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A conductive distributed Bragg reflector (DBR) fabricated on PET substrate using the single indium tin oxide (ITO) material is proposed. The large index contrast of the DBRs was obtained by depositing alternating layers of dense and porous ITO films. The high refractive index of the dense ITO films was achieved by long-throw radio-frequency magnetron sputtering technique at room temperature. On the other hand, the porous ITO films with low refractive index were fabricated by supercritical CO2 (SCCO2) treatment at 60 °C. The index contrast of the dense and porous ITO films as larger as 0.59 at blue spectral range was obtained. For the 4.5-period ITO DBR fabricated on PET substrate, the reflectance and sheet resistance of 85.1% and 47 Ω/◻ were achieved at 475 nm.
© 2014 Optical Society of America
1. Introduction
The most advanced organic electronics for commercial applications nowadays are highly efficient, bright and colorful displays based on organic light emitting diodes (OLEDs) [1,2]. Significant progress has also been made in realization of organic photovoltaic cells for low-cost solar energy generation [3,4]. Recent researches of organic electronics are focused on engineering simple processing technologies of organic compounds and ability of depositing organic thin films on various substrates. However, the ultimate challenge of the organic electronics for future applications lies in the ability to manufacture organic devices at a very low cost. Even though the cost of organic materials is low, dominant costs of the organic electronics primarily come from device fabrication and packaging. Therefore successful applications of the organic electronics will be realizing their low-cost potential through innovative fabrication technology of the devices on inexpensive, large-area substrates, such as plastics.
Recently, applications of distributed Bragg reflector (DBR) for flexible electronics are extensively investigated, such as for light-trapping of thin-film organic solar cells and for display and lighting of OLEDs. Significant enhancement of device performance via DBRs has been successfully demonstrated [5,6]. Conventionally, fabrication of DBRs often requires stacking different dielectric thin films, such as TiO2/SiO2 and Al2O3/HfO2 bilayers, on plastic substrates [7,8]. The use of these bilayers is advantageous because they can provide exceptional wide bandwidth and high reflectivity with few pairs. However, these dielectric DBRs are non-conductive. Additional processing steps can be required in order to make contacts to the device surface [9]. In addition, thermal treatments are generally used to improve the quality of the organic thin films and metal oxides [10,11]. Unfortunately, the use of plastic substrates limits the window of annealing temperatures in organic devices. Indeed, higher temperature will increase thermal strain at the polymer-dielectric interface and can significantly reduce the integrity of the metal oxides and therefore the performance of the device.
In this work, a DBR electrode fabricated entirely from ITO on polyethylene terephthalate (PET) substrates for organic electronics is proposed. The DBR electrode consists of alternating layers of dense ITO films and porous ITO films. The dense ITO films were prepared by long-throw radio frequency (rf) magnetron sputtering technique. In a long-throw sputtering system, the thickness uniformity of the deposited film is significantly improved because the angular distribution of the sputtered particles caused by annulus erosion of the target can be washed out due to collision with other particles during their movement towards the substrate. The sputtered particles can be completely thermalized and will reach the substrate by diffusion provided that a large target-to-substrate distance is used [12]. In addition, this technique is particularly useful for low temperature processes since the substrate is away from the target, and therefore heating effect of plasma is relieved.
On the other hand, the porous ITO films were obtained by extracting organic matters from the gel-coated films using supercritical CO2 (SCCO2) fluids [13]. A small amount of polar co-solvents, such as Isopropyl alcohol (IPA), was added to the non-polar CO2 fluids. This resulted in a higher polarity supercritical mixed solvent and greatly enhanced the solubility of polar solutes in the mixed solvents [14] even at a low process temperature. The proposed DBR electrodes are far more advanced than the conventional dielectric reflectors in terms of functionality because they combine wavelength-selective reflectance with transparent and conductive properties of ITO. To the best of our knowledge, there have been no published demonstrations of a high-performance conductive DBR fabricated entirely from ITO on a flexible substrate.
2. Experimental
The conductive DBRs were fabricated by stacking the porous ITO/sputtered ITO bilayers on 3 cm square, 300-μm-thick glass and 250-μm-thick PET substrates. The root-mean-square (rms) surface roughness of the glass and PET substrates were 0.5 nm and 6.0 nm, respectively. The dense ITO films were prepared by the rf magnetron sputtering at room temperature with a target-to-substrate distance of 18 cm. The ITO target composition was 90% In2O3 and 10% SnO2 in weight. The internal stress of the sputtered ITO films was minimized by adjusting the rf power of the sputtering at an operating pressure of 1.5 mtorr. On the other hand, the porous ITO films were formed by sol-gel spin coating followed by the SCCO2/IPA treatment [15]. The ITO sol-gel solution was obtained from Kojundo Chemical Laboratory. After coating, the ITO samples were loaded into the SCCO2 processing chamber. High purity carbon dioxide was introduced into the chamber using a high pressure syringe pump. A small amount of IPA was added into the chamber followed by injection of pressurized liquid CO2 at a required pressure. The operation pressure of the treatment was 2000 psi, and the chamber temperature was kept at 60°C.
