FCA-FTIR
4.4 Solar cells made from UMG silicon in an industrial stan- stan-dard process
Having assessed the main influences on the UMG wafers from mono- and multicrystalline ingots, in the following, representative solar cell results are shown. The intention of this chapter is to highlight the typical properties of the solar cells made from the UMG mate-rial presented so far. It is not aimed at an assessment of the efficiency potential of UMG-Si solar cells nor are possibilities for process optimizations discussed, which are the topic of several other works (see e.g. [45]).
4.4.1 Solar cells made from UMG Cz-silicon wafers
For the first batch of UMG silicon solar cells, wafers were taken from five ingot positions distributed from seed to tail end from the crystals “UMG Cz 1” (100% UMG-Si feedstock content) and “UMG Cz 2” (blend of 50% UMG-Si, 50% virgin grade Si). For reference, wafers were taken from ingot “UMG Cz 3” (100% virgin grade Si) as well as highly pure Cz wafers from a different wafer supplier (“ISE Ref”).
All wafers were processed in the same run in the PV-TEC laboratory at the Fraunhofer ISE involving the standard process steps in the following order22:
• SC1 wafer cleaning
• Alkaline texturization
• Phosphorus emitter diffusion, sheet resistance of 65 Ω/sqr
• Phosphorus silicate glass etch back
• SiN antireflection coating (PECVD)
• Screen printing of the AgAl solder pads on the rear side
• Screen printing of the Al rear side metallization (intended for back surface field-formation)
• Screen printing of the Ag front side metallization
• Fast firing of the metal contacts
• Laser edge isolation
• Degradation of the solar cells by light soaking for at least 15 hrs.
No optimization of the solar cell processing was tackled.
The solar cell results in the degraded state are shown in Figure 4.2723. We want to focus on the most important findings:
The open circuit voltage VOC gives for all three crystals around 610 mV; the ISE reference shows the relatively low VOC obtained in this solar cell run. Nevertheless, since both the carrier lifetime and the base net doping concentration – which both vary strongly be-tween the three crystals – significantly influence the open circuit voltage, a significant
22 Industrial solar cell run in the PVTEC was the collaboratory work of several engineers, supervised by G.
Emanuel.
23 Analysis of the solar cell results by G. Emanuel and J. Geilker.
difference in the VOC was initially expected. To identify the origin of this coincidence, the open circuit voltage was calculated as a function of the effective diffusion length (the car-rier lifetime) by means of a simple two-diode model with the net doping concentration of the different ingots as input parameter24 and compared to measurements in Figure 4.28.
This simulation approach was by no means intended to yield correct values, as some simplifications like a constant short circuit current for all solar cells were assumed. How-ever, the graph indicates that the reason for the same VOC-level of all materials may lie in the superposition of the counteracting effects of the diffusion length and the base resis-tivity on the voltage: While with increasing net doping a higher VOC is expected for a given diffusion length, the latter decreases the VOC due to the carrier lifetime-limiting boron-oxygen defect which is linearly proportional to p0 in this resistivity range.
As the reduced diffusion length affects the collection efficiency of the pn-junction, the short circuit current is significantly lower (~3 mA/cm2) in the 100% UMG-Si ingot com-pared to the reference “UMG Cz 3”.
This JSC-decrease already accounts for a reduction in the solar cell efficiency of about 1%
abs. of ingot “UMG Cz 1”. In addition, the solar cells from this crystal suffer from the in-creased saturation currents Jrev1 and Jrev2 which may account for another 1% abs. reduc-tion in the efficiency.
In total, while the reference solar cells show good solar cell efficiencies of around 17%, which is approximately the currently attained efficiency in industry on Cz wafers, the so-lar cells made from a silicon blend of UMG-Si and highly pure Si (“UMG Cz 2”) reach only 16% and the solar cells made entirely from UMG-Si only about 15%. This can mainly be attributed to the Cz-related boron-oxygen defect.
Hence, a reduction of the net doping concentration coming with a lower content of both boron and phosphorus in the UMG silicon feedstock is highly desirable. However, note that due to the rapid technological progress promoted by the feedstock producers, the materials investigated in the course of this thesis are meanwhile outdated. Current feed-stock of most manufacturers contains significantly less dopants [101].
24 Simulations were performed by J. Greulich.
Figure 4.27: Solar cell results in the degraded state of the wafers from the UMG silicon Cz crystals UMG 1 and 2, and the reference ingot “UMG Cz 3” versus ingot position (1=seed end). For comparison, the ISE reference solar cells were made from highly pure standard Cz wafers from a different wafer supplier. After ref. [54].
Figure 4.28: Measured open circuit voltage versus the measured diffusion length (calcu-lated from carrier lifetime measurements in the degraded state) of the UMG Cz ingots as well as the reference sample. The lines represent the VOC(Leff) relation of various net doping concentrations p0 simulated by means of a simple two-diode model24. The net doping concentrations correspond to the average p0-values measured on the different crystals.
4.4.2 Solar cells made from UMG mc-silicon wafers
Since multicrystalline silicon generally does not contain similarly high amounts of oxygen, the Cz-defect does not play an important role in most mc-Si wafers.
Therefore, in this section UMG mc-Si solar cells made from ingot “UMG mc 1” are ana-lyzed and compared to reference solar cells made from ingot “Ref mc 1”. They were proc-essed in the frame of the project SolarFocus in an standard screen printing process in an industrial facility. The details were not disclosed, but with a high probability they are similar to the process described in the previous subsection.
The results are presented in Figure 4.29.
As a consequence of the higher net doping concentration in the UMG silicon wafers, the open circuit voltage of the UMG-Si solar cells exceeds the VOC of the reference solar cells by about 10 mV.
By contrast, the lower SRH-minority carrier lifetime results in a short circuit current which is reduced by approx. 2.5 mA/cm2, which explains the lower efficiency in the UMG-Si solar cells of about 0.7% absolute.
Figure 4.29: Solar cell results obtained on the multicrystalline UMG silicon wafers “UMG mc 1” and on the mc-Si reference wafers “Ref mc 1” versus ingot height. The red dashed line marks the approximate position of the changeover from p- to n-type con-ductivity in ingot “UMG mc 1”25.
The impairment of the minority carrier lifetime can be traced back to the metallic impuri-ties which remain in the multicrystalline silicon ingots after solidification. As the compari-son with the similarly metal-contaminated mc reference crystal shows, the transition metals by themselves do not have such a deleterious effect; however, in conjunction with a high net doping concentration, the minority carrier lifetime suffers.
While this chapter has centered mostly around the implications of high boron and phos-phorus concentrations, the following will focus on the behavior of transition metals during the high-temperature steps in the solar cell process.
25 Solar cell parameter measurements performed within the industrial facility and provided for the general use of the project consortia.