Metal Evaporation-Induced Degradation of Fullerene Acceptors in

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Metal Evaporation-Induced Degradation of Fullerene Acceptors in Polymer/Fullerene Solar Cells Wenchao Huang, Eliot Gann, Lars Thomsen, Anton Tadich, Yi-Bing Cheng, and Christopher R. McNeill ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10957 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 23, 2015

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ACS Applied Materials & Interfaces

Metal Evaporation-Induced Degradation of Fullerene Acceptors in Polymer/Fullerene Solar Cells Wenchao Huanga, Eliot Gannab, Lars Thomsenb, Anton Tadichb, Yi-Bing Chenga, Christopher R. McNeill a* a

Department of Materials Science and Engineering, Monash University, Victoria 3800,

Australia b

Australian Synchrotron, 800 Blackburn Rd, Victoria 3168, Australia

(Corresponding author’s email: [email protected]) KEYWORDS: polymer solar cells, interface, PCBM, thermal evaporation, NEXAFS spectroscopy

ACS Paragon Plus Environment


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Abstract: Surface-sensitive NEXAFS spectroscopy is used to probe the interaction between low work function metal electrodes and fullerene derivatives in organic solar cells. Evaporation of either Ca or Al electrodes onto films of the fullerene derivatives (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and indene-C60 bisadduct (ICBA) leads to a dramatic change in the observed NEXAFS spectrum. The observed changes cannot be explained only in terms of interfacial electronic doping or charge transfer, but rather point to the formation of new chemical bonds that destroy the extensive electron delocalization on the C60 cage. A combination of ex-situ and in-situ ultrahigh vacuum measurements indicates that metal evaporation results in a change in the electronic structure of PCBM that then facilitates chemical degradation and oxidation in the presence of oxygen. In order to investigate the effect of this chemical interaction on the device performance, rather than use thermal deposition, a unique transfer method is used to laminate the Al electrode to the top of polymer blend, in which case, the chemical degradation of the fullerene is not observed. Device performance of P3HT/PCBM blend solar cells in which the top metal electrode has either been thermally evaporated or transferred has also been compared. These results highlight that chemical as well as electronic interactions between metals and organic semiconductors must be considered.


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1. Introduction Organic photovoltaics are continuing to receive strong interest with the power conversion efficiency of organic solar cells now exceeding 10%.1 From a manufacturing perspective, solution processability is a particularly attractive feature, with blends based on a conjugated polymer as the donor and a functionalized fullerene as the acceptor producing the highest performing solution-processable organic system.2 Interfaces play an important role in organic photovoltaic devices, especially the interfaces between the active layer and the electrodes.3 Organic solar cells typically adopt a sandwich-style structure, built upon a semitransparent indium-tin oxide (ITO) electrode with glass or flexible foil as the substrate. For

the

so-called

“standard”

geometry,

a

thin

interfacial

layer

of

poly(3,4-

ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is typically deposited on top of the ITO electrode before deposition of the active semiconducting layer. The top contacts are then generally deposited via thermal evaporation of a metal such as aluminium (Al) or calcium (Ca). Inverted devices can also be realized by depositing zinc oxide4 or a work function altering layer (such as polyethylenimine ethoxylated (PEIE)5-7) to switch the polarity of the ITO electrode. The top contact for inverted devices often then employs a thin oxide layer (for example MoOx8) and a Silver (Ag) contact deposited again by vacuum evaporation. It is well-known for both standard and inverted configurations that the proper choice of electrode and interfacial layer materials is critical to achieving optimum solar cell performance,9-14 as these affect the open-circuit voltage achieved and the fill-factor sustained by the device. Under the metal-insulator-metal (MIM) model, the open circuit voltage of a device is determined by the workfunction offset of the two electrodes.14 Careful research has shown that in certain circumstances (namely Ohmic contacts) the open circuit voltage of polymer solar cells does not vary as predicted by the MIM model, and is instead relatively insensitive to the electrode work function.9,

14, 15

This observation is explained by charge



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transfer from the electrode into the semiconductor, leading to pinning of the Fermi level of the electrode material to the semiconductor band edge.14 Such an explanation in terms of semiconductor device physics neglects however to consider any chemical changes that may also be occurring at the interface between the organic layer and the vacuum-deposited top contacts. In our previous study, we found a change in Near-Edge X-ray Absorption FineStructure (NEXAFS) spectra of poly(3-hexylthiophene) P3HT/ electron acceptor (6,6)phenyl-C61-butyric acid methyl ester (PCBM) blends following thermal evaporation of Al electrodes.16-18 By measuring transitions of core electrons (such as those in the carbon 1s orbital) to antibonding π* and σ* orbitals, NEXAFS spectroscopy is sensitive to changes in electronic structure brought about by both interfacial charge transfer and interfacial chemical reactions.19 Recently Mauger et al. noted that vacuum deposition Al or Ca on top of neat films of PCBM leads to a pronounced change in the observed carbon K-edge NEXAFS spectra.20 Mauger et al. explained the changes of the PCBM spectra upon metal evaporation as being purely electronic (doping) in origin,20 however we find the nature and magnitude of the changes suggest otherwise. In this paper, we present an in-depth NEXAFS-based investigation of the interaction between vacuum-deposited metal electrodes and PCBM on both neat films and blends with the donor polymer (P3HT) as relevant to devices.20,

