Characterization of ZnO Interlayers for Organic Solar Cells: Correlation

Subscriber access provided by Weizmann Institute of Science

Article

Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin Film Morphology and Device Performance Kai-Lin Ou, Ramanan Ehamparam, Gordon A MacDonald, Tobias Stubhan, Xin Wu, R. Clayton Shallcross, Robin E. Richards, Christoph J. Brabec, S. Scott Saavedra, and Neal R. Armstrong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02792 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin Film Morphology and Device Performance Kai-Lin Ou,† Ramanan Ehamparam,† Gordon MacDonald,† Tobias Stubhan,‡ Xin Wu,† R. Clayton Shallcross,† Robin Richards,† Christoph J. Brabec,‡ S. Scott Saavedra,† and Neal R. Armstrong†, * † Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721 ‡Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-AlexanderUniversity Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany.

To whom correspondence should be addressed: [email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 41

ABSTRACT. This report focuses on the evaluation of the electrochemical properties of both solution-deposited, sol-gel (sg-ZnO) and sputtered zinc oxide (sp-ZnO) thin films, intended for use as electron-collecting interlayers in organic solar cells (OPVs). In the electrochemical studies (voltammetric and impedance studies), we used ITO over coated with either sg-ZnO or sp-ZnO interlayers, in contact with either plain electrolyte soluitons, or solutions with probe redox couples. The electroactive area of exposed ITO under the ZnO interlayer was estimated by characterizing the electrochemical response of just the oxide interlayer, and the charge transfer resistance (Rct) from solutions with the probe redox couples, Compared to bare ITO, the effective electroactive area of ITO under sg-ZnO films was ca. 70%, 10%, and 0.3% for 40, 80, and 120 nm sg-ZnO films. More compact sp-ZnO films required only 30 nm thicknesses to achieve an effective electroactive ITO area of ca. 0.02%. We also examined the electrochemical responses of these same ITO/ZnO heterojunctions over-coated with device thickness pure poly(3hexylthiophehe)

(P3HT),

and

donor/acceptor

blended

active

layers

(P3HT:PCBM).

Voltammetric oxidation/reduction of pure P3HT thin films on ZnO/ITO contacts showed that pinhole pathways exist in ZnO films that permit dark oxidation (ITO hole injection into P3HT). In P3HT:PCBM active layers, however, the electrochemical activity for P3HT oxidation is greatly attenuated, suggesting PCBM enrichment near the ZnO interface, effectively blocking P3HT interaction with the ITO contact. The shunt resistance (Rp), obtained from dark currentvoltage (J/V) behavior in full P3HT/PCBM OPVs, was dependent on both: i) the porosity of the sg-ZnO or sp-ZnO films (as revealed by probe molecule electrochemistry) and; ii) the apparent enrichment of PCBM at ZnO/P3HT:PCBM interfaces, both effects conveniently revealed by electrochemical characterization. We anticipate that these approaches will be applicable to a wider array of solution-processed interlayers for “printable” solar cells.


2

Page 3 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Keywords: organic solar cells, zinc oxide, charge selective interlayers, voltammetry, impedance spectroscopy, phase separation


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 41

1. Introduction Interlayer thin film materials are often introduced between the electrical contacts and active layers in many “printable” solar cells, including organic solar cells (OPVs), to provide for selective electron- or hole-harvesting.1–8 Inverted configuration OPVs typically use an electronharvesting interlayer, such as ZnO or TiO2, on a transparent conducting oxide contact (e.g. indium-tin oxide, ITO), with an emphasis on interlayers which can be solution processed from sol-gel or nanoparticle precursors.1,3,4,9–17 ZnO and TiO2 interlayers are typically used as electron-harvesing interlayers, since their conduction band energies are positioned close to the LUMO transport levels of OPV electron acceptor components and can provide for good suppression of hole-harvesting and interface recombination at those contacts, owing to their large bandgap energies.3–5,7,8 Additional properties required for electron-harvesting interlayers include: i) high electron motilities and appropriate doping levels and dopant distribution to minimize series resistance and electrostatic barriers to electron harvesting;18–23 ii) minimization of pathways (pinhlles) permitting direct contact of the donor component with the electronharvesting interlayer, which can lower shunt resistance (Rp) and OPV efficiency.3,4,7,24 Optimization of these properties is sensitive to processing conditions which control micro/nanostructure, near-surface chemical composition and electronic properties of the oxide.11,19,25 Vacuum processing of metal oxide films by chemical vapor deposition (CVD), atomic layer deposition (ALD), or RF/DC sputtering (SP) can provide for fine control of nucleation and growth of quite dense ZnO films.26–29 ZnO and TiO2 interlayers are often formed by processing from solution precursors.11,12,25,30–35 Deposition typically occurs via dip/spin-coating precursors (such as zinc acetate) and post-annealing to evaporate solvents and decompose precursor ligands, proceeding from a “xerogel-like” film to a more densified oxide.8,36–38 Ultimately it is desired to


4

Page 5 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


“print” or spin-coat these layers at thicknesses, and processing temperatures that ensure good electrical performance in one deposition cycle.39 Metal oxide nucleation, however, begins in the liquid phase, where interactions between precursor molecules dominate over molecule-to-substrate interactions. “Island” growth of the oxide films may dominate,26 and a mesoporous structure with high surface area can be obtained. Solution-deposited films may not be conformal with the underlying substrate, with persistent partially decomposed, “ZnO-precursor” states, leading to high pinhole densities.37,38 Multiple deposition cycles and/or deposition of thicker ZnO films may be required, The pinhole density of films in a layer-structured PV device is often indirectly evaluated through the device shunt resistance, Rp.40 Previous studies established the photo-shunt resistance (Rp under illumination) as a consequence of, and correlated with, interface defects.41 Measuring Rp requires a full device architecture, which does not necessarily lead to identification of where the defect occurs. In this report, we demonstrate easily implemented electrochemical methods (cyclic voltammetry, impedance spectroscopies) to semi-quantitatively evaluate the physical imperfections, i.e., pinholes for ZnO, processed from sol-gel precursors, requiring multiple coatings (1x, 2x, 3x) to build a suitable ZnO interlayer. We compare the electrochemical properties of sg-ZnO films with those obtained by RF sputtering, and then correlate their electrochemical behavior with device J/V behavior and values of Rp. Similar approaches have been recently demonstrated by Bard and coworkers for characterization of ALD TiO2 films.42 Cyclic voltammetry is used first for the evaluation of background (capacitive) current responses for ITO/ZnO samples in contact with non-aqueous electrolyte solutions to evaluate the double-layer capacitive current at the ITO/electrolyte interface and the capacitive current in the “chemical capacitance” region of ZnO.43,44 Probe molecules are added to these solutions,


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 41

whose redox potentials span the equivalent bandgap energy of the ZnO film, and we estimate the ITO exposed area through faradaic currents (voltammetry) and impedance responses. Lastly we show that the voltammetric responses of pure poly(3-hexylthiophene) (P3HT) or P3HT:PCBM blend films, deposited on these ZnO interlayers, provide an indication of the degree to which the donor polymer can penetrate to the underlying ITO contact. For mixed P3HT:PCBM active layers, oxidation of P3HT through the pinhole pathways in the ZnO thin films is significantly suppressed, suggesting preferential PCBM enrichment near the ZnO interlayer, which can provide as much improvement in device response as the formation of a dense, pin-hole-free ZnO film. The porosity of these oxide films, as revealed by electrochemical measurements, inversely correlates with the shunt resistance, Rp, extracted from dark J/V curves for full device platforms. We expect that this approach to evaluation of film properties will be applicable to the characterization of several other interlayer materials, and are the type of protocols that could be easily implemented with films created by roll-to-roll (low temperature) deposition processes.39,45,46 EXPERIMENTAL 2.1 Synthesis of sg-ZnO solution In ambient air, ethanolamine (0.223 ml, >99.9% Sigma-Aldrich) and 2-methoxyethanol (3.0 ml, >99.5%, Aldrich) were loaded in a clean 20 ml vial. After minimal mixing, zinc acetate dihydrate (0.826 g, ≥98%, Sigma-Aldrich) and 2-methoxyethanol (2.0 ml, >99.5%, Aldrich) were sequentially added into the resultant solution. The vial was then sealed with a plastic lid and transferred onto a 60 oC hot plate with 800 rpm stirring. After 30 minute heating process, the zinc acetate dihydrate solid was totally dissolved and the solution became transparent. The solution was filtered through a 0.45 µm syringe filter prior to use.


