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Mater Renew Sustain Energy (2015) 4:12 DOI 10.1007/s40243-015-0055-8 ORIGINAL PAPER Limitations of ultra-thin transparent conducting oxides for integration into plasmonic-enhanced thin-film solar photovoltaic devices 1 2 3 3 • • • • Jephias Gwamuri Ankit Vora Rajendra R. Khanal Adam B. Phillips 3 2 2 2 • • • • Michael J. Heben Durdu O. Guney Paul Bergstrom Anand Kulkarni 1,2 Joshua M. Pearce Received: 20 April 2015 / Accepted: 3 July 2015 / Published online: 16 July 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract This study investigates ultra-thin transparent thicker (bulk) films deposited under the same conditions, conducting oxides (TCO) of indium tin oxide (ITO), alu- (2) the films are delicate due to their thickness, requiring minum-doped zinc oxide (AZO) and zinc oxide (ZnO) to very long annealing times to prevent cracking, and (3) determine their viability as candidate materials for use in reactive gases require careful monitoring to maintain sto- plasmonic-enhanced thin-film amorphous silicon solar ichiometry. The results presented here found a trade-off photovoltaic (PV) devices. First a sensitivity analysis of the between conductivity and transparency of the deposited optical absorption for the intrinsic layer of a nano-disk films. Although the sub 50 nm TCO films investigated patterned thin-film amorphous silicon-based solar cell as a exhibited desirable optical properties (transmittance greater function of TCO thickness (10–50 nm) was performed by than 80 %), their resistivity was too high to be considered simulation. These simulation results were then used to as materials for the top contact of conventional PV devices. guide the design of the experimental work which investi- Future work is necessary to improve thin TCO properties, gated both optical and electrical properties of ultra-thin or alternative materials, and geometries are needed in (10 nm on average) films simultaneously deposited on both plasmonic-based amorphous silicon solar cells. The sta- glass and silicon substrates using conventional rf sputter- bility of ultra-thin TCO films also needs to be experi- ing. The effects of deposition and post-processing param- mentally investigated under normal device operating eters on material properties of ITO, AZO and ZnO ultra- conditions. thin TCOs were probed and the suitability of TCOs for integration into plasmonic-enhanced thin-film solar PV Keywords Transparent conducting oxide (TCOs) devices was assessed. The results show that ultra-thin Plasmonics Solar photovoltaics Indium tin oxide Zinc TCOs present a number of challenges for use as thin top oxide Aluminum-doped zinc oxide contacts on plasmonic-enhanced PV devices: (1) optical and electrical parameters differ greatly from those of Introduction & Joshua M. Pearce Despite the material, sustainability, economic and technical pearce@mtu.edu benefits of thin-film solar photovoltaic (PV) devices [1–3], Department of Materials Science and Engineering, Michigan conventional crystalline silicon (c-Si) modules dominate Technological University, 1400 Townsend Dr., Houghton, the market [4]. The cost of c-Si PV has fallen to the point MI 49931-1295, USA that the balance of systems (BOS) and thus the efficiency Department of Electrical and Computer Engineering, of the modules plays a major role in the levelized cost of Michigan Technological University, 1400 Townsend Dr., electricity for solar [5]. There is thus a clear need to Houghton, MI 49931-1295, USA improve the efficiency of thin-film devices further [6]. Wright Center for Photovoltaic Innovation and Recent developments in plasmonics theory promise new Commercialization, Department of Physics and Astronomy, methods with great potential to enhance light trapping in School of Solar and Advanced Renewable Energy, University thin-film PV devices [7–14]. To fully exploit these of Toledo, Toledo, OH 43606, USA 123 12 Page 2 of 11 Mater Renew Sustain Energy (2015) 4:12 potential benefits offered by plasmonic-based devices, performed using COMSOL Multiphysics RF module v4.3b TCOs with high transmittance (low loss) and low enough on the optical absorption in the i-a-Si:H layer of nano-disk resistivity are to be used as device top contacts. However, patterned thin-film a-Si:H solar cells (NDPSC) shown in for current transparent conducting oxides (TCOs) to be Fig. 1a[15]. These simulation results are used to guide the successfully integrated into the novel proposed plasmonic- experimental work which investigated both optical and enhanced PV devices, ultra-thin TCOs films are required electrical properties of ultra-thin (10 nm on average) films [14]. For example, simulations by Vora et al. showed a simultaneously deposited on both glass and silicon sub- 19.65 % increase in short circuit