Open-Circuit Voltage Modulations on All-Polymer Solar Cells by Side

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Open-Circuit Voltage Modulations on All-Polymer Solar Cells by Side Chain Engineering on 4,8-Di(thiophen-2yl)benzo[1,2-b:4,5-b#]dithiophene-Based Donor Polymers Birhan A. Abdulahi, Xiaofeng Xu, Petri Murto, Olle Inganäs, Wendimagegn Mammo, and Ergang Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00562 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Applied Energy Materials

Open-Circuit Voltage Modulations on All-Polymer Solar Cells by Side Chain Engineering on 4,8-Di(thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-Based Donor Polymers Birhan A. Abdulahi,†,‡,┴ Xiaofeng Xu, §,┴ Petri Murto,† Olle Inganäs,§ Wendimagegn Mammo,*,‡ Ergang Wang*,† †

Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden. E-mail: [email protected]

‡

Department of Chemistry, Addis Ababa University, P.O. Box 33658, Addis Ababa, Ethiopia. E-mail: [email protected]

§

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

KEYWORDS: acceptor polymers, all-polymer solar cells, 4,8-Di(thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene, donor polymers, open-circuit voltage, organic photovoltaics, side chain engineering

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ABSTRACT: In recent years, all-polymer solar cells (all-PSCs), incorporating active layers based on blends of electron-donor (D) and acceptor (A) polymers, have drawn attention because of the advantages they hold in the flexibility of choosing the D:A combinations to modulate their energy levels and to improve their overall open-circuit voltages (Voc)s and power conversion efficiencies (PCE)s. Voc is one of the key parameters for the determination of the PCEs of PSCs. In this work, we synthesized six donor polymers with three different side chains appended to the 4,8-di(thiophen-2yl)benzo[1,2-b:4,5-b']dithiophene (BDT) units. By substituting carbon with sulfur and silicon atoms at the 5-position of the thiophenes attached to the BDT units, the highest occupied molecular orbital (HOMO) levels of the donor polymers could be successfully lowered. As anticipated, the Vocs of the resulting all-PSCs increased along with the lowering of the HOMO levels of the donor polymers. Among the six all-PSCs, the PBDT-BDD:PNDI-T10 all-PSC realized a balance between the photovoltage and photocurrent, where a decent PCE of 5.6% was obtained with a Voc of 0.9 V and a photocurrent of 10.5 mA/cm2.

INTRODUCTION The advantages of polymer solar cells (PSCs), such as light weight, flexibility, low cost and simplicity of manufacture by using roll-to-roll techniques, have motivated researchers to develop new conjugated materials and optimize device architectures toward practical applications.1-2 Nowadays, various conjugated polymers are used as electron donors (D) in conjunction with fullerene derivatives (PCBM) as electron acceptors (A) in PSCs, where the power conversion efficiencies (PCEs) of single junction PSCs have reached over 11%.3-4 Open-circuit voltage (Voc)


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is one of the most important parameters that directly affects the PCEs of PSCs.5 In general, Voc is proportional to the energy difference between the highest occupied molecular orbital (HOMO) level of a donor (D) and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (A).6 To improve the Vocs of PSCs, the HOMO levels of the donors should be down-shifted, while the LUMO levels of the acceptors should be up-shifted. For polymer:PCBM solar cells, since the commonly used acceptors like PC61BM and PC71BM exhibit fixed LUMO levels, synthetic efforts have been directed at down-shifting the HOMO levels of donor polymers.7 Introducing electronegative atoms such as fluorine or electron-withdrawing groups such as cyano groups on the polymer backbones is one of the strategies used to lower the HOMO levels.8-10 Side chain engineering by modulating the electron-donating properties of the pendent atoms also plays a role in lowering the HOMO levels of donor polymers.11-13 Meanwhile, efforts are also made to develop emerging non-fullerene acceptor materials to match with donor polymers to boost the Vocs and the PCEs in non-fullerene organic solar cells.13-15 Recently, a non-fullerene organic solar cell incorporating an alkylsilyl-substituted donor polymer and a small molecular acceptor achieved a high Voc of 0.94 V and a PCE of 11.4%,13,16 which revealed that organic acceptors including small molecular and polymeric materials have the potential to replace PCBM.17-18 Such acceptor polymers are designed by taking into account complementary absorption spectra, proper energy levels and easy synthetic procedures. Currently, naphthalene diimide (NDI) is the most widely used building block for the preparation of acceptor polymers used in all-PSCs.19-31 In 2016, we developed an acceptor polymer, PNDI-T10, by introducing small amounts of thiophene units in the backbones of poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5'-(2,2'-bithiophene)] (N2200).32-33 A high PCE of 7.6% was achieved for the binary PTB7-Th:PNDI-T10 all-PSC. Afterwards, an outstanding PCE of 9% was recorded in a ternary all-PSCs based on two donor polymers and PNDI-T10.22 Encouragingly, all-PSCs fabricated by 3 ACS Paragon Plus Environment


