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Thin semiconductor films for radiative cooling applications
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Solar Energy Materials & Solar Cells 80 (2003) 283–296
Thin semiconductor films for radiative cooling applications K.D. Dobsona,1, G. Hodesa, Y. Mastaib,* a
Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel b Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received 14 January 2002; received in revised form 22 April 2003; accepted 14 June 2003
Abstract Passive cooling systems use simple low-cost techniques to provide summer comfort in warm climates and can also be used to keep food, liquids and other materials at temperatures below ambient. Radiative cooling devices require a convective shield that should reject solar radiation but be transparent to mid-IR thermal radiation. In this paper, chemical solution deposition of thin semiconductor films (PbS and PbSe) onto polyethylene foils for radiative cooling applications is described. Optical and structural characterizations of the films were performed using UV–Vis, FTIR spectroscopies, X-ray diffraction and electron microscopy. Additionally, pigmented shields, which exhibit good radiation scattering properties, were prepared by incorporation of ZnS or ZnO into polyethylene. We also studied a combination of pigmented polyethylene foils coated with thin films of PbS that show improved optical properties for cooling purpose. r 2003 Elsevier B.V. All rights reserved. Keywords: Radiative cooling; Chemical solution deposition; Polyethylene pigmentation; Semiconductor thin films
1. Introduction Passive (or radiative) cooling has many potential applications [1–3], including cold storage of food, seeds and medicines, climatization of buildings and water *Corresponding author. Bar-Ilan University, Department of chemistry, 52900 Ramat-Gan, Israel. Tel.: +972-3 531 7681; Fax: +972-3 535 1250. E-mail address: [email protected] (Y. Mastai). 1 Current address :Institute of Energy Conversion, University of Delaware Newark, DE 19716, USA. 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2003.06.007
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desalinization. The phenomenon of radiative cooling uses the fact that the thermal energy emitted by a clear sky in the ‘‘window region’’ (8–13 mm) is much less than the thermal energy emitted by a blackbody at ground air temperature in this wavelength range. Hence, a surface on the earth facing the sky experiences an imbalance of outgoing and incoming thermal radiation and cools to below the ambient air temperature. While this concept can work well at night, assuming a relatively dry atmosphere, the solar energy input during the day, which is normally much greater than that radiated out, causes heating of the system. To prevent this, a shield is needed to cover the radiating surface in order to block solar radiation during the day as well as to prevent convective mixing in the cooled space. The ideal radiation shield would completely reflect solar radiation, but allow complete transmission in the ‘‘atmospheric-window’’ region. Solar radiation should preferably be reflected, as any absorbed radiation will be converted to heat somewhere in the system. Two different approaches for the design of shield have been previously proposed [2]. Firstly, the introduction of optical scattering materials into shield substrates, and secondly, coating of shield substrates with high solar reflector films. Low band gap semiconductors, such as Te, PbS and PbSe, can exhibit high reflectivity and are expected to block solar radiation, while being transparent in the ‘‘atmospheric window’’ if intrinsic. Vacuum evaporation is generally used to deposit such films. In this work, we use the chemical solution deposition (CSD) technique for film preparation. CSD has been used for a long time [4,5] to deposit semiconductor thin films, particularly, in the area of solar energy conversion [6] and is it ideally suited for the production of large area thin films. The basis of CSD is the slow formation of a semiconductor, either by reaction of a slowly produced anion with free metal ions or colloidal metal hydroxide, or by slow dissociation of a chalcogen complex. In CSD, thin semiconductor films are deposited on substrates immersed in dilute solutions containing metal ions and a source of (usually) hydroxide, sulfide or selenide, either as ions or as a complex species. The use of metal complexing agents is required mainly to prevent bulk hydroxide precipitation in the usually alkaline solutions used. Many different chalcogenide semiconductor thin films [4,5], including PbSe and PbS, have been prepared by the CSD technique [7]. The CSD of PbSe is very well established, including an understanding of growth mechanisms and the effects of temperature, pH and concentration. Furthermore, we have shown that for CSD, PbSe [8–10] transmission/absorption properties in the UV/Vis/near IR region can be controlled due to quantum size effects [11]. Therefore, this method appears promising for the design of radiative cooling shields, where specific optical properties are required. Our aim in this work is to investigate different approaches for the design of shields for radiative cooling, focusing mainly on films deposited by CSD. CSD of PbS and PbSe thin films were studied under various experimental conditions, with emphasis on their optical properties as shields for passive cooling devices. Additionally, we explored alternative shield designs, in which pigmented polyethylene foils were coated with PbS thin films. For this, we prepared pigmented polyethylene foils (with non-absorbing pigments ZnS and ZnO), and the optical properties of those films subsequent to PbS deposition were analyzed. This approach shows, to some extent, improved optical properties for passive cooling application.
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2. Experimental 2.1. Materials and methods Low-density polyethylene (LDPE), ZnO, ZnS, Te, TiO2, carbon, C60, Sb2O3, KMnO4, KOH trisodium citrate, potassium nitrilotriacetate, lead acetate, sodium selenosulfate and thiourea were all analytical grade or better and were used as received. All aqueous solutions were prepared using milliQ (18 MO) water. 2.2. Pigmented polyethylene foils For the preparation of pigmented foils, LDPE and the pigment powders (ZnO, ZnS, Te, TiO2, carbon, C60 or Sb2O3) were mixed together in an extruder and heated to 210 C. Subsequently, the mixtures were pressed into films at 165 C and 10,000 psi for 3 min. In all films, the pigment volume fraction was 5% and the nominal film thickness was ca. 50 mm. 2.3. CSD of PbSe and PbS films Thin commercial LDPE (ca. 50 mm) foil was used as substrates for film deposition. The foil was cut to a size of 5 cm 5 cm, cleaned with detergent and rinsed well with water. Deposition of PbSe and PbS films by CSD was carried out according to literature methods [8,7,10,12]. In short, trisodium citrate (TSC) or potassium nitrilotriacetate (NTA) were used as complexing agents. Aqueous stock solutions of 0.5 M lead acetate, 1 M TSC, 0.7 M K3NTA, 0.5 M thiourea and 0.2 M sodium selenosulfate (Na2SeSO3) were used. Deposition solutions were prepared by diluting the lead acetate solution with water, adding the complexing agents solution and adjusting the pH to 10 with aqueous KOH. Finally, Na2SeSO3 or thiourea was added and pH adjusted to 10.8–11. The composition of the final solution was 60 mM Pb2+, 50 mM Na2SeSO3 and 160 mM (for low-complexant (LC) metal ion ratio) or 320 mM (for high-complexant (HC) metal ion ratio) TSC or with 60–70 mM NTA. The polyethylene substrates were placed in the CSD solutions for typically 8–24 h periods at various temperatures (5–85 C). The thickness of the deposited films was calculated based on the weight change of the polyethylene before and after the film deposition. Thicker films (>200 nm thick) tended to have poor adhesion and were prone to cracking; therefore, permanganate (KMnO4) surface treatment of the polyethylene was carried out prior to CSD to improve film adhesion as well as homogeneity [13]. For KMnO4 surface treatment, the polyethylene sheets were immersed in a 10% KMnO4 solution for 24 h at room temperature and then washed with water. 2.4. Characterization methods All FTIR spectra were recorded as direct (specular) transmission or reflection, while the UV/Vis/near IR spectra were obtained as diffuse transmission and
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reflectance. Diffuse reflection and transmission spectroscopy measurements in the UV/ Vis/near IR region were carried out on a Jasco V-570 spectrophotometer equipped with an integrating sphere. Spectra were recorded at room temperature between 200 and 2000 nm, with a scanning speed of 100 nm/min. FTIR spectra were recorded on a Bruker IFS66 spectrometer. The measurements were performed with normal incident light of 2.5–25 mm wavelength. The optical spectra were collected from one side of deposited films, the other side was removed with a swab moistened with nitric acid and water (1:4). It should be noted that the FTIR (window region) spectra are not diffuse measurements and, therefore, the measured transmission values should be considered as minimum ones. Scanning electron microscopy (SEM) images were obtained using a JEOL GMC 6400 microscope at an acceleration voltage of 20 kV. In order to be able to compare the characteristics of optical properties of different films, we will define some optical functions. We follow the definition of Nilsson et al. [14] for solar reflectance (RSol) as the average of the spectral reflectance of the films over the entire solar spectrum, as given by Eq. 1: Z l2 Z l2 RSol ¼ RðlÞW ðlÞ dl= W ðlÞ dl; ð1Þ l1
l1
where W ðlÞ is the solar spectrum AM 1.5 [15] and RðlÞ is the spectral reflectance of the film (and l is the wavelength). Similarly, we define the solar transmittance (TSol) and the solar absorption (ASol) as given by Eqs. (2) and (3), respectively, Z l2 Z l2 TðlÞW ðlÞ dl= W ðlÞ dl; ð2Þ TSol ¼ l1
ASol ¼ 1 ðRSol þ TSol Þ:
l1
ð3Þ
Applying the same calculation for the atmospheric window (8–13 mm) is a more complex issue, as the spectral characteristics of the atmospheric window are highly dependent on climate and, in particular on humidity. For simplicity we use atmospheric spectrum for clear sky as reported by Berdahl and Fromberg [16], and in the same way, T8–13, R8–13 and A8–13 are calculated for each type of sample. Table 1 gives the above optical function values for all samples reported in this work.
