SSRL paper draft10 SI final

1

Supporting Information

Surface Characterization of Polythiophene:Fullerene Blends on

Different Electrodes using Near Edge X-ray Absorption Fine

Structure

Andreas F. Tillack†, Kevin M. Noone†, Bradley A. MacLeod†, Dennis Nordlund

‡, Kenneth P. Nagle

§,

Joseph A. Bradley§, Steven K. Hau**

, Hin-Lap Yip**††

, Alex K.-Y. Jen**††, Gerald T. Seidler§, David

S. Ginger*†

Contents:

SI1: Data normalization and PCBM density determination

SI2: Fit model and error estimation

SI3: Fullerene investigation

SI4: Beam damage evaluation

† Department of Chemistry, University of Washington, Seattle, Washington 98195-1700

‡ Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road MS69, Menlo Park, CA 94025

§ Department of Physics, University of Washington, Seattle, Washington 98195- 1560

** Department of Materials Science and Engineering, University of Washington, Seattle,

Washington 98195-2120

†† Institute of Advanced Materials and Technology, University of Washington, Seattle,

Washington 98195-2120

2

Supplementary Information 1: Data normalization and PCBM density

determination

Data normalization: A common way to correct for experimental variations (i.e. beam drift, sample

alignment) is to normalize spectra of chemically similar species (in our case Carbon based polymers) to

a constant height in the region above the ionization continuum (>330 eV).1 If the effective sampling

depth of the measurement technique is roughly constant across samples in the study, the recorded

polymer spectra treated this way are set to a fixed Carbon number density. We follow the same

procedure by normalizing our spectra in the region 308-309.6 eV. The normalization region was chosen

so that all of our recorded spectra are normalized identically because most spectra were recorded up to

only 310 eV (with control measurements up to 370 eV) in order to conserve measurement time.

PCBM density: In order to extract composition ratios from the linear decomposition of the measured

NEXAFS spectra onto the P3HT and PCBM reference spectra, one must know the mass densities (and

hence carbon densities) of each material. Erwin et al2 determined the density of P3HT using atomic

force microscopy and Rutherford backscattering spectroscopy to be ρP3HT=1.33±0.07 g/cm3. The Carbon

number density n can be expressed as:

ρ/MW

Nn C= (1)

Here, CN is the number of Carbon atoms per molecule, MW its molecular weight and ρ the bulk

density. The Carbon number density ratio PCBMHTP nn 3=γ in conjunction with the density of P3HT can

then be used to determine the density of the PCBM. We find a Carbon number density ratio

01.070.0 ±=γ , calculated as the ratio of the measured average NEXAFS intensities of reference TEY

P3HT and PCBM spectra in the region above 330 eV (ionization continuum). Using this ratio we obtain

for the density of PCBM:

308.041.1)3()(

)(

)3( 3

cm

gHTP

C

C

PCBMHTPMW

PCBMMW

PCBMN

HTPN±==

γ

ρρ (2)

3

Using molecular weights of mol

g3.170 , and

mol

g8.910 for P3HT (per repeat) and PCBM, respectively.

This density corresponds to a volume per PCBM molecule of 3Å)601070( ± which is in good

agreement with theoretical predictions for the crystal structure of PCBM previously reported.3,4

Supplementary Information 2: Fit model and error estimation

Fit model: The blend spectrum at a given energy, )(EBlend , is modeled as a linear combination of

P3HT and PCBM spectra:

)()(3)( EPCBMbEHTPaEBlend ⋅+⋅= (3)

Where a and b are fit coefficients, and )(3 EHTP and )(EPCBM are the respective component

spectra at a given energy. To be able to report true volume percentages one has to account for the

different Carbon number densities of P3HT and PCBM as mentioned above. We determined the

reference normalization intensity ratio 'γ , calculated as the ratio of average intensities of P3HT and

PCBM spectra over the same region used for data normalization (308 to 309.6 eV), of a set of reference

P3HT and PCBM spectra on four different samples each under ideal measurement conditions. While

this ratio is calculated similarly to the Carbon number density ratio defined above, the spectral region

over which the ratio is calculated still contains the trailing edges of resonance features of P3HT and

