Comparison of solar cell technologies

In the proceeding sections the high quality of the epilayers grown in the RTCVD100 has been demonstrated. Now the focus is put on a correlation between solar cell preparation technique and epitaxial solar cell performance. The influence of emitter formation and metallization technique on the epitaxial material and the associated solar cell parameters are discussed.

Before comparing the process technologies in terms of solar cell characteristics, the process steps for both emitter and contact formation techniques are described in more detail to enable an understanding of possible effects.

Emitter formation

- TF emitter (Tube Furnace): emitter diffusion from a POCl₃ source takes place in a closed tube furnace under an atmosphere containing nitrogen, POCl₃ and oxygen. A chemical reaction of oxygen and the phosphorus containing compounds on the heated sample surface leads to the formation of phosphorus silicate glass (PSG), serving as source for the phosphorus diffusion. The diffusion of highly doped emitters was carried out at temperatures around 875°C for 20 min.

- CBF emitter (Conveyor Belt Furnace): the phosphorus containing paste “Soltech P101” was homogeneously screen printed on the front side of the wafer. After drying, in-line diffusion was carried out in an infrared heated RTC conveyor belt furnace at 925°C under oxygen atmosphere.

The total process time for this step was 15 min.

Both emitter formation techniques finish with a PSG-etch using hydrofluoric acid (HF).

Contact formation

- EC (Evaporated Contacts): the standard cleanroom process at Fraunhofer ISE employs contact formation by photolithography and subsequent evaporation of the contact metal. For the emitter front contacts a combination of Ti/Pd/Ag and in addition electroplating is used whereas aluminum is evaporated on the rear surface. The photolithographic emitter contact grid used for these solar cells is optimized for an 80 Ω/sq emitter. The fingers and bus bar are tapered with a contact width of 10 and 20 µm in case of the fingers and 60 and 120 µm for the bus bar. After electroplating the finger width was determined to approximately 100 µm with a final height of 20 µm. The entire process ends with a sinter step at 400°C for 35 min.

- SPC (Screen Printed Contacts) : the standard screen printing process starts with the deposition of an antireflection PECVD-SiNx at 346°C for 10 min resulting in a layer thickness in the range of 55 nm. The front and rear contacts are screen printed using Ag-paste and Al-pase respectively.

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After drying, rapid thermal firing is employed for contact formation. The RTF process includes a burn out of the organics which remained in the paste and a subsequent firing step at 780°C for few seconds. The final width and height of the front contacts were measured to 100 µm and 8 µm respectively.

Figure 6.5: Mean values of illuminated I/V parameters calculated for all process types.

The mean values for the solar cell parameters determined from illuminated characteristics were calculated for all solar cell types. Figure 6.5 gives an overview on the results. For solar cells with screen printed contacts but without SiN_x layer (3b and 5b) no statistical evaluation was carried out.

As expected, the standard cleanroom process 1 shows the overall best solar cell performance with an electrical efficiency of 14.8% due to an optimized emitter and contact grid. Only slightly lower mean efficiencies were achieved for solar cells with CBF emitter and screen printed contacts (4) while the efficiencies of the remaining solar cell types are all in the range of 12%.

Solar cells with screen printed contacts (3a and 5a) feature lower fill factors compared to solar cells with evaporated contacts. This is a typical characteristic associated to contact formation by screen printing which results in increased contact and series resistance. From fitted dark I/V characteristics the series resistance was determined to values well above 1 Ωcm² for solar cells with screen printed contacts. The application of evaporated contacts led to constant fill factors above 78%, comparable to those determined for process 1 and independent from emitter formation technique.

Considering the open-circuit voltage, the combination of CBF emitter with SPC (5a) results in a mean value comparable to process 1. The lowest VOC is observed for solar cells with TF emitter and EC (2) with a loss of -16 mV compared to process 5a. Keeping the emitter formation technique fixed, it can be observed that the application of screen printing for contact formation instead of evaporation leads in general to larger open-circuit voltages (2 vs. 3a and 4 vs. 5a). For TF and CBF emitter an increase of 11 mV and 7 mV respectively is calculated for screen printing compared to evaporation.

On the other hand, if the contact formation technique is fixed, an increased V_OC is achieved if a CBF emitter is used instead of a TF emitter (2 vs. 4 and 3a vs. 5a). In this case, a rise of 9 mV and 6 mV is observed in VOC for evaporated contacts and screen printed contacts respectively, when replacing the TF emitter by a CBF emitter.

Both features can be readily explained by the different contact and emitter formation techniques. The application of a SiNx layer in case of screen printed contacts leads to an effective passivation of the surface, emitter and bulk region thus reducing the recombination activity and therefore increasing VOC. The results further show that the application of a CBF emitter is beneficial for V_OC, indicating that a general difference exists between the characteristics of the two emitter types. A possible explanation for the low performance of the TF emitter might be that high phosphorus surface concentrations or a large density of phosphorus precipitates led to an enhanced recombination in the emitter and space charge region.