3. Results and discussion 3.1. Polyethylene substrate Thin polyethylene foils are, at present, the only practical substrate for use as radiation shields. Polyethylene is non-absorbing, except for characteristic absorption bands at 2.4, 3.4, 6.8 and 13.7 mm [17]. Other materials either absorb too strongly in the window region or, as for high band gap semiconductors, are not feasible for large area application. Therefore, a detailed investigation of the optical properties of the polyethylene foils is important. We have investigated several different types of low and high-density polyethylene, both commercial and prepared in our laboratory. All
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Table 1 Values of the optical functions, R8–13, A8–13, T8–13, RSol, TSol and ASol, of various materials for application in radiative cooling devices. (Optical functions values, R8–13, A8–13, T8–13, RSol, TSol and ASol as defined in the text.) Samples
TSol
RSol
ASol
T8–13
R8–13
A8–13
LDPE 50 mm Pigmented ZnS (50 mm) Pigmented ZnO (50 mm) PbSe HC NTA (200 nm) T=25 C PbS HC NTA (200 nm) T=25 C PbS HC TSC (150 nm) T=25 C Pigment ZnS coated with PbS film Pigment ZnO LDPE coated with PbS film
0.891 0.346 0.403 0.09 0.168 0.138 0.04 0.03
0.078 0.540 0.429 0.274 0.468 0.372 0.331 0.286
0.031 0.114 0.168 0.636 0.364 0.490 0.629 0.684
0.813 0.641 0.582 0.508 0.741 0.642 0.488 0.406
0.156 0.193 0.184 0.186 0.154 0.143 0.162 0.155
0.031 0.166 0.234 0.306 0.105 0.215 0.350 0.439
commercial samples show an appreciable absorption at ca. 1100 cm1, which is not characteristic of pure polyethylene, and is attributed to an impurity that is present in those films. For this reason, we prepared polyethylene substrate by conventional polymerization processes in which impurities can be controlled. These films included LDPE and high-density polyethylene (HDPE) and copolymers of polyethylene. The HDPE showed unacceptable absorption in the window region (at 1086 and 1265 cm1), while the copolymer (1:1 ratio of HDPE and LDPE) exhibited a slightly higher transmission than the normal low-density samples and was somewhat more rigid, which can prevent convection losses in the cooled volume. However, commercial LDPE films were selected as the substrate material due to its availability and constant optical properties, which were difficult to obtain for the homemade samples. The FTIR transmission spectrum of pure LDPE foil (ca. 50 mm) used in this work is shown in Fig. 1 curve A. The average transmission in the atmospheric window is ca. 80% (T8–13=0.813), and the greatest part of the loss in transmission is due to reflectance (R8–13=0.156). As was noted in the experimental section, we applied a modified KMnO4 pretreatment of the polyethylene, as previously reported by Pramanik et al. [13] to improve adhesion of thicker films. The nature of this surface treatment has been discussed in detail elsewhere [18]. In short, the KMnO4 pre-treatment forms a brown transparent MnO2 films, which does not affect the FTIR spectrum to any noticeable extent. However, it did reduce transmission in the solar range, mainly due to absorption rather than reflection. This characteristic is considered disadvantageous for radiative cooling, but could be beneficial if the subsequently deposited films exhibit high reflection. However, in practice, this is not expected to make an appreciable difference to shield properties, and treatment of the polyethylene was used prior to deposition of thick films (>200 nm thick). 3.2. Pigmented polyethylene foils Polymeric foils containing non-absorbing pigment offer the possibility to combine high reflectance of solar radiation with high transmittance in the
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80
A
B
70 60 50
C
40 30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Fig. 1. FTIR transmission spectra of 50 mm. (A) Pure LDPE foil; (B) Pigmented polyethylene films containing 5% ZnS; (C) Pigmented polyethylene films containing 5% ZnO.
‘‘atmospheric-window’’ region [19–22]. Usually, finely divided materials with high refractive indices are used as pigments. These materials, due to their small particle size, scatter short wavelengths more efficiently than longer wavelengths (note Rayleigh scattering efficiency pl4 ). The application of polyethylene foils for radiative cooling, pigmented with materials such as ZnS, ZnO and TiO2, has been theoretically and experimentally considered [23,24]. Those studies have shown that pigmented polyethylene films, using pigments such as ZnS, can exhibit high solar reflection, while being highly transparent in the IR region [14]. However, the optical properties of pigmented foils are a function of the volume fraction and the crystal size of the pigment. Usually, high volume fraction (>20%) of the pigment material is required in order to achieve the desired optical properties. On the other hand, it is difficult to obtain homogeneous pigmented films of high pigment volume fraction due to aggregation of the pigment material during preparation. Recently [25], we have shown that polyethylene foils pigmented with TiO2 nanoparticles have favorable optical characteristics, for use as solar radiation shields in radiative cooling devices. However, in the current work, we limited our research to films of low pigment volume fraction pigmented with sub-micron particles. We have prepared pigmented polyethylene foils containing 5% (by weight) of pigment. The pigment materials initially investigated were ZnO, ZnS, Te, carbon, C60 (fullerene) and Sb2O3. The two most promising materials were ZnS and ZnO, with Sb2O3 also exhibiting reasonable behavior. Figs. 1 (curves B and C) and 2 show the FTIR and the UV/Vis/near IR transmission and reflectance spectra of ZnS and ZnO films. The average transmission in the window region is fairly high in both samples, i.e. T8–13 for ZnS films is 0.641. The transmission drops toward the solar region, reaching zero at ca. 400 nm for both materials. The ZnS-pigmented sample shows a lower transmission over the solar region than the ZnO-pigmented sample; for instance at
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Transmission / Reflectance (%)
100 90 B
80 70 60
D
50 40 30
C
20
A
10 0
400
600
800 1000 1200 1400 1600 Wavelength (nm)
Fig. 2. UV/Vis/near IR diffuse transmission and reflection spectra of the same ZnS and ZnO films as in Fig. 1. (A) Transmission ZnS; (B) Reflection ZnS; (C) Transmission ZnO; (D) Reflection ZnO.
