3.1 Introduction
3.2.1. POCl 3 Diffusion Front Surface Field
First we adopt heavy doping POCl3 diffusion and heavy doping POCl3 diffusion with wet oxide growing and HF treatment to remove dead layer at the front surface for test structure, after samples covered by PECVD SiNx, we compare two processes by minority carrier lifetime measurement, implied open circuit voltage and saturation current density, as shown in Fig.3.1 to Fig. 3.4, we could realize that for minority carrier lifetime, and implied open circuit voltage at 1 sun, heavy doping with wet oxide growing and HF treatment are overwhelmingly higher than normal heavy doping process, and saturation current for heavy doping is extremely higher than heavy doping with wet oxide growing and HF
treatment, we attribute this result to doping concentration is reduced at the front side due to heavy doped silicon bulk is oxidized and then removed by HF treatment, therefore the diffusion length of minority carriers is enhanced significantly because of the lower defect density created by overdosing impurities which exceeds solid solubility. POCl3
850℃ with drive in 15min was adopted for heavy doping diffusion, and wet oxide was growing under the condition of 800℃ for 10 minutes.
Due to heavy doping POCl3 diffusion leads to lifetime degradation and wet oxide growing with HF treatment need more time and thermal process, light doping POCl3 diffusion profile is another way to achieve excellent front surface passivation. Next we comparing light doping diffusion profile with PSG remained or removed by HF treatment, the result of lifetime measurement, surface recombination velocity and implied open circuit voltage were tested after deposition of SiNx capping layer as anti-reflection layer, the results are shown in Fig.3.5 to Fig.3.7, the sample with light doping POCl3 diffusion and PSG passivation shows higher minority carrier lifetime, implied open circuit voltage at one sun and lower surface recombination velocity due to the passivation provided by PSG, light doping POCl3 diffusion profile is performed under the
condition of 20 minutes POCl3 diffusion without drive in whether temperature does not exceed 700 ℃ . We should notice that POCl3
diffusion is adopted for front surface field and front surface passivation which is texture structure to reduce the reflection of incident light. The difference between POCl3 diffusion applied for texture surface and polish surface is worth researching. For this purpose, POCl3 diffusion 20 minutes without drive in at 650℃, 675℃ was applied for test structure for lifetime measurement , the result are shown in Fig.3.8, Fig.3.9, the minority carrier lifetime of polish sample is higher than texture sample, this phenomenon could be explained that the surface area of texture surface is about two-times larger than polish side, hence the heavy doping region is double, so is the quantity of defect [3.3]. For the purpose of finding out the optimism of light doping POCl3 diffusion with PSG and PECVD SiNx as front surface passivation at relatively low process temperature (650℃ to 700℃), we set light doping POCl3 diffusion at 650℃, 675℃, 700℃ for 20minutes as our experiment, the result of minority carrier lifetime measurement are shown in Fig.3.10, although the values are closed, but we could still identify light doping POCl3 diffusion at 675℃ for 20 minutes is the optimism. Finally the comparison of
saturation current density of heavy doping with wet oxide growing 10 minutes at 700℃with HF treatment and light doping POCl3 diffusion with PSG at 650℃, 675℃, 700℃ is shown in Fig.3.11, we could realize light doping profile is superior for front surface field and front surface passivation.
3.2.2. Emitter Passivation By Atomic Layer Deposition (ALD) Al
2O
3For n-type IBC silicon solar cell, we normally deposit SiO2 to cover p-type emitter region as passivation layer, however, the existence of
which is deposited by ALD has state-of-the-art quality, furthermore after annealing, high density negative fix charge would appear at the interface of Si substrate and Al2O3 layer with ultrathin SiOx which could enhance
field effect passivation at p-type emitter region Fig.3.12 [3.4], according to the research, the fixed charge density is up to 1012 ~1013 cm-2, which is able to be achieved that many researchers have also reported [3.4] [3.5].
However, the optimism of passivation is existed for different annealing temperature due to two kinds of passivation mechanism which are chemical passivation and field effect passivation. The dangling bond at the interface is the origin of defect which would trap minority carriers then creating recombination, however dangling bond can be passivated by hydrogen [3.4].