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using PBDTTS-FTAZ as a donor and PNDI-T10 as an acceptor achieved Vocs in the order of 0.88 V, higher than the Vocs of the PBDTTS-FTAZ:PCBM solar cells.34-35 In addition, we have performed a direct comparison on all-PSCs and polymer:PCBM solar cells, where the corresponding all-PSCs showed simultaneously higher Vocs of 1.1 V and higher PCEs of 8.0%, as compared to the polymer:PCBM solar cells. These results reveal the advantages of flexibility in choosing the donor and acceptor combinations for all-PSCs to modulate the energy levels and boost the Vocs and PCEs.36 In this work, we synthesized and characterized six alternating polymers by incorporating 4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene butyloctyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene

(BDT), (BDTS),

4,8-bis(5-((2and

4,8-bis(5-

(tributylsilyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTSi) units as electron-donating building

blocks

and

c']dithiophene-4,8-dione

1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)-4H,8H-benzo[1,2-c:4,5(BDD)

and

5,6-bis(dodecyloxy)-4,7-di(thiophen-2-

yl)benzo[c][1,2,5]thiadiazole (TBT) units as electron-deficient building blocks. The BDT-, BDTSand BDTSi-based polymers were designed to study the influences of the side chains on the energy levels and the device parameters, while the use of the electron-deficient units BDD and TBT was inspired by the reports from Sun et al.37 and Hou et al.38, respectively. PNDI-T10 was used as the acceptor polymer in blends with the six donor polymers, due to its up-lying LUMO energy level of −4.05 eV and reduced crystallinity and backbone rigidity, as compared to N2200.32 The effects of the side chains on the thermal transitions, optical and electrochemical properties of the donor polymers, and the photovoltaic properties of the all-PSCs were investigated. The HOMO levels of the donor polymers were successfully down-shifted by changing the side chains from alkyl to alkylthio and trialkylsilyl groups, which improved the Vocs of the corresponding all-PSCs. The charge transport mobilities, exciton dissociation, and film morphologies were also studied to 4 ACS Paragon Plus Environment

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understand the reasons behind the varied photovoltaic parameters and why the PBDT-BDD:PNDIT10 all-PSCs achieved the best photovoltaic performance.

RESULTS AND DISCUSSION Synthesis of monomers and polymers Scheme 1. Chemical structures of the donor and acceptor polymers

The chemical structures of the polymers are shown in Scheme 1. The BDTS and BDTSi monomers were synthesized by modifying the procedures reported in the literature (Scheme S1 and Scheme S2, Supporting Information).11,13 The 1H and 13C NMR spectra of the monomers are shown in Figure S1, Supporting Information. The Stille condensation polymerization was used to synthesize the polymers (Scheme S3 and Table S1, Supporting Information). All polymers were readily soluble in chlorobenzene (CB) and o-dichlorobenzene (oDCB). We found that the silicon containing polymers PBDTSi-BDD and PBDTSi-TBT were more soluble in chloroform (CF) than PBDT-BDD, PBDTS-BDD, PBDT-TBT and PBDTS-TBT. This may be due to the fact that bulky 5 ACS Paragon Plus Environment


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tertiary silicon side chains prevent molecular packing and the Si-C bonds also give more flexibility than C-C bonds. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) at 150 °C, using 1,2,4-trichlorobenzene as the eluent. As summarized in Table 1, all polymers present relatively high molecular weights. Table 1. Molecular weights, decomposition temperatures and optical properties of the polymers PDI

Tda (°C)

εmaxb (Lg-1cm-1)