800 nm, the transmission of the ZnS is 28% while that of the ZnO is 40%. The loss in transmission is due to reflection, a desirable property for a shield. The better reflection of the ZnS may be due to its somewhat higher dielectric constant. Previous studies of the (photo) chemical stability of the pigmented foils have shown that ZnS [26] is not sufficiently stable for outdoor application over extended periods. However, ZnO, due to its long-term stability, has been recommended for use as a pigment material. We have performed preliminary investigations of the outdoor stability of ZnS- and ZnO-pigmented foils. Exposure of these foils, ZnO and ZnS, to sunlight for a period of a few weeks resulted in a minor decrease ca. 3% of the ‘‘atmospheric window’’ transmission. This lower than expected change in the IR transmission of the films is probably because of the low volume fraction of the pigments that limits the degradation processes of films. It should be pointed out that polyethylene foils with higher volume fraction typically, 15–20%, exhibit improved optical properties in contrast to our films but are likely to have poor (photo)chemical stability. 3.3. CSD of PbSe and PbS on polyethylene foils It is possible, at least in principle, to achieve high transparency for intrinsic semiconductors at wavelengths larger than that corresponding to the absorption edge (Eg o l). Low band gap semiconductors, such as PbSe (Eg = 0.27 eV), PbS (Eg = 0.37 eV) and Te [27] (Eg = 0.32 eV), are potential alternatives for shield materials. If thick enough, these materials should block the solar spectrum and, if intrinsic, have high transparency in the ‘‘atmospheric-window’’ region. In addition, their high dielectric constants imply high reflectivity. Recently we have used Te films [18], which exhibit high transmission in the mid-IR and high blocking in the solar spectral region, which make Te a suitable shield material for radiative cooling
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devices. In that work, a method for the preparation of Te films on polyethylene foils using room-temperature decomposition of electrochemically generated hydrogen telluride was described. In view of the encouraging results for Te films, the use of other low band gap semiconductors prepared using the chemical solution deposition method seems promising. For that reason, we chose to investigate the optical properties of PbS and PbSe films deposited under varying deposition parameters, in particular the nature of the complexing agent, the ratio between complex and metal ion concentrations, film thickness and deposition temperature. Before describing the specific optical properties of the films, we will point out some general conclusions on the CSD of those films. First, overall homogenous large-area thin films can be deposited at relatively low temperatures up to 50 C, while deposition at higher temperatures resulted in powdery deposits under the range of conditions used by us. These deposits are more scattering (not necessarily bad for the present purpose) but are also less adherent. Homogeneity of the deposited films was dependent on the film thickness. Up to 400 nm thickness resulted, in general, in uniform optical and structural properties. Although films of higher thickness could be deposited, they displayed variable optical properties with typically higher IR absorption. PbSe films could be deposited with large variation in crystallite size from a few nm up to micrometers by control of the deposition parameters (see Refs. [7–10] for details). On the contrary, PbS films exhibit polycrystalline structure with average crystal sizes of ca. 300 nm as measured SEM and estimated from by X-ray diffraction. Moreover, varying the deposition parameters, such as temperature, had negligible effects on crystal size. This is rather unexpected, especially in views of the results on the CSD of PbSe, which indicate that the deposition mechanism of PbS is somewhat different from that of PbSe. It should be noted that very recently, chemical bath deposition of nanocrystalline PbS thin films was reported [28,29]. The deposited PbS and PbSe films display large variations in their optical properties, depending on the experimental parameters. However, we will discuss here only films that exhibit favorable optical properties as solar shields, namely high (‘‘window’’) IR transmission combined with high solar blocking. In general, films deposited under conditions where an ion-by-ion deposition occurred [8–10], namely high complexant metal ion ratio and at low temperatures, 20–30 C, exhibited good optical quality for passive cooling applications. Fig. 3 (curves A and B) shows the FTIR transmission spectra of PbS films (ca. 200 nm thick) chemically deposited at 25 C from NTA and TSC baths, respectively. The window transmission for films deposited from the NTA bath is very high ca. 70% (T8–13 = 0.721) and also shows low absorption (A8–13 = 0.105). Such optical values are suitable for passive cooling use. Films deposited from a TSC bath under identical conditions show somewhat lower window transmission (T8–13 = 0.610) due to increase in IR absorption. The UV/Vis/near IR transmission and reflectance spectra of those films are shown in Fig. 4a and b. The total (specular + diffuse) reflectance, in both films, over most of the solar region is rather high; RSol = 0.468 for PbS deposited from HC NTA and RSol = 0.372 for PbS deposited from HC TSC baths. The film transmission is a critical parameter for solar shields as it determines heating of the cooled space by solar radiation. As can be seen from Fig. 4,
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100 90 A
Transmission (%)
80 70
B
60 50 C
40 30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Transmission / Reflectance (%)
Fig. 3. FTIR transmission spectra of CSD PbSe and PbS thin films on LDPE foil (50 mm) deposited at T = 25 C. (A) PbS films (200 nm) from HC NTA bath; (B) PbS films (200 nm) from HC TSC bath; (C) PbSe film (210 nm) from HC NTA bath. 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
A Reflectance
B C A B Transmission
400
600
C 800
1000 1200 1400 1600
Wavelength (nm)
Fig. 4. UV/Vis/near IR diffuse transmission and total reflection spectra of the same films as in Fig. 3. (A) PbS films (200 nm) from HC NTA bath; (B) PbS films (200 nm) from HC TSC bath; (C) PbSe film (210 nm) from HC NTA bath.
the transmission is reasonably high in the near IR region and falls to a low average value in the visible region (ca. 15%). The ‘‘atmospheric-window’’ transmission for PbSe films deposited from NTA (Fig. 3 curve C) is lower (T8–13 = 0.508) compared with the transmission of the PbS films. Moreover, the PbSe film shows lower reflection (RSol = 0.274) in the UV/Vis/ near IR region (Fig. 4 curve C), in spite of the higher dielectric constant of PbSe. Attempts to further reduce the solar transmission by increasing film thickness was of limited success, due mainly to the fact that the solar reflectivity for both PbS and
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PbSe decreased with further increase in film thickness. This phenomenon has been observed in another study [12] and was explained by change of film morphology. It should be noted that other films prepared under different conditions often exhibited considerable absorption in the atmospheric-window region and in a few cases, also low reflection, in the solar region. It is clear that the optical properties of the films are very preparation dependent. In general, the optical properties of thin semiconductors films can be related to different factors such as surface morphology, crystal size and size distribution, doping and the presence of impurities. Low transmission in the atmospheric-window region may be the result of doping, resulting in an increase in free carrier concentration and thus in free carrier reflection and absorption in the IR [30]. For example, PbS films deposited from an ion-by-ion NTA bath at 25 C exhibit high IR transmission (see Fig. 3a), while deposition from a bath at the same composition but at 55 C resulted in films of low IR transmission (T8–13 = 0.345, R8–13 = 0.123). X-ray diffraction of such low IR transmission films revealed additional diffraction peaks at 2W = 26.20 , 29.28 and 20.71 , which are assigned to Pb(OH)2 and elemental sulfur. The presence of these impurities may scatter light or dope the films, and could explain the low transmission observed in the IR spectra. Furthermore, surface morphology is an important factor in determining the optical properties of semiconductor films. SEM images of PbS films deposited from NTA baths at 25 C and 55 C are shown in Fig. 5a and b, respectively. The film deposited at 25 C is composed of a background of smooth, spherical particles, 100–200 nm in size, fairly distributed, together with some large aggregates of these particles. The 55 C film, in contrast, contains what appear to be crystals of several hundred nm in size, unevenly distributed on a finer background. The latter is expected to be more highly scattering due to these relatively large, irregularly distributed crystals. This can therefore explain the higher scattering we observe for the highertemperature films.