The mean values in short-circuit current are equal for process 4 and process 1 while process 2, 3a and 5a feature considerably lower values, all in a similar range. Using screen printed contacts, the emitter formation technique has only little impact on JSC. CBF and TF emitter result in almost identical mean values (3a vs. 5a). On the other hand, a significant boost in JSC can be observed for evaporated contacts, if a CBF emitter is used instead of a TF emitter (4 vs. 2).

Increased shadowing and series resistance loss determine the short-circuit current densities if contact formation is done by screen printing. This characteristic is clearly reflected when comparing process 4 and 5a, where a difference of almost 3 mA/cm² in JSC is observed in favor of the solar cells with evaporated contacts. A similar difference is expected for process 2 and 3, where both emitters are formed by POCl₃ diffusion but different contact formation techniques are applied. Instead, both processes result in similar short-circuit current densities. To explain this discrepancy the following assumption concerning TF and CBF emitter is made: compared to the CBF emitter the POCl3 diffused emitter features an increased surface concentration and is less deep. In addition the sheet resistance might be slightly different for both emitters, with the CBF emitter probably featuring a lower value.

Considering process 2 and 4 the detrimental decrease in J_SC for the TF emitter can then be attributed to an enhanced recombination velocity in the emitter region. The internal quantum efficiency measurements graphed in Figure 6.6 confirm this statement. The solar cell with CBF emitter shows an overall better internal quantum efficiency compared to the solar cell with TF emitter. While the increase in red response is comparatively low and might be either a consequence of the CBF diffusion or a slight deviation in base layer thickness, the difference in the short-wavelength range is apparent.

Up to a wavelength of 600 nm the solar cell with CBF emitter shows a substantially higher IQE than the solar cell with TF emitter, possibly due to different emitter characteristics.

In Figure 6.7 the IQE characteristics for solar cells prepared by process 3a and 5a are compared.

Again, the CBF emitter shows an overall better spectral response. However, compared to Figure 6.6 the difference between both emitter types is less pronounced and in the mid-wavelength range they even show almost identical values.

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0.0 0.2 0.4 0.6 0.8 1.0

EC with

TF emitter (2) CBF emitter (4)

IQE

λ

[nm]

Figure 6.6: Internal quantum efficiency measured for epitaxial solar cells with different emitter diffusion techniques. In both cases, contact formation was done by evaporation.

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0.0 0.2 0.4 0.6 0.8 1.0

SPC with RTF through SiN_x TF emitter (3a) CBF emitter (5a)

IQE

λ

[nm]

Figure 6.7: IQE for solar cells with screen printed metallization and different emitter types.

To evaluate the effect of the passivating SiNx layer on spectral response, IQE measurements were also carried out on solar cells of process 3b and 5b. The resulting curves are depicted in Figure 6.8 and compared to the IQE characteristics obtained for corresponding solar cells with SiN_x (type a).

The application of a SiN_x layer on a TF emitter leads only to a minor improvement in IQE for short wavelengths (Figure 6.8, left). In contrast, a substantial rise in blue response is obtained for the CBF emitter, if a passivating silicon nitride layer is deposited on the solar cell surface (Figure 6.8, right). In both cases, a slight increase in red response is observed if a SiNx layer is applied, indicating a bulk passivation of the base layer region. Similar to the improvement in the short-wavelength range the effect is more pronounced for the CBF emitter.

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Figure 6.8: Effect of RTF through SiN_x on internal quantum efficiency studied for two different emitter formation techniques. Left: TF emitter with screen printed contacts. Right: CBF emitter with screen printed contacts.

Considering the solar cell parameters for TF emitter, contact formation with RTF through SiN_x yields an increase in V_OC by 29 mV and an increase in J_SC by 7.3 mA/cm² compared to contact formation by evaporation. For the CBF emitter the improvement is even more drastic with a rise in V_OC by 35 mV and in JSC by 8.2 mA/cm².

The results from IQE and illuminated I/V characteristics show that the CBF emitter can be better passivated by the SiN_x layer, indicating a lower surface concentration compared to the TF emitter and thus further confirming the assumption made on the shape of the emitter profiles.

In Figure 6.9 the impact of contact formation on spectral response can separately be studied for both emitter types. Concerning the TF emitter the internal quantum efficiency characteristics suggests that the mere process of contact formation by screen printing (without RTF through SiN_x) considerably improves the emitter and bulk region of the solar cell. For the CBF emitter almost the inverse effect is observed: the process of screen printing deteriorates the emitter but not the bulk.

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Figure 6.9: Internal quantum efficiency for solar cells with TF emitter (left) and CBF emitter (right) and evaporated as well as screen printed contacts.

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A conclusions which can be drawn from these results is that the process of screen printing seems to influence the solar cells with TF emitter in terms of an improved minority carrier lifetime in emitter and bulk. Temperature and duration of the firing step are very short compared to the emitter diffusion from POCl₃ source and therefore a change of the emitter properties as a consequence of the firing process seems to be unlikely. At present, the mechanism leading to the observed improvement is not clear.

To quantify the CBF and TF emitter profiles, SIMS and Stripping Hall measurements were ordered but not completed at the end of this work.