BORON DIFFUSION METHOD SUITABLE FOR HETEROJUNCTION BACK CONTACT SOLAR CELLS

Abstract

A boron diffusion method suitable for HBC solar cells is provided. In a propelling step, a nitrogen and oxygen hybrid propelling mode is used, and a propelling time is increased, such that a junction depth can be effectively increased and a surface concentration can be effectively reduced. Moreover, a nitrogen propelling time is included, such that a concentration of boron in BSG is reduced, and the BSG is easier to be removed completely. For N-type solar cells, the method has excellent front surface doping capability, a thinner dead layer, and good light absorption capability. In addition, the method uses a horizontal diffusion mode and has good single-side performance, and utilizes a gravity press mode to make silicon wafers be placed back-to-back in horizontal grooves.

Claims

1. A boron diffusion method suitable for heterojunction back contact (HBC) solar cells, comprising: step (1) of boating entering, comprising: placing silicon wafers in a boat and then performing an entering operation for the boat, wherein during performing the entering operation for the boat, a furnace tube has a temperature of 700 degrees Celsius (° C.)-750° C. and simultaneously is maintained with 2000 standard cubic centimeter per minute (sccm) of nitrogen (N.sub.2) for continuous blowing; step (2) of heating-up and leak detection, comprising: heating-up after the boat enters the furnace tube until a target temperature of 800-850° C., vacuumizing while the heating-up, turning off a vacuum pump and performing leak detection with a leak detection requirement of less than 5 millibars per minute (mbar/min) after completion of the vacuumizing; and after completion of the leak detection, turning on the vacuum pump to adjust a pressure in the furnace tube until a target vacuum degree of 100-500 mbar; step (3) of pre-oxidation, comprising: after reaching the target temperature and the target vacuum degree, introducing 2000 sccm-10000 sccm of oxygen (O.sub.2) for pre-oxidation for 10-30 minutes (min); step (4) of evacuating, comprising: after completion of the pre-oxidation, evacuating the furnace tube for an evacuation time of 3-10 min until a target pressure of less than 200 mbar; step (5) of boron source introduction and deposition, comprising: introducing BCl.sub.3 gas and oxygen for an introduction time of 5-20 min as per a gas volume ratio of the BCl.sub.3 gas to the oxygen is 1:1˜1:3; step (6) of heating-up, comprising: after completion of boron source introduction and deposition, evacuating the furnace tube until a target pressure of less than 100 mbar and heating-up after completion of the evacuating until another target temperature of 1000° C.; step (7) of propelling, comprising: after reaching the another target temperature, alternately introducing N.sub.2 and O.sub.2 for propelling according to a ratio of a N.sub.2 propelling time to a O.sub.2 propelling time being 1:2˜1:3, a flow rate of the N.sub.2 being 3 standard liter per minute (SLM)˜10 SLM and a flow rate of the O.sub.2 being 5 SLM˜20 SLM in each time of propelling; and cyclically performing the propelling for 2-5 times; and step (8) of cooling down and boat exiting, comprising: cooling down and then performing an exiting operation for the boat, thereby completing a diffusion process.

2. The boron diffusion method suitable for HBC solar cells according to claim 1, wherein in the step (2), a heating rate is 10° C./min.

3. The boron diffusion method suitable for HBC solar cells according to claim 1, wherein in the step (7), after reaching the another target temperature, introducing 3000 sccm of the nitrogen for 10 min, introducing 5000 sccm of the oxygen for 25 min, introducing 3000 sccm of the nitrogen for 10 min, and introducing 5000 sccm of the oxygen for 25 min sequentially in that order.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 illustrates an electrochemical capacitance-voltage (ECV) diagram of boron diffusions respectively in a method according to an embodiment of the invention and a method of comparative example 1.

[0027] FIG. 2 illustrates photoluminescence (PL) pictures of photovoltaic solar cells obtained after boron diffusions using a method of a comparative example 1 and a method of the embodiment 1 of the invention, respectively.

[0028] FIG. 3 illustrates test positions during sampling and testing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Technical solutions of the invention will be described below in detail with reference to the accompanying drawings. It should be indicated that embodiments described below are merely detailed description of the invention and should not be construed as limiting the invention.

Embodiment 1

[0030] A boron diffusion method suitable for HBC solar cells includes steps as follows.

[0031] (1) Step of boat entering. Specifically, silicon wafers after being texturing cleaned are placed into a horizontal insertion-type quartz boat, the silicon wafers are inserted into horizontal quartz grooves in a back-to-back manner (e.g., adjacent two of the silicon wafers are placed in the same horizontal quartz groove in the back-to-back manner), and then the quart boat is performed with an entering operation to enter a quartz furnace tube. When the quartz boat enters the quartz furnace tube, the quartz furnace tube has a temperature of 700 degrees Celsius (° C.), and simultaneously is maintained with 2000 standard cubic centimeter per minute (sccm) of nitrogen (N.sub.2) for continuous blowing.