Fig. 5. SEM images of PbS films deposited on polyethylene from HC NTA chemical baths at (scale bar = 1 mm) (A) 25 C; (B) 55 C.
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To summarize this section, PbS and PbSe could be chemically deposited with optical properties reasonably suitable for passive cooling application. However, the experimental range in which such properties could be achieved is rather limited. Also, although much of the blocking behavior of the films in the solar region is due to reflection, a significant proportion of the incident radiation in the solar region is either absorbed or transmitted by the films. 3.4. PbS films deposited on pigmented polyethylene The pigmented foils or the PbS and PbSe shields investigated in this work were still not sufficiently blocking in the solar range for device application. However, a combination of pigmented polyethylene, coated with PbSe or PbS, may be expected to provide improved optical and cooling properties. The concept of this approach is to combine high solar reflection or scattering from the upper side of the shield, due to the pigment films, and to absorb (and reflect some of) the remaining light in the lower semiconductor film. Alternatively, the absorbing semiconductor film could be the upper layer. This configuration would be applicable only under a high rate of heat removal from the shield. For this purpose, we investigated CSD of PbS films onto ZnS- and ZnOpigmented polyethylene. We selected PbS films deposited from NTA for further investigation since those films exhibited high solar reflection. Optical measurements were carried out using two different configurations: in one, the PbS coating facing away from the incident light beam and the other with the PbS coating facing the incident light beam. Figs. 6 and 7 show the FTIR and UV/Vis/near IR transmission and reflection spectra of ZnS- and ZnO-pigmented foils coated with PbS films deposited from NTA baths at 25 C. In general, a small decrease ca. 20% of transmission in the ‘‘atmospheric-window’’ range is noted in both films and moreover the transmission spectra are identical regardless of the configuration of the 70 60
Transmission (%)
A 50 40
B
30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Fig. 6. FTIR transmission spectra of pigmented polyethylene coated with CSD PbS films (ca 200 nm) deposited at T = 25 C from HC NTA baths. (A) ZnS; (B) ZnO.
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A
Transmission / Reflectance (%)
40 35
Reflectance
B
30
C
25 20
Reflectance
D
15 E
10
F
5 Transmission
0 400
600
800 1000 1200 Wavelength (nm)
1400 1600
Fig. 7. UV/Vis/near IR diffuse transmission and total reflection spectra of the same pigmented films as in Fig. 6. (A) ZnS-reflection spectra PbS coating facing away from light beam; (B) ZnO-reflection spectra PbS coating facing away from light beam; (C) ZnS-reflection spectra PbS coating facing light beam; (D) ZnO-reflection spectra PbS coating facing light beam; (E) ZnO-transmission spectra PbS coating facing away from light beam; (F) ZnS-transmission spectra PbS coating facing away from light beam.
optical measurements. However, transmission in the solar region is drastically reduced to the point where the samples are considered to be absorbing (see Fig. 7). For example, the transmission of ZnS-pigmented films at 800 nm is ca. 40%. However, following coating with PbS the film transmission decreased to ca. 3.5%. The nature of the transmission spectra measured in the two optical configurations is almost identical; therefore, we present only the transmission spectra in which the PbS coatings face away from the incident beam as illustrative cases. Reflection in the solar region is also decreased, but not to the same extent as the transmission and furthermore the quality of the reflection spectra depended strongly on the optical configuration of the films. In Fig. 7, the reflection spectra of PbScoated ZnS- and ZnO-pigmented polyethylene are shown using both measurement configurations. It can be seen that the reflection is considerably higher when the PbS faces away from the incident light beam than when it faces it. A consideration of the two geometries leads to an expected difference in the reflection spectra (reflection characteristics dominated by the ZnS(O)-pigmented polyethylene for the facing away geometry and by the PbS for the facing geometry). However, these PbS-coated scattering foils do appear to possess the best properties for use in radiation shields. Further experimental work on reducing the longer wavelength transmission, increasing the reflectivity and fabricating actual devices using these films is required.
4. Conclusions We have shown that chemical solution deposited semiconductor films can offer an alternative to pigmentation of polymer foils in the design of shields for radiative
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cooling applications. Thin PbS and PbSe films were deposited onto polyethylene foil from baths of different compositions and varying conditions. The optical properties of these films were measured in the UV/Vis/near IR (solar) and mid-IR (atmospheric-window) regions. Some of these films were shown to possess suitable optical properties for use as shields. Pigmented (scattering) shields have also been made by incorporating ZnS and ZnO into the polyethylene substrate. While these shields are not sufficiently blocking by themselves, a combination of PbS chemically deposited onto the pigmented polyethylene exhibited a substantial decrease of radiative transmission in the solar region.
Acknowledgements Y. Mastai thanks Prof. G. Levin for the assistance in the preparation of the pigmented polyethylene foils and for helpful discussions. We are grateful to Dr. D. Katzen of the Holon Academic Institute of Technology for suggestions and discussions regarding the manuscript.
References [1] M. Martin, in: J. Cook (Ed.), Passive Cooling, MIT Press, Cambridge, MA, 1989. [2] C.G. Granqvist, T.S. Eriksson, in: C.G. Granqvist (Ed.), Materials for Solar Energy Conversion Systems, Pergmon Press, 1991. [3] P.C. Agrawal, Int. J. Energy Res. 16 (1992) 101. [4] K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, in: G. Hass, M.H. Francombe, J.L. Vossen (Eds.), Physics of Thin Film, Vol. 12, Academic Press, New York, 1982, p. 201. [5] C.D. Lokhande, Mater. Chem. Phys. 27 (1991) 1. ! A. Castillo, I.T. Ayala, O. Gomezdaza, [6] P.K. Nair, M.T.S. Nair, V.M. Garc!ıa, O.L. Arenas, Y. Penna, A. S!anchez, J. Campos, H. Hu, R. Su!arez, M.E. Rincon, Sol. Energy Mater. Sol. Cells 52 (1998) 313. [7] G. Hodes, in: Chemical Solution Deposition of Semiconductor Films. Marcel Dekker Inc., New York, 2003. [8] S. Gorer, A. Albu-Yaron, G. Hodes, J. Phys. Chem. 99 (1995) 16442. [9] S. Gorer, A. Albu-Yaron, G. Hodes, Chem. Mater. 7 (1995) 1243. [10] S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338. [11] A.D. Yoffe, Adv. Phys. 42 (1993) 173. [12] I. Pop, C. Nasuc, V. Ionescu, E. Indrea, I. Bartu, Thin Solid Films 307 (1997) 240. [13] P. Pramanik, S. Bhattacharya, J. Mater. Sci. Lett. 6 (1987) 1106. [14] T.M.J. Nilsson, G.A. Niklasson, C.G. Granqvist, Sol. Energy Mater. Sol. Cells 28 (1992) 175. [15] R.E. Bird, R.L. Hulstrom, L.J. Lewis, Solar Energy 30 (1983) 563. [16] P. Berdahl, R. Fromberg, Solar Energy 29 (1982) 299. [17] S. Catalanotti, V. Cuomo, D. Piro, V. Silvestrini, G. Troise, Sol. Energy 17 (1975) 83. [18] T. Engelhard, E.D. Jones, I. Viney, Y. Mastai, G. Hodes, Thin Solid Films 370 (2000) 101. [19] D. Michell, K.L. Biggs, Appl. Energy 5 (1979) 216. [20] A. Andretta, B. Bartoli, B. Coluzzi, V. Cuomo, J. Phys. (Paris) C (1981) 423. [21] T.M.J. Nilsson, G.A. Niklasson, Sol. Energy Mater. Sol. Cells 37 (1995) 93. [22] G.A. Niklasson, T.S. Erilksson, Proc. Soc. Photo-Opt. Instrum. Eng. 1016 (1988) 89. [23] W.E. Vargas, E.M. Lushiku, G.A. Niklasson, T.M.J. Nilsson, Sol. Energy Mater. Sol. Cells 54 (1998) 343.
