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Supporting Information to

Initiator free crosslinking of oxetane functionalized low bandgap polymers: An approach towards stabilized bulk heterojunction

solar cells

Philipp Knauer1, Tobias Hahn2, Anna Köhler2,3, Peter Strohriegl1,3*

1Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, Germany2Experimental Physics II, Department of Physics, University of Bayreuth, Germany3Bayreuth Institute of Macromolecular Science (BIMF), University of Bayreuth, 95440 Bayreuth, Germany

*Corresponding author: peter.strohriegl@uni-bayreuth.de

A. Cationic ring-opening polymerization of oxetane

RO

O+ H

RO

OH

+

RO

O

RO

OH

RO

OA

A A

R OO

+ n

OO

RORO

nA

RO

OH

R =

S SN

SN

O

O

n

H13C6 C6H13

Figure S 1. Mechanism of the cationic ring-opening polymerization of oxetane after reference 1.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2016

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B. Crosslinking in the presence of PCBM

The crosslinkable polymer PFDTBTOx and the fullerene derivative PCBM (1:2 w/w, 30 mg ml-1) were

mixed in a solution of chlorobenzene. After spin coating and drying, absorption spectra were

recorded. In an inert argon atmosphere films were exposed to TFA at 100 °C followed by storage in

vacuum at 60 °C for 30 minutes. To estimate the crosslinking efficiency, the optical densities before

and after rinsing with solvent were compared.

The absorption spectra are shown in Figure S 2. In the range of 280 to 400 nm most of the optical

density is lost after rinsing. However, the remaining absorption spectrum matches the spectrum of

neat PFDTBTOx. The low bandgap polymer is crosslinked successfully, remaining as insoluble layer.

All the loss of optical density during rinsing is assigned to PCBM. The low-molar mass acceptor is not

incorporated into the polymer network by chemical bonds. PCBM is easily washed out of the thin

samples, leaving behind the polymer scaffold.

300 400 500 600 700 800

PFDTBTOx:PCBM 1:2 after rinsing

1.4

1.2

1.0

0.8

0.6

0.4

0.2

optic

al de

nsity

wavelength /nm

0.0

300 400 500 600 700 800

PFDTBTOx:PCBM 1:2 after rinsing

1.4

1.2

1.0

0.8

0.6

0.4

0.2

optic

al de

nsity

wavelength /nm

0.0

reference PFDTBTOx

Figure S 2. Crosslinking of PFDTBTOx in a blend with PCBM. The sample was exposed to TFA vapor at 100 °C for five minutes. The reference spectrum of PFDTBTOx was normalized to the maximum at 530 nm.

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C. Accelerated aging

Blends of PFDTBTOx:PCBM and PFDTBT:PCBM were cast on glass slides. The films were annealed at

100 °C under inert atmosphere for up to 100 hours. By polarization microscopy the samples were

checked for PCBM aggregates.

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PFDTBTOx:PCBM 1:2 crosslinked in TFA vapor

as cast

after 100 h at 100 °C

Figure S 3. Optical micrographs of a PFDTBTOx:PCBM 1:2 blend. The films were crosslinked in TFA vapor prior to annealing at 100 °C for 100 hours.

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PFDTBTOx:PCBM 1:2

as cast

after 100 h at 100 °C

Figure S 4. Optical micrographs of a PFDTBTOx:PCBM 1:2 blend.

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PFDTBT:PCBM 1:2

as cast

after 100 h at 100 °C

Figure S 5. Optical micrographs of a PFDTBT:PCBM 1:2 blend.

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D. Accelerated aging of BHJ solar cells

The current-voltage (J-V) characteristics of the solar cells were measured for every single interval of

annealing. Figure S 6 shows the J-V curves.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

-1.5

-1.0

-0.5

0.0

0h 15min 1h 8h 30h 100h

PFDTBTOx (TFA crosslinked)

curre

nt de

nsity

/mA

cm-2

Voltage /V

a)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0-2.0

-1.5

-1.0

-0.5

0.0

0h 15min 1h 8h 30h 100h

PFDTBTOx (thermally crosslinked)

curre

nt de

nsity

/mA

cm-2

voltage /V

b)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2.0

-1.5

-1.0

-0.5

0.0

PFDTBT (non-crosslinked)

0h 15min 1h 8h 30h 100h

curre

nt de

nsity

/mA

cm-2

voltage /V

c)

Figure S 6. J-V characteristics from accelerated aging tests of the TFA crosslinked PFDTBTOx (a), thermally crosslinked PFDTBTOx (b), and non-crosslinkable PFDTBT (c) in a polymer:PCBM blend with a 1:2 ratio. For each material combination four solar cells were measured.

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0 10 20 30 40 50 60 700.00.10.20.30.40.50.60.70.80.9

PFDTBTOx (x-linked) PFDTBTOx PFDTBT

PCE (

%)

annealing time at 100 °C /min

a)

0 20 40 60 80 1000.00.10.20.30.40.50.60.70.80.9c)

PFDTBTOx (x-linked) PFDTBTOx PFDTBT

PCE (

%)

annealing time at 100 °C /h

0 10 20 30 40 50 60 700.00.20.40.60.81.01.21.41.61.82.0

PFDTBTOx (x-linked) PFDTBTOx PFDTBT

PCE (

norm

alize

d)

annealing time at 100 °C /min

b)

0 20 40 60 80 1000.00.20.40.60.81.01.21.41.61.82.0

PFDTBTOx (x-linked) PFDTBTOx PFDTBT

PCE (

norm

alize

d)

annealing time at 100 °C /h

d)

60 minutes at 100 °C

100 hours at 100 °C

Figure S 7. Development of the PCE for the TFA crosslinked PFDTBTOx, thermally crosslinked PFDTBTOx and non-crosslinkable PFDTBT in a 1:2 blend ratio with PCBM. At the top the PCE during the first 60 minutes of annealing at 100 °C is displayed. At the bottom the development for up to 100 hours at 100 °C is shown. Absolute PCE values are shown on the left (a, c). On the right the relative PCE normalized to the starting value is displayed (b, d). For each system four different cells were measured.

References1 M. C. Gather et al., Adv. Funct. Mater., 2007, 17, 191–200.