Wideband perfect light absorbers covering visible and near-infrared solar optical radiations are significant for solar thermal energy systems. In this work, titanium and silicon dioxide multilayer thin film structures were investigated for perfect light absorption in wideband solar optical radiation spectral range from 400 nm to 2500 nm wavelength. Wideband solar light absorbers with different numbers of titanium and silicon dioxide layers and different layer thicknesses were designed, fabricated and characterized. It is found that the optimal wideband solar light absorber consists of only six titanium and silicon dioxide layers that form two cascade optical nanocavities, and increasing the number of optical cavities does not improve optical absorption in the solar spectrum. A super-wideband solar light absorber 97.97% total absorptance over the wavelength range from 400 nm to 1700 nm has been experimentally demonstrated.
© 2016 Optical Society of America
Single junction solar cells are widely used for solar energy harvesting. However, traditional single junction solar cells have limited energy conversion efficiency due to the Shockley-Queisser (SQ) limit. The efficiency of an ideal single junction solar cell cannot exceed 41% under direct solar illumination at the room temperature (300K) . Alternatively, a solar thermophotovoltaic (STPV) energy conversion systems potentially can overcome the SQ limit . STPV systems potentially have higher solar energy conversion efficiency than traditional single junction solar cells [3,4].The theoretical limit of maximal efficiency of STPV system was predicted as high as 85% . However, the efficiency of a recently demonstrated STPV system was only 3.2% . Fundamentally, an ideal STPV system requires a high efficiency wideband solar light absorber for harvesting solar optical energy. Previously, various light absorbing structures, such as multilayer metal-dielectric thin films [6–12], cermet [13–17], and black silicon [18–22], have been investigated. Multilayer metal-dielectric thin film structures were reported to achieve 95% total optical energy absorptance within the wavelength range from 400 nm to 1000 nm [6,7] and in the wavelength range from 400 nm to 1200 nm . Also, wideband solar light absorbers were reported to have near perfect absorption in the wavelength range from 250 nm to 1200 nm  and in the wavelength range from 350 nm to 750 nm [10,11]. Solar light absorbers covering visible range and the long tail of infrared of solar spectrum have never been investigated. However, since a significant amount of solar optical energy is in the spectral range beyond 1200 nm wavelength, an ideal solar light absorber should absorb solar energy radiations beyond 1200 nm. In this work, wideband solar light absorbers made of titanium and silicon dioxide thin film cascade cavity structures were investigated for absorbing solar optical energy in the wavelength range from visible to infrared up to 2500 nm and experimentally demonstrated a near unit total optical energy absorption over the solar spectral range from 400 nm to 1700 nm wavelength.
2. Wideband perfect light absorber structure and simulations
The cascade optical cavity multilayer absorber device structure is illustrated in Fig. 1. Starting from the top, a thin SiO2 layer, called “anti-reflection” (AR) layer, is to reduce the optical reflection and protect the device. Then, Ti and SiO2 thin films form optical cavities that create multiple resonance modes for trapping light and enhancing absorption. A 200 nm optically thick Ti layer deposited on a silicon wafer substrate at the bottom is used to form the last optical cavity and also to prohibit light transmission to the substrate. As it is shown in Fig. 1, N cavities structure has 2(N+1) layers.
To calculate optical absorptance (A) in the structure, we first calculated reflectance (R) and transmittance (T) by using the Transfer Matrix method . For TE polarization of angular incidence, the dynamical matrix Di in the medium i is,Eq. (4) as,
We first calculate absorption in the solar absorber structure by changing thickness of each layers (metal, dielectric and antireflection layers) and then changing number of optical cavities for achieving maximal absorption in the wavelength range from 400 nm to 2500 nm. We first change the metal layer thickness while fixing the thickness of anti-reflection layer at 110 nm, the number of cascade cavities at 2, and the thickness of dielectric layer at 50 nm, 80 nm, 110 nm, and 140 nm, respectively. Calculated energy absorptance versus wavelength and titanium layer thickness for different SiO2 layer thicknesses are shown in Fig. 2. Absorptance values over 95% are enclosed by a white dash line. In Fig. 2, it is seen that the optical absorptance increases as increasing titanium layer thickness from 0 nm to 10 nm. Further increasing titanium layer thickness reduces optical absorptance. The device with 10 nm titanium layer absorbs maximal energy in the wavelength range from 400 nm to 2500 nm.