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[24] W.E. Vargas, J. Appl. Phys. 88 (2000) 4079. [25] Y. Mastai, Y. Diamant, S.T. Aruna, A. Zaban, Langmuir 17 (2001) 7118. [26] J. Brookes, T.A. Compton, Development of Radiative Cooling Materials; Phase III, DOE- Report, Energy Materials Research Company, Final Report, 1985. [27] P. Grenier, Rev. Phys. Appl. 124 (1979) 97. [28] E. Pentia, L. Pintilie, I. Matei, T. Botila, E. Ozbay, J. Optoelectron Adv. M. 3 (2001) 525. [29] C. Wang, W.X. Zhang, X.F. Qian, X.M. Zhang, Y. Xie, Y.T. Qian, Mater. Lett. 40 (1999) 255. . [30] K.A. Jackson, W. Schroter (Eds.), Handbook of Semiconductor Technology, Vol. 1, Wiley-VCH, Weinheim, 2000.
Solar Energy Materials & Solar Cells 80 (2003) 283–296
Thin semiconductor films for radiative cooling applications K.D. Dobsona,1, G. Hodesa, Y. Mastaib,* a
Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel b Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received 14 January 2002; received in revised form 22 April 2003; accepted 14 June 2003
Abstract Passive cooling systems use simple low-cost techniques to provide summer comfort in warm climates and can also be used to keep food, liquids and other materials at temperatures below ambient. Radiative cooling devices require a convective shield that should reject solar radiation but be transparent to mid-IR thermal radiation. In this paper, chemical solution deposition of thin semiconductor films (PbS and PbSe) onto polyethylene foils for radiative cooling applications is described. Optical and structural characterizations of the films were performed using UV–Vis, FTIR spectroscopies, X-ray diffraction and electron microscopy. Additionally, pigmented shields, which exhibit good radiation scattering properties, were prepared by incorporation of ZnS or ZnO into polyethylene. We also studied a combination of pigmented polyethylene foils coated with thin films of PbS that show improved optical properties for cooling purpose. r 2003 Elsevier B.V. All rights reserved. Keywords: Radiative cooling; Chemical solution deposition; Polyethylene pigmentation; Semiconductor thin films
1. Introduction Passive (or radiative) cooling has many potential applications [1–3], including cold storage of food, seeds and medicines, climatization of buildings and water *Corresponding author. Bar-Ilan University, Department of chemistry, 52900 Ramat-Gan, Israel. Tel.: +972-3 531 7681; Fax: +972-3 535 1250. E-mail address: [email protected] (Y. Mastai). 1 Current address :Institute of Energy Conversion, University of Delaware Newark, DE 19716, USA. 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2003.06.007
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desalinization. The phenomenon of radiative cooling uses the fact that the thermal energy emitted by a clear sky in the ‘‘window region’’ (8–13 mm) is much less than the thermal energy emitted by a blackbody at ground air temperature in this wavelength range. Hence, a surface on the earth facing the sky experiences an imbalance of outgoing and incoming thermal radiation and cools to below the ambient air temperature. While this concept can work well at night, assuming a relatively dry atmosphere, the solar energy input during the day, which is normally much greater than that radiated out, causes heating of the system. To prevent this, a shield is needed to cover the radiating surface in order to block solar radiation during the day as well as to prevent convective mixing in the cooled space. The ideal radiation shield would completely reflect solar radiation, but allow complete transmission in the ‘‘atmospheric-window’’ region. Solar radiation should preferably be reflected, as any absorbed radiation will be converted to heat somewhere in the system. Two different approaches for the design of shield have been previously proposed [2]. Firstly, the introduction of optical scattering materials into shield substrates, and secondly, coating of shield substrates with high solar reflector films. Low band gap semiconductors, such as Te, PbS and PbSe, can exhibit high reflectivity and are expected to block solar radiation, while being transparent in the ‘‘atmospheric window’’ if intrinsic. Vacuum evaporation is generally used to deposit such films. In this work, we use the chemical solution deposition (CSD) technique for film preparation. CSD has been used for a long time [4,5] to deposit semiconductor thin films, particularly, in the area of solar energy conversion [6] and is it ideally suited for the production of large area thin films. The basis of CSD is the slow formation of a semiconductor, either by reaction of a slowly produced anion with free metal ions or colloidal metal hydroxide, or by slow dissociation of a chalcogen complex. In CSD, thin semiconductor films are deposited on substrates immersed in dilute solutions containing metal ions and a source of (usually) hydroxide, sulfide or selenide, either as ions or as a complex species. The use of metal complexing agents is required mainly to prevent bulk hydroxide precipitation in the usually alkaline solutions used. Many different chalcogenide semiconductor thin films [4,5], including PbSe and PbS, have been prepared by the CSD technique [7]. The CSD of PbSe is very well established, including an understanding of growth mechanisms and the effects of temperature, pH and concentration. Furthermore, we have shown that for CSD, PbSe [8–10] transmission/absorption properties in the UV/Vis/near IR region can be controlled due to quantum size effects [11]. Therefore, this method appears promising for the design of radiative cooling shields, where specific optical properties are required. Our aim in this work is to investigate different approaches for the design of shields for radiative cooling, focusing mainly on films deposited by CSD. CSD of PbS and PbSe thin films were studied under various experimental conditions, with emphasis on their optical properties as shields for passive cooling devices. Additionally, we explored alternative shield designs, in which pigmented polyethylene foils were coated with PbS thin films. For this, we prepared pigmented polyethylene foils (with non-absorbing pigments ZnS and ZnO), and the optical properties of those films subsequent to PbS deposition were analyzed. This approach shows, to some extent, improved optical properties for passive cooling application.
