Alkyl Surface Treatments of Planar Zinc Oxide in Hybrid Organic

Apr 10, 2012 - SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway. ABSTRACT: Hybrid organic/inorganic solar cells have not lived up to their ...
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Alkyl Surface Treatments of Planar Zinc Oxide in Hybrid Organic/ Inorganic Solar Cells C. G. Allen,† D. J. Baker,‡ T. M. Brenner,‡ C. C. Weigand,§ J. M. Albin,‡ K. X. Steirer,‡ D. C. Olson,∥ C. Ladam,⊥ D. S. Ginley,∥ R. T. Collins,‡ and T. E. Furtak*,‡ †

Department of Physics, Nanoco Technologies, Ltd., Manchester, U.K. Colorado School of Mines, Golden, Colorado 80401, United States § Norwegian University of Science and Technology, NO-7491 Trondheim, Norway ∥ National Renewable Energy Laboratory, Golden, Colorado 80401, United States ⊥ SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway ‡

ABSTRACT: Hybrid organic/inorganic solar cells have not lived up to their potential because of poor interface properties. Interfacial molecular layers provide a way of adjusting these devices to improve their performance. We have studied a prototypical system involving poly(3-hexylthiophene) (P3HT) on planar zinc oxide (ZnO) films that have been modified with two types of molecules having identical 18-carbon alkyl chain termination and different surface attachments: octadecanethiol (ODT) and octadecyltriethoxysilane (OTES). We examined the functionalized surfaces using water contact angle measurements, Kelvin probe measurements, infrared absorbance spectroscopy, and atomic force microscopy. These have shown that OTES forms disordered incomplete monolayers, while ODT is prone to develop multilayered islands. Both treatments enhance polymer ordering. However, inverted solar cell devices fabricated with these treated interfaces performed very differently. ODT improves the short circuit current (JSC), open circuit voltage (VOC), and power conversion efficiency (η), while these parameters all decrease in devices constructed from OTES-treated ZnO. The differences in VOC are related to modifications of the surface dipole associated with deposition of the two types of alkyl molecules, while changes in JSC are attributed to a balance between charge transfer blocking caused by the saturated hydrocarbon and the improved hole mobility in the polymer.

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INTRODUCTION Organic photovoltaic (OPV) devices are of considerable recent interest because of their potential for low cost production through low temperature solution processing and roll-to-roll manufacturing on flexible substrates.1,2 Current attention is directed toward improving device efficiencies while maintaining simple fabrication methods to enable widespread use of OPVs. An important aspect of optimizing the efficiency in these systems involves controlling the nanoscale structural properties of the absorber layers. This arises because of the fundamentally different way in which carrier generation and collection occurs in organic devices relative to their inorganic semiconducting counterparts.3 Photon absorption in an OPV device results in a strongly bound electron−hole pair, or exciton, which must be dissociated to create free carriers. It is most desirable if dissociation occurs at the boundary between an electron conducting acceptor and a hole conducting donor (a heterojunction interface), so that the resulting carriers can be separately collected after transport in different phases. Exciton diffusion lengths are very small, on the order of nanometers, in organic materials.4,5 Therefore, the heterojunction interfaces must be located within an exciton diffusion length of the regions where solar photons are absorbed. The most successful polymeric solar cells accomplish this through the solubility © 2012 American Chemical Society

properties of a mixture of a donor polymer and an organic acceptor. These materials separate into a bicontinuous, interpenetrating network of donor and acceptor phases, termed a bulk heterojunction (BHJ), that are subdivided on a size scale that enables effective carrier collection. The prototypical BHJ device is a blend of the light-absorbing polymer, poly(3hexylthiophene) (P3HT), and the electron acceptor, [6,6]phenyl-C61-butyric acid methyl ester (PCBM).6 Recent progress involving low-band-gap polymers blended with C70 derivatives has raised the power conversion efficiency (η) of BHJ OPV devices to well above 8%.7 However, there is no guarantee that the morphology of a phase-separated system will provide effective conduction pathways for carriers. The morphology can also change with time or thermal cycling. In addition, carrier mobilities in organic materials are typically very low. An alternative design for an OPV device involves explicit fabrication of the carrier collecting phase in the form of a metal oxide network with open channels perpendicular to the substrate. This inorganic network can be infiltrated with the Received: December 5, 2011 Revised: March 30, 2012 Published: April 10, 2012 8872