To find the optimal SiO2 layer thickness for wideband absorption, we fixed the top anti-reflection layer thicknesses at 110 nm, the number of optical cavities at 2, and the thickness of metal layer at 5 nm, 10 nm, 15 nm, and 20 nm, respectively. Fig. 3 shows calculated optical absorptance versus wavelength and dielectric layer thickness for different thicknesses of titanium layer. Optical absorptance values over 95% are enclosed by a white dash line. Simulation results show that the optimal thickness of SiO2 layer for absorption is 110 nm. The multiple absorption bands can be seen in Fig. 3(a) and Fig. 3(b) for thin Ti layers (5 nm and 10 nm). The multiple absorption bands are due to the coupling of cascade cavity modes. The wide absorption bands caused by strong coupling for titanium layer thickness of 5 nm and 10 nm red-shift as the SiO2 layer thickness increases as indicated in Figs. 3(a) and 3(b), which is due to the red-shift of the individual cavity resonance modes. For thick Ti layers of 15 nm and 20 nm, since coupling of two cavities is weak, only one absorption band can be seen in the wavelength range from 400 nm to 2500 nm.
To find the top AR layer thickness for maximal absorption, we calculated optical absorptance versus wavelength and anti-reflection layer thickness. We fixed the thickness of Ti layer at 10 nm, the thickness of SiO2 layer at 110 nm and the number of cascade cavities at 2. Optical absorptance values over 95% are enclosed by a white dash line circle. It is seen that anti-reflection layer thickness of 110 nm gives the best absorption as shown in Fig. 4. It also can be seen in Fig. 4 that absorption spectrum red-shifts as the AR layer thickness increases. The top SiO2 AR layer controls the coupling from free space to the first optical cavity. Under critical coupling condition, reflection can be reduced to zero.
Next, we investigated how the number of Ti/SiO2 layers affects absorption by fixing the top AR layer thickness at 110 nm, Ti layer thickness at 10 nm, SiO2 layer thickness at 110 nm, and bottom Ti layer thickness at 200 nm. Calculated absorption spectra with different number of cascade cavities (1, 2, 3, and 4) are plotted and shown in Fig. 5. It can be seen in Fig. 5 that optical absorption of one-cavity structure decreases dramatically in infrared region (the green line curve). Absorption of three-cavity structure is below 90% in the wavelength range from 1500 nm to 2200 nm (the blue line curve). The absorption of four-cavity structure is below 90% in the wavelength range from 1600 nm to 2500 nm (the black line curve).The two-cavity device gives best absorption in the wavelength range from 400 nm to 2500 nm (the red line curve). The super-wideband perfect absorption in the two-cavity device is caused by coupled multiple Fabry-Perot resonance modes in the structure. Previously, it has been reported that the coupled Fabry-Perot cavities can significantly enhance the absorption and increase the absorption bandwidth . Earlier, dielectric-metal periodic thin film structures made of SiO2/Cu/SiO2 was reported to achieve near perfect absorption in visible spectral range  and increasing the number of dielectric-metal-dielectric layers, i.e. number of cavities, slightly increases the absorption. However, for the materials system in this work, Ti/SiO2 multilayer structures with different number of cascade optical cavities have completely different behavior of absorption. Our calculations indicate that two cascade optical cavities gives best absorption in the wavelength range from 400 nm to 2500 nm, and increasing number of cavities more than two does not improve absorption in the solar spectrum.