current (J ) for nano- strates (with a thermally grown oxide layer. The effects of SC cylinder patterned solar cell (NCPSC) in which the ITO deposition and post-processing parameters on material layer thickness was kept at 10 nm to minimize the parasitic properties of ITO, AZO and ZnO ultra-thin TCOs were Ohmic losses and simultaneously act as a buffer layer probed and the suitability of TCOs for integration into while helping to tune the resonance for maximum plasmonic-enhanced thin-film solar PV devices was absorption [14]. TCOs such as the most established indium assessed. From these results some of the limitations of thin tin oxide (ITO), aluminum-doped zinc oxide (AZO) and TCOs for plasmonic optical enhancement of thin-film PV zinc oxide (ZnO) are standard integral materials in current were identified. thin-film solar PV devices [15–18]. Bulk material proper- ties for common TCOs including ITO have been well The optical effects of TCO thickness researched and documented for different processing con- ditions and substrates [15, 16, 19–23]; however, this is not Sensitivity analysis for the proposed silver nano-disk pat- the case for ultra-thin TCOs. The few exceptions include terned solar cell (NDPSC) was performed in the Sychkova et al. [24], who reported both optical and elec- 300–750 nm spectral range to determine the optimum ITO trical properties of 9–80 nm ITO films deposited by pulsed layer thickness which would promote maximum enhance- DC sputtering varied with thickness and showed a general ment and minimize Ohmic losses. Having a TCO spacer increase in resistivity with decrease in film thickness [24]. layer with as low as possible Ohmic losses is desirable for Other notable studies on ultra-thin ITO films using various efficient coupling of light from the silver nano-discs into deposition techniques include the following: Chen et al. the active layers of the device. The results are shown in who used filtered cathodic vacuum arc (FCVA) to deposit Fig. 1b and theoretically show 10 nm films offer the best 30–50 nm on heated quartz and Si substrates [25]; Tseng absorption and hence the greatest potential to improve and Lo, who used DC magnetron sputter for efficiency in plasmonic-based PV devices. From these 34.71–71.64 nm ITO film on PET (polyethylene tereph- results, AZO and ITO offer the best potential due to lower thalate) [26]; Kim et al. who used RF magnetron sputter for Ohmic losses and ZnO, despite having the greatest Ohmic films between 40 and 280 nm deposited on PMMA sub- losses among the three TCOs, is still promising particularly strate heated at 70 C[27]; Alam and Cameron, who used for the sub 20 nm films since its absorption ([250 W/m ) sol–gel process for 50–250 nm film deposited on titanium is still higher than that expected of a standard PV device. dioxide film [20]; and Betz et al. who used planar DC magnetron sputtering for 50, 100 and 300 nm films on glass substrates [28]. The results from these few thin TCO Experimental details studies reveal a pattern in which resistivity increases rapidly as film thickness decreases from 50 to 10 nm. The focus of the study was to investigate ways of The electrical properties of ITO thin films depend on the improving material properties of ultra-thin TCOs for inte- preparation method, the deposition parameters used for a gration into plasmonic-enhanced thin-film solar PV devices given deposition technique and the subsequent heat treat- by studying the effects of different process parameters on ments. Key factors for the low resistivity have not been both optical and electrical properties of sub 50 nm films. A clearly documented because of the complex structure of the comparative study of the three most commonly used TCOs unit cell of crystalline In O formed by 80 atoms and the in thin-film commercial solar cells is undertaken, and a 2 3 complex nature of the conducting mechanisms in poly- more in-depth study of ITO is performed. crystalline films [29]. The issue is further complicated by the large number of processing parameters, even for a Sample preparation and fabrication single technique. To probe these challenges and to determine if ITO, AZO Samples of ITO with thickness ranging from 10 to 50 nm and ZnO are viable candidate materials for use in plas- were deposited on both glass and n-doped silicon (with a monic-enhanced thin-film PV devices, sensitivity analysis 32-nm thermally grown oxide layer) substrates using rf on TCO thickness (10–50 nm) versus absorption was sputter deposition techniques previously described in refs 123 Mater Renew Sustain Energy (2015) 4:12 Page 3 of 11 12 Fig. 1 a Structure of the NDPSC with an enlarged unit cell, absorption in the active regions of plasmonic PV devices varies with b absorption as a function of ITO, ZnO and AZO thickness. The TCO type and