The Role of Stabilizing Surfactant on Capacitance, Charge and Ion

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Organic Electronic Devices

The Role of Stabilizing Surfactant on Capacitance, Charge and Ion Transport in Organic Nanoparticle-Based Electronic Devices Mohsen Ameri, Mohammed Al-Mudhaffer, Furqan Almyahi, Georgia Fardell, Melissa Marks, Alaa Al-Ahmad, Adam Fahy, Thomas Andersen, Daniel C Elkington, Krishna Feron, Michael Dickinson, Feridoun Samavat, Paul C. Dastoor, and Matthew James Griffith ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19820 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Applied Materials & Interfaces

The Role of Stabilizing Surfactant on Capacitance, Charge and Ion Transport in Organic Nanoparticle-Based Electronic Devices

Mohsen Ameri,a,b Mohammed Al-Mudhaffer,a,c Furqan Almyahi,a,c Georgia C. Fardell,a Melissa Marks,a Alaa Al-Ahmad,a,c Adam Fahy,a Thomas Andersen,a Daniel C. Elkington,a Krishna Feron,a,d Michael Dickinson,a Feridoun Samavat,b Paul C. Dastoor,a Matthew J. Griffitha,*

a Centre

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

b Department

of Physics, Bu-Ali Sina University, Hamedan, Iran.

c Department

of Physics, College of Education for Pure Sciences, University of Basrah, Iraq

d

CSIRO Energy, Newcastle, NSW, 2300, Australia

CORRESPONDING AUTHOR:

Matthew J. Griffith; Email: [email protected],

Phone: +61 2 4033 9191

KEYWORDS

Capacitance, charge transport, nanoparticle, organic electronic, photodiode, surfactant

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Abstract. Deposition of functionalized nanoparticles onto solid surfaces has created a new revolution in electronic devices. Surface adsorbates such as ionic surfactants or additives are often used to stabilize such nanoparticle suspensions, however, little is presently known about the influence of such surfactants and additives on specific electronic and chemical functionality of nanoparticulate electronic devices. This work combines experimental measurements and theoretical models to probe the role of an ionic surfactant on the fundamental physical chemistry and electronic charge carrier behavior of photodiode devices prepared using multicomponent organic electronic nanoparticles. A large capacitance was detected, which could be subsequently manipulated using the external stimuli of light, temperature and electric fields. It was demonstrated that analyzing this capacitance through the framework of classical semiconductor analysis produced substantially misleading information on the electronic trap density of the nanoparticles. Electrochemical impedance measurements demonstrated that it is actually the stabilizing surfactant that creates capacitance through two distinct mechanisms, each of which influenced charge carrier behavior differently. The first involved a dipole layer created at the contact interfaces by mobile ions, a mechanism that could be replicated by addition of ions to solution cast devices and was shown to be the major origin of restricted electronic performance. The second mechanism consisted of immobile ionic shells around individual nanoparticles and was shown to have a minor impact on device performance as it could be removed upon addition of electronic charge in the photodiodes through either illumination or external bias. The results confirmed that the surfactant ions do not create a significantly increased level of charge carrier traps as has been previously suspected, but rather, the critical issue to optimize the performance is preventing the diffusion of mobile ions through the nanoparticulate film and their accumulation at contacts.


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1. Introduction Nanoparticles created from a multitude of different materials have attracted intense interest as fundamental building blocks in numerous areas of science and technology. This interest arises from an often radical difference in chemical, electrical, optical, and structural properties of nanostructured materials compared to their bulk material counterparts. These different nanoscale properties are determined by size-dependent quantum effects. The ability to tune these nanomaterial properties by modulating the nanoparticle size and shape has allowed innovative breakthroughs in catalysis,1-3 quantum dots,4-6 biosensors and medical diagnostics,7-9 transistors,10 photovoltaics,11 and printed electronics.12-14 Successful incorporation of nanoscale components into useful devices requires control of how nanoparticles interact with their surroundings, including solvent molecules, other nanoparticles and bulk surfaces.15 Nanoparticle size, shape and surface functionality is typically controlled by introducing a stabilizing ligand or surfactant which coats the outer surface of the particles and prevents their subsequent aggregation.16-22 There is an extraordinary amount of literature that has probed the role of surfactants and stabilizing ligands in dictating nanoparticle size, shape, interparticle spacing, and specific surface chemistry.15,

23-28

However, despite these advances, there remains a significant lack of understanding regarding how these stabilizing materials influence the macroscale electronic performance of fully fabricated devices.29-31 In order to unlock the full potential of nanoparticulate electronic devices, a better understanding of how the stabilizing species interact with the other organic and inorganic components of these systems to influence charge carrier processes is urgently required.

There has been significant interest in creating electronic devices from organic semiconducting materials such as conjugated polymers and carbon allotropes for several decades. A major advantage of these organic semiconductors is the ability to coat the materials from inexpensive 3 ACS Paragon Plus Environment


solutions and inks at ambient temperatures across large areas using high throughput printing techniques.32-35 However, such organic semiconductors face limitations with poor charge generation and transport due to their low dielectric constants and often require halogenated and/or aromatic organic solvents for processing, which are harmful to both the environment and human health. An elegant strategy to address both of these limitations is to pre-fabricate the organic semiconductors into discrete nanoparticles of donor-acceptor blends dispersed in green solvents. Such nanoparticles allow tunability of the internal nanostructure through established colloidal chemistry, where an ambipolar surfactant can be added to the organic materials in the role of a stabilizing species to create a nanoscale miniemulsion.36-37 This approach has been utilized to fabricate a range of devices, including transistors,38 sensors,39-40 light emitting diodes41 and photovoltaic devices42-44 from organic semiconductor nanoparticles. However, the performance of these nanoparticulate devices does not yet match the levels of their traditional halogenated solventprocessed analogues. Whilst attention is being focused towards manipulating fabrication-tomorphology relationships by controlling both the amount and surface chemistry of surfactant,45-46 there has been very little attention directed towards understanding the influence of the surfactant ions on the fundamental physical chemistry in organic nanoparticulate devices. Indeed, a major limitation in the current understanding of organic nanoparticles is the failure of the probe methods that explore the electronic properties of semiconductors to account for the ionic surfactants used to stabilize the nanoparticles. This has led to a range of conclusions and assumptions regarding the detrimental role of the surfactants that creates a difficult environment in which to rationally optimize the materials design and device functionality.

In this work, we examine the influence of stabilizing species that are in the specific form of surfactant ions on the device physics of a nanoparticulate organic electronic photodiode prepared


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using the benchmark donor and acceptor materials regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Elucidation of the ionic-specific capacitance and its photophysical origin was achieved by combining theoretical analysis and experimental impedance and capacitance-frequency measurements. Measurements were applied to a range of different device configurations in the presence and absence of a sodium dodecylsulphate (SDS) surfactant. Through exposure of these photodiodes to various external stimuli, including light, temperature and electric fields, a generalized model explaining the influence of surfactants on nanoparticle device physics was developed. This model provides insight into universal design guidelines for nanoparticulate organic electronic devices prepared with any potential donor and acceptor blends.