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modification of a surface dipole layer. Some reports have appeared in which similar modifications of ZnO charge collection layers were reported.28−30 However, general interface design flexibility has not yet been demonstrated with ZnO. Some progress has recently been made with molecules having a phosphonic acid headgroup.31,32 However, organic acids frequently etch the surface of ZnO, making it necessary to employ different attachment methods for interface modifiers. This has motivated studies of molecular layers bonded to ZnO with thiol moieties33 as well as through siloxane networks.34 It has been shown that surface modification of ZnO with alkanethiols improved JSC of bilayer P3HT/ZnO devices.35 This was somewhat unexpected, since a close-packed alkyl monolayer would be expected to act as an insulator.36 The results are interesting in light of reports showing that alkanedithiol additives, mixed with P3HT, improved the bulk structural order in BHJ devices.37 Crystalline-like regions in P3HT have improved π−π interchain electronic coupling as well as increased conjugation lengths along the polymer chains. Improved order has been shown to increase exciton diffusion lengths38 and hole mobilities39 in P3HT. The influence of alkanethiols as surface treatments for ZnO, in improving the order of P3HT at the interface, has been confirmed through grazing incidence X-ray diffraction.40 It has been proposed that the improved transport properties of devices fabricated from treated ZnO overcompensated for the charge transfer inhibition characteristics of an insulating monolayer.35 However, the details of this balance are currently not understood. It is also not clear how these alkyl treatments influence the other characteristics of the interface between the polymer and the metal oxide. What is needed are comparative investigations of different organic layers on ZnO to help reveal details about the factors dominating charge generation and extraction at this interface. This was the goal in a characterization of 1-hexanethiol and 1hexanephosphonic acid layers deposited on polycrystalline ZnO and single crystals of ZnO.41 Since both molecules in that study had the same terminal group, it was possible to attribute differences to the different surface attachment reactions. In this comparison, the thiol-based layers were shown to be more defective than layers bonded to ZnO through phosphonate chemistry. This may help to explain how it is possible for alkanethiols to actually improve the performance of photovoltaic devices, since a defective layer would have been a poor insulator. Our objective is to further explore how organic molecule surface modifications of ZnO alter the structure and charge transfer properties of the interface with P3HT. Here we report on the comparison between two molecules having the same alkyl chain terminal group, but different surface attachment groups. These are octadecanethiol (ODT) [CH3(CH2)17SH] and octadecyltriethoxysilane (OTES) [CH 3 (CH 2 ) 17 Si(OCH2CH3)3]. We expect these materials to interact with ZnO in substantially different ways. The sulfur−zinc bond is sufficiently strong to cause dissociative adsorption for some thiols.42 So, it is expected that ODT should attach to ZnO through exposed Zn sites. By contrast, functionalization with OTES occurs through a condensation reaction involving preexisting surface hydroxyl groups.34,43 This results in a strong covalent attachment, which is different from the weaker bond expected with the thiol. An additional difference is the tridentate configuration of OTES, which enables multiple surface bonds in a single molecule, and cross-linking between

light-absorbing organic polymer by simple spin-coat processing. Theoretical models of different ordered nanostructures show that this is an optimal configuration for organic solar cells.8 Various embodiments of this concept have been demonstrated, including designs based on mesoporous titania9 as well as nanorod arrays of TiO2,10 CdTe,11 and ZnO.12 Zinc oxide is a particularly promising system, since it can be fabricated into a range of ordered nanostructures, including nanorod arrays using a variety of processing methods,13 and electron mobilities in ZnO are several orders of magnitude higher than they are in typical organic electron transport materials.14,15 In the most common device configuration ZnO is grown on indium−tin oxide (ITO) coated glass. The device operates under so-called inverted conditions, in which the oxide collects electrons and holes migrate through the polymer to a high work function metal electrode.16 Solar cells fabricated from ZnO nanorods infiltrated with P3HT demonstrate photocurrents that are several times higher than planar ZnO/ P3HT devices.17 However, hybrid organic/inorganic nanorod solar cells have yet to achieve power conversion efficiencies near to those of BHJ cells based on polymer blends.18 The most likely cause of unrealized performance in these devices is suppressed charge transfer from the polymer into the metal oxide. This has been attributed to improperly aligned energy states at the interface, disordered polymer at the interface, and polymer that does not fully contact the nanorods in the array.19 Some of these issues can be addressed through polymer processing by increasing the drying time during spincoating and annealing the device after the polymer has dried. These procedures encourage polymer ordering and improve the infiltration of the polymer into the metal oxide network.20 Additional improvement can be achieved through management of the polymer/ZnO interface. A simple way to accomplish this is through organic monolayers adsorbed on the oxide. Treatment of ZnO nanorods with an organic layer (phenyltrichlorosilane) having a low surface energy was shown to improve wetting of the nanorods by the polymer (P3HT). However, the organic layer completely blocked charge transfer.19 It is likely that the phenyltrichlorosilane, which is very reactive, cross-linked to form an insulating polymer multilayer on the nanorods. More success was achieved with a surfactant layer (polyoxyethylene(12) tridecyl ether) at the ZnO interface, which improved the open-circuit voltage VOC.19 However, the short-circuit current (JSC) of model devices was sufficiently reduced so as to keep the overall efficiency low. Some strategies to improve charge transfer in ZnO nanorod organic devices have been drawn from organic layers that worked effectively in dye-sensitized solar cells. In these cases the molecules (an amphiphilic polypyridylruthenium complex (Z907)21 or mercurochrome22) adsorbed on the oxide improved the efficiencies of the devices. This was attributed to introduction of unoccupied intermediate energy states between the conduction band edge of ZnO and the lowest unoccupied molecular orbital (LUMO) of the polymer (P3HT), which reduce carrier recombination at the interface. Some organic dyes additionally improve the optical absorption characteristics of ZnO nanorod organic devices, leading to efficiencies as high as η = 1.02%,23 which is still much smaller than BHJ devices. Considerable control of interfacial energetics should be possible with surface modifiers.24−26 This has been successfully demonstrated on TiO2,27 where carboxylic acids were used to vary the work function of the oxide through introduction or 8873

