The Contribution of Fullerene Photocurrent Generation to Organic

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C: Physical Processes in Nanomaterials and Nanostructures

The Contribution of Fullerene Photocurrent Generation to Organic Solar Cell Performance Nicolas C. Nicolaidis, Mohammed F Al-Mudhaffer, John Holdsworth, Xiaojing Zhou, Warwick J. Belcher, and Paul C. Dastoor J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01439 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

The Contribution of Fullerene Photocurrent Generation to Organic Solar Cell Performance Nicolas C. Nicolaidisa, Mohammed F. Al-Mudhaffera,b, John L. Holdswortha, Xiaojing Zhoua, Warwick J. Belchera and Paul C. Dastoora

a Centre

for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, Australia.

b Department

of Physics, College of Education for Pure Science, University of Basrah, Basrah, Iraq.

Abstract Light harvesting is the critical first step in the generation of photocurrent in solar photovoltaic devices. In the case of organic photovoltaic devices based on binary blends of conjugated polymer donors and fullerene acceptors, light harvesting can occur both at the conjugated polymer and in the fullerene components. As such, both elements in the blend contribute to the generation of photocurrent. In this paper, optical modelling is used to compare the relative light harvesting of the polymer and fullerene components for binary active layers consisting of P3HT:PC61BM, P3HT:PC71BM and P3HT:ICBA. We show that the relative contribution of light harvesting and photocurrent generation by the fullerene component is greater than previously considered and can rise to as high as 30 % of the total photocurrent generated by the binary blend system.

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Introduction Organic photovoltaic cells are widely predicted to play a significant part of any future energy economy, due to their inherent ability to provide low cost, flexible solutions for sustainable energy generation that can be readily integrated into a variety of products1-4. Typically, the best cells reported exhibit power conversion efficiencies of upwards of 10 %5,6. For devices containing active layers that comprise 1:1 blends of poly (3-hexylthiophene) (P3HT) and a fullerene electron, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) the efficiency limit is lower at

5%7,8.

The conventional explanation of the operation of these devices is that light absorption by the polymer generates an exciton9 which then diffuses for

10 nm whereupon if it encounters a donor-acceptor

interface the bound pair will split or else recombine10,11. Absorption occurs perpendicular to the conjugation backbone at energies determined by the Peierls distortion12. Organic semiconducting materials typically have optical absorption coefficients that are an order of magnitude higher than that of silicon13,14. When designing solar cells from less-absorbing materials, the active layer thickness is increased, typically above 100 µm, and light-trapping layers are incorporated to allow scattered light to increase the light absorption over longer path lengths. Wavelengths of light with lower material absorption cross-section may contribute if the apparent path length is sufficiently long15. By contrast, in highly-absorbing OPV devices, the layer thickness is typically 100 nm, which means that, although the exciton generation rate does not decay exponentially, optical interference effects can influence the amount of light that is absorbed. Although the fullerene’s dual role as an electron acceptor and the provider of a continuous electron transport network in polymer blend devices is well established, its function in primary charge generation is less well recognized16. Interestingly, one of the first investigations of photocurrent 2 ACS Paragon Plus Environment

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generation in organic photovoltaic bilayer solar cells was by Pettersson et al. who showed that the external quantum efficiency (EQE) spectra could only be accurately modelled by including a photocurrent contribution from light absorbed by the C60 layer17. Discussion around the fullerene contribution was subsequently marginalized as researchers focused primarily on the light absorption and charge generation of the polymer component in bilayer and bulk heterojunction organic solar cell architectures18–20. Subsequently, Dastoor et al. showed that for poly[2-methoxy-5-(2’-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV):fullerene blends there was a significant fullerene photocurrent contribution. For MEH-PPV and related polyphenylenevinylene polymers the optimal polymer:fullerene ratio is 1:4, and it was shown that for these devices the fullerene component contributed over half of the total generated photocurrent11. However, despite this work, reviews of device function have continued to focus only on the generation of excitons by light absorbed by the polymer component2123.

