In recent decades, there has been increasing concern about the impact of climate change on the earth. Various countries are actively developing sustainable energy technologies, of which solar cell is one of the new energy sources with the most attention. This thesis is based on poly−Si(O_x) cells consisting of SiO_x/poly−Si(O_x) passivating contact. Various methods have been investigated to mitigate the TCO (IWO) deposition induced passivation degradation and to optimize the screen printing process.

First, approaches to reduce the passivation degradation due to IWO deposition were explored. The power density and working pressure in IWO deposition were optimized. With utilizing 1.23 W/cm² power density and 5×10⁻³ mbar working pressure, the 𝑖𝑉_𝑂𝐶 degradation was reduced to 4.7 mV for 𝑛⁺ sample (NAOS-SiO_x) and to 7.9 mV for 𝑝⁺ sample after deposition of 75 nm IWO. Besides, hydrogenated amorphous silicon and AZO were used as buffer layers for the 𝑝⁺ SiO_x/poly−Si(O_x) samples, finding that they were effective in reducing sputtering damage. Then, the optimal post-annealing condition was investigated, which turned out to be vacuum annealing at 400 ^◦𝐶 for 30 min, recovering the 49 mV and 60 mV 𝑖𝑉_𝑂𝐶 for 𝑛⁺ sample (thermal-SiO_x) and 𝑝⁺ sample, respectively. Based on the above experiments, the first cells were prepared with a maximum efficiency of 17.4%.

Second, the existing screen printing process in the lab was optimized. The 0.37% organic solvent was added into the silver paste, reducing the viscosity of paste. The snap-off distance was changed to 0.02 mm in order to improve the continuity of printed grids. Moreover, the squeegee speed was optimized to 30 mm/s, limiting the spreading of silver paste to 57.3 𝜇m, much narrower than the initial spreading distance of 155.3 𝜇m.

Finally, a batch of poly−Si(O_x) cells were prepared by applying the above various optimized conditions. The cell with optimized IWO and optimized screen printing process performed best with an champion efficiency of 18.9%. ... Read more

Transparent conductive oxides (TCOs) play important roles in information and energy technologies. In the photovoltaic (PV) community, they are normally required to provide sufficient lateral carrier transport towards metal electrodes at the illumination side. Thus, a trade­off between optical and electrical
properties is of critical importance in relevant device fabrications. So far, within the PVMD group, the investigation on TCOs has been mainly focused from an experimental perspective. In this thesis, a firstprinciples approach based on DFT software is investigated to obtain the self­consistent opto­-electrical properties of TCOs.

A comprehensive study is carried out to understand the fundamentals of DFT software, as this is crucial to get reliable results from it. Within the PVMD group, the DFT method is applied on the indium­oxide (IO) host material. It was found that referable results for the (partial) density of states and band
structure could be obtained for this structure using the PBE exchange­correlation (XC) functional. The dielectric function could be obtained by combining the PBE and HSE06 XC functional through the PHSmethod.

Based on a preliminary validation of IO, two case studies are carried out. In these case studies, it is investigated if DFT can be used to compare the opto­-electrical properties of different doping types and ratios. This is done for post­transition metals (Sn), transition metals (W and Mo) and anionic doping
(F). By observing the partial density of states of each element, it was found that significant hybridization of dopant states with the CBM of the IO host states occurs for the cases of Sn­ and F­doping.
Such disturbance in the host conduction band may lead to detrimental influences to the opto­-electrical properties of corresponding TCOs. However, in the cases of W­ and Mo­doped TCOs at commonly used doping concentrations, no hybridization between dopant states and the host conduction band was observed. Furthermore, physical parameters of different TCOs at different doping levels are extracted and compared, such as band gap, effective electron mass, work function and dielectric functions. These results may provide supportive and indicative information for the experimental work.

This thesis work has successfully introduced DFT calculation into TCO investigations within our research group. Although the preliminary results were not sufficiently accurate to predict the opto­-electrical properties of the TCOs in a quantitative way, the qualitative trend can still be used as guidance and support for explaining experimental results. However, many challenges still remain, especially for determining some optical properties like the band gap and dielectric function. Further research is still needed to improve the proposed method. ... Read more

Carrier-selective passivating contacts (CSPC) have been proven to be effective in suppressing recombination losses at metal-silicon interface in high-efficiency crystalline silicon solar cells. The poly-Si-based CSPCs consisting of SiOX/poly-Si stacks are used in this thesis due to their compatibility with high thermal budgets. CSPCs with thin poly-Si layers are preferred since thicker poly-Si layers induce significant parasitic absorption at device level. The bifacial solar cell design could provide a higher energy yield than monofacial solar cells and largely reduce the consumption of metals such as silver, indium due to the contribution of reflected light. The objective of this thesis is to fabricate double-side textured copper (Cu)-plated bifacial poly-Si solar cells. It is achieved by developing thin poly-Si contacts, addressing the TCO sputtering-induced passivation damage, and optimizing the Cu-plating processes.
Firstly, the passivation of the poly-Si symmetric samples and solar cells was optimized. 16 nm-thick n+ poly-Si and 16 nm-thick p+ poly-Si symmetric samples with intrinsic layer grown through PECVD demonstrated good passivation quality indicated by their iVOC of 713 mV and 665 mV respectively. To further improve the passivation quality of the p+ poly-Si, the layer thickness and the doping gas flow ratios were varied. Best passivation was achieved for a solar cell precursor with 42 nm-thick p+ poly-Si layers. The optimum doping gas flow ratio was found to be SiH4-B2H6=20/15 sccm. The highest iVOC achieved was 692 mV.
TCO sputtering caused a severe passivation drop of ~90 mV in solar cell precursors. Post-deposition annealing treatments and 2-step TCO sputtering techniques were used to reduce such a passivation loss. However, it was challenging to reproducibly obtain solar cell precursors with good passivation quality before metallization step. The initial solar cell with double side TCO use showed cell parameters were: VOC 398 mV, FF 56.22%, JSC 30.55 mA/cm2 and η 6.85% (from n-side illumination).
To maintain a good passivation quality of the solar cell precursor, an 8 nm-thick MoOX buffer layer was introduced on top of the p+ poly-Si layer. This approach effectively reduced the passivation drop from 91 mV to 12 mV. However, the MoOX layer was observed to strongly react with the solution which was used for silver seed layer removal in Cu-plating metallization procedure. Different approaches were tested to obtain a well-plated p+ poly-Si side. As for the n+ poly-Si side, a TCO-free design was deployed. To ensure a good adhesion of the plated Cu fingers, a Ti/Ag (8 nm/192 nm) seed layer was employed before Cu-plating step. Moreover, an additional SiOX layer, which acts as the anti-reflection coating, was deposited on the n-side of a complete solar cell. With these adjustments, the solar cell performance was improved to: VOC 610 mV, FF 64.98%, JSC 36.95 mA/cm2 and η 14.64% (from n-side illumination).
Furthermore, with reducing the Ti thickness in the metal seed layer to 2 nm. The solar cell performance was further improved to VOC 611 mV, FF 69.58%, JSC 36.16 mA/cm2 and η 15.38% (from n-side illumination). The bifaciality factor is 89%.
... Read more