εmaxc (×104 cm-1)

λmaxd (nm)

Ege (eV)

92.0

2.3

396

53.9

6.0

615

1.82

PBDTS-BDD

38.4

2.8

328

50.5

5.1

622

1.84

PBDTSi-BDD

60.0

2.4

413

49.9

5.1

604

1.81

PBDT-TBT

36.1

2.6

303

50.5

4.9

626

1.79

PBDTS-TBT

53.0

2.5

282

39.0

5.6

632

1.79

PBDTSi-TBT

76.4

2.4

313

43.0

4.6

616

polymer

Mn (kDa)

PBDT-BDD

a

b

1.81 c

Td is decomposition temperature. the maximal absorption coefficient in oDCB. the maximal absorption coefficient in thin films. d absorption peaks in thin films. e optical band gap.

Thermal properties In order to evaluate the thermal stabilities of the polymers, thermogravimetric analysis (TGA) was conducted. TGA plots of the polymers are shown in Figure S2a, Supporting Information. As summarized in Table 1, the decomposition temperature (Td) of PBDT-BDD, PBDTS-BDD and PBDTSi-BDD were 396 °C, 328 °C and 413 °C, respectively, while the Td of PBDT-TBT, PBDTS-TBT and PBDTSi-TBT were 303 °C, 282 °C and 313 °C, respectively. The lower Td of the TBT-based polymers, as compared to the BDD-based polymers, might be due to thermal degradation of the alkoxy groups on the TBT units.39 In general, the Tds over 280 °C indicated that the six donor polymers are stable enough for photovoltaic applications. As observed from the 6 ACS Paragon Plus Environment

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differential scanning calorimetry (DSC) measurements, none of the donor polymers presented detectable thermal transitions in the temperature range of 0–300 °C, whereas clear melting and crystallization transitions were detectable from the acceptor PNDI-T10.32 It was inferred that the six donor polymers were less crystalline than the acceptor PNDI-T10 and did not afford strong crystals in the pristine solids (Figure S2b, Supporting Information).

Optical properties

6 5 4 3 2 1

PBDT-BDD PBDTS-BDD PBDTSi-BDD PBDT-TBT PBDTS-TBT PBDTSi-TBT PNDI-T10

(b) 50 40 30 20 10

0

0

800

400

500

Wavelength (nm)

4 3 2 1 0 400

500

600

700

(d) -3.0

700

800

-3.06 -3.02 -3.03 -3.14 -3.22 -3.19

-3.5 -4.0 -4.5 -5.0

PBDTS-BDD

5

PBDT-BDD

PBDT-BDD:PNDI-T10 PBDTS-BDD:PNDI-T10 PBDTSi-BDD:PNDI-T10 PBDT-TBT:PNDI-T10 PBDTS-TBT:PNDI-T10 PBDTSi-TBT:PNDI-T10

(c)

Energy level (eV) (vs Vacuum)

6

600

Wavelength(nm)

-5.5 -6.0

800

-4.05

PNDI-T10

700

PBDTSi-TBT

600

PBDTS-TBT

500

PBDT-TBT

400

PBDTSi-BDD

7

60

PBDT-BDD PBDTS-BDD PBDTSi-BDD PBDT-TBT PBDTS-TBT PBDTSi-TBT PNDI-T10

(a)

Absorption coefficient (Lg-1cm-1)

Absorption coefficient (X104 cm -1)

8

Absorption coefficient (X104 cm -1)

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


-5.81 -5.83 -5.87 -5.90 -5.95 -6.00

Wavelength (nm)

-6.36

Figure 1. (a) Absorption coefficients of the donor and acceptor polymers in thin films, (b) Absorption coefficients of the donor and acceptor polymers in the oDCB solution, (c) Absorption coefficients of thin films of the donor:acceptor blends (donor:acceptor = 1:1), (d) Energy level diagram of the polymers from SWV measurements. 7 ACS Paragon Plus Environment


