This work demonstrated a-Si:H thin-film solar cells with backside TiO2 / SiO2 distributed Bragg reflectors (DBRs) for applications involving building-integrated photovoltaics (BIPVs). Selectively transparent solar cells are formed by adjusting the positions of the DBR stop bands to allow the transmission of certain parts of light through the solar cells. Measurement and simulation results indicate that the transmission of blue light (430 ~500 nm) with the combination of three DBR mirrors has the highest increase in conversion efficiency.
© 2012 OSA
Solar cells are the cleanest, safest, and most inexhaustible among all alternative energies for electricity generation. Among the many materials used to manufacture solar cells include silicon, CdS / CdTe [1,2], Copper indium gallium selenide (CIGS) [3,4], organic polymers , PbSe quantum dots (QDs) , and InAs QDs . Silicon-based solar cells with hydrogenated amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) are the most ideal and abundant materials of those mentioned above. A reason for their advantage is the bandgaps for a-Si:H and μc-Si:H are equivalent to 1.7 eV and 1.1 eV, respectively, which are extremely close to the theoretical values  for ideal tandem solar cells  with maximum conversion efficiency. The μc-Si:H single cell has a wider optical response under the sun illumination and a higher conversion efficiency than a-Si:H . Additionally, a-Si:H has the property of light-induced degradation known as the Staebler-Wronski-effect , which restricts the typical film thickness of 200 ~400 nm. In brief, while μc-Si:H solar cells are characterized by a high conversion efficiency with a long and stable operation time, a-Si:H solar cells are inexpensive for production, large area panel integration, and flexibility [12,13].
The absorption coefficient spectra of amorphous and microcrystalline silicon  reveal that the absorption coefficients decrease with the red shift of wavelength. If film thickness is restricted for defect free and high quality purposes, silicon-based solar cells in the wavelength ranges of the red and near-IR parts fail to totally absorb incident light once. Of the many studies that have attempted to improve light absorption in the absorber layer by increasing the optical path length, one method uses back reflectors in the bottom of cells to reflect the light into the absorber layer again. Although metallic back reflectors can also act as electrodes, metals suffer from intrinsic absorption losses incurred at interface. This absorption can be reduced by adding the optical spacing layer [14,15]. One-dimensional distributed Bragger reflectors (DBRs) with a high reflectance (~100%) can double the path length over a wide range of incident angles and wavelengths. The broadband back reflectors can be achieved by connecting with two DBR mirrors . The above mentioned back reflectors are also integrated with in-plane periodic structures at the bottom interface of cells. Like metallic periodic nano-hole or nano-pillar arrays [17–20], dielectric DBR with 1-dim diffractive grating [21–23] or two dimensional photonic crystal [24,25] are examples of integration between in-plane periodic and vertical mirror structures. A previous study theoretically estimated the power conversion efficiencies under the same p-i-n structures for dielectric and metal back reflectors . The metallic back reflector with one-dimensional grating has additional surface plasmon losses and exhibits a lower efficiency than that of a planar metal one.
A recent study indicated that a-Si:H has the application of building-integrated photovoltaics (BIPV) offers the electricity generation, shielding, thermal insulation, and semi-transparency without wasting capital and space . Dielectric DBR has light transparency within a portion of incident angles or wavelengths , whereas metallic BR suffers from a lack of light transparency. In addition to offering light transparency partially at a visible wavelength, DBRs with n-type a-Si:H / ZnO:Al  can also act as a back contact. The selectivity transparent and conducting photonic crystals (STCPC) rear contact  composed of indium tin oxide (ITO) films and antimony-doped tin oxide (ATO) nanoparticles also offer a certain amount of light transparency. A previous study designed a backscattering layer with elliptical metal nanoparticles  to transmit light with a normal incidence and reflect the incident light with an angle into the solar cells. These back reflectors have also been used for BIPV applications [29,30]. The photonic design principle to reduce thermodynamic losses and enhance solar cell efficiency was also discussed .
In this work, dielectric distributed Bragg reflectors put on the backside of n-type ZnO transparent contact are used to improve the energy conversion efficiency of BIPV and select colors of light passing through the BIPV simultaneously. The select colors of light for BIPV can be achieved within the gap between two neighboring stop band edges. Thus, the landscape can be observed on the other side of the BIPV, and there is no influence on the operation of ZnO contact.