The surface morphology of the ITO films prepared by the long throw sputtering and the SCCO2/IPA treatment was characterized using scanning electron microscope (SEM) and atomic force microscope (AFM) system with a scan size of 5 × 5 μm2 at non-contact tapping mode. The cross sectional images of the ITO DBRs were obtained by using a focused ion beam (FIB) SEM system. The optical and electrical characteristics of the ITO films and DBRs were obtained with the white-light ellipsometer, UV–vis spectroscopy system, and four-point probe system.
3. Results and discussion
The dispersive n(λ) and κ(λ) relations of the sputtered ITO film at room temperature and the SCCO2/IPA treated film at 60 °C on PET substrates are given in Fig. 1. . The refractive indices of the sputtered ITO and the porous ITO were 2.11 and 1.52 at a wavelength of 475 nm. The index contrast of the bilayer as large as 0.59 was obtained. In addition, the extinction coefficient of the sputtered ITO at an rf power of 30 W was around 0.01 at a wavelength ranging from 400 to 800 nm. In comparison with that of the sputtered sample, the extinction coefficient of the SCCO2/IPA treated film was relatively small (< 0.004), indicating a negligible absorption loss of the porous ITO at blue spectrum range. Relatively large extinction coefficient of the sputtered ITO can be caused by oxygen deficiency of the film.
Fig. 1 The dispersive n(λ) and κ(λ) relations of the sputtered ITO film at room temperature and the SCCO2/IPA treated film at 60 °C.
Figure 2 shows the AFM photos of the ITO surfaces prepared by the (a) long-throw sputtering at 30 W and the (b) SCCO2/IPA treatments at 2000 psi on PET substrates. The AFM photos of the ITO samples on glass substrates are also shown in the insets of the figure for comparison. The rms surface roughness of the ITO samples prepared by the sputtering and SCCO2/IPA treatment on PET substrates was 2.9 nm, and 1.1 nm, as compared with 0.3 nm and 1.2 nm, respectively, of the ITO samples on glass substrates. As expected, the ITO films prepared by the long-throw sputtering on glass substrate exhibited the best surface flatness The porous ITO films with a higher surface roughness were obtained on both PET and glass substrates. This is because voids were formed from dissolving of organic materials of the sol-gel ITO after the SCCO2/IPA treatments. Intriguingly, poor PET surface roughness did not seem to greatly affect the surface morphology of the films after the treatments. However, the ITO sample prepared by the long-throw sputtering on PET substrate showed the highest surface roughness. The poor surface roughness of the sputtered ITO on PET can be attributed to faithful replication of surface geometry of the substrate by the long-throw sputtering technique. On the contrary, liquid-phase deposition of the sol-gel spin coating could improve the surface roughness by smoothing uneven structures on the substrate surface. Nevertheless, the obtained surface roughness of the films on PET substrate was low, and therefore is useful to efficient stacking of the ITO bilayers for high performance conductive DBRs.
Fig. 2 The AFM photos of the ITO surfaces on PET substrates prepared by the (a) rf sputtering at 30 W and the (b) SCCO2/IPA treatments at 2000 psi.
The FIB-SEM image of the ITO DBRs with 4.5-period ITO bilayers on glass substrate is shown in Fig. 3.The porous ITO films (dark regions) were clearly observed with well-defined interfaces to the sputtered ITO layers (bright regions). The quarter-wave thickness of the porous ITO film and sputtered ITO film were 78 nm and 56 nm. The rms surface roughness of 1.8 nm was obtained for the 4.5-period DBR on the glass substrate, as compared with 3.0 nm of the DBR on the PET substrate. In addition, no significant DBR film cracking and peeling were observed by bending the PET substrates on a cylinder with a diameter of 6 cm and by scotch tape pull test in air.
