Impact of Fullerene Mixing Behavior on the Microstructure

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Impact of Fullerene Mixing Behavior on the Microstructure, Photophysics and Device Performance of Polymer/Fullerene Solar Cells Wenchao Huang, Naresh Chandrasekaran, Shyamal K. K. Prasad, Eliot Gann, Lars Thomsen, Dinesh Kabra, Justin M Hodgkiss, Yi-Bing Cheng, and Christopher R. McNeill ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10404 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

Impact of Fullerene Mixing Behavior on the Microstructure, Photophysics and Device Performance of Polymer/Fullerene Solar Cells Wenchao Huang,a Naresh Chandrasekaran,abc Shyamal Prasad,de Eliot Gann,af Lars Thomsen,f Dinesh Kabra,b Justin M. Hodgkiss,de Yi-Bing Chenga and Christopher R. McNeilla* a

Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia b Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India c IITB-Monash Research Academy, IIT Bombay, Mumbai, 400076, India d MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand e School of Chemical and Physical Sciences, Victoria University of Wellington, New Zealand f Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia Corresponding email: [email protected]

Keywords: Organic solar cells, fullerenes, morphology, mixing behavior, photophysics, device physics

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Abstract: Here a comprehensive study of the influence of polymer:fullerene mixing behavior on the performance, thin-film microstructure, photophysics and device physics of polymer solar cells is presented. In particular, blends of the donor polymer PBDTTT-EFT with the acceptor PC71BM that exhibit power conversion efficiencies over 9% are investigated. Through tuning of the fullerene concentration in PBDTTT-EFT:PC71BM blends the impact of fullerene mixing behavior is systematically investigated via a combination of synchrotron based X-ray scattering and spectroscopy techniques. The impact of fullerene loading on photophysics and device physics is further explored with steady-state photoluminescence measurements, ultrafast transient absorption spectroscopy and transient photovoltage measurements. In the low fullerene concentration regime (< 50 wt.%) most fullerene molecules are dispersed in the polymer matrix resulting in severe geminate and non-geminate recombination due to a lack of pure fullerene aggregates and percolating pathways for charge separation and transport. In the high fullerene concentration regime (> 70 wt.%) large fullerene domains result in incomplete PC71BM exciton harvesting with the presence of fullerene molecules also disrupting the molecular packing of polymer crystallites. The optimum fullerene concentration of ~ 60 – 67 wt.% balances the requirements of charge generation and charge collection. These findings demonstrate that controlling the fullerene concentration in the mixed phase and optimizing the balance between pure and mixed phases are critical for maximizing the efficiency of highly mixed polymer/fullerene solar cells.


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1. Introduction As the consumption of energy has increased dramatically in the past decade, intensive efforts have been devoted to developing photovoltaic technologies which directly convert sunlight into electric power. Among the various technologies under development, organic solar cells (OSCs) have emerged as a promising technology for harnessing

energy

due

to

their

potential

low

manufacturing

cost,

environmentally-friendly fabrication and chemically-tunable properties. The steady advancement of the power conversion efficiencies (PCEs) of OSCs to over 10% has been achieved by a combination of materials development and optimization of active layer morphology.1-4 The development of low band-gap polymers such as the PBDTTT

family

based

on

benzo[1,2-b:4,5-b’]dithiophene

(BDT)

and

thieno[3,4-b]thiophene (TT) units has provided active layers with a broad absorption profile across the visible spectrum particularly when paired with the electron acceptor PC71BM.5-8 Tuning the LUMO energy levels of the polymer donor to be as close as possible to the LUMO energy level of the fullerene acceptor via (for example) side chain modification has enabled optimization of the open circuit voltage.5-6 Among the PBDTTT

family,

the

polymer

poly[[2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2[(2-eth ylhexyl)carbonyl]thieno[3,4-b]thiophenediyl (PBDTTT-EFT), also called PTB7-Th, in particular has recently attracted intense attention due to the excellent device performance in both polymer/fullerene and polymer/polymer solar cells.1-2, 4, 8-15