6

Page 7 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2 The deposition of sg-ZnO and sp-ZnO on ITO substrates (1) sg-ZnO: Pre-cut ITO substrates (ca. 15Ω/sq, Colorado Concept Coating, LLC) were cleaned with micro-fiber cloth using Triton-X 100 detergent, and sonicated in Triton-X 100 detergent, nanopure water, and pure ethanol, sequentially. Cleaned ITO substrates were kept in pure ethanol for storage. ITO substrates were dried under a N2 stream before use. The ITO were treated with 10.5 watts O2 plasma at ca. 400 mtorr for 10 minutes (Harrick plasma, Model PDC32G) before each deposition. The deposition of sg-ZnO film onto ITO substrate was carried out through spin coating (3000 rpm, 1 minute). The deposited film was seated in a petro-dish with lid on and transferred to a 280 oC heat oven for 10 minutes thermal annealing. Multiple spin coating and annealing cycles (2, 3 times, named 2x and 3x) were performed to obtain thicker sg-ZnO films. (2) sp-ZnO: The pre-cleaned and plasma treated ITO (as discussed above) was loaded into the sputtering chamber (Kurt J. Lesker, Model AXXIS), and pumped down to a base pressure of ca. 1 x 10-6 torr. Sputtered films were formed via RF sputtering, where the applied bias between anode and cathode is flipped at a radio frequency of ca. 10 MHz to prevent charge build-up on the ZnO target (99.999%, Kurt J. Lesker). During sputtering the magnetron power was operated at 100 W with the background pressure set to ca. 4.6 x 10-3 torr by adjusting the carrier gas flow rate. The carrier gas is composed of 99% argon and 1% oxygen. The deposited 30 nm film thickness was calibrated with atomic force microscopy (AFM) through the height difference at mask-covered regions versus uncovered regions. 2.3 Film characterization (1) Film morphology and micro-structure : cross-section images of ITO/sg-ZnO and ITO/spZnO films were obtained with field emission scanning electron microscopy (FE-SEM – Hitachi,


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 41

Model 4800). The samples were coated with a few nanometers of sputtered Pt to prevent charging during the analysis. The morphology of sg-ZnO and sp-ZnO deposited on ITO was characterized via tapping mode AFM (Dimension 3100 - Veeco instruments). X-ray diffraction (XRD) of each thin film was obtained on a Philips X'PERT MPD diffractometer using a Cu Kα (0.154 nm) light source. The diffractometer was set at 0.02o and 0.05o for sg-ZnO/glass and sp-ZnO/ITO, respectively with the diffraction angle range (2ϴ) scanned between 25- 75o. (2) Electrochemical studies: Cyclic voltammetry (CV) was used to evaluate the pinhole density and hole blocking ability of ZnO films on ITO. All the electrochemical measurements were conducted using a potentiostat (CHI 660c) and a standard three-electrode electrochemical cell with ZnO/ITO as working electrode (area = 0.671 cm2), platinum gauze as counter electrode and Ag/Ag+ (BASi, 10 mM AgNO3 in 0.1M tetrabutylammonium hexafluorophosphate (TBAPF6) electrolyte) pseudo-reference electrode. Probe molecules: N,N’-Bis(3-methylphenyl)-N,N’diphenylbenzidine (TPD) (99%, Aldrich, (recrystallized before used), Ferrocene (Fc) (>98%, Fluka), Decamethylferrocene (Me10Fc) (97%, Sigma-Aldrich), and 1, 1’-DimthylFerrocene (98%, Aldrich) were prepared to be 0.5 mM in 0.1 M (TBAPF6) (98%, Aldrich) in acetonitrile (HPLC grade, EMD) was used as the supporting electrolyte. Electrochemical impedance spectroscopy (EIS) was carried out using the same electrochemical cell with AC potential modulation amplitudes of 10 mV and a frequency range of 104 to 1Hz in 0.5 mM TPD in 0.1 M TBAPF6 acetonitrile solutions. The DC potential applied for the EIS experiment was 0.42 V (mid-point potential of first redox process of TPD) and -0.1 V (outside of redox window of TPD) vs. Ag/Ag+ non aqueous reference electrode. The EIS data was fitted in the frequency range of 10 kHz to 1 Hz using Garmy Echem Analyst (Version 5.65) software.


8

Page 9 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The deposition of pure P3HT or P3HT:PCBM blends on ITO/ZnO substrates were carried out in a N2 filled glovebox, P3HT (electronic grade, Rieke Metals) and P3HT:PCBM (99.5%, NanoC) solutions were prepared in 1,2-dichlorobenzene at 15 mg/mL and 36 mg/ml (1: 0.8 wt%), respectively. The solutions were stirred at 60 oC for at least 3 hours before being used. The active layer film deposition was carried out by spin coating on ZnO/ITO substrates (700 rpm, 1 minute). The deposited films were seated in a petri-dish with the lid covered for overnight and then the samples were annealed at 100 oC (P3HT:PCBM/ITO) and 150 oC (P3HT/ITO) for ten minutes. 2.4. OPV Device characterization The same procedures for P3HT:PCBM BHJ layer deposition onto sg-ZnO/ITO and spZnO/ITO, as described above, were used. To complete the full BHJ device configuration, PEDOT:PSS was deposited onto the OPV active layer. 1 wt% Triton X-100 surfactant was added to PEDOT:PSS solution to improve the PEDOT:PSS wettability to the blend layer. The mixed PEDOT:PSS solution was spun on the blend layer in air at 4000 rpm for 1 minute and rinsed with ethanol, isopropyl alcohol for multiple cycles (generally, 4 to 5 cycles, depending upon ambient humidity) until the PEDOT:PSS layer completely wet the blend layer. The deposited samples were transferred into the N2 filled glovebox and annealed at 110 oC for 10 minutes. Ag (99.99%, Kurt J. Lesker) top electrodes with 100 nm film thickness were thermally evaporated on the substrates under < 10-6 torr. A shadow mask was used during the Ag deposition to confine the cell area (0.125 cm2). 2.5 Device J/V characterization The dark and light J/V measurements were carried out using a source meter (Keithley, model 2400) and in-house software (National Instruments Labview 8.2) in a N2 filled glovebox


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 41

(Mbraun Labmaster). The scan voltage was swept from -2 V to 2 V with 10 mV steps. A 300 W Xe arc lamp (Newport) was used as a light source, light was passed through a neutral density filter (Thorlabs), an AM 1.5 filter (Melles Griot), and a 40o output angle optical diffuser (Newport), subsequently before illuminating OPV devices. The illuminating light intensity at the device plane was calibrated by a flat response thermopile (Newport) and a calibrated silicon photodiode (Newport – Model 818-SL with OD3 Attenuator) to achieve 100 mW/cm2. 3. RESULTS AND DISCUSSION 3.1 Film morphology and porosity We compare the cross sectional FE-SEM images of sp-ZnO and sg-ZnO films on ITO in Figure 1 (a), (b), while Figure 1 (c), (d) show schematic views of a P3HT:PCBM layer with the ZnO/ITO contact. sp-ZnO films are generally conformal with the ITO substrate, and quite compact, consistent with layered growth mechanisms for deposition.26 The sg-ZnO films are typically much rougher with significant apparent porosity when viewed in cross-section, consistent with what is expected from island growth mechanisms.26 These observed structural and morphology differences are consistent with their AFM-determined surface roughness root mean square (RMS) value: ca. 7.3 nm (sg-ZnO) vs. ca. 2.5 nm (sp-ZnO)), as shown in Supporting Information, Figure S1. Figures 1 (c), (d) show proposed interactions between P3HT:PCBM blend active layers and sp-ZnO versus sg-ZnO interlayers, emphasizing the role that physical defects in the sg-ZnO interlayers might play in providing pathways for hole-harvesting/injection at exposed ITO/P3HT interfacial regions. As will be discussed below, the 40 nm sg-ZnO layer (typical for OPV interlayers, spin-cast at 3000 rpm, one cycle), shows the least hole-blocking ability in the electrochemical data and leads to the highest leakage currents (lowest Rp) in OPV devices.


