsc and FF (Table S10 and S13). PThTD2:PC71BM and PSeTD2:PC71BM blend films with DIO show ideal phase separation with sufficient D/A interfaces. The bicontinuous interpenetrating network in the blend films favors exciton dissociation and charge carrier transport, enhancing Jsc and FF. The mobilities for the blend films were measured by space charge limited current (SCLC) method (Fig. S9). The hole mobilities for PThTD1, PSeTD1, PThTD2 and PSeTD2 are 4.06×10-4, 9.79×10-4, 5.98×10-4 and 2.53×10-3 cm2V-1s-1, respectively. To further understand high hole mobilities for selenophene polymers, the Table 2 Performance data for inverted polymer:PC71BM solar cells.
polymers PThTD1 PSeTD1 PThTD2 PSeTD2
Voc [V] 0.87 0.83 0.85 0.85
Jsc [mA/cm2] 9.67 10.92 11.95 13.55
FF [%] 71.59 68.67 67.96 71.06
a
Fig. 4 AFM phase images for PThTD2/PC71BM films without DIO (a) and with 3 vol% DIO (b); PSeTD2/PC71BM films without DIO (c) and with 3 vol% DIO (d).
polymer films were studied by X-ray diffraction (XRD). All polymers exhibited two diffraction peaks at 3.97-4.58° and ~25° (Fig. S10 and Table S15). The diffraction peaks at ~25° indicated that the dspacing for π-π stacking for all polymers is ~3.55 Å. The interlayer dspacings for PThTD1 and PSeTD1 (~22 Å) are longer than those of PThTD2 and PSeTD2 (~19.4 Å), which might be due to the longer side chains (OD vs HD).17 Selenophene polymers, especially PSeTD2, possess better lamellar packing than thiophene polymers, explaining well the higher hole mobilities for PSeTD1:PC71BM and PSeTD2:PC71BM blend films. In conclusion, four D-A copolymers based on novel fused-ring acceptor units have been developed. These materials possess good solubility, medium optical bandgaps, and low HOMO energy levels. An 8.18% PCE was obtained from inverted PSeTD2:PC71BM solar cells, which is the record for D-A copolymers using selenophene as the donor unit. Our results indicate that extending conjugation length of acceptor units and replacing thiophene donor unit with selenophene unit can effectively reduce optical bandgaps of D-A copolymers and enhance solar cell performance synergistically. This work was supported by National Natural Science Foundation of China (U1401244, 21374025 and 21402009).
Notes and references
PCE [%]a 6.02 (5.86±0.15) 6.22 (5.96±0.16) 6.90 (6.65±0.18) 8.18 (7.95±0.19)
Average PCEs and standard deviations in the brackets were obtained from 10 cells.
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Four D-A copolymers based on new fused-ring acceptor units were developed, and an
donor unit.
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8.18% PCE record was demonstrated for D-A copolymers using selenophene as the
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This article can be cited before page numbers have been issued, to do this please use: L. Ding, J. Cao, C. Zuo, B. Du and X. Qiu, Chem. Commun., 2015, DOI: 10.1039/C5CC04375A.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Hexacyclic lactam building blocks for highly efficient polymer solar cells Received 00th January 20xx, Accepted 00th January 20xx
a
a
b
a
Jiamin Cao,‡ Chuantian Zuo,‡ Bin Du,* Xiaohui Qiu* and Liming Ding*
a
DOI: 10.1039/x0xx00000x www.rsc.org/
Two hexacyclic lactam building blocks, TD1 and TD2, and four D-A copolymers have been developed. Compared with thiophene copolymers, selenophene analogues PSeTD1 and PSeTD2 possess medium optical bandgaps, better packing, higher hole mobilities, enhanced EQE, and higher Jsc. Inverted PSeTD2:PC71BM solar cells gave a decent PCE of 8.18%, which is the record for D-A copolymers using selenophene as the donor unit. Polymer solar cells (PSCs) have advantages of low cost, flexibility, lightweight, and easy fabrication.1 The power conversion efficiencies (PCEs) of PSCs are determined by three parameters: open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). Voc is proportional to the difference between the HOMO energy levels of the donor materials and the LUMO energy levels of the acceptor materials.2 Jsc correlates with sunlight harvesting capability, charge carrier mobility, and morphology of the active layers.3 Charge carrier mobility and lifetime, morphology affect FF significantly.3,4 Polycyclic D-A copolymers are promising donor materials because of extended conjugation length, reduced bandgap, enhanced π-electron delocalization, strong π-π stacking, and high hole mobility.5 As a result, polycyclic D-A copolymers can achieve high PCEs via increasing Jsc and FF and maintaining high Voc. However, many efforts focused on polycyclic donor units, and excellent polycyclic acceptor units are rare, such as naphthobisthiadiazole, naphthobistriazole, isoindigo, pyridophenanthridine, and dihydropyrroloindoledione.6 Recently, a pentacyclic lactam acceptor unit thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline5,11(4H,10H)-dione (TPTI) was first reported by our group.7 D-A copolymer PThTPTI possesses good light absorption, deep HOMO energy level, partial crystallinity, and good hole-transporting property.7a However, the photovoltaic performance was limited by the moderate optical bandgap of 1.86 eV with an absorption onset