Figure 4 shows the reflectance and transmittance spectra of the DBRs comprising 1.5- to 4.5-period ITO bilayers on PET and glass substrates at a wavelength ranging from 400 to 800 nm. The designed Bragg wavelength of the DBRs is at 475 nm. The center wavelength of the DBR can be detuned by simply changing the optical thickness of the sputtered ITO and porous ITO films to select any modes in visible spectrum [16]. It is obvious that the reflectivity significantly increases with the period of the ITO bilayers for both DBRs. In addition, the DBRs exhibited strong Bragg reflections and wide stop bands owing to large index contrast between the sputtered ITO and porous ITO. In Fig. 4(a), the maximum optical reflectivity of the 4.5-period DBR on PET substrate was 85.1% at 469 nm and a stop band of 125 nm. The reflectivity measurements were further complemented by optical transmittance measurements, as shown in Fig. 4 (b). The transmittance of the 4.5-period DBR at 469 nm is 14.2% suggesting that optical losses were less than 1%. Compared with the DBR on PET substrate, the optical performance was improved for the DBR fabricated on glass substrates. As shown in Fig. 4(c) and 4(d), the optical reflectance and transmittance of the 4.5-period DBR on glass substrate was 87.5% and 12.1% at 478 nm, respectively. The optical loss of the DBR of less than 0.5% was obtained. The relatively small Bragg wavelength shift observed for the DBR on glass substrate can be attributed to better optical thickness control of the bilayer. Further, good uniformity of the reflectivity was obtained with a standard deviation of 3% on 3 cm square glass substrate, as compared with 4% of the DBR on PET substrate. The results indicated excellent control of the optical thickness of the bilayers during the stacking process on both the PET and glass substrates.
Fig. 4 The reflectance and transmittance spectra of the DBRs comprising 1.5 to 4.5 periods ITO bilayers on (a), (b) PET substrates and on (c),(d) glass substrates at a wavelength ranging from 400 to 800 nm.
However, the scale-up of our results for large-area applications requires further attention on size and geometric effect of the substrate primarily because the spin coating process was used to fabricate the porous ITO films. One of the problems in spin coating is the geometric effects of the substrate in the corners. An air barrier plate [17] can be applied to create a partially saturated atmosphere above the substrate, hence minimizing evaporation and allowing the centrifugal forces to level the fluid at the periphery of the wafer as well as in the corners.
In addition to the optical performance of the DBRs, the proposed high-reflectivity coating can also function as a transparent contact electrode. The resistivity of the sputtered ITO film and the SCCO2/IPA treated ITO film at the quarter-wave thickness on PET substrates were 8.0 × 10−4 Ωcm and 3.0 Ωcm, respectively. Figure 5 shows the electrical characteristics of the ITO DBRs on PET and glass substrates with number of the bilayer periods. The sheet resistances of the DBRs comprising 1.5 to 4.5-period bilayers on PET substrates were 96, 75, 58, and 47 Ω/◻, respectively. The sheet resistances were found to inversely proportional to the total thickness of the periods, indicating the stacking process did not significantly change the effective resistivity (~2.7 × 10−3 Ωcm) of the bilayers. A slight increase of the effective resistivity of the DBR with the periods was primarily caused by non-uniform coating of the ITO bilayers on the rough PET substrates. The effective resistivity of the ITO bilayer can be reduced by improving the conductivity of the porous ITO. This can be achieved by increasing the operation pressure of the SCCO2 treatment. The higher pressure of the treatment can collapse the porous structure of the film resulting in a higher film density, and therefore a better film conductivity. However, the optical reflectivity of the DBR can be degraded because the index contrast of the bilayer is reduced. On the other hand, the DBRs comprising 1.5 to 4.5-period ITO bilayers on glass substrates were 94, 71, 50, and 39 Ω/◻, respectively. Improved sheet resistance and stable effective resistivity of the films suggest excellent control of the optical thickness of the bilayers on glass substrates. The obtained results suggested that the higher surface roughness of the substrates could influence the bilayer stacking process and degrade both the optical and electrical properties of the DBRs.
4. Conclusions
In summary, we have demonstrated the conductive DBRs formed by stacking the dense and porous ITO films on both the PET and glass substrates. Large index contrast of the bilayers was achieved by producing the dense ITO films using the long-throw sputtering system and the porous ITO films via the SCCO2/IPA treatment. For the 4.5-period DBR fabricated on PET substrates, the Bragg reflectivity and sheet resistance of 85.1% and 47 Ω/◻ were obtained. In addition, the measured optical properties of the DBRs agree very well with the calculation results, indicating excellent control of the bilayer stacking process. The proposed low temperature technique offers a unique alternative for fabricating high performance conductive DBRs on flexible substrate, and may well lead to novel applications for optoelectronic devices.
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