Stability of SiNX/SiNX double stack antireflection coating for single crystalline silicon solar cells
© Lee et al; licensee Springer. 2012
Received: 10 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Double stack antireflection coatings have significant advantages over single-layer antireflection coatings due to their broad-range coverage of the solar spectrum. A solar cell with 60-nm/20-nm SiNX:H double stack coatings has 17.8% efficiency, while that with a 80-nm SiNX:H single coating has 17.2% efficiency. The improvement of the efficiency is due to the effect of better passivation and better antireflection of the double stack antireflection coating. It is important that SiNX:H films have strong resistance against stress factors since they are used as antireflective coating for solar cells. However, the tolerance of SiNX:H films to external stresses has never been studied. In this paper, the stability of SiNX:H films prepared by a plasma-enhanced chemical vapor deposition system is studied. The stability tests are conducted using various forms of stress, such as prolonged thermal cycle, humidity, and UV exposure. The heat and damp test was conducted for 100 h, maintaining humidity at 85% and applying thermal cycles of rapidly changing temperatures from -20°C to 85°C over 5 h. UV exposure was conducted for 50 h using a 180-W UV lamp. This confirmed that the double stack antireflection coating is stable against external stress.
Silicon nitride films are widely used in semiconductor device industries as well as in photovoltaic industries due to their strong durability, good dielectric characteristics, and resistance against corrosion by water [1, 2]. Hydrogenated silicon nitride films can improve reflectance and surface passivation .
A single-layer antireflection coating is known to be unable to cover a broad range of the solar spectrum [4, 5], and using double-layer antireflection coating is considered. There have been reports of using double-layer antireflection coatings of two different materials, such as MgF2/CeO2, SiO2/TiO2, MgF2/TiO2, SiO2/SiN, and MgF2/ZnS [6–8]. Two materials with different refractive indices are stacked together for double stack antireflection coating. This may be more vulnerable to outside stress. Solar cells operate in an external environment, and it is important that the surface of the solar cells endures various kinds of physical conditions. Thus, the antireflection film of solar cells should have strong resistance against a number of stress factors. SiNX:H thin film is often used as antireflection coatings. Its stability against ultraviolet light should be verified since it absorbs most of the ultraviolet light of the short wavelength region . SiNX:H thin films deposited by plasma-enhanced chemical vapor deposition [PECVD] contain about 8% to approximately 30% (atom) hydrogen and are easily affected by moisture. Thus, the analysis of the stability against various stresses is necessary. However, little research has been conducted on the stability of SiNx used as antireflection coating [ARC] or solar cells.
In this paper, the stabilities of SiNX:H thin films deposited under various conditions and double stack SiNX:H thin films with different refraction indices are studied by applying different kinds of stress. Solar cells with double stack antireflection coatings are fabricated, and their characteristics are analyzed.
Single layers of SiNX:H thin films are first studied to find the appropriate deposition conditions and to verify the stability and reliability of the double stack antireflection coating. A p-type crystalline silicon wafer with a sheet resistance of 1 to approximately 3 Ω cm and < 100 > orientation is used as the substrate for the deposition of thin films. The wafer is doped with phosphorous in a furnace using a conventional POCl3 diffusion source at 830°C for 7 min. Phosphorus silicate glass [PSG] is removed by dipping the wafer in 10% hydrofluoric acid [HF] solution for 30 s. The drive-in process is conducted for 25 min at 860°C. Next, a second doping process at 810°C is followed for 7 min. PSG is removed by dipping the wafer in 10% HF solution for 30 s. SiNx deposition is conducted in the environment of N2 at 450°C with a radio frequency [RF] power of 180 mW/cm2. The ratio of SiH4:NH3 is varied. The flow rate of NH3 is fixed at 200 sccm, and the flow rate of SiH4 is varied. Double stack SiNX:H with refractive indices ranging from 1.9 to 2.3 is prepared. All the samples are co-fired in a conveyer belt furnace. The effective minority carrier lifetimes are determined with the microwave photo conductance decay technique via quasi-steady state photoconductance using the WCT-120 silicon wafer lifetime detector (Sinton Consulting Inc., Boulder, CO, USA) before and after applying a stress. Fourier transform infrared spectroscopy [FT-IR] characteristics are measured using Shimadzu IR Prestige-21 (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan).