21

Extending the observations of Mauger et al., we have found that pronounced NEXAFS spectral changes remain in the samples even after the evaporated Al and Ca electrodes are carefully removed. Our results indicate that the interfacial interaction between fullerene molecules and low workfunction metals such as Ca and Al is chemical (i.e. a permanent changing of the bonding environment within the top layer), and not purely electronic. Using a combination of surface-sensitive and bulk sensitive NEXAFS spectroscopy we also show that these effects are confined to the surface of the film. Furthermore, by studying


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polymer/fullerene blends (not just fullerene layers) we show that this effect exists in bulk heterojunction blends used in actual devices. Finally, we assess the potential impact of this chemical interaction on device performance.

2. Results and Discussion Figure 1 (a,b) presents the carbon K-edge NEXAFS spectra of neat PCBM films along with PCBM films deposited with either 2 nm of Al or 2 nm of Ca. For these measurements, 2 nm of Al or Ca was deposited in a high vacuum chamber (~ 10-6 mbar) inside a nitrogen glovebox that is also used for device fabrication. These films were subsequently transferred to the ultrahigh vacuum (UHV) endstation of the soft X-ray beamline22 at the Australian Synchrotron for NEXAFS analysis. Figure 1 presents Total Electron Yield (TEY) spectra, which typically have a surface sensitivity of ~ 3 nm.23 The NEXAFS spectrum of PCBM is dramatically changed upon deposition of Al or Ca, as shown by the loss of the rich, near-edge structure that largely derives from the C60 constituent of PCBM.24 In particular, the oscillator strength of the lowest energy transition at 284.3 eV associated with the 1s → LUMO transition is dramatically reduced, as well as the oscillator strength of the two peaks at 285.7 eV and 286.1 eV associated with the 1s → LUMO+1 and 1s → LUMO+2 transitions. In both cases, new resonant transitions appear, with peaks appearing at 285.0 eV and 288.5 eV for the Al-coated sample, and at 288.5 eV and 290.2 eV for the Ca-coated sample. The NEXAFS spectra of Al-coated and Ca-coated PCBM films annealed at 150 ˚C for 10 minutes after deposition are also presented. The spectra of these annealed films are similar to the unannealed films in that the same peaks are present, but now exhibit slight changes in their relative intensities. In contrast to what is observed for PCBM films deposited with Ca and Al, little change in the NEXAFS spectrum of PCBM is observed when 2 nm of Ag or MoOx is deposited, see Figure 1 (c,d).



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The dramatic changes in the NEXAFS spectra of PCBM with Al and Ca deposition, and the absence of any change when Ag or MoOx are deposited, may seem consistent with electronic interactions between Al or Ca with PCBM, as the electron affinities and work functions of Al and Ca are sufficiently low to enable charge transfer to PCBM, whereas the higher work functions of Ag and MoOx preclude a similar charge transfer.20 However, the nature and degree of the observed spectral changes are inconsistent with a purely electronic interaction. For such an interaction one may expect a reduction in the oscillator strength of the 1s-LUMO peak, but higher energy peaks, particularly peaks above 290 eV associated with transitions to σ* states should remain unchanged. In contrast, after deposition of Al or Ca, the entire NEXAFS spectrum is changed including a loss of structure above 290 eV and the appearance of new peaks, indicative not just of an electronic interaction but also of the formation of new chemical bonds. The peak at 288.5 eV is commonly encountered in carbon K edge NEXAFS spectroscopy, and is associated with the presence of carbonyl (C=O) groups,19 suggesting that Al deposition has induced a chemical change in the molecules at the surface associated with the presence of oxygen. The observed NEXAFS spectrum of PCBM after Ca deposition (with an additional peak at 290.2 eV and broad peak at ~ 300 eV) bears a remarkable similarity to that of Ca carbonate (CaCO3).25 Thus from a spectroscopic point of view, the observed changes in the NEXAFS spectrum of PCBM upon deposition of Al or Ca point to a pronounced change in the surface chemistry. NEXAFS spectra were also acquired after the removal of Al or Ca using either NaOH or HCl, see Figure 2. (Such a treatment itself does not cause a change in the NEXAFS spectrum of pure PCBM, see Figure S1, and was found to completely remove all Al (or Ca) as confirmed by X-ray photoelectron spectroscopy, see Figure S2). After removal of Al or Ca, a different spectrum again is observed, with a single 1s-π* peak observed at 285 eV and a broad σ* structure, with an absence of a strong carbonyl peak. The NEXAFS spectra of the Al-removed and Ca-removed