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adjacent molecules. The head groups of OTES should form a siloxane network that is bonded to Zn sites through oxygen links. Our primary samples were thin films of polycrystalline ZnO, grown from a sol−gel precursor. This material is easy to produce under conditions that are compatible with low-cost solar cell fabrication and is similar to other studies where similar material was used.35,40,44 We examined the characteristics of these films, modified with either ODT or OTES, using water contact angle (CA) tests, Kelvin probe measurements, Fourier transform infrared (FTIR) transmission spectroscopy, and atomic force microscopy (AFM). We also studied how the surface modifications altered the interaction between ZnO and P3HT using optical transmission spectroscopy of very thin polymer layers in the UV−vis range. We measured the photovoltaic performance of ZnO/P3HT bilayer devices to compare the influences of ODT and OTES treatments. In this paper we are not reporting results obtained from experiments on ZnO nanorod arrays. However, as part of our study, we have investigated how the effects of the surface modifiers change after thermal annealing, a step that is commonly used to assist infiltration of the polymer into nanorod arrays.

Deposition of OTES followed a method that was originally developed for silica substrates43 and has been shown to create reproducible molecular layers on sol−gel ZnO films.34 The substrates were rinsed with acetone and ethanol and were then hydroxylated through exposure to UV light from a mercury lamp in a commercial (Jelight) ultraviolet-ozone (UVO) cleaner for 15 min.49 The power density from the most intense line in the spectrum, at 254 nm, was 20 mW cm−2 at the location of the samples. Hydroxylation occurs very rapidly on UV-treated ZnO through dissociative adsorption of atmospheric water. These films were immersed for 90 min in a 0.045 M solution of OTES in toluene, which also contained an nbutylamine catalyst (0.072 M). Ethoxysilanes are relatively unreactive, compared to chlorosilanes, so the tendency of oligomers, which might deposit to produce a multilayer polymeric insulator, to form in the bulk solution is minimal. The catalyst promotes the dehydration reaction on the surface by interacting with surface-attached hydroxyl groups.34 The deposition was performed at 45 °C, which was controlled by surrounding the container with a water bath. The samples were then thoroughly rinsed with toluene and acetone and then blown dry with nitrogen. They were then baked at 110 °C in air for 60 min to promote completion of the condensation reaction and to encourage siloxane cross-linking, after which they were rinsed with acetone and were blown dry. We report the average and standard deviation of multiple CA measurements on untreated and functionalized planar ZnO films. The sessile drop had volumes ranging from 1.5 to 2.5 μL. For each measurement the outline of the drop in the recorded image was fit to a constrained curve using a polynomial spline, making it possible to objectively identify the CA through the intersection of the spline with the horizontal contact line of the drop.50 Relative work function measurements were performed in air, in the dark, for untreated and functionalized planar ZnO films on ITO substrates using a KP Technology SKP5050 Kelvin Probe System. The recorded values were referenced to a gold calibration standard. The results for the functionalized surfaces are reported as work function changes, relative to untreated ZnO. The untreated sample was measured immediately after it was rinsed with deionized water, acetone, and ethanol and then baked for 20 min at 150 °C. The vibrational fingerprints of ODT and OTES adsorbed on planar ZnO films on Si were determined through normalincidence transmission, obtained with a Thermo Fisher 6700 FTIR spectrometer equipped with a liquid nitrogen cooled Hg−Cd−Te detector and a KBr beamsplitter. For each measurement, 800 scans at a resolution of 2 cm−1 were averaged. The absorbance was calculated with respect to a spectrum acquired from an untreated ZnO film. We compared these results to an absorbance spectrum of a well-ordered cadmium stearate monolayer on silicon. This molecule, which is terminated with an alkyl chain containing 17 carbon atoms, was deposited on both sides of double-side polished silicon with a Langmuir−Blodgett (LB) trough using methods described elsewhere.34 The spectrum of the LB layer was multiplied by one-half to produce the spectrum of a reference monolayer. This allowed the relative cross sections of the symmetric and antisymmetric methylene modes to be determined in a highquality single monolayer, which were then used to make quantitative estimates of the amount of ODT and OTES on treated ZnO surfaces.