thickness. Theoretically, at small film thicknesses results simulated using COMSOL show how the useful optical Ohmic losses decrease and useful optical absorption increases [15] [30–32]. A 99.99 % 4-inch pressed ITO (Sn O:In O AZO was processed using a Perkin–Elmer Model 2 2 3 10:90 % wt) target was used, and an average base pressure 2400-8 J rf sputter deposition system using an 8-inch -8 of 7 9 10 torr was achieved before deposition. Both the (203.2 mm) target. The rf power was kept at 500 W, argon glass and silicon substrates were ultrasonically cleaned in flow rate at 18.0 sccm, oxygen rate of 2.0 sccm and process -3 isopropanol for 5 min. All other process parameters such as pressure at 7.3 9 10 torr. The system was initially -8 target bias [900 V (ITO and ZnO) and -500 V (AZO)] and pumped to a base pressure of 6.0 9 10 torr. The process substrate distance (75 mm) were kept constant through the parameters are summarized in Table 1. experiment. Substrates and target were sputter pre-cleaned To investigate the effects of post-processing treatment in an argon environment for 5 min before each run. The on both optical and electrical effects, additional samples of protocol for pre-cleaning is described in Ref. [29]. To ITO films on sodalime glass (SLG) substrates were pro- investigate substrate dependency, ITO was deposited on a cessed using a different instrument [33] to obtain a pair of pair of substrates for 1 min with 0 % oxygen ratio and film samples with varying thicknesses from 10 to 50 nm in 100 W rf power. ZnO samples were processed at rf power steps of 10 nm. The system is a four-gun sputtering system of 100 W on glass and silicon substrates in an argon with a target to substrate spacing of approximately 4 .An environment and 0 % oxygen in the same system as ITO ITO (90 % In O /10 % SnO from Lesker) target was used. 2 3 2 using a stoichiometric 99.99 % 4-inch pressed ZnO target. The material was sputtered using 100 W rf under 4 mTorr -3 The process pressure was maintained at 7.1 9 10 torr of Ar. Deposition time was varied for film thickness with and the deposition rate was calculated to be 8 nm/min. 36 s resulting in 10 nm (*3 A/sec). This deposition rate 123 12 Page 4 of 11 Mater Renew Sustain Energy (2015) 4:12 Table 1 Summary of process parameters for the TCOs Sample name TCO Substrate type RF power (W) Target bias (V) Process gases flow rates (sccm) Film thickness (nm) Ar O 0A ITO Glass 100 900 10 0 9.55 0B Si/SiO 10.02 1A Glass 19.92 1A Si 19.75 1B Glass 10.23 1C 20.01 1D 30.79 1E 39.70 1F 50.03 2A ZnO Glass 9.51 2A Si/SiO 10.05 2 2 2B 20.01 2C 29.72 2D 38.98 2E 48.31 3A AZO Glass 500 500 18.0 2.0 12.16 3B Si/SiO 11.93 3C 20.39 3D 30.04 3E 40.63 was determined by depositing for a set amount of time and VB-400). In each case, a standard scan was performed measuring the resulting film thickness using stylus pro- ranging from 300 to 1000 nm in increments of 10 nm for filometry (Veeco Dektak 150). the 65,70 and 75 incident angles. Random detailed One sample for each as-deposited pair was divided into scans were performed for the quality check purposes three samples using a diamond scriber. The three pieces although they are normally not necessary for isotropic were then annealed separately at 400 C for 10, 20 and samples. Ellipsometry analysis was performed following 30 min, respectively, using UHP forming gas (FG) (95 % the process by Synchkova [24]. Intensity measurements N /5 % H from Air Gas) in a sealed (by vacuum coupling were carried out using the VASE for normal transmission 2 2 components) quartz tube inside a tube furnace. The furnace incidence (0 reflection angle) for the three TCOs on glass was equilibrated at the heating temperature prior to sample substrates for the same wavelength range as above. A baseline scan was obtained for the clean SLG substrate first introduction. The samples were placed in the quartz tube; then the tube was purged with FG at 5 scfm for 5 min—this followed by the main data scans using baseline data. Both was approximately four exchanges of tube volume. After the baseline and the data scans were acquired in close purging, the samples were introduced into the hot zone successions to minimize errors due to light source intensity with a vacuum-sealed push rod, and the flow rate was fluctuations. reduced to approximately 150 sccm for the duration of Electrical characterization was performed using a four- heating. After heating, the samples were removed from the point probe system consisting of ITO optimized tips con- hot zone and cooled by increasing the gas flow. After sisting of 500 micron tip radii set to 60 g pressure and an characterization, the sample previously annealed at 400 C RM3000 test unit from Jandel Engineering Limited, UK. for 30 min was further annealed at 500 C for 10 min. The sheet resistance of the 10 and 20 nm TCOs on glass and on silicon substrates with a spacer oxide layer was Optical and electrical characterization process determined by direct measurement for both forward and reverse currents. For each TCO on glass sample, a mean The film thickness measurements and optical characteri- sheet resistance value from three random points was used zation were carried out using spectroscopic ellipsometry in the final results whilst a mean of only two points was (J.A Woollam Co UV–VIS V-VASE with control module used for the TCO on Si samples since they were smaller. 