3.6 Simulation studies: reducing parasitic absorption from highly doped poly-Si layers

The excellent results from the earlier sections clearly demonstrate the potential of deploying double-sided passivated contacts for next-generation silicon solar cell

#### Figure 14.

Comparison of the ECV profile for 10-nm-thick poly-Si(n<sup>+</sup> ) layer on both planar and textured silicon surfaces. Additionally included is the ECV profile for a thick poly-Si(n<sup>+</sup> ) capping layer for comparison purpose. A higher dopant in-diffusion is observed for the textured surfaces, which partially explains the lower measured implied-VOC values.

## Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

concepts, which in this work had been entirely realized on commercially available industrial tools. However, as mentioned earlier, one of the key issues if contact passivation is to be applied front-side also is to minimize parasitic absorption within the highly doped front-side poly-Si capping layer. Highly doped poly-Si is similar to transparent conductive oxide (TCO) layers deployed for silicon heterojunction solar cells, non-zero extinction coefficients, resulting in the inevitable parasitic absorption. This is even more pronounced, if applied front-side, thereby directly reducing the absorbable photogeneration current in the silicon wafer bulk (as the incident light is then first entering the parasitically absorbing poly-Si capping layer before entering the silicon wafer).

Hence, the objective of this section is to utilize an appropriate numerical calculation method to determine the parasitic absorption as a function of the (rear or front side) poly-Si capping layer thickness and then subsequently predict the corresponding solar cell efficiency potential of the correspondingly optimized passivated contact, being rear-side-only or front- and rear-side deployed in a solar cell. To address the above, the simulation program SunSolve™, available on PV Lighthouse [53], was utilized to study the impact of the doped poly-Si capping layer thickness on the maximum absorbable current density within the silicon wafer bulk Jabsorbed, cell. Besides calculating Jabsorbed, cell, the various optical losses can also be determined (i.e., front-reflected, front-escaped, rear-escaped, parasitic absorption in each layer, edge absorption) for the investigated solar cell precursors in this work.

To enhance the accuracy of the optical calculations, ellipsometry measurements were performed on all in-house fabricated samples, i.e., measuring our deployed dielectric films (SiNx, SiOx, AlOx) as well as our optimized doped poly-Si capping layers, followed by a fitting and extraction of the wavelength-dependent optical refractive indices (n, k). These wavelength-dependent refractive indices were then imported into the SunSolve™ simulation program for a more realistic prediction of the current loss analysis, based on our own developed contact passivation layers. As an example, Figure 15 shows the fitted wavelength-dependent refractive indices for the doped poly-Si layers in this work, which is further compared to the crystalline silicon reference [73]. As seen, the doped poly-Si layers do exhibit a higher extinction coefficient (k) compared to a c-Si reference within the visible to nearinfrared region (400–900 nm). This again indicates that parasitic absorption is inevitable and should be minimized by thickness reduction while not compromising on the passivation quality. Further optimization work should also try to reduce the extinction coefficient of the poly-Si capping layers itself, i.e., by changing its chemical composition.

The current loss analysis results for a rear-side passivated contact solar cell (using SunSolve™) are shown in Figure 16. In order to account for internal back reflection, a local full-area metal contact scheme has been assumed (see Figure 16) (this can be realized by local laser ablation, forming contact openings in the SiNx passivation layer and a subsequent full-area metallization).

It can be seen that for a solar cell with a conventional front-side boron-diffused junction and a rear-side electron-selective passivated contact (thermal-SiOx/poly-Si(n<sup>+</sup> )), the parasitic absorption arising from the rear-side poly-Si(n<sup>+</sup> ) capping layer can be directly addressed by reducing the rear-side poly-Si film thickness (e.g., the parasitic absorption current loss reduces from 0.55 mA cm<sup>2</sup> for a 250-nm-thick poly-Si(n<sup>+</sup> ) layer to 0.02 mA cm<sup>2</sup> for a 10-nm-thick poly-Si(n<sup>+</sup> ) layer). The reduction in the parasitic absorption directly enhances the potentially absorbable current in the wafer bulk (Jabsorbed, cell), which in this case improves from 40.7 to 41.1 mA cm<sup>2</sup> .

Interestingly, it was observed that for a poly-Si capping layer thickness lower than 25 nm, the <sup>J</sup>absorbed, cell saturates at 41.1 mA cm<sup>2</sup> . On hindsight, we would

To provide more insights, ECV measurements were performed on the thin poly-

) layers on both the textured and planar surfaces and compared to the thick reference as shown in Figure 14. The following observations can be made: (i) the

) than the thicker counterpart (<sup>2</sup> <sup>10</sup><sup>20</sup> cm<sup>3</sup>

capping further enhances the overall passivation quality, such that the textured and planar lifetime structures now exhibit an improvement in the implied-VOC by 17 and 8 mV, which is a relative improvement of 2.5 and 1.1%, respectively.

To summarize, we have demonstrated on a textured silicon surface the ability to obtain an excellently passivating SiNx-capped electron-selective passivated contact

)) with sufficiently thin poly-Si(n<sup>+</sup>

The excellent results from the earlier sections clearly demonstrate the potential of deploying double-sided passivated contacts for next-generation silicon solar cell

) layer thickness to 70 nm (i.e., observing a drastic drop in

to reduce the parasitic absorption issue while maintaining excellent passivation qualities (implied-VOC values exceeding 700 mV). Re-optimizing the diffusion recipe for an ultrathin LPCVD of intrinsic poly-Si layer therefore solves the limitations encountered when using slow silicon etch technology, which limited the

3.6 Simulation studies: reducing parasitic absorption from highly doped

planar surface, which could partially explain the lower measured implied-VOC

values for the former (i.e., 686 mV as compared to 719 mV).

) layer exhibits a higher phosphorus dopant concentration

) layer on the textured surface exhibits a higher dopants in-diffusion than the

) capped samples, an additional symmetrical SiNx

) layer on both planar and textured silicon

) capping layer for comparison purpose.

); and (ii) the poly-

) thickness (10 nm)

Si(n<sup>+</sup>

Si(n<sup>+</sup>

thin poly-Si(n<sup>+</sup>

Silicon Materials

(<sup>5</sup> <sup>10</sup><sup>20</sup> cm<sup>3</sup>

Similar to the thick poly-Si(n<sup>+</sup>

passivation quality for thinner layers).