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samples resemble amorphous carbon.26 The fact that the NEXAFS spectrum of pristine PCBM is not recovered after electrode removal indicates that the interaction between Al and PCBM, and Ca and PCBM, is not purely electronic in nature (eg. doping or charge transfer) but rather must be an irreversible chemical change involving the formation of new bonds at the interface. (We note that in organic semiconductor systems that the addition or removal of an electron will result in a rearrangement of the local bonding environment. Thus electronic doping in organic semiconductors will involve a ‘chemical’ change to some extent. However such subtle changes in bonding are reversible, in contrast to the pronounced irreversible changes we observe here.) In order to investigate the interaction between vacuum evaporated Al and PCBM in the absence of oxygen (trace amounts of oxygen and water will still be present in a high vacuum environment of ~ 10-6 mbar), NEXAFS measurements were performed on PCBM films that had Al thermally deposited within an ultrahigh vacuum preparation chamber (pressure ~ 1 x 10-10 mbar) at the soft X-ray beamline, see Figure 3, thus removing any potential contaminants such as trace oxygen and water from interacting with the thin films of Al. (The data shown in figure 3 were acquired with partial electron yield detection which has a slightly higher surface sensitivity than TEY measurement.) Progressive changes in the NEXAFS spectrum of PCBM with UHV deposition of Al are observed but are more subtle than those observed for samples where Al was evaporated ex-situ. In particular, a systematic reduction in the oscillator strength for the peaks labeled 1, 2, 3 and 4 is observed, following the numbering of Brumboiu et al.24 Peak 1 is associated with the 1s-LUMO transition, while peaks 2, 3 and 4 result from electronic transitions to higher energy unoccupied orbitals in the fullerene cage, with the side chain carbon atoms also contributing to peak 4.24 The peak labeled S is not predicted by theoretical calculations and has been associated by Brumboiu et al as being due to the surface modification of PCBM molecules.24 Interestingly the intensity



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of this peak is not affected as strongly by Al evaporation and becomes more prominent with increased Al evaporation as the intensities of the neighboring peaks diminish. Above 290 eV the shape of the spectrum remains largely unaffected (although with a reduction in intensity that could be due to the presence of the Al overlayer) suggesting that the σ* structure has not been significantly changed. Hence the observed changes in the NEXAFS spectrum of PCBM with UHV deposition of Al are more consistent with an electronic interaction. However Matz et al., who performed UHV Raman spectroscopy measurements of C60 films evaporated with Al in situ,27 found direct evidence for the presence of a C60 anion radical (C60•−) as well as amorphous carbon created by further degradation of C60•−. This result indicates that chemical degradation of the fullerene cage into amorphous carbon can occur even in UHV which may not be as obvious with NEXAFS spectroscopy. Hence the electron transfer following Al deposition initiates the degradation of PCBM, initially into an Al+/PCBM•− salt which is then further unstable to oxidation. The ex-situ NEXAFS spectra of PCBM sample after the removal of Al and Ca is similar to the amorphous carbon, which shows agreement with Matz’s observation.26 For the ex-situ prepared samples, oxidation is likely to occur even during metal evaporation in the relatively poor vacuum chamber or during sample transportation from glovebox to the beamline, with such oxidation only prevented in a UHV system. As well as the surface sensitive partial electron yield (PEY) measurements presented in Figure 3(a), bulk sensitive fluorescence yield (FY) data were also acquired (which have not previously been reported) as shown in Figure 3(b,c). The FY NEXAFS spectrum of PCBM are unaffected by the Al evaporation confirming that the chemical degradation is indeed confined to the surface. Interestingly there is very little oscillator strength at 285 eV (peak S) which confirms the assignment of this peak by Brumboiu et al.24 as a surface modification.