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2. Experimental 2.1. Materials and methods Low-density polyethylene (LDPE), ZnO, ZnS, Te, TiO2, carbon, C60, Sb2O3, KMnO4, KOH trisodium citrate, potassium nitrilotriacetate, lead acetate, sodium selenosulfate and thiourea were all analytical grade or better and were used as received. All aqueous solutions were prepared using milliQ (18 MO) water. 2.2. Pigmented polyethylene foils For the preparation of pigmented foils, LDPE and the pigment powders (ZnO, ZnS, Te, TiO2, carbon, C60 or Sb2O3) were mixed together in an extruder and heated to 210 C. Subsequently, the mixtures were pressed into films at 165 C and 10,000 psi for 3 min. In all films, the pigment volume fraction was 5% and the nominal film thickness was ca. 50 mm. 2.3. CSD of PbSe and PbS films Thin commercial LDPE (ca. 50 mm) foil was used as substrates for film deposition. The foil was cut to a size of 5 cm 5 cm, cleaned with detergent and rinsed well with water. Deposition of PbSe and PbS films by CSD was carried out according to literature methods [8,7,10,12]. In short, trisodium citrate (TSC) or potassium nitrilotriacetate (NTA) were used as complexing agents. Aqueous stock solutions of 0.5 M lead acetate, 1 M TSC, 0.7 M K3NTA, 0.5 M thiourea and 0.2 M sodium selenosulfate (Na2SeSO3) were used. Deposition solutions were prepared by diluting the lead acetate solution with water, adding the complexing agents solution and adjusting the pH to 10 with aqueous KOH. Finally, Na2SeSO3 or thiourea was added and pH adjusted to 10.8–11. The composition of the final solution was 60 mM Pb2+, 50 mM Na2SeSO3 and 160 mM (for low-complexant (LC) metal ion ratio) or 320 mM (for high-complexant (HC) metal ion ratio) TSC or with 60–70 mM NTA. The polyethylene substrates were placed in the CSD solutions for typically 8–24 h periods at various temperatures (5–85 C). The thickness of the deposited films was calculated based on the weight change of the polyethylene before and after the film deposition. Thicker films (>200 nm thick) tended to have poor adhesion and were prone to cracking; therefore, permanganate (KMnO4) surface treatment of the polyethylene was carried out prior to CSD to improve film adhesion as well as homogeneity [13]. For KMnO4 surface treatment, the polyethylene sheets were immersed in a 10% KMnO4 solution for 24 h at room temperature and then washed with water. 2.4. Characterization methods All FTIR spectra were recorded as direct (specular) transmission or reflection, while the UV/Vis/near IR spectra were obtained as diffuse transmission and
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reflectance. Diffuse reflection and transmission spectroscopy measurements in the UV/ Vis/near IR region were carried out on a Jasco V-570 spectrophotometer equipped with an integrating sphere. Spectra were recorded at room temperature between 200 and 2000 nm, with a scanning speed of 100 nm/min. FTIR spectra were recorded on a Bruker IFS66 spectrometer. The measurements were performed with normal incident light of 2.5–25 mm wavelength. The optical spectra were collected from one side of deposited films, the other side was removed with a swab moistened with nitric acid and water (1:4). It should be noted that the FTIR (window region) spectra are not diffuse measurements and, therefore, the measured transmission values should be considered as minimum ones. Scanning electron microscopy (SEM) images were obtained using a JEOL GMC 6400 microscope at an acceleration voltage of 20 kV. In order to be able to compare the characteristics of optical properties of different films, we will define some optical functions. We follow the definition of Nilsson et al. [14] for solar reflectance (RSol) as the average of the spectral reflectance of the films over the entire solar spectrum, as given by Eq. 1: Z l2 Z l2 RSol ¼ RðlÞW ðlÞ dl= W ðlÞ dl; ð1Þ l1
l1
where W ðlÞ is the solar spectrum AM 1.5 [15] and RðlÞ is the spectral reflectance of the film (and l is the wavelength). Similarly, we define the solar transmittance (TSol) and the solar absorption (ASol) as given by Eqs. (2) and (3), respectively, Z l2 Z l2 TðlÞW ðlÞ dl= W ðlÞ dl; ð2Þ TSol ¼ l1
ASol ¼ 1 ðRSol þ TSol Þ:
l1
ð3Þ
Applying the same calculation for the atmospheric window (8–13 mm) is a more complex issue, as the spectral characteristics of the atmospheric window are highly dependent on climate and, in particular on humidity. For simplicity we use atmospheric spectrum for clear sky as reported by Berdahl and Fromberg [16], and in the same way, T8–13, R8–13 and A8–13 are calculated for each type of sample. Table 1 gives the above optical function values for all samples reported in this work.
3. Results and discussion 3.1. Polyethylene substrate Thin polyethylene foils are, at present, the only practical substrate for use as radiation shields. Polyethylene is non-absorbing, except for characteristic absorption bands at 2.4, 3.4, 6.8 and 13.7 mm [17]. Other materials either absorb too strongly in the window region or, as for high band gap semiconductors, are not feasible for large area application. Therefore, a detailed investigation of the optical properties of the polyethylene foils is important. We have investigated several different types of low and high-density polyethylene, both commercial and prepared in our laboratory. All
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Table 1 Values of the optical functions, R8–13, A8–13, T8–13, RSol, TSol and ASol, of various materials for application in radiative cooling devices. (Optical functions values, R8–13, A8–13, T8–13, RSol, TSol and ASol as defined in the text.) Samples
TSol
RSol
ASol
T8–13
R8–13
A8–13
LDPE 50 mm Pigmented ZnS (50 mm) Pigmented ZnO (50 mm) PbSe HC NTA (200 nm) T=25 C PbS HC NTA (200 nm) T=25 C PbS HC TSC (150 nm) T=25 C Pigment ZnS coated with PbS film Pigment ZnO LDPE coated with PbS film
0.891 0.346 0.403 0.09 0.168 0.138 0.04 0.03
0.078 0.540 0.429 0.274 0.468 0.372 0.331 0.286
0.031 0.114 0.168 0.636 0.364 0.490 0.629 0.684
0.813 0.641 0.582 0.508 0.741 0.642 0.488 0.406
0.156 0.193 0.184 0.186 0.154 0.143 0.162 0.155
0.031 0.166 0.234 0.306 0.105 0.215 0.350 0.439
commercial samples show an appreciable absorption at ca. 1100 cm1, which is not characteristic of pure polyethylene, and is attributed to an impurity that is present in those films. For this reason, we prepared polyethylene substrate by conventional polymerization processes in which impurities can be controlled. These films included LDPE and high-density polyethylene (HDPE) and copolymers of polyethylene. The HDPE showed unacceptable absorption in the window region (at 1086 and 1265 cm1), while the copolymer (1:1 ratio of HDPE and LDPE) exhibited a slightly higher transmission than the normal low-density samples and was somewhat more rigid, which can prevent convection losses in the cooled volume. However, commercial LDPE films were selected as the substrate material due to its availability and constant optical properties, which were difficult to obtain for the homemade samples. The FTIR transmission spectrum of pure LDPE foil (ca. 50 mm) used in this work is shown in Fig. 1 curve A. The average transmission in the atmospheric window is ca. 80% (T8–13=0.813), and the greatest part of the loss in transmission is due to reflectance (R8–13=0.156). As was noted in the experimental section, we applied a modified KMnO4 pretreatment of the polyethylene, as previously reported by Pramanik et al. [13] to improve adhesion of thicker films. The nature of this surface treatment has been discussed in detail elsewhere [18]. In short, the KMnO4 pre-treatment forms a brown transparent MnO2 films, which does not affect the FTIR spectrum to any noticeable extent. However, it did reduce transmission in the solar range, mainly due to absorption rather than reflection. This characteristic is considered disadvantageous for radiative cooling, but could be beneficial if the subsequently deposited films exhibit high reflection. However, in practice, this is not expected to make an appreciable difference to shield properties, and treatment of the polyethylene was used prior to deposition of thick films (>200 nm thick). 3.2. Pigmented polyethylene foils Polymeric foils containing non-absorbing pigment offer the possibility to combine high reflectance of solar radiation with high transmittance in the
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80
A
B
70 60 50
C
40 30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Fig. 1. FTIR transmission spectra of 50 mm. (A) Pure LDPE foil; (B) Pigmented polyethylene films containing 5% ZnS; (C) Pigmented polyethylene films containing 5% ZnO.