(thermal-SiOx/poly-Si(n<sup>+</sup>

obtainable poly-Si(n<sup>+</sup>

poly-Si layers

Figure 14.

110

implied-VOC values.

Comparison of the ECV profile for 10-nm-thick poly-Si(n<sup>+</sup>

surfaces. Additionally included is the ECV profile for a thick poly-Si(n<sup>+</sup>

A higher dopant in-diffusion is observed for the textured surfaces, which partially explains the lower measured

a 250-nm-thick poly-Si to 0.78 mA cm<sup>2</sup> with a 10-nm-thick poly-Si layer. Additionally, there was also a clear increasing trend in the front-escaped current density from 1.91 mA cm<sup>2</sup> with a 250-nm-thick poly-Si to 2 mA cm<sup>2</sup> with a 10-nmthick poly-Si layer. These two effects were found to limit the potential Jabsorbed, cell

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach…

Hence, for the purpose of device integration, our numerical findings suggest that when considering tunnel oxide/poly-Si(doped) passivated contacts at the rear surface, it would suffice to shrink down the rear-side poly-Si thickness to 25 nm (thinner layers will not further improve the photogeneration current Jabsorbed, cell within the silicon wafer). However, a thicker rear-side poly-Si layer may be more suited to accommodate screen-printed, industrial fire-through metal contacts, without damaging the interface passivation (see the next section). Hence, a trade-off of between these two requirements is needed and to be investigated

Figure 17 presents a pie chart summary for the current loss analysis of the simulated solar cell structure shown in Figure 16, which adopts a rear-side poly-

tioned in the earlier sections. Based on this single-sided (rear-side) passivated contact solar cell structure, the parasitic absorption contribution by the rear-side

the incident photon current density is absorbed by the silicon wafer (88.72%), although this could be further enhanced when better front-side anti-reflection coatings are available for deployment (currently, a front-reflected current density loss of 4.66% is calculated for our in-house deployed thin-film AlOx/SiNx antireflection stack). The second highest current loss channel is the front-escaped current density at 4.32%. Please note that this loss channel cannot be reduced: Photons which are desired to enter the silicon wafer will also be able to leave it. Actually, the higher the percentage loss due to front surface escape, the better the optical performance of the solar cell. The metal grid at the front and rear accounts for a total current loss of 2.05% based on our in-house available screen designs. Taking all optical current losses into account, the maximum absorbable photon

) capping layer with an experimentally realizable thickness of 10 nm as men-

) layer leads to a negligible low 0.04% of the total AM1.5G incident

Extending the analysis from a solar cell with a single rear-side-only passivated contact toward double-sided passivated contacts, the same current loss analysis

Pie chart representing the SunSolve™ current loss analysis for the solar cell structure sketched in Figure 16,

the total incident current density of 46.32 mA cm<sup>2</sup> for the utilized AM1.5G solar spectrum. For a rear-sideonly passivated contact solar cell, the maximum absorbable photocurrent density in the silicon wafer is

) capping layer exhibits a negligible parasitic absorption of 0.04% at 0.02 mA cm<sup>2</sup>

, amounting to 0.02 mA cm<sup>2</sup> only. The bulk of

.

) capping layer thickness of 10 nm. In this case, the

, out of

in case of deploying very thin rear-side poly-Si capping layers.

DOI: http://dx.doi.org/10.5772/intechopen.85039

in the future work.

current density of 46.32 mA cm<sup>2</sup>

current density in the silicon wafer is 41.1 mA cm<sup>2</sup>

featuring an experimentally realizable rear-side poly-Si(n<sup>+</sup>

41.1 mA cm<sup>2</sup> (88.72% of the incoming solar spectrum).

Si(n<sup>+</sup>

poly-Si(n<sup>+</sup>

Figure 17.

113

rear-side poly-Si(n<sup>+</sup>

#### Figure 15.

Optical indices extracted for the doped poly-Si layers in this work, based on ellipsometry measurements and its subsequent fitting by the Tauc-Lorentz model. Also included is the crystalline silicon optical index data for reference. The poly-Si films do exhibit higher extinction coefficient values than the c-Si wafer bulk within the visible to near-infrared regions (400–900 nm), clearly indicating the need to optimize the poly-Si capping layer thickness in order to reduce parasitic absorption.

#### Figure 16.

Numerically calculated absorbed photogeneration current in the silicon solar cell bulk (Jabsorbed, cell) and the parasitic absorption contributed by the rear-side poly-Si(n<sup>+</sup> ) capping layer (Jabsorbed, parasitic (rear-polySi)), as a function of its thickness from 250 nm down to 0 nm, for a rear-side passivated contact solar cell, adopting a conventional front-side boron-diffused emitter junction, and the investigated rear-side electron-selective passivated contacts (tunnel oxide/poly-Si(n<sup>+</sup> )). Reducing the rear-side poly-Si(n<sup>+</sup> ) layer thickness leads to a significant reduction on parasitic absorption (up to 0.55 mA cm<sup>2</sup> ) and a corresponding gain in the photogeneration current Jabsorbed, cell (up to 0.4 mA cm<sup>2</sup> ). Interestingly, Jabsorbed, cell saturates for a poly-Si(n<sup>+</sup> ) layer thickness lower than 25 nm, despite a further reduction of rear-side parasitic absorption (see text).

expect that as the poly-Si capping layer thickness reduces, photons which were not absorbed in the first pass within the cell bulk would now have an increased probability of being parasitically absorbed at the rear-side metal contacts. Indeed, the numerical calculations confirm that hypothesis in which the calculated parasitic absorption within the rear-side metal contacts increases from 0.73 mA cm<sup>2</sup> with

## Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

a 250-nm-thick poly-Si to 0.78 mA cm<sup>2</sup> with a 10-nm-thick poly-Si layer. Additionally, there was also a clear increasing trend in the front-escaped current density from 1.91 mA cm<sup>2</sup> with a 250-nm-thick poly-Si to 2 mA cm<sup>2</sup> with a 10-nmthick poly-Si layer. These two effects were found to limit the potential Jabsorbed, cell in case of deploying very thin rear-side poly-Si capping layers.