In the case of the P3HT:PCBM system, the role of the fullerene component in the photocurrent generation process is readily observed by the presence of a peak in the EQE spectrum between 340 – 350 nm. In addition, the effect of photocurrent generation by the fullerene component is commonly exploited by the use of C70-based fullerenes to enhance spectral sensitivity since the change from spherical to cylindrical symmetry splits formerly degenerate energy levels; leading to increased absorption in comparison with C60-based fullerenes8. Indeed, in light responsive organic field effect transistors based on C70, 80 – 90 % of the photocurrent is generated by hole transfer from the fullerene component (channel II) rather than electron transfer from the polymer donor component (channel I)24. Another path to enhancing device efficiency via the fullerene is using different functionalising groups to tune the bandgap25. However, again despite these observations, the role of charge generated by the



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fullerene component is almost universally neglected in discussions regarding the operation of polymerfullerene devices. In this paper, we characterise the optical absorption and charge generation of P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM bulk heterojunction devices. Using optical modelling, we demonstrate that there is a substantial contribution to the overall photocurrent from channel II charge transfer in all three cases. We show that up to 30 % of the photocurrent in these 1:1 blends can arise from charge generation by the fullerene component; highlighting the importance of channel II processes in the operation of OPV devices. Results and Discussion The reflectance and transmission spectra of pure films of P3HT and fullerene were used to obtain the dielectric function of the polymer and fullerene components separately as a sum of model oscillator functions, as previously reported26. Using this approach it is possible to identify the characteristic oscillators for the P3HT and fullerene components independently27. In order to determine the contributions of the polymer and fullerene components to the generated photocurrent, the optical absorption characteristics of the multilayered functional devices were modelled to obtain the true absorption spectra for the active layer inside the device29. Previously, the dielectric function for the PCBM component in the blend has been obtained by simply subtracting the subset of P3HT oscillators from the complete set of oscillators used to model the P3HT:PCBM dielectric function27. The assumption that the dielectric of the blend can be treated as a linear superposition of the polymer and fullerene components is reasonable since the coupling between the two components results in a relatively small change in absorption that is primarily located at the band edge28. However, for the P3HT:ICBA and P3HT:PC70BM blends there are regions of significant overlap between the oscillators 4 ACS Paragon Plus Environment

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associated with the polymer and fullerene components, which can result in one component being over represented in comparison to the other. For example, the longer decay tail into the visible in the ICBA case, and degeneracy splitting in the PC70BM case, both result in significant overlap in the absorption of the fullerene and polymer components of the imaginary part of the blend dielectric function. In order to deconvolute the blend dielectric function better, additional spectral ellipsometric data was collected for the P3HT:ICBA and P3HT:PC70BM materials. However, spectral ellipsometry is a reflection based technique and thus is surface sensitive30. As such, the spectral ellipsometric data is less sensitive to the detailed absorption at the band edge, which primarily manifests in transmission. In particular, an issue that can arise when determining the optical constants by only using reflection at non normal incidence is that the band edge is simply not observed. Typically, models generated in this manner have a longer roll off at the band edge than observed experimentally; potentially creating a disconnect between the optical model and the predominate manner in which the films are measured (i.e. near normal incidence transmission) and their application (i.e. near normal incidence reflection). By constraining the material model across near and non normal incidence, a model that is comparable to the typical measurement, intended application and existing optical model can be achieved. The model obtained is therefore a compromise between the reflection and ellipsometry measurements with respect to the dielectric background. As a consequence, there is a band edge discrepancy in ICBA and PC70BM data that arises from a mismatch of the model with the spectral ellipsometry data. However, this mismatch is restricted to the region above the band edge and the model fit to the spectral ellipsometry data is excellent in the 300 – 600 nm region. To determine the dielectric function, a baseline of 10 % of the P3HT

– *

transition is first removed to allow the polymer region to have a high energy tail similar to its pure dielectric spectra. Subsequently, the pure fullerene dielectric function was fitted to the blend dielectric function in the fullerene-dominant 300 – 400 nm region using non-linear least squares fitting. We find 5 ACS Paragon Plus Environment


that the order of fitting fullerene first and then subsequently fitting the polymer dielectric function is optimal, since the long-chain P3HT is considerably more optically complicated in comparison to the small molecule fullerenes. The remainder of the spectrum was then assigned to P3HT, which is smoothed in the PCBM dominated region as there are no oscillators present in this part of the optical spectrum for the pristine polymer, and the fullerene component is recalculated.

