Carrier selective passivating contacts (CSPC) are promising contact structures for high efficiency silicon solar cells. They provide silicon surface passivation as well as high carrier selectivity. In this thesis, the bifacial interdigitate back contacted cell concept (bifacial IBC) is combined with the interdigitated back-contacted (IBC) structure with the bifacial concept for the purpose of further improve the solar cell output. It is enabled by poly-SiOX as CSPCs. The main aim of this thesis project is the preparation of proof of concept bifacial IBC cells. In this work, two process flowcharts for bifacial IBC solar cell fabrication are presented: (1) poly-SiOX fingers patterned by ion-implantation through photoresist masking layer, (2) wet chemical patterning of in-situ doped poly-SiOX. In the first process, the optimization of poly-SiOx passivating contact was performed by varying the intrinsic a-SiOx:H layer thickness. The best passivation results obtained from this approach was 646 mV, and 623 mV for the n+ and p+ layer, respectively. In the second process, the thickness of doped p+ and n+ poly-SiOx layers was also optimized for the symmetrical test samples based on the requirement of bifacial IBC solar cell flowchart. As a consequence, a 25 nm thick p+ poly-SiOx layer and a 40 nm thick n+ poly-SiOx layer recorded highest iVOC performance. Next, the hydrogenation treatment is applied to further optimize the samples passivation quality. This results in an overall passivation of 714 and 730 mV for p+ and n+ poly-SiOX symmetrical samples, owed to the fact that the high hydrogen content in SiNX layer boosting the passivation properties of the poly-SiOX passivating contacts.These findings of the optimized poly-SiOX passivation contacts are used to fabricate the bifacial-IBC solar cell. As a result, using the wet-etching patterning of in-situ poly-SiOX CSPC flow chart, a demonstration proof of this cell principle is obtained with VOC of 649 mV, FF of 44,3 %, JSC of 40,7 mA/cm2, and efficiency of 12% for the best solar cell. The losses analysis of such cell performance was also conducted.

Crystalline silicon solar cells based on poly-Si Tunnelling Oxide Passivating Contacts (TOPCon) is becoming one of the most promising solar cell structures that enable both high efficiency and low cost. The record efficiency for the Front-Back contacted (FBC) cell with TOPCon structure is 25.7 %. By moving both the metal contacts to the back side, the so-called interdigitated back contact (IBC) approach, the solar cell efficiency can be improved significantly due to the absence of optical shading from the front metal contact. Further, by narrowing the width of the metal fingers present on the rear side of an IBC solar cell, light illumination can also be made possible from the rear side. This makes the IBC solar cell a bifacial IBC solar cell. The objective of this thesis work is to optimize the Carrier Selective Passivating Contacts (CSPCs) with an ultra-thin thermal SiOX.

Carrier selective passivating contact (CSC) is considered to be a promising direction for surface passivation research because it performs passivation for both non-contacted and contacted areas. Conventional heavy doping CSCs are subject to a few drawbacks such as significant Auger recombination, parasitic absorption, complicated processing and relatively high fabrication cost. An alternative approach is by using transition metal oxides (TMO) where nm-thick metal oxides are introduced with easy and cost effective processes to realize the asymmetric conductivity for charge carriers. In this project, we investigated the passivation property and carrier selectivity of two types of TMOs, namely MoOx as hole transport layer (HTL) and TiOx as electron transport layer (ETL).

The performance of MoOx based HTL is characterized at device level. By optimizing the metallization process, interfacial (i)a-Si:H layer thickness, textured surface pre-treatment and annealing conditions, an ultimate PCE of 17.60% is achieved, with Voc being 655 mV, Jsc being 38.36 mA/cm2 and FF being 70.40%.

The passivation quality of TiOx based ETL is characterized both by symmetric test for the passivation properties and at device level. First, the passivation property of single TiOx layer and NAOS-SiO2/TiOx stacks are investigated. With the optimization of layer thickness and FGA conditions, a Voc of ~680 mV is obtained in both cases. After which, we investigated the passivation degradation caused by changing a new TiO2 source material. We conclude that the passivation degradation is mainly attributed to the non-uniform TiOx coating resulted from the reduced free mean path of evaporated TiOx due to TiO2 outgassing during deposition. Meanwhile, we also studied the influence of different interfacial layer between c-Si and TiOx such as (i)a-Si:H thin layer where a Voc of ~660mV is achieved by using new TiO2 source. Eventually, all optimized ETL structures are tested in FBC solar cells with p+ poly-SiOx at the front side as HTL. The results demonstrate that all cells are showing similar passivation quality (Voc=~575 mV) and the champion cell of 14.21% efficiency is achieved in c- Si/TiOx stack, with Jsc being 30.53 mA/cm2 and FF being 74.97%.