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EXPERIMENTAL PROCEDURE All solvents used for depositions and rinses, including acetone, ethanol, toluene, and chloroform, were of 99% purity or greater. Water was deionized on site using an Aqua Solutions Solution 2000 system to a resistance of greater than 18 MΩ. The planar ZnO films were fabricated as follows. A solution of 0.75 M zinc acetate dihydrate (99.999%, Aldrich) and 0.75 M ethanolamine (99%, Aldrich) in 2-methoxyethanol (98% anhydrous, Aldrich) was mixed on a 60 °C hot plate for 30 min. It was then spincoated for 60 s at 2000 rpm onto a clean substrate (either native oxidized Si, microscope slide glass, or patterned ITOcoated glass (Thin Film Devices)) following the method of Ohyama et al.45 as discussed by Allen et al.34 Prior to use, the substrates were sonicated in acetone for 10 min and then in isopropyl alcohol for 10 min, before photochemical treatment in a commercial ultraviolet-ozone (UVO) cleaner (Jelight) for 20 min. After spin-coating, the sol−gel film was reacted by placing the substrate on a 300 °C hot plate for 10 min in air to form polycrystalline ZnO. The resulting layers were 30 nm thick (as determined by a stylus profilometer) with a wurtzite crystal structure having a weak (002) texture (as determined by X-ray diffraction) with additional contributions from (100) and (101) textures. The ZnO films were characterized before and after surface modification with atomic force microscopy (AFM). Images were collected in tapping mode (Veeco, Nanoscope III) using tips with high aspect ratios and 125 μm cantilevers (Nanoworld). The average grain size was 20 nm or smaller, and the rms surface roughness was ∼2 nm. We used the same deposition conditions for ODT as were employed by Monson et al.35 The ZnO-coated substrates were first rinsed with acetone followed by ethanol and then baked in air on a 150 °C hot plate for 30 min to remove physisorbed water from the surface.46 The substrates were then cooled on an aluminum cold plate, blown with nitrogen, and immersed in a 1 mM solution of ODT (98%, Aldrich) in ethanol. The samples were removed after 72 h, rinsed with ethanol, and then blown dry with a stream of nitrogen gas. This procedure is the common practice for functionalizing a variety of surfaces with alkanethiols.47,48 8874

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P3HT (regioregular, electronic grade, Rieke) films were spincoated from chloroform solutions having different concentrations. In each case the polymer was dissolved by active stirring at 60 °C for 12 h after which the solutions were allowed to cool before spin-coating. Immediately before P3HT deposition the substrates that had been functionalized with ODT or OTES were rinsed with acetone and with ethanol and were then dried with nitrogen. The untreated control samples were thermally treated, at 150 °C for 30 min, to evaporate physisorbed water and allowed to cool just prior to being coated with P3HT. Very thin layers (10 nm) of P3HT were produced from a 1 g L−1 solution, which was spin-coated at 2000 rpm. These layers were deposited on untreated and on modified planar films of ZnO on glass to enable measurement of UV−vis absorbance spectra (using a Cary-V spectrophotometer) from layers that were thin enough to be sensitive to polymer order near the ZnO interface. The details of these spectra revealed how the surface modifications influenced the conformation of the polymer adjacent to the oxide.40 The thickness of the polymer in these samples was similar to exciton diffusion lengths, so they provide a way of examining the critical interfacial region in hybrid solar cells. For the photovoltaic devices we used a thicker P3HT layer (∼60 nm). This was spincoated from a 15 g L−1 solution at 800 rpm onto both untreated and modified ZnO films, which had been spin-coated onto patterned ITO-coated glass substrates. To complete the photovoltaic devices a back contact of 100 nm of silver was deposited by thermal evaporation through a shadow mask at a rate of 2 Å s−1. The active area of each of the six devices on a patterned substrate was 0.1 cm2. The completed device used in this work had the structure glass/ ITO/ZnO/P3HT/Ag. The devices were stored in air for 16 days, after which their photovoltaic performance was tested. Aging treatments of this type have been shown to improve the performance of similar devices, presumably through an increase of the work function of the Ag contact caused by oxidation of the metal.51−53 We measured curent−voltage (J−V) curves in air with a Keithley 238 power meter on a Spectrolab XT-10 solar simulator, which was calibrated for AM1.5 illumination with a light intensity of 100 mW cm−2 using a reference silicon solar cell. In devices fabricated with untreated ZnO the variations from sample to sample in the short-circuit current (JSC) and open-circuit voltage (VOC) were ±0.05 mA cm−2 and ±0.05 V, respectively. In devices with surface modified ZnO these variations were reduced to ±0.03 mA cm−2 and ±0.01 V. Some of the samples with very thin P3HT layers, and some of the photovoltatic devices, were annealed in air at 220 °C (above the glass transition temperature and near the melting temperature of P3HT) to further identify the influence of alkyl surface modifications on the character of the ZnO/P3HT interface. A treatment of this type is commonly used to promote infiltration into the nanorod network to improve the extent of the effective heterojunction interface. There is evidence that annealing also improves the mobility in ZnO nanowires. However, annealing also detrimentally disrupts the order within the polymer next to the untreated ZnO.20