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0.0 400

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Figure 1: Imaginary component of the dielectric functions and deconvolutions for P3HT:fullerene active layers. (a) P3HT:PC60BM, (b) P3HT:ICBA and (c) P3HT:PC70BM. All films are 85 nm thick and annealed as described in the experimental section. The upper (dash-dot) and lower (solid) lines show the real ( 1) and imaginary ( 2) parts respectively of the P3HT:fullerene dielectric function. The blue dash line and the red dotted line represent the deconvoluted fullerene (

F 2

) and P3HT (

P 2

) components of the

imaginary part of the complex dielectric function. The fullerene component absorption in the PC70BM is enhanced due the change in energy levels, compared to the more symmetric PC60BM.


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The calculated dielectric functions for the three P3HT:fullerene active layers are shown in Figure 1. The spectra reveal an increasing contribution of the fullerene component to the imaginary part of the dielectric function in the 400 – 700 nm region of the spectrum in going from PC60BM to ICBA to PC70BM. Below 400 nm the absorption due to PC60BM and PC70BM is comparable with ICBA absorption being the greatest. For wavelengths above 700 nm, absorption due to the P3HT:PC60BM blend is lowest, whereas that arising from the P3HT:PC70BM and P3HT:ICBA blends are slightly higher and about the same as each other. Thus, on the basis of these spectra, the absorption of the polymer and fullerene components in the blends are essentially complementary in nature, except for the PC70BM where there is significant competitive absorption in the 400 – 700 nm region. The modelling data is in good agreement with previous studies of the photoabsorption characteristics of these blend materials. The P3HT:ICBA blend exhibits a slight hypsochromic shift in

max

(400 – 700 nm) compared to P3HT:PC60BM

in agreement with previous studies25. By contrast, the P3HT:PC70BM shows little shift in

max

but there

is increased photon absorption in the 400 – 700 nm region as previously reported31. 2

Using the electric field distribution ( E z ) obtained from the model for the P3HT:fullerene device structure, together with the imaginary parts of the dielectric functions for the P3HT ( (

F 2

P 2

) and fullerene

) components, allows the absorption distributions to be calculated for the individual components

since the absorption (Qi) of the ith component is simply given by: Qi

where

0

1 2

0

is the permittivity of free space,

component and

i 2

Ez

i 2

2

(1)

is the imaginary parts of the dielectric function for the ith

is the angular frequency of the light.



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Figure 2 shows the calculated spatial distribution of light absorbed by the P3HT and fullerene components in the P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM 85 nm active layer blends, within the complete device architecture, as a function of wavelength. The device structure is complicated having a reflective back electrode and thus a higher potential for interference within the device architecture. As such, the modelled absorption in the layer is assigned according to the partition in Figure 1; ensuring that any absorption enhancement or changes are assigned to the materials most likely to responsible for the absorption at that wavelength. As expected, the active layer absorption distribution for the 85 nm thick P3HT:PC60BM active layer exhibits a double mode structure consisting of: (1) a P3HTdominated (450 – 600 nm) surface mode that occurs within the first 50 nm from the transparent electrode (top) interface, and (2) a weak PC60BM-dominated (300 – 350 nm) bulk mode that occurs at depths greater than 50 nm from the top interface27. For the P3HT:ICBA system, a similar double mode structure is preserved but, in this case, the surface mode is deeper and the peak is shifted to lower wavelengths. In addition, there is a relative increase in the magnitude of the fullerene bulk modes. By contrast, for the P3HT:PC70BM system the absorption is peaked much closer to the top electrode for both modes, with the fullerene mode again being stronger than that observed for PCBM.


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The effect of light interference effects in the active layer is illustrated most dramatically in Figure 3, which shows the corresponding light absorption maps for the 200 nm thick P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM blends. In particular, for all three blend systems there is a strong surface mode and weaker bulk mode for both the polymer and fullerene components. The shape and position of the surface and bulk modes are quite similar for all of the blends. Consequently, there is a pronounced absorption minimum in all of the active layers, which covers the entire wavelength range. The position of this minimum does not change greatly with blend composition; lying at a depth of 75 nm from the top interface of the active layer. Clearly, for these architectures, the Ez term in Equation 1 has a significant effect on the absorption distribution within the active layer; highlighting the potential risk of simply relying on UV-vis data of the blend film for the actual device active layer absorption.