Ideally, solar light absorbers should have wideband perfect absorption at a wide angular range of incidence for both TE and TM polarizations. To investigate angular performance of wideband absorption, we calculated optical absorptance in a two-cavity structure versus the angle of incidence and wavelength for TE polarization, TM polarizations and un-polarized light. The device consists of 110 nm SiO2, 10 nm Ti, 110 nm SiO2, 10 nm Ti, 110 nm SiO2, and 200 nm Ti on a silicon wafer substrate. The calculation results are shown in Fig. 6(a), 6(b) and 6(c) for TE polarization, TM polarization, and un-polarized light, respectively. Optical absorptance values over 95% are enclosed by a white dash line. For TE polarization, absorptance slightly decreases in the near infrared region as increasing incident angle from 0 degree to 80 degree. For TM polarization, absorptance is over 90% when incident angle is less than 80 degree. For un-polarized light, optical absorptance slightly decreases in the near infrared region as increasing incident angle from 0 degree to 80 degree. Similar angular dependence of absorption was reported in thin film solar light absorbers previously [6,8,9].
3. Experimental results
To fabricate wideband solar light absorbers, a Denton sputtering system was used to deposit silicon dioxide (SiO2) and titanium (Ti) thin films on silicon wafer substrates. For deposition of Ti layers, we set DC power at 200 watts, argon pressure at 5 mTorr, and pre-sputtering conditioning time at 120 seconds. For deposition of SiO2 dielectric layers, we set RF power at 200 watts, argon pressure at 5 mTorr, and pre-sputter conditioning time at 180 seconds.
For fabricating two-cavity solar light absorbers, a 200 nm Ti layer on silicon wafer was first deposited. Then, a 110 nm SiO2 layer, a 10 nm Ti layer, a 110 nm SiO2 layer, and a 10 nm Ti layer, were deposited sequentially on the 200 nm Ti film surface. Finally, a 110 nm thickness of SiO2 layer was deposited as the anti-reflection layer on top of the device.
Also, four sets of devices were fabricated. In each set of devices, we varied only one parameter but leave other three parameters constants. Four absorber devices with four different Ti layer thicknesses of 5 nm, 10 nm, 15 nm, and 20 nm were first fabricated and then four devices with four SiO2 dielectric layer thicknesses of 50 nm, 80 nm, 110 nm, and 140 nm were fabricated afterwards. Also we fabricated four devices with four different anti-reflection layer thicknesses of 50 nm, 80 nm, 110 nm and 140 nm, and four devices with four different numbers of optical cavities (N= 1, 2, 3, and 4).
Optical reflectance spectra from all fabricated devices were measured with a broadband light and two optical spectrometers (StellarNet, Inc.) for covering the wavelength range from 400 nm to 1700 nm. The measured reflectivity from a glass wafer was used as the reference to obtain the absolute optical reflectance (R) from fabricated devices. The optical reflectance was obtained by normalizing measured reflection from fabricated device to measured reflection from a glass wafer and then multiplying the calculated reflectivity of 7% from the glass wafer. Absorptance (A) was obtained by using the law of energy conservation.