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To explore the generality of this electron-transfer induced degradation, we also examined the interaction between evaporated metals and the fullerene derivative indene-C60 bisadduct (ICBA).28 Figure 4 presents the NEXAFS spectra of neat ICBA films along with ICBA films coated with 2 nm of Al or Ca. As in the case of PCBM, deposition of 2 nm of Al or Ca leads to a loss in of the near-edge structure characteristic of the ICBA molecule and the appearance of new resonant peaks. Strikingly, despite ICBA having a NEXAFS spectrum distinct from PCBM, the NEXAFS spectrum of ICBA coated with either Al (or Ca) closely matches the NEXAFS spectrum of PCBM coated with Al (or Ca). The similar destruction of the rich characteristic NEXAFS spectra of these two fullerene derivatives when coated with Al or Ca indicates that the surface chemical structure is modified to a similar state in both cases by metal-induced degradation. Similar to PCBM, coating with a higher work function material such as MoOx results in no change in the observed NEXAFS spectrum (see figure S3). To investigate whether this interaction between low workfunction metals and PCBM is also present in polymer/fullerene blends employed in solar cells (previous studies of fullerene degradation have only focused on neat films) we have studied the effect of Al deposition

(and

subsequent

removal)

on

the

NEXAFS

spectrum

of

poly(3-

hexylthiopehene):PCBM blends, shown in Figure 5. Control experiments on neat P3HT films indicate that the deposition and removal of Al has no effect on the NEXAFS spectrum of P3HT, see Figure S4. The NEXAFS spectrum of an as-cast P3HT:PCBM blend (no Al deposited) largely resembles that of P3HT (see Figure S4) indicating a surface layer rich in P3HT, consistent with previous reports.16, 29 From XPS and NEXAFS compositional analysis a surface composition of 15 wt. % PCBM for the as-cast blend film is calculated within the top 3 nm of the film.16 The NEXAFS spectrum of the ‘pre-annealed’ sample (that is, a sample that was annealed at 150 ˚C for 10 minutes without Al deposition) is essentially



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identical to the as-cast sample with the P3HT-rich surface layer remaining intact. In contrast, the NEXAFS spectrum of the ‘post-annealed’ sample (which has had a layer of Al deposited, followed by annealing at 150 ˚C for 10 minutes, followed by the removal of the Al) is vastly different, resembling that of a neat PCBM film that has had an Al layer deposited and subsequently removed (see Figure 2(a)). The observed NEXAFS spectrum indicates that (as for the neat PCBM sample) the PCBM at the interface has chemically decomposed after reacting with Al. Fluorescence yield NEXAFS spectra (Figure 5(b)) of the as-cast, preannealed and post-annealed samples are all very similar confirming that the metal evaporation induced chemical degradation of PCBM is confined to the blend/Al interface. Given the different samples depths of FY and PEY NEXAFS this observation suggests the chemical degradation is confined to the first few nanometers of the film. We have also checked for chemical degradation in a low-band gap polymer:fullerene blend, namely that based on PBDTTT-EFT:PC71BM.6 Identical chemical degradation effects are seen (see Figure S5) indicating that the effects are not exclusive to P3HT:fullerene blends. Additional evidence of a chemical change at the film/Al interface is provided by water-contact angle measurements (Figure 6). As-cast films of neat P3HT and PCBM exhibit water contact angles of 106.2˚ and 86.8˚ respectively. The as-cast P3HT/PCBM blend also exhibits a water contact angle of 106.2˚ consistent with this surface being rich in P3HT. After coating with Al, annealing and then Al removal, the neat P3HT film retains a relatively high contact angle of 92.5˚. In contrast, the water contact angle measured for the neat PCBM film after post-annealing and Al removal drops significantly to 57.9˚ indicating a significant change in hydrophobicity. Likewise, the water contact angle of the post-annealed P3HT/PCBM blend drops from 106.2˚ for the as-cast blend sample to 65.9˚, consistent with the XPS results that show the surface being primarily made up of decomposed PCBM


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molecules. These observations are all consistent with a significant change in the interfacial chemistry of PCBM when coated with Al. In order to assess the impact of such an interfacial interaction between Al and PCBM on device performance, we have compared the device performance of P3HT/PCBM solar cells where the top Al electrode has either been thermally evaporated, or laminated using a polydimethylsiloxane (PDMS) stamping process. Figure 7 schematically depicts the Al lamination process, whereby an Al film is first evaporated onto a trichloro(octadecyl)silane (OTS)-coated glass and then delaminated and picked up with a PDMS stamp. This delaminated Al film is then transferred to a P3HT/PCBM blend and subsequently annealed following a standard device fabrication protocol. By performing NEXAFS spectroscopy on neat PCBM films (shown in Figure 8), it is seen that an Al film laminated in this fashion does not react chemically with the PCBM film. The reason for this lack of chemical interaction is likely due to the passivation through oxidation of the top of the aluminum layer deposited on OTS-coated glass which then interfaces with the active layer. (Note that although this transfer process has been conducted in an inert glove-box, oxidation of the top aluminium surface will still occur due to the residual oxygen content in the box.) Figure 9 compares the device performance of ITO/PEDOT:PSS/P3HT:PCBM/Al devices where the Al electrode has either been thermally evaporated or laminated as depicted in figure 7. In both cases the devices were ‘post-annealed at 150 ˚C for 10 minutes after top electrode deposition. Whilst the two devices exhibit very similar external quantum efficiency (EQE) spectra, the device where the Al has been transferred rather than evaporated shows a substantially lower fill-factor (FF) than the evaporated device under simulated sunlight, see Table 1. The transferred device also exhibits a lower short-circuit current (JSC) and open-circuit voltage (VOC) resulting in an overall power conversion efficiency (PCE) of 1.9 % compared to 3.3 % for the evaporated device. Thus although evaporated Al is found to cause chemical