PCBM. It is expected to be slightly larger because PCBM trails off at lower energies than P3HT. We

find 03.079.0' ±=AEYγ and 02.081.0' ±=TEYγ for AEY and TEY measurements respectively. Then:

⋅+⋅−⋅

+−= )(

')(3)1(

')1()( EPCBMEHTPEBlend

γ

ββ

γββ

α (4)

Here, α is a scaling factor, β the PCBM volume fraction, and 'γ the reference P3HT to PCBM

normalization ratio. By comparing (3) and (4) the PCBM volume fraction can be obtained from the

spectral fractions determined by the fits:

4

γ

γβα

⋅+

⋅=+=

ba

bba ; (5)

In both AEY and TEY compositional fits, shown in Figures 1 and 2 of the main text, the scaling factor

α is close to 1.0, typically varying by less than 3% indicating very good agreement between theoretical

model and experiment.

Error estimation: Least squares fits were performed to the average blend spectra shown in

Supplementary Figures 1 and 2. The standard deviations of these spectra enable the calculation of the

reduced chi-squared value 2redχ as a goodness-of-fit estimator:

∑=

−=

N

i C

ii

red

i

EBlendC

DOF 12

22 ))((1

σχ (6)

Here, DOF is the number of degrees of freedom (number of data points minus number of parameters),

while iC and iCσ represent the average Blend spectrum at a given photon energy iE and its standard

deviation, respectively. As presented in the main paper in Figures 1 and 2 2redχ for the fits in this report

are typically slightly larger than 1 although a good match between model and data is observed based on

the residuals. This suggests that the standard deviations of the blend spectra alone are not sufficient to

describe the overall error in the measurement. In addition, the asymptotic standard errors calculated by

the fitting routine for the fractions a and b in the fit function (3) are likely to underestimate the errors

of these parameters because the standard deviations of the component spectra (as part of the model

function) are not evaluated during the fitting routine’s error estimation.

Thus, in order to estimate better errors for the obtained parameters a and b the reduced chi-squared

2redχ value was calculated in the parameter space around their best-fit values. According to Lampton et

al5, for a two-parameter fit the contour with a change in the reduced chi-square value of 61.42 =∆ redχ

encloses the region in parameter space which corresponds to 90% confidence ( 3.22 =∆ redχ for 68%

5

confidence), independent of the original best-fit value of 2redχ . The changes relative to the best-fit value

of the reduced chi-square values are plotted in Supplementary Figures 3 and 4 along with contour plots

to examine the confidence areas in parameter space. To determine individual confidence intervals for the

fit parameters we use the extrema of the 90% (68%) confidence contour.

The resulting errors along with the fit parameters a and b for the fits used in this report are listed in

Supplementary Table 1. The fit parameters are then used in equations (5) to calculate the resulting

properties α , a scaling factor, and β , the PCBM volume fraction. The scaling factor α should be equal

to one within the error if the model describes the data well and the error is realistic. According to the

errors calculated using error propagation of equation (5) this is the case.

a a error b b error α α error β in % β error in %

ITO unannealed AEY 0.948 0.045

(0.032) 0.051

0.043

(0.031) 0.999

0.062

(0.044) 4.1

3.3

(2.4)

ITO unannealed TEY 0.777 0.020

(0.014) 0.188

0.020

(0.019) 0.965

0.028

(0.020) 16.3

1.5

(1.1)

ITO annealed AEY 0.951 0.044

(0.031) 0.049

0.041

(0.029) 1.001

0.060

(0.042) 3.9

3.1

(2.2)

ITO annealed TEY 0.823 0.017

(0.012) 0.160

0.016

(0.011) 0.984

0.023

(0.016) 13.6

1.2

(0.9)

ZnO annealed AEY 0.952 0.027

(0.019) 0.048

0.036

(0.026) 1.000

0.045

(0.032) 3.8

2.7

(2.0)

ZnO annealed TEY 0.858 0.011

(0.008) 0.144

0.010

(0.007) 1.002

0.015

(0.010) 11.9

0.7

(0.5)