10

Page 11 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Cross-section FE-SEM images of (a) sp-ZnO/ITO and (b) sg-ZnO/ITO with P3HT:PCBM layers above the ZnO interlayer. sp-ZnO films are generally conformal with the ITO substrate, while sg-ZnO films, even after annealing, demonstrate considerable porosity and rough surfaces. (c) and (d) are the hypothesized schematic views for polymer semiconductor active layer/ZnO/ITO heterojunctions for sp-ZnO and sg-ZnO interlayers, respectively.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 41

The crystallinity of ZnO is sensitive to its processing conditions, such as the choice of precursors, acid or base modifiers and annealing time/temperatures.37 Using thin film X-ray diffraction we observed only broad and weak (002), (110) and (112) reflections in sg-ZnO films (Figure S2 (a)) due to the low processing temperature (280oC). In order to get highly crystalline sg-ZnO films processing temperatures at or above 400oC are usually needed.37 For the more crystalline sp-ZnO films a strong (002) orientation growth was observed (Figure S2 (b)), in which the c-axis is mainly perpendicular to the substrate.37 The coherence length for the diffraction peaks in sg-ZnO and sp-ZnO was estimated to be ca. 8 nm and ca. 15 nm, respectively using the Scherer relationship,47 with the peak width calculated from their major Bragg peaks (sg-ZnO: (110); sp-ZnO: (200)). 3.2 Electrochemical characterization: double layer and chemical capacitance Displacement currents observed in the voltammetric characterization of sg-ZnO/ITO and spZnO/ITO contacts were used as an indirect measure of “electroactive area” of both sg-ZnO/ITO and sp-ZnO/ITO thin films (Figure 2). Cyclic voltammetry was conducted in CH3CN/0.1M TBAPF6 solutions, at sweep rates of 1 V/s over potentials from +0.8 to -0.9 volts vs. Ag/Ag+. These potentials, referenced to the vacuum scale, were selected to stay within the bandgap region, below ECB of ZnO,1 (ca. -4.4 eV versus vacuum).1,7,48 In the bandgap region below ECB for ZnO the current density can be described by: 49

where υ is sweep rate (volts/sec); Cd (Farads/cm2) for a planar electrode would be described by the double-layer capacitance at the ITO/electrolyte interface (assuming penetration of electrolyte to that contact), t is time; Rs is solution resistance.


12

Page 13 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Cyclic voltammograms of sg-ZnO/ITO, sp-ZnO/ITO and sg-ZnO/sp-ZnO/ITO heterojunctions in contact with 0.1 M TBAPF6 in anhydrous acetonitrile. The inset shows the similarity of the capacitive response of the sp-ZnO/ITO and sg-ZnO/sp-ZnO/ITO samples. For all measurements, 1000 mV/s scan rate was used.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 41

At a bias negative of -0.5 volts Ag/Ag+ (close to ECB for ZnO) additional background currents (“chemical capacitance”) are observed due to charge accumulation in “trap states” just below the conduction band edge,44 which rise exponentially with increasingly negative applied potential, as charge density and interfacial capacitance increases. Bisquert et al. have combined theoretical modeling with experimental data acquired from cyclic voltammetry (CV) and impedance spectroscopy to characterize the electrochemical responses of chemical capacitance in TiO2 electrodes.43,44,50–52 This charge trapping/detrapping in high surface area metal oxides is dictated by any localized state present on an oxide surface resulting from structural disorder and the polycrystalline/amorphous nature of the oxide.53,54 The distribution of these trap states can be described by:51,55

where NL is the electrode total volume density of traps; To is a temperature parameter to describe the broadening of the exponential distribution; E - ECB is the energy of the trap state with respect to the conduction band energy; kB is the Boltzmann constant. Note that the current to charge this chemical capacitance is nearly proportional to oxide surface area.56 We characterized the chemical capacitance of dense sg-ZnO and porous sg-ZnO by filling the states close to or within Ecb of ZnO. Both sg-ZnO/ITO and sp-ZnO/ITO thin films show exponential growth in current at applied bias negative of ca. -0.4 V vs. Ag/Ag+, however, for the sp-ZnO sample, the capacitive current normalized to electrode area is ca. 10 x lower than sg-ZnO at -0.8 V vs. Ag/Ag+, attributable to the surface area difference between sg-ZnO (large) and sp-ZnO (small) thin films.56–59 Without


14

Page 15 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


knowing the absolute value of surface area, however, we note the difficulty in comparing the trap state density per surface area of both samples. For applied bias positive of 0 V vs. Ag/Ag+ the nearly constant capacitive current has been previously attributed to the ITO/electrolyte double-layer capacitance.44,60 From the steady-state current and using eq. (1), the double-layer capacitance was determined to be 10 and 0.80 µF/cm2 for sg-ZnO and sp-ZnO films on ITO, respectively (determined from the current difference at 0.3 V vs. Ag/Ag+ in forward and reverse sweep, which is ca. 2x the product of Cd and sweep rate, 1 V/s). The difference in this double-layer capacitance reflects the differences in degree of penetration of electrolyte to the ITO surface for sg-ZnO versus sp-ZnO films. sp-ZnO forms a dense and conformal film with the underlying ITO substrate. To confirm our explanations for the double-layer and chemical capacitance difference between these two ZnO film types, we prepared a layered ZnO interlayer with sp-ZnO (30 nm) deposited on an ITO substrate and a sg-ZnO film (40 nm) on top of the sp-ZnO film (ITO/sp-ZnO/sgZnO), as depicted in Figure 2. From this layered sample, we expect sp-ZnO with a low pinhole density would produce a small double-layer capacitance, while sg-ZnO with high surface area would contribute to high exponential current in chemical capacitance region negative of applied potentials representing ECB. The voltammogram of ITO/sp-ZnO/sg-ZnO films are also shown in Figure 2. As expected, layered ZnO films on ITO show the capacitive response of sp-ZnO at potentials positive of 0 volts (0.79 µF/cm2 - comparable to 0.80 µF/cm2 in sp-ZnO) and the chemical capacitance response of sg-ZnO at potentials negative of that bias This suggests that an optimized combination of a compact sp-ZnO layer (to block hole injection) in between ITO and large surface area sg-ZnO layer (to facilitate electron extraction) may be beneficial in a variety of


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 41

device platforms, and this approach will be communicated elsewhere. It is notable that a similar strategy as has been used in the design of TiO2 photoanodes in both dye-sensitized and perovskite solar cell configurations.61–66 3.3 Voltammetric and electrochemical impedance characterization of hole-blocking capabilities of ZnO films using solution probe redox couples Voltammetric oxidation/reduction of solution probe molecules, and impedance spectroscopies of these same electrolyte solutions, can be used to semi-quantitatively describe the pinhole density and fractional coverage (1-θ) of bare ITO surface exposed under these ZnO films, provided that the redox potentials of the probe molecules are energetically within the band gap of ZnO, where charge transfer events should not occur in the dark.1 We recently demonstrated this approach for several different probe molecules in the characterization of CVD TiO2 thin films on ITO contacts, intended for use as electron selective OPV interlayers.67 Defects and pinholes in the oxide layer allow electroactive species to diffuse to the ITO surface where redox processes generate Faradaic.68 Decamethylferrocene (Me10Fc), 1,1’-dimethylferrocene (Me2Fc)) and N,N′bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) undergo oxidation/reduction over a potential range in excess of 1 volts at potentials which lie within the band gap region of ZnO,

67

so that

voltammetric oxidation/reduction peaks should only be observable because of the presence of pinhole defects in ZnO films. The cyclic voltammograms of Me2Fc, Me10Fc and TPD using bare ITO and sg-ZnO/ITO deposited with one (1x, ca. 40 nm), two (2x, ca. 80 nm), and three (3x, ca. 120 nm) 3000 rpm spin-coating cycles, and a 30 nm sp-ZnO/ITO film are shown in Figure 3. Peak separation in such cyclic voltammograms are related to the areal density (as a fraction of a monolayer, 1- θ) of exposed electroactive sites.69–73


16

Page 17 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. The cyclic voltammograms of 0.5 mM TPD, Me2Fc, and Me10Fc (left to right) using bare ITO (a) and one (1x, ca. 40 nm) (b), two (2x, ca. 80 nm) (c), and three (3x, ca. 120 nm) (d) spin-coating cycle sg-ZnO/ITO and (e) 30 nm sp-ZnO/ITO as working electrode. For all measurements, 100mV/s scan rate was used.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

The voltammetric responses of 1x sg-ZnO film (Figure 3 (b)) are nearly identical to those for bare ITO (Figure 3 (a)) with good symmetry in the voltammetric response, and little attenuation in peak current, indicating 40 nm sg-ZnO layers do not significantly block the charge transfer of these redox probes. The charge blocking ability of spin-cast sg-ZnO films improves as the number of spin-coating cycles increases. For 2x and 3x sg-ZnO films (Figure 3 (c), (d)) the decreased peak current magnitudes at these voltammetric sweep rates suggest that 1- θ drops below ca. 1%.69–73 Secondary voltammetric peaks were found for both the 2x and 3x sg-ZnO film, in the range of the chemical capacitance region, which may be an indication of charge transfer through trap states just below ECB. To our knowledge the electrochemical characterization of ZnO films in the bandgap energy regime, below ECB, has not been previously reported. The attenuation of voltammetric activity for these probe redox couples was, as expected, significantly better for even thin sp-ZnO thin films (Figure 3 (e), note that the current scale is 50x smaller than the other voltammograms). Electrochemical impedance spectroscopy (EIS) allows for a more quantitative evaluation of 1- θ and the distribution of electroactive sites. In general, to determine the relative coverage of blocked areas (θ), the impedance data were fit with the Randles model, as shown in Figure 4 (a),74 where θ is estimated from previously derived relationships which use the charge transfer resistance (Rct) of blocked and unblocked electrodes:75–78

For our EIS studies the probe molecule, TPD, was chosen since its first redox potential is ca. 1 eV positive of ECB of ZnO on the electrochemical scale, as shown in Figure 3. Any faradaic events can be mainly attributed to probe molecule diffusion through pinholes.