Hence, for the purpose of device integration, our numerical findings suggest that when considering tunnel oxide/poly-Si(doped) passivated contacts at the rear surface, it would suffice to shrink down the rear-side poly-Si thickness to 25 nm (thinner layers will not further improve the photogeneration current Jabsorbed, cell within the silicon wafer). However, a thicker rear-side poly-Si layer may be more suited to accommodate screen-printed, industrial fire-through metal contacts, without damaging the interface passivation (see the next section). Hence, a trade-off of between these two requirements is needed and to be investigated in the future work.

Figure 17 presents a pie chart summary for the current loss analysis of the simulated solar cell structure shown in Figure 16, which adopts a rear-side poly-Si(n<sup>+</sup> ) capping layer with an experimentally realizable thickness of 10 nm as mentioned in the earlier sections. Based on this single-sided (rear-side) passivated contact solar cell structure, the parasitic absorption contribution by the rear-side poly-Si(n<sup>+</sup> ) layer leads to a negligible low 0.04% of the total AM1.5G incident current density of 46.32 mA cm<sup>2</sup> , amounting to 0.02 mA cm<sup>2</sup> only. The bulk of the incident photon current density is absorbed by the silicon wafer (88.72%), although this could be further enhanced when better front-side anti-reflection coatings are available for deployment (currently, a front-reflected current density loss of 4.66% is calculated for our in-house deployed thin-film AlOx/SiNx antireflection stack). The second highest current loss channel is the front-escaped current density at 4.32%. Please note that this loss channel cannot be reduced: Photons which are desired to enter the silicon wafer will also be able to leave it. Actually, the higher the percentage loss due to front surface escape, the better the optical performance of the solar cell. The metal grid at the front and rear accounts for a total current loss of 2.05% based on our in-house available screen designs. Taking all optical current losses into account, the maximum absorbable photon current density in the silicon wafer is 41.1 mA cm<sup>2</sup> .

Extending the analysis from a solar cell with a single rear-side-only passivated contact toward double-sided passivated contacts, the same current loss analysis

#### Figure 17.

expect that as the poly-Si capping layer thickness reduces, photons which were not absorbed in the first pass within the cell bulk would now have an increased probability of being parasitically absorbed at the rear-side metal contacts. Indeed, the numerical calculations confirm that hypothesis in which the calculated parasitic absorption within the rear-side metal contacts increases from 0.73 mA cm<sup>2</sup> with

Numerically calculated absorbed photogeneration current in the silicon solar cell bulk (Jabsorbed, cell) and the

Optical indices extracted for the doped poly-Si layers in this work, based on ellipsometry measurements and its subsequent fitting by the Tauc-Lorentz model. Also included is the crystalline silicon optical index data for reference. The poly-Si films do exhibit higher extinction coefficient values than the c-Si wafer bulk within the visible to near-infrared regions (400–900 nm), clearly indicating the need to optimize the poly-Si capping layer

function of its thickness from 250 nm down to 0 nm, for a rear-side passivated contact solar cell, adopting a conventional front-side boron-diffused emitter junction, and the investigated rear-side electron-selective

layer thickness lower than 25 nm, despite a further reduction of rear-side parasitic absorption (see text).

)). Reducing the rear-side poly-Si(n<sup>+</sup>

) capping layer (Jabsorbed, parasitic (rear-polySi)), as a

) and a corresponding gain in the

). Interestingly, Jabsorbed, cell saturates for a poly-Si(n<sup>+</sup>

) layer thickness leads to a

)

parasitic absorption contributed by the rear-side poly-Si(n<sup>+</sup>

photogeneration current Jabsorbed, cell (up to 0.4 mA cm<sup>2</sup>

significant reduction on parasitic absorption (up to 0.55 mA cm<sup>2</sup>

passivated contacts (tunnel oxide/poly-Si(n<sup>+</sup>

thickness in order to reduce parasitic absorption.

Figure 16.

112

Figure 15.

Silicon Materials

Pie chart representing the SunSolve™ current loss analysis for the solar cell structure sketched in Figure 16, featuring an experimentally realizable rear-side poly-Si(n<sup>+</sup> ) capping layer thickness of 10 nm. In this case, the rear-side poly-Si(n<sup>+</sup> ) capping layer exhibits a negligible parasitic absorption of 0.04% at 0.02 mA cm<sup>2</sup> , out of the total incident current density of 46.32 mA cm<sup>2</sup> for the utilized AM1.5G solar spectrum. For a rear-sideonly passivated contact solar cell, the maximum absorbable photocurrent density in the silicon wafer is 41.1 mA cm<sup>2</sup> (88.72% of the incoming solar spectrum).

approach was applied to front- and rear-side passivated contact solar cells, exhibiting an optically negligible rear-side capping layer thickness of 3 nm, as experimentally realized. As sketched in Figure 18, this solar cell structure consists of a front-side textured surface with our developed electron-selective (tunnel oxide/poly-Si(n<sup>+</sup> )) passivated contacts, and a rear-side planar surface with our developed hole-selective (tunnel oxide/poly-Si(p<sup>+</sup> )) passivated contacts. This is followed by the standard dielectric coatings (SiOx, SiNx, AlOx) at both surfaces to serve both passivation and anti-reflection purposes, prior to the screen-printed firethrough metal contacts at both sides. Adopting an experimentally realizable rearside poly-Si(p<sup>+</sup> ) capping layer thickness of 3 nm (see earlier section), the influence of the front-side poly-Si capping layer thickness on the Jabsorbed, cell is investigated. Figure 18 shows that the parasitic absorption by the front-side poly-Si capping layer has a much more severe and significant impact on the remaining absorbable current density in the solar cell bulk (Jabsorbed, cell). If contact passivation is applied frontside, <sup>J</sup>absorbed, parasitic (front poly-Si) is as high as 20.8 mA cm<sup>2</sup> for a 250-nm-thick poly-Si(n<sup>+</sup> ) layer, and it reduces to 1 mA cm<sup>2</sup> for a 5-nm-thick poly-Si(n<sup>+</sup> ) layer. Front-side poly-Si layer thickness reduction therefore directly translates into a significant gain in Jabsorbed, cell, approximately by the same amount (i.e., increasing from 21 to 40.3 mA cm<sup>2</sup> ).