ZnO & C60 annealed AEY 0.951 0.026

(0.019) 0.047

0.023

(0.016) 0.999

0.035

(0.025) 3.8

1.8

(1.3)

ZnO & C60 annealed TEY 0.858 0.020

(0.014) 0.137

0.021

(0.015) 0.995

0.029

(0.020) 11.4

1.6

(1.1)

Supplementary Table 1: Fit fractions with resulting model parameters from equation (5), errors are for

90% (68%) confidence interval calculated as described

6

0

0.5

1

1.5

2

2.5

3

-0.08-0.04 0 0.04 0.08

0

0.5

1

1.5

2

2.5

3

-0.15-0.1-0.05 0 0.05 0.1

inte

nsity [a

.u.]

a) ITO/PEDOT:PSS/P3HT(1):PCBM(0.95)

unannealed

b) ITO/PEDOT:PSS/P3HT(1):PCBM(0.95)

annealed

inte

nsity [

a.u

.]

AEY AEYStandard Deviation Standard Deviation

0

0.5

1

1.5

2

2.5

3

280 285 290 295 300 305 310-0.06-0.04-0.02 0 0.02 0.04 0.06

energy [eV]

Standard Deviation

c) ITO/ZnO/P3HT(1):PCBM(0.6)

AEY 0

0.5

1

1.5

2

2.5

3

280 285 290 295 300 305 310

-0.04

0

0.04

0.08

energy [eV]

Standard Deviation

d) ITO/ZnO/C /P3HT(1):PCBM(0.6)60

AEY

Supplementary Figure 1: Average AEY blend spectra with residuals for individual spectra used to calculate the average and standard deviation (grey envelope)

0

0.5

1

1.5

2

2.5

3

-0.04 0 0.04 0.08Standard Deviation

0

0.5

1

1.5

2

2.5

3

-0.08-0.04 0 0.04 0.08Standard Deviation


unannealed


annealed

TEY TEY

inte

nsity [

a.u

.]in

tensity [a

.u.]

0

0.5

1

1.5

2

2.5

3

280 285 290 295 300 305 310-0.02-0.01 0 0.01 0.02 0.03

energy [eV]

Standard Deviation

c) ITO/ZnO/P3HT(1):PCBM(0.6)

TEY 0

0.5

1

1.5

2

2.5

3

280 285 290 295 300 305 310-0.08-0.04 0 0.04 0.08

energy [eV]

Standard Deviation


TEY

Supplementary Figure 2: Average TEY blend spectra with residuals for individual spectra used to calculate the average and standard deviation (grey envelope)

7


AEY

annealed

-100

-50

0

50

100

-4 -2 0 2 4

b d

evia

tion [perc

ent]

a deviation [percent]

0

50

100

150

200

250

300

350

-80

-60

-40

-20

0

20

40

60

80

-4 -2 0 2 4

b d

evia

tion [perc

ent]


0

50

100

150

200

250

300

350

400


AEY

unannealed

-100

-50

0

50

100

-4 -3 -2 -1 0 1 2 3 4

b d

evia

tion [perc

ent]


0

100

200

300

400

500

600c) ITO/ZnO/P3HT(1):PCBM(0.6)

AEY -80

-60

-40

-20

0

20

40

60

80

-4 -3 -2 -1 0 1 2 3 4

b d

evia

tion [perc

ent]


0

50

100

150

200

250

300d) ITO/ZnO/C /P3HT(1):PCBM(0.6)60

AEY

68% confidence

90% confidence

Supplementary Figure 3: Change of the reduced chi-squared 2redχ value in parameter space (deviations

from best-fit value in percentage of value) for AEY component fits, dashed black lines indicate 5,4,3,2,12 =∆ redχ from the center ( 02 =∆ redχ ), solid red line encloses 90% confidence area

( 61.42 =∆ redχ ) and ( 02 =∆ redχ ), solid yellow line encloses 68% confidence area ( 3.22 =∆ redχ )

8


TEY-10

-5

0

5

10

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

b d

evia

tio

n [

pe

rce

nt]


