Patent Application
20120214319
This invention relates to improving the passivation effect of a substrate with a film, and more particularly to silicon oxide thin films on a silicon substrate.
Crystalline silicon solar cell remains the most popular product in the photovoltaic industry in spite of the challenge from other low cost but low efficiency product such as thin film solar cell. The trend to go for thinner wafer calls for the application of advanced solar cell design. PERC (Passivated Emitter and Rear Cell) structure developed in 1980's are one of the most popular approaches for low cost high efficiency solar cell production, which has been scaled up by Suntech as the Pluto solar cell.
Surface passivation is a vitally important issue for PERC design. Surface passivation may be described as a process which reduces the density of available electronic states present at the surface of a semiconductor, thereby limiting hole and electron recombination possibilities. A high surface recombination velocity of electron and hole reduces the light generated current extracted by the solar cell therefore lower the cell efficiency. The so called “dangling bonds” in an incomplete surface usually act as the recombination centers for the hole and electron generated at the surface or approaching to the surface from inside. Surface passivation attempts to erase or disable these recombination centers. There are a few ways to accomplish surface passivation, including dielectric film coating on the surface to satisfy the dangling bonds, using an electric field to repel the minority carriers from the surface, or a combination thereof.
Another method for the passivation of silicon (Si) surfaces is the thermal oxidation at high temperature (˜1000° C.). Thermal oxide is perfect for the rear surface passivation with an “alneal” process (an annealing process for thermal oxide coated by Aluminum film). However, the high temperature as well as long time process during the thermal oxide growth can severely degrade the bulk carrier lifetime and are undesirable from production cost and throughput considerations. Hence, significant effort has been devoted in recent years to the development of low temperature (<500° C.) surface passivation schemes as an alternative to the high temperature oxidation of silicon. One successful approach is plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (SiNx). PECVD grown SiNx is currently popular in the Si solar cell manufacturing process due to the ability to provide both anti-reflectance and surface passivation of the cell. Other alternatives of dielectric material include Al2O3 grown by atomic layer deposition (ALD), amorphous Si, and the like.
Liquid phase deposition (LPD) silicon oxide represents a low cost process to deposit silicon oxide on silicon at nearly room temperature, by preventing using high temperature furnace or large vacuum deposition chamber. However, the as-deposited silicon samples usually show poor surface passivation effect, for example, low minority carrier lifetime. The following description describes a process that enhances the surface passivation of substrates with poor surface passivation.
In an illustrative implementation, a film deposited on substrate may originally have a high surface recombination velocity (SRV). By annealing in a gas ambient containing O2 or O2− at high temperature annealing and/or annealing in a Forming Gas (FG) at mild temperature, the SRV is extremely suppressed and the minority carrier lifetime shows orderly increased. Additionally, the passivation may be achieved using the same production steps normally applied to the solar cell to create its top and bottom metal contacts, and no additional heating cycles are required. The synergistic nature of this technology with existing cell fabrication steps will greatly simplify the standard silicon solar cell manufacturing process.
The foregoing has outlined rather broadly various features of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter.
For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein:
a and 1b show effective minority carrier lifetime comparison between the as-deposited sample, after O2 annealing, and after O2+FG annealing for p-type wafers and n-type wafers;
a and 2b show measured effective lifetime of samples after six weeks of storage in an ambient cleanroom for p-type wafers and n-type wafers;
Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.
Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing particular embodiments of the disclosure and are not intended to be limiting thereto. While most of the terms used herein will be recognizable to those of ordinary skill in the art, it should be understood that when not explicitly defined, terms should be interpreted as adopting a meaning presently accepted by those of ordinary skill in the art.
A thin film deposited on substrate, such as by a LPD method at near room temperature, may have a high surface recombination velocity (SRV). Nonlimiting examples of a thin film may include metal oxides with a formulation as MxOy or LxMyOz (where L and M are metal elements, O is oxygen element); metal sulfides with a formulation as MxSy (M is metal elements, S is sulfur element); and metal selenides with a formulation MxSey (M is metal elements, Se is selenium element). For example, thin films, such as silicon oxide (SiOx), SiO2, TiO2, ZrO2, In2O3, SnO2, BaTiO3, ZnS, Bi2Se3, and/or the like, may be placed on a silicon substrate in solar cells. By treatment of O2 high temperature annealing and/or mild temperature Forming Gas (FG) annealing, the SRV is extremely suppressed and the minority carrier lifetime shows orderly increased. In other implementations, O2 may be substituted with any gas ambient that contains O2 or O2−, such as, but not limited to, purified air, purified oxygen, N2 and O2 mixture, purified DI water steam, or the like. In other implementations, the FG may be substituted with any gas ambient that contains H2 or H+, such as, but not limited to, purified H2, purified DI water steam, or the like. The first annealing step in O2 ambient at 700-1050° C. may be for a duration of 30-120 seconds. The second annealing step in a Forming Gas at 500° C. may be for a duration of 300 seconds or greater. It will be recognized that annealing duration is highly dependent on temperature. In some implementations, annealing duration for the second annealing steps may be 60 seconds or greater. In some implementations, the annealing temperature may be in the range of approximately 200-600° C. This passivation is achieved using the same production steps normally applied to the solar cell to create its top and bottom metal contacts, and no additional heating cycles are required. The synergistic nature of this technology with existing cell fabrication steps will greatly simplify the standard silicon solar cell manufacturing process.
The O2 annealing process may preferably performed in a fast firing furnace in a Si solar cell product line designed for the metal contact so that no additional heat cycles are needed. Considering that the optimal temperature/dwelling time might be harsh for currently widely used screen printed metal pastes that work with SiNx, alternative electrode materials may be desired, such a metal paste material, suitable deposited metal film, or the like that works reasonably during the O2 annealing process.