‘‘atmospheric-window’’ region [19–22]. Usually, finely divided materials with high refractive indices are used as pigments. These materials, due to their small particle size, scatter short wavelengths more efficiently than longer wavelengths (note Rayleigh scattering efficiency pl4 ). The application of polyethylene foils for radiative cooling, pigmented with materials such as ZnS, ZnO and TiO2, has been theoretically and experimentally considered [23,24]. Those studies have shown that pigmented polyethylene films, using pigments such as ZnS, can exhibit high solar reflection, while being highly transparent in the IR region [14]. However, the optical properties of pigmented foils are a function of the volume fraction and the crystal size of the pigment. Usually, high volume fraction (>20%) of the pigment material is required in order to achieve the desired optical properties. On the other hand, it is difficult to obtain homogeneous pigmented films of high pigment volume fraction due to aggregation of the pigment material during preparation. Recently [25], we have shown that polyethylene foils pigmented with TiO2 nanoparticles have favorable optical characteristics, for use as solar radiation shields in radiative cooling devices. However, in the current work, we limited our research to films of low pigment volume fraction pigmented with sub-micron particles. We have prepared pigmented polyethylene foils containing 5% (by weight) of pigment. The pigment materials initially investigated were ZnO, ZnS, Te, carbon, C60 (fullerene) and Sb2O3. The two most promising materials were ZnS and ZnO, with Sb2O3 also exhibiting reasonable behavior. Figs. 1 (curves B and C) and 2 show the FTIR and the UV/Vis/near IR transmission and reflectance spectra of ZnS and ZnO films. The average transmission in the window region is fairly high in both samples, i.e. T8–13 for ZnS films is 0.641. The transmission drops toward the solar region, reaching zero at ca. 400 nm for both materials. The ZnS-pigmented sample shows a lower transmission over the solar region than the ZnO-pigmented sample; for instance at
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Transmission / Reflectance (%)
100 90 B
80 70 60
D
50 40 30
C
20
A
10 0
400
600
800 1000 1200 1400 1600 Wavelength (nm)
Fig. 2. UV/Vis/near IR diffuse transmission and reflection spectra of the same ZnS and ZnO films as in Fig. 1. (A) Transmission ZnS; (B) Reflection ZnS; (C) Transmission ZnO; (D) Reflection ZnO.
800 nm, the transmission of the ZnS is 28% while that of the ZnO is 40%. The loss in transmission is due to reflection, a desirable property for a shield. The better reflection of the ZnS may be due to its somewhat higher dielectric constant. Previous studies of the (photo) chemical stability of the pigmented foils have shown that ZnS [26] is not sufficiently stable for outdoor application over extended periods. However, ZnO, due to its long-term stability, has been recommended for use as a pigment material. We have performed preliminary investigations of the outdoor stability of ZnS- and ZnO-pigmented foils. Exposure of these foils, ZnO and ZnS, to sunlight for a period of a few weeks resulted in a minor decrease ca. 3% of the ‘‘atmospheric window’’ transmission. This lower than expected change in the IR transmission of the films is probably because of the low volume fraction of the pigments that limits the degradation processes of films. It should be pointed out that polyethylene foils with higher volume fraction typically, 15–20%, exhibit improved optical properties in contrast to our films but are likely to have poor (photo)chemical stability. 3.3. CSD of PbSe and PbS on polyethylene foils It is possible, at least in principle, to achieve high transparency for intrinsic semiconductors at wavelengths larger than that corresponding to the absorption edge (Eg o l). Low band gap semiconductors, such as PbSe (Eg = 0.27 eV), PbS (Eg = 0.37 eV) and Te [27] (Eg = 0.32 eV), are potential alternatives for shield materials. If thick enough, these materials should block the solar spectrum and, if intrinsic, have high transparency in the ‘‘atmospheric-window’’ region. In addition, their high dielectric constants imply high reflectivity. Recently we have used Te films [18], which exhibit high transmission in the mid-IR and high blocking in the solar spectral region, which make Te a suitable shield material for radiative cooling
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devices. In that work, a method for the preparation of Te films on polyethylene foils using room-temperature decomposition of electrochemically generated hydrogen telluride was described. In view of the encouraging results for Te films, the use of other low band gap semiconductors prepared using the chemical solution deposition method seems promising. For that reason, we chose to investigate the optical properties of PbS and PbSe films deposited under varying deposition parameters, in particular the nature of the complexing agent, the ratio between complex and metal ion concentrations, film thickness and deposition temperature. Before describing the specific optical properties of the films, we will point out some general conclusions on the CSD of those films. First, overall homogenous large-area thin films can be deposited at relatively low temperatures up to 50 C, while deposition at higher temperatures resulted in powdery deposits under the range of conditions used by us. These deposits are more scattering (not necessarily bad for the present purpose) but are also less adherent. Homogeneity of the deposited films was dependent on the film thickness. Up to 400 nm thickness resulted, in general, in uniform optical and structural properties. Although films of higher thickness could be deposited, they displayed variable optical properties with typically higher IR absorption. PbSe films could be deposited with large variation in crystallite size from a few nm up to micrometers by control of the deposition parameters (see Refs. [7–10] for details). On the contrary, PbS films exhibit polycrystalline structure with average crystal sizes of ca. 300 nm as measured SEM and estimated from by X-ray diffraction. Moreover, varying the deposition parameters, such as temperature, had negligible effects on crystal size. This is rather unexpected, especially in views of the results on the CSD of PbSe, which indicate that the deposition mechanism of PbS is somewhat different from that of PbSe. It should be noted that very recently, chemical bath deposition of nanocrystalline PbS thin films was reported [28,29]. The deposited PbS and PbSe films display large variations in their optical properties, depending on the experimental parameters. However, we will discuss here only films that exhibit favorable optical properties as solar shields, namely high (‘‘window’’) IR transmission combined with high solar blocking. In general, films deposited under conditions where an ion-by-ion deposition occurred [8–10], namely high complexant metal ion ratio and at low temperatures, 20–30 C, exhibited good optical quality for passive cooling applications. Fig. 3 (curves A and B) shows the FTIR transmission spectra of PbS films (ca. 200 nm thick) chemically deposited at 25 C from NTA and TSC baths, respectively. The window transmission for films deposited from the NTA bath is very high ca. 70% (T8–13 = 0.721) and also shows low absorption (A8–13 = 0.105). Such optical values are suitable for passive cooling use. Films deposited from a TSC bath under identical conditions show somewhat lower window transmission (T8–13 = 0.610) due to increase in IR absorption. The UV/Vis/near IR transmission and reflectance spectra of those films are shown in Fig. 4a and b. The total (specular + diffuse) reflectance, in both films, over most of the solar region is rather high; RSol = 0.468 for PbS deposited from HC NTA and RSol = 0.372 for PbS deposited from HC TSC baths. The film transmission is a critical parameter for solar shields as it determines heating of the cooled space by solar radiation. As can be seen from Fig. 4,
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100 90 A
Transmission (%)
80 70
B
60 50 C
40 30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Transmission / Reflectance (%)
Fig. 3. FTIR transmission spectra of CSD PbSe and PbS thin films on LDPE foil (50 mm) deposited at T = 25 C. (A) PbS films (200 nm) from HC NTA bath; (B) PbS films (200 nm) from HC TSC bath; (C) PbSe film (210 nm) from HC NTA bath. 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
A Reflectance
B C A B Transmission
400
600
C 800
1000 1200 1400 1600
Wavelength (nm)
Fig. 4. UV/Vis/near IR diffuse transmission and total reflection spectra of the same films as in Fig. 3. (A) PbS films (200 nm) from HC NTA bath; (B) PbS films (200 nm) from HC TSC bath; (C) PbSe film (210 nm) from HC NTA bath.
