Surface Treatment of NiO Hole Transport Layers for Organic Solar Cells

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010 1649

Surface Treatment of NiO Hole TransportLayers for Organic Solar Cells

Joseph J. Berry, N. Edwin Widjonarko, Brian A. Bailey, Ajaya K. Sigdel, David S. Ginley, and Dana C. Olson

(Invited Paper)

Abstract—Recent advances in the power-conversion efficiencyof organic photovoltaics (OPVs) has largely been realized throughthe development of conjugated polymer absorber materials thatprovide for increased overlap with the solar spectrum as wellas proper energy level offset with the electron acceptor. Theseallow for increased photocurrent and photovoltage, thus result-ing in increased performance. Such systems could further be im-proved through the application of contact materials that havebeen tuned to minimize losses in carrier and potential losses atthe charge-extraction interfaces. To date, these devices continueto use contacts that have not been optimized for the specific ac-tive layer components employed. Here, we demonstrate that theelectrical and contact properties of NiO can be tuned throughcareful control of the deposition parameters as well as throughsurface treatments. The effects of the NiO thin-film processingand properties are investigated for application as a hole transportlayer (HTL) in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyricacid methyl ester OPV devices. Devices based on the NiO HTLsdemonstrate equal performance to those employing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HTLs. Further-more, the NiO HTLs enable the application of zinc-oxide-basedmaterials as transparent electrodes.

Index Terms—Hole transport layer (HTL), metal oxide, organicphotovoltaic (OPV).

Manuscript received December 7, 2009; revised March 8, 2010; acceptedMarch 10, 2010. Date of publication July 15, 2010; date of current versionDecember 3, 2010. This work was supported by the U.S. Department ofEnergy’s Office of Energy Efficiency and Renewable Energy PhotovoltaicsProgram through the Seed Fund Program. The work of J. J. Berry, N. E.Widjonarko, B. A. Baily, A. Miedaner, D. S. Ginley, and D. C. Olson wassupported by the U.S. Department of Energy and the National Renewable En-ergy Laboratory under Contract DOE-AC36-08GO28308. The work of J. J.Berry, A. K. Sigdel, D. S. Ginley, and D. C. Olson was supported by the Centerfor Interface Science: Solar-Electric Materials (CIS:CEMA), an Energy Fron-tier Research Center funded by the U.S. Department of Energy, Office of BasicSciences, under Award DE-SC0001084. The work of J. J. Berry, D. S. Ginley,and D. C. Olson was supported by the Center for Energy Efficient Materials, anEnergy Frontier Research Center funded by the U.S. Department of Energy, Of-fice of Science, Office of Basic Energy Sciences under Award DE-SC0001009.

J. J. Berry, D. S. Ginley, and D. C. Olson are with the National RenewableEnergy Laboratory, National Center for Photovoltaics, Golden, CO 80401 USA(e-mail: [email protected]).

N. E. Widjonarko is with the National Renewable Energy Laboratory, Na-tional Center for Photovoltaics, Golden, CO 80401 USA, and also with theDepartment of Physics, University of Colorado, Boulder, CO 80309 USA.

B. A. Bailey and A. K. Sigdel are with the National Renewable Energy Lab-oratory, National Center for Photovoltaics, Golden, CO 80401 USA, and alsowith the Department of Physics and Astronomy, University of Denver, Denver,CO 80208 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2010.2049347

I. INTRODUCTION

IMPROVEMENTS in the lifetime and efficiency of organicsolar cells are critical to the development of organic photo-