Accordingly, the pie chart current loss analysis for the double-sided passivated contact solar cell structure depicted in Figure 3 is shown in Figure 19 for the case of an experimentally realizable front-side poly-Si(n<sup>+</sup> ) capping layer thickness of 10 nm and an experimentally realizable rear-side poly-Si(p<sup>+</sup> ) capping layer thickness of 3 nm. Figure 19 shows that the presence of the 10-nm-thick front-side poly-Si(n<sup>+</sup> ) capping layer contributes to a comparatively higher parasitic absorption loss (4.32%) than the rear-side poly-Si(p<sup>+</sup> ) capping layer (0.01%), based on a total incident current density of 46.32 mA cm<sup>2</sup> (AM1.5G spectrum). The remaining potentially absorbable current density within the solar cell bulk stands at 85.56% (39.6 mA cm<sup>2</sup> ), which is 3.16% lower than a rear-side-only passivated contact scheme. Hence, it is clear that although double-sided passivated contact solar cells could deliver excellent passivation on both sides of the wafer (thereby reaching higher open-circuit voltages VOC than rear-side-only passivated contact solar cells or conventional diffused solar cells), there is still a trade-off with increased frontside parasitic absorption, demanding more optimization efforts.

3.7 Compatibility of screen printing (using conventional screen-printing

Pie chart representing the SunSolve™ current loss analysis for the double-sided passivated contact solar cell

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach…

) capping layer can be neglected, based on a total incident current density of 46.32 mA cm<sup>2</sup> for the utilized AM1.5G solar spectrum. For the double-sided passivated contact solar cell, the maximum absorbable photocurrent density in the silicon wafer is now 39.6 mA cm<sup>2</sup> (85.56% of the

) capping layer thickness of 10 nm. The 10-nm-thin front-side poly-Si(n<sup>+</sup>

In earlier sections, the feasibility of the electron-selective and hole-selective passivated contacts has been demonstrated, both on symmetrical lifetime test structures and asymmetrical solar cell precursors as sketched in Figure 11 (in the asdeposited state and after an additional symmetrical SiNx capping). The remaining solar cell fabrication step would be the formation of metal contacts toward these thin-film passivated contacts, without damaging the passivation quality underneath these contacts. As a first attempt, conventional metal contacting schemes, i.e., screen printing, as commonly deployed for conventional silicon solar cells

(exhibiting double-sided diffused junctions), were performed on our lifetime and solar cell precursors. In particular, we tested our industrial in-house fire-through and non-fire-through screen-printing pastes, based on Ag, Ag/Al, or Al material formulations. The corresponding results were compared to a nonindustrial research

In summary, so far, using conventional screen-printing pastes, screen printing

(I) A fire-through Ag paste (as conventionally used to contact n-doped silicon

formably, without any issues, i.e., exhibiting a low contact resistance (13 mΩ cm<sup>2</sup>

(top, left)). The investigated fire-through Ag paste is suitable for rear-side

and no void issues or punch-through effects underneath the contact (see Figure 20

Deploying industrial screen printing for rear-side-only passivated contact solar cells, we currently reach a solar cell efficiency of 21.7%, using our 250-nm "stan-

) capping layers down to a thickness of 150 nm; however, it

) contact passivation layers (see Figure 21 and

) layers, i.e., requiring a poly-Si(n+

) capping layer thickness of 3 nm and a front-

), whereas parasitic absorption within the 3-nm

) capping layer still

) capping layer, as outlined in some

)

)

) layers. The

) layers con-

pastes) on our developed passivated contact layers

structure sketched in Figure 18, featuring a rear-side poly-Si(p<sup>+</sup>

DOI: http://dx.doi.org/10.5772/intechopen.85039

contributes to the parasitic absorption (4.32% at 0.2 mA cm<sup>2</sup>

reference contact, deploying thermally evaporated Ag contacts.

SEM results presented in Figure 20 sum up these observations.

thickness of 150 nm or larger. So far, it does not work on poly-Si(p<sup>+</sup>

material) is able to contact our standard 250-nm-thick poly-Si(n<sup>+</sup>

works only on comparatively thick poly-Si(n<sup>+</sup>

fails to contact our ultrathin 10-nm poly-Si(n+

contacting poly-Si(n<sup>+</sup>

dard" rear-side SiOx/poly-Si(n<sup>+</sup>

more detail later.

Table 8).

115

Figure 19.

side poly-Si(n<sup>+</sup>

rear-side poly-Si(p<sup>+</sup>

incoming solar spectrum).

#### Figure 18.

Numerically calculated photon current absorption for a double-sided passivated contact solar cell. The rear-side hole-selective poly-Si(p<sup>+</sup> ) capping layer thickness is fixed at 3 nm, while the front-side electron-selective poly-Si(n+ ) capping layer thickness is varied from 0 nm to 250 nm. Front-side parasitic within the poly-Si(n<sup>+</sup> ) capping layer (Jabsorbed, parasitic (front poly-Si)) has a severe impact on the absorbable photon current density within the silicon wafer (Jabsorbed, cell).

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

#### Figure 19.

approach was applied to front- and rear-side passivated contact solar cells, exhibiting an optically negligible rear-side capping layer thickness of 3 nm, as experimentally realized. As sketched in Figure 18, this solar cell structure consists of a front-side textured surface with our developed electron-selective (tunnel

followed by the standard dielectric coatings (SiOx, SiNx, AlOx) at both surfaces to serve both passivation and anti-reflection purposes, prior to the screen-printed firethrough metal contacts at both sides. Adopting an experimentally realizable rear-

of the front-side poly-Si capping layer thickness on the Jabsorbed, cell is investigated. Figure 18 shows that the parasitic absorption by the front-side poly-Si capping layer has a much more severe and significant impact on the remaining absorbable current density in the solar cell bulk (Jabsorbed, cell). If contact passivation is applied frontside, <sup>J</sup>absorbed, parasitic (front poly-Si) is as high as 20.8 mA cm<sup>2</sup> for a 250-nm-thick

) layer, and it reduces to 1 mA cm<sup>2</sup> for a 5-nm-thick poly-Si(n<sup>+</sup>

Accordingly, the pie chart current loss analysis for the double-sided passivated contact solar cell structure depicted in Figure 3 is shown in Figure 19 for the case of

ness of 3 nm. Figure 19 shows that the presence of the 10-nm-thick front-side poly-

incident current density of 46.32 mA cm<sup>2</sup> (AM1.5G spectrum). The remaining potentially absorbable current density within the solar cell bulk stands at 85.56%

scheme. Hence, it is clear that although double-sided passivated contact solar cells could deliver excellent passivation on both sides of the wafer (thereby reaching higher open-circuit voltages VOC than rear-side-only passivated contact solar cells or conventional diffused solar cells), there is still a trade-off with increased front-

Numerically calculated photon current absorption for a double-sided passivated contact solar cell. The rear-side

) capping layer thickness is varied from 0 nm to 250 nm. Front-side parasitic within the poly-Si(n<sup>+</sup>

capping layer (Jabsorbed, parasitic (front poly-Si)) has a severe impact on the absorbable photon current density

) capping layer thickness is fixed at 3 nm, while the front-side electron-selective poly-

) capping layer contributes to a comparatively higher parasitic absorption loss

), which is 3.16% lower than a rear-side-only passivated contact

Front-side poly-Si layer thickness reduction therefore directly translates into a significant gain in Jabsorbed, cell, approximately by the same amount (i.e., increasing

).