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In the fabrication of solution-processed polymer solar cells, the donor and acceptor materials are typically dissolved in a common solvent. During film deposition, enthalpic interactions favor phase separation, whilst entropic consideration favor a mixing.16 In addition to thermodynamics, kinetic considerations also play a critical role in determining the resultant thin-film morphology. As the solvent evaporates, the increasing molecular interactions drive phase separation, but the polymer chains also become less mobile as the concentration increases with the morphology eventually becoming kinetically frozen in a non-equilibrium morphology. Intensive studies have revealed that as-cast polymer/fullerene active layers typically consist of (at least) three phases: a pure, crystalline polymer phase; a fullerene-rich phase consisting largely of fullerene aggregates or clusters; and a mixed-amorphous phase containing disordered polymer chains with molecularly-mixed fullerene molecules.16-22 Phase separation, aggregation and crystallization of the polymer and fullerene to form a nanoscale bicontinuous network is required to facilitate the collection of both electron and holes at the relevant electrodes. Charge carrier transport across pure crystalline structures is much more efficient than through a mixed or an amorphous phase because of longer free paths and fewer defects, which significantly alleviate non-geminate charge recombination.23-26 On the other hand, disordered mixed phases also have a significant impact on device performance. As the exciton diffusion length is limited to ~ 10 nm, the mixed phase ensures that the excitons created in the mixed phase have a high probability of dissociation by reaching a donor/acceptor interface


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before recombining.27-28 In addition, the energetic offset between the disordered and ordered domains facilitate charge transfer from the mixed phase into the ordered pure phase, which helps to prevent geminate recombination.21, 29-30 Not surprisingly then, the ability to tune the mixing behavior of polymer/fullerene blends is critical in optimizing device performance. In addition to promoting exciton separation efficiency, a high loading of fullerene in the mixed phase is beneficial for electron transport. Recent investigations have shown that charge transport in the mixed phase is strongly influenced by the domain purity with partial miscibility of polymer and fullerene regarded as being crucial for efficient solar cell performance.31-32 A partial miscibility value above the percolation threshold for electron transport is regarded as being necessary to transport electrons out of the mixed phase.20 For fullerene concentrations below the percolation threshold individual fullerene molecules act as morphological electron traps to the determinate of charge transport and collection. Kesava et al. for example investigated the effect of blend ratio on the morphology and photogeneration in PGeBTBT/PC71BM mixtures. In this study it was found that while increased fullerene loading did not significantly affect the size of morphological features, the higher resulting fullerene concentration in the mixed phase was essential for efficient charge photogernetaion.32 He et al. studying PTB7-Th:PC71BM blends found variation of the blend ratio of the active layer can be used to effectively tune the band tailing.1 He et al. reported a continuous increase in open circuit voltage as a function of fullerene concentration attributed to a reduction in band tailing with an increase in fullerene concentration. Based on this



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idea, He et al. achieved a certified PCE of 9.9% based on an optimized PTB7-Th:PC71BM blend. Although the influence of polymer:fullerene blend ratio has been studied by several independent groups,1,

31-32

there has to date been no

comprehensive study to build up the in-depth relationship between the thin-film microstructure, ultrafast photophysics and device physics of PBDTTT-EFT:PC71BM blends. Due to batch-to-batch variations in polymer quality, and group-to-group variations in film preparation, there is value in performing these varied and specialized characterization techniques on the same blend films that have also been validated as high performing in actual solar cell devices. In this paper, we use the high efficiency blend of the polymer PBDTTT-EFT (also called PTB7-Th) with PC71BM (molecular structure shown in Figure 1) as a model to provide a clear picture of the relationship between the morphology (such as fullerene concentration in the mixed phase, fullerene domain size, and the ratio between mixed phase and pure fullerene phase), photophysics and device physics. The morphology of the polymer blends are examined by a combination of synchrotron based techniques such as grazing incidence wide angle X-ray scattering (GIWAXS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant soft X-ray scattering (R-SoXS).

The photophysics

of the blends

is probed with steady-state

photoluminescence measurements combined with ultrafast transient absorption spectroscopy. Finally, device physics is probed with transient photovoltage measurements. By combining multiple advanced characterization techniques this


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study provide clear guidance on how to optimize the device via control of the fullerene mixing behavior. 2. Experimental Section Materials PBDTTT-EFT (batch number YY7222) and PC71BM were purchased from 1-Material and Nano-C

respectively.