123 Mater Renew Sustain Energy (2015) 4:12 Page 5 of 11 12 All samples were imaged for film quality and a com- Results and discussion positional analysis was done using a Hitachi S4700 field emission scanning electron microscope (FE-SEM). Atomic TCOs characterization force microscopy (AFM) was performed using a Veeco Dimension 3000 equipment with cantilever tips The transmittance and resistivity measurement results for (Tap300Al-G) on a 1:1 acquisition aspect ratio. The field of the TCOs are discussed below. view was 2 lm at 512 pixel width and scans performed at a speed of 0.5 Hz. Three randomly selected fields of view Transmittance were acquired per sample and the analyzed areas were limited near to the center of the sample. Roughness anal- Figure 2 below shows how transmittance of the TCOs ysis was then performed on a defect-free region. studied varied within the 300–1000 nm wavelength range. Transmittance results support the sensitivity analysis results. For the 20-nm films, AZO has greater than 90 % Theory and calculations transmittance for the 300–1000 wavelength range, whilst ITO and ZnO show an average transmittance greater than The theoretical derivations of both the resistivity and 80 and 70 %, respectively, in the same spectral range. attenuation coefficient of the ITO films are highlighted in Sects. 3.1 and 3.2 below to explain the underlying pro- Sheet resistance cesses contributing to the results reported in this paper. The resistivity of the 20 nm as-deposited TCO films on Resistivity measurements SLG substrates are shown in Table 2. ZnO, despite having the worst transmittance (Fig. 2), has the lowest resistivity Sheet resistance measurement was used to obtain the among the three TCOs being compared here and AZO has resistivity: the highest resistivity value. ITO has transmittance com- qL L parable to that of AZO and its resistivity is slightly higher R ¼ ¼ R ; ð1Þ tW W than that of ZnO, making it the most promising candidate material for plasmonic-based devices. where R is the resistance, R is the sheet resistance, and L, Table 3 shows the dependence of ITO sheet resistance W and t are the sheet length, width and thickness, with substrate type and thickness. There was a marked respectively. difference between the readings on the 10- and 20-nm Si As the film thickness is measured, the bulk resistivity q samples; however, there was no discernible difference (in ohm cm) can be calculated by multiplying the sheet between the readings on the 10 and 20 nm on glass. There resistance by the film thickness in cm: were very small amounts of fluctuation which can be q ¼ R t ð2Þ expected on high resistance samples, and it was more prominent on the Si samples. The readings reversed well, Transmittance indicating that the film was uniform, with the worst cor- relation on the 10-nm Si sample. This is the limit of four- To determine the true transmittance of the TCOs, it was point probe capability. The 10-nm ITO on glass showed the necessary to perform a correction on the experimental data to compensate for losses due to both surface reflection and absorption due to the glass substrate. It is assumed light passing through the glass substrate undergoes attenuation according to Beer-Lambert’s law: a t g g I ¼ I e ; ð3Þ g o where I and I represent the initial incident intensity and 0 g intensity through the glass substrate, a and t are the g g attenuation coefficient of the glass and glass thickness, respectively. The total normalized transmittance, T is given by T ¼ 1 A R ð4Þ where A and R represent the total absorbance and reflec- Fig. 2 Transmittance results for 20 nm thick ITO, ZnO and AZO tance, respectively. films 123 12 Page 6 of 11 Mater Renew Sustain Energy (2015) 4:12 Table 2 Resistivity of 20 nm as-deposited ITO, AZO and ZnO films on SLG substrates Sample Substrate Thickness (nm) Sheet resistance, R (X/h) 9 10 Resisitivity, q (X cm) -3 ITO Glass 20 623 1.3 9 10 -3 AZO Glass 20 876 1.7 9 10 -4 ZnO Glass 20 390 7.8 9 10 Table 3 Sheet resistance of various as-deposited TCO samples Sample Substrate Thickness (nm) Input current Sheet resistance, R (X/h) 9 10 Resisitivity, q (X cm) -4 ITO Glass 10 100 nA 830 8.3 9 10 -3 20 623 1.3 9 10 -4 Si 10 1 lA 422 4.2 9 10 -4 20 83.9 1.7 9 