10 nm and an experimentally realizable rear-side poly-Si(p<sup>+</sup>

side parasitic absorption, demanding more optimization efforts.

an experimentally realizable front-side poly-Si(n<sup>+</sup>

developed hole-selective (tunnel oxide/poly-Si(p<sup>+</sup>

)) passivated contacts, and a rear-side planar surface with our

) capping layer thickness of 3 nm (see earlier section), the influence

)) passivated contacts. This is

) capping layer thickness of

) capping layer (0.01%), based on a total

) capping layer thick-

) layer.

)

oxide/poly-Si(n<sup>+</sup>

Silicon Materials

side poly-Si(p<sup>+</sup>

poly-Si(n<sup>+</sup>

Si(n<sup>+</sup>

Figure 18.

Si(n+

114

hole-selective poly-Si(p<sup>+</sup>

within the silicon wafer (Jabsorbed, cell).

(39.6 mA cm<sup>2</sup>

from 21 to 40.3 mA cm<sup>2</sup>

(4.32%) than the rear-side poly-Si(p<sup>+</sup>

Pie chart representing the SunSolve™ current loss analysis for the double-sided passivated contact solar cell structure sketched in Figure 18, featuring a rear-side poly-Si(p<sup>+</sup> ) capping layer thickness of 3 nm and a frontside poly-Si(n<sup>+</sup> ) capping layer thickness of 10 nm. The 10-nm-thin front-side poly-Si(n<sup>+</sup> ) capping layer still contributes to the parasitic absorption (4.32% at 0.2 mA cm<sup>2</sup> ), whereas parasitic absorption within the 3-nm rear-side poly-Si(p<sup>+</sup> ) capping layer can be neglected, based on a total incident current density of 46.32 mA cm<sup>2</sup> for the utilized AM1.5G solar spectrum. For the double-sided passivated contact solar cell, the maximum absorbable photocurrent density in the silicon wafer is now 39.6 mA cm<sup>2</sup> (85.56% of the incoming solar spectrum).

## 3.7 Compatibility of screen printing (using conventional screen-printing pastes) on our developed passivated contact layers

In earlier sections, the feasibility of the electron-selective and hole-selective passivated contacts has been demonstrated, both on symmetrical lifetime test structures and asymmetrical solar cell precursors as sketched in Figure 11 (in the asdeposited state and after an additional symmetrical SiNx capping). The remaining solar cell fabrication step would be the formation of metal contacts toward these thin-film passivated contacts, without damaging the passivation quality underneath these contacts. As a first attempt, conventional metal contacting schemes, i.e., screen printing, as commonly deployed for conventional silicon solar cells (exhibiting double-sided diffused junctions), were performed on our lifetime and solar cell precursors. In particular, we tested our industrial in-house fire-through and non-fire-through screen-printing pastes, based on Ag, Ag/Al, or Al material formulations. The corresponding results were compared to a nonindustrial research reference contact, deploying thermally evaporated Ag contacts.

In summary, so far, using conventional screen-printing pastes, screen printing works only on comparatively thick poly-Si(n<sup>+</sup> ) layers, i.e., requiring a poly-Si(n+ ) thickness of 150 nm or larger. So far, it does not work on poly-Si(p<sup>+</sup> ) layers. The SEM results presented in Figure 20 sum up these observations.

(I) A fire-through Ag paste (as conventionally used to contact n-doped silicon material) is able to contact our standard 250-nm-thick poly-Si(n<sup>+</sup> ) layers conformably, without any issues, i.e., exhibiting a low contact resistance (13 mΩ cm<sup>2</sup> ) and no void issues or punch-through effects underneath the contact (see Figure 20 (top, left)). The investigated fire-through Ag paste is suitable for rear-side contacting poly-Si(n<sup>+</sup> ) capping layers down to a thickness of 150 nm; however, it fails to contact our ultrathin 10-nm poly-Si(n+ ) capping layer, as outlined in some more detail later.

Deploying industrial screen printing for rear-side-only passivated contact solar cells, we currently reach a solar cell efficiency of 21.7%, using our 250-nm "standard" rear-side SiOx/poly-Si(n<sup>+</sup> ) contact passivation layers (see Figure 21 and Table 8).

#### Figure 20.

SEM images taken for poly-Si(n<sup>+</sup> ) and poly-Si(p<sup>+</sup> ) layers contacted via conventional screen printing, using various commercially available pastes: (i) bifacial fire-through pastes, i.e., Ag paste for contacting n-doped Si and Al paste for contacting p-doped Si, and (ii) non-fire-through Ag/Al pastes, using laser ablation to form local contact openings prior to screen printing. Screen printing works only in case of contacting moderately thick (150–250 nm) electron-extracting poly-Si(n<sup>+</sup> ) capping layers. In all other cases, issues like void formation or a local "punch through" of the metal paste (locally contacting the c-Si wafer instead of the poly-Si capping layer) occur.