Polyethylenimine ethoxylated

(PEIE),

1,8-diiodooctane (DIO) and anhydrous solvents were supplied by Sigma-Aldrich. Device fabrication Devices were fabricated in an inverted structure of ITO/PEIE/active layer/MoO3/Ag.4,

33

The layer of PEIE was first spin-coated from solution of

methoxyethanol (0.4 wt%) and then was thermally annealed at 100 °C for 10 mins. PBDTTT-EFT and PC71BM with 33 wt.%, 50 wt.%, 60 wt.%, 67 wt.% and 75 wt.% PC71BM were dissolved in a co-solvent of chlorobenzene (97 vol.%) and DIO (3 vol.%) in a nitrogen glove box. The concentration of PBDTTT-EFT in the solution was kept at 8 mg/mL, with all films having a similar thickness between 70 nm and 90 nm. The active layers were deposited on PEIE-coated substrates under nitrogen at a spin speed of 2000 rpm for 60s. Subsequently layers of MoO3 and Ag with thicknesses of 15 nm and 100 nm respectively were deposited in a vacuum evaporation chamber. Devices were encapsulated prior to removal from the glove box and testing. Device characterization



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The PCE of solar cells was measured under 1 sun AM 1.5G with a Photo Emission Tech model SS50AAA solar simulator. The intensity of the sunlight simulator was adjusted by a calibrated silicon photodiode with KG3 filter (PV measurements). The current-voltage curves were collected using a Keithley 2635 source meter. External quantum efficiency (EQE) was measured as a function of wavelength by dispersing light from a tungsten filament. The light was split by a monochromator (Oriel Cornerstone 130) with the intensity of less than 1 mW cm-2. The short circuit current was recorded by a Keithley 2635 sourcemeter. Steady-state optical spectroscopy Samples for Ultraviolet-Visible (UV-Vis) and PL spectroscopy were deposited on quartz substrates. The absorption spectra of all samples were collected with a Lambda 950 spectrophotometer from PerkinElmer. Photoluminescence spectroscopy was performed on a Horiba Jobin Yvon Fluorolog spectro fluorometer. Structural characterization GIWAXS experiments were conducted at the Australian Synchrotron on the SAXS/WAXS beamline.34 All the samples were deposited on silicon wafers following the same procedure as device fabrication. Samples were irradiated by 9 keV X-rays at a fixed incident angle of 0.16° and the 2D scattering patterns were recorded by a Dectris Pilatus 1M detector. The incident angle of X-ray was close to the critical angle of the polymer film but below the critical angle of silicon wafer, minimizing the background signal from substrate. The X-ray exposure time was 1 s such that no film


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damage was identified. The results were analyzed by an altered version of the NIKA 2D based in IgorPro.35-36 NEXAFS spectra were measured at the soft X-ray (SXR) beamline at the Australian Synchrotron.37 The carbon edge spectra were collected in a scan from 280 eV to 320 eV. Total electron yield (TEY) spectra were measured by recording the drain current flowing to the sample. Spectra were normalized by the “stable monitor method”, followed by background subtraction and then normalized to 1 at 320eV and 0 at 280 eV, similar to previous studies.38-39 A singular value decomposition method was used to fit the spectra of the blend film with the spectra of the pristine polymer and fullerene.40-41 Resonant soft X-ray scattering was collected at beamline 11.0.1.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory.42 Films were cast on poly(styrene)-co-styrene sodium sulfonate (NaPSS)–coated glass substrates and were then floated off in deionized water, followed by transferring onto 100 nm thick SiN2 windows. The X-ray energy of 284.0 eV was chosen by calculating the scattering materials contrast between PBDTTT-EFT and PC71BM.42 Scattered photons were collected by a Princeton PI-MTE in-vacuum CCD detector with 27.6 um x 27.6 um pixels. Two scattering patterns of 100 sec exposure time were collected at 30 mm and 150 mm sample to detector distances respectively, and combined in software. Two dimensional scattering patterns were reduced to one dimensional profiles by using a customized version of NIKA.42 Transient absorption spectroscopy



