Temperature dependence of the 10.6-µm reflectivity of ITO-coated silicon

Temperature dependence of the 10.6-μm reflectivity of ITO-coated silicon R. B. Goldner Tufts University, Electrical Engineering Department, Medford, Massachusetts 02155. Received 20 January 1977. In a previous Letter1 it was shown that tin-doped indium oxide (ITO)-coated silicon is a useful selective absorber for solar energy conversion applications. Reported in this Letter are the results of measurements that have been made on the temperature dependence of the 10.6-μm wavelength reflec­ tivity of ITO-coated silicon, from room temperature to 475°C. The results indicate that ITO-coated silicon retains its solar selective absorbing properties at least up to 475°C. This is in agreement with data on the temperature independence (from room temperature to about 400° C) of their reflectivity of indium oxide films deposited on quartz substrates, as re­ ported earlier by Kryzhanovskii.2 Shown in Fig. 1 is a diagram of the configuration used for the measurements. Samples (same as used in the previously reported work1) were mounted on a heated stage with an im­ bedded thermocouple and cartridge heaters, all of which were in an evacuated chamber. A mechanical vacuum pump was utilized to evacuate the chamber to pressures ≤IOO μm Hg (0.1 Torr), and a BaF2 window was placed in front of the sample. A CO2 laser, radiating approximately 3-5 W at 10.6 μm, irra­ diated the sample after passing through a ZnSe beam splitter and the BaF2 window. The beam diameter at the sample was less than 2 mm. A shutter was manually translated to block alternately the reflected sample (Rs) and reflected reference 808 APPLIED OPTICS / Vol. 16, No. 4 / April 1977 Fig. 1. Configuration for measuring the temperature dependence of 10.6-μm reflectivity of ITO-coated silicon. Fig. 2. Normalized 10.6-μm reflectivity vs temperature for ITO- coated silicon. (- - -): linear regression fit. R(T = 25°C) ≈ 92%. (Rr) beams; the reference beam was the beam reflected from the ZnSe beam splitter. Initially, a measurement was made of the sample 25°C absolute reflectivity [R(T = 25°C)]. This was done by measuring separately the incident and reflected sample beams, with no beam splitter or window in front of the sample. For the sample whose results are shown in Fig. 2 [sample (b) of Ref. 1], the 25° C absolute reflectivity was ap­ proximately 92%. Following the absolute reflectivity mea­ surement, with the beam splitter and window in front of the sample and the chamber evacuated, the ratio Rs/Rr was re­ corded as a function of sample temperature. The results are shown in Fig. 2, obtained by normalizing to (Rs/Rr) at 25°C. The maximum attainable temperature was limited by the cartridge heaters. The indicated uncertainty corresponds to variations in the laser output power, which was not stabi­ lized. Since the Hall effect mobility for ITO decreases with tem­ perature,3 (in the temperature range explored in these ex­ periments), it can be hypothesized that the relatively tem­ perature-insensitive 10.6-μm reflectivity (or optical conduc­ tivity) is associated with a thermally induced increase in the free electron concentration, probably caused by an increase in oxygen vacancies. In conclusion, because of the room temperature reflectivity vs wavelength results reported earlier1 and since the present results indicate that the 10.6-μm reflectivity decreases by less than 10% (or the 10.6-μm emissivity remains below 0.2), for temperatures as high as 475°C ITO-coated silicon should be an effective selective absorber for relatively high temperature solar energy conversion applications. The author gratefully acknowledges laboratory assistance from and useful discussions with Haim M. Haskal. References 1. R. B. Goldner and H. M. Haskal, Appl. Opt. 14, 2328 (1975). 2. B. P. Kryzhanovskii, Opt. Spektrosk. 10, 682 (1961). 3. J. H. W. DeWit, J. Solid State Chem. 8, 142 (October 1973). April 1977 / Vol. 16, No. 4 / APPLIED OPTICS 809