"punch-through" areas (see Figure 20 (top, right)). This in turn leads to local shunting (in case of using an n-type wafer) and to a severe degradation of contact

This issue can be likely attributed to the presence of the Al alloy within the paste, which is typically responsible for forming the back surface field regions in conventional silicon solar cells. Al alloying is known to partially consume crystalline silicon

firing of the screen-printed Ag/Al paste, leading to the just outlined local "punch-

(III) A non-fire-through pure Al paste (as conventionally used to contact a p-doped silicon wafer in order to form locally Al-alloyed back-surface-field (BSF) regions within the wafer) was found to create large voids in several regions (see

tacts, leading to a drastic drop in contact passivation quality and measured device

ultraviolet wavelength of 330 nm, the onset of laser fluence for optimized SiNx

preserved after laser ablation (as indicated by photoluminescence imaging), and the SiNx is fully ablated (as indicated by optical microscope imaging) (see Figure 22). However, the paste composition of the screen-printing paste has to be altered, in order to enable a subsequent damage-free contacting of our (thick or ultrathin)

) layers. Corresponding research activities, in cooperation with a paste

It is possible to use femtosecond laser ablation, in order to create damage-free local contact openings (i.e., locally ablating the overlaying SiNx layer without dam-

) capping layers will be consumed upon contact

) capping layer). Using a femtosecond laser at an

. Within the optimized process window, the lifetime is

) passivated con-

) capping layer

passivation quality, as evident from the final measured cell Voc values.

I–V data of rear-side passivated contact solar cells, with a varying rear-side poly-Si(n<sup>+</sup>

Figure 20 (bottom, right)) and to consume the entire poly-Si(p<sup>+</sup>

material: thus, our thin poly-Si(p<sup>+</sup>

aging the underlying poly-Si(p<sup>+</sup>

manufacturer, are currently initiated.

ablation is 0.08 J cm<sup>2</sup>

through" effects.

Cell type: thickness of rear poly-Si(n<sup>+</sup>

Table 8.

Table 9.

thickness.

metallization.

Cell type: thickness of rear poly-Si(n<sup>+</sup>

)

)

Voc (mV)

DOI: http://dx.doi.org/10.5772/intechopen.85039

250 nm 678 39.7 80.5 21.7

Voc (mV)

250 nm 672 39.4 80.2 21.3 150 nm 676 39.8 80.7 21.7 100 nm 660 39.6 80.2 21.0

Jsc (mA cm<sup>2</sup> )

FF (%)

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach…

I–V data of the rear-side passivated contact record cell, deploying conventional bifacial screen printing for

Jsc (mA cm<sup>2</sup> ) FF (%)

Eff (%)

Eff (%)

performance.

poly-Si(p<sup>+</sup>

117

#### Figure 21.

(a) Measured I–V curve of our current rear-side passivated contact record efficiency cell, exhibiting a rear-side wet-chemically formed SiOx tunnel layer, a 250-nm poly-Si(n<sup>+</sup> ) capping layer, and a conventionally formed (boron-diffused, AlOx/SiNx passivated and screen printed) front-side contact. (b) The correspondingly measured external/internal quantum efficiency, EQE, IQE, and the measured reflectance for the same cell.

Reducing the rear-side poly-Si(n+ ) capping layer thickness (separate batch, hereby only reaching 21.3% for the solar cell with the 250-nm-thick poly-Si reference layer), we were able to observe a clear increase in short-circuit current density (see Table 9). By thinning down the rear-side poly-Si(n<sup>+</sup> ) capping layer thickness from 250 nm down to 150 nm, using etch-back technology, we gain 0.4 mA cm<sup>2</sup> in short-circuit current density, reaching again a best cell efficiency of 21.7%. Up to a thickness of 150 nm, the poly-Si(n<sup>+</sup> ) thinning did neither significantly affect the open-circuit voltage Voc nor the fill factor of the solar cell (compare Table 8). However, the samples with a 100-nm rear-side poly-Si capping layer exhibit a drop in Voc (15 mV). This resulted from a local punch through of the screen-printed metal paste, similar to the SEM image as shown in Figure 20 (top, right).

(II) A fire-through Ag/Al paste (as conventionally used to contact p-doped silicon material) could not contact our standard 250-nm-thick poly-Si(p<sup>+</sup> ) layers properly: There are several regions where the paste is observed to consume the poly-Si(p<sup>+</sup> ) layer, causing a thinning of the poly-Si(p<sup>+</sup> ) layer and some local

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

#### Table 8.

I–V data of the rear-side passivated contact record cell, deploying conventional bifacial screen printing for metallization.


#### Table 9.

I–V data of rear-side passivated contact solar cells, with a varying rear-side poly-Si(n<sup>+</sup> ) capping layer thickness.

"punch-through" areas (see Figure 20 (top, right)). This in turn leads to local shunting (in case of using an n-type wafer) and to a severe degradation of contact passivation quality, as evident from the final measured cell Voc values.

This issue can be likely attributed to the presence of the Al alloy within the paste, which is typically responsible for forming the back surface field regions in conventional silicon solar cells. Al alloying is known to partially consume crystalline silicon material: thus, our thin poly-Si(p<sup>+</sup> ) capping layers will be consumed upon contact firing of the screen-printed Ag/Al paste, leading to the just outlined local "punchthrough" effects.

(III) A non-fire-through pure Al paste (as conventionally used to contact a p-doped silicon wafer in order to form locally Al-alloyed back-surface-field (BSF) regions within the wafer) was found to create large voids in several regions (see Figure 20 (bottom, right)) and to consume the entire poly-Si(p<sup>+</sup> ) passivated contacts, leading to a drastic drop in contact passivation quality and measured device performance.

It is possible to use femtosecond laser ablation, in order to create damage-free local contact openings (i.e., locally ablating the overlaying SiNx layer without damaging the underlying poly-Si(p<sup>+</sup> ) capping layer). Using a femtosecond laser at an ultraviolet wavelength of 330 nm, the onset of laser fluence for optimized SiNx ablation is 0.08 J cm<sup>2</sup> . Within the optimized process window, the lifetime is preserved after laser ablation (as indicated by photoluminescence imaging), and the SiNx is fully ablated (as indicated by optical microscope imaging) (see Figure 22). However, the paste composition of the screen-printing paste has to be altered, in order to enable a subsequent damage-free contacting of our (thick or ultrathin) poly-Si(p<sup>+</sup> ) layers. Corresponding research activities, in cooperation with a paste manufacturer, are currently initiated.

Reducing the rear-side poly-Si(n+

wet-chemically formed SiOx tunnel layer, a 250-nm poly-Si(n<sup>+</sup>

a thickness of 150 nm, the poly-Si(n<sup>+</sup>

poly-Si(p<sup>+</sup>

116

Figure 20.

Silicon Materials

occur.

Figure 21.