Measured optical absorptance spectra from fabricated devices with different layer thicknesses are plotted and shown in Fig. 7. The absorption spectral range is from 400 nm to 1700 nm because it is limited by the range of our optical spectrometer. Fig. 7(a) shows measurement results for different Ti layer thicknesses. It can be seen in Fig. 7(a) that 10 nm thick Ti device gives best optical absorption in the spectral range. The optical absorptance is over 95% in wavelength range from 400 nm to 1700 nm. Device pictures taken under ambient light are shown as the inset in Fig. 7(a). The 10 nm Ti layer device is seen to have an absolute black color. Dark blue colors are seen from other devices. Fig. 7(b) shows measured optical absorptance from devices of different SiO2 layer thicknesses. It can be seen in Fig. 7(b) that the 110 nm thick SiO2 device gives best optical absorptance in the spectral range. Pictures of fabricated devices are shown as the insets of Fig. 7(b). Devices with 80 nm and 110 nm SiO2 layers exhibit absolute black colors. Dark blue colors are seen from devices with 50 nm and 140 nm SiO2 layers. Measured optical absorptance spectra from devices with different anti-reflection layer thicknesses are shown in Fig. 7(c). It can be seen in Fig. 7(c) that the device with 110 nm AR coating gives best absorption in the wavelength range from 400 nm to 1700 nm. Pictures of four fabricated devices are shown as the insets in Fig. 7(c). Absolute black colors are seen from the devices with 80 nm and 110 nm anti-reflection layer thicknesses. Dark blue colors are seen from the devices with 50 nm and 140 nm anti-reflection layer thicknesses. Absorptance spectra from devices of different numbers of optical cavities (1, 2, 3, and 4) were measured. Measurement results are shown in Fig. 7(d). It is seen in Fig. 7(d) that the two-cavity device gives best absorptance in the wavelength range of 400 nm-1700 nm. Pictures of the devices are shown as insets in Fig. 7(d). Dark colors are seen from all devices, indicating perfect light absorption over visible spectrum.
Total optical absorptance (α) over the spectral range from λ1 to λ2 of a wideband absorber can be calculated by using the solar optical radiation spectrum Lsun(λ) and the optical absorption spectrum A(λ) of the device. The total absorptance (α) over the wavelength range from λ1 to λn is defined as,
With measured absorption data of the best absorber device with two cavities, we calculated total absorptance over the wavelength range from λ1= 400 nm to λn= 1700 nm by using Eq. (7). The total absorptance of our best device is 97.97% over this spectral range. With simulated absorption data of the best structure, we also calculated the total absorptance. The calculated absorptance is 96.99% over the wavelength range from 400 nm to 1700 nm and is 96.07% over the wavelength range from 400 nm to 2500 nm.
Measured and simulated total optical absorptance results are summarized in Table 1. It can be seen in Table 1 that the fabricated best absorber has a total absorptance of 97.97% over the spectral range from 400 nm to 1700 nm wavelength. It is also seen in Table 1 that measured total absorptance is slightly higher than simulated total absorptance in the same wavelength range. This is due to the additional absorption loss in Ti and SiO2 thin film materials caused by the imperfect material deposition processes.
In this work, wideband solar light absorbers consisting of titanium (Ti) and silicon dioxide (SiO2) multilayer thin films were investigated for absorbing solar optical radiations from visible to near-infrared. It was found that the best solar light absorber consists of only six Ti and SiO2 layer thin films that form two cascade optical nanocavities. Increasing the number of optical nanocavities to more than two does not give better performance of absorption for solar light. With simulations, we have shown that the two-cavity structure absorber can absorb 96.07% solar optical energy over a super-wideband solar spectral range from 400 nm to 2500 nm wavelength. Experimentally, we demonstrated a wideband solar light absorber that absorbs 97.97% optical energy over the solar optical spectral range from 400 nm to 1700 nm wavelength. It was explained that the wideband solar light absorption is due to the coupling of multiple Fabry-Perot resonance modes in the multiple cavities when the titanium metal layer thickness is small and chosen properly. The titanium layer thickness is critical for achieving wideband absorption. The cavity SiO2 dielectric layer thickness controls the location of the absorption band. Changing SiO2 dielectric layer thickness shifts the absorption band. The demonstrated super-wideband solar light absorbers are significant for solar-to-thermal energy harvesting and future thermophotovoltaic solar energy systems.
This work was partially supported by the National Science Foundation under the award no.1158862. J. Chen acknowledges the support from the Alabama Graduate Research Scholars Program (GRSP). J. Guo acknowledges the Individual Investigator Distinguished Research Award from the University of Alabama in Huntsville. Correspondence should be sent to J. Guo via email@example.com.
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