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degradation of PCBM at the top electrode interface resulting in the formation of mid-gap states,27 in devices, this interaction does not appear to be severely detrimental to device performance and leads to superior performance relative to a laminated Al film. Thus although the laminated Al leaves PCBM at the surface unaffected, charge extraction is not as effective. As it is well-established that interfacial layers can have a significant effect on thin-film organic solar cell performance,9, 10, 13, 30 the superior performance of the evaporated device may be attributed to the superior properties of chemically degraded PCBM at the evaporated blend/Al interface resulting in improved charge extraction. Of course this is not necessarily a fair comparison since the laminated device might suffer from microvoids due to imperfect lamination however the similar EQE and JSC values suggest that the physical quality of the lamination is quite good. It has been established that evaporation of interlayer materials such as LiF before the evaporation of Aluminum leads to improved device properties relative to aluminum-only devices,31 which suggests that evaporation of LiF might be effective in mitigating interfacial chemical degradation and enabling good electrical contact between PCBM and the electrode. However to our knowledge a NEXAFS study on the PCBM/LiF interface has not been performed, with there still being the possibility that the deposition of vacuum-evaporated LiF onto PCBM will also lead to interfacial chemical degradation. So while there may be several factors that result in the poor performance for the laminated device, we speculate that the formation of mid-gap states through the evaporation-induced chemical degradation may facilitate a superior contact between PCBM and aluminum. This chemical interaction facilitates the formation of an Ohmic contact between Al and the active layer and hence a stronger built-in electric field.14 This increased internal electric field then leads to a faster sweep out of electrons resulting in reduced recombination and higher device efficiency. 3. Conclusion


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Using NEXAFS spectroscopy we have investigated the interaction between evaporated low work function metals on the surface chemistry of fullerene derivatives used in organic solar cells. Rather than a purely electronic interaction between evaporated metals and fullerenes, evaporation of Al or Ca onto films of PCBM or ICBA leads to a pronounced, irreversible chemical interaction between the metal and the fullerene that destroys the extensive electronic delocalization around the C60 cage. Removal of the Al or Ca film results in a NEXAFS spectrum that is distinct to the starting material confirming that a chemical reaction indeed has taken place. Water contact angle measurements confirm a change in interfacial surface chemistry, with PCBM and P3HT/PCBM blend films becoming more hydrophilic with post-annealing and subsequent electrode removal. By comparing the operation of solar cells where the Al electrode was either thermally evaporated or deposited via PDMS transfer, superior performance of the device with the evaporated electrode compared to the laminated electrode suggests that the interfacial chemical interaction between Al and PCBM may be beneficial for device performance, leading to improved charge extraction. These results highlight fundamental differences between organic semiconductors and inorganic semiconductors, where within organic semiconductors, chemical interactions with metal components must be considered in addition to purely electronic interactions. While this study has focused on the interaction between PCBM and low work function metals, the interaction between PCBM and common interlayer materials such as LiF, ZnO and PEIE would help to clarify how ubiquitous interfacial chemical degradation between PCBM and electron collecting interlayer materials is. 4. Experimental Section Sample Preparation: P3HT (4002-E) was supplied by Rieke Metals Inc. and PCBM was purchased from Nano-C. Neat PCBM films were spin-coated onto highly-doped silicon wafers from chloroform solution (20mg/ml PCBM). P3HT/PCBM blend films were prepared