10 highest resistivity whilst the lowest resistivity value was around UV wavelengths the transmittance for the as-de- recorded for the 20-nm Si substrate sample. The results are posited ITO shifts down below that of the annealed sam- further confirmed by the nature of the microstructure ples (i.e., at small wavelengths annealing is more effective observed by SEM (vide infra) for these samples. as the annealed samples are more transparent). This is a well-known phenomenon (Burstein–Mess shift) which is a ITO characterization result of ITO optical band gap shifting towards higher energies when annealed in FG or H gas. This is attributed Transmittance measurements for ITO to increase in carrier concentration and is well documented [25]. In addition, it appears that among the annealed Transmittance measurements for ITO samples deposited on samples, 20 min gives the optimum transmittance for SLG substrates are shown in Fig. 3. All transmittance thicknesses below 50 nm, especially at large wavelengths. values were normalized as given in Eq. (4). It can be noted There observed trend means that the use of thinner that there is no discernible difference between the as-de- (10 nm), more transmitting and low loss (Ohmic losses) posited and the heat-treated samples particularly for the films will result in more light being coupled into the 30-, 40- and 50-nm films. However, it is also interesting to underlying i-a-Si:H layer rather than being absorbed in the note that for the 10- and 20-nm films, the as-deposited films TCO layer as is the case with thicker film ([20 nm). have the highest transmittance with the 10-nm film being almost 100 % transmitting throughout the visible spectra. Electrical characterization For the 40-nm film, annealing at 400 C for 20 min gives the best transmittance. Generally it is observed that heat- Figure 4 shows the dependence of sheet resistance on film treated ITO films in FG environment improve transmit- thickness, annealing temperature and time. Films annealed tance in the UV region of the spectra. for 20 min give the lowest resistivity and show the same As-deposited thinner ITO samples (10 and 20 nm) have trend as those annealed for 30 min whilst the as-deposited the transmittance greater than 95 %. It is interesting to note resistivity versus thickness trend is similar to films that the 40-nm film sample does not seem to follow this annealed for 10 min. Results here show that annealing in general trend, particularly the sample annealed for 20 min. FG lowers the resistivity. The lowest resistivity of -4 This sample film shows the greatest increase in mean approximately 4 9 10 Xcm is for the 40-nm film roughness (vida infra) when all other films’ roughness is annealed for 20 min. The highest resistivity value for the decreasing and it also has the best transmission for all the annealed samples is for the 20-nm film annealed for 40-nm film samples. The general trend is that the overall 10 min. transmittance curve for the as-deposited ITO shifts down with increasing film thickness (i.e., the as-deposited film Film morphology and roughness becomes less transparent with increasing thickness as expected). Around visible spectrum and at higher wave- Effect of substrate on ultra-thin ITO films lengths, the transmittance for the as-deposited ITO approaches that of the annealed ITO (i.e., annealing is not Figure 5 shows results from SEM scans showing the sur- much effective here in improving transparency). However, face morphology for both 10 and 20 nm as-deposited ITO 123 Mater Renew Sustain Energy (2015) 4:12 Page 7 of 11 12 Fig. 3 Transmittance spectra for ITO as-deposited and annealed films on sodalime glass for a 10 nm, b 20 nm, c 30 nm, d 40 nm and e 50 nm ITO thickness films. Figure 5a–c shows that the film surface is relatively films were of good quality. The AFM roughness results are smooth and predominantly amorphous in nature. Figure 5d summarized in Table 4. shows signs of grains development. The AFM analysis results are shown in Fig. 6. Effect of annealing time on ultra-thin ITO films The results in Fig. 6 show how mean roughness values of ITO vary with substrate type and are in agreement with When ultra-thin ITO films were subjected to post-pro- the results shown in Fig. 5. It can be observed in these cessing treatment at 400 C in a FG environment, different images that ITO tends to form uniform features on silicon treatment times produced different effects. with no evidence of defects. This is not the case with ITO The as-deposited films mean roughness for this second on glass substrate which, despite having finer features batch of ITO samples were observed to vary between 0.67 (10 nm film) exhibits some larger defects. These defects and 0.85 nm. The 10-nm film had the smallest mean seem to increase with the increase in film