18

Page 19 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) The equivalent circuit (Randles model) for the impedance data fitting. CE is the counter electrode; WE is the working electrode; Rs is series resistance; RCT is charge transfer resistance; CPEdl is constant phase element for double layer capacitance; W is Warburg diffusion element. The Nyquist plots of (b) bare ITO, (c) 1x sg-ZnO/ITO, (d) 2x sg-ZnO/ITO, (e) 3x sgZnO /ITO, and (f) sp-ZnO/ITO using 0.5 mM TPD with 0.1 M TBAPF6 supporting electrolyte in acetonitrile solution. The potential was set at 0.42 V vs. Ag/Ag+. The experimental data is shown by circle (○) and the fit is shown as solid line (-).


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

Figure 4 (b), (c), (d), (e), and (f) show the Nyquist (Z’’ vs Z’) plots and the fitting curves (using the Randles model, as shown in Figure 4(a)) for bare ITO, 1x, 2x, and 3x sg-ZnO, and 30 nm sp-ZnO/ITO, respectively. The fitting results for each component in the Randles model are summarized in Table 1. In Figure 4(b) and 4(c), a nearly complete semicircle in the high frequency region (104 to 102 Hz) and a linear region in the low frequency region (102 to 1 Hz) were found. In the high frequency region, the impedance frequency response can be seen as the contribution of a combined parallel RCT and double layer capacitance (Cdl) and when RCT is small compared to the capacitive reactance (χc) of Cdl, a distinct semicircle appears. These data suggest that the charge transfer process is kinetically controlled at the ITO/electrolyte interface, and the blocking effect from the ZnO film is minimal. In the low frequency region, a linear characteristic means semi-infinite linear diffusion of the redox species is achieved, with a Warburg diffusion element (W) modeling this diffusion behavior.79–81 In Figure 4(d), (e) and (f), only a semicircle or a fraction of a semicircle with no low frequency linear region was observed, suggesting that kinetically controlled charge transfer dominates the impedance response over this frequency range, and an increase in RCT is observed (Table 1). The RCT of 1x sg-ZnO (290 Ω cm2) is at the same scale with slightly larger value than that of bare ITO (222 Ω cm2), meaning charge transfer of TPD at 1x sg-ZnO/ITO is not significantly inhibited by the physical barrier of the 1x sg-ZnO top layer. For thicker sg-ZnO film from multiple depositions, we observed greater than 10 times enhancement in RCT (2x sg-ZnO: ca. 1.9 x 103 Ω cm2; 3x sg-ZnO: 6.7 x 104 Ω cm2). For the denser sp-ZnO films RCT is ca. 1.2 x 106 Ω cm2). Using Eq. 3 we estimated (1- θ) for each ZnO interlayer (Table 1), dropping from ca. 70 % (1x sg-ZnO) to 10 % (2x) to 0.3 % (3x) as sg-ZnO film thickness increases, whereas for 30 nm sp-ZnO 1- θ = ca. 0.02%. We show below that these estimates of exposed ITO contact correlated with actual Rp in OPV platforms.


20

Page 21 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrode active Rs (Ω cm2)

Rct (Ω cm2)

CPEDL (μF cm-‐2)

CPEDL (n)

area (1- θ) w. r. t. ITOa (%)

ITO

79 ±6

202±83

6.46±1.35

0.93±0.03

n/a

1x sg-ZnO (ca. 40 nm)

79±6

290±84

6.39±0.43

0.96±0.01

70

2x sg-ZnO (ca. 80 nm)

79±2

1939±144

9.84±1.19

0.92±0.01

10

3x sg-ZnO (ca. 120nm)

87±4

67037±29883

8.78±1.04

0.93±0.01

0.3

sp ZnO (30 nm)

77±4

1219633±270102

0.64±0.03

0.97±0.01

0.02

Table 1. Modelling impedance characteristics of the charge transfer between TPD probe molecule with bare ITO and sg-ZnO layers and sp-ZnO covered ITO working electrodes based on the Randles model, as shown in Figure 5 (a). The electrode active area was determined by the numerical ratio of Rct of ITO over Rct of ZnO.81 Average and standard deviation of three samples were reported here.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

3.4 Voltammetric characterization of the charge blocking properties of ZnO films overcoated with prototypical donor polymers (P3HT) and with P3HT:PCBM active layers Voltammetric oxidation of P3HT films deposited on either sg-ZnO/ITO or sp-ZnO/ITO contacts provide another means of assessing the charge harvesting selectivity of these ZnO interlayers.

An

ideal

electron

selective

interlayer

should

provide

both

electrical

selectivity,1,3,67,82–85 and physically separation of the hole-transporting component of the active layers from the underlying contact.4,7,34,67,86–88 We deposited both pure P3HT and blends of P3HT:PCBM onto the sg-ZnO/ITO and sp-ZnO/ITO contacts described above, and evaluated the oxidation of P3HT using voltammetry in contact with acetonitrile (a non-solvent) electrolyte solutions.89 We hypothesized that, since the HOMO transport level of P3HT is near 5 eV vs. vacuum, with poor electronic coupling with ZnO energy levels,1,7 if the density of mid gap states within the ZnO band gap is small, the voltammetric oxidation response (both onset potential and current density) would be dictated strictly by the extent of P3HT penetration through pinholes and charge transfer with the underlying ITO contact. Cyclic voltammograms from the first scan of the spin-coated P3HT layer on sg-ZnO/ITO, spZnO/ITO and bare ITO are shown in Figure 5 (a). The voltammetric oxidation of deposited P3HT films on conductive substrates, as seen for bare ITO shows a sharp onset for current flow and a voltammetric peak at ca. +0.2 volts (vs. A/Ag+), associated with the sub-population of P3HT chains that are the easiest to oxidize (longest conjugation length, largest degree of regioregularity), and the size and mobility of the counter ion which must be incorporated into the oxidized polymer film (PF6-).90–92 In sg-ZnO/ITO cases, the onset potential for oxidation for otherwise identical P3HT films becomes increasingly positive as sg-ZnO film thickness increases, with multiple spin-cast deposition cycles (ca. 0.37 V, 0.45 V, and 0.72 V for oxidation


22

Page 23 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. The cyclic voltammograms of P3HT (a) and P3HT:PCBM (BHJ) (b) films deposited on bare ITO and one (1x, ca. 40 nm), two (2x, ca. 80 nm), and three (3x, ca. 120 nm) spincoating sg-ZnO/ITO and 30 nm sp-ZnO/ITO. The (b) inlet shows the magnification of trace oxidative current of 2x and 3x sg-ZnO/ITO and sp-ZnO/ITO. The applied potential is in a range of P3HT oxidation which is in the band gap of ZnO. For all measurements, 100 mV/s scan rate was used.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

of P3HT on 1x, 2x, and 3x sg-ZnO). The 3x sg-ZnO film is effectively blocking hole-injection into the P3HT layer over the energy window required for adequate OPV response.1,67 It is unlikely that this onset potential difference links to sg-ZnO/P3HT energy alignment since the very top layer of sg-ZnO, regardless of spin-coating cycle, was processed through the same method, i.e., annealing at 280 oC for 10 minute. Rather, we hypothesize that these differences in onset potential for polymer oxidation correlate with the restricted access of polymer to the ITO contact, and restricted access of the negative counter ions that must penetrate the polymer film during oxidation, in order to maintain electroneutrality. Interestingly, once oxidation of the P3HT film is initiated, regardless of onset potential, the total oxidative charge transferred is nearly the same, suggesting that oxidation occurs laterally throughout the regions in the P3HT film that were electroactive for the porous ZnO layers, once hole-injection has been initiated at the P3HT/ITO (pinhole) interface.92,93 This oxidation process of course must be accompanied by the incorporation of counter ions from solution, to maintain electroneutrality, and does not involve the entire P3HT film. Reduction of the voltammetrically oxidized P3HT films occurs at nearly the same potentials, and with comparable current densities, suggesting that, once oxidized P3HT chains are created within the film, they can all be reduced at nearly the same energy. When these same experiments were conducted with sp-ZnO films there was almost no detectable oxidation/reduction activity, consistent with the near absence of defects that would permit access of P3HT chains to the ITO substrate. The same electrochemical characterizations were extrapolated to P3HT:PCBM blends deposited on bare ITO, and sg-ZnO/ITO, and sp-ZnO/ITO contacts (Figure 5 (b)). On bare ITO the voltammetric response for these P3HT:PCBM films is similar to that seen for pure P3HT films, suggesting that P3HT domains in the blend film extend to the ITO contact and that