There are other benefits for Al2O3 passivation. SiNx and SiO2 show the degradation of effective lifetime after UV irradiation which means surface passivation goes worse than before, however Al2O3 appears improved effective lifetime after long term light illumination [3.6] [3.7], this is a significant phenomenon for solar cell. Moreover, Al2O3 grown by thermal ALD with PECVD SiNx capping layer is proved to own higher thermal stability and longer effective lifetime than single Al2O3 layer even for sample with capping layer without anneal for Al2O3 [3.8].
In our experiment, we test two kind of annealing temperature in O2
for 30 minutes which are 300℃ and 400℃ to the same ALD Al2O3
thickness which is 10nm.
First, the result of minority carrier lifetime measurement, surface recombination velocity and implied open circuit voltage of single Al2O3
layer are shown in Fig.3.13 to Fig.3.15, sample with 400℃ annealing treatment presents better quality than sample with 300℃ annealing treatment, this result can be explained by the top view of the test sample from microscope as shown in Fig.3.16, Fig.3.17, the sample with 300℃
annealing treatment shows more blistering part of surface than sample with 400℃ annealing treatment, however, in our fabrication process, ultrathin Al2O3 layer is protected by SiNx capping layer which is deposited by PECVD, for this reason, the passivation of Al2O3/SiNx stacking layer is investigated, the sample with SiNx capping layer after Al2O3 annealing at different temperature have been tested, the result of minority carrier lifetime measurement, surface recombination velocity and implied open circuit voltage are shown in Fig.3.18 to Fig.3.20, we could clear understand the passivation of Al2O3 annealing at 300℃ with SiNx capping layer is better than sample at 400℃ annealing with SiNx capping layer, the comparison of surface recombination velocity of single Al2O3 layer and Al2O3 with SiNx capping layer at the same annealing
temperature is shown in Fig.3.21, Fig.3.22, the surface recombination velocity is improved for both of sample, this is due to the hydrogen from NH3 passivates the interface and blistering area of sample, and Al2O3
layer with 300℃ annealing shows higher blistering area density, hence hydrogen passivation is more effectively for Al2O3 layer with 300℃
annealing.
3.3 Open Circuit Voltage Comparison
The conditions we mentioned above are all applied for IBC solar cell device, the device condition is shown in Table 3.1 All the discussion in section 3.2.1 and 3.2.2 is about minority carrier lifetime which would influence open circuit voltage seriously, so we start from open circuit voltage analysis.
Fist, the comparison of open circuit voltage and shunt resistance for the solar cell device which spacer is 50 um with the same heavy doping POCl3 diffusion treatment (850℃,drive in 15 minutes ), but different wet oxide growing time (10 minutes, 30 minutes) is shown in Fig.3.23, with longer wet oxide growing time, the device shows higher open circuit voltage and larger shunt resistance, the result can be speculated from
lifetime measurement we had already tested. Next, we compare the open circuit voltage and shunt resistance of device with different light doping POCl3 diffusion, as shown in Fig.3.24 to Fig.3.29, we have already known light doping POCl3 diffusion with 675℃ is the optimism on lifetime measurement, for solar cells with Al2O3 annealing at 400℃, devices with 675℃ light doping POCl3 diffusion demonstrate higher open circuit voltage as we expected. However, for the device at 300℃
annealing condition, device with light doping POCl3 diffusion shows poor open circuit voltage and low shunt resistance, this might be caused by the mistake in fabrication process, in fact, all devices with Al2O3 annealing at 300℃ and light doping POCl3 at 675℃ demonstrate low open circuit voltage and low shunt resistance which means serious leakage mechanism is existed in our device.