2. Distributed Bragg reflector structure designs and reflectance measurements
DBR which consists of transparent oxides, TiO2 and SiO2, is placed on the backside of ZnO for the contact of silicon thin-film solar cells. The colors of light reflected back to BIPV are determined by the overlap between the spectral reflectance of DBR and a visible light spectrum. Figure 1(a), (b) , and(c) illustrate an example of wavelength selectivity transparent DBR by combining two mirrors. The green and red light are mainly reflected back into solar cell by the DBR1, and the wavelengths of light below 500 nm are not reflected entirely as shown in Fig. 1(a). The select colors of light can be achieved by adding another DBR to reflect most of transmission light. The blue light are mainly reflected by DBR2 as shown in Fig. 1(b). By controlling the positions of stop bands for DBR1 and DBR2, the select colors of light can be passed through the BIPV. Figure 1(c) shows a transmission between two stop bands at a wavelength of 500 nm, and light around this wavelength can be detected by visual inspection. It is also possible to combine more DBRs not only to create many transmission peaks inside the whole visible spectrum but also to increase the efficiency of solar cells.
Experiments in this work considered a combination of three DBR mirrors, and simulation of spectral reflectance for TiO2 / SiO2 alternating pairs were obtained by using the transfer matrix method [32,33]. The simulation also considered material absorption (e.g., imaginary part of refractive index) and wavelength dispersion. Each layer thickness di is equal to λ/4ni, where di and ni denote the thickness and refractive index of material i, and λ represents the target wavelength situated at the midpoint of stop band. The target wavelength is also called Bragg wavelength, denoted as λB. Amorphous Silicon thin-film solar cells were fabricated by using the very high frequency plasma-enhanced chemical vapor deposition system (VHF-PECVD, 40 MHz) at 200 °C. The cell structure includes a 10 nm a-Si p-layer, a 200 nm light absorbing a-Si layer and a 15 nm a-Si n-layer, which were deposited on a commercial Asahi glass substrate. The TCO layer (ZnO:Al) was then deposited by pulsed dc magnetron sputtering for electrodes. Three DBRs were deposited on the glass-based thin-film solar cells (ZnO / p-i-n a-Si:H / ZnO:Al) by an optical-monitored coating system. The following measurement of spectral reflectance contains the DBR structure and a-Si:H solar cells, whereas the simulation considers only the DBR structure and n-type ZnO contact at normal incidence.
Figure 2(a) illustrates the DBR structure with a combination of three mirrors. The Bragg wavelength λB2 and λB3 for the purpose of reflecting red and near-IR light are equal to 600 nm and 810 nm, respectively. The refractive indices of TiO2/SiO2 under these Bragg wavelengths are approximately equal to 2.49/1.45. Meanwhile, the Bragg wavelength of top DBR λB1 is shifted to a shorter wavelength of 370 nm to create a transparent region between two stop bands. From the top to bottom, the Bragg wavelengths and number of pairs are 370 nm / five pairs, 600 nm / four pairs, and 810 nm / two pairs. The thicknesses of TiO2 / SiO2 in three DBRs are 31/60 nm, 60/101 nm and 83/138 nm, respectively. The ZnO layer functions as the back electrode of thin film solar cells. The incident light is transmitted through the ZnO contact and then reflected by the DBR structure. Figure 2(b) compares the reflectance spectra between simulation and measurement results. The high reflectance of Bragg reflectors caused by constructive interference of electromagnetic waves at ZnO surface can be achieved in the entire visible light spectrum, except for the wavelength range of 430 ~500 nm. The light transmission through BIPV is approximately 70% in the wavelength range of 430 ~500 nm. The estimated transmitted solar irradiance from 430 nm to 500 nm without considering a-Si:H absorption is about 71.8 W/m2. Figure 2(c) is the image of the cell with the DBRs under the white light source. The blue color from the cell indicates that the DBR structures take a strong effect to select the color of the Si thin film solar cell. The transmitted color of the cell could also be manipulated by varying the bottom DBR structures which can select different display colors of the BIPV cell.
