Interdigitated back contact (IBC) architecture can yield among the highest silicon wafer-based solar cell conversion efficiencies. Since both polarities are realized on the rear side, there is a definite need for a patterning step. Some of the common patterning techniques involve photolithography, inkjet patterning, and laser ablation. This work introduces a novel patterning technique for structuring the rear side of IBC solar cells using the enhanced oxidation characteristics under the locally laser-doped n⁺⁺ back surface field (BSF) regions with high-phosphorous surface concentrations. Phosphosilicate glass layers deposited via POCl₃ diffusion serve as a precursor layer for the formation of local heavily laser-doped n⁺⁺ BSF regions. The laser-doped n⁺⁺ BSF regions exhibit a 2.6-fold increase in oxide thickness compared to the nonlaser-doped n⁺ BSF regions after undergoing high-temperature wet thermal oxidation. The utilization of oxide thickness selectivity under laser-doped and nonlaser-doped regions serves two purposes in the context of the IBC solar cell, first patterning rear side and second acting as a masking layer for the subsequent boron diffusion. Proof-of-concept solar cells are fabricated using this novel patterning technique with a mean conversion efficiency of 20.41%.

@en

The doctoral dissertation focuses on developing cost-effective, yet industrially viable, high-efficiency solar cells using APCVD technology. The following are some of the key highlights of the work: 1) A novel patterning and masking technique for fabricating IBC solar cells. The patterning technique capitalizes on the enhanced oxidation properties present under the laser-doped phosphorous BSF regions. 2) We fabricated diffused junction IBC solar cells with conversion efficiencies comparable to the commercially available full-tube diffused ZEBRA IBC solar cells, using the APCVD layer to form the doped regions. We use the high-temperature co-annealing step to prevent BRL formation, drive-in dopants of both polarities, and grow in-situ SiO2 at the Si/dopant glass interface, which later serves as a universal passivation stack. 3) This work demonstrates the concept of in-situ annealing used to crystallize boron-doped amorphous silicon layers deposited by APCVD to form boron-doped polysilicon layers for the fabrication of cost-effective solar cell concepts based on passivating contacts. 4) This work also delves into understanding the origins and mitigation strategies for the blistering of polysilicon layers upon fast-firing. We demonstrate that the use of a bi-layer stack with a low hydrogen content, such as PECVD silicon oxide or phosphosilicate glass, between the poly-Si and PECVD silicon nitride layers significantly improves the firing stability of the phosphorous-doped poly-Si layers.@en

Even though interdigitated back contact (IBC) architecture produces the most efficient solar cells, it is difficult to make them cost-effective and industrially viable. Therefore, single-sided atmospheric pressure chemical vapor deposition (APCVD) is investigated for the fabrication of IBC solar cells because it reduces the overall thermal budget, simplifies wet bench processing, and requires no additional masking layer. For the fabrication of a full APCVD IBC solar cell, a very lightly doped front surface field (FSF) of 650 Ω/sq, a heavier doped back surface field (BSF) of 100 Ω/sq and a moderately doped emitter of 250 Ω/sq was used. The high-temperature annealing step is partially done in an oxygen (O₂) environment to (i) drive in dopants, (ii) prevent the formation of a boron-rich layer in case of p⁺ doped c-Si, and (iii) grow an in-situ SiO₂ at the Si/dopant glass interface. The etch rate difference between the in-situ grown SiO₂ and the doped glass layer is utilized to etch the doped glass completely. The retained in-situ SiO₂ after etching is capped with plasma-enhanced chemical vapor deposited (PECVD) SiN_x for the passivation of both polarities of IBC solar cells. A full APCVD IBC solar cell precursors (i.e. before metallization) obtained implied open-circuit voltage (iV_oc) of 714 mV and emitter saturation current density (J_0s) of 17 fA/cm². At the device level, a full APCVD IBC solar cell achieved a conversion efficiency of 22.8% with V_oc of 696 mV and short-circuit current density J_SC of 41.3 mA/cm². These parameters are comparable to the commercially available full-tube diffused ZEBRA® IBC solar cells.

@en

In this work, we developed an in situ annealing process to crystallize boron-doped amorphous silicon [a-Si(p+)] layers deposited by atmospheric pressure chemical vapour deposition (APCVD) to form boron-doped polycrystalline silicon [poly-Si(p+)] layers. The influence of the temperature profiles during a-Si(p+) inline deposition on structural, electrical, and passivation properties was studied in detail. The results show that a-Si(p+) layers can be successfully crystallized by fine-tuning the temperature profiles in the postdeposition zones of the APCVD tool. It was observed that the hydrogenation processes during the fast firing play a significant role in enhancing the passivation quality as well as the electrical properties of the in situ annealed poly-Si(p+) layers. The sheet resistance (Rsh) and implied open circuit voltage (iVoc) of the best in situ annealed poly-Si(p+) layers were found to be comparable to the ones that were ex situ annealed in the tube furnace at 950 $^{\circ }$C for 30 min. The sheet resistance of 200 $\Omega$/$\square$ could be obtained on 150-nm thick poly-Si(p+) layers with an (iVoc) of 718 mV. The use of this novel in situ annealing process to form poly-Si(p+) layers opens a new horizon for a lean process sequence without the additional high-temperature annealing step for fabricating solar cells concepts based on passivating contact.

@en

A novel non-destructive and contactless approach for measuring the base resistivity of emitter-diffused, partially-processed wafers is introduced. The method is based on temperature-dependent resistivity analysis and referred as temperature-dependent resistivity slope model (TRSM). It is developed for p-type boron-doped silicon wafers used in industrial applications with base resistivities ranging from 1 to 5 Ωcm. A sensitivity analysis is carried out on TRSM to determine the limits of this simple approach and the results show that TRSM can determine the base resistivity with an accuracy of 90%. The main limiting factor is the reproducibility from the measuring tool with a mean error of 6% on the TRSM results. Finally, the base resistivity of emitter-diffused, partially-processed wafers (in this work referred as precursor wafers) obtained from TRSM is compared to two reference approaches and showed mean error percentages of less than 10%.@en