A temperature cycle with a maximum of 80°C and a minimum of -20°C within 5 h is used to test the stability against the temperature of SiNX:H thin films. Twenty temperature cycles, i.e., 100 h, are applied to see the effect of the constantly changing temperature, fixing the humidity at 85%.
The samples are exposed to ultraviolet light using a 180-W UV lamp to test the stability against ultraviolet rays. First, they are exposed to the UV light for 5 min six times; 15 min for the next six times; 30 min for the next six times; 1 h, five times; and then 10 h, five times.
Finally, the solar cells are fabricated. A 5-in. p-type crystalline CZ-Si solar-grade wafer of around 200 μm thick having a specific resistance of around 1 to approximately 3 Ω cm with < 100 > orientation is used as the substrate for the deposition of thin films. A 2% NaOH solution is used for pyramidal texturing of the Si wafer. The wafers are dipped for 25 min in the 2% NaOH etching solution maintained at 84°C to approximately 86°C. All textured p-type silicon wafers are then doped with phosphorus in a furnace using a conventional POCl3 diffusion source first at 830°C for 7 min. PSG is removed by dipping the wafer in 10% HF solution for 30 s. Then, the drive-in process is conducted at 860°C for 25 min. The second doping is done at 810°C for 7 min. The SiNX film is then deposited on the substrate using the PECVD technique. During deposition, RF power, plasma frequency, pressure, and substrate temperature are maintained at 180 mW/cm2, 13.56 MHz, 0.5 to approximately 0.8 Torr, and 450°C, respectively. The gas flow rates of NH3 and N2 are maintained at 200 sccm and at 85 sccm, respectively, for the double stack antireflection coating of the SiNX:H film on the silicon wafer; whereas, the SiH4 flow rate is set at 20 and 80 sccm for each layer. Back metallization is conducted with a standard aluminum paste using the screen-printing technique. The samples are then baked and co-fired in a conveyer belt furnace. The effective carrier lifetime and efficiency characteristics are measured using Sinton WCT-120 (Sinton Consulting Inc., Boulder, CO, USA) and Pasan cell tester CT 801 (Pasan Measurement Systems, Neuchâtel, Switzerland). Reflectance characteristics are measured using Scinco S-310 (Scinco S-310, Seoul, Korea).
Results and discussion
Solar cell characteristics
The double stack film has a lifetime of 50 μs, which is not worse compared with other thin films. The lifetime of the film with a refractive index of 2.0 decreases from 66 μs to 54 μs after 50 h of UV exposure. Its decay rate is 18.2%. The lifetime of the double stack film decreases by 13.1% from 50 μs to 44 μs. For other thin films with refractive indices from 1.9 to approximately 2.3, the average lifetime decay rate is 18.7%, which is higher than the double stack film by more than 5%. This proves that the solar cells with double stack antireflection film are stable against UV light.
A thin film with a refractive index of 2.3 has a high reflectivity of 3%, and a film with a refractive index of 1.9 has a reflectivity below 2%. In all cases, the reflectivity remains almost unchanged after the heat and damp test. This means that the thin films fabricated by PEVCD are not directly affected by the rapid change in temperature and the humidity.
It is known that solar cells with double stack antireflection coating have better efficiency than those with single-layer ARC. The same results are obtained in our experiments. The solar cell with a 60-nm/20-nm SiNX:H double stack antireflection coating has 17.8% efficiency, while that with an 80-nm SiNX:H single-layer antireflection coating has 17.2% efficiency. The improvement of the efficiency is due to the effect of better passivation and better antireflection of the double stack antireflection coating. However, studies on the stability against outside environment for double stack ARC are seldom conducted.
The effects of temperature, humidity, and UV exposure on the SiNX:H thin films with different gas ratios were investigated, and the stability of the double stack antireflection coating thin film was examined. First, single-layer antireflection coatings were studied to establish the deposition conditions, and the results were applied to the double stack antireflection coating. The passivation of the thin films with various refractive indices was also studied. After the temperature and humidity test for 100 h, the carrier lifetime of the thin film decreased by 7.5%. The lifetime decreased by 13.1% after the UV exposure test. These are better results than those obtained for the average of single layers, 8.9% after the heat and damp test and 18.72% after UV exposure. The stability of double stack antireflection coatings has been experimentally confirmed.
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0018397).
This research was also supported by the World Class University (WCU) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10029-0).
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