2. Experimental 2.1.

Nanoparticle ink preparation.

Regioregular P3HT was synthesized at the University of Newcastle. Synthetic procedures have been previously reported,47 and produced a polymer with a molecular weight of Mn = 22.7 kg mol−1 and a PDI of 1.44. PCBM was purchased from Solenne BV, Netherlands, Anhydrous chloroform and SDS surfactant were supplied from Sigma Aldrich. The P3HT:PCBM nanoparticle (NP) dispersion was prepared with a ratio of 1:1 using the miniemulsion method reported previously.48 The NP suspensions were then subjected to successive dialysis treatments to concentrate the samples and remove excess SDS. Inks prepared in this study used a number of dialysis steps ranging from 4-6 depending upon the amount of excess surfactant desired in the ink suspension. Dialysis was performed using ultra centrifuge dialysis tubes (Millipore (10 kDa MWCO), with samples spun at 4000 rpm for 7 min. 5 ACS Paragon Plus Environment


2.2. Samples

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X-ray Photoelectron Spectroscopy were

prepared

by

spin-coating

a

formulation

consisting

of

poly(3,4-

ethylenedioxythiophene) with a poly(styrene sulphonate) anionic counterion (PEDOT:PSS; Baytron P) film onto a float glass substrate that had been dried at 150°C for 20 minutes. A nanoparticulate P3HT:fullerene acceptor film was then spun onto the PEDOT:PSS film and mounted to a sample holder using a 6 mm strip of conducting carbon adhesive coating (Ted Pella Inc.), with no direct grounding applied. X-ray photoelectron spectroscopy (XPS) measurements were obtained by illuminating the samples with a non-monochromatic X-ray source (Omnivac) using Al Kα radiation (1486.6 eV), and the subsequent photoemission was collected by an SES2002 analyzer (Scienta). Survey scans were carried out with a pass energy of 100 eV, while regions scans were performed using a pass energy of 20 eV and 200 meV steps. The working pressure in the analysis chamber was 2.5 x 10-9 mBar. Samples were measured directly as prepared, after an initial heat treatment at 110°C for 10 minutes, and then after further heat treatments at 140°C for both 4 and 16 minutes. Spectra are charge corrected to C1s (C-C, C-H, adventitious carbon) set to 284.8 eV. 2.3. Photodiode Fabrication. Patterned indium-doped tin oxide (ITO) coated glass substrates (Xin Yan Technology Ltd; 15 Ω/□) were cleaned by successive sonications in milli-Q water, acetone and isopropanol. PEDOT:PSS films were deposited onto the ITO-glass substrates at a spin speed of 5000 rpm and subsequently dried at 150°C for 15 min. For nanoparticulate photodiode (NP-PD) devices, the aqueous P3HT:PCBM NP ink was deposited onto the PEDOT:PSS interlayer by spin coating at 1750 rpm. For molecularly blended photodiode (MB-PD) reference devices, the initial chloroform solution


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of P3HT:PCBM with a ratio of 1:0.8 was deposited onto the PEDOT:PSS interlayer by spin coating at 950 rpm. The photoactive films were dried at 110°C for 5 min and the film thicknesses were determined to be (95±10 nm) for NP-PD films and (170±15 nm) for MB-PD after measurement with an Alpha Step 500 profilometer (KLA-Tencor). Calcium/aluminium (Ca/Al) electrodes were subsequently evaporated onto the active layers under vacuum (3 × 10-7Torr). The thicknesses of the Ca and Al layers were measured to be 30 nm and 100 nm respectively using a quartz crystal microbalance. After thermal deposition of Ca/Al cathodes, devices were annealed at 140°C for 4 min. SDS surfactant was also introduced to reference MB-PD devices (MB/SDS-PD) through addition of surfactant into the PEDOT:PSS solution prior to interlayer deposition. The SDS was added at concentrations of 1.25 mg mL-1 (“high SDS”) and 0.70 mg mL-1 (“low SDS”) in order to quantitatively match the amount of SDS surfactant present in the corresponding NPPD devices.

2.4. Photodiode Characterisation Capacitance–frequency and impedance spectra were measured using a Keysight LCR meter (model E4980A) over the range of 20 Hz to 1 MHz with an AC probe oscillation amplitude of 30 mV. A white light diode was employed to provide illumination, adjusted to an approximate light bias intensity of ∼100 mW/cm2 by modifying the intensity to produce a device short circuit current equal to that measured under AM1.5 simulated sunlight conditions. The electric field stimulus was introduced by varying the applied DC bias voltage in dark. Equivalent circuit modelling was performed with EIS Spectrum Analyser software.49 Current density–voltage (J–V) measurements were conducted using a Newport Class AAA solar simulator with an AM 1.5 filter. The light intensity was adjusted to 100 mW cm2 by a silicon reference solar cell (FHG-ISE) covered with a



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KG5 cut-off filter. A Keithley 2400 sourcemeter was used to record the J–V data. 6 repeat samples were prepared for every photodiode measured, with variations in the measured outputs indicated as standard errors where appropriate.

3. Results and Discussion 3.1 Influence of Ions on Analysis of Charge Carriers in Nanoparticle Devices Figure 1a depicts the structure of the NP-PD and MB-PD devices employed in this study. The corresponding J–V curves of these two devices are presented in the inset of Figure 1b, and show a clear performance limitation for the NP-PD devices as reported in previous studies.43, 48, 50 The power conversion efficiencies of the MB-PD and NP-PD devices were measured to be 3.2% and 1.0%, respectively (Table 1). To analyze the nature of this performance limitation, the capacitance–frequency spectra of the MB-PD and NP-PDs were measured between frequencies of 20 Hz and 1 MHz in the dark with no DC bias voltage applied (Figure 1b). Capacitance-frequency measurements are a well-known technique for exploring the behavior of various types of charges within a semiconductor material. Recently, this technique has been used to study organic photodiodes, investigating phenomena including the molecular doping level,51

trap defect

detection and quantification,52-56 and charge carrier kinetics.57-58 The technique has also been employed to analyze the ionic contribution in doped semiconductors59-60 and to isolate ion-induced capacitance in perovskite solar cells.61-62 It is therefore an ideal technique to probe any potential photophysical processes associated with the surfactant ions in the nanoparticulate organic electronic devices. The MB-PD device shows a standard dark capacitance-frequency spectrum consistent with past studies in the literature.52 The capacitance response is spectrally flat in the frequency domain from 10 Hz to 100 kHz, dropping away at higher frequencies as the series


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resistance begins to dominate the capacitive impedance. As the film thickness is smaller than the depletion width of the polymer for both devices (wdepletion ≈ 150 nm), this response is attributed to the geometric capacitance of the device and corresponds to a dielectric constant of 3.5, in agreement with reported values for these materials.63-64

Figure 1: (a) A schematic diagram illustrating the photodiode device architecture and active layer morphology for molecularly blended (MB) and nanoparticulate (NP) devices. (b) The capacitance per unit volume measured in the dark at short circuit conditions as a function of frequency for MBPD (black circles) and NP-PD (red squares) devices. The inset shows the corresponding device current density-voltage curves in the dark (open symbols) and under illumination (solid symbols). We note that the error margin for this measurement is smaller than the size of the data points on the plots.