[0032] (2) Step of heating-up and leak detection. Specifically, the silicon wafers and the quartz boat after entering the quartz furnace tube are heated up with a heating rate of 10 degrees Celsius per minute (° C./min) until a target temperature of 850° C., and the furnace tube is vacuumized through a vacuum pump while the heating-up. After the vacuumizing is completed, the vacuum pump is turned off and vacuum leak detection is performed as per a leak detection requirement of less than 5 millibars per minute (mbar/min). After the leak detection is completed, the vacuum pump is turned on to adjust a pressure in the furnace tube, and a target vacuum degree is 200 mbar.

[0033] (3) Step of pre-oxidation. Specifically, after reaching the target temperature and the target vacuum degree, 2000 sccm of oxygen is introduced for pre-oxidation for 10 min, and a thin silicon oxide layer is grown on a surface of each the silicon wafer through the pre-oxidation. The thin silicon oxide layer serves as a barrier layer, which can prevent the surface of the silicon wafer from being damaged by chloride generated by decomposition of BCl.sub.3 gas.

[0034] (4) After the pre-oxidation is completed, the furnace tube is evacuated to remove oxygen in the furnace tube, and a target pressure is less than 200 mbar, so as to prevent excessive oxygen from remaining in the furnace tube and thereby prevent excessive oxygen to consume a source of BCl.sub.3 in a next step of source introduction.

[0035] (5) Step of boron source introduction and deposition. Specifically, BCl.sub.3 gas and oxygen (O.sub.2) are introduced as per a certain ratio, a gas volume ratio of BCl.sub.3 to O.sub.2 is 1:1, and an introduction time is 20 min. In particular, a relatively low ratio may cause that BCl.sub.3 cannot react with O.sub.2 completely, resulting in excessive self-decomposition of BCl.sub.3, and thus a large amount of elemental boron is deposited on the surface of silicon wafer to form a loss layer, and meanwhile, Cl.sub.2 generated by the decomposition is very easy to corrode the surface of the silicon wafer. Whereas, a relatively high ratio may cause insufficient concentration of BCl.sub.3, resulting in uneven distribution of gas sources in the quartz furnace tube, and thereby resulting in poor uniformity of whole tube.

[0036] (6) Step of heating-up. Specifically, after the boron source introduction and deposition is completed, the quartz furnace tube is evacuated again to remove BCl.sub.3 and oxygen in the furnace tube until a target pressure of less than 100 mbar. After the evacuating is completed, heating-up is performed until a target temperature of 1000° C.

[0037] (7) Step of propelling. Specifically, after reaching the target temperature of 1000° C., going to the step of propelling. In the step of propelling, nitrogen (N.sub.2) and oxygen (O.sub.2) are alternately introduced for propelling, which includes: introducing 3000 sccm of nitrogen for 10 min, introducing 5000 sccm of oxygen for 25 min, introducing 3000 sccm of nitrogen for 10 min, and introducing 5000 sccm of oxygen for 25 min sequentially in that order.

[0038] (8) Step of cooling down and boat exiting. Specifically, cooling down to 800° C. and then the quartz boat is performed with an exiting operation to exit from the furnace tube, thereby completing the boron diffusion.

Embodiment 2

[0039] A boron diffusion method suitable for HBC solar cells includes steps as follows.

[0040] (1) Step of boat entering. Specifically, silicon wafers after being texturing cleaned are placed into a horizontal insertion-type quartz boat, the silicon wafers are inserted into horizontal quartz grooves in a back-to-back manner, and then the quart boat is performed with an entering operation to enter a quartz furnace tube. When the quartz boat enters the quartz furnace tube, the quartz furnace tube has a temperature of 750° C., and simultaneously maintained with 2000 sccm of N.sub.2 for continuous blowing.

[0041] (2) Step of heating-up. Specifically, the silicon wafers and the quartz boat after entering the quartz furnace tube are heated up with a heating rate of 10° C./min until a target temperature of 800° C., and the furnace tube is vacuumized through a vacuum pump while the heating-up. After the vacuumizing is completed, the vacuum pump is turned off and vacuum leak detection is performed as per a leak detection requirement of less than 5 mbar/min. After the leak detection is completed, the vacuum pump is turned on to adjust a pressure in the furnace tube until a target vacuum degree of 500 mbar.