0

50

100

150

200

250

300

350c) ITO/ZnO/P3HT(1):PCBM(0.6)

TEY -20

-15

-10

-5

0

5

10

15

20

-3 -2 -1 0 1 2 3

b d

evia

tio

n [

pe

rce

nt]


0

100

200

300

400

500

600

700

800

-15

-10

-5

0

5

10

15

-3 -2 -1 0 1 2 3

b d

evia

tio

n [

pe

rce

nt]


0

100

200

300

400

500

600


TEY

unannealed

68% confidence

90% confidence

-15

-10

-5

0

5

10

15

-3 -2 -1 0 1 2 3

b d

evia

tio

n [

pe

rce

nt]


0

50

100

150

200

250

300

350

400

450

500


TEY

annealed

Supplementary Figure 4: Change of the reduced chi-squared 2redχ value in parameter space for TEY

component fits, dashed black lines indicate 5,4,3,2,12 =∆ redχ from the center ( 02 =∆ redχ ), solid red line

encloses 90% confidence area ( 61.42 =∆ redχ ) and ( 02 =∆ redχ ), solid yellow line encloses 68%

confidence area ( 3.22 =∆ redχ )

Supplementary Information 3: Fullerene investigation

In order to judge the quality of our pristine PCBM samples and to determine the shape of “good” PCBM

spectra for our study we compare average AEY and TEY spectra of PCBM and treated PCBM

(Supplementary Figure 5) to C60. The C60 spectra recorded agree with previously reported spectra.6,7 The

treated PCBM sample was subjected to ambient conditions overnight and illuminated under a UV lamp

for four hours afterwards.

AEY and TEY spectra are in good qualitative agreement for the respective fullerene. In all fullerene

spectra we observe the typical four split π* resonances6,7 of the C60 fullerene around 284.5 eV, 285.8 eV,

9

286.3 eV, and 288.3 eV. Additionally, in all fullerene AEY spectra there is a resonance present around

286.8 eV (between π3* and π4*) which is not observed in the TEY spectra.

There are notable differences between the spectra of C60, PCBM, and treated (heavily oxidized)

PCBM. The σ* region (~290-308 eV) exhibits decreasing contrast in the following order: C60, PCBM,

and treated PCBM. The first three π* resonances decrease in intensity in the same order. Interesting to

note is that the peak to peak ratios of the second to third π* peak are opposite for C60 than for PCBM.

Also, the first and fourth π* resonances of C60 are slightly shifted towards lower energies compared with

both PCBM spectra.

Furthermore, in both AEY and TEY PCBM spectra there is an additional resonance visible at 285.0

eV. This peak does not appear in the C60 spectrum. At this energy a likely candidate is a C1s→π* C=C

resonance. The only additional source of C=C bonds in pristine PCBM is its phenyl group. Indeed it has

been observed that the first π* resonance of benzene-like groups is around 285.0 eV.8,9 The loss in

intensity of the first three π* resonances (peaks at 284.5 eV, 285.8 eV, and 286.3 eV) compared to the

pristine PCBM spectrum points to photoinduced damage of the fullerene cage. For pure C60 this was

reported by Itchkawitz et al.10

280 285 290 295 300 305 310

energy [eV]

AEY

TEY

C60PCBM

PCBM treated

1

2 34

C60

PCBM

PCBM treated

Supplementary Figure 5: Average AEY and TEY spectra of PCBM (red line), C60 (green line), and treated PCBM (blue line); the four π* resonances are labeled from 1-4

10

Supplementary Information 4: Beam damage evaluation

Beam damage studies were performed on P3HT(1):PCBM(0.95) blend, pure P3HT, and pure PCBM

samples on ITO as well as on gold evaporated on SiO2. We monitored spectral changes for

approximately two hours. During this timeframe, we saw changes in the spectral features of our

polymers. The overall change in resonance heights was approximately 10% for all samples during the

observed timeframe. The spectral changes typically evolved faster initially, with the largest change in

resonance heights of ~4% after the first measurement (~35 min, accumulated dose ~8 kJ/cm2).