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by spin-coating from a solution of P3HT and PCBM dissolved with a 1:1 weight ratio in dichlorobenzene. The different cathode electrodes were thermally-deposited on the top with the rate of 0.5 Å/s in a pressure of 10-6 mbar or better. For samples prepared in UHV, a rate of 0.02 Å/s was used with a pressure of 1 × 10-10 mbar. For Al transferred samples, the Al electrode were first thermally evaporated on OTS-coated glass and then picked up and transferred with a PDMS stamp. Al and Ca electrodes were removed by etching in dilute NaOH and HCl solutions respectively, followed by rinsing in deionized water and isopropanol. NEXAFS Spectroscopy and XPS: Surface-sensitive NEXAFS spectroscopy and XPS measurements were performed at the Soft X-ray Spectroscopy Beamline at the Australian Synchrotron.22 Nearly perfectly linearly polarized photons (P ≈ 1) from an APPLE II elliptical polarised undulator X-ray source with high spectral resolution of E/∆E ~ 10 000 were focused into an ultrahigh vacuum chamber to a spot size ∼0.4 × 0.1 mm at the sample. Photon flux at the carbon edge was ~ 5 × 1010 photons per second per 200 mA of ring current. X-ray absorption was measured using Total Electron Yield (TEY), Partial Electron Yield (PEY) and Fluorescence Yield (FY) methods. TEY was detected by measuring the drain current to the sample upon x-ray illumination. PEY detection was performed with a channeltron detector. FY detection was with a multi-channel plate fluorescence yield detector A fresh spot on the sample was used for each spectrum with the acquisition time minimised (0.5 s per 0.1 eV step) to prevent beam damage. The recorded signals were normalized to the incident photon flux using the “stable monitor method,” in which the sample signal is compared consecutively to a clean reference sample and the time variation in flux is measured via a gold mesh.32 The normalized spectra were scaled by normalising the pre-edge (at 280 eV) to zero and the post-edge (at 320 eV) to one, effectively normalizing to the total carbon content of the material. The photon energy was calibrated by measuring the NEXAFS


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spectrum of highly oriented pyrolytic graphite (HOPG) simultaneously to the sample signal and normalising to the exciton peak at 291.65 eV.33 All NEXAFS spectra were measured with an x-ray angle of incidence of 55˚ to negate orienational alignment effects. For XPS measurements an X-ray energy of 1486.7 eV was used with photoelectrons detected using a SPECS Phoibos 150 Hemispherical Analyzer with a pass energy of 40 eV. Water Contact Angle Measurements: A DataPhysics OCA20 was used for water contact angle measurements. A water droplet was placed on the polymer or the organic semiconductor film with the contact angle calculated from the CCD image using SCA202 software package. Device Fabrication and Characterization: Devices were prepared on PEDOT:PSS-coated ITO-glass substrates with all fabrications steps carried out in a nitrogen-filled glove box. A solution of P3HT blended with PCBM was prepared by dissolving P3HT and PCBM in dichlorobenzene with a weight ratio of 1 to 1. P3HT/PCBM blend films were spin-coated onto the PEDOT:PSS-coated ITO-glass substrates. Al electrodes were either thermally evaporated in vacuum or transferred via PDMS stamping. Thermal annealing was performed after electrode deposition by placing devices on a hotplate at 150˚C for 10 minutes. Devices were encapsulated with epoxy resin before removal from the glove box and testing. Device performance under simulated sunlight was performed using a Photo Emission Tech. model SS50AAA solar simulator with the current-voltage curves measured with a Keithley 2635 source meter. The intensity of the solar simulator was set using a calibrated silicon reference cell with a KG3 glass filter (PV Measurements Inc.). External quantum efficiency (EQE) was measured as a function of wavelength by dispersing light from a tungsten filament (Newport 250 W QTH) through a monochromator (Oriel Cornerstone 130) with a spot size smaller than the device active area. Light intensities of less than 1 mW cm−2 were used with short-circuit current recorded using a Keithley 2635 source measure unit. The system was calibrated by



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placing a calibrated photodiode (Thorlabs FDS-100CAL) in the device under test position and referencing the intensity measured to that of another silicon photodiode that samples a portion of the beam via a beamsplitter and serves to account for any intensity fluctuations.

Supporting Information The supporting information is available free of charge on the ACS Publications website. NEXAFS spectra of control PCBM samples, XPS spectrum of PCBM film with removed Al, NEXAFS and XPS spectra of P3HT, TEY NEXAFS spectra of PBDTTT-EFT/PC71BM films.

Acknowledgements C.R.M. acknowledges support from the Australian Research Council (FT100100275) and veski. This research was undertaken on the soft X-ray beamline at the Australian Synchrotron, Victoria, Australia.

References (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 47). Prog. Photovolt. Res. Appl. 2015, DOI: 10.1002/pip.2728. (2). Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A. Yang, Y., 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 66426671. (3) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450-1472. (4) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679-1683. (5) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Brédas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B. A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Science 2012, 336, 327-332. (6) Huang, W.; Gann, E.; Thomsen, L.; Dong, C.; Cheng, Y.-B.; McNeill, C. R. Unraveling the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT-EFT. Adv. Energy Mater. 2015, 5, 1401259.