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Figure 4 shows the light absorption as a function of wavelength of the P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM blend devices for active layer thicknesses of 85 nm and 200 nm. The absorption spectra were obtained by integrating the absorption maps shown in Figure 2 and Figure 3 with respect to device thickness across the active layer. Figure 4 also includes the imaginary component of the dielectric function (absorption profile) for the corresponding blend materials alone (i.e. not in the full device architecture). The spectra demonstrate that for all of the active layer blends measured here the incorporation of the active layer into the device architecture results in a new absorption feature at around 400 nm. In the case of P3HT:PC60BM, this feature is shared between the two components, whereas for the P3HT:ICBA and P3HT:PC70BM blend devices the new absorption feature occurs primarily in the fullerene component. The feature is not correlated with a physical transition in P3HT or PCBM but is observed to shift to lower wavelength by 5 – 20 nm as the modeled thickness is increased from 85 to 200 nm. As such, we deduce that the feature must arise from an interference effect due to the overlap of incident and reflected partial waves in the device.


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Figure 5: EQE spectra as a function of wavelength for P3HT:fullerene devices with different active layer thicknesses. (a) P3HT:PC60BM, (b) P3HT:ICBA and (c) P3HT:PC70BM active layers with an overall thickness of 85 nm. (d) P3HT:PC60BM, (e) P3HT:ICBA and (f) P3HT:PC70BM active layers with an overall thickness of 200 nm. The solid line represents the total EQE spectrum. The blue dash line and the red dotted line represent the deconvoluted fullerene and P3HT components of the EQE spectrum.


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Figure 6:

2

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(grey solid line), device active layer absorption (dashed line) and EQE (black solid line)

spectra as a function of wavelength for P3HT:fullerene devices with different active layer thicknesses. (a) P3HT:PC60BM, (b) P3HT:ICBA and (c) P3HT:PC70BM active layers with an overall thickness of 85 nm. (d) P3HT:PC60BM, (e) P3HT:ICBA and (f) P3HT:PC70BM active layers with an overall thickness of 200 nm. All spectra have been normalized to the maximum value in the 400 – 700 nm region. The vertical red line shows the position of the device active layer absorption feature at

400nm.

Figure 6 compares the shapes of the normalized blend absorption spectra ( 2), device active layer absorption and EQE) spectra for P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM blend devices for active layer thicknesses of 85 nm and 200 nm. Focusing on the 400 – 600 nm region, the EQE spectra are broadened at low wavelengths, particularly in the region around

400 nm where there is reduced

absorption (close to a minimum) in the blend absorption spectra ( 2). By contrast, the device active layer absorption spectra shows an enhanced maximum in this region, which is not present in the blend absorption spectra, as highlighted by the red vertical lines in Figure 6. The shape of the EQE in this part of the spectrum lies intermediate between the profiles of the

2

and device absorption spectra,

indicating that the broadening of the EQE spectrum arises from the enhanced absorption due to interference effects in the device architecture. For the PC60BM blend, the absorption enhancement at 400 nm is partitioned equally between the P3HT and fullerene components and exhibits the greatest enhancement to EQE. On the other hand, for the ICBA and PC70BM blends the absorption enhancement arises almost entirely from the fullerene component and the EQE enhancement is much lower. On the basis of these observations, it is clear that there is an enhancement in the photocurrent production around 400 nm due to interference effects in these P3HT:fullerene devices that is not apparent in the absorption spectra of the material blends alone. However, this enhancement is not as 16 ACS Paragon Plus Environment

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great as predicted by the modelled active layer absorption within the device. Hence, we can infer that excitons generated on the fullerene component in these devices are not fully harvested. Indeed, this inference is supported by the observation that the greatest enhancement to the photocurrent generated in the 400 nm region occurs in the P3HT:PC60BM blend, where the absorption of both components are comparable at 400 nm. Given that the charge transport matrix is common to charges generated via both channel 1 and channel 2 pathways, we conclude that the decreased harvesting of fullerene excitons must arise from reduced charge generation; suggestive of morphologies (e.g. aggregates32,33) within the active layer where separation of fullerene based excitons is hindered.