The absorption coefficients of the donor and acceptor polymers in thin films and in the oDCB solution are shown in Figure 1a and Figure 1b, respectively. The donor polymers present comparable absorption coefficients in thin films, which are much higher than that of the acceptor polymer. The main absorption bands of the six donor polymers were in the wavelength region of 500–700 nm. The acceptor polymer PNDI-T10 showed two distinct absorption bands in the wavelength region of 350–450 nm and 600–800 nm, which could stem from excitations with the π-π* manifold, and correspond to transitions within local NDI and intramolecular charge transfer (ICT) character, respectively.32 Each of the donor polymers showed complementary absorptions with the acceptor polymer, which led to a good coverage of the solar irradiation.40 It is worth noting that among the six donor polymers, PBDT-BDD exhibited the highest absorption coefficients both in the thin film and in the solution (Table 1). Figure 1c reveals that both the donor and acceptor polymers contributed to the absorption of each blend film, but the clear absorption coefficient differences of the donor and acceptor polymers indicated that the donor polymers still dominated the absorption of the blend films. In addition, temperature-dependent absorption was measured to study the aggregation behavior of the donor polymers in solution (Figure S3, Supporting Information). When the oDCB solutions were heated from 25 °C to 85 °C, all six polymers presented clear blue-shifted absorption profiles, decreased absorption maxima, and reduced vibronic shoulders, indicating the presence of strong supramolecular interactions of the polymers in oDCB at an ambient temperature.41-42 Electrochemical properties In order to evaluate how the side chains affect the energy levels of the six donor polymers, both cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements were performed (Figure S4a and Figure S4b, Supporting Information). The HOMO and LUMO levels of the 8 ACS Paragon Plus Environment

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polymers were calculated from the SWV measurements, by using the oxidation and reduction peak potentials relative to ferrocene/ferrocenium (Fc/Fc+) redox couple. The oxidation potentials of PBDT-BDD, PBDTS-BDD and PBDTSi-BDD were 0.77 V, 0.82 V, and 0.87 V in the SWV curves, corresponding to HOMO levels of –5.90 eV, –5.95 eV and –6.00 eV, respectively. Similarly, HOMO levels of PBDT-TBT, PBDTS-TBT, and PBDTSi-TBT were calculated to be – 5.81 eV, –5.83 eV and –5.87 eV, respectively. Figure 1d compares the HOMO and LUMO energy levels of the donor polymers and PNDI-T10. Thus, Figure 1d, and Figure S4c, Supporting Information, revealed that the HOMO levels of the six polymers followed the same trend both in the SWV and CV measurements. Inclusion of sulfur and silicon atoms on the side chains of the BDT units gradually lowered the HOMO levels of both the BDD- and the TBT-based polymers. The sulfur atoms on the pendent thiophenes of the BDTS unit serve as moderate π-electron acceptors, due to the presence of the empty 3d-orbitals in divalent sulfur atoms.11,34,43 This renders BDTS as a weaker electron-donating unit relative to the BDT unit, accounting for the deeper HOMO levels of the alkylthio-substituted polymers, as compared to the alkyl substituted polymers.11,44-45 Moreover, by introducing the silicon atoms on the BDT units, the HOMO levels of the trialkylsilyl-substituted polymers were further decreased due to the interactions of low-lying σ* orbitals of silicon and π* orbitals of the thiophene units.13,46-47 Between the two series of polymers, slightly higher HOMO levels were found in the TBT-based polymers as compared to the BDD-based polymers. LUMO levels of around –3.0 eV and –3.2 eV were found for the BDDand TBT-based polymers, respectively, which should have been determined by the electrondeficient units BDD and TBT. Photovoltaic properties