[0042] (3) Step of pre-oxidation. Specifically, after reaching the target temperature and the target vacuum degree, 5000 sccm of oxygen is introduced for pre-oxidation for 20 min, and a thin silicon oxide layer is grown on a surface of the silicon wafer through the pre-oxidation. The thin silicon oxide layer serves as a barrier layer, which can prevent the surface of the silicon wafer from being damaged by chloride generated by decomposition of BCl.sub.3 gas.

[0043] (4) After the pre-oxidation is completed, the furnace tube is evacuated to remove oxygen in the furnace tube, and a target pressure is less than 200 mbar, so as to prevent excessive oxygen from remaining in the furnace tube and thereby prevent excessive oxygen to consume a source of BCl.sub.3 in a next step of source introduction.

[0044] (5) Step of boron source introduction and deposition. Specifically, BCl.sub.3 gas and O.sub.2 are introduced as per a certain ratio, a gas volume ratio of BCl.sub.3 to O.sub.2 is 1:3, and an introduction time is 10 min. In particular, a relatively low ratio may cause that BCl.sub.3 cannot react with O.sub.2 completely, resulting in excessive self-decomposition of BCl.sub.3, and thus a large amount of elemental boron is deposited on the surface of silicon wafer to form a loss layer, and meanwhile, Cl.sub.2 generated by the decomposition is very easy to corrode the surface of the silicon wafer. Whereas, a relatively high ratio may cause insufficient concentration of BCl.sub.3, resulting in uneven distribution of gas sources in the quartz furnace tube, and thereby resulting in poor uniformity of whole tube.

[0045] (6) Step of heating-up. Specifically, after the boron source introduction and deposition is completed, the quartz furnace tube is evacuated again to remove BCl.sub.3 and oxygen in the furnace tube until a target pressure of less than 100 mbar. After the evacuating is completed, heating-up is performed until a target temperature of 1000° C.

[0046] (7) After reaching the target temperature of 1000° C., going to a step of propelling. Specifically, in the propelling step, N.sub.2 and O.sub.2 are alternately introduced for propelling, which includes: introducing 4000 sccm of nitrogen for 10 min, introducing 6000 sccm of oxygen for 30 min, introducing 4000 sccm of nitrogen for 10 min, and introducing 6000 sccm of oxygen for 30 min sequentially in that order.

Embodiment 3

[0047] A boron diffusion method suitable for HBC solar cells includes steps as follows.

[0048] (1) Step of boat entering. Specifically, silicon wafers after being texturing cleaned are placed into a horizontal insertion-type quartz boat, the silicon wafers are inserted into horizontal quartz grooves in a back-to-back manner, and then the quart boat is performed with an entering operation to enter a quartz furnace tube. When the quartz boat enters the quartz furnace tube, the quartz furnace tube has a temperature of 720° C., and simultaneously maintained with 2000 sccm of N.sub.2 for continuous blowing.

[0049] (2) Step of heating-up and leak detection. Specifically, the silicon wafers and the quartz boat after entering the quartz furnace tube are heated up with a heating rate of 10° C./min until a target temperature of 820° C., and the furnace tube is vacuumized through a vacuum pump while the heating-up. After the vacuumizing is completed, the vacuum pump is turned off and vacuum leak detection is performed as per a leak detection requirement of less than 5 mbar/min. After the leak detection is completed, the vacuum pump is turned on to adjust a pressure in the furnace tube, and a target vacuum degree is 100 mbar.

[0050] (3) Step of pre-oxidation. Specifically, after reaching the target temperature and the target vacuum degree, 6000 sccm of oxygen is introduced for pre-oxidation for 10 min, and a thin silicon oxide layer is grown on a surface of the silicon wafer through the pre-oxidation. The thin silicon oxide layer serves as a barrier layer, which can prevent the surface of the silicon wafer from being damaged by chloride generated by decomposition of BCl.sub.3 gas.

[0051] (4) After the pre-oxidation is completed, the furnace tube is evacuated to remove oxygen in the furnace tube, and a target pressure is less than 200 mbar, so as to prevent excessive oxygen from remaining in the furnace tube and thereby prevent excessive oxygen to consume a source of BCl.sub.3 in a next step of source introduction.

[0052] (5) Step of boron source introduction and deposition. Specifically, BCl.sub.3 gas and oxygen (O.sub.2) are introduced as per a certain ratio, a gas volume ratio of BCl.sub.3 to O.sub.2 is 1:2, and an introduction time is 15 min. In particular, a relatively low ratio may cause that BCl.sub.3 cannot react with O.sub.2 completely, resulting in excessive self-decomposition of BCl.sub.3, and thus a large amount of elemental boron is deposited on the surface of silicon wafer to form a loss layer, and meanwhile, Cl.sub.2 generated by the decomposition is very easy to corrode the surface of the silicon wafer. Whereas, a relatively high ratio may cause insufficient concentration of BCl.sub.3, resulting in uneven distribution of gas sources in the quartz furnace tube, and thereby resulting in poor uniformity of whole tube.