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TA spectroscopy was carried out using the system described in references.22, 43 All data is read out shot-to-shot, whereby differential transmission (ΔT/T) spectra are obtained by comparing alternate shots collected with and without the pump. Values greater than 2.5 standard deviations away from the mean at each wavelength were removed prior to averaging. This process was completed at each time delay to give the final time and wavelength dependent TA surface. Excitation fluences were kept to a few µJ/cm2 to suppress bimolecular processes, unless intensity dependent effects were being specifically probed. The long delay measurements were recorded with matching excitation fluence and then the kinetics scaled to match in the overlapping time region. Transient photovoltage measurements Transient photovoltage measurements were performed by illuminating devices with a constant background intensity high power white LED (Thorlabs MWWHL3). A small light perturbation was provided by a Kingbright L-7104VGC-H green LED (525 nm wavelength) driven by a function generator (Agilent 33533A). A pulse width of 500 ns was chosen for the small perturbation with the rise and fall time of the LED to be ~< 100 ns measured using a Thorlabs DET10A/M Si photo detector. The transient response of the cell under test was recorded on an Agilent Technologies Infiniivision DSO-X

3032A

digital

oscilloscope

(DOSC).

For

transient

photovoltage

measurements cells were connected to the DOSC with a termination of 1 MΩ to achieve open circuit condition. The intensity of the background white LED was changed to achieve different VOC condition of the device. To measure the transient


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photocurrent response the device was connected to the DOSC with a 50 Ω termination to achieve a short circuit condition.

3. Results and Discussion 3.1 Device performance The photovoltaic performance of PBDTTT-EFT:PC71BM solar cells with different fullerene concentrations are summarized in Figure 2. (I-V curves and external quantum efficiency (EQE) spectra are plotted in Figure S2) At low PC71BM loading (33 wt.%), PBDTTT-EFT devices exhibit a poor PCE of only of 3.3%. The PCE then monotonically increases with PC71BM weight fraction from 33 wt.% to 60 wt.%, see Figure 2 (a). A PCE of over 9.4% is achieved in the blend with 60 wt.% to 67 wt.% PC71BM. As the PC71BM weight fraction continues to increase from 67 wt.% to 75%, the PCE decreases slightly to 8.8%. Figure 2 (b) presents the photocurrent density as a function of PC71BM weight percentage, which is measured with and without applying a reverse bias voltage (-1 V). In both measurements, the photocurrent density exhibits the highest value at 60 wt.% PC71BM. The difference in photocurrent measured with and without a bias voltage is largest for low PC71BM fractions, both in an absolute and relative sense, indicating that the low efficiency of PBDTTT-EFT:PC71BM solar cells is associated with a voltage-dependent loss mechanism such as geminate recombination or bimolecular recombination.44 Such difference becomes smaller as the PC71BM loading is increased, indicating that voltage-dependent recombination is alleviated at higher PC71BM concentrations. As shown in Figure 2 (c), the open circuit voltage



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remains constant at ~ 0.8 V between 33 wt.% and 75 wt.% PC71BM, suggesting no significant effect of ‘band tailing’ over this range of fullerene loadings.1 The fill factor follows the same trend as power conversion efficiency, achieving the highest value at 67 wt.% PC71BM. For fullerene concentrations above 67 wt.% PC71BM, the loss in photocurrent is mainly attributed by the polymer observed from UV-vis (Figure S1) and EQE (Figure S2 (b)) spectra. As the PC71BM loading is increased, a decrease in light absorption from polymer is observed. However, it cannot entirely explain the observed drop in efficiency. To further investigate the mechanism behind device performance is needed. 3.2 Thin film microstructure 3.2.1 GIWAXS The molecular packing within the polymer-fullerene blend has been characterized with GIWAXS. From the 2D scattering patterns of the pristine polymer film (Figure S3), we identify the broad peak located at q = 1.59 Å-1 as the π-π stacking peak of PBDTTT-EFT, and identify the peak at q = 0.27 Å-1 as the alkyl stacking peak. The orientation of these peaks, with the π-π stacking peak largely out-of-plane (OOP) and the alkyl stacking peak largely in-plane (IP), indicates that the polymer exhibits a “face-on” orientation,4 with a face-on alignment considered to be advantageous for hole transport across the active layer.3, 45 One dimensional OOP and IP line profiles extracted from the 2D scattering images are plotted in Figure 3. At the lowest PC71BM loading (33 wt.%) no new peaks are evident with only subtle changes in the polymer peaks observed. As the concentration