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Table 1: Device performance parameters for both a NP-PD and reference MB-PD organic photovoltaic device. Morphology

Voc (V)

Jsc (mA cm-2)

Fill Factor

Efficiency (%)

Nanoparticle

0.365 ± 0.030

6.93 ± 0.45

0.400 ± 0.015

1.0 ± 0.2

Molecularly Blended

0.540 ± 0.030

8.66 ± 0.65

0.675 ± 0.015

3.2 ± 0.3

In contrast, Figure 1b demonstrates that the capacitance spectra of the NP-PD device is markedly different to that of the MB-PD device. The capacitance is no longer constant with respect to frequency, but rather exhibits distinct low (20-1 kHz), mid (1k Hz – 10 kHz) and high (greater than 10 kHz) frequency regions that contribute to the capacitance response of the devices. In particular, a large increase in the capacitance is observed when transitioning from the midfrequency to the low- frequency regions. Typically, elevated low frequency capacitance in organic electronic devices is attributed to enhanced charge carrier traps in the semiconductor materials. Such capacitance is then related directly to the density of states, g(E):52, 65 𝑢0 𝐶 = 𝑞2 𝑔(𝐸) 𝑢𝑒𝑥𝑡

(Equation 1)

𝑢0

where q is the elementary charge and the ratio 𝑢𝑒𝑥𝑡 is the voltage drop in the AC perturbation over the space charge region of the junction. Changing the frequency of the probe signal, subsequently alters the energy depth within the semiconductor band at which the AC voltage systematically traps and de-traps charges. Thus these measurements provide an energy spectroscopy tool to systematically explore the behavior of faster carriers at high frequencies, slower moving mobile charges at intermediate frequencies and deeply trapped charges at low frequencies to be uniquely


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identified. The density of states distribution, g(E), can then be obtained from the derivative of the capacitance-frequency spectra:65-66 𝑔(𝐸) = ―

𝛽 𝑉𝐵𝐼 𝑑𝐶(𝜔) 𝑞𝐴𝑑𝑘𝐵𝑇𝑑 𝑙𝑛(𝜔)

(Equation 2)

where VBI is the built-in potential, ω is the angular frequency of perturbation, kB the Boltzmann constant, T the absolute temperature, and A and d are the film photoactive area and thickness respectively. β is a small correction which accounts for the drop in the AC probe voltage with distance from the electrode contacts. Since this factor is not known precisely in these measurements, we have set β to 1 in our analysis, which previous reports have shown will provide a minimum bound for the density of states.66 Derivation of the density of states (DOS) using Equation 2 and the derivative of capacitance-frequency data from Figure 1 shows the MB-PD device exhibits a g(E) distribution consistent with previous reports (Supporting Information, Figure S1). The DOS shows an exponential dependence at small trap energies representing the tail states of the polymer HOMO and a Gaussian distribution of states at deeper trap energies, consistent with previous reports of organic semiconductors.66-67 The total density of deeply trapped states was found to be 5.6 x 1015 cm-3 from fits to the DOS (Figure S1). The NP-PD device exhibits a similar shape, although now the apparent density is increased by a factor of 3 for shallow traps and more than an order of magnitude for deeper traps (7.0 x 1016 cm-3). Interpreted with standard semiconductor physics, this analysis appears to suggest the addition of surfactant molecules into the organic materials to create the NP-PD devices creates a large increase in the number of electronic trap states in the semiconductor materials. Such conclusions are apparently supported by the density of defect states extracted from a Mott-Schottky analysis performed on the capacitance measured as a function of varying bias voltage. Here the apparent doping density in the dark can be extracted under certain boundary conditions from the slope of C-2 vs V,68 providing 11 ACS Paragon Plus Environment


values of 7.5 x 1015 cm-3 for the MB-PD and 1.8 x 1017 cm-3 that are consistent with the trap density from the DOS analysis (Figure S2). However, the semiconductor physics that underpins the analysis from these widely applied techniques fails to account for the additional charged surfactant ions present in the NP-PD device. Indeed these stabilized nanoparticles are actually more analogous to organic mixed ion-electron conducting polymers such as PEDOT:PSS in that both electronic charge movement through semiconductor bands and ionic movements through a bulk film have important roles in the conduction mechanims.69-71 Critically, a full understanding of the origins of material capacitance in such systems remains an unclear. The apparent “density of trap states” may therefore not be at all related to electronic states in the transport bands of the semiconducting materials. Indeed, calculation of the hole current alone that is expected from the derived DOS and the Fermi-Dirac occupation probability produces a value far in excess of the total measured photocurrent for the NP-PD device (18 mA cm-2). This situation is not physically realizable and indicates that the DOS analysis and trap density values derived from standard semiconductor measurements cannot explain the electronic behavior of organic electronic materials with ionic character. This finding has significant implications for analysis of electronic devices incorporating organic nanoparticles, which often assume additional charge trapping due to the presence of surfactant.24, 36, 72-73

3.2 Origin of Surfactant Capacitance in Nanoparticle Devices To first identify the origins of the distinct capacitance signals from Figure 1, modified capacitor devices were prepared by casting either a pure PEDOT:PSS layer, or a PEDOT:PSS ink prepared by incorporating SDS ions into the ink and then casting this mixed dielectric between an ITO anode and Ca/Al cathode. The C-f spectra for these devices are shown in Figure 2. The


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PEDOT:PSS/SDS mixed dielectric device shows a strong rise in the capacitance at mid to low frequency, mimicking the response of the NP-PD devices observed in Figure 1b. In contrast, the device prepared with a pure PEDOT:PSS dielectric shows a capacitance an order of magnitude lower than the PEDOT:PSS/SDS device, with little dependence on frequency in the range of 101106 Hz. Since the pure PEDOT:PSS device does not exhibit any frequency dependent capacitance, the low frequency response of the PEDOT:PSS/SDS device must arise predominantly from the SDS ions.

Figure 2: The capacitance per unit volume measured in the dark as a function of frequency for modified capacitor consisting of either a pure PEDOT:PSS layer, or a PEDOT:PSS layer with incorporated SDS surfactant sandwiched between ITO and aluminum electrodes. We note that the error margin for this measurement is smaller than the size of the data points on the plots.