voltaics (OPVs) as a viable technology for an inexpensive re-newable energy source. One of the important interfaces in OPVdevices is between the transparent electrode and the organic ac-tive layer, where an interfacial material can both enhance theelectric field across the active layer and aid in charge extractionat the contact. Often, OPV devices employ an organic hole trans-port layer (HTL), such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), to modulate the inter-face between the indium tin oxide (ITO) transparent electrodeand the active layer consisting of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk het-erojunction (BHJ). The PEDOT:PSS material serves as a se-lective contact for holes by blocking electron transport to theITO electrode and allowing for a low contact resistance forhole collection. However, PEDOT:PSS has numerous draw-backs with respect to morphology, uniformity, chemistry, sta-bility, and scalability [1]–[5]. Here, we report our studies onthe effects of deposition parameters and surface modification ofnickel oxide (NiO) thin films as alternative inorganic HTL ma-terials. The results from P3HT:PCBM BHJ devices using NiOHTLs are compared with those from control devices employ-ing a PEDOT:PSS HTL. Our studies stem from the promisingdemonstration of NiO as an HTL by Irwin et al. [6]. Here,we expand on these results and correlate the deposition con-ditions and/or postprocessing of the NiO with the physicalproperties and device performance of this novel HTL mate-rial in traditional BHJ devices. By examining these aspectsof the NiO HTL in an integrated fashion, we demonstrate theability of this material to be used as a template HTL, wherethe structural and electronic properties can be tailored as afunction of the deposition and processing processes. The abilityto tune the contact properties allows the HTL to be matchedto the requirements of new active layer materials other thanthe traditional P3HT:PCBM examined here for optimizationof the in-device performance of these systems. We also useplasma surface treatments of the NiO films, which manifestin significant modification of the in-device behavior of theNiO HTLs. Specifically, we examine the device performancecharacteristics in the context of the work function (ϕw ) andquantified as a function of the NiO deposition conditionsand postprocessing employed for the HTL preparation. Thisinformation permits the location of the valance-band elec-tronic edge to be inferred, thus permitting us to effectively

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1650 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010

perform band engineering at the electrode/active layer interface[7].

II. NiO DEPOSITION

The films for this study were made by pulsed laser de-position (PLD) using a 243-nm KrF laser. The pulse powerfor these experiments was maintained at between 200 and220 mJ/pulse to deliver an energy density of 5.2 J/cm2 perpulse after optical losses. A fixed target to substrate distancewas used for the experiments of ≈ 12.5 cm. The PLD vacuumsystem has a base pressure of <10−7 torr, with working pres-sures of up to 3 × 10−1 torr. The vacuum and working pressureswere monitored using ion and/or convectron gauges. Substratetemperatures were calibrated using a type-k thermocouple onthe front side of the substrate. Film thickness was determinefor the PLD films using calibration samples with subsequentprofilometry and/or ellipsometry to establish the deposition rateper pulse at the atmospheric deposition conditions of interest.Device substrates consisted of patterned ITO on glass obtainedfrom thin-film devices, onto which an NiO HTL was depositedfrom an NiO target over a ∼3.5 cm2 using an aluminum shadowmask. The NiO deposition parameters, including the oxygenpartial pressure and substrate temperature, were varied over arange of conditions subject to the constraints imposed by theITO electrode.

III. HTL FILM CHARACTERIZATION

After deposition, the NiO films were examined using stan-dard characterization techniques including, optical reflectionand transmission measurements, and work-function measure-ments using an electrostatic voltmeter. While this technique isnot as rigorous as ultraviolet photoemission spectroscopy (UPS)on unpatterned substrates, we have found that the ϕw of the elec-trostatic measurements to be highly correlated with both stan-dard vacuum UPS and more recently available atmospheric UPSmeasurements. These measurements indicate tuning of the ϕw

between ∼4.5 and 5.6 eV depending on the deposition condi-tions, and subsequent, surface treatments. Specifically, we havemeasured an approximate 0.8 eV increase in work functionimmediately after O2 plasma treatment. Tuning the depositionconditions then provides an additional range to controlling theresultant work function. The thickness of the NiO films was eval-uated using stylus profilometry on calibration samples, whichwere also examined for structural properties. The morphologyof selected NiO HTLs were measured using atomic force mi-croscopy prior to device fabrication. Structural studies of theNiO HTLs were carried out on deposition calibration substratesas well as the device and using X-ray diffraction (XRD). TheXRD spectra were acquired on an X-ray diffractometer equippedwith a large area 2-D multiwire array proportional detector us-ing Cu Kα radiation. This system permitted the examination ofindividual areas on a device substrate so that the HTL could beexamined both before and after device fabrication. Examinationof the NiO before and after device fabrication, as well as boththe glass and ITO on glass, were examined and appeared similarin structure.