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AM1.5 Current Spectrum (mA/cm2/ m)

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Figure 7: Contribution of P3HT and PCBM to the photocurrent generated under incident AM1.5 solar radiation for P3HT:fullerene devices with different active layer thicknesses. (a) P3HT:PC60BM, (b) P3HT:ICBA and (c) P3HT:PC70BM active layers with an overall thickness of 85 nm. (d) P3HT:PC60BM, (e) P3HT:ICBA and (f) P3HT:PC70BM active layers with an overall thickness of 200 nm. The solid line represents the total AM1.5 spectrum. The blue dash line and the red dotted line represent the deconvoluted fullerene and P3HT components of the AM1.5 spectrum.


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Figure 7 plots the contribution of the P3HT and fullerene components to the total photocurrent generated under incident AM1.5 solar radiation for the two different active layer thicknesses. These data were calculated by convoluting the calculated EQE spectra with the AM1.5 spectrum and show that the contribution to the total photocurrent from the fullerene component in all three cases is significant, especially at wavelengths below 500 nm. Integrating under the graphs in Figure 7 provides a measure of the total photocurrent that would be generated by the polymer and fullerene components under AM1.5 conditions and the results are summarized in Table 1 and Table 2.

In agreement with previous measurements27, the total photocurrent generated by the devices varies with active layer thickness. For the P3HT:PC60BM and P3HT:PC70BM devices, the 200 nm thick devices produce a greater total current density (9.8 mA/cm2 and 7.0 mA/cm2) than the 85 nm thick devices (6.8 mA/cm2 and 5.0 mA/cm2). By contrast, the photocurrent produced by the 200 nm thick P3HT:ICBA device (3.4 mA/cm2) is much lower than that generated by the 85 nm thick device (5.8 mA/cm2). The calculated short circuit current densities are in good agreement with the corresponding measured short circuit current densities under AM1.5 conditions (average error ~ ± 6 %). Additionally, it is possible to calculate the fraction of the current density that is generated by the polymer and fullerene components in the blend under AM1.5 illumination for each of the active layer thicknesses. As shown in Table 1 and Table 2, the photocurrent contribution from the fullerene component ranges from 14 – 32 % of the total current density generated by the device across both thicknesses.



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Table 1: Comparison of device characteristics for P3HT:fullerene devices with an active layer thickness of 85 nm.

Parameter

P3HT:PC60BM P3HT:ICBA P3HT:PC70BM

Open circuit voltage (Voc)

(V)

0.58

0.78

0.57

(mA cm-2)

6.8

5.8

5.0

0.51

0.58

0.58

(%)

2.0

2.6

1.7

P3HT AM1.5 current density

(mA cm-2)

5.5

4.1

4.4

Fullerene AM1.5 current density

(mA cm-2)

1.0

1.6

2.0

Percent P3HT AM1.5 current density

(%)

85

72

69

Percent fullerene AM1.5 current density

(%)

15

28

31

Short circuit current density (Jsc) Fill factor (FF) Power conversion efficiency (PCE)

Table 2: Comparison of device characteristics for P3HT:fullerene devices with an active layer thickness of 200 nm.

Parameter

P3HT:PC60BM P3HT:ICBA P3HT:PC70BM

Open circuit voltage (Voc)

(V)

0.58

0.72

0.54

(mA cm-2)

9.8

6.3

7.0

0.52

0.51

0.62

(%)

3.0

2.31

2.35

P3HT AM1.5 current density

(mA cm-2)

7.9

4.5

4.2

Fullerene AM1.5 current density

(mA cm-2)

1.3

1.8

2.0

Percent P3HT AM1.5 current density

(%)

86

71

68

Percent fullerene AM1.5 current density

(%)

14

29

32

Short circuit current density (Jsc) Fill factor (FF) Power conversion efficiency (PCE)


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We have previously reported that PC60BM makes a significant contribution to the photocurrent generated in a number of polymer:PC60BM systems. For the MEH-PPV:PCBM system, where the polymer:fullerene ratio is 1:4, we showed that the fullerene contributes approximately 50 % of the generated photocurrent27. In the case of out earlier studies of the P3HT:PC60BM 1:1 system, we showed that the fullerene generates 13 % of the generated photocurrent11; in good agreement with the 14 – 15 % reported here. This work shows that the contribution of the fullerene to the overall photocurrent generated in the devices actually increases as we move to other fullerenes. In the case of ICBA and PC70BM, excitons generated on the fullerene component contribute around 30 % of the total photocurrent produced by the device, for both the 85 and 200 nm thick active layers studied here. This extra photocurrent from the fullerene component arises, in part, from enhanced absorption in the active layer due to interference effects in the device architecture.