The data in Figure 2 strongly implies that the SDS surfactant is the origin of a large capacitive response in organic electronic films. The precise location of the SDS ions within the nanoscale structure of the device now becomes of critical importance. In the traditional colloidal formation process, the stabilizing surfactant will adhere to the organic materials to stabilize the blended nanoparticles and are likely to then become immobile. However, excess SDS ions not required for this stabilization process may remain mobile within the active layer and can potentially diffuse to different spatial locations within the device.45 Establishing whether the ions can move within the solid film becomes an important factor to establish in order to understand how the capacitive response of the ions will influence the device performance. In order to investigate if SDS ions exhibit any substantial solid state diffusion, nanoparticulate P3HT:fullerene acceptor films were probed using X-ray photoelectron spectroscopy (XPS). The high surface sensitivity of the XPS technique allows a quantitative analysis of the chemical and electronic state of the top 1-5 nm of the nanoparticulate films. XPS spectra were acquired for samples isolated at different stages of thermal treatment throughout the device fabrication, including directly after preparation, after an initial heat treatment at 110°C for 10 minutes to dry the films and commence annealing,74 and after further annealing heat treatments at 140°C for either 4 or 16 minutes to fully complete polymer crystallization. The full spectrum survey scans show evidence of carbon (1 s peak), oxygen (1 s peak), sulphur (2s and 2p peaks) and sodium (1s peak) as expected from the materials in the NP films (Figure S3). There is no evidence of any silicon or indium peaks in the full spectrum survey scans, and as the technique is highly surface sensitive we have therefore concluded that the films are densely packed and form pinhole-free NP photoactive layers. To begin to identify chemical changes in the films under various treatment regimes, higher resolution spectra (region scans) for the C1s, O1s, S2p and Na1s transitions were collected (Figure 3 and Figures S4-S5). The region


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scans allowed a determination of the relative percentage concentrations of the four elements for each sample, with the sodium signal considered a unique identifier of the SDS ions in the nanoparticle film (Figure 3(b)).

Figure 3: (a) XPS region scan spectra focusing on the Na 1s energy region for P3HT:PCBM nanoparticulate films after various thermal treatments. (b) The atomic concentration of sodium extracted from the XPS scans plotted for each different film treatment condition. These results in Figure 3(b) show that the sodium atomic composition is substantially increased (from 1.6% to 4.5%) after the initial 110°C heat treatment, before it then falls significantly with a



standard annealing treatment at 140°C for 4 minutes. Further heating of the film at 140 °C for a total time period of 16 minutes does not have any additional influence on the sodium surface concentration. The carbon (C1s) peak shows contributions from sp3 and sp2 carbon-carbon bonds as well as contributions from carbon-oxygen and carbon-sulphur bonds. The composition of these peaks are attributed predominantly to the P3HT and fullerene acceptor, and show negligible changes with the various thermal treatments (Table S2). The oxygen (O1s) peak shows evidence of hydroxides, organic oxygen, sulphate group and signals for both bulk film and surface water species, consistent with previous reported peak fitting energy values (Figure S3).75-77 Here the sulphate peak at 532.5 eV is used as another chemical marker for SDS, and we note that the sulphate peak shows the same initial rise in concentration when films are heated to at 110 °C, with a subsequent fall at 140 °C as observed in Figure 3 (Table S3 and Figure S6). The O1s peak also reveals the presence of both free water in the bulk of the film and adsorbed water at the surface of the films. As the temperature is increased, the bulk water decreases whilst the surface water concentration systematically increases, indicative of water moving to the surface to dry via evaporation (Figure S6). The deconvolution of the sulphur (S2p) spectra employs a spin orbit splitting of the 2p3/2 and 2p1/2 doublets of 1.2 eV, and a 2:1 area ratio (respectively). Based on similar fitting in the literature, the spectra have been deconvoluted into 5 separate components (each a doublet).78-79 Two components were identified to be associated with the SDS surfactant (167.7/168.9 eV and 169.3/170.5 eV), and a further two were associated with the sulphur atom in the thiophene ring of the P3HT polymer in both neutral (163.5/164.7 eV) and partially oxidized (164.7/165.9 eV) forms (Figure S3). A shake-up doublet (167.0/168.2) was also identified, consistent with the literature.78-79 Again we note that the distinct SDS sulphur signals show an initial rise in surface concentration when films are heated to at 110 °C, with a subsequent fall at 140 °C (Table S4 and Figure S6). The XPS data indicates that SDS ions experience solid state 16 ACS Paragon Plus Environment

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diffusion through the active layer to the nanoparticulate film interfaces during thermal treatment. We have previously shown that thermally-induced changes in the NP film morphology only begin to occur at a temperature of 120 °C,80 and that furthermore, at temperatures exceeding this value the changes in the NP film morphology initiate a movement of SDS ions away from the top film surface.81-82 This established behavior is consistent with the data in Figure 3(b) and Figure S6, where the SDS surface concentration is lower after heat treatment at 140 °C than it is in the initially prepared films. At the lower treatment temperature of 110 °C, the morphology-induced movement of SDS has not yet been triggered, and the main change to the films is the removal of residual water through the top surface of the films. This is confirmed by the rise in the surface water signal from the O1s peak (Figure S6). We hypothesize that as this water rises to the film surface, it carries associated solvated mobile SDS molecules through the polar sulphate head group. As such, the XPS data reveals that the SDS ions are highly mobile inside the device under typical fabrication conditions.

Figure 4a shows the structure of the MB/SDS-PD devices prepared by adding SDS surfactant to the reference MB-PD architecture. The dark capacitance-frequency spectra and corresponding photodiode current density-voltage curves for devices prepared with either no (MB-PD), a low (0.7 mg mL-1) or a high (1.25 mg mL-1) SDS concentration in the PEDOT:PSS ink are shown in Figure 4b. These values were selected to provide equivalent amounts of SDS surfactant to those present in NP-PD devices examined later in this study. As the SDS content in the PEDOT:PSS layer is systematically increased, the low frequency capacitance of the OPV devices shows a corresponding increase. This observation is consistent with the earlier determination of SDS ions as the origin of the low frequency capacitance. As the SDS content in the PEDOT:PSS layer is increased, the electronic performance of the MB-PD devices also shows a systematic decrease 17 ACS Paragon Plus Environment


(Figure 4b inset and Table S8). This change is driven by a reduction in the short circuit photocurrent with no significant variation in the open circuit voltages observed with varying SDS content. The built-in electric field of the devices, determined from the voltage at which the dark and photocurrent values are equivalent, also shows no change with SDS content, indicating that the electric field driving charge extraction is unaltered by the changing SDS content (Figure S7

Figure 4: (a) A schematic diagram illustrating the modified photodiode structure where SDS surfactant ions have been added to the PEDOT:PSS hole transport layer. (b) The capacitance per unit volume measured in the dark at short circuit conditions as a function of frequency for the MB/SDS-PD devices with no SDS (black diamonds), a low SDS content in the PEDOT:PSS layer (red circles), and a high SDS content in the PEDOT:PSS layer (blue squares). The inset shows the corresponding device current density-voltage curves under AM1.5 illumination. We note that the error margin for this measurement is smaller than the size of the data points on the plots. 18 ACS Paragon Plus Environment

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and Table S8). These observations suggest that the surfactant accumulates at the hole extracting interface and substantially restricts the charge extraction through this layer to the anode. This conclusion is supported by observing the photocurrent at larger reverse bias values, where the device without SDS shows a stable photocurrent with minimal change from short circuit to -2 V, where the devices with SDS show substantial photocurrent increases as the reverse bias is increased (Figure S4), consistent with a photocurrent extraction problem.