The photovoltaic properties of the all-PSCs were evaluated by using conventional solar cells with the configuration of ITO/PEDOT:PSS (40 nm)/active layer/LiF (0.6 nm)/Al (90 nm). The current density–voltage (J–V) curves of the all-PSCs are shown in Figure 2a and Figure 2c and their photovoltaic parameters are summarized in Table 2. PNDI-T10 was used as the acceptor polymer.32 The all-PSCs based on the two trialkylsilyl-substituted donor polymers, PBDTSiBDD:PNDI-T10 and PBDTSi-TBT:PNDI-T10 exhibited Vocs of 0.98 V and 0.90 V, respectively, higher than the Vocs of the all-PSCs containing alkylthio- and alkyl-substituted polymers. The Voc variations are in good agreement with the HOMO level alignments of the donor polymers. In spite of achieving high Vocs, the corresponding short-circuit current density (Jsc) values of the PBDTSiBDD:PNDI-T10 and PBDTSi-TBT:PNDI-T10 all-PSCs were lower compared to the all-PSCs based on the alkylthio-substituted and the alkyl-substituted donor polymers. As a result, moderate PCEs of 3–4% were obtained from these two all-PSCs. On the other hand, the PBDT-BDD:PNDIT10 all-PSC achieved a higher PCE of 5.6%, where a higher Jsc of 10.5 mA/cm2 and a Voc of 0.90 V was recorded. PBDT-TBT:PNDI-T10-based all-PSCs also demonstrated a decent PCE of 4.8% due to a high Jsc of 11.7 mA/cm2. In addition, moderate Voc and Jsc values were recorded from the all-PSCs based on the alkylthio-substituted donor polymers. We noted that the trend in the variation of Jsc was opposite to that of Voc among the six all-PSCs, while little changes in fill factor (FF) were found. The PBDT-BDD:PNDI-T10 all-PSC appeared to achieve a good balance between Voc and Jsc, which gave the best photovoltaic performance. The optimization studies of the all-PSCs based on the best-performing PBDT-BDD:PNDI-T10 system are summarized in Table S2, Supporting Information, exhibiting how the D:A ratios and thicknesses of the active layers affected the device performance. To understand the reasons behind this, we investigated the charge transport mobilities, quenching efficiencies, and morphologies of the blend films as described in the next sections. 10 ACS Paragon Plus Environment

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80

2

(a)

PBDT-BDD:PNDI-T10 PBDTS-BDD:PNDI-T10 PBDTSi-BDD:PNDI-T10

70

(b)

PBDT-BDD:PNDI-T10 PBDTS-BDD:PNDI-T10 PBDTSi-BDD:PNDI-T10

60

-2

50

EQE (%)

Current density (mA/cm2)

0

-4 -6

40 30

-8

20

-10

10

-12 -0.2

0.0

0.2

0.4

0.6

0.8

0 300

1.0

400

500

600

700

800

Wavelength (nm)

Voltage (V) 2

80

(c)

PBDT-TBT:PNDI-T10 PBDTS-TBT:PNDI-T10 PBDTSi-TBT:PNDI-T10

0

70

-2

60

-4

50

EQE (%)

Current density (mA/cm2)

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


-6

PBDT-TBT:PNDI-T10 PBDTS-TBT:PNDI-T10 PBDTSi-TBT:PNDI-T10

(d)

40 30

-8

20

-10 10

-12 -0.2

0.0

0.2

0.4

0.6

0.8

0 300

1.0

400

Voltage (V)

500

600

700

800

Wavelength (nm)

Figure 2. (a) J–V curves for BDD-based all-PSCs, (b) EQE curves for BDD-based all-PSCs, (c) J– V curves for TBT-based all-PSCs, (d) EQE curves for TBT-based all-PSCs. Table 2. Photovoltaic properties of the all-PSCs thickness [nm]

Voc [V]

Jsc [mA/cm2]

FF

PCE [%]

Eloss

PBDT-BDD:PNDI-T10

84

0.90

10.5 (10.3)a

0.59

5.6

0.65

PBDTS-BDD:PNDI-T10

78

0.97

7.2 (6.9)

0.56

3.9

0.58

PBDTSi-BDD:PNDI-T10

76

0.98

6.1 (5.7)

0.62

3.7

0.57

PBDT-TBT:PNDI-T10

74

0.77

11.7 (11.6)

0.53

4.8

0.78

PBDTS-TBT:PNDI-T10

81

0.86

9.0 (8.8)

0.55

4.3

0.69

PBDTSi-TBT:PNDI-T10

79

0.90

6.9 (6.7)

0.48

3.0

0.65

donor:acceptor

a

Photocurrents obtained by integrating the EQE with the AM1.5G spectrum are given in the parentheses 11 ACS Paragon Plus Environment


