Supplementary Figure 1. (a) Thermogravimetric analysis (10 °C min-1) and (b) differential

scanning calorimetry (5 °C min-1) of EH-IDTBR and O-IDTBR measured under nitrogen.

Thermograms are offset vertically for clarity.

Supplementary Figure 2. Chemical structures and minimum energy conformations of (a)

FBR and (b) IDTBR calculated (with methyl replacing n-octyl or 2-ethylhexyl groups) using

Gaussian (B3LYP/6-31G*) to visualize the LUMO and HOMO distributions.

0 50 100 150 200 250

Hea

t Flo

w, E

ndo

up

Temperature (°C)

O-IDTBR EH-IDTBR

heating

cooling

100 200 300 400 500 60020

30

40

50

60

70

80

90

100

110

We

igh

t L

oss

(%

)

Temperature (°C)

EH-IDTBR O-IDTBR

a b

HOMO

LUMOLUMO

a" b"

S

S

R R

R RN SN

NSN

SN S N

S

O

O

S

R = n-octyl O-IDTBRR= 2-ethylhexyl EH-IDTBR

R R

NS N

NSNSN

S N

O

S

O

SR = n-octyl FBR

HOMO

Supplementary Figure 3. (a) Absorption coefficients α of EH-IDTBR in the thin film

compared with a selection of low bandgap donor polymers (structures shown in

Supplementary Figure 16), where α was calculated with the equation α =1/d*ln(1/T); (b) UV-

vis absorption spectra of O-IDTBR thin films spin-coated from 10 mg ml-1 chlorobenzene

solution, as-cast and with 10 min annealing at different temperatures.

Supplementary Figure 4: Normalised UV-vis absorption spectra of as-cast IDTBR thin

films compared with that of PC60BM.

400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

Ab

sorp

tion

Wavelength (nm)

as-cast 110 °C 120 °C 130 °C 140 °C

400 500 600 700 8000

1

2

3

4

5

PTB7 PCE-10 PCE-11 EH-IDTBR

Abs

orpt

ion

Coe

ffic

ient

(105 c

m-1)

Wavelength (nm)

a b

300 400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

Thi

n Fi

lm A

bsor

ptio

n

Wavelength (nm)

O-IDTBR EH-IDTBR PC60BM

Supplementary Figure 5: (a) J-V characteristics and (b) EQE spectra for IDTBR:P3HT

devices compared to reference PC60BM:P3HT device measured at 100 mW cm-2 illumination.

Supplementary Figure 6. J-V characteristics of O-IDTBR:P3HT devices with different

active areas measured under 100 mW cm-2 illumination.

a b

400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

EQ

E

Wavelength (nm)

EH-IDTBR:P3HT O-IDTBR:P3HT PC60BM:P3HT

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-15

-12

-9

-6

-3

0

3C

urr

ent D

ensi

ty (m

A c

m-2

)

Voltage (V)

EH-IDTBR:P3HT O-IDTBR:P3HT PC60BM:P3HT

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-18

-15

-12

-9

-6

-3

0

3

Cur

rent

Den

sity

(mA

cm

-2)

Voltage (V)

0.045 cm2

0.15 cm2

0.75 cm2

1.5 cm2

Supplementary Figure 7. Specular XRD of (a) O-IDTBR and (b) EH-IDTBR films as-cast

and with annealing at 110 °C and 130 °C.

Supplementary Figure 8. (a) Specular XRD of FBR films as-cast and with annealing at

110 °C and 130 °C; (b) DSC first heating cycles measured at 5 °C min-1 on drop-cast samples

of FBR, P3HT and P3HT:FBR (1:1). Thermograms are offset vertically for clarity.