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of PC71BM is increased above 50 wt.%, an isotropic ring located at qz=1.35Å is observed in the 2D scattering pattern which is assigned to the presence of pure PC71BM aggregates in the blend film.19 As the PC71BM concentration further increases to 75 wt.%, the scattering intensity of the PC71BM peak shows a systematic increase in scattering intensity, while both the OOP π-π stacking and IP alkyl stacking peaks of the polymer show a monotonic decrease, indicating that the crystallization of polymer is hindered as more PC71BM is added. To examine the changes in the crystallographic properties of both PBDTTT-EFT and PC71BM in more detail, the d-spacings and the coherence lengths of the relevant peaks were calculated by peak fitting and use of Bragg’s law and Scherrer equation respectively.46 The calculated values are summarized in Figure 4. As shown in Figure 4 (a), it is interesting to note that a decrease in d-spacing of the polymer IP alkyl stacking is observed as PC71BM is mixed in with the polymer, approaching a constant value at PC71BM loadings above 50 wt.%. A similar reduction of d-spacing in the side-chain stacking direction is also observed in PTB7 blends, which is explained by fullerene-induced interdigitation of polymer side-chains.47 The coherence length of polymer stacking along the IP alkyl stacking direction shows a monotonic decrease from 8.3 nm (0 wt.% PC71BM) to 7.1 nm (75 wt.% PC71BM) evidence that the presence of PC71BM slightly hinders the crystallization of polymer. Additionally, a broader orientational distribution of polymer alkyl stacking is observed in the blend films. Figure 4 (b) displays the calculated Herman’s orientation parameter of the alkyl stacking peak. Hermann’s parameter of the alkyl stacking peak ( S ) is calculated by: 48



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π /2

2 1 ∫ I ( χ ) cos ( χ ) sin( χ )d χ S = (3 0 π /2 − 1) 2 I ( χ ) sin( χ )d χ

∫

0

(1) where I ( χ ) is the scattering intensity as a function of polar angle. S can take values that vary between -0.5 and 1, where S = -0.5 corresponds to scattering completely within the plane parallel to the substrate (face-on), S = +1 corresponds to the scattering completely perpendicular to the substrate (edge-on), while S = 0 indicates no orientational preference. The neat polymer films exhibits an S-value of S = -0.095, indicating a slightly face-on configuration. PBDTTT-EFT chains start to lose its preferential orientation as PC71BM molecules are introduced. S increases to 0 in films with 75 wt.% PC71BM, showing a fully random orientation. Figure 4 (c) presents the variation of d-spacing and coherence of the PC71BM (200) peak (at q=1.35 Å-1) as a function of PC71BM fraction. The PC71BM aggregates in the films exhibit a relatively constant d-spacing of ~ 0.46 nm while the coherence length shows a steady increase in the blend films as a function of PC71BM loading, accompanied by an increase in scattering intensity. The picture provided by GIWAXS is that the introduction of PC71BM hinders polymer crystallization and preferential orientation of the polymer, with PC71BM aggregates forming with PC71BM concentration above 33 wt.%. 3.2.2 Resonant soft X-ray scattering Resonant soft X-ray scattering (R-SoXS) was additionally performed to quantitatively determine domain purity and domain size, see Figure 5.3, 49-50 The data shown in Figure 5, collected at 284.0 eV, provides the maximum scattering contrast


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between PBDTTT-EFT and PC71BM along with minimal background scattering (see Figure S4 for the calculated contrast function). The characteristic size scale is calculated as lc = 2π / q where q is the peak position. Interestingly despite there being no evidence for PC71BM aggregates from the GIWAXS results for a PC71BM loading of 33 wt.%, a clear peak is observed in the R-SoXS pattern of the 33 wt.% blend corresponding to an lc of 28.2 nm. As the PC71BM loading is increased, a more intense peak is observed which shifts to lower q. The shifting in peak position to lower q indicates a coarser morphology indicating that the fullerene loading is influencing the characteristic morphological length scales. For the 60 wt.% blend, the q value corresponds to an lc of 33.5 nm, while for 75 wt.% blend,