It is known that, Vocs and PCEs of organic solar cells are affected by the photon energy losses (Eloss), which are calculated by using the equation Eloss = Eg – eVoc, where Eg is the lowest optical gap of the light absorber. The empirical threshold of Eloss for organic solar cells was suggested to be 0.6 eV.48-49 Since the Vocs of the all-PSCs were gradually improved by incorporating sulfur and silicon atoms onto the donor polymers, a reduced Eloss was found in this work. Specifically, the PBDTSi-BDD:PNDI-T10 all-PSCs featured a low Eloss of 0.57, which is below the empirical threshold of 0.6 eV and among the lowest energy losses reported from non-fullerene organic solar cells.50-52 This indicates that the introduction of silylalkyl side chains could be a viable design strategy for donor polymers to maximize Voc of the resulting PSCs. In order to evaluate the spectral responses of the all-PSCs and the accuracies of the photocurrents from the J–V measurements, external quantum efficiency (EQE) measurements were conducted. As shown in Figure 2b and Figure 2d, each EQE spectrum revealed that both the donor and acceptor polymers contributed to the photocurrent, which is consistent with the absorption spectra of the blend films. It is clear that the PBDT-BDD:PNDI-T10 and PBDTTBT:PNDI-T10 all-PSCs presented higher EQE responses. The photocurrents calculated by integrating the EQE spectra with AM 1.5G solar spectrum are in good agreement with the corresponding Jsc obtained from the J–V measurements, with mismatches less than 5%. The electron (µe) and hole (µh) mobilities of the all-PSCs were measured by using the space charge limited current (SCLC) method (Table 3 and Figure S5, Supporting Information). The PBDTSi-BDD:PNDI-T10 blend film exhibited lower µh, while all the other blends showed decent µh and µe in the order of 10−5 cm2 V−1 s−1. Relatively balanced µh and µe were found for the four blends containing alkyl- and alkylthio-substituted donor polymers, whereas the two blends containing tributylsilyl-substituted donor polymers showed slightly less balanced µh and µe. Since 12 ACS Paragon Plus Environment

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unbalanced µh and µe can increase bimolecular recombination, this can be one of the reasons which partially contributed to the reduced photocurrents in the PBDTSi-BDD:PNDI-T10 and PBDTSiTBT:PNDI-T10 all-PSCs.32,53 Table 3. SCLC hole and electron mobilities and PL quenching efficiencies of the blend films donor:acceptor

SCLC µh (cm2 V−1 s−1

SCLC µe (cm2 V−1 s−1)

µh/µe

∆PLDa (%)

∆PLAb (%)

PBDT-BDD:PNDI-T10

2.6×10−5

7.6×10−5

0.34

89

86

PBDTS-BDD:PNDI-T10

1.8×10−5

6.4×10−5

0.28

84

79

PBDTSi-BDD:PNDI-T10

2.0×10−6

1.4×10−5

0.14

79

75

PBDT-TBT:PNDI-T10

5.0×10−5

4.9×10−5

1.02

92

88

PBDTS-TBT:PNDI-T10

5.4×10−5

3.9×10−5

1.38

83

78

PBDTSi-TBT:PNDI-T10

2.4×10−5

1.3×10−5

1.84

82

72

a

b

∆PLD is PL quenching efficiency of the blends relative to each donor polymer, ∆PLA is PL quenching efficiency of the blends relative to the acceptor. PL quenching characterization

700

800

900

1000

1100

600

700

Wavelength (nm)

700

800

900

Wavelength (nm)

900

1000

Relative PL intensity (a.u.) 600

1100

1000

1100

Relative PL intensity (a.u.)

PBDTS-TBT PNDI-T10 PBDTS-TBT:PNDI-T10

600

700

800

900

700

800

900

1000

1100

Wavelength (nm)

Wavelength (nm)

PBDT-TBT PNDI-T10 PBDT-TBT:PNDI-T10

600

800

PBDTSi-BDD PNDI-T10 PBDTSi-BDD:PNDI-T10

1000

1100

Wavelength (nm)

PBDTSi-TBT PNDI-T10 PBDTSi-TBT:PNDI-T10


600

PBDTS-BDD PNDI-T10 PBDTS-BDD:PNDI-T10



PBDT-BDD PNDI-T10 PBDT-BDD:PNDI-T10


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


600

700

800

900

1000

Wavelength (nm)

Figure 3. PL quenching measurements of the polymer:polymer blends (donor:acceptor = 1:1). 13 ACS Paragon Plus Environment