5 10 15 20 25 300

1

2

Inte

nsi

ty (

103 c

oun

ts)

2θ (degrees)

O-IDTBR as cast O-IDTBR annealed 110 °C O-IDTBR annealed 130 °C

5 10 15 20 25 300

1

2

Inte

nsit

y (1

03 cou

nts

)

2θ (degrees)

EH-IDTBR as cast EH-IDTBR annealed 110 °C EH-IDTBR annealed 130 °C

5 10 15 20 25 300

1

2

Inte

nsi

ty (1

03 co

unts

)

2θ (degrees)

FBR as cast FBR annealed 110 °C FBR annealed 130 °C

50 100 150 200 250

Hea

t Flo

w, E

ndo

Up

Temperature (°C )

FBR P3HT:FBR (1:1) P3HT

a b

Supplementary Figure 9. Chi-Q plots of (top) EH-IDTBR and EH-IDTBR:P3HT blend

(bottom) O-IDTBR and O-IDTBR:P3HT blend, for which the diffraction intensity is

integrated in two ranges of Chi = 87.5-92.5˚ and 40-50˚ and plotted against the whole wave

vector Q.

Supplementary Figure 10. Line cuts from GIXRD chi-Q plots of (a) EH-IDTBR and EH-

IDTBR:P3HT blend at 87.5-92.5°; (b) O-IDTBR and O-IDTBR:P3HT blend at 87.5-92.5°;

(c) O-IDTBR and O-IDTBR:P3HT blend at 40-50°.

EH-IDTBR EH-IDTBR:P3HT

O-IDTBR:P3HT O-IDTBR

0.4 0.6 0.8 1.0 1.2 1.4 1.6

104

105

106

107

Inte

nsity

(a.u

.)

Q (Å- 1)

O-IDTBR O-IDTBR:P3HT

87.5-92.5°

0.4 0.6 0.8 1.0 1.2 1.4 1.6

104

105

106

107

Inte

nsity

(a.u

.)

Q (Å-1)

O-IDTBR O-IDTBR:P3HT

40-50°

0.4 0.6 0.8 1.0 1.2 1.4 1.6

104

105

106

107

108

87.5-92.5°

Inte

nsi

ty (a

.u.)

Q (Å- 1)

EH-IDTBR EH-IDTBR:P3HT

a b c

Supplementary Figure 11. Current-voltage characteristics of electron-only devices of

different thickness in log-lin representation for (a) O-IDTBR:P3HT and (b) EH-

IDTBR:P3HT blends and of hole-only devices of different thickness for (c) EH-

IDTBR:P3HT blends. The solid lines represent fits to the experimental data (open triangles in

(a) and (b) and open dots in (c)) according to the Mott-Gurney law assuming space charge-

limited currents in these devices. Note that for EH-IDTBR:P3HT the hole mobility is about

two orders higher than the electron mobility.

c"

b"

-1 0 1 2 3 4 5 6

10-1

100

101

102

103

Cur

rent

Den

sity

(mA

cm

-2)

Voltage (V)

100 nm: 2.7x10-4 cm2V-1s-1

130 nm: 3.0x10-4 cm2V-1s-1

170 nm: 6.8x10-4 cm2V-1s-1

ITO/PEDOT:PSS/EH-IDTBR:P3HT/Au

0 1 2 3 410-5

10-4

10-3

10-2

10-1

100

101

102

Cur

rent

Den

sity

(mA

cm

-2)

Voltage (V)

190 nm: 6.1x10-6 cm 2V-1s-1

160 nm: 5.8x10-6 cm 2V-1s-1

130 nm: 3.2x10-6 cm 2V-1s-1

ITO/PEDOT:PSS/Al/EH-IDTBR:P3HT/Al

0 1 2 3 4

10-2

10-1

100

101

190 nm: 2.1x10-6 cm 2V-1s-1

160 nm: 4.7x10-6 cm 2V-1s-1

130 nm: 3.4x10-6 cm 2V-1s-1Cur

rent

Den

sity

(mA

cm

-2)

Voltage (V)

ITO/PEDOT:PSS/Al/O-IDTBR:P3HT/Al

a"

Supplementary Figure 12: Photoluminescence spectra of EH-IDTBR, O-IDTBR and

annealed EH-IDTBR:P3HT (130 °C for 10 min) and O-IDTBR:P3HT (130 °C for 20 min)

blends excited at 680 nm. Note that P3HT does not absorb at this excitation wavelength. All

the spectra are corrected for film absorption.