24

Page 25 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation (hole-injection) in unimpaired. Using the thinnest (1x) sg-ZnO/ITO contact, P3HT oxidation is still observable, but with a significant shift in onset potential to ca. 0.65 V vs. Ag/Ag+. For 2x- and 3x-ZnO interlayer films P3HT oxidation is almost completely blocked. These observations are consistent with inhibited access of P3HT chains to the bare ITO regions as a result of the addition of PCBM.7,10,94,95 It has been proposed that PCBM interfacial enrichment can occur during deposition of P3HT:PCBM blends on both ZnO and TiO2 interlayers, via spin coating and drying. The low surface energy P3HT donor component tends to segregate toward the top surface, while the PCBM acceptor is enriched at the interlayer (ZnO, TiO2)/ITO interface.3,7 This phase separation has been verified to differing extents by AFM,95–97 TEM,95–98 XPS,96,97 and X-ray scattering.99,100 Our results demonstrate that this type of PCBM enrichment can be easily verified in BHJ blends using voltammetric studies which are likely to be easily extrapolated to other polymer/small molecule blend active OPV layers. 3.5 J/V characteristics of inverted BHJ device using ZnO films and correlation with their charge blocking ability The physical and electrochemical properties of both the sg-ZnO and sp-ZnO interlayer films correlate well with both the dark J/V response in OPV platforms. We use Rp, measured from the dark J/V behavior (Figure 6), as a quantitative indication of the degree of active layer penetration to the ITO contact (shorting) as has been applied widely for a variety of PV platforms,40,101–103 where Rp determines how much current passes around the diode created by the rectifying active layer, under reverse bias.86,87,104,105


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

Figure 6. Dark J/V in log-linear scale in full-bias region (a) and linear J/V in reverse-bias region (b) for ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag bulk heterojunction OPVs using one (1x, ca. 40 nm), two (2x, ca. 80 nm) and three (3x, ca. 120 nm) spin-coating cycle(s) of sg-ZnO and 30 nm sp-ZnO electron transport layer. The leakage current and shunt resistance (Rp, summarized in Table 2) determined in the reverse bias show as strong correlation with the hole blocking abilities in the electrochemical studies.


26

Page 27 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We determined the Rp of ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag platforms and correlated Rp with ZnO pinhole density. In the dark logJ/V current (Figure 6 (a)), devices with sg-ZnO interlayers created with multiple deposition cycles or sp-ZnO all showed good rectification (>1000x at ± 1V).106 For these devices the diode ideality factor (determined from the 0.0 – 0.5 volt forward bias response) is nearly identical, the series resistance (RS) determined from 0.5 to 1.0 volts is lower (and nearly identical) for the devices using sg-ZnO interlayers, but a factor of ca. 2x higher for devices using sp-ZnO interlayers, and the current densities at biases approaching space charge limited currents (2.0 volts and beyond) were higher for the devices using sg-ZnO interlayers. The clearest differences in performance that could be correlated with pinhole and defect density was seen in the negative bias region where the current is a combination of reverse saturation current (Jo) from the diode and any other leakage current passing through Rp. Jo is typically on the order of 10-5 to 10-6 mA/cm2 or smaller83,107–109 and Rp dominates at far negative reverse bias -- estimation of Rp is achieved by taking the inverse instantaneous slope.110 Leakage currents of the devices using sg-ZnO interlayers become lower as the number of spincoating steps (film thickness) increases (Figure 6 (b) and Table 2), meaning that thicker more dense sg-ZnO films (3x, 120 nm) block leakage currents more effectively. sp-ZnO interlayers with lower pinhole densities at only 30 nm film thickness, show comparable or lower leakage currents compared to 120 nm sg-ZnO interlayers. The level of improvement of Rp, however, is less significant than the change in ZnO thin film pinhole density revealed by impedance spectroscopies, suggesting that there may be additional leakage pathways introduced with the PEDOT:PSS/Ag cathode. It has been reported recently that thermally evaporated Ag atoms are


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

Rp (Ω/cm2) 1x sg-ZnO

2x sg-ZnO

3x sg-ZnO

sp-ZnO

(ca. 40 nm)

(ca. 80 nm)

(ca. 120nm)

(30 nm)

-1.8V

230 KΩ

450 KΩ

830 KΩ

400 KΩ

-1.5V

400 KΩ

550 KΩ

900 KΩ

890 KΩ

-1.2V

520 KΩ

680 KΩ

1080 KΩ

1320KΩ

-1V

720 KΩ

790 KΩ

1200 KΩ

1420KΩ

Table 2. Summary of shunt resistance (Rp) for ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag bulk heterojunction OPVs using various sg-ZnO layers and sp-ZnO. The determination of Rp is taken the inverted slope of dark J/V curves at -1.8, -1.5, -1.2 and -1V.


28

Page 29 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


able to percolate into PEDOT:PSS polymer networks,111,112 which can lower the overall Rp in OPV platforms112 Under illumination we note that the optimum sg-ZnO interlayer thickness proves to be in the range 30 - 40 nm for these inverted configuration devices,11,113 i.e. 2x (80 nm) and 3x (120 nm) sg-ZnO interlayers produced somewhat diminished device performance ( Figure S3 and Table S1) even with larger values of Rp, suggesting that series resistance effects may begin to dominate with these thicker interlayers. 4. Conclusion We have shown here that electrochemical methods can be used to routinely characterize PVrelevant electron-transport metal oxide interlayers: sg-ZnO and sp-ZnO films. These electrochemical methods can provide for rapid optimization of interlayers in terms of microstructure controlling pinhole density, without the need for full fabrication of PV platforms. We envision these electrochemical techniques can be applied for many kinds of metal oxides on a flexible TCO substrate during roll-to-roll processing, especially for the evaluation of pinhole density created from physical /imperfections caused by the processing steps. Studying ZnO/ITO charge transfer with probe molecules and solid-state active layer thin films, we showed a strong correlation with the leakage currents in the full BHJ OPV device: the more electrochemically porous sg-ZnO films also allow for P3HT penetration to the ITO substrate, correlating with the high leakage current (small Rp) at reverse bias in device platforms. Thinner, but denser sp-ZnO interlayers effectively block polymer penetration, resulting in low leakage current and large Rp in the PV platform. These protocols can be extended to other materials platforms relevant for “printable” solar cells, and those studies will be reported subsequently.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

ACKNOWLEDGMENTS This work was supported as part of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001084 (K.-L.O., R.M., G.M., X.W., R.C.S., R.C., N.R.A.). The authors also gratefully acknowledge the support of the Cluster of Excellence “Engineering of Advanced Materials (EAM)”, Energy Campus Nuremberg (EnCana, Solarfactory), “Synthetic Carbon Allotropes” (SFB 953) project, DFG research training group GRK 1896 and the Erlangen Graduate School in Advanced Optical Technologies (SAOT) at the University of Erlangen-Nuremberg, which is funded by the German Research Foundation (DFG) within the framework of its “Excellence Initiative”. Supporting Information. The AFM data and XRD patterns of sg-ZnO and sp-ZnO/ITO , and the BHJ device light J/V curves and characteristics are in Supporting information. This material is available free of charge via the internet at http://pubs.acs.org REFERENCES (1)

Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Selective Interlayers and Contacts in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 1337–1350.

(2)

Litzov, I.; Brabec, C. Development of Efficient and Stable Inverted Bulk Heterojunction (BHJ) Solar Cells Using Different Metal Oxide Interfaces. Materials 2013, 6, 5796–5820.

(3)

Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials for Organic Solar Cells. J. Mater. Chem. 2010, 20, 2499–2512.

(4)

Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371–1388.

(5)

Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285–310.


30

Page 31 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6)

Tada, A.; Geng, Y.; Nakamura, M.; Wei, Q.; Hashimoto, K.; Tajima, K. Interfacial Modification of Organic Photovoltaic Devices by Molecular Self-Organization. Phys. Chem. Chem. Phys. 2012, 14, 3713–3724.

(7)

Yip, H.-L.; Jen, A. K.-Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994–6011.

(8)

Zilberberg, K.; Meyer, J.; Riedl, T. Solution Processed Metal-Oxides for Organic Electronic Devices. J. Mater. Chem. C 2013, 1, 4796–4815.

(9)

Kyaw, a. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L. An Inverted Organic Solar Cell Employing a Sol-Gel Derived ZnO Electron Selective Layer and Thermal Evaporated MoO3 Hole Selective Layer. Appl. Phys. Lett. 2008, 93, 221107.

(10)

Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed Small-Molecule Solar Cells with Inverted Structure. Adv. Mater. 2013, 25, 2397–2402.

(11)

Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679–1683.

(12)

White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted BulkHeterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517.

(13)

Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. High Performance Polythiophene/fullerene Bulk-Heterojunction Solar Cell with a TiOx Hole Blocking Layer. Appl. Phys. Lett. 2007, 90, 163517.