1100


In order to evaluate the efficiency of exciton dissociation and to understand the reasons behind the differences in Jsc, photoluminescence (PL) spectra of the neat polymers and the polymer:polymer films were measured. As depicted in Figure 3, the PL peaks of the six neat donor polymers were around 690 nm, while the acceptor polymer PNDI-T10 showed a PL with a peak around 835 nm. In all blend films, the acceptor PNDI-T10 showed incomplete quenching of the PL, and the PL intensities of the donor polymers varied at the specific wavelength region in each of the blends. The steady-state PL quenching efficiency (∆PL) could be calculated by using Equation 1,32,54

∆PL = 1 −

PL blend PL polymer

(1)

where PLblend and PLpolymer are the integral PL counts of the blends and the neat polymer films, respectively. As summarized in Table 3, both the ∆PLD and ∆PLA of the blend films decreased as the donor polymers changed from the alkyl-, to the alkylthio- and trialkylsilyl-substituted polymers. Clearly, the higher ∆PLD and ∆PLA of the PBDT-BDD:PNDI-T10 and PBDTTBT:PNDI-T10 blends revealed more efficient exciton dissociation between the donor and acceptor polymers, which should be one of the key reasons leading to the higher Jsc extracted from all-PSC devices containing these two systems. The ∆PL values of PBDTS-BDD:PNDI-T10 and PBDTS-TBT:PNDI-T10 blends were between the values determined for blends based on the alkyland trialkylsilyl-substituted polymers. These results are in reasonable agreement with the moderate photocurrents recorded from the all-PSCs. The two peaks around 690 and 840 nm in the PL spectra of PBDTSi-BDD:PNDI-T10 and PBDTSi-TBT:PNDI-T10 blends indicated that the excitons generated in the donor and acceptor were not completely quenched by each other. The inefficient exciton dissociation might be associated with the undesired film morphology, which can partially explain the inferior photocurrents of these two all-PSCs.54-55


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Film morphology

Figure 4. Tapping mode AFM topography images (5×5 µm2) of the polymer:polymer blends (donor:acceptor = 1:1). Atomic force microscopy (AFM) was employed to study the surface roughness and phase separation of the blend films. Figure 4 depicts that, in general, the six blend films featured very smooth surfaces, where the root mean-square (RMS) roughnesses of the BDD-based polymer blends were around 1.7 nm, and the corresponding values for the TBT-based polymer blends were around 0.8 nm. Although the donor polymers appeared to undergo strong interactions in solution, none of the blend films showed large domains and phase separation after casting on the ITO/PEDOT:PSS substrates. The AFM results indicated that the presence of different atoms on the side chains of the BDT units brought about very little influences on the surface morphologies of the blend films. Excluding the effects pertaining to blend morphology, we infer that the key reasons that led to the high Jsc of the PBDT-BDD:PNDI-T10 all-PSC should be the strong photon absorption and the efficient exciton dissociation, where the former is supported by the high 15 ACS Paragon Plus Environment


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absorption coefficient of the PBDT-BDD:PNDI-T10 blend and the latter by the PL quenching measurements.

CONCLUSIONS In summary, we synthesized six alternating polymers, comprising BDT, BDTS, or BDTSi as the electron-donating units and BDD or TBT as the electron-withdrawing units, to study the effects of the side chains of the donor polymers on the photovoltaic performances of the all-PSCs. The HOMO levels of the donor polymers were down-shifted when the CH2 groups at 5-position of the thiophenes attached to the BDT units were successively replaced by sulfur and silicon atoms. Gratifyingly, PBDTSi-BDD and PBDTSi-TBT showed Vocs of 0.98 V and 0.90 V, respectively, which are among the highest Vocs obtained from all-PSCs to date. However, the Jscs of these two systems were limited by inefficient exciton dissociation. On the other hand, the PBDTBDD:PNDI-T10 all-PSC achieved a good balance between Voc and Jsc, where a high Voc of 0.90 V, a high Jsc of 10.5 mA/cm2, and a decent FF of 0.59 synergistically led to the highest PCE of 5.6% among the six all-PSCs. This work provided useful guidelines for the modulation of HOMO levels of donor polymers by side chain engineering and thus the Vocs of the resulting all-PSCs. Further improvement of the performances of all-PSCs can be expected from optimization of device architecture and film morphologies of the all-PSCs based on the sulfur- and silicon-substituted polymers.