Fig. 1. Illustration of an OPV device. The layer of interest for this proposalis the HTL, which both aids in hole extraction from the active layer as well asacts as an electron-blocking layer thereby preventing contact between the n-typematerial and the TCO electrode.

IV. DEVICE FABRICATION

After NiO PLD film deposition, the samples were subse-quently exposed to a variety of surface treatments, as discussedshortly, after which a P3HT:PCBM active layer was depositedfrom a 1:1 blend by weight (50 mg/mL total concentration in1,2-dichlorobenzene) via spin-casting in a nitrogen-filled glove-box with less than 1 ppm of O2 and water using the proce-dure outlined by Li et al. [8]. After the deposition of the ac-tive layer, the samples are annealed at 110 ◦C for 10 min inthe glovebox and are subsequently transferred, without expo-sure to atmosphere, to the thermal evaporator for the deposi-tion of calcium (20 nm) and aluminum (100 nm) electrodes.The basic architecture of these studies is shown in Fig. 1. Foreach series of samples, a control device using the conventionalITO/PEDOT:PSS/P3HT:PCBM/Ca/Al device architecture wasprocessed in parallel to ensure consistent performance of thenon-NiO HTL device and to permit an appropriate evaluationof the NiO HTL materials.

V. VARIATION IN HTL DEPOSITION CONDITIONS

AND DEVICE PERFORMANCE

The deposition conditions of the NiO films were optimized inan iterative fashion with the initial NiO depositions performedunder an O2 pressure of 2.4 × 10−2 torr and substrate tempera-ture (Tsub ) of 100 ◦C. The thickness was then optimized, with athickness of 10 nm yielding the best devices consistent with thereport of Irwin et al. [6]. The experiments in which the Tsub wasaltered from room temperature to 400 ◦C were then performed.For both the initial optimization and substrate-temperature de-pendence, studies devices were fabricated using our standardprotocol, which includes an O2 plasma cleaning of the ITO sub-strate prior to deposition of the solution-processed PEDOT:PSSand active layers. In this case, the surface of the NiO films wasalso plasma-cleaned prior to the active layer and electrode de-position steps. The device results for these samples are shownin Fig. 2 with a summary of the device parameters shown in Ta-ble I. The data from this set of devices indicate a clear decreasein the fill factor (FF) due to the appearance of a double-diodebehavior in the current–voltage curves. The double-diode char-acteristic is indicative of the creation of an additional barrier thatinhibits current extraction near the open-circuit voltage (VOC )due to decreased electric field resulting in an enhanced recom-bination. This type of double-diode behavior has been attributed

BERRY et al.: SURFACE TREATMENT OF NiO HOLE TRANSPORT LAYERS 1651

Fig. 2. Device data for NiO films deposited as a function of temperature(from 100 to 400 ◦C) at a fixed O2 partial pressure of 2.4 × 10−2 (100 ◦CHTL-deposited 5 × 10−3 torr).

TABLE ISUMMARY OF DEVICE METRICS FOR DEVICE SHOWN IN FIG. 1

to the formation of an effective dipole at the interface resultingin the modification of the electric field at oxide/polymer inter-face [9], [10]. The series resistance estimated at high forwardbias indicates either a modest decrease in the conductivity of theNiO or an increase in contact resistance at the NiO/active layerinterface with increasing Tsub from 100 to 300 ◦C. This is con-sistent with the modest decrease in short-circuit current (JSC ).However, the device characteristics change dramatically whenTsub changes from 300 to 400 ◦C, where the degradation of theconductivity of ITO may also be contributing to the poor de-vice performance. Examining the structure of the NiO on thesesamples, we find that, at elevated growth temperatures, the NiOdisplays crystallinity on the glass portion of the device. Data forthe NiO films on glass is shown in Fig. 3, which includes theXRD patterns for a thick (100 nm) NiO film deposited on glass ata substrate temperature of 100 ◦C. In the XRD examinations ofthe NiO films on device (glass/ITO) and glass substrates showsa lack of crystallinity for NiO on glass when deposited at roomtemperature (data not shown). At the elevated Tsub depositionconditions examined, the films displayed the traditional NiOcrystal structure, as shown in Fig. 3(c), observed with some de-gree of texturing apparent in for the (1 1 1) peak, as indicated bythe experimental data. We note that for the thinner (10 nm) films,