(14)

Boix, P. P.; Ajuria, J.; Etxebarria, I.; Pacios, R.; Garcia-Belmonte, G.; Bisquert, J. Role of ZnO Electron-Selective Layers in Regular and Inverted Bulk Heterojunction Solar Cells. J. Phys. Chem. Lett. 2011, 2, 407–411.

(15)

Ecker, B.; Egelhaaf, H.; Steim, R. Understanding S-Shaped Current − Voltage Characteristics in Organic Solar Cells Containing a TiO. J. Phys. Chem. C 2012, 116, 16333–16337.

(16)

Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317, 222–225.

(17)

Larsen-Olsen, T. T.; Bundgaard, E.; Sylvester-Hvid, K. O.; Krebs, F. C. A Solution Process for Inverted Tandem Solar Cells. Org. Electron. 2011, 12, 364–371.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

(18)

Min, J.; Zhang, H.; Stubhan, T.; Luponosov, Y. N.; Kraft, M.; Ponomarenko, S. a.; Ameri, T.; Scherf, U.; Brabec, C. J. A Combination of Al-Doped ZnO and a Conjugated Polyelectrolyte Interlayer for Small Molecule Solution-Processed Solar Cells with an Inverted Structure. J. Mater. Chem. A 2013, 1, 11306–11311.

(19)

Oh, H.; Krantz, J.; Litzov, I.; Stubhan, T.; Pinna, L.; Brabec, C. J. Comparison of Various Sol–gel Derived Metal Oxide Layers for Inverted Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 2194–2199.

(20)

Stubhan, T.; Salinas, M.; Ebel, A.; Krebs, F. C.; Hirsch, A.; Halik, M.; Brabec, C. J. Increasing the Fill Factor of Inverted P3HT:PCBM Solar Cells Through Surface Modification of Al-Doped ZnO via Phosphonic Acid-Anchored C60 SAMs. Adv. Energy Mater. 2012, 2, 532–535.

(21)

Trost, S.; Zilberberg, K.; Behrendt, A.; Polywka, A.; Görrn, P.; Reckers, P.; Maibach, J.; Mayer, T.; Riedl, T. Overcoming the “Light-Soaking” Issue in Inverted Organic Solar Cells by the Use of Al:ZnO Electron Extraction Layers. Adv. Energy Mater. 2013, 3, 1437–1444.

(22)

Aprilia, A.; Wulandari, P.; Suendo, V.; Hidayat, R.; Fujii, A.; Ozaki, M. Influences of Dopant Concentration in Sol–gel Derived AZO Layer on the Performance of P3HT:PCBM Based Inverted Solar Cell. Sol. Energy Mater. Sol. Cells 2013, 111, 181– 188.

(23)

Zhang, H.; Stubhan, T.; Li, N.; Turbiez, M.; Matt, G. J.; Ameri, T.; Brabec, C. J. A Solution-Processed Barium Hydroxide Modified Aluminum Doped Zinc Oxide Layer for Highly Efficient Inverted Organic Solar Cells. J. Mater. Chem. A 2014, 2, 18917–18923.

(24)

Weickert, J.; Dunbar, R. B.; Hesse, H. C.; Wiedemann, W.; Schmidt-Mende, L. Nanostructured Organic and Hybrid Solar Cells. Adv. Mater. 2011, 23, 1810–1828.

(25)

Bao Foong, T. R.; Singh, S. P.; Sonar, P.; Ooi, Z.-E.; Chan, K. L.; Dodabalapur, A. ZnO Layers for Opto-Electronic Applications from Solution-Based and Low-Temperature Processing of an Organometallic Precursor. J. Mater. Chem. 2012, 22, 20896–20901.

(26)

Venables, J. A.; Spiller, G. D. T.; Hanbuchen, M. Nucleation and Growth of Thin Films. Rep. Prog. Phys. 1984, 47, 399–459.

(27)

Ritala, M.; Niinisto, J. Chemical Vapour Deposition Precursors, Processes and Applications; Jones, A. C.; Hitchman, M. L. The Royal Society of Chemistry: London, 2009; Chapter 4, pp. 158–206.

(28)

Choy, K. Chemical Vapour Deposition of Coatings. Prog. Mater. Sci. 2003, 48, 57–170.


32

Page 33 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29)

Pan, J.; Leygraf, C.; Thierry, D.; Ektessabi, a M. Corrosion Resistance for Biomaterial Applications of TiO2 Films Deposited on Titanium and Stainless Steel by Ion-BeamAssisted Sputtering. J. Biomed. Mater. Res. 1997, 35, 309–318.

(30)

Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger, a. J. New Architecture for High-Efficiency Polymer Photovoltaic Cells Using Solution-Based Titanium Oxide as an Optical Spacer. Adv. Mater. 2006, 18, 572–576.

(31)

Hau, S. K.; Yip, H.-L.; Baek, N. S.; Zou, J.; O’Malley, K.; Jen, A. K.-Y. Air-Stable Inverted Flexible Polymer Solar Cells Using Zinc Oxide Nanoparticles as an Electron Selective Layer. Appl. Phys. Lett. 2008, 92, 253301.

(32)

Kuwabara, T.; Kawahara, Y.; Yamaguchi, T.; Takahashi, K. Characterization of InvertedType Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by Ac Impedance Spectroscopy. ACS Appl. Mater. Interfaces 2009, 1, 2107–2110.

(33)

Kuwabara, T.; Sugiyama, H.; Kuzuba, M.; Yamaguchi, T.; Takahashi, K. Inverted BulkHeterojunction Organic Solar Cell Using Chemical Bath Deposited Titanium Oxide as Electron Collection Layer. Org. Electron. 2010, 11, 1136–1140.

(34)

Li, C.-Y.; Wen, T.-C.; Lee, T.-H.; Guo, T.-F.; Huang, J.-C.-A.; Lin, Y.-C.; Hsu, Y.-J. An Inverted Polymer Photovoltaic Cell with Increased Air Stability Obtained by Employing Novel Hole/electron Collecting Layers. J. Mater. Chem. 2009, 19, 1643–1647.

(35)

Park, M.-H.; Li, J.-H.; Kumar, A.; Li, G.; Yang, Y. Doping of the Metal Oxide Nanostructure and Its Influence in Organic Electronics. Adv. Funct. Mater. 2009, 19, 1241–1246.

(36)

Ho, Y.-S.; Lee, K.-Y. Fabrication of Highly Oriented (002) ZnO Film on Glass by Sol–gel Method. Thin Solid Films 2010, 519, 1431–1434.

(37)

Znaidi, L. Sol–gel-Deposited ZnO Thin Films: A Review. Mater. Sci. Eng. B 2010, 174, 18–30.

(38)

Miller, J. B.; Hsieh, H.-J.; Howard, B. H.; Broitman, E. Microstructural Evolution of Sol– gel Derived ZnO Thin Films. Thin Solid Films 2010, 518, 6792–6798.

(39)

Krebs, F. C.; Gevorgyan, S. a.; Alstrup, J. A Roll-to-Roll Process to Flexible Polymer Solar Cells: Model Studies, Manufacture and Operational Stability Studies. J. Mater. Chem. 2009, 19, 5442–5451.

(40)

Kunz, O.; Wong, J.; Janssens, J.; Bauer, J.; Breitenstein, O.; Aberle, A. G. Shunting Problems Due to Sub-Micron Pinholes in Evaporated Solid-Phase Crystallised Poly-Si Thin-Film Solar Cells on Glass. Prog. Photovoltaics Res. Appl. 2009, 17, 35–46.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 41

(41)

Waldauf, C.; Scharber, M. C.; Schilinsky, P.; Hauch, J. a.; Brabec, C. J. Physics of Organic Bulk Heterojunction Devices for Photovoltaic Applications. J. Appl. Phys. 2006, 99, 104503.

(42)

Satpati, A. K.; Ji, L.; Yu, E. T.; Bard, A. J. Electrochemical Monitoring of TiO 2 Atomic Layer Deposition by Chronoamperometry and Scanning Electrochemical Microscopy. Chmemistry Mater. 2013, 25, 4165–4172.

(43)

Fabregat-Santiago, F.; Randriamahazaka, H.; Zaban, A.; Garcia-Cañadas, J.; GarciaBelmonte, G.; Bisquert, J. Chemical Capacitance of Nanoporous-Nanocrystalline TiO2 in a Room Temperature Ionic Liquid. Phys. Chem. Chem. Phys. 2006, 8, 1827–1833.

(44)

Bisquert, J.; Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Barea, E. M.; Palomares, E. A Review of Recent Results on Electrochemical Determination of the Density of Electronic States of Nanostructured Metal-Oxide Semiconductors and Organic Hole Conductors. Inorganica Chim. Acta 2008, 361, 684–698.