Conclusions Extensive optical modelling and EQE measurements have been used to determine the relative contribution from the polymer and fullerene components to photocurrent generation in P3HT:PC60BM, P3HT:ICBA and P3HT:PC70BM bulk heterojunction devices. The optical models show that interference effects play a key role in determining the optical absorption of thin OPV devices. We demonstrate that the channel II photocurrent generation pathway (hole transfer from the fullerene) makes a significant contribution to the total photocurrent produced by P3HT:fullerene OPV devices. This work highlights the need to consider the photocurrent generated by the acceptor (fullerene) component when describing the operation of organic solar cells.



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Experimental Methods Device fabrication: Glass substrates with patterned ITO were solvent and ozone cleaned. A 35 – 50 nm layer of PEDOT:PSS was applied by spin casting and the substrates were then annealed for 30 – 35 minutes at 140 °C. The annealing and subsequent fabrication steps were conducted in an inert environment. A 1:1 blend of P3HT and fullerene was dissolved in chloroform (PC60BM) or odichlorobenzene (ICBA and PC70BM). The solution was filtered (0.2 film), and then spin cast on top of the annealed PEDOT:PSS to give active layers of thickness 85 nm and 200 nm for each blend. A 100 nm thick aluminium cathode layer was then deposited by evaporation. The samples were then thermally annealed for 4 minutes at 140 °C (P3HT:PC60BM) or solvent annealed for 30 minutes (P3HT:ICBA and P3HT:PC70BM). The device fabrication parameters (such as thickness, solvent, annealing and component ratio) were not optimised for device efficiency. Instead, the conditions were optimised to produce standardised 85 nm and 200 nm thick films with minimal surface roughness thus allowing direct comparison between the physical structures in the ideal layer structures assumed for the subsequent modelled optical measurements. Device characterisation: Short circuit current density was obtained from the device current-voltage (IV) curves measured under AM1.5 illumination (Newport Oriel class A model 91160A solar simulator) using a Keithley sourcemeter (model 2400). The light intensity was measured to be 90 mW cm-2 by a silicon reference solar cell (FHG-ISE). External quantum efficiency (EQE) measurements were conducted in an inert environment. In normal atmosphere a chopped arc lamp source coupled to an Oriel Cornerstone scanning monochromator produced 2 nm bandwidth monochromatic light. The light is then coupled via a quartz fibre bundle into the glove box where it is then focused to illuminate the cell area with no overfilling. The EQE system was capable of transmitting light in the 310 - 400 nm region to 22 ACS Paragon Plus Environment

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allow the fullerene contribution to be directly measured. The illuminated device output electrodes are connected to a Stanford Research Systems SR830 lock-in amplifier. A reference diode measuring a reflection of the beam off a glass wedge is used to monitor any source intensity fluctuations. The monitor data is acquired by a separate lock-in amplifier (Ithaco Dynatrac) with absolute spectral corrections via a look-up table generated from a reference scan which was performed prior to the measurement of the EQE. Output signals from both phase-locked loop measurements are recorded on a PC as is the illumination wavelength. Optical characterisation: An ultraviolet-visible absorption spectrophotometer (UV-vis, Varian Cary 6000i) was used to measure the reflection and transmission of the relevant component layers spin coated on quartz slides, before and after annealing. Spectral ellipsometry (Woollam M-2000 Spectroscopic Ellipsometer) was used to measure

and over the wavelength range of 210 - 1000 nm.

Samples were measured using incident angles of 55° to 75° using 20 revolutions of the analyser per measurement. Acknowledgements The University of Newcastle is gratefully acknowledged for a PhD scholarship (NN). This work was performed in part at the Materials and NSW node of the Australian National Fabrication Facility; a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers.



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The Contribution of Fullerene Photocurrent Generation to Organic

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