Table 1. Water Contact Angle and Work Function (Relative to Untreated ZnO) on Planar Sol−Gel ZnO treatment

contact angle (deg)

relative work function (meV)

untreated ODT OTES

60 ± 6 109 ± 11 97 ± 13

0 −344 ± 30 +231 ± 66

treatment and nine separate samples for the OTES treatment. The contact angle measurements were performed on the same four samples for the ODT treatment and 12 samples for the OTES treated surfaces. Three to five separate contact angle determinations were performed on each sample. ODT and OTES on planar ZnO had water CAs of 109° and 97°, respectively. The values are equivalent to those that have been previously measured on similar systems.47,54−58 The untreated ZnO displayed a much smaller CA of 60°. A low CA is associated with a higher surface energy, which is a characteristic of a polar surface that is oleophobic (hydrophilic). The modified surface should be more oleophilic (hydrophobic), which should assist infiltration of a polymer solution into a nanorod array. The CAs that are typically measured from wellordered monolayers with close-packed alkyl chains nominally perpendicular to the surface are usually close to 110°, due to the exposed methyl groups at the ends of the chains.59 Usually the CA decreases as the coverage and ordering of a monolayer decreases. This is because more of the high surface energy substrate is exposed and also because the disordered alkyl chains expose more methylene, rather than methyl, groups. Our experience is that OTES does not completely cover the surface of sol−gel ZnO.34 This explains the somewhat lower CA on the OTES-treated sample. While the CA of 109° observed on the ODT-treated sample is consistent with a high quality fully packed monolayer, it is also true that surface topography influences the effective CA.60 Rougher surfaces lead to higher CAs. Some ODT samples displayed CAs as high as 120°. CAs above 120° have also been reported by Monson et al.35 for sol− gel ZnO treated with hexadecanethiol. These values are much higher than can be achieved on a flat, methyl-terminated surface. This means that the CAs were influenced by nanoscale irregularities that were an intrinsic feature of the as-grown ZnO film and/or roughness due to island-like deposition of the molecular layer as discussed below. Since the CA measured on samples treated with ODT was larger than on samples treated with OTES, we can conclude from the CA results that either the coverage of the ODT was greater than that of the OTES or the roughness of the ODT layer was larger than that of the OTES, or both. Table 1 also shows the results of Kelvin probe measurements, which are reported in terms of the work function relative to an untreated ZnO film. These demonstrated a systematic trend. The ODT-modified samples had an average work function that was reduced by 344 meV, while OTES modification led to an increase of the work function, which averaged 231 meV. These changes were associated with a modification of the interface dipole (μ⃗ ). In the case of ODT functionalization the positive end of the dipole change (Δμ⃗ ) pointed outward from the interior of the ZnO, while Δμ⃗ on the OTES functionalized samples pointed toward the ZnO. The FTIR absorbance of planar ZnO on Si, modified with ODT and OTES, is shown in Figure 1. The spectral range that is displayed includes features from the symmetric (νs) and asymmetric (νa) methylene stretch modes along with small

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RESULTS AND DISCUSSION Contact angle measurements confirmed that both ODT and OTES were present on planar ZnO films. The results are shown in Table 1. Experimental uncertainties were established from multiple measurements. The work function measurements were performed on four separate samples for the ODT 8875

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The smaller value of the integrated intensity for OTES, compared to the LB layer, is consistent with the higher frequencies of the peak positions. Both features of the data can be explained by a molecular layer with disordered alkyl chains that may also not completely cover the surface. However, the absorbance from the ODT layer exhibits an integrated intensity that is much larger than what is seen in the spectrum from the LB layer. We have consistently observed this trend, with many different samples. These results indicate that there are significantly more ODT molecules on the ZnO than are found in a single, close-packed monolayer. It is interesting to note that ODT multilayers have been observed with similar treatments of both copper oxide65 and gold oxide.66 Further evidence for ODT multilayers is provided by the results of AFM measurements. The topographical images of the untreated ZnO and the ZnO samples with ODT or OTES layers were difficult to distinguish. However, the phase images (shown in Figure 2) provided some revealing distinctions. The

Figure 1. Infrared absorbance of ODT (inverted triangles) and OTES (circles) on sol−gel ZnO thin films on Si. Also shown is the absorbance of a close-packed and well-ordered monolayer of Cd stearate, deposited with LB methods (squares). The peaks near 2850 and 2920 cm−1 correspond to the symmetric and antisymmetric methylene C−H stretch modes of the alkyl chain; the smaller peak near 2960 cm−1 corresponds to the methyl C−H antisymmetric stretch mode.