the q value

corresponds to an lc value of 44.7 nm. The optimum value of lc = 33.5 nm for the 60 wt.% blend is consistent with values of lc of ~ 30 nm measured for other high efficiency blends based on polymers or small molecules such as PTB7, PffBT4T and p-DTS(FBTTh2)2.10,14,21 For 75 wt.% blends the domain spacings are significantly larger than the exciton diffusion length which will have implications for exciton harvesting efficiency, as confirmed by the photophysics measurements below. Since no PC71BM aggregates are observed in the 30 wt.% blend with GIWAXS, and since this blend also exhibits very high quenching efficiencies for both polymer and fullerene excitons (vide infra) the R-SoXS peak observed for this blend is likely due to local variations in the concentration of molecularly dispersed PC71BM molecules. 3.2.3 NEXAFS spectroscopy



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The chemical composition of the top surface of PBDTTT-EFT:PC71BM blends was investigated by surface-sensitive electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectra of PBDTTT-EFT: PC71BM blends with different weight ratios are shown in Figure S5. The chemical composition was obtained by fitting blend NEXAFS spectra to a linear combination of pristine PBDTTT-EFT and PC71BM spectra, see Figure 6.40 At low PC71BM loadings (33 wt.%), the chemical composition at the top surface is identical to the chemical composition of the solution. As the fraction of PC71BM increases from 33 wt.% to 75 wt.%, the PC71BM fraction at top surface shows a continuous increase but with the discrepancy between the chemical composition of the solution and that of the top surface becomes larger. To explain this phenomenon, we propose that NEXAFS spectroscopy is measuring the PC71BM content of the mixed phase, with a skin layer of this phase existing at the top surface that covers the relatively pure PC71BM domains that are embedded within the continuous mixed phase.51 Thus NEXAFS spectroscopy is directly measuring the composition of the mixed phase. At low PC71BM loading (33 wt.%), the measured surface concentration matches the overall blend ratio as there are few PC71BM aggregates that would reduce the concentration of the mixed phase relative to the solution concentration. As the PC71BM loading increases, PC71BM aggregates begin to form within the film with the measured surface composition deviating more and more from the solution concentration. Interestingly the PC71BM concentration of the mixed phase appears to saturate at ~ 50 wt.% similar to the value measured for MDMO-PPV:PC61BM blends.52


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3.3 Photophysics 3.3.1 Steady-state photoluminescence Steady-state photoluminescence measurements are presented in Figure S6. PBDTTT-EFT exhibits emission from 670 nm with a peak at 770 nm, while PC71BM exhibits weaker emission with two peaks at 720 nm and 800 nm. The PL quenching efficiency has been calculated and is plotted as a function of PC71BM loading in Figure 7. All blends show a high quenching efficiency over the region between 650nm and 850nm. This strong quenching can be explained by the rapid electron transfer from polymer to PC71BM. Two different excitation wavelengths were used in the PL measurement: 532 nm and 625 nm. For an excitation wavelength of 625 nm, the majority of photoluminescence stems from the polymer due to the low absorption strength of PC71BM at 625 nm (see Figure S7). In blends with low concentration of PC71BM, following excitation at 625 nm emission is mainly from the semicrystalline portions of the polymer, as the amorphous polymer chains are expected to be intimately mixed with fullerene. As more PC71BM molecules are introduced, the quenching efficiency shows a slight increase from 99.3 % (with 33 wt.% PC71BM) to a peak value of 99.6 % (with 60 wt.%), followed by a decrease to 99.1 % (with 75wt.% PC71BM). The increase in quenching with PC71BM loading going from 33 wt.% to 60 wt.% is consistent with the increased fullerene composition in the mixed phase as measured by NEXAFS spectroscopy. While the quenching efficiency slightly decreases above 60 wt.% this is likely due to emission from PC71BM aggregates, with PC71BM absorbing a non-negligible fraction at this high PC71BM