the transmission is reasonably high in the near IR region and falls to a low average value in the visible region (ca. 15%). The ‘‘atmospheric-window’’ transmission for PbSe films deposited from NTA (Fig. 3 curve C) is lower (T8–13 = 0.508) compared with the transmission of the PbS films. Moreover, the PbSe film shows lower reflection (RSol = 0.274) in the UV/Vis/ near IR region (Fig. 4 curve C), in spite of the higher dielectric constant of PbSe. Attempts to further reduce the solar transmission by increasing film thickness was of limited success, due mainly to the fact that the solar reflectivity for both PbS and
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PbSe decreased with further increase in film thickness. This phenomenon has been observed in another study [12] and was explained by change of film morphology. It should be noted that other films prepared under different conditions often exhibited considerable absorption in the atmospheric-window region and in a few cases, also low reflection, in the solar region. It is clear that the optical properties of the films are very preparation dependent. In general, the optical properties of thin semiconductors films can be related to different factors such as surface morphology, crystal size and size distribution, doping and the presence of impurities. Low transmission in the atmospheric-window region may be the result of doping, resulting in an increase in free carrier concentration and thus in free carrier reflection and absorption in the IR [30]. For example, PbS films deposited from an ion-by-ion NTA bath at 25 C exhibit high IR transmission (see Fig. 3a), while deposition from a bath at the same composition but at 55 C resulted in films of low IR transmission (T8–13 = 0.345, R8–13 = 0.123). X-ray diffraction of such low IR transmission films revealed additional diffraction peaks at 2W = 26.20 , 29.28 and 20.71 , which are assigned to Pb(OH)2 and elemental sulfur. The presence of these impurities may scatter light or dope the films, and could explain the low transmission observed in the IR spectra. Furthermore, surface morphology is an important factor in determining the optical properties of semiconductor films. SEM images of PbS films deposited from NTA baths at 25 C and 55 C are shown in Fig. 5a and b, respectively. The film deposited at 25 C is composed of a background of smooth, spherical particles, 100–200 nm in size, fairly distributed, together with some large aggregates of these particles. The 55 C film, in contrast, contains what appear to be crystals of several hundred nm in size, unevenly distributed on a finer background. The latter is expected to be more highly scattering due to these relatively large, irregularly distributed crystals. This can therefore explain the higher scattering we observe for the highertemperature films.
Fig. 5. SEM images of PbS films deposited on polyethylene from HC NTA chemical baths at (scale bar = 1 mm) (A) 25 C; (B) 55 C.
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To summarize this section, PbS and PbSe could be chemically deposited with optical properties reasonably suitable for passive cooling application. However, the experimental range in which such properties could be achieved is rather limited. Also, although much of the blocking behavior of the films in the solar region is due to reflection, a significant proportion of the incident radiation in the solar region is either absorbed or transmitted by the films. 3.4. PbS films deposited on pigmented polyethylene The pigmented foils or the PbS and PbSe shields investigated in this work were still not sufficiently blocking in the solar range for device application. However, a combination of pigmented polyethylene, coated with PbSe or PbS, may be expected to provide improved optical and cooling properties. The concept of this approach is to combine high solar reflection or scattering from the upper side of the shield, due to the pigment films, and to absorb (and reflect some of) the remaining light in the lower semiconductor film. Alternatively, the absorbing semiconductor film could be the upper layer. This configuration would be applicable only under a high rate of heat removal from the shield. For this purpose, we investigated CSD of PbS films onto ZnS- and ZnOpigmented polyethylene. We selected PbS films deposited from NTA for further investigation since those films exhibited high solar reflection. Optical measurements were carried out using two different configurations: in one, the PbS coating facing away from the incident light beam and the other with the PbS coating facing the incident light beam. Figs. 6 and 7 show the FTIR and UV/Vis/near IR transmission and reflection spectra of ZnS- and ZnO-pigmented foils coated with PbS films deposited from NTA baths at 25 C. In general, a small decrease ca. 20% of transmission in the ‘‘atmospheric-window’’ range is noted in both films and moreover the transmission spectra are identical regardless of the configuration of the 70 60
Transmission (%)
A 50 40
B
30 20 10 0
3 4 5 6 7 8 9 10 11 12 13 14 15 16 Wavelength (µm)
Fig. 6. FTIR transmission spectra of pigmented polyethylene coated with CSD PbS films (ca 200 nm) deposited at T = 25 C from HC NTA baths. (A) ZnS; (B) ZnO.
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A
Transmission / Reflectance (%)
40 35
Reflectance
B
30
C
25 20
Reflectance
D
15 E
10
F
5 Transmission
0 400
600
800 1000 1200 Wavelength (nm)
1400 1600
Fig. 7. UV/Vis/near IR diffuse transmission and total reflection spectra of the same pigmented films as in Fig. 6. (A) ZnS-reflection spectra PbS coating facing away from light beam; (B) ZnO-reflection spectra PbS coating facing away from light beam; (C) ZnS-reflection spectra PbS coating facing light beam; (D) ZnO-reflection spectra PbS coating facing light beam; (E) ZnO-transmission spectra PbS coating facing away from light beam; (F) ZnS-transmission spectra PbS coating facing away from light beam.
optical measurements. However, transmission in the solar region is drastically reduced to the point where the samples are considered to be absorbing (see Fig. 7). For example, the transmission of ZnS-pigmented films at 800 nm is ca. 40%. However, following coating with PbS the film transmission decreased to ca. 3.5%. The nature of the transmission spectra measured in the two optical configurations is almost identical; therefore, we present only the transmission spectra in which the PbS coatings face away from the incident beam as illustrative cases. Reflection in the solar region is also decreased, but not to the same extent as the transmission and furthermore the quality of the reflection spectra depended strongly on the optical configuration of the films. In Fig. 7, the reflection spectra of PbScoated ZnS- and ZnO-pigmented polyethylene are shown using both measurement configurations. It can be seen that the reflection is considerably higher when the PbS faces away from the incident light beam than when it faces it. A consideration of the two geometries leads to an expected difference in the reflection spectra (reflection characteristics dominated by the ZnS(O)-pigmented polyethylene for the facing away geometry and by the PbS for the facing geometry). However, these PbS-coated scattering foils do appear to possess the best properties for use in radiation shields. Further experimental work on reducing the longer wavelength transmission, increasing the reflectivity and fabricating actual devices using these films is required.
4. Conclusions We have shown that chemical solution deposited semiconductor films can offer an alternative to pigmentation of polymer foils in the design of shields for radiative
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cooling applications. Thin PbS and PbSe films were deposited onto polyethylene foil from baths of different compositions and varying conditions. The optical properties of these films were measured in the UV/Vis/near IR (solar) and mid-IR (atmospheric-window) regions. Some of these films were shown to possess suitable optical properties for use as shields. Pigmented (scattering) shields have also been made by incorporating ZnS and ZnO into the polyethylene substrate. While these shields are not sufficiently blocking by themselves, a combination of PbS chemically deposited onto the pigmented polyethylene exhibited a substantial decrease of radiative transmission in the solar region.
Acknowledgements Y. Mastai thanks Prof. G. Levin for the assistance in the preparation of the pigmented polyethylene foils and for helpful discussions. We are grateful to Dr. D. Katzen of the Holon Academic Institute of Technology for suggestions and discussions regarding the manuscript.
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