Page 16 of 29

Page 17 of 29

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


(7) Huang, W.; Gann, E.; Xu, Z.-Q.; Thomsen, L.; Cheng, Y.-B.; McNeill, C. R. A Facile Approach to Alleviate Photochemical Degradation in High Efficiency Polymer Solar Cells. J. Mater. Chem. A 2015, 3, 16313-16319. (8) Schmidt, H.; Flügge, H.; Winkler, T.; Bülow, T.; Riedl, T.; Kowalsky, W. Efficient Semitransparent Inverted Organic Solar Cells with Indium Tin Oxide Top Electrode. Appl. Phys. Lett. 2009, 94, 243302. (9) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M. Effect of Metal Electrodes on the Performance of Polymer:Fullerene Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2004, 85, 970-972. (10) Knesting, K. M.; Ju, H.; Schlenker, C. W.; Giordano, A. J.; Garcia, A.; Smith, O. N. L.; Olson, D. C.; Marder, S. R.; Ginger, D. S. ITO Interface Modifiers Can Improve VOC in Polymer Solar Cells and Suppress Surface Recombination. J. Phys. Chem . Lett. 2013, 4, 4038-4044. (11) Zhang, Z.-G.; Qi, B.; Jin, Z.; Chi, D.; Qi, Z.; Li, Y.; Wang, J. Perylene Diimides: A Thickness-Insensitive Cathode Interlayer for High Performance Polymer Solar Cells. Energy & Environ. Sci. 2014, 7, 1966-1973. (12) Gupta, V.; Kyaw, A. K. K.; Wang, D. H.; Chand, S.; Bazan, G. C.; Heeger, A. J. Barium: An Efficient Cathode Layer for Bulk-heterojunction Solar Cells. Scientific Reports 2013, 3, 1965. (13) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Effect of LiF/Metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002, 80, 1288-1290. (14) Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.; Rispens, M. T. Cathode Dependence of the Open-Circuit Voltage of Polymer:Fullerene Bulk Heterojunction Solar Cells. J. Appl.Phys. 2003, 94, 6849-6854. (15) Gadisa, A.; Svensson, M.; Andersson, M. R.; Inganäs, O. Correlation Between Oxidation Potential and Open-Circuit Voltage of Composite Solar Cells Based on Blends of Polythiophenes/ Fullerene Derivative. Appl. Phys. Lett.2004, 84, 1609-1611. (16) Huang, W.; Gann, E.; Cheng, Y.-B.; McNeill, C. R. In-Depth Understanding of the Morphology–Performance Relationship in Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 14026-14034. (17) McNeill, C. R.; Ade, H. Soft X-ray Characterisation of Organic Semiconductor Films. J. Mater. Chem. C 2013, 1, 187-201. (18) Huang, W.; Huang, F.; Gann, E.; Cheng, Y.-B.; McNeill, C. R. Probing Molecular and Crystalline Orientation in Solution-Processed Perovskite Solar Cells. Adv. Funct. Mater. 2015, 25, 5529-5536. (19) Stöhr, J. NEXAFS Spectroscopy. Springer: Berlin, 1992. (20) Mauger, S. A.; Chang, L.; Friedrich, S.; Rochester, C. W.; Huang, D. M.; Wang, P.; Moulé, A. J. Self-Assembly of Selective Interfaces in Organic Photovoltaics. Adv. Funct. Mater. 2013, 23, 1935-1946. (21) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C., Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006-7043. (22) Cowie, B. C. C.; Tadich, A.; Thomsen, L. The Current Performance of the Wide Range (90-2500 eV) Soft X‐ray Beamline at the Australian Synchrotron. AIP Conf. Proc. 2010, 1234, 307-310. (23) Chua, L.-L.; Dipankar, M.; Sivaramakrishnan, S.; Gao, X.; Qi, D.; Wee, A. T. S.; Ho, P. K. H. Large Damage Threshold and Small Electron Escape Depth in X-ray Absorption Spectroscopy of a Conjugated Polymer Thin Film. Langmuir 2006, 22, 8587-8594.