[0053] (6) Step of heating-up. Specifically, after the boron source introduction and deposition is completed, the quartz furnace tube is evacuated again to remove BCl.sub.3 and oxygen in the furnace tube until a target pressure of less than 100 mbar. After the evacuating is completed, heating-up is performed until a target temperature of 1000° C.

[0054] (7) Step of propelling. Specifically, after reaching the target temperature of 1000° C., going to the step of propelling. In the step of propelling, N.sub.2 and O.sub.2 are alternately introduced for propelling, which includes: introducing 3000 sccm of nitrogen for 10 min, introducing 5000 sccm of oxygen for 30 min, introducing 3000 sccm of nitrogen for 10 min, and introducing 5000 sccm of oxygen for 30 min sequentially in that order.

[0055] (8) Step of cooling down and boat exiting. Specifically, cooling down to 800° C. and then the quartz boat is performed with an exiting operation to exit from the furnace tube, thereby completing the boron diffusion.

Comparative Example 1

[0056] In this comparative example, specific steps of boron diffusion are shown in TABLE 1 below.

[0057] As seen from FIG. 1, the ECV results show that: the result obtained through the diffusion method according to the illustrated embodiment of the invention is that the junction depth is increased and the surface concentration is lower than that of the boron diffusion in the comparative example. A difference between latter parts of FIG. 1 of the two methods is mainly caused by different junction depths, and the surface concentration mainly depends on a concentration at the junction depth of 0-0.05 micrometers (μm).

TABLE-US-00001 TABLE 1 Comparison of steps between the method of the comparative example 1 and the method according to the invention Boron diffusion methods Method of the comparative example Method according to the invention Boat entering Boat entering Heating-up Heating-up Leak detection Leak detection Pre-oxidation Pre-oxidation 850° C. boron source introduction 850° C. boron source introduction and deposition and deposition Heating-up Heating-up 1000° C. oxygen propelling 1000° C. nitrogen propelling (40 min) (10 min) Cooling down 1000° C. oxygen propelling (25 min) Boat exiting 1000° C. nitrogen propelling (10 min) 1000° C. oxygen propelling (25 min) Cooling down Boat exiting

TABLE-US-00002 TABLE 2 Comparison of doping concentrations and minority carrier lifetimes Comparative items Embodiment 1 Comparative example 1 doping concentration (cm.sup.−3) 1.00E+19 2.00E+19 minority carrier lifetime (μs) 512 357 Junction depth (μm)  0.9  0.7

TABLE-US-00003 TABLE 3 Diffusion sheet resistances corresponding to different regions of the furnace tube and uniformity of sheet resistance of intra-wafer Comparative example 1 Embodiment 1 furnace furnace furnace furnace furnace furnace Regions head middle tail head middle tail Sheet 109 118 108 111 110 110 resistances 111 113 112 109 111 111 of 5 spots 106 118 115 111 111 111 (Ω) 117 105 110 112 112 112 106 116 118 111 111 111 Averaging 106 105 108 111 111 111 Unifor- 5.0% 5.7% 4.4% 1.50% 1.80 1.97% mity

[0058] As seen from TABLE 2 and FIG. 1, the new boron diffusion method according to the illustrated embodiment of the invention can obtain a deeper PN junction and a lower surface doping concentration, the lower surface doping concentration can reduce a surface recombination rate of the solar cell and improve open-circuit voltage and current, and the deeper junction depth can enhance a field effect of a front surface of an N-type HBC solar cell and is beneficial to an improvement of fill factor (FF).

[0059] Before unloading of wafers (i.e., removing the silicon wafers from the quartz boat), the silicon wafers at different positions of the quartz boat are sampled and tested. Test positions are shown in FIG. 3, and test results are shown in TABLE 3. It can be seen from the data shown in TABLE 3 that: the method according to the illustrated embodiment of the invention has excellent intra-wafer and inter-wafer diffusion uniformity, which can further improve the consistency of PN junctions and facilitate lateral transport of carriers. FIG. 2 illustrates PL (photoluminescence) pictures of photovoltaic solar cells obtained after the boron diffusions using the method of the comparative example 1 and the method of the embodiment 1 of the invention, respectively. As seen from FIG. 2, the illustrated embodiment of the invention has better PL brightness and uniformity.

[0060] Apparently, the illustrated embodiments are merely some embodiments of the invention, and not all embodiments of the invention. Based on the illustrated embodiments of the invention, all other embodiments obtained by those of ordinary skill in the art without creative work should fall within the scope of protection of the invention.