Figure 5 shows the current density-voltage curves and dark capacitance-frequency spectra of NPPD devices prepared with a successively increasing amount of dialysis to vary the SDS content in the same manner as the MB-PD devices examined earlier. Increasing dilution factors of 3x103, 2x104 and 1x105 were used to produce “high”, “medium” and “low” SDS content in the NP-PD devices, respectively. We have previously quantified the effect of these dialysis treatments on the inks, reporting that the “free” (mobile) SDS content in the active layer ink decreases from 1.15 mg mL-1 for high SDS content to 0.86 mg mL-1 for medium SDS content and down to 0.63 mg mL-1 for low SDS content under these dialysis conditions.46 These three inks were selected as our previous studies found that the particle size was invariant at 40 ± 9 nm across this ink series. Furthermore, Zeta potential measurements of the inks performed in this work reveal a stable value of -32 ± 4 mV for all three of the NP inks (Figure S8). Although calculation of the true surface charge from the Zeta potential is famously problematic,83 the Zeta potential can be used to calculate the interfacial charge present at the outer edge of the Stern layer. For this purpose the Grahame equation is employed under the assumption that the potential at the slipping plane (Zeta potential) approximates the potential at the interface of the Stern and diffuse layers around the nanoparticles.84 Performing this calculation shows the three inks exhibit relatively invariant values



of interfacial charge densities of 0.4 – 0.44 μC cm-2 (Table S7), indicating a constant value of the surfactant surface charge in all three inks. These values are in close agreement with the theoretical surface charge that is calculated for a 40 nm nanoparticle by assuming an SDS surface footprint of

Figure 5: (a) The current density-voltage curves measured for NP-PD devices prepared with a low (red circles), medium (green diamonds) and high (blue squares) SDS surfactant content. J– V curve were measured in the dark (open symbols) and under AM1.5 illumination (Solid symbols). (b) The capacitance per unit volume at short circuit conditions as a function of frequency measured in the dark (open symbols) and under illumination (solid symbols) for NP-PD devices prepared with a low (red circles), medium (green diamonds) and high (blue squares) SDS surfactant content. We note that the error margin for this measurement is smaller than the size of the data points on the plots. 20 ACS Paragon Plus Environment

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5.5 nm2, as previously reported for this surfactant at water-oil nanodroplet interfaces.85 Utilizing these parameters returns a theoretical surface charge of 2.5 μC cm-2, consistent with previous measurements for this surfactant.86 This value would not be expected to vary with the SDS concentration across the range of values in the nanoparticle inks of this study, which matches the observation from the experimentally measured parameters. The theoretical value is also expected to be slightly higher than the experimental value, as it does not take into account the charge screening in the Stern layer that is included in the experimental measurement. These parameters therefore show that the three inks do not vary in their amount of surface charge or particle size, but rather only in the concertation of mobile SDS present in the ink dispersions.

The J-V curves in Figure 5a indicate a significant decrease in both the open circuit voltage and short circuit photocurrent of devices with increasing SDS content (Table S9). This decreased photocurrent is accompanied by a 50% decrease in the built in voltage of devices as the SDS content is increased from the low to high levels. This result implies that the SDS ions have a large influence on the built-in field in the NP-PD devices, in contrast to the previous result for the MB/SDS-PD devices. Furthermore the capacitance frequency spectra for the NP-PD devices show the opposite trend to that previously observed for the MB/SDS-PD. Figure 5b indicates that as the SDS content in the NP-PD devices is increased, the low frequency capacitance shows a significant decrease. This result is in direct contrast to the MB/SDS-PD devices, and coupled with the substantially different performance trends of the NP and MB/SDS-PD devices with additional SDS content, provides a clear conclusion that although there is a second unique influence of the SDS ions in the NP-PD devices.



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3.3 Mechanism of Surfactant Capacitance in Nanoparticle Devices Considering the cumulative inferences drawn from the data in Figures 1-5, it can be concluded that the observed capacitive response in NP-PD devices is fundamentally different from the normal MB-PD device response in that both the photogenerated charges and surfactant ions contribute to the capacitance. How the surfactant generates capacitance and whether the ionic and electronic capacitance mechanisms are interdependent remains an issue for further investigation. It was noted in Figure 5b that the ionic capacitance response of the NP-PD devices is reduced when the excess SDS ion concentration is increased. This observation cannot be explained using the classic electrochemical Stern model of a capacitive double layer with an inner compact (Stern) layer and an outer diffuse layer. This is because the two layers act as capacitors in series such that the total capacitance is given by:

(Equation 3)

1 𝐶𝑡𝑜𝑡𝑎𝑙

=

1 𝐶𝑆𝑡𝑒𝑟𝑛

+

1 𝐶𝑑𝑖𝑓𝑓𝑢𝑠𝑒

Thus any increase in the ion concentration will increase either CStern or Cdiffuse (or both), leading to an enhancement of the total capacitance.87 Assuming that the SDS ions and the polymer/fullerene nanoparticles establish an electrical double layer in the solid state, the mechanism by which capacitance is generated is clearly more complex than a simplistic planar electrode model, and may need to consider additional factors such as the chemical affinity between the adsorbed SDS ions and nanoparticles, ion transportation in a confined solid state system, and the Ohmic resistance associated with the insulating surfactant molecules. Towards this purpose, the specific electronic and ionic contributions to the NP-PD device capacitance were examined in further detail by applying various external stimuli in order to elucidate their respective effects on the electron transfer reactions that influence device performance.


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Figure 6 shows the capacitance-frequency spectra of both a MB/SDS-PD and the highest performing NP-PD device from Figure 5a. Three distinct regions can be observed in these plots, each of which is influenced in a different manner upon subsequent illumination. The high frequency region (105-106 Hz) exhibits the same value of capacitance for all illumination intensities. At this high AC probe frequency, the response of photogenerated charge and ions in the active layer are both frozen out and the capacitance trends to a common value representing the geometric capacitance of the device. Between 103 and 104 Hz, the capacitance response is observed to increase with increasing illumination for both the MB/SDS-PD device and the NP-PD device. This response is typical for a doped semiconductor, and has been previously demonstrated to relate to the chemical capacitance (trapped photogenerated charges) in organic electronic devices.51 The capacitance-frequency spectrum of a reference MB-PD device without any SDS was also measured and found to exhibit the same increase in capacitance in the mid-frequency region (Figure S9), confirming the enhanced capacitance with increasing illumination intensity is related to accumulated chemical capacitance in the polymer and fullerene materials.88 At frequencies of 101-102 Hz, the capacitance of the MB/SDS-PD device shows a small increase with increasing illumination (Figure 6b), however, the NP-PD device exhibits the opposite trend in this frequency region, displaying a substantial decrease with increasing illumination intensity (Figure 6a).