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

(24) E. Brumboiu, I.; Anselmo, A. S.; Brena, B.; Dzwilewski, A.; Svensson, K.; Moons, E. Near-edge X-ray Absorption Fine Structure Study of the C60-Derivative PCBM. Chem. Phys. Lett. 2013, 568–569, 130-134. (25) Lee, J. R. I.; Han, T. Y.-J.; Willey, T. M.; Wang, D.; Meulenberg, R. W.; Nilsson, J.; Dove, P. M.; Terminello, L. J.; van Buuren, T.; De Yoreo, J. J. Structural Development of Mercaptophenol Self-Assembled Monolayers and the Overlying Mineral Phase during Templated CaCO3 Crystallization from a Transient Amorphous Film. J. Am. Chem. Soc. 2007, 129, 10370-10381. (26) Buijnsters, J. G.; Gago, R.; Redondo-Cubero, A.; Jiménez, I. Hydrogen Stability in Hydrogenated Amorphous Carbon Films with Polymer-Like and Diamond-Like Structure. J. Appl. Phys.2012, 112, 093502. (27) Matz, D. L.; Ratcliff, E. L.; Meyer, J.; Kahn, A.; Pemberton, J. E. Deciphering the Metal-C60 Interface in Optoelectronic Devices: Evidence for C60 Reduction by Vapor Deposited Al. ACS Appl. Mater. Interfaces 2013, 5, 6001-6008. (28) He, Y.; Chen, H.-Y.; Hou, J.; Li, Y. Indene−C60 Bisadduct: A New Acceptor for High-Performance Polymer Solar Cells. J. Am. Chem. Soc. 2010, 132, 1377-1382. (29) Xue, B.; Vaughan, B.; Poh, C.-H.; Burke, K. B.; Thomsen, L.; Stapleton, A.; Zhou, X.; Bryant, G. W.; Belcher, W.; Dastoor, P. C. Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells. J. Phys. Chem. C 2010, 114, 15797-15805. (30) Woo, S.; Hyun Kim, W.; Kim, H.; Yi, Y.; Lyu, H.-K.; Kim, Y. 8.9% Single-Stack Inverted Polymer Solar Cells with Electron-Rich Polymer Nanolayer-Modified Inorganic Electron-Collecting Buffer Layers. Adv. Energy Mater. 2014, 4, 1301692. (31) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Effect of LiF/Metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002, 80, 1288-1290. (32) Watts, B.; Thomsen, L.; Dastoor, P. C. Methods in Carbon K-edge NEXAFS: Experiment and Analysis. J. Elec. Spec. & Rel. Phen. 2006, 151, 105-120. (33) Watts, B.; Ade, H. A Simple Method for Determining Linear Polarization and Energy Calibration of Focused Soft X-ray Beams. J. Elec. Spec. & Rel. Phen. 2008, 162, 49-55.


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Figure 1. TEY NEXAFS spectra of PCBM films coated with (a) 2 nm of Al, (b) 2 nm of Ca, (c) 2 nm of Ag and (d) 2 nm of MoOx. In each subfigure a pristine PCBM NEXAFS spectrum is provided for comparison. Both as-cast “AC” (that is, not annealed) and thermally annealed “TA” samples are presented in parts (a) and (b).



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Figure 2.TEY NEXAFS spectra of PCBM films after the removal of (a) evaporated Al film, or (b) evaporated Ca film. In both cases spectra for samples both as-cast “AC” or thermally annealed “TA” are presented.


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Figure 3. Evolution of the NEXAFS spectrum of PCBM upon deposition of Al in UHV: (a, b) surface-sensitive partial electron yield measurement; (c, d) bulk-sensitive fluorescence yield measurement.



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Figure 4. NEXAFS spectra of ICBA films coated with (a) Al and (b) Ca.


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Figure 5. (a) TEY NEXAFS spectra and (b) FY NEXAFS spectra of P3HT/PCBM blend films that were either as-cast, annealed at 150 ˚C for 10 minutes without an electrode (“preannealing”), or annealed at 150 ˚C for 10 minutes with an Al top electrode that was subsequently removed (“post-annealing”).



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Figure 6. Water contact angle measurements of as-cast (a,c,e) and post-annealed (b,d,f) films of neat P3HT (a,b), neat PCBM (c,d) and P3HT/PCBM blend (e,f).


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Figure 7. Schematic of the transfer technique for laminating an Al film as the top electrode in a P3HT/PCBM solar cell.



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Figure 8. NEXAFS spectra of PCBM films that have had an Al electrode laminated and subsequently removed, compared to the NEXAFS spectrum of a pristine PCBM film. Both as-cast “AC” and thermally annealed “TA” cases are presented.


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Figure 9. Device performance of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells where the top Al electrode has either been thermally evaporated or transferred by the PDMS stamping process. (a) current-voltage characteristics under 100 mW/cm2 AM1.5G illumination and (b) external quantum efficiency spectra.

Table 1. Photovoltaic performance of ITO/PEDOT:PSS/P3HT:PCBM/Al devices (averaged over 4 devices)



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Device type

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

Evaporated

0.63

8.91

0.59

3.29 (±0.18)

Transferred

0.60

7.80

0.40

1.86 (±0.46)


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TOC


Metal Evaporation-Induced Degradation of Fullerene Acceptors in

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