The results of the open circuit voltage with different Al2O3 annealing temperature for light doping POCl3 diffusion at 650℃, 675℃, 700℃ are shown in Fig.3.30 to Fig.3.38 for all the spacer conditions, the devices with Al2O3 annealing at 300℃ generally demonstrate higher open circuit voltage and larger shunt resistance, except for device with light doping POCl3 at 675℃
3.4 Short Circuit Current Density Comparison
The short circuit current has relation to the emitter fraction in solar cell [3.2], we design three kinds of spacer for solar cell device, which are 50um, 100um, 150um, the emitter fraction is increasing with smaller spacer, accompanying with lower series resistance, the relationship of spacer, short circuit current and series resistance is shown in Fig.3.39 to Fig.3.44.
3.5 I-V curve and efficiency
After the comparison of open circuit voltage and short circuit current, I-V curve and efficiency for the solar cell with heavy doping POCl3
diffusion and the best result of light doping POCl3 with Al2O3 passivation are shown in Fig.3.45 to Fig.3.47, Table.3.2, Table.3.3, we could clearly discover that for light doping POCl3 diffusion with Al2O3 passivation condition, the best solar cell is for 675℃ POCl3 diffusion with Al2O3
emitter passivation layer at 400℃ annealing 30 minutes in O2 ambient, which efficiency could achieve 7.37%. However, the efficiency of the solar cell with heavy doping POCl3 diffusion and Al2O3 passivation at
annealing temperature 400 ℃ could achieve 7.59%. This can be attributed to the accident during device fabrication, during the process the HF treatment before ALD Al2O3 deposition, photoresist is applied for texture surface to protect SiNx anti reflection layer and PSG passivation, the photoresist we adopted is FH-6400, the photoresist could not cover the texture surface completely during spin coating process, hence SiNx and PSG was etched by HF, the front surface after HF treatment is shown in Fig.3.48, Fig.3.49, for the following HF treatment in fabrication process, we adopted AZ-4620 for front surface protection.
1E15 2E15
Fig. 3.1 lifetime of heavy doping POCl3 diffusion
Fig. 3.2 lifetime of heavy doping POCl3 diffusion with wet oxide growing
400 500 600 700 800 0.1
1 10
heavy doping
heavy doping with wet oxide
Il lu mi n a ti on (s u n )
Fig. 3.3 implied open circuit voltage for heavy doping POCl3 diffusion w/wo wet oxide growing
Fig. 3.4 saturation current density for heavy doping POCl3 diffusion w/wo wet oxide growing
1E15 1E16
light doping with PSG with SiNx
Fig. 3.5 lifetime of light doping w/wo PSG
Fig. 3.6 SRV of light doping w/wo PSG
650 700 750 800 1
2 3 4
Il lu mi n ati on ( su n )
Implied Open Circuit Voltage (mV)
light doping with SiNx
light doping with PSG and SiNx
Fig. 3.7 implied open circuit voltage of light doping w/wo PSG
Fig. 3.8 lifetime of light doping at 650℃ on texture/polish surface
Fig. 3.9 lifetime of light doping at 675℃ on texture/polish surface
Fig. 3.10 lifetime of light doping at 650℃, 675℃, 700℃ and heavy doping 850℃
Fig. 3.11(a) saturation current for light doping with different temperature and heavy doping with wet oxide growing
Fig. 3.11(b) saturation current for light doping with different temperature and heavy doping with wet oxide growing
Fig. 3.12 interface band diagram and TEM image [3.4]
Fig. 3.13 lifetime of Al2O3 layer at different annealing temperature
Fig. 3.14 SRV of Al2O3 layer at different annealing temperature
Fig. 3.15 implied open circuit voltage of Al2O3 layer at different annealing temperature
Fig. 3.16 OM image of Al2O3 layer at 300℃ annealing
Fig. 3.17 OM image of Al2O3 layer at 400℃ annealing
Fig. 3.18 lifetime of Al2O3 at different annealing temperature with SiNx capping layer
Fig. 3.19 SRV of Al2O3 at different annealing temperature with SiNx capping layer
Fig. 3.20 implied open circuit voltage of Al2O3 at different annealing temperature with SiNx capping layer
Fig. 3.21 SRV of Al2O3 at same 300℃ temperature w/wo SiNx capping layer
Fig. 3.22 SRV of Al2O3 at same 400℃ temperature w/wo SiNx capping layer
Fig. 3.23 open circuit voltage & shunt resistance comparison for heavy doping with different wet oxide growing time
Fig. 3.25 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 100um, annealing at 300℃
Fig. 3.24 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 50um, annealing at 300℃