Supplementary Figure 13. Femtosecond-transient absorption spectra of (a), (b) EH-

IDTBR:P3HT and (c), (d) O-IDTBR:P3HT blends and pristine counterparts excited at 680

nm with 2µJ cm-2 density in N2 atmosphere at (a), (c) 1 pico-second (exciton signature) and

(b), (d) 6 nano-second (polaron signature).

700 725 750 775 8000

1x107

2x107

3x107

4x107

5x107

6x107P

hot

olu

min

esce

nce

(a.u

.)

Wavelength (nm)

P3HT EH-IDTBR EH-IDTBR:P3HT

750 800 850 900 950 1000 1050 11000.0

0.5

1.0

1.5

2.0

2.5

Ph

otol

umin

esce

nce

(a.u

.)

Wavelength (nm)

P3HT O-IDTBR O-IDTBR:P3HT

a" b"

Supplementary Figure 14. Oxidative stability of O-IDTBR:P3HT devices (normalised PCE

values) compared with other high performace polymer:fullerene systems (polymer structures

shown). Devices were exposed to air over the course of 1200 hr.

Supplementary Figure 15. Optical microscopy of O-IDTBR:P3HT blends in comparison

with PC60BM:P3HT blends. Films were prepared on ITO/ZnO coated glass substrates

according to procedures for the device active layers, and then annealed for 1 h under inert

atmosphere.

0 50 100150200 800 12000.00.10.20.30.40.50.60.70.80.91.01.11.2

PC60BM:P3HT (1:1) O-IDTBR:P3HT (1:1) PC70BM:PTB7 (1:1.5) PC70BM:PCE-10 (1:1.5) PC70BM:PCE-11 (1:1.4)

N

orm

alis

ed P

CE

time in air (hours)

S

S

O

O

S

SF

OO

n

PTB7 S

S

S

SF

OO

n

PCE-10

S

S

NS

N

F FS

S

S

S

C10H21 C8H17

C10H21C8H17

n

PCE-11

S

C6H13

nP3HT

Supplementary Table 1. Optoelectronic properties of PC60BM as measured in this study.

ε [104 M-1 cm-1]a) λmax film [nm] b) Eg opt. [eV]b) EA [eV] c) IP [eV] d)

0.39 (400 nm) 333 2.05 4.10 6.15

Measurements were carried out in a) CHCl3 solution; b) thin film spin-coated from 10 mg ml-1

chlorobenzene solution; c) cyclic voltammetry carried out on the as-cast thin film with 0.1 M

TBAPF6 electrolyte in acetonitrile; d) estimated from the EA and the optical Eg.

Supplementary Table 2. Photovoltaic characteristics of PC60BM:P3HT reference devices

Jsc [mA cm-2] Voc [V] FF PCE [%]

PC60BM:P3HT 9.59 0.58 0.67 3.73

Supplementary Table 3. Photovoltaic performance of O-IDTBR:P3HT OPV devices tested for differerent active areas under 100 mW cm-2 illumination.

Area of Device Jsc [mA/cm2] Voc [V] FF PCE [%] 0.045 cm2 14.1 0.73 0.62 6.4 0.15 cm2 13.9 0.72 0.63 6.3 0.75 cm2 11.9 0.73 0.55 4.8 1.5 cm2 11.1 0.73 0.53 4.3

Supplementary Table 4: Device parameters for the cells used in photo-CELIV measurements measured under 100 mW cm-2 illumination.

Jsc [mA cm-2] Voc [V] FF PCE [%]

O-IDTBR:P3HT 13.5 0.73 0.60 5.91

EH-IDTBR:P3HT 12.0 0.76 0.61 5.56