of 666 nm. Further lowering the bandgap and widening the absorption spectra favour to harvest more photons and enhance the efficiency. Hence, two bigger hexacyclic lactam acceptor units TD1 and TD2 with extended conjugation length were developed (Scheme 1). The new units can be regarded as the derivatives of TPTI by replacing benzene ring with thieno[3,2-b]thiophene (TT). Fusing rigid and coplanar TT into conjugated building blocks can extend the effective conjugation length, leading to lower optical 8 bandgaps. Moreover, the ordered packing and strong interchain interactions caused by TT can improve charge carrier mobility, 9 which is beneficial to high Jsc. Another effective strategy to reduce bandgaps is replacing thiophene with selenophene. Compared with sulfur atom (2.58), selenium atom possesses smaller electronegativity (2.55), therefore selenophene exhibits lower aromatic resonance energy (1.25 eV) than that of thiophene (1.26 eV) and stronger electron-donating capability, leading to longer effective conjugation length and reduced optical bandgap.10 The larger, easily polarizable Se atom and the strong intermolecular Se···Se interaction can facilitate interchain charge transfer, thus enhancing hole mobility.11 In a word, replacing thiophene with selenophene can increase Jsc and FF, stabilize Voc, thus enhancing
Fig. 1 Structures for D-A copolymers.
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Scheme 1 Synthetic routes for monomers and copolymers.
PCEs. In this work, four D-A copolymers PThTD1, PSeTD1, PThTD2, and PSeTD2 using TD1 or TD2 as acceptor units and thiophene (Th) or selenophene (Se) as donor units were developed (Fig. 1). Solar cells based on selenophene polymers show higher PCEs than cells of thiophene polymers, and the inverted PSeTD2:PC71BM solar cells gave a decent PCE of 8.18%. The centrosymmetric TD1 and TD2 were constructed by six fused aromatic rings with a thieno[3,2-b]thiophene at the center, two thiophenes at the end, and two pyridones as the bridges. In order to endow monomers and polymers with good solubility, 2octyldodecyl (OD) or 2-hexyldecyl (HD) as side chains were introduced onto TD1 or TD2, respectively. As shown in Scheme 1, monomer synthesis started from bromination of diethyl thieno[3,2b]thiophene-3,6-dicarboxylate (1)12. Compound 2 was hydrolyzed to produce compound 3, which reacted with oxalyl chloride, and then with N-(2-octyldodecyl)thiophen-3-amine or N-(2hexyldecyl)thiophen-3-amine to produce 4a or 4b, respectively. TD1 or TD2 was obtained via intramolecular coupling reaction. In 1H NMR spectra, two double peaks at 7.51-7.12 ppm confirm the chemical structures of TD1 and TD2 (Fig. S1 and Fig. S2). Monomers TD1-Br and TD2-Br were synthesized via bromination of TD1 or TD2, respectively. Polymerization of TD1-Br or TD2-Br with 2,5bis(trimethylstannyl)thiophene or 2,5bis(trimethylstannyl)selenophene using Pd(PPh3)4 as the catalyst in toluene refluxing for 24 h gave four target copolymers. The crude products were purified by Soxhlet extraction sequentially with methanol and hexane to remove monomers and low MW polymers, followed by extraction with chloroform. The chloroform extract was concentrated and added dropwise into methanol to precipitate the polymers. All polymers possess good solubility in common organic solvents at room temperature, such as chloroform, chlorobenzene, ο-dichlorobenzene, and toluene. The number-average molecular weights (Mn) of PThTD1, PSeTD1, PThTD2, and PSeTD2 are 58.1, Table 1 Optical and electrochemical data for the polymers
polymers PThTD1 PSeTD1 PThTD2 PSeTD2
λsolution [nm] 617 632 619 637
λfilm [nm] 579,630 597,651 578,629 602,656
Egopt [eV]a 1.82 1.75 1.82 1.75
HOMO [eV] -5.36 -5.34 -5.30 -5.31
LUMO [eV] -2.56 -2.70 -2.63 -2.74
Egec [eV]b 2.80 2.64 2.67 2.57
Fig. 2 Absorption spectra for polymer films.