SEM images taken for poly-Si(n<sup>+</sup>

(150–250 nm) electron-extracting poly-Si(n<sup>+</sup>

) and poly-Si(p<sup>+</sup>

various commercially available pastes: (i) bifacial fire-through pastes, i.e., Ag paste for contacting n-doped Si and Al paste for contacting p-doped Si, and (ii) non-fire-through Ag/Al pastes, using laser ablation to form local contact openings prior to screen printing. Screen printing works only in case of contacting moderately thick

local "punch through" of the metal paste (locally contacting the c-Si wafer instead of the poly-Si capping layer)

(see Table 9). By thinning down the rear-side poly-Si(n<sup>+</sup>

) capping layer thickness (separate batch,

) layers contacted via conventional screen printing, using

) capping layers. In all other cases, issues like void formation or a

) thinning did neither significantly affect the

) layer and some local

) capping layer thickness

) capping layer, and a conventionally formed

) layers

hereby only reaching 21.3% for the solar cell with the 250-nm-thick poly-Si reference layer), we were able to observe a clear increase in short-circuit current density

(a) Measured I–V curve of our current rear-side passivated contact record efficiency cell, exhibiting a rear-side

(boron-diffused, AlOx/SiNx passivated and screen printed) front-side contact. (b) The correspondingly measured external/internal quantum efficiency, EQE, IQE, and the measured reflectance for the same cell.

from 250 nm down to 150 nm, using etch-back technology, we gain 0.4 mA cm<sup>2</sup> in short-circuit current density, reaching again a best cell efficiency of 21.7%. Up to

open-circuit voltage Voc nor the fill factor of the solar cell (compare Table 8). However, the samples with a 100-nm rear-side poly-Si capping layer exhibit a drop in Voc (15 mV). This resulted from a local punch through of the screen-printed metal paste, similar to the SEM image as shown in Figure 20 (top, right).

(II) A fire-through Ag/Al paste (as conventionally used to contact p-doped silicon material) could not contact our standard 250-nm-thick poly-Si(p<sup>+</sup>

properly: There are several regions where the paste is observed to consume the

) layer, causing a thinning of the poly-Si(p<sup>+</sup>

#### Figure 22.

(Left) Sketch of the laser process to locally laser ablate SiNx on top of thin-film poly-Si, in order to form local contact openings for further metallization, aiming at conventional screen printing, using a non-fire-through paste. (Right) Photoluminescence images and optical microscope images inside the opening, taken for a screened range of laser fluence.

(IV) As expected, our research reference thermal evaporated Ag contacts were able to form damage-free conformal low resistivity contacts to our developed SiOx/poly-Si(p<sup>+</sup> ) and ALD-AlOx/poly-Si(p<sup>+</sup> ) passivated contacts, thereby enabling a nonindustrial full-area reference contact on hole-extracting poly-Si(p<sup>+</sup> ) capping layers [34].

is not the case for the 250-nm-thick "standard" layers. ECV measurements confirm

passivated contacts, in the as deposited state, after additional SiNx deposition and after a fast-firing belt furnace

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach…

Comparison of the measured passivation quality for the asymmetrical lifetime test structures comprising a textured front and a planar rear surface, passivated with electron-extracting thermal-SiOx/10 nm-poly-Si(n<sup>+</sup>

diffused into the silicon wafer bulk, thereby effectively reducing field-effect passivation and thus the observed lifetimes of the samples. More detailed investigations

Thus, more efforts to render our ultrathin contact passivation layers firing stable, i.e., by deploying lower peak firing temperatures and/or changing the chemical composition of the ultrathin LPCVD of poly-Si capping layers, are necessary. Furthermore, efforts to optimize the composition of the screen-printing paste itself, in order to be able to successfully contact ultrathin poly-Si layers using screen

printing, will be undertaken. An alternative work plan is to investigate lowtemperature inline plating, as a possible approach to contact our ultrathin

3.8 Cell efficiency potential prediction: single-sided versus double-sided

side contacting scheme is better than a bifacial contacting scheme.

As already indicated in the introduction part, we can determine a practical solar cell efficiency potential of our investigated solar cell structures, adopting either a rear-sideonly passivated contact scheme or a double-sided passivated contact scheme. Using Brendel's model [54], and explicitly considering measured front-side contact resistance and contact recombination parameters (i.e., the combined front-side saturation current density J0, front, combing the contributions from both the non-metallized/passivated regionsJ0, non-metal and from the metallized regionsJ0, metal), it is possible to calculate a practical solar cell efficiency potential as a function of the rear-side passivated contact layer properties, i.e., the rear-side recombination current density J0, rear and the rearside contact resistance Rc, rear of the rear-side passivated solar cell contact. By fixing the front-side J0, front and Rc, front contributions, iso-efficiency contour plots can be calculated as a function of the rear-side J0, rear and the rear-side Rc, rear (thereby generalizing Brendel's model [54]). The goal of the cell efficiency prediction is twofold: (1) to determine if adopting a double-sided passivated contacts scheme is better than the single-sided (rear) passivated contact scheme and (2) to determine if a full-area rear-

Firstly, Figure 24 shows a comparison of solar cells with a rear-side-only passivated contact scheme, comprising a conventional front-side textured surface with a boron-diffused emitter, passivated by a standard AlOx/SiNx double-layer

) capping layer have out-

)

that after fast-firing, the dopants within the poly-Si(n<sup>+</sup>

temperature treatment. The straight lines are a guide to the eyes.

DOI: http://dx.doi.org/10.5772/intechopen.85039

are currently ongoing.

Figure 23.

SiOx/poly-Si contact passivation layers.

contact passivation

119

As just outlined above, using our conventional screen-printing metal pastes and fast-firing conditions, thus far we were not able to successfully contact holeextracting poly-Si(p<sup>+</sup> ) layers as well as ultrathin 10-nm electron-extracting poly-Si(n<sup>+</sup> ) layers. Thus, a closer attention toward (i) an optimization of the metal paste itself, i.e., tuning its chemical composition, and (ii) an optimization of the fastfiring conditions, applied after screen printing, in order to form a low resistivity contact, is necessary.