EXPERIMENTAL Materials Characterization The intermediate compounds and monomers were characterized by 1H NMR (400 MHz) and

13

C

NMR (100 MHz) using a Varian Inova 400 MHz NMR spectrometer. Tetramethylsilane was used


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as the internal reference. The molecular weights of the polymers were determined by size exclusion chromatography (SEC) using 1,2,4-trichlorobenzene as the eluent at 150 °C with a polystyrene standard calibration. Thermogravimetric analysis (TGA) was carried out on a METTLER TOLEDO thermogravimetric analyzer TGA/DSC 3+, from 50 °C to 550 °C at a heating rate of 10 °C /min under N2 flow. Differential scanning calorimetry (DSC) was carried out on a METTLER TOLEDO differential scanning calorimeter DSC 2, from 0 °C to 300 °C at a heating and cooling rate of 10 °C/min under N2 flow. A Perkin Elmer Lambda 900 UV-Vis-NIR spectrometer was used to measure the absorptions of the polymers. Cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements were done on a CH-Instruments 650A electrochemical workstation using a three-electrode system. Platinum wires were used for both working and counter electrodes while Ag/Ag+ was used as reference electrode calibrated with ferrocene/ferrocenium

couple

(Fc/Fc+).

The

supporting

electrolyte

was

tetrabutylammoniumhexafluorophosphate (Bu4NPF6) solution (0.1 M) in anhydrous acetonitrile. Steady-state photoluminescence (PL) spectra were recorded with a Shamrock sr-303i-B spectrograph from Andor Tech., coupled to a Newton EMCCD Si array detector. PL spectra were excited using a Xe flash lamp at a wavelength of 540 nm. Atomic force microscopy (AFM) images were recorded in tapping mode using an NT-MDT NTEGRA Prima scanning probe microscope in the AEK-2002 acoustic enclosure and NT-MDT NSG01 AFM tips with resonant frequencies of ca. 200 kHz. PSC fabrication and characterization The device structure of the all-PSCs was glass/ITO/PEDOT:PSS/active layer/LiF/Al. As a buffer layer, PEDOT:PSS (Baytron P VP Al 4083) was spin-coated onto ITO-coated glass substrates, followed by annealing at 150 °C for 15 mins to remove water. The thickness of the PEDOT:PSS



layer was around 40 nm, as determined by a Dektak 6 M surface profilometer. The active layer, consisting of a blend of a donor polymer and an acceptor polymer, was spin-coated from oDCB solution onto the PEDOT:PSS layer. The concentration of the solution was 12 mg/mL. The thickness of the active layer was determined by a Dektak 6 M surface profilometer. The prepared active layers were directly transferred to a vapor deposition system mounted inside a glove box. LiF (0.6 nm) and Al (90 nm) were used as the top electrodes and deposited via a mask under vacuum onto the active layer. The accurate area of each all-PSC was 4.6 mm2, which was defined by the overlap of the ITO and metal electrode and measured by using a microscope. The device parameters were calculated from the J–V curves recorded by a Keithley 2400 source meter under the illumination of an AM 1.5G solar simulator with an intensity of 100 mW cm−2 (Model SS50A, Photo Emission Tech., Inc.). The light intensity was determined using a standard silicon photodiode. For the EQE measurements, the currents were recorded by using a Keithley 485 picoammeter under monochromatic light (MS257) illumination through the ITO side of the allPSCs. The current was recorded as the voltage over a 50 Ω resistance and was converted to EQE profile by comparing the data with a calibrated silicon reference cell.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Synthesis and characterization of monomers and polymers, TGA and DSC measurements, temperature-dependent absorption, electrochemical measurements, hole and electron mobilities. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] 18 ACS Paragon Plus Environment

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* E-mail: [email protected] Author Contributions ┴

B. A. Abdulahi and X. Xu contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS E. Wang thanks the Swedish Research Council, the Swedish Research Council Formas, Chalmers Area of Advance Energy and Graphene Center, The Knut and Alice Wallenberg Foundation and National Natural Science Foundation of China (21728401) for financial support. W. Mammo and B. A. Abdulahi acknowledge financial support from the International Science Programme (ISP), Uppsala University, Sweden. O. Inganäs and X. Xu acknowledge financial support from the Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant. P. Murto thanks the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 607585 (OSNIRO) for financial support.

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