For P3HT (green traces in Supplementary Figure 6), spectral changes were an increase of the π*(C=C)

resonance at 285.4 eV and a decrease of the σ*(C-H, C-S) resonance at 287.5 eV. This behavior, the

formation of C=C double bonds (leading to cross-linking) and dehydrogenation of C-H bonds has

previously been reported on alkanethiolate monolayers.11,12

PCBM (blue traces) exhibits a decrease of its π*(C=C) resonances evidencing damage to the fullerene

cage and an increase of the resonance around 285.0 eV. Smaller in magnitude, these changes are similar

to the ones observed between heavily oxidized and pristine PCBM films above.

Spectral changes of the P3HT(1):PCBM(0.95) blend (red traces) are predominantly of P3HT type

described before. It is interesting to note that the leading edge of the blend spectra’s first peak which

contain the first π*(C=C) resonance of PCBM diminishes in agreement with the beam damage of

PCBM.

Although we observed spectral changes due to beam damage the magnitude of these changes is

sufficiently small as not to significantly alter the conclusions concerning composition ratios. Each

measurement was undertaken on a new sample spot which has not been exposed to the beam before and

was multiple millimeters (beam spot size is approximately 0.1 mm x 1 mm) away from any previous

measurements on the same sample. In conclusion, because of the observed low beam damage, because

11

the samples and references were taken under similar exposure, and because of moving to a “fresh” spot

for each spectrum, we conclude that artifacts in composition ratios arising from beam damage are

negligible in the spectra used for the present report.

280 285 290 295 300 305 310energy [eV]

PCBM

P3HT

P3HT(1):PCBM(0.95)

AEY

PCBM

P3HT

P3HT(1):PCBM(0.95)

TEY

280 285 290 295 300 305 310energy [eV]

Supplementary Figure 6: AEY and TEY spectra revealing beam damage of PCBM, P3HT, and P3HT(1):PCBM(0.95) on gold substrate (arrows indicate peak progression with increasing exposure time)

References

(1) Stoehr, J. NEXAFS spectroscopy; Springer-Verlag: Berlin ; New York, 1992. (2) Erwin, M. M.; McBride, J.; Kadavanich, A. V.; Rosenthal, S. J. Thin Solid Films 2002, 409, 198-205. (3) Napoles-Duarte, J. M.; Reyes-Reyes, M.; Ricardo-Chavez, J. L.; Garibay-Alonso, R.; Lopez-Sandoval, R. Physical Review B 2008, 78, 035425. (4) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Chemical Communications 2003, 2116-2118. (5) Lampton, M.; Margon, B.; Bowyer, S. Astrophysical Journal 1976, 208, 177-190. (6) Terminello, L. J.; Shuh, D. K.; Himpsel, F. J.; Lapianosmith, D. A.; Stoehr, J.; Bethune, D. S.; Meijer, G. Chemical Physics Letters 1991, 182, 491-496. (7) Yi, L.; Agren, H.; Gelmukhanov, F.; Guo, J. H.; Skytt, P.; Wassdahl, N.; Nordgren, J. Physical Review B 1995, 52, 14479-14496. (8) Hitchcock, A. P.; Fischer, P.; Gedanken, A.; Robin, M. B. Journal of Physical Chemistry 1987, 91, 531-540. (9) Yang, M. X.; Xi, M.; Yuan, H. J.; Bent, B. E.; Stevens, P.; White, J. M. Surface Science 1995, 341, 9-18. (10) Itchkawitz, B. S.; Long, J. P.; Schedelniedrig, T.; Kabler, M. N.; Bradshaw, A. M.; Schlogl, R.; Hunter, W. R. Chemical Physics Letters 1995, 243, 211-216. (11) Mueller, H. U.; Zharnikov, M.; Voelkel, B.; Schertel, A.; Harder, P.; Grunze, M. Journal

of Physical Chemistry B 1998, 102, 7949-7959. (12) Zharnikov, M.; Geyer, W.; Golzhauser, A.; Frey, S.; Grunze, M. Physical Chemistry

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SSRL paper draft10 SI final