contributions from methyl stretch modes. Also shown is the spectrum recorded from a single monolayer of cadmium stearate deposited on Si by Langmuir−Blodgett (LB) methods and reproduced here from Allen et al.34 A stearate LB layer involves close-packed, well-ordered alkyl chains that have nearly the same number (16) of methylene groups as ODT and OTES, which have 17. The locations of the peak frequencies of the methylene C−H stretch modes provide information about the conformation of the alkyl chains.61,62 Densely packed molecular layers with trans extended alkyl groups have peak frequencies for νs centered between 2846 and 2850 cm−1, while the νa peak frequencies are centered between 2915 and 2920 cm−1. The spectrum from the well-ordered LB layer exhibits frequencies in this range, as expected. By contrast, liquidlike and highly disordered alkyl groups have peak frequencies that are shifted to larger values. For example, νs is found at 2856 cm−1 and νa appears at 2928 cm−1 in liquid polyethylene.63 Thus, the alkyl chains of the ODT molecular layer, with peak frequencies at νs = 2851 cm−1 and νa = 2919 cm−1, had a greater fraction of their structure in the trans conformation than did those in the OTES layer, with νs = 2854 cm−1 and νa = 2925 cm−1. This is consistent with OTES exhibiting a disordered layer of covalently bonded molecules, while ODT shows islandlike multilayer behavior, as discussed next, in which well-ordered domains are separated by regions of lower coverage. It is also possible to use the magnitude of the integrated absorbance within the methylene range (from 2835 to 2950 cm−1) as an estimate of the relative coverage of the molecular layers. This method is sensitive to the orientation of methylene groups, which have their infrared transition moment in the H− C−H plane. The alkyl chains in an LB layer are nearly perpendicular to the sample surface. Consequently, the methylene planes are parallel to the optical polarization of the infrared radiation, a configuration that maximizes the strength of the absorbance. Randomly oriented chains will couple less effectively to the infrared radiation, resulting in a smaller absorbance.64 Therefore, the LB layer has an integrated absorbance in the methylene range that is close to the upper limit of what should be observed for a single monolayer.

Figure 2. AFM phase images of sol−gel ZnO thin films deposited on native oxidized silicon: (top) as grown, (lower left) after modification with ODT, (lower right) after modification with OTES.

phase of the oscillation of the AFM cantilever is related to tip adhesion and the Young’s modulus of a flat surface and/or to the derivative of the topography. Phase images can therefore be used to distinguish between different chemical species, provided they have differing mechanical properties. The phase image of the untreated ZnO surface was somewhat variable, over a range of about 5°. This is understandable, given that the sol−gel process leads to a fine-grained polycrystalline film with exposed crystal faces having a variety of orientations. The OTES modification removes much of this variability, as shown by the low-contrast phase image of that sample in Figure 2. This would indicate that the OTES-functionalization created a more uniform surface through a combination of the siloxane network attached to the ZnO and the disordered alkyl chains. A different conclusion is reached from a consideration of the phase image of the ODT modified sample in Figure 2. Here we observed a mottled image with a lateral length scale of ∼100 nm. This is consistent with a surface that was not uniform, as would be the case if multilayered island structures existed on that sample. In spite of these differences between the ODT and OTES layers, both surface treatments improved the ordering of thin layers of P3HT, as implied from measurements of their UV−vis absorbance spectra. These are shown in Figure 3. The primary 8876

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previously been recognized and attributed to interchain disorder.70 By contrast, absorbance spectra of P3HT on samples treated with both types of alkyl-chain molecules demonstrated a more pronounced shoulder after annealing than before, with no blue shift. The results with ODT confirm the observation of Monson et al.35 Seeing the same effect with OTES supports the conclusion that alkyl chains in both ODT and OTES have a decoupling effect allowing the polymer to order in a way that is similar to its behavior when deposited on glass.40 This suggests that annealing to enhance nanorod array infiltration need not also disrupt the polymer order at the ZnO interface, provided the detrimental interaction between the ZnO and the polymer can be mitigated with a surface modifier. In order to study the effects on device performance, a bilayer device structure was chosen. This device architecture limits photocarrier generation mostly to the donor/acceptor region that is strongly influenced by the surface modification of ZnO. Although bilayer devices are expected to demonstrate low power conversion efficiencies, short exciton diffusion lengths limit charge generation to this thin interfacial region for which the bilayer architecture is well suited. Photovoltaic performances of the bilayer devices with and without the ZnO surface modifiers are shown in Figure 5. The performance metrics of

Figure 3. Normalized UV−vis absorbance spectra of thin layers (∼10 nm) of P3HT on an as-grown sol−gel ZnO film as well as on films treated with ODT and OTES. Both surface treatments lead to structure in the spectra that is associated with increased π-stacking within the polymer.

peak between 450 and 650 nm was caused by exciton generation throughout the polymer film. The emergence of a shoulder at 615 nm is associated with intrachain and interchain interactions that develop in ordered domains in P3HT.67 These domains also contain polymer chains with longer conjugation lengths, which reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) causing a shift of the spectrum to longer wavelengths.68,69 The characteristics of polymer ordering are observed in the spectra from P3HT on surfaces that were treated with both types of molecules. These features are suppressed in the spectrum from P3HT that was spin-coated on the untreated ZnO films. The surface modifiers decoupled the ZnO from the P3HT, which allowed selforganization of the polymer as the solvent evaporated. The distinction in the morphology of the polymer on treated and untreated ZnO was more pronounced after the samples were annealed in air at 220 C, as shown in Figure 4. The less pronounced shoulder at 615 nm and the blue shift of the absorbance indicate that annealing thin P3HT films on untreated ZnO reduces structural order. This effect has