Fig. 3. XRD pattern for a 100-nm NiO film. (a) Image taken with 2-D imagingdetector for a 2.3-h integration, with the class substrate background subtractedout. (b) Conventional θ–2θ scan obtained by integrating at χ = 0 across imagein (a). (c) Powder diffraction pattern for NiO with lines at 2θ = 34.28◦ (1 1 1)and 2θ = 43.25◦ (2 0 0) peaks.

the crystalline XRD pattern observed in Fig. 3 is still present,although the XRD strength of the signal is diminished, thusmaking an assessment of the texture difficult. The weakness ofthe XRD signal and the resolution of the detector have precludeda more comprehensive assessment of the crystal volume fractionin the device films to be compared for the Tsub range examined.When taken in its totality, the XRD data suggests that the in-creased series resistance observed in the devices is likely due toan increase in contact resistance at the NiO/BHJ interface.

To determine the impact, if any, of the plasma treatment ofthe NiO, a set of three NiO films were deposited near the basevacuum of the system 2.6 × 10−6 torr and another set of threefilms were prepared at an O2 pressure of 5.3 × 10−2 torr. Bothset of films were deposited at a fixed Tsub of 200 ◦C. Paired filmsfrom each set were then subjected to different surface treatments.The first pair was simply solvent-cleaned using an ultrasonicbath in acetone followed by isopropyl alcohol. In contrast, thesecond pair was subjected to an argon plasma cleaning for 5 minat 155 W and 8 × 10−1 torr, while the final pair was subjectedto the oxygen plasma cleaning for 5 min at 155 W at 8 × 10−1

torr. The device data for the two sets of samples are presented inFig. 4(a) displaying the devices in which the low-pressure NiOwas used, while Fig. 4(b) shows those for NiO deposited in the5.3 × 10−2 torr O2 atmosphere. It is clear from the device datathat the surface treatment generates a significant change in the


Fig. 4. Device data for NiO films with a fixed Tsub of 200 ◦C deposited at an O2 pressure of (a) 2.6 × 10−6 torr and (b) 5.3 × 10−2 torr. Parallel films fromeach set were then subjected to differing surface treatments. These were subsequently (circles) solvent cleaned, (diamonds) argon plasma cleaned, or (triangles)oxygen plasma cleaned.

JV characteristics of the devices. In the samples deposited nearthe base vacuum pressure, the changes in the characteristics aredramatic, but fundamentally mirror the devices based on NiOfilms deposited in an O2 atmosphere. The two main features ofthe data are the change in behavior between the solvent-cleanedsamples and the similarity of the devices base on the O2 plasma-cleaned NiO films.

The difference in the solvent-cleaned samples results fromthe difference in the O2 incorporation rate during film deposi-tion, some variation in the observed work function is observed,which we presume results from changes in carrier concentration.However, the resistivity of all the films are in excess of whatcan be measure on our existing four point prove experimen-tal system. The differences resulting from systematic changesin the deposition conditions are reflected in the other aspectsof the device characteristics in which we see evidence for thepresence of a mismatch in the work function of the HTL tothe active layer, resulting in an increased barrier for carrier ex-traction from the active layer. This behavior is seen for HTLswith work functions shallower than the highest occupied molec-ular orbital (HOMO) level of the electron-donor material [11].Although the solvent-cleaned sample deposited in an O2 at-mosphere displays a less severe suppression of the device FFdue to this contact barrier, the presence of this feature in thedata indicate that the O2 present may be insufficient to raise thework function to lower the contact resistance for hole extractionfrom the electron-donor material. However, despite the originalshortcomings of the as-deposited NiO HTLs, these deficienciesare mitigated, and then, largely wiped away by the Ar and O2plasma treatments, respectively. In the case of the O2 plasma-treated NiO devices, we observe that while the modification ofthe NiO surface dominates the device characteristics, the under-lying electronic properties imposed by the original deposition

Fig. 5. Average device data for (red) optimized PLD NiO HTL compared to(black) PEDOT:PSS.

conditions of the NiO are still evident in the more subtle aspectsof the JV curves, namely in the difference in JSC of the de-vices in comparison to the PEDOT:PSS controls. As mentionedpreviously, we measure a distinct shift in the work function ofthe NiO films immediately after plasma treatment. Over time,we observe that the work function decreases, but not to a levelas low as measured prior to O2 plasma treatment, thus indicat-ing there are both permanent and temporary components to thechange in the electronic structure of the film.