3. Measurement results and EQE simulation
This work also investigated the photovoltaic performance of BIPV by measuring current-voltage (I-V) curve and the external quantum efficiency (EQE) for a single solar cell with 1x1 cm2 size. The current-voltage (I-V) test of the cell was first performed under AM1.5 illumination (1 Sun) at room temperature, and the external quantum efficiency (EQE) spectrum of each cell was measured in the wavelength range of 300~800 nm. The p-i-n a-Si:H thin film sandwiched between ZnO and ZnO:Al deposited on glass are the reference cells. Figure 3(a) shows the measured characteristic of current density versus voltage (J-V) for the above mentioned DBR structure. The short-circuit photon current density yields from the value of 11.8 mA/cm2 (without DBR) to 13.5 mA/cm2 (with DBR). An efficiency of 4.66% is achieved for the BIPV, which is higher than that for the cells without DBR (3.678%). Figure 3(b) shows the corresponding EQE curves with or without DBR structure. The spectral resolution of measurement is equal to 10 nm. EQE of BIPV and reference cell are depicted by green and blue curves respectively. The kink of green EQE curve originates from the fabrication errors of quarter-wavelength layer thicknesses. The increased efficiency almost starts from the wavelength of 480 nm, implying that the additional mirrors that reflected the light in the wavelengths shorter than 480 nm fail to increase the conversion efficiency. The overall improved efficiency is defined as the difference in area of blue curve to that of red curve. According to our measurement result, the conversion efficiency for BIPV integrated with three type DBR mirrors can be increased up to 14.7%.
This work further confirmed the reliability of measurement results by using APSYS  to perform two-dimensional Finite Element simulations of the EQE spectra for this DBR structure. The APSYS simulator can model the electrical properties in connection with the light propagation simultaneously. The light propagation was determined by the transfer matrix method. Additionally, the electrical properties were obtained by Poisson’s equation as well as the current continuity equations for electrons and holes. The simulated structure consists of DBR / ZnO (800 nm) / n-a-Si:H (20 nm) / i-a-Si:H (200 nm) / p-a-Si:H (15 nm) / ZnO:Al (400 nm). The refractive indices and absorption coefficients of TiO2, SiO2, ZnO, a-Si:H and ZnO:Al are set as the functions of wavelength during simulations. The wavelength dependent refractive indices and absorption coefficients are shown in Fig. 4(a) and 4(b). The wavelength dependent refractive index and absorption coefficient of a-Si are obtained from the macro file of simulation software. Additionally, the thicknesses of alternating pairs are modified based on the fabrication result. AM1.5 sun illumination is normally incident on the top of the device, and the incident power transmission through the top surface is set to 100% without reflection. In this work, the electrons and holes were produced by illumination in the i-a-Si:H absorber layer, and the carrier transport was mainly by the drift in the electric field of PN junction. The doping of p-type and n-type a-Si is set as 5 × 1025 m−3. The electron affinity of p-i-n is set as 4 eV. The relative permittivity of p-i-n layers are set as 7.2, 11.9, and 11.9, respectivrly. The other electrical parameters for simulation of a-Si also obtained from the database of APSYS as shown in reference [35–37]. The parameters of mobility  for a-Si are shown in Table 1 . The electron mobility and hole mobility of ZnO are set as 100 and 25 cm2/V-sec. The electron affinity of ZnO is set as 4.3 eV. EQE enhancement is defined as the ratio of the number of collected charges to the number of incident photons. Figure 4(c) shows the simulated external quantum efficiencies for this BIPV integrated with three types of DBR mirrors. Simulation results indicate that the conversion efficiency is increased by 13.8%. EQE simulations for this DBR structure agree well with our measurement results.
In this study, the multiple DBRs structure was designed to increase quantum efficiency and select display color of the BIPV thin film solar cell. This work has compared three DBR back reflectors with TiO2/SiO2 on the a-Si:H thin-film solar cells. Adjusting the positions of one or more DBRs’ stop bands to overlap most of the visible spectrum allows for the selected wavelengths range out of the stop band or within the gap between two neighboring stop band edges to pass through BIPV. We also investigated the photovoltaic performance of BIPV by measuring current-voltage (I-V) curve and the external quantum efficiency (EQE) for the thin film cell with the DBRs. The Current results indicate that the optimum overall efficiency improvement in measurement and simulation is with three DBR mirrors, which transmits light in the wavelength range of 430 ~500 nm and reflects the others. Moreover, the landscape in the front of BIPV can be observed by visual inspection with overall EQE enhancement 14.7% in relation to the reference cell in our experiment.
The authors would like thank the Center for Nano Science & Technology, National Chiao Tung University (NCTU) for the fabrication facilities support. This study is based on research supported by the National Science Council (NSC) of ROC, Taiwan under Grant No. NSC-99-2112-M-001-033-MY3, NSC 100-3113-E-110-006- and by the Grant of the Academia Sinica, Taiwan.
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