Nowadays, silicon heterojunction (SHJ) solar cell is one of the most promising photovoltaic technologies thanks to the outstanding passivation quality from the a-Si:H layers. Together with the interdigitated-back-contacted (IBC) architecture, it enables the highest efficient, 26.7%, single junction c-Si solar cell. However, the mass production of such high efficient solar cells is limited, due to the complexity of the solar cell processes and the involved expensive TCO layer(s). The objective of this thesis is to develop high efficiency, simple processed IBC-SHJ solar cells. To accomplish this goal, a comprehensive study ‘from layer to device’ is conducted: firstly, the focus is on the contact stacks deposited via PECVD, which includes intrinsic and doped hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon oxide (nc-SiOx:H) thin-film layers; Then the optimized passivation contact stacks are used in the front back contacted (FBC) solar cells, with which the factors that limit the fill factor (FF) and open-circuit voltage (VOC) are identified; Lastly, a simplified process is developed to fabricate tunneling IBC-SHJ solar cells. The influences of PECVD deposition parameters on passivation quality and carrier selectivity of the passivation and contact layer stacks were intensively studied. With the optimized 6 nm thick intrinsic a-Si:H layer an effective lifetime over 3 ms and implied-VOC (iVOC) beyond 720 mV are achieved on double side textured c-Si. Enhanced passivation qualities with iVOC of 729 mV is obtained by adding the field effect passivation from the optimized n-type a-Si:H and nc-SiOx:H layers on top of the excellent chemical passivation induced by the optimized 10 nm thick intrinsic a-Si:H layer. On the other hand, with the optimized intrinsic a-Si:H passivation layer, the deposition of p-type a-Si:H or nc-SiOx:H does not deteriorate the overall passivation quality. Besides that the optimized p-type nc-SiOx:H layer exhibits excellent activation energy of 51.4 meV, which closes to the optimal value for a high efficient hole selectivity, and a dark conductivity of 0.174 S/cm, which is high enough to facilitate the hole transport. Research on FBC-SHJ solar cells reveals that thicker p-type nc-SiOx:H layer is essential to ensure a smaller/no drop from SunsVoc to VOC, which is related to minority carrier collection. Besides, such a thick doped nc-SiOx:H layers can effectively shield the device precursor from the influence of ITO’s field effect and keep the overall passivation quality after ITO sputtering. Accordingly, the best FBC device shows promising results in terms of SunsVoc with 727 mV and 734 mV, pFF of 0.862 and 0.841, measured before and after metallization, respectively. By implementing 3 nm n-type nc-Si:H instead of directly n-type nc-SiOx:H in contact with ITO, FF improves from 0.56 to 0.73. The best manufactured FBC-SHJ solar cell (7.84 cm2) exhibits VOC of 710 mV, EQE short-circuit current density (JSC,EQE) of 39.4 mA/cm2, FF of 0.73 and efficiency of 20.4%. For IBC-SHJ solar cells manufacture, the lift-off patterning approach is proved to be not suitable for processing double side textured cells, mainly due to the fact that the doped nc-SiOx thin film alloys is not HF resistant. However, by applying this non-HF resistant property of the doped nc-SiOx alloys, a novel wet-chemical approach for processing tunneling IBC-SHJ solar cell is developed. This patterning approach allows to simplify the process. And the tunneling structure avoids the patterning step of the p-type nc-SiOx:H layer. The first preliminary IBC device demonstrated with this approach exhibits VOC of 659 mV, JSC of 41.30 mA/cm2, FF of 0.67, and efficiency of 18.2%. Further optimization on the thickness of the intrinsic a-Si:H layers induces an excellent VOC of 719 mV with an average VOC of 715 mV over 7 cells, JSC over 41 mA/cm2. However, the low FF (<0.60) is limiting the cells performance, which is mainly attribute to the low conductivity of the n-type nc-SiOx:H layers.