Possible variations may include, but are not limited to:
The following experimental examples are provided for illustrative purposes only. The various aspects described in the examples merely represent exemplary implementations. It will be recognized by one of ordinary skill in the art that various changes can be made in the implementations described without departing from the spirit and scope of the present disclosure.
Sample Preparation:
The reagent solution for the LPD growth of silica was prepared by saturating a ratio of 1 liter of 3 M hexafluorosilicic acid (H2SiF6) with 60 g 0.007 μm fumed silica powder at room temperature. After overnight saturation, the solution was filtered, first with a course VWR Grade 315 fluted filter for 25 μm particle retention, then with the Millipore Stericap system using 0.22 μm filters. The solution was then diluted to 1 M by adding 18 MOhm DI water.
The addition of water initiated the reaction and precipitated the silica according to
H2SiF6+2H2O→SiO2↓+6HF
Both N-type doped and P-type doped silicon wafers with a resistivity of about 3 Ohm-cm and a thickness of about 525 μm were used. The silicon wafers cleaned by standard procedures were immersed in the solution at a temperature of 30° C. The silicon dioxide film was deposited on the wafers with a growth rate about 40 nm per hour. A series of SiOx film thickness (7.3 nm˜167.4 nm) were obtained by controlling the growth time. The refractive index of the as-deposited film was about 1.43 which is slightly lower than that of thermal oxide (n˜1.46).
While the experimental examples discussed specifically utilized SiOx films deposited on silicon wafers using a LPD process, the systems and methods for improving the passivation effect substrates may be utilized on any suitable films formed by any deposition process.
Post-Annealing:
The as-deposited sample was placed in a programmable rapid thermal processer to undergo annealing in O2 and Forming Gas ambient according to the parameters listed in Table 1. There was about one hour of interval between the two steps of annealing to allow the intermediate characterization. For comparison, single step of Forming Gas annealing, and O2/Forming Gas two steps of annealing were also performed using about the same parameters.
Characterization:
To evaluate the passivation effect of the LPD SiOx, a quasi-steady state photoconductance (QSSPC) lifetime measurement was performed with Sinton WCT120 at an injection level of 1×1015 cm−3 for each sample before and after each step of annealing. Some of the samples were tracked for months to identify the stability of the passivation effect in standard cleanroom ambient. secondary ion mass spectrometry (SIMS) measurements was performed for certain samples to explore the film composition variation induced by the annealing.
Results:
a and 1b show the effective minority carrier lifetime comparison between the as-deposited sample, after O2 annealing only, and after O2+FG (two steps) annealing. The samples were measured immediately (in approximately ten minutes) after they were taken out of the annealing chamber. As can be seen, the lifetime increases mildly (up to 6 times) after O2 annealing alone and increases sharply (about 20 times for N-type wafers and about 2 orders for P-type wafers) after O2+FG annealing. It is theorized that annealing in O2 can substitute F content in the as deposited LPD-SiOx film with O, leading to a more purified SiOx structure that has fewer electron and hole trap centers. Theoretically, with the high temperature annealing in O2 ambient, the weak Si—F bonds are driven out, leaving only strong Si—F bonds in the film. Therefore, trap concentration relating to the incorporation of F in a SiO2 film is reduced. On the other hand, the atomic hydrogen can diffuse to the Si/SiOx interface during FG annealing to reduce the interface state density by reacting with the dangling bonds. In brief, the annealing in O2 and FG causes an orderly smaller surface recombination velocity due to the reduction of trap concentrations at the interface and oxide film thus increases the lifetime significantly for the minority carriers.
a and 2b show O2 annealing temperature dependent effective minority carrier lifetime of p-type wafers and n-type wafers. With the FG annealing condition fixed, the dependence of the effective lifetime on O2 annealing temperature and dwelling time has been examined.
The stability of the annealing effect has been examined by tracking the effective lifetime of the post-annealed samples, as shown in
By enhancing the surface passivation effect, the process will improve the device performance of the silicon solar cell that use LPD deposited silicon dioxide as the first layer coating on its surfaces. The process makes the LPD deposited silicon dioxide comparable to thermal oxide in term of surface passivation effect of Si substrate, potentially promoting the industrial application of LPD silicon dioxide to reduce the cost of the Si solar cell.
LPD deposited silicon dioxide is a low temperature process to achieve dielectric thin film on Si substrate, potentially reducing the energy consumption and the wafer thickness used in the fabrication of crystalline Si solar cells. The effective lifetime of minority carriers is a critical index to evaluate the passivation effect. To our best knowledge, there seems no report to date about how the minority carrier lifetime can be increased by annealing for the Si substrate with the LPD deposited silicon dioxide film. Our experiments demonstrated for the first time that the effective lifetime could be significantly improved by the annealing process compared to as-deposited samples, which represents a new feature. We believe that both the interface state density and the trap density in the film were significantly reduced after annealed in O2 and FG subsequently.
Implementations described herein are included to demonstrate particular aspects of the present disclosure. It should be appreciated by those of skill in the art that the implementations described herein merely represent exemplary implementation of the disclosure. Those of ordinary skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific implementations described and still obtain a like or similar result without departing from the spirit and scope of the present disclosure. From the foregoing description, one of ordinary skill in the art can easily ascertain the essential characteristics of this disclosure, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt the disclosure to various usages and conditions. The implementations described hereinabove are meant to be illustrative only and should not be taken as limiting of the scope of the disclosure.
This application claims the benefit of U.S. Provisional Patent Application No. 61/442,461 to Yuanchang Zhang, filed on Feb. 14, 2011, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61442461 | Feb 2011 | US |