70.8, 33.2, and 35.5 kDa, with PDI of 5.78, 3.58, 4.41, and 2.63, respectively (Table S1). The decomposition temperatures (Td, 5% wt loss) of the four polymers are 439, 429, 442, and 443 °C, respectively, suggesting excellent thermal stability for solar cell application (Fig. S3). The absorption spectra for polymers PThTD1, PSeTD1, PThTD2, and PSeTD2 in chloroform and as thin films are presented in Fig. S4 and Fig. 2, and the corresponding optical data are listed in Table 1. The polymers show two absorption peaks both in solution and in solid state. In solution, four polymers showed the long-wavelength absorption peaks at 617, 632, 619, and 637 nm, while in solid state the long-wavelength peaks locate at 630, 651, 629, and 656 nm, respectively, displaying red shifts of 13, 19, 10, and 19 nm, respectively. Selenophene-based polymers exhibited remarkable 11 red-shifts, which might be due to stronger π-π stacking. Selenophene-based polymers PSeTD1 and PSeTD2 possess narrower optical bandgaps of 1.75 eV than thiophene analogues (1.82 eV). The electrochemical properties for the four polymers were investigated by cyclic voltammetry (CV) and all potentials were + corrected against Fc/Fc (Fig. S5). The energy levels of polymers were calculated according to empirical formulas: HOMO or LUMO = onset onset 13 or Ered + 4.8) eV. All polymers exhibited similar HOMO −(Eox energy levels of -5.36~-5.30 eV. PSeTD1 and PSeTD2 showed deeper LUMO energy levels (-2.70 and -2.74 eV) than PThTD1 and PThTD2 (-2.56 and -2.63 eV) (Table 1). The bandgap reduction for PSeTD1 and PSeTD2 results from the lowering of their LUMO energy levels. Solar cells with a conventional structure of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al were firstly investigated. The best devices for the four polymers had a D/A ratio of 1:0.8 (w/w), an active layer thickness of 110~125 nm, 3 vol% 1,8diiodooctane (DIO) additive in chlorobenzene, and took no thermal annealing. PThTD1, PSeTD1, PThTD2 and PSeTD2 solar cells gave PCEs of 5.95%, 6.03%, 6.86% and 7.73%, respectively (Fig. S6 and Table S14). Inverted solar cells with a structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag were also fabricated using the optimized conditions for conventional cells. The J-V curves and EQE spectra for the best cells are shown in Fig. 3 and the performance
Egopt = 1240/λonset; bEgec = LUMO - HOMO.
a
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Fig. 3 J-V curves (a) and EQE spectra (b) for inverted polymer:PC71BM solar cells.