Based on this surface treatment data, we begin to gain amore comprehensive understanding of the physical require-ments for the optimizing the NiO as a replacement for thePEDOT:PSS HTL in the OPV devices. By omitting the plasmatreatment before the deposition of the active layer on the NiO


Fig. 6. AFM images of device substrates sampled a several locations on the substrate.

films and increasing the O2 atmosphere, it is possible to sig-nificantly improve the performance of the NiO films to reachan equivalent to the plasma-cleaned samples. However, giventhe significant impact of the plasma-cleaning step on the HTLin device performance, a reexamination of the optimized HTLdeposition conditions was required. Based on the results forthe substrate temperature studies shown in Fig. 1 and Table I,we also examined room temperature deposition of the NiOfilms over a range of partial pressures with an optimum deviceachieved for a room temperature Tsub and an O2 atmosphere of8.2 × 10−2 torr. The results for an optimized room temperatureNiO HTL deposited on ITO are shown in Fig. 5. In addition, forthe optimized NiO films, we are able to obtain devices of su-perior FF and identical JSC to the PEDOT:PSS control devices.These devices result in equivalent η to the controls, but we donot see an overall improvement in η due to a modest decrease inthe VOC . For these devices, we have not been able to detect anycrystallinity in the 10 nm NiO HTL films via XRD for acquisi-tion time in excess of 2 hrs. Atomic force microscope imagesof the ITO, NiO, and NiO on ITO all on the device substrateglass are shown in Fig. 6. Surface roughness analysis indicatesthat the NiO/Glass has an rms roughness value of 0.3 nm, whilethe ITO/Glass has an rms roughness value of 1.0 nm over a500 nm2 area. The composite NiO/ITO/Glass has an interme-diate rms roughness value of 0.5 nm over the same area. Thisindicates that the NiO layer helps to smooth the ITO surface.

Overall, the NiO-based devices result in very similar per-formance relative to PEDOT:PSS controls. However, we notethat the typical device performance from the ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al devices fabricated in our laboratory, as acontrol, is of η = ∼4%, which leaves little room for improveddevice performance.

VI. ENABLING ZnO-BASED TRANSPARENT

CONTACT MATERIALS

The use of alternative transparent conducting oxide (TCO)materials based on ZnO have been demonstrated as anodes

Fig. 7. Device data for optimized PLD NiO HTL on (red) ITO and (black)GZO.

in organic light-emitting diodes and hybrid OPVS (h-OPV),as well as electron transport layers (ETLs) in “inverted”ITO/ZnO/P3HT:PCBM/Ag OPV devices [10], [12], [13]. How-ever, the use of these ZnO-based TCOs as anodes in standardOPV architecture has been precluded due to the use of PE-DOT:PSS as the HTL. By substituting an inorganic HTL forthe highly acidic PEDOT:PSS suspension, which would oth-erwise eviscerate these polycrystalline TCO materials, we areable to successfully use ZnO-based TCO materials. Fig. 7 showdevices in which Ga-doped ZnO (GZO) with similar sheet resis-tance (Rs = 12 Ω/sq.) and optical properties similar (T > 85%in the visible) to that of the standard ITO used in our canoni-cal BHJ OPV devices. With an NiO HTL used in place of thetraditional ITO/PEDOT:PSS combination, it is possible to con-struct a standard BHJ device in which the holes are extractedthrough the TCO contact. Using the same deposition conditionsthat resulted in the ITO/NiO performance shown in Fig. 5, wehave successfully realized equivalent performance from OPV


Fig. 8. Energy structure and band-bending simulations including the electric field across the physical height for devices with (a) no HTL, (b) intrinsic 10 nmlayer of NiO at 4.7 eV, (c) same as (b), but doped throughout the layer to 1 × 1018 cm−3 , and (d) device as in (b), but doped to 1 × 1018 cm−3 only at the top5 nm.

devices employing a GZO electrode without the need for addi-tional optimization. This resulting devices show that either ITOor GZO electrodes can be used effectively, and at the same time,demonstrates the utility of the oxide HTL to provide chemicalstability of the interfaces, thus decoupling the TCO interfacialproperties of the TCO from the device performance.