The response for the MB/SDS-PD is typical of a standard MB-PD device, and is consistent with the presence of deep trap states in the semiconductor materials. However, the value of the capacitance is significantly larger than a reference MB-PD device without SDS ions. This may indicate either an altered dielectric constant due to the presence of the SDS ions, or alternatively, a double layer being formed at the PEDOT:PSS/active layer interface which acts to prevent hole



Figure 6: The capacitance per unit volume of an (a) NP-PD device, and (b) MB/SDS-PD device measured at short circuit conditions as a function of frequency with various levels of illumination. The inset shows the normalized efficiency of the device at each light level. We note that the error margin for this measurement is smaller than the size of the data points on the plots.

extraction, increasing the trap occupancy and resulting in an increased capacitance value. In contrast, the NP-PD devices exhibit a substantial decrease compared to the dark capacitance as the light level is increased up to a limit of 0.1 Suns, where the low frequency response subsequently saturates and does not change with further increases in illumination intensity. This frequency 24 ACS Paragon Plus Environment

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region was earlier demonstrated to be dominated by the response of the SDS ions in the active layer, and the data in Figure 6a provides further confirmation that this low-frequency capacitance cannot be ascribed to polymer defects or chemical capacitance since it exhibits the opposite of the trend with increasing illumination to that observed to both the standard reference MB-PD and MB/SDS-PD device. The insets to Figure 6a and 6b show the normalized efficiency of the NP-PD and MB/SDS-PD devices as a function of the light intensity. The NP-PD device exhibits a doubling of the power conversion efficiency as the light intensity is increased from 0.01 to 0.1 Suns, and then remains constant up to a value of 1 Sun. In contrast the MB/SDS-PD device shows no change in efficiency with variation in light intensity. As both devices show the same trend in capacitance changes with illumination intensity in the high and mid frequency where the active layer trap occupancy dominates the signal, the difference in efficiency changes with illumination intensity between the two devices is attributed to the contrasting low frequency capacitance changes with illumination intensity. It is therefore concluded that the ionic surfactant is detrimental to the charge transfer reactions associated with photogeneration, transport and recombination in the NP-PD devices. However, as this signal disappears with increasing illumination intensity, it is inferred that it is possible to reduce the influence of the surfactant substantially through external stimuli. Indeed, the capacitance-frequency spectra of the NP-PD and MB/SDS-PD trend to equivalent values and spectral shape under 1 Sun illumination, implying that the unique ionic influence on NP-PD device performance is almost entirely removed under illumination. A similar trend is observed when the external stimuli of bias voltage is used in place of illumination. The application of a DC bias in the dark provides an alternative method to fill the density of electronic states in the polymer and fullerene with electronic charge. The application of a forward bias voltage to NP-PD and reference MB-PD devices was found to produce the same trends in the various frequency



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regions as illumination (Figure S10), confirming the results with varying illumination intensity that the capacitance changes in the mid frequency region originate from electronic charges whilst thise in the low frequency region originate from an ionic response. The capacitance in the mid frequency region is observed to increase linearly with both a decreasing effective built in potential and increasing illumination intensity for the MB-PD and NP-PD devices (Figure S11), consistent with Equation 2, where for a fixed density of states the product of the effective built in potential, 𝑑𝐶

VBI, and the derivative of the capacitance with respect to frequency, 𝑑𝑙𝑛(𝜔), should remain constant.

To clearly separate the influence of electronic and ionic contributions to the device capacitance of NP-PDs, the dependence of the dark capacitance on temperature was probed (Figure 7). This thermal treatment can have several influences. At higher temperatures the active layer morphology can expand, resulting in a change in the geometric capacitance as the dielectric layer thickness is altered.89 The thermal energy can also induce ionic movement through an Arrhenius activated process90 or chemical capacitance, Cμ, by promoting excess free charge carriers to the conductance and valance bands respectively through thermally activated de-trapping. By keeping the temperature between 20-60 °C the first two mechanisms become negligible as the temperature is well below the active layer blend glass transition temperature (thus no morphologically induced expansion is expected) and the thermal energy is an order of magnitude below the activation energy of 50-80 kJ mol-1 for molecular diffusion in polymers.91 Therefore, only thermally activated contributions to the electronic trap capacitance are probed, where 𝐶𝑡𝑟𝑎𝑝 is described by92: 𝜇 𝛿𝑛𝑡𝑟𝑎𝑝

= 𝑞2𝛿𝐸𝐹 𝐶𝑡𝑟𝑎𝑝 𝜇

(Equation 4)

𝑛,𝑝

with ntrap representing the excess charge concentration and EFn,p the quasi-Fermi level generated


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Figure 7: The capacitance per unit volume of an (a) NP-PD, and (b) MB-PD reference device as a function of frequency in the dark at various temperatures. The corresponding Arrhenius plots showing the relationship between capacitance and inverse temperature at frequencies of 100 Hz and 3.1 kHz are shown in (c) for an NP-PD device and (d) for the MB-PD reference device.

by the excess charge carriers in either the conduction (EFn) or valence bands (EFp). Using an exponential distribution of trap states, this equation then transforms to: = 𝑞2 𝐶𝑡𝑟𝑎𝑝 𝜇

𝛼𝑁𝑡𝑟𝑎𝑝 𝑘𝐵𝑇

exp

[

𝛼(𝐸𝑣 ― 𝐸𝑡𝑟𝑎𝑝) 𝑘𝐵𝑇

]

(Equation 5)

where α is the characteristic exponential tail parameter (T/T0), and Ntrap is the total density of trap states in the semiconducting materials. By recognizing that the frequency of the AC perturbation is equal to the trap depth (Eω – Ev) by the relation: 27 ACS Paragon Plus Environment


𝜔0

( )

𝐸𝜔 ― 𝐸𝑣 = 𝑘𝑇 𝑙𝑛

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𝜔

(Equation 6)

where ω is the perturbation frequency and ω0 the attempt to escape frequency, then Equation (5) can be converted into the following form specific to the parameters controlled in the capacitancefrequency measurements: = ―𝑞2 𝐶𝑡𝑟𝑎𝑝 𝜇

𝛼𝑁𝑡𝑟𝑎𝑝 𝜔 𝛼 𝑘𝐵𝑇

( ) 𝜔0

(Equation 7)

This expression allows the total density of trap states to be extracted from an Arrhenius plot of Cμ against 1/T at any given perturbation frequency. Figure 7a demonstrates that both the NP-PD device and the MB-PD device both show a consistent increase in the mid-frequency capacitance with increasing temperature at 3.1 kHz, however, the trend in the low frequency capacitance at 100 Hz is obscured by the large ionic capacitance, making extraction of the electronic charge density inaccurate at this frequency. The Arrhenius plots of Cμ against 1/T are linear for both the NP-PD in the mid-frequency region, in agreement with equation 7. The total density of states for the trapped charges in the semiconducting materials of each device can be extracted from the slope of the Arrhenius plots at 3.1 kHz where the electronic charge has been isolated and the ionic charge frozen out. This analysis produces values of Ntrap = (4.9 ± 0.7) × 1019 cm-3 and (2.0 ± 0.5) × 1019 cm-3 for the NP-PD and MB-PD respectively. This data indicates the total available trap state density is higher by a factor of 2-3 in the NP-PD device compared to the MB-PD device, a much smaller difference than the order of magnitude typically found using classical density of states analysis such as Figure S1 . These comparable values imply a density of trap states of the same order of magnitude in each device when probed at 3.1 kHz, where mobile and shallow trapped charges dominate the response. This finding confirms that electronically, the nanoparticulate devices are broadly equivalent to the solution cast MB-PD reference in the energy regime closer


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to the band edges. The results are in contrast to previous studies of nanoparticulate organic electroni materials, where the ionic response of the surfactant stabilized nanoparticles dominates and the density of trap states in the semiconducting materials is incorrectly determined to be orders of magnitude greater than the reference MB-PD device.