To address the latter, an asymmetric lifetime test structure, featuring a textured front surface and a planar rear surface, symmetrically passivated contact by our ultrathin (10 nm) electron-selective, thermal-SiOx/poly-Si(n<sup>+</sup> ) passivated contact layers, was utilized. The passivation quality of these samples in the as-deposited state was measured first, followed by a symmetrical deposition of the passivation/anti-reflective SiNx film, and its passivation quality was remeasured. Then, the samples were subjected to different fast-firing peak temperatures (650, 660, 680, 700, 720, 740, 760°C), thereby mimicking different fast-firing conditions after screen printing, and the resulting final passivation quality was remeasured again (see Figure 23).

For our ultrathin 10-nm SiOx/poly-Si(n<sup>+</sup> ) contact passivation layers, a severe degradation of passivation quality after fast-firing is observed (see Figure 23). In the as-deposited state, our asymmetrical lifetime test structures with electronselective passivated contacts were exhibiting good passivation quality with average <sup>τ</sup>eff/J0/iVoc values of 3.3 ms /35.4 fA cm<sup>2</sup> /704 mV. Upon the subsequent symmetrical SiNx capping, the passivation quality got further enhanced, i.e., reaching excellent average <sup>τ</sup>eff/J0/iVoc values of 7.3 ms /14.4 fA cm<sup>2</sup> /720 mV. However, after an additional short high-temperature treatment, in case of using our ultrathin 10-nm electron-extracting contact passivation layers, the passivation quality drops significantly. For example, if we adopt a fast-firing peak temperature of 740°C (which is currently utilized for our conventional double-sided diffused silicon solar cells), a drastic drop in passivation quality occurs, in which the τeff/J0/iVoc values degrade to 1.2 ms/91 fA cm<sup>2</sup> /684 mV. This effect is less severe, but still significant, if lower fast-firing peak temperatures can be deployed (see Figure 23). It seems like our current ultrathin electron-selective passivation layers are not firing stable, especially if deploying high peak firing temperatures (they still do outperform conventionally diffused front-side contacts, though). Interestingly, this Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

Figure 23.

(IV) As expected, our research reference thermal evaporated Ag contacts were

(Left) Sketch of the laser process to locally laser ablate SiNx on top of thin-film poly-Si, in order to form local contact openings for further metallization, aiming at conventional screen printing, using a non-fire-through paste. (Right) Photoluminescence images and optical microscope images inside the opening, taken for a screened

As just outlined above, using our conventional screen-printing metal pastes and

) layers. Thus, a closer attention toward (i) an optimization of the metal paste

To address the latter, an asymmetric lifetime test structure, featuring a textured front surface and a planar rear surface, symmetrically passivated contact by our

) layers as well as ultrathin 10-nm electron-extracting poly-

) passivated contacts, thereby enabling a

) contact passivation layers, a severe

/704 mV. Upon the subsequent sym-

/684 mV. This effect is less severe, but still signifi-

) capping

) passivated contact

able to form damage-free conformal low resistivity contacts to our developed

fast-firing conditions, thus far we were not able to successfully contact hole-

itself, i.e., tuning its chemical composition, and (ii) an optimization of the fastfiring conditions, applied after screen printing, in order to form a low resistivity

layers, was utilized. The passivation quality of these samples in the as-deposited

passivation/anti-reflective SiNx film, and its passivation quality was remeasured. Then, the samples were subjected to different fast-firing peak temperatures (650, 660, 680, 700, 720, 740, 760°C), thereby mimicking different fast-firing conditions after screen printing, and the resulting final passivation quality was remeasured

degradation of passivation quality after fast-firing is observed (see Figure 23). In the as-deposited state, our asymmetrical lifetime test structures with electronselective passivated contacts were exhibiting good passivation quality with average

metrical SiNx capping, the passivation quality got further enhanced, i.e., reaching excellent average <sup>τ</sup>eff/J0/iVoc values of 7.3 ms /14.4 fA cm<sup>2</sup> /720 mV. However, after an additional short high-temperature treatment, in case of using our ultrathin 10-nm electron-extracting contact passivation layers, the passivation quality drops significantly. For example, if we adopt a fast-firing peak temperature of 740°C (which is currently utilized for our conventional double-sided diffused silicon solar cells), a drastic drop in passivation quality occurs, in which the τeff/J0/iVoc values

cant, if lower fast-firing peak temperatures can be deployed (see Figure 23). It seems like our current ultrathin electron-selective passivation layers are not firing

outperform conventionally diffused front-side contacts, though). Interestingly, this

stable, especially if deploying high peak firing temperatures (they still do

nonindustrial full-area reference contact on hole-extracting poly-Si(p<sup>+</sup>

) and ALD-AlOx/poly-Si(p<sup>+</sup>

ultrathin (10 nm) electron-selective, thermal-SiOx/poly-Si(n<sup>+</sup>

For our ultrathin 10-nm SiOx/poly-Si(n<sup>+</sup>

<sup>τ</sup>eff/J0/iVoc values of 3.3 ms /35.4 fA cm<sup>2</sup>

degrade to 1.2 ms/91 fA cm<sup>2</sup>

118

state was measured first, followed by a symmetrical deposition of the

SiOx/poly-Si(p<sup>+</sup>

range of laser fluence.

Silicon Materials

extracting poly-Si(p<sup>+</sup>

contact, is necessary.

again (see Figure 23).

layers [34].

Figure 22.

Si(n<sup>+</sup>

Comparison of the measured passivation quality for the asymmetrical lifetime test structures comprising a textured front and a planar rear surface, passivated with electron-extracting thermal-SiOx/10 nm-poly-Si(n<sup>+</sup> ) passivated contacts, in the as deposited state, after additional SiNx deposition and after a fast-firing belt furnace temperature treatment. The straight lines are a guide to the eyes.

is not the case for the 250-nm-thick "standard" layers. ECV measurements confirm that after fast-firing, the dopants within the poly-Si(n<sup>+</sup> ) capping layer have outdiffused into the silicon wafer bulk, thereby effectively reducing field-effect passivation and thus the observed lifetimes of the samples. More detailed investigations are currently ongoing.

Thus, more efforts to render our ultrathin contact passivation layers firing stable, i.e., by deploying lower peak firing temperatures and/or changing the chemical composition of the ultrathin LPCVD of poly-Si capping layers, are necessary. Furthermore, efforts to optimize the composition of the screen-printing paste itself, in order to be able to successfully contact ultrathin poly-Si layers using screen printing, will be undertaken. An alternative work plan is to investigate lowtemperature inline plating, as a possible approach to contact our ultrathin SiOx/poly-Si contact passivation layers.