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loading. With excitation at 532 nm, emission from both polymer and fullerene can be expected with PC71BM absorbing a significant portion of the light at this wavelength (roughly 4 times as much for a 50 wt.% loading, see Figure S1 for UV-vis spectra, and Figure S7 for decomposition analysis of UV-vis absorption spectra). With 532 nm excitation, the quenching efficiency shows a continuous decrease as a function of PC71BM concentration from 99.1 % (33 wt.%) to 94.9 % (75 wt.%), indicating reduced exciton dissociation efficiency for higher PC71BM concentrations associated with larger and purer fullerene domains. It is plausible that energy transfer from fullerene to the polymer (prior to charge transfer) can account for some of the fullerene quenching. However, the TA data (below) does not show any evidence of an intermediate polymer exciton population, meaning that if this two-step process does occur, the final charge transfer step would be much faster than the preceding energy transfer from fullerene to polymer. 3.3.2 Transient absorption spectroscopy The effect that fullerene loading has on charge generation and recombination kinetics can be resolved via transient absorption (TA) spectroscopy. Our TA system can resolve dynamics from femtosecond to microsecond timescales and from visible to NIR frequencies, making it ideally suited to resolving exciton and charge dynamics in OPV blends.22, 43 TA spectral features are clearly seen for the blend with 33 wt% fullerene shown in Figure 8 (a). The spectra are dominated by a ground state bleach (GSB) feature (∆T/T>0) that corresponds to loss of polymer ground state absorption in the 1.6 - 2.1 eV range, and a photoinduced absorption peak (∆T/T 2) is common in OPV devices and is often explained in terms of the presence of energetic disorder.60 However variations in the spatial distribution of charge carrier concentration in thin (< 100 nm) active layer devices can also contribution to value of ϕ great than 2.61 While all the devices used in this study are about 70 nm thick the reaction order for these devices are significantly different, suggesting different orders of recombination. The reaction order decreases from 9.9 for 33 wt. % PC71BM to 3.4 for 75 wt.% PC71BM consistent with the presence of more energetic disorder and a higher t-DOS in PBDTTT-EFT:PC71BM blends with lower fullerene concentrations. 3.5 Discussion



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In the above we have provided a comprehensive analysis of the influence of fullerene loading on the performance, microstructure, photophysics and device physics of high-performance PBDTTT-EFT:PC71BM blends. From the above results we can separate the effect of fullerene concentration into three regimes: low concentration (below 50 wt.%), medium concentration (~ 50 wt.% to 70 wt.%) and high concentration (above 70 wt.%). In the low concentration regime, a significant fraction of fullerene molecules exists in the mixed phase. Although exciton dissociation is highly efficient in this highly blended morphology, the lack of pure fullerene aggregates hinders charge separation (as evidenced by the TA results) and those charges that do separate experience severe non-geminate recombination. While beneficial for exciton dissociation, the intimate mixing of polymer and fullerene results in a lack of fullerene percolation pathways and is characterized by a high degree of energetic disorder.20 Space-charge limited current (SCLC) measurements of electron mobility (see Table S1) confirm a relatively low electron mobility in the 33 wt.% PC71BM blend, with a value of 1.2 × 10-6 cm2/Vs recorded. As more fullerene molecules are introduced, the PC71BM concentration in the mixed phase increases to a maximum of ~ 50 wt. % with fullerene-rich clusters forming as observed by GIWAXS and R-SoXS. The formation of these PC71BM aggregates results in improved fullerene mobility (7.2 × 10-6 cm2/Vs at 60 wt.% PC71BM loading) and significantly reduced energetic disorder enabling charges to separate and be more mobile. Device performance also experiences a significant improvement as more


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fullerene molecules are introduced, following the observation by Kesava et al. PCE reaches an overall peak value at around 67 wt.%.32 In the high PC71BM concentration regime the fullerene domain size shows a rapid increase in size greatly exceeding the exciton diffusion length. The drop in PCE for PC71BM loadings of > 70 wt.% can largely be understood in terms of increased exciton recombination and reduced polymer light absorption. Due to the large domain size, the exciton dissociation is not as efficient as in the blends with smaller domain size. Photoluminescence originating from fullerene molecules shows a significant increase with PC71BM loadings above 60 wt.% with TA spectroscopy also observing delayed charge generation due to large PC71BM domains. Charge collection efficiency, at least in the fullerene phase, continues to improve with increasing fullerene loading (mobility increases to 1.7 × 10-5 cm2/Vs for 75 wt.% PC71BM), although a slight decrease in FF is observed going from 67 wt.% to 75 wt.% PC71BM. While a clear explanation for this decrease in FF is not apparent, it is important to note that in the polymer phase high PC71BM loadings have a negative effect on the molecular packing of the polymer, with reduced coherence lengths and a more random orientation of polymer crystallites. Thus while charge collection in the fullerene phases continues to increase with fullerene loading into the high concentration regime, hole collection in the polymer phase suffers.