VII. MODELING ELECTRIC-FIELD STRENGTH THROUGH

CHANGES IN CARRIER CONCENTRATION

An intriguing aspect of these studies is the fact that despite thecorrelation between the device performance and the enhancedwork function provided by the NiO HTL, the best-performing

devices were based on NiO HTLs with much more negative workfunction than the expected optimal value of 5.0 eV. For workfunctions deeper than this, ignoring effects of surface chemistry,one might anticipate the creation of a barrier for hole extrac-tion from the BHJ to the ITO and a corresponding increase inrecombination at the interface resulting in reduced device per-formance. However, experimentally, we find that the higher thework function of the NiO HTL, as measured prior to depositionof the active layer, the better the devices performed. This maybe the result of a number of different factors. First, as describedearlier, the O2 plasma treatment of these films results in sig-nificant enhancements in the device performance. Exactly whatthe O2 plasma does to the surface of the films is yet unclear. It


is possible that it is simply removing organic contaminants orany remaining solvent at the surface, revealing the higher workfunction of the NiO beneath. Another plausible explanation isthat the surface energy of the film is changed, thus resulting inenhanced wetting and/or chemical interactions and the forma-tion of a superior interface for selective extraction of holes. Itis also possible that a surface oxidation process is changing thework function and the doping of the surface of the NiO film,where the “bulk” of the film lies at a lower work function forbetter ohmic contact to the TCO. Essentially, the thin regionwhere work function is far too high is thin enough that carrierscan easily tunnel through it. There may also be pinning of theNiO Fermi level to the donor material much like that has beenobserved for very low work function cathode materials whenin contact with fullerene acceptors [14]. These types of mech-anisms may place more importance on the electric field, andtherefore, the effective doping profile in the NiO layer than theactual work function of the layer. In fact, since carrier lifetimesare short within the active layer, the electric field does helpto direct charge diffusion in the active layer resulting in im-proved carrier extraction. Before recombination can occur, thehigh electric field would allow the carriers to be removed fromthe interface and accelerated toward the TCO in the electrode.Higher fields would therefore correspond to enhanced chargetransport and collection. A simple model was compiled in orderto elucidate the effects of the electric field and band-bendingand to access the impact of different NiO doping over a rangeof physically realizable levels. These calculations, as shown inFig. 8, demonstrate the potential for surface doping to signifi-cantly modify the electric fields, and hence, carrier extractionat the HTL/P3HT interface. While instructive, these crude andoverly simplistic models suggest that other mechanisms, similarto those examined for enhancing h-OPV, may offer an avenue forimproving device performance through careful control of the in-terface doping, surface chemistry, and other physical propertiesof the NiO [12].

VIII. CONCLUSION

We have demonstrated the use of an NiO as a tunable HTLfor OPV devices through control of deposition and processingparameters. The surface treatment through plasma processingresults in equivalent or improved performance of NiO as com-pared to PEDOT:PSS. The use of postprocessing techniquesmay enable nontraditional processing technologies to be usedto deposit and optimize NiO or other oxide-based HTLs. Wehave demonstrated that the performance of devices based onNiO HTLs is equivalent to power conversion efficiencies ofPEDOT:PSS-based devices. We have also demonstrated the useof NiO to enable the use of GZO-based anodes in a tradi-tional BHJ device incompatible with the canonical PEDOT:PSSHTL. The role of the plasma processing is likely to enhance theelectric-field effects created by doping at the NiO layer.

ACKNOWLEDGMENT

The authors would like to thank Prof. S. Shaheen for valuablediscussion and for the use of the AFM facilities at the Universityof Denver.

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Surface Treatment of NiO Hole Transport Layers for Organic Solar Cells