(45)

Andersen, T. R.; Larsen-Olsen, T. T.; Andreasen, B.; Böttiger, A. P. L.; Carlé, J. E.; Helgesen, M.; Bundgaard, E.; Norrman, K.; Andreasen, J. W.; Jørgensen, M.; Krebs, F. C. Aqueous Processing of Low-Band-Gap Polymer Solar Cells Using Roll-to-Roll Methods. ACS Nano 2011, 5, 4188–4196.

(46)

Lilliedal, M. R.; Medford, A. J.; Madsen, M. V.; Norrman, K.; Krebs, F. C. The Effect of Post-Processing Treatments on Inflection Points in Current–voltage Curves of Roll-toRoll Processed Polymer Photovoltaics. Sol. Energy Mater. Sol. Cells 2010, 94, 2018– 2031.

(47)

Music, S.; Gotic, M.; Ivanda, M.; Popovic, S.; Turkovic, A.; Furic, K.; Trojko, R.; Sekulic, A.; Furic, K. Chemical and Microstructural Properties of TiO2 Synthesized by Sol-Gel Procedure. Mater. Sci. Eng. B 1997, 47, 33–40.

(48)

Donald, W. a; Leib, R. D.; O’Brien, J. T.; Bush, M. F.; Williams, E. R. Absolute Standard Hydrogen Electrode Potential Measured by Reduction of Aqueous Nanodrops in the Gas Phase. J. Am. Chem. Soc. 2008, 130, 3371–3381.

(49)

Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New Youk, NY, 2000.

(50)

Fabregat-santiago, F.; Bisquert, J.; Zaban, A.; Salvador, P. Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2 /Electrolyte System by Impedance Methods. J. Phys. Chem. B 2002, 106, 334–339.

(51)

Fabregat-Santiago, F.; Barea, E. M.; Bisquert, J.; Mor, G. K.; Shankar, K.; Grimes, C. a. High Carrier Density and Capacitance in TiO2 Nanotube Arrays Induced by Electrochemical Doping. J. Am. Chem. Soc. 2008, 130, 11312–11316.


34

Page 35 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52)

Fabregat-santiago, F.; Mora-sero, I.; Bisquert, J. Cyclic Voltammetry Studies of Nanoporous Semiconductors . Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte. J. Phys. Chem. B 2003, 107, 758–768.

(53)

Nayak, P. K.; Garcia-Belmonte, G.; Kahn, A.; Bisquert, J.; Cahen, D. Photovoltaic Efficiency Limits and Material Disorder. Energy Environ. Sci. 2012, 5, 6022–6039.

(54)

Hoye, R. L. Z.; Ehrler, B.; Böhm, M. L.; Muñoz-Rojas, D.; Altamimi, R. M.; Alyamani, A. Y.; Vaynzof, Y.; Sadhanala, A.; Ercolano, G.; Greenham, N. C.; Friend, R. H.; MacManus-Driscoll, J. L.; Musselman, K. P. Improved Open-Circuit Voltage in ZnOPbSe Quantum Dot Solar Cells by Understanding and Reducing Losses Arising from the ZnO Conduction Band Tail. Adv. Energy Mater. 2014, 131544.

(55)

Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R. The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. Chemphyschem 2012, 13, 2824–2875.

(56)

Berger, T.; Lana-villarreal, T.; Monllor-Satoca, D.; Gomez, R. An Electrochemical Study on the Nature of Trap States in Nanocrystalline Rutile Thin Films. J. Phys. Chem. C 2007, 111, 9936–9942.

(57)

Berger, T.; Anta, J. A.; Morales-flo, V. Electrons in the Band Gap : Spectroscopic Characterization of Anatase TiO2 Nanocrystal Electrodes under Fermi Level Control. J. Phys. Chem. C 2012, 116, 11444–11455.

(58)

Berger, T.; Lana-Villarreal, T.; Monllor-Satoca, D.; Gómez, R. Charge Transfer Reductive Doping of Nanostructured TiO2 Thin Films as a Way to Improve Their Photoelectrocatalytic Performance. Electrochem. commun. 2006, 8, 1713–1718.

(59)

Boschloo, G.; Fitzmaurice, D. Spectroelectrochemical Investigation of Surface States in Nanostructured TiO2 Electrodes. J. Phys. Chem. B 1999, 4, 2228–2231.

(60)

Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Bogdanoff, P.; Zaban, A. Effect of Contamination on the Electronic Structure and Hole-Injection Properties of MoO3/organic Semiconductor Interfaces. J. Electrochem. Soc. 2003, 150, E293–E298.

(61)

Park, N. Organometal Perovskite Light Absorbers Toward a 20 % E Ffi Ciency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423–2429.

(62)

Kim, H.-S.; Lee, J.-W.; Yantara, N.; Boix, P. P.; Kulkarni, S. a; Mhaisalkar, S.; Grätzel, M.; Park, N.-G. High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO2 Nanorod and CH3NH3PbI3 Perovskite Sensitizer. Nano Lett. 2013, 13, 2412–2417.

(63)

Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B. Mesoscopic CH 3 NH 3 PbI 3 /TiO 2 Heterojunction Solar Cells. J. Am. Chem. Soc. 2012, 134, 17396–17399.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

(64)

Wang, H.; Chen, L.; Su, W.; Chung, J.; Hwang, B. Effect of the Compact TiO 2 Layer on Charge Transfer between N3 Dyes and TiO 2 Investigated by Raman Spectroscopy. J. Phys. Chem. C 2010, 114, 3185–3189.

(65)

Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gratzel, M. Organic Dye for Highly Efficient Solid-State Dye-Sensitized Solar Cells. Adv. Mater. 2005, 17, 813–815.

(66)

Burke, A.; Ito, S.; Snaith, H.; Bach, U.; Kwiatkowski, J.; Grätzel, M. The Function of a TiO2 Compact Layer in Dye-Sensitized Solar Cells Incorporating “Planar” Organic Dyes. Nano Lett. 2008, 8, 977–981.

(67)

Ou, K.-L.; Tadytin, D.; Xerxes Steirer, K.; Placencia, D.; Nguyen, M.; Lee, P.; Armstrong, N. R. Titanium Dioxide Electron-Selective Interlayers Created by Chemical Vapor Deposition for Inverted Configuration Organic Solar Cells. J. Mater. Chem. A 2013, 1, 6794–6803.

(68)

Kavan, L.; Regan, B. O.; Kay, A.; Grgtzel, M. Preparation of TiO2 ( Anatase ) Films on Electrodes by Anodic Oxidative Hydrolysis of TiCl3. J. Electroanalitical Chem. 1993, 346, 291–307.

(69)

Amatore, C.; Savéant, J. M.; Tessier, D. Charge Transfer at Partially Blocked Surfaces. J. Electroanal. Chem. Interfacial Electrochem. 1983, 147, 39–51.

(70)

Amatore, C.; Savéant, J. M.; Tessier, D. Kinetics of Electron Transfer to Organic Molecules at Solid Electrodes in Organic Media. J. Electroanal. Chem. Interfacial Electrochem. 1983, 146, 37–45.

(71)

Holt, K. B.; Bard, A. J.; Show, Y.; Swain, G. M. Scanning Electrochemical Microscopy and Conductive Probe Atomic Force Microscopy Studies of Hydrogen-Terminated BoronDoped Diamond Electrodes with Different Doping Levels. J. Phys. Chem. B 2004, 108, 15117–15127.

(72)

Duo, I.; Fujishima, A.; Comninellis, C. Electron Transfer Kinetics on Composite Diamond (sp3)–graphite (sp2) Electrodes. Electrochem. commun. 2003, 5, 695–700.

(73)

Brumbach, M.; Veneman, P. A.; Marrikar, F. S.; Schulmeyer, T.; Simmonds, A.; Xia, W.; Lee, P.; Armstrong, N. R. Surface Composition and Electrical and Electrochemical Properties of Freshly Deposited and Acid-Etched Indium Tin Oxide Electrodes. Langmuir 2007, 23, 11089–11099.

(74)

Randles, J. E. B. KINETICS OF RAPID ELECTRODE REACTIONS. Discuss. Faraday Soc. 1947, 1, 11–19.


36

Page 37 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(75)

Campuzano, S.; Pedrero, M.; Montemayor, C.; Fatás, E.; Pingarrón, J. M. Characterization of Alkanethiol-Self-Assembled Monolayers-Modified Gold Electrodes by Electrochemical Impedance Spectroscopy. J. Electroanal. Chem. 2006, 586, 112–121.

(76)

Finklea, H.; Snider, D. A.; Fedyk, J. Passivation of Pinholes in Octadecanethiol Monolayers on Gold Electrodes by Electrochemical Polymerization of Phenol. Langmuir 1990, 6, 371–376.

(77)

Finklea, H.; Snider, D. A.; Fedyk, J.; Virginia, W.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Characterization of Octadecanethiol-Coated Gold Electrodes as Microarray Electrodes by Cyclic Voltammetry and Ac Impedance Spectroscopy. Langmuir 1993, 9, 3660–3667.