3.4 Influence of Surfactant Capacitance on Performance in Nanoparticle Devices The preceding sections demonstrated that the solid state semiconducting behavior of nanoparticulate films is comparable to solution cast films, however, the ionic surfactant creates a unique capacitance in NP-PD devices. A similar phenomenon has been observed in other types of nanostructured electronic devices with mobile ions. For instance, a large low-frequency dark capacitance has been reported in perovskite solar cells, which has been ascribed to electrode polarization induced by mobile ions.93-94 It is therefore critical that the influence of this ionic capacitance upon device performance can be clarified and controlled. Towards this purpose, the capacitance observed in the nanoparticulate devices and its specific influence on device performance was analyzed using electrochemical impedance spectroscopy. The Nyquist plots constructed from EIS measurements for an NP-PD device and a MB/SDS-PD are presented in Figure 8. The measured impedance data was fitted with an equivalent electric circuit model that is illustrated in Figure 8a. This equivalent circuit models the underlying device behavior using two parallel RC elements as has been previously reported.95-96 The first RC element, representng the bulk transport of charge carriers through the film, is composed of a transport resistance, Rtr and a geometric capacitance (Cg). The second RC element models the photoinduced changes to the device under illumination, represented by a chemical capacitance (Cμ) and recombination resistance (Rrec). The ionic contribution to capacitance is then modelled with an additional Debye



dielectric relaxation component comprising a dielectric capacitance and a constant phase element to mimic a frequency dependent dielectric polarization as has been previously used in devices with ionic character.48, 61 The fits generated from this equivalent circuit show close agreement with the measured impedance data under a range of different illumination levels for both the MB/SDS-PD and NP-PD devices, with parameters extracted from the fits recorded in Tables S5 and S6.

The geometric capacitance was determined to be 1.7 nF for the MB/SDS-PD device and 1.6 nF for the NP-PD device, equating to a blend dielectric constant of 3.8 and 3.6 respectively. These values are in agreement with previous reports for standard P3HT:PCBM solution cast devices without ionic surfactant present.64,

97

This is in contrast to the apparent elevated dielectric constant

determined from the dark capacitance-frequency spectra in Figures 4-6, which were clearly influenced by the dielectric capacitance of mobile ions in the devices. This finding of equivalent blend dielectric constants provides further evidence that the electronic nature of the semiconducting materials are equivalent in both the NP-PD and MB-PD structure. Figure 9 plots the observed decrease in the device power conversion efficiency and the values of the chemical capacitance and dielectric capacitance against the light intensity. All results are normalized to the observed values under 1 Sun illumination intensity. The NP-PD show a strong correlation between decreasing device performance and increasing dielectric and chemical capacitance at lower light intensities. This result implies that the large ionic capacitance creates substantial problems with charge extraction at lower light intensities, resulting in an increased chemical capacitance from accumulated photocharge and a subsequent drop in device performance. As the light intensity is increased beyond 0.1 Suns the dielectric capacitance approaches a constant value of 5.6 nF, and


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the chemical capacitance reduces to its minimum value. In contrast, the MB/SDS-PD device exhibits a relatively constant dielectric capacitance of ~6.5 nF for all light intensities, with the

Figure 8: (a) The equivalent electrical circuit used to fit the NP and MB/SDS-PD devices and account for the ionic surfactant. Nyquist plots of the impedance data measured under varying 31 ACS Paragon Plus Environment


illumination levels for (b) a MB/SDS-PD device, and (c) an NP-PD device containing SDS in the active layer. Measured data is displayed as open symbols and the equivalent circuit fits as continuous lines. We note that the error margin for this measurement is smaller than the size of the data points on the plots.

Figure 9: Variation in the chemical capacitance (Cμ) and ionic capacitance (Cdr) extracted from fits of the impedance data measured at short circuit to the equivalent circuit shown in Figure 8 as the illumination intensity is changed for (a) an NP-PD device, and (b) a MB/SDS-PD device.

chemical capacitance showing an increasing value with increased light intensity and the power conversion efficiency remaining constant. This result is consistent with the observations from Figure 6, but produces further insight into the mechansims of the ionic capacitance. 32 ACS Paragon Plus Environment

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Addition of ions to the interface layer of a solution cast MB-PD structure increases the dielectric capacitance, but this effect is not influenced by photoinduced charge within the active layer. This implies an accumulation of ions at the electrode interfaces. This mechansim is likely to remain present in the NP-PD devices, however, there is a second contribution to the ionic capacitance which is removed upon addition of photoinduced charge. One proposed emchanims for this result is a residual shell of immobile surfactant ions that remain around the nanoparticles in the active layer. This ionic charge can be screened by the addition of photogenerated charge to the semiconductor material, indicating a limited influence on the device performance. The impedance data can also be utilized to provide insight into the charge carrier tranport in the nanoparticulate photodiodes. Since device thickness is 100 nm and the typical depletion width for a doped P3HT semiconductor is ~150 nm, the semiconductor physics will follow the metal-insulator-metal model as has been previously reported.98 Assuming that drift in the built-in field will dominate the tranport, the carrier mobility can be described by: 𝜇 =

d 𝜏𝑡𝑟𝐸

=

d2 𝜏𝑡𝑟𝑉𝐵𝐼

(Equation 8)

where d is the film thickness, τtr the transit time of charges through the film determined from the impedance data as RtrCμ, and VBI is the built-in voltage. Charge carrier mobility values calculated using Equation 8 and the transit time from the impedance data in Figure 9 revealed values of (3.4 ± 0.4) × 10-4 cm2V-1s-1 and (1.0 ± 0.3) × 10-4 cm2V-1s-1 for the MB/SDS-PD and NP-PD respectively. This analysis implies a slightly reduced mobility in the NP-PD device, which is quantitatively consistent with the difference between MB-PD and NP-PD devices in previous reports.48 We attribute this reduced mobility to the thin layer of immobile surfactant ions that remain on the nanoparticle surface after annealing, however, previous work has shown this reduced 33 ACS Paragon Plus Environment


mobility can only account for approximately 20% of the measured performance drop in NP-PD devices in comparison to a MB-PD reference.48 The influence of surfactant on the device performance thus appears to be dominated by a bulk ionic mechanism related to mobile surfactant ions.