Fig. 3.26 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 150um, annealing at 300℃
Fig. 3.27 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 50um, annealing at 400℃
Fig. 3.28 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 100um, annealing at 400℃
Fig. 3.29 open circuit voltage & shunt resistance comparison for light doping with different temperature, spacer 150um, annealing at 400℃
Fig. 3.30 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 50um, POCl3 diffusion 650℃
Fig. 3.31 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 100um, POCl3 diffusion 650℃
Fig. 3.32 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 150um, POCl3 diffusion 650℃
Fig. 3.33 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 50um, POCl3 diffusion 675℃
Fig. 3.34 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 100um, POCl3 diffusion 675℃
Fig. 3.35 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 150um, POCl3 diffusion 675℃
Fig. 3.36 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 50um, POCl3 diffusion 700℃
Fig. 3.37 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 100um, POCl3 diffusion 700℃
Fig. 3.38 open circuit voltage & shunt resistance comparison for annealing with different temperature, spacer 150um, POCl3 diffusion 700℃
Fig. 3.39 short circuit current & series resistance comparison for different spacer, annealing 300℃, POCl3 diffusion 650℃
Fig. 3.40 short circuit current & series resistance comparison for different spacer, annealing 300℃, POCl3 diffusion 675℃
Fig. 3.41 short circuit current & series resistance comparison for different spacer, annealing 300℃, POCl3 diffusion 700℃
Fig. 3.42 short circuit current & series resistance comparison for different spacer, annealing 400℃, POCl3 diffusion 650℃
Fig. 3.43 short circuit current & series resistance comparison for different spacer, annealing 400℃, POCl3 diffusion675℃
Fig. 3.44 short circuit current & series resistance comparison for different spacer, annealing 400℃, POCl3 diffusion700℃
Fig. 3.45 I-V curve comparison of heavy doping with different wet oxide growing time, spacer 50um
Fig. 3.46 I-V curve comparison of heavy doping w/wo Al2O3 passivation
Fig. 3.47 I-V curve comparison of light doping POCl3 diffusion at 675℃
and Al2O3 annealing at 400℃
Fig. 3.48 poor uniformity of photoresist coating on texture surface
Fig. 3.49 SiNx and PSG etched by HF treatment due to poor uniformity of photoresist at texture surface
Fig. 3.50 damage of front side of device
Sample A
Table 3.2 comparison of heavy doping device with different treatment Table 3.1 split table of device fabrication
Table 3.3 comparison of light doping POCl3 diffusion at675℃ and Al2O3
annealing at 400℃ with different spacer
Chapter 4
Conclusion and Future Work
4.1 Conclusion
We have successfully proved light doping POCl3 diffusion and PSG layer provide state-of-art passivation comparing to heavy doping POCl3
diffusion from minority carrier lifetime measurement, and the effect Al2O3 layer passivation from test structure, although the result is not outstanding showing on the true IBC solar cell device due to some problems in the fabrication process, the reason of SiNx and PSG etched during HF dip treatment was explained at the end of Chapter 3, and the solution is adopted immediately for the following fabrication process, however the damage could not be recovered. as long as we improved the fabrication process, high efficiency IBC solar cell can be achieved
4.2 Future Work
For recent IBC silicon solar cell, point contact and metal finger which is made by Ti, Pt, Ag stack deposition are guaranteed to achieved better metal contact, reducing contact recombination and resistance
simultaneously. According to the reason we mentioned above, changing metal material and designing new mask for point contact are what we could research in the future, in addition to metal, Al2O3 is also a key issue we consider to researching deeply, different Al2O3 thickness, the effect of Al2O3 for n-type region are worth-attention. Except for all of these, based on mask design, the emitter area passivated by Al2O3 is almost equal to n-type region passivated by SiNx, hence the passivation provided by Al2O3 might not be obvious, this is another problem we could improve in future study.
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