(78)

Diao, P.; Guo, M.; Tong, R. Characterization of Defects in the Formation Process of SelfAssembled Thiol Monolayers by Electrochemical Impedance Spectroscopy. J. Electroanal. Chem. 2001, 495, 98–105.

(79)

Niu, L.; Li, Q.; Wei, F.; Chen, X.; Wang, H. Electrochemical Impedance and Morphological Characterization of Platinum-Modified Polyaniline Film Electrodes and Their Electrocatalytic Activity for Methanol Oxidation. J. Electroanal. Chem. 2003, 544, 121–128.

(80)

Choi, Y.-J.; Chung, Y.-D.; Baek, C.-Y.; Kim, K.-W.; Ahn, H.-J.; Ahn, J.-H. Effects of Carbon Coating on the Electrochemical Properties of Sulfur Cathode for Lithium/sulfur Cell. J. Power Sources 2008, 184, 548–552.

(81)

Janek, R. P.; Fawcett, W. R.; Ulman, A. Impedance Spectroscopy of Self-Assembled Monolayers on Au(111): Sodium Ferrocyanide Charge Transfer at Modified Electrodes. Langmuir 1998, 14, 3011–3018.

(82)

Savva, A.; Neophytou, M.; Koutsides, C.; Kalli, K.; Choulis, S. A. Synergistic Effects of Buffer Layer Processing Additives for Enhanced Hole Carrier Selectivity in Inverted Organic Photovoltaics. Org. Electron. 2013, 14, 3123–3130.

(83)

Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff, E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S.; Berry, J. J.; Olson, D. C. Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers. Adv. Energy Mater. 2011, 1, 813–820.

(84)

Hau, S. K.; Yip, H.-L.; Acton, O.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Interfacial Modification to Improve Inverted Polymer Solar Cells. J. Mater. Chem. 2008, 18, 5113– 5119.

(85)

Steim, R.; Choulis, S. a.; Schilinsky, P.; Brabec, C. J. Interface Modification for Highly Efficient Organic Photovoltaics. Appl. Phys. Lett. 2008, 92, 093303.

(86)

Kippelen, B.; Brédas, J.-L. Organic Photovoltaics. Energy Environ. Sci. 2009, 2, 251–261.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

(87)

Armstrong, N. R.; Carter, C.; Donley, C.; Simmonds, A.; Lee, P.; Brumbach, M.; Kippelen, B.; Domercq, B.; Yoo, S. Interface Modification of ITO Thin Films: Organic Photovoltaic Cells. Thin Solid Films 2003, 445, 342–352.

(88)

Stubhan, T.; Ameri, T.; Salinas, M.; Krantz, J.; Machui, F.; Halik, M.; Brabec, C. J. High Shunt Resistance in Polymer Solar Cells Comprising a MoO[sub 3] Hole Extraction Layer Processed from Nanoparticle Suspension. Appl. Phys. Lett. 2011, 98, 253308.

(89)

Roncali, J. Conjugated Poiy(thiophenes): Synthesis, Functionalizatlon, and Applications. Chemi. ReV. 1992, 92, 711–738.

(90)

Ratcliff, E. L.; Jenkins, J. L.; Nebesny, K.; Armstrong, N. R. Films for Photovoltaic Applications. Chem. Mater. 2008, 20, 5796–5806.

(91)

Skompska, M. Quartz Crystal Microbalance Study of Electrochemical Deposition of poly(3-Dodecylthiophene) Films on Au Electrodes. Electrochim. Acta 2000, 45, 3841– 3850.

(92)

Skompska, M.; Szkurłat, A. The Influence of the Structural Defects and Microscopic Aggregation of poly(3-Alkylthiophenes) on Electrochemical and Optical Properties of the Polymer Films: Discussion of an Origin of Redox Peaks in the Cyclic Voltammograms. Electrochim. Acta 2001, 46, 4007–4015.

(93)

Skompska, M.; Szkurlat, A.; Kowal, A.; Szklarczyk. Spectroelectrochemical and AFM Studies of Doping - Undoping of Poly ( 3-Hexylthiophene ) Films in Propylene Carbonate and Aqueous Solutions of LiClO 4. Langmuir 2003, 19, 2318–2324.

(94)

Cheun, H.; Berrigan, J. D.; Zhou, Y.; Fenoll, M.; Shim, J.; Fuentes-Hernandez, C.; Sandhage, K. H.; Kippelen, B. Roles of Thermally-Induced Vertical Phase Segregation and Crystallization on the Photovoltaic Performance of Bulk Heterojunction Inverted Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 3456–3460.

(95)

Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45–61.

(96)

Xu, Z.; Chen, L.-M.; Yang, G.; Huang, C.-H.; Hou, J.; Wu, Y.; Li, G.; Hsu, C.-S.; Yang, Y. Vertical Phase Separation in Poly(3-Hexylthiophene): Fullerene Derivative Blends and Its Advantage for Inverted Structure Solar Cells. Adv. Funct. Mater. 2009, 19, 1227–1234.

(97)

Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y. Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells. Adv. Funct. Mater. 2008, 18, 1783–1789.

(98)

Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Influence of Blend Microstructure on Bulk Heterojunction Organic Photovoltaic Performance. Chem. Soc. Rev. 2011, 40, 1185– 1199.


38

Page 39 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(99)

Chiu, M.; Jeng, U.; Su, C.; Liang, K. S.; Wei, K. Simultaneous Use of Small and Wide Angle X ray Techniques to Analyze Nanometerscale Phase Separation in Polymer Heterojunction Solar Cells. Adv. Mater. 2008, 20, 2573–2578.

(100) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.; Williams, S. P. PolymerFullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2010, 22, 3839–3856. (101) Dauwe, S.; Mittelstadt, L.; Metz, A.; Hezel, R. Experimental Evidence of Parasitic Shunting in Silicon Nitride Rear Surface Passivated Solar Cells. Prog. Photovoltaics Res. Appl. 2002, 10, 271–278. (102) Python, M.; Vallat-Sauvain, E.; Bailat, J.; Dominé, D.; Fesquet, L.; Shah, A.; Ballif, C. Relation between Substrate Surface Morphology and Microcrystalline Silicon Solar Cell Performance. J. Non. Cryst. Solids 2008, 354, 2258–2262. (103) Jones, E. W.; Barrioz, V.; Irvine, S. J. C.; Lamb, D. Towards Ultra-Thin CdTe Solar Cells Using MOCVD. Thin Solid Films 2009, 517, 2226–2230. (104) Moliton, A.; Nunzi, J.-M. How to Model the Behaviour of Organic Photovoltaic Cells. Polym. Int. 2006, 55, 583–600. (105) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Influence of the Molecular Weight of Poly(3-Hexylthiophene) on the Performance of Bulk Heterojunction Solar Cells. Chem. Mater. 2005, 17, 2175–2180. (106) Placencia, D.; Wang, W.; Gantz, J.; Jenkins, J. L.; Armstrong, N. R. Highly Photoactive Titanyl Phthalocyanine Polymorphs as Textured Donor Layers in Organic Solar Cells. J. Phys. Chem. C 2011, 115, 18873–18884. (107) Foertig, a.; Rauh, J.; Dyakonov, V.; Deibel, C. Shockley Equation Parameters of P3HT:PCBM Solar Cells Determined by Transient Techniques. Phys. Rev. B 2012, 86, 115302. (108) Wu, B.; Wu, X.; Guan, C.; Fai Tai, K.; Yeow, E. K. L.; Jin Fan, H.; Mathews, N.; Sum, T. C. Uncovering Loss Mechanisms in Silver Nanoparticle-Blended Plasmonic Organic Solar Cells. Nat. Commun. 2013, 4, 2004. (109) Lee, J. H.; Cho, S.; Roy, A.; Jung, H.-T.; Heeger, A. J. Enhanced Diode Characteristics of Organic Solar Cells Using Titanium Suboxide Electron Transport Layer. Appl. Phys. Lett. 2010, 96, 163303. (110) Yoo, S.; Domercq, B.; Kippelen, B. Intensity-Dependent Equivalent Circuit Parameters of Organic Solar Cells Based on Pentacene and C60. J. Appl. Phys. 2005, 97, 103706. (111) Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. M. Degradation Mechanisms in Organic Photovoltaic Devices. Org. Electron. 2012, 13, 432–456.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 41

(112) Weickert, J.; Sun, H.; Palumbiny, C.; Hesse, H. C.; Schmidt-Mende, L. Spray-Deposited PEDOT:PSS for Inverted Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 2371–2374. (113) Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Polymer Solar Cells That Use Self-Assembled-Monolayer- Modified ZnO/Metals as Cathodes. Adv. Mater. 2008, 20, 2376–2382.


40

Page 41 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Figure:


41

Characterization of ZnO Interlayers for Organic Solar Cells: Correlation

Recommend Documents