Figure 5. Photovoltaic response of typical planar devices illuminated with a simulated AM1.5 (100 mW cm−2) solar spectrum.

the devices are presented in Table 2. Bilayer devices are expected to demonstrate low power conversion efficiencies due to the short exciton diffusion length, which limits the active zone to a small thickness of polymer adjacent to the ZnO. Table 2. Photovoltaic Performance of Planar Devices as Functions of the Surface Treatments of the Sol−Gel ZnO Layers Prior to Spin-Coating with P3HT and the Thermal History of the P3HT Prior to Deposition of the BackContact

Figure 4. Normalized UV−vis spectra of thin layers (∼10 nm) of P3HT on treated and untreated sol−gel ZnO films after a 220 °C anneal. The spectral features associated with π-stacking are enhanced on the chemically modified samples. On the untreated ZnO film these features are entirely missing, and there is a significant shift of the spectrum toward shorter wavelengths. 8877

surface treatment

thermal history

VOC (V)

JSC (mA/cm2)

FF (%)

η (%)

untreated ODT treated OTES treated untreated ODT treated OTES treated

not annealed not annealed not annealed annealed annealed annealed

0.363 0.368 0.297 0.403 0.447 0.302

0.666 0.791 0.399 0.397 0.701 0.353

42.0 44.6 40.2 47.8 44.4 44.4

0.102 0.130 0.048 0.076 0.139 0.047

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However, such structures allow the interfacial region, which is influenced by the surface modifiers, to be probed. A typical device fabricated with untreated ZnO demonstrated a short-circuit current (JSC) of 0.67 mA cm−2 and an opencircuit voltage (VOC) of 0.36 V. With a fill factor of 42% the power conversion efficiency (η) was typically 0.10%. This is similar, within experimental variation, to what has been reported for bilayer devices of this type.44 For example, the performance of our untreated bilayer device was comparable to results from Monson et al., who studied devices with a similar structure, where JSC was 0.28 mA cm−2 and VOC was 0.305 V.35 The differences could be caused by the different thicknesses of the polymer layers in the earlier devices, which were 80 nm, compared to our 60 nm. In any case, the important issue in our study is the changes that were observed when the ZnO surfaces were modified by the organic layers. As shown in Figure 5, neither surface treatment completely blocked charge transfer in this system, even though alkyl chains have previously been show to act as electron transfer barriers.36 In fact, the ODT treatment improved JSC to 0.791 mA cm−2, which is qualitatively consistent with the study by Monson et al.35 However, our results show that this behavior is not entirely due to the alkyl chains on the terminal ends of the surface modifiers. If that were the case, then we would have observed a similar improvement in the device fabricated with OTEStreated ZnO. Instead, the photovoltaic results in Figure 5 show that the OTES treatment significantly reduced both JSC and VOC, compared to the device with the untreated ZnO surface. Initially, the effects of ODT and OTES treatments seem contradictory. However, our IR absorbance measurements show that much more than a molecular monolayer of ODT was present on the ODT-treated sol−gel ZnO prior to spin-coating with P3HT. The AFM images showed that the ODT layer was irregular with significant heterogeneity. It is possible that these structures are not retained in the bilayer device since spincoating a solution of P3HT in chloroform could alter and may dissolve the multilayer ODT deposit. To provide insight about this issue, we prepared an ODT-treated sol−gel ZnO film on silicon and measured the IR absorbance before and after spincoating with pure chloroform. This showed that the amount of ODT on the surface was not changed by exposure to chloroform in this manner. However, before and after AFM measurements showed that the chloroform treatment did reorganize the morphology of the ODT deposit so as to reduce the size of the larger islands. This test implies that the multilayer coating was preserved in the planar device. Our experiments do not preclude the possibility that a monolayer of ODT was present in addition to the multilayer islands. However, an islandlike deposit would allow more efficient charge transport across the interface in regions between the islands where there was little or no ODT. These characteristics would have improved the performance of the devices with ODT-treated ZnO, compared to devices containing OTES-treated ZnO where the covalently bonded OTES was more strongly attached leading to a more uniform (although somewhat defective) layer on the ZnO surface. When the planar devices were annealed, with a heat treatment similar to what is commonly used to assist with the infiltration of the polymer into an inorganic oxide network, the JSC value measured on the untreated device dropped dramatically, to 0.397 mA cm−2 (Figure 6), while η was reduced to 0.076%. By contrast, the photovoltaic performance of the devices that included either type of surface modifier changed

Figure 6. Photovoltaic response of typical planar devices that had been subjected to a heat treatment at 250 °C. Testing conditions were the same as in Figure 5.