4. Conclusion



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We have systematically investigated the influence of PC71BM loading on the performance,

microstructure,

photophysics

and

device

physics

of

PBDTTT-EFT:PC71BM blends. From the influence of PC71BM concentration on device performance we have separated the effect of PC71BM concentration into three regimes: (i) the low PC71BM concentration regime (< 50 wt.%) where polymer and fullerene remain largely well-mixed resulting in poor charge separation efficiency and poor electron transport; (ii) the medium PC71BM concentration regime (~ 50 wt.% to 70 wt.%) which optimizes device performance where PC71BM clusters form that facilitate charge separation and the concentration of the mixed phase is sufficient to provide percolation pathways to the electrodes; and (iii) the high PC71BM concentration regime (> 70 wt.%) where the PC71BM domains become over-coarse resulting in significant PC71BM exciton recombination and polymer crystallites are most disordered. The results presented here clearly show that in low crystallinity polymer blends, control of the chemical composition of the mixed phase and the size of fullerene-rich domains are key factors in determining geminate and non-geminate charge recombiantion. Supporting information UV-vis, I-V curve, EQE, 2D GIWAXS images, Scattering contrast, NEXAFS spectra, steady state photoluminescence

Acknowledgements


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This work was support by the Australian Research Council (FT100100275, DP130102616). W.H. acknowledges support from the Monash University Postgraduate Publications Awards. W.H. and Y.B.C acknowledges support from Australian Center for Advanced Photovoltaics (ACAP) and Australian Renewable Energy Agency (ARENA). Part of this research was performed at the SAXS/WAXS and soft X-ray beamlines at the Australian Synchrotron, Victoria, Australia, and at beamline 11.0.1.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. C.R.M. and E.G. acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. N.C. and D.K. would like to acknowledge the funding agency "DST-India (SB/S3/ME/037/2014) for partial funding support



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Figure 1. Molecular structure of (a) PBDTTT-EFT and (b) PC71BM


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Figure 2. Device performance based on PBDTTT-EFT/PC71BM blend with different PC71BM wt.%. (a) Power conversion efficiency (b) photo-generated current with and without bias voltage (-1 V) (c) open circuit current and (d) fill factor.



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Figure 3. Line profiles of PBDTTT-EFT: PC71BM blend cut from 2D GIWAXS patterns. (a) out-of-plane (OOP) direction and (b) in-plane (IP) direction.


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Figure 4.The fit parameters from GIWAXS (a) D-spacing and coherence length of polymer PBDTTT-EFT in-plane alkyl stacking peak (b) S value of PBDTTT-EFT in-plane alkyl stacking peak and (c) D-spacing and coherence length of PC71BM.



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Figure 5. Lorentz-corrected RSOXS scattering profiles of PBDTTT-EFT:PC71BM blends with different PC71BM wt.%. Scattering profiles were taken at the X-ray energy of 284.0 eV.


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Figure 6. The chemical compositions on the top surface, fit from NEXAFS spectra.



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Figure 7. PL quenching efficiency as a function of PC71BM wt.% in the film deposition solution. Excitation wavelengths are indicated in the legend.


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Figure 8. (a) Series of transient absorption spectra for a thin film blend of PBDTTT-EFT:PC71BM (33 wt%) at various times after excitation at 532 nm (100 fs, ~3 µJ/cm2 for ∆t < 6 ns, and 600 ps excitation, 3 µJ/cm2 for ∆t > 6 ns). The shaded regions represent GSB and polaron features, whose integrated normalized kinetics are shown in parts (b) and (c) for the same blend as well as for blends with 50 wt% and 60 wt% PC71BM.



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Figure 9. Parameters extracted from transient photovoltage measurement. (a) Small perturbed carrier lifetime τ∆n as function of bias. (b) Charge carrier density (n) vs. applied bias fitted with the equation 2. (c) τ∆n as the function of total charge carrier density in the cells showing power law dependence.


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Impact of Fullerene Mixing Behavior on the Microstructure

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