To complete the data available for establishing capacitance mechanisms, we have also measured the long-term stability of the photodiode devices. Degradation studies under ISOS-L-2 conditions were carried out for both MB-PD and NP-PD devices. The J-V curve as a function of time and

Figure 10: The temporal stability of device parameters (a) power conversion efficiency (η), (b) open circuit voltage (Voc), (c) short circuit current density (Jsc), and (d) device fill factor (FF) for both the MB-PD and NP-PD devices. All parameters have been normalized to their initial value upon commencing the stability testing.

normalized degradation curves for the three types of devices can be seen in Figure 10. The curves are normalized at to their initial value upon commencing the stability testing. Each point represents the average of six devices from two different substrates for each active layer material. From the 34 ACS Paragon Plus Environment

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normalized data it can be seen that the active layer material has an impact on the temporal response of the degradation, with the NP-PD devices degrading to 65 % of initial performance within 1 day, whilst the MB-PD devices are more stable, showing a slower decline to 65 % of the initial performance over a 3 day period. This initial “burn-in” degradation is driven by losses in both the Voc (20% loss) and the Jsc (15 % loss), with the fill factor reaming stable in both devices. After this initial burn-in, both the MB-PD and NP-PD devices show good stability with no further degradation in performance observed out to a 10 day time period. The results suggest that there is little differences in the long term stability of the NP-PD and MB-PD devices, indicating that the ionic influence on capacitance in the NP-PD devices occurs within the first 24 hours of fabrication.

The origins and mechanism of additional capacitance in ligand-stabilized organic electronic nanoparticles has now become much clearer. Electronically the nanoparticulate films behave in a very similar manner to solution cast films, with a slightly elevated density of trap states. However, the surfactant ions then generate a capacitance that influences the devices in a further two distinct manners, each of which have a differing impact on charge carrier functionality. The mobile ions that are present in the solid state film due to an excess of stabilizing surfactant in the nanoparticulate ink can move towards the electrodes, causing a polarization double layer that impedes charge extraction at the electrodes (Figure 11a). This mechanism provides a fundamental restriction on the device performance that cannot be altered through illumination and photogeneration of additional charge. This phenomenon has been previously observed in perovskite solar cells, and was verified in this work by replication of the ionic electrode polarization through addition of surfactant ions to an interlayer confined to the electrode interface.



This polarization causes a reduced photocurrent in the devices as charge cannot be efficiently extracted at electrodes due to an increase in the energetic barrier for extraction. In contrast the

Figure 11: Schematic illustration showing (a) ionic accumulation at the interface layers of the OPV device, and (b) the influence of annealing and increasing dialysis on the ionic shells surrounding individual P3HT:PCBM nanoparticles.

voltage is largely unaltered, indicating that the built in electric field is not screened by these ionic charges at the electrodes. The second mechanism occurs at the local nanoparticle level, and involves a combination of immobile ions anchored to the surface of the nanoparticles and mobile ions in the film creating a dielectric shell around the nanoparticles (Figure 11b). This capacitance mechanism causes both a reduced photocurrent and photovoltage, as the built-in electric field is screened by the ionic shells. Both increasing ink dialysis to remove free ions and thermal annealing 36 ACS Paragon Plus Environment

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post-treatments to sinter the particles together into a continuous network reduce the thickness of this ionic shell, resulting in a higher apparent ionic capacitance due to a reduced dielectric thickness. However, the total number of ions contributing to this second capacitance mechanism is significantly reduced in optimized devices with high dialysis and annealing treatments, such that injection of charge into the nanoparticles by either illumination or bias voltage in the dark can negate the effect of the ionic charge. This results in a significant reduction in this second local nanoparticle capacitance, such that its influence on device performance is removed. These findings suggest that the stabilizing ligands used in nanoparticulate materials for electronic devices have a significant impact on device performance only when the mobile ion concentration is significant and the electrode polarization effect can dominate. If the treatment is optimized to produce stabilized nanoparticles with only thin shells of immobile ions, then the device performance will not be restricted.

4. Conclusions. This work has examined the influence of stabilizing ionic surfactants on the solid state photophysics of nanoparticulate organic photodiodes. In particular, the specific influence of the ionic surfactant on internal charge carrier processes in organic electronic materials was elucidated in detail. A large low frequency capacitive response was detected for nanoparticulate devices which was shown to arise from the ionic surfactant. Some aspects of this capacitive response could be reproduced by introducing ionic surfactant into the electrode interlayer of a solution cast device, which created a dipole layer at the electrode interfaces. This was further supported by demonstrating that the mobile ionic surfactant can diffuse through the organic film to be detected at an electrode interface. The nanoparticulate devices were subsequently shown to exhibit two



specific mechanisms that generate capacitance; a general electrode dipole formation and a further capacitance arising from thin ionic layers at the interface of the individual nanoparticles. This second interparticle mechanism was shown to have a negligible impact on device performance as it could be removed upon addition of electronic charge in the photodiodes through either illumination or external bias. The electronic density of trap states and charge carrier mobility in the nanoparticulate devices were found to be of the same order of magnitude as the solution cast devices, with performance limitations in the surfactant stabilized films attributed to the accumulation of mobile surfactant ions at the electrodes of devices. These results imply that the surfactant ions do not create a significantly increased level of charge carrier traps as has been previously suspected, but rather the trade-off between the presence of sufficient surfactant for nanoparticle film formation and minimization of the mobile surfactant content to prevent accumulation at extracting electrodes is the critical issue to optimize the performance. The challenge for the new generation of electronic devices based upon stabilized nanoparticulate materials is to optimize the film formation properties to remove any mobile surfactant ions during or after the fabrication stage.

Acknowledgements This research was primarily funded by the Australian Research Council through its Discovery Grant scheme (Project DP140104083). The work was also performed in part at the Materials 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. MFA, FA and AA-A thank the Higher Committee for Education Development in Iraq (HCED) for provision of a PhD scholarships. MA gratefully acknowledges


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the Iran National Science Foundation (INSF) for the award of a PhD fellowship fund under project No. 95014898.

Supporting Information Some expanded data to compliment conclusions in this manuscript is provided as Supporting Information. This data includes density of states determined by capacitance spectroscopy, MottSchottky plots of selected photodiode devices, XPS full spectrum survey scans and expanded region scans for NP-PD films, summarized electrical performance data and associated current density – voltage plots for both MB-PD and NP-PD devices with varied surfactant content, zeta potential data for NP inks, capacitance-frequency data for both MB-PD and NP-PD devices in the dark under different bias voltages, and the equivalent circuit fitting parameters extracted from electrochemical impedance spectroscopy measurements for both MB-PD and NP-PD films.

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TABLE OF CONTENTS GRAHIC


The Role of Stabilizing Surfactant on Capacitance, Charge and Ion

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