mean roughness roughness value whilst the 20-nm film had the largest and thickness. Despite the presence of a few dust particles value. This may be due to the presence of surface defect on the sample surface, results confirmed that the sputtered features which seem to be more pronounced on the 20-nm 123 12 Page 8 of 11 Mater Renew Sustain Energy (2015) 4:12 10-nm film resulting in the mean roughness increasing from the initial value of approximately 0.7–1.9 nm. The effects of annealing different ITO films in forming gas at 400 C for 10, 20 and 30 min on films surface roughness are compared and summarized in Fig. 7. The detailed study on ITO showed some dependency of electrical properties and surface roughness with substrate type which is consistent with results from previous studies on slightly thicker films. Also ITO films on glass show a high degree of surface defects and finer amorphous-like features which may explain the high and oscillating values of sheet resistance on these films. Films grown on Si substrate have uniform, but large features. However, the same films have higher resistivity values. All samples, as- deposited and annealed, have a transmittance value greater than 80 % with the as-deposited films being superior except for the 40-nm films for which the annealing for 20 min gives the best transmittance. Further analysis of samples shows films annealed for 20 min generally have the lowest resistivity and lower roughness values. Future work is needed to improve other TCOs such as AZO and ZnO and to engineer new high-conductivity low- loss materials for integration into plasmonic devices. AZO exhibited a transmittance superior to that of ITO while ZnO Fig. 4 Variation of resistivity with ITO film thickness for (a) as- had the best sheet resistance among the three TCOs being deposited/room temperature (RT) (b) annealed films for 10, 20 and 30 min compared. Further investigative work is needed to find the balance between films with useful resistivity and accept- able Ohmic losses in plasmonic-based PV devices. Future film compared to all the other samples. Generally, all work should focus on different processing techniques such samples show a varying degree of dust particles’ presence as DC sputtering as well as exploring other post-processing and potential artifacts. Sections of the film samples which environments. exhibited heavy dust particles’ (and any other contami- nants) presence, striations and potential artifacts that were not consistent with other areas on the sample were exclu- Conclusions ded from the analysis. The images show a sharp increase in the mean rough- Ultra-thin TCOs and in particular ITO present a number of ness for generally all films after 10 min of heat treatment. challenges for use as thin top contacts on plasmonic-en- Whilst the film roughness is small for both the 10- and hanced PV devices. First, both ultra-thin TCO optical and 20-nm films, it is observed to increase by a factor of two electrical parameters differ greatly from those of thicker for the 30- to 50-nm film thickness samples. There is a (bulk) films deposited under the same conditions. Second, trend for all films showing a decrease in mean roughness they are delicate due to their thickness, requiring very long after 20 min of post-processing treatment with the 30-nm annealing times to prevent film cracking. The reactive film showing the greatest decrease from approximately 1.9 gases (usually oxygen or hydrogen) require careful moni- to 1 nm. Evidently, annealing for 30 min results in a slight toring to avoid over-oxidizing or over-reducing the film as improvement in film roughness for the 30- to 50-nm range it impacts their stoichiometry. There is a trade-off between of film. However, the thinner films (10 and 20 nm) show conductivity and transparency of the deposited films. The great deterioration in film mean roughness when annealed sub 50 nm thick TCO films investigated exhibited desir- for longer periods of time (30 min or greater). This can be able optical properties (transmittance greater than 80 %), explained by the onset of islands on both of these films. which makes them viable for plasmonic PV devices Island formation is more pronounced on the quasi 2D 123 Mater Renew Sustain Energy (2015) 4:12 Page 9 of 11 12 Fig. 5 FESEM images for (a) 10 nm ITO on glass, (b) 10 nm ITO on silicon (with oxide spacer), (c) 20 nm ITO on glass and (d) 20 nm ITO on silicon (with oxide spacer) Fig. 6 AFM images for as- deposited (a) 10 nm ITO on glass, (b) 10 nm ITO on silicon (with oxide spacer), (c)20 nm ITO on glass and (d) 20 nm ITO on silicon (with oxide spacer). 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Materials for Renewable and Sustainable Energy – Springer Journals
Published: Jul 16, 2015
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