very little as a result of the thermal treatment. This effect has been observed before, in a device containing ODT-treated ZnO.35 Here we see that OTES also stabilized the ZnO/P3HT device performance during thermal annealing. The reorganization of the polymer during this step is thought to depend on how the polymer interacts with the ZnO and hence is closely related to the effect of heat treatment on local polymer order, as discussed above. Untreated surfaces are quite polar and contain a variety of reactive sites that could pin the polymer during annealing, thus inhibiting the thermal diffusion necessary for crystallization. The result is an amorphous network with low hole mobility leading to low JSC. Both types of alkyl surface functionalization provided a buffer between ZnO and the polymer. This enabled the polymer chains to more easily reorganize during the thermal treatment, as is evident from our UV−vis absorbance results. This helped to retain the mobility of holes in the annealed devices. The absence of the blue shift could also be important in the spectral response of photovoltaic devices, since it allows more of the solar spectrum to be absorbed in the polymer. In the case of the OTES-treated devices there is an additional detrimental effect. The value of VOC is much lower than what is observed in the devices fabricated with untreated ZnO. This effect is robust and not influenced by the thermal treatment. We speculate that the siloxane attachment introduces a change in the interface dipole that increases the effective work function of the ZnO. This would decrease the energy difference between the HOMO of P3HT and the conduction band of ZnO, which would decrease VOC. This interpretation is consistent with our Kelvin probe data in which OTES treatments of ZnO surfaces always increased the work function of the samples. In a previously published study it was shown that the performance of ZnO/P3HT hybrid devices depended on the nature of the bare ZnO surface.44 The report showed that devices fabricated on UVO-treated ZnO exhibited a significantly lower VOC compared to those fabricated on ZnO without the UVO treatment. The difference was attributed to modification of the interfacial dipole during the annealing step. However, our results show that the shift of VOC occurs on devices that were not annealed. It is possible that the UVO treatment alone is responsible for the drop in VOC. However, it is equally likely that the damaging effect is also associated with the interface dipole resulting from the OTES attachment. It is clear that the UVO treatment leaves a detrimental dipole at the surface. The 8878

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OTES either leaves that unchanged or puts a similar detrimental dipole in its place. Recent studies of the ZnO/ P3HT interface using X-ray and ultraviolet photoemission showed that sol−gel ZnO has a dipole moment pointing (negative to positive) from the polymer into the oxide.71 Our proposal is that the OTES processing increases the magnitude of this intrinsic dipole. This could involve radiation-induced defects, indirect changes in the coverage of dissociated water, or a charge distribution associated with siloxane bonding. Additional investigations are currently under way to clarify this effect.

Notes

CONCLUSIONS In this study we investigated chemical modification of ZnO polycrystalline films with two different molecules, ODT and OTES, which are terminated with the same alkyl chain but possess different attachment groups. Both types of alkyl modification decouple the ZnO from the P3HT to enable enhanced π-stacking within the polymer during solvent evaporation at room temperature. The decoupling also allows the polymer to further organize during annealing, which is necessary to promote polymer infiltration of a ZnO nanorod array. With ZnO that has not been chemically treated this annealing step leads to poor polymer morphology adjacent to the oxide and poor photovoltaic performance, as determined by measurements on planar structures. It is likely that the organic molecules in both cases inhibit some charge transfer by creating a partially insulating barrier. However, the arrangements of molecules in the layers still provide pathways for conduction of electrons from the polymer to the ZnO. There may be a tradeoff between improvements in charge collection as a result of the improved interfacial polymer morphology and reduced charge transfer through blocked regions of the interface. This suggests that there is an optimal coverage of the alkyls on the surface or that a different surface termination might improve ordering without negatively affecting charge transfer. The two chemical modifiers influence the performance of bilayer ZnO/P3HT photovoltaic devices in very different ways. Notably, VOC in devices built from ODT-treated ZnO increases, particularly after the annealing step, while devices containing OTES-treated ZnO exhibit reduced VOC values, compared to devices with untreated ZnO. These changes are correlated with changes in the ZnO work function, as determined by Kelvin probe experiments. The OTES treatment increases the work function of the oxide, compared to an untreated surface, while the work function of the oxide decreases after functionalization with ODT. Our results show that improved ordering at the ZnO interface is simply the result of exposed methyl and methylene groups since both molecular treatments give virtually the same improvement in order. They also show, however, that energetics effecting device performance, such as the work function and the conduction band HOMO offset, depend on the nature of the attachment and the nanomorphology. For OTES treated ZnO, performance is reduced as a result of an unfavorable surface dipole, giving lower VOC and JSC than ODT, which attaches through thiol bonds and exhibits an islandlike nature.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge valuable discussions with, and assistance from, George Radziszewski, Joseph Dahdah (deceased), David Wood, Michael Ratzloff, Andrea Yocom, Gang Chen, Matt Lloyd, and Matthew Bergren. This report is based on work supported by the National Science Foundation under Grants DMR-0606054 and DMR-0907409 and by the NSF-spponsored Renewable Energy Materials Research Science and Engineering Center under DMR-0820518.

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