data are listed in Table 2. High Voc of 0.83~0.87 V were obtained for all solar cells due to the deep HOMO energy levels. Inverted cells gave higher PCEs than conventional cells because of enhanced Jsc, 14 which results from higher transmittance of ZnO. Polymers with shorter side chains exhibited higher efficiencies (6.90% for PThTD2 and 8.18% for PSeTD2) than polymers with longer side chains (6.02% for PThTD1 and 6.22% for PSeTD1). Shorter side chains favour polymer backbone packing, leading to higher hole mobility and photocurrent.15 Compared with thiophene analogues, selenophenebased polymers show enhanced EQE (Fig. 3b). The best inverted PSeTD2:PC71BM solar cells gave a PCE of 8.18%, with a Jsc of 13.55 mA/cm2, a Voc of 0.85 V, a FF of 71.06%, and over 65% EQE at 471668 nm. 8.18% is the highest PCE for D-A copolymers using selenophene as the donor unit.16 The integrated photocurrent from EQE spectra are 9.33, 10.73, 11.43 and 12.90 mA/cm2 for PThTD1, PSeTD1, PThTD2 and PSeTD2, respectively, which are consistent with those from J-V measurements. Morphologies of the blend films were studied by atomic force microscope (AFM). As shown in Fig. 4 and Fig. S7-S8, DIO addition can effectively reduce the surface roughness for all blend films. The RMS roughnesses were reduced from 5.89 nm to 2.08 nm for PThTD2:PC71BM blend film, from 6.28 nm to 2.57 nm for PSeTD2:PC71BM blend film. PThTD2:PC71BM and PSeTD2:PC71BM blend films without DIO additive show large domains (Fig. 4a and 4c), which significantly reduce D/A interface, are unfavorable for exciton dissociation, leading to low Jsc and FF (Table S10 and S13). PThTD2:PC71BM and PSeTD2:PC71BM blend films with DIO show ideal phase separation with sufficient D/A interfaces. The bicontinuous interpenetrating network in the blend films favors exciton dissociation and charge carrier transport, enhancing Jsc and FF. The mobilities for the blend films were measured by space charge limited current (SCLC) method (Fig. S9). The hole mobilities for PThTD1, PSeTD1, PThTD2 and PSeTD2 are 4.06×10-4, 9.79×10-4, 5.98×10-4 and 2.53×10-3 cm2V-1s-1, respectively. To further understand high hole mobilities for selenophene polymers, the Table 2 Performance data for inverted polymer:PC71BM solar cells.
polymers PThTD1 PSeTD1 PThTD2 PSeTD2
Voc [V] 0.87 0.83 0.85 0.85
Jsc [mA/cm2] 9.67 10.92 11.95 13.55
FF [%] 71.59 68.67 67.96 71.06
a
Fig. 4 AFM phase images for PThTD2/PC71BM films without DIO (a) and with 3 vol% DIO (b); PSeTD2/PC71BM films without DIO (c) and with 3 vol% DIO (d).
polymer films were studied by X-ray diffraction (XRD). All polymers exhibited two diffraction peaks at 3.97-4.58° and ~25° (Fig. S10 and Table S15). The diffraction peaks at ~25° indicated that the dspacing for π-π stacking for all polymers is ~3.55 Å. The interlayer dspacings for PThTD1 and PSeTD1 (~22 Å) are longer than those of PThTD2 and PSeTD2 (~19.4 Å), which might be due to the longer side chains (OD vs HD).17 Selenophene polymers, especially PSeTD2, possess better lamellar packing than thiophene polymers, explaining well the higher hole mobilities for PSeTD1:PC71BM and PSeTD2:PC71BM blend films. In conclusion, four D-A copolymers based on novel fused-ring acceptor units have been developed. These materials possess good solubility, medium optical bandgaps, and low HOMO energy levels. An 8.18% PCE was obtained from inverted PSeTD2:PC71BM solar cells, which is the record for D-A copolymers using selenophene as the donor unit. Our results indicate that extending conjugation length of acceptor units and replacing thiophene donor unit with selenophene unit can effectively reduce optical bandgaps of D-A copolymers and enhance solar cell performance synergistically. This work was supported by National Natural Science Foundation of China (U1401244, 21374025 and 21402009).
Notes and references
PCE [%]a 6.02 (5.86±0.15) 6.22 (5.96±0.16) 6.90 (6.65±0.18) 8.18 (7.95±0.19)
Average PCEs and standard deviations in the brackets were obtained from 10 cells.
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Four D-A copolymers based on new fused-ring acceptor units were developed, and an
donor unit.
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8.18% PCE record was demonstrated for D-A copolymers using selenophene as the
