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High efficiency silicon solar cells
The main problem with standard PV cell technology is their highly recombinative metal contact [44], thus new approaches to increase the efficiency of single junction silicon cells rely on so-called “passivating contacts”. Passivating contact solar cells employ thin passivating layers in between the c-Si absorber and the metal contacts to play simultaneously contacting and passivating roles. The two predominant technologies for passivating contacts are the poly-silicon based approaches (e.g. TOPCon [45], POLO [46]) and the silicon heterojunction solar cell (SHJ). The TOPCon structure employs a diffused emitter at the front surface, and a very thin (<20 Å) tunnel oxide combined with a poly-Si layer at the rear surface [47] (see Figure 15). The tunnel oxide passivates very effectively dangling bonds at the c-Si surface, and if thin enough, allows for efficient transport (either by tunneling or through “pinholes” conduction [48]) and therefore generates no important transport losses. The poly-Si, which is typically highly doped, is a very good selective contact thanks to its high conductivity and to the band bending it induces in the absorber. It however leads to substantial free carrier absorption, reason why it is usually put at the rear surface, and complicates its integration in both side poly-Si based contacts devices [49].
Historically, the first passivating contact structure that reached high efficiencies was the SHJ cell, but we will discuss it in the next chapter. Additionally, both the TOPCon and the SHJ concepts have been derived in back-contact architectures, which enables better due to the absence of shading at the front surface and have reached very high efficiencies [46], [50]. To achieve even higher efficiencies in the near-future, beyond that of the theoretical limit of single-junction c-Si cells, silicon-based tandem solar cells are a very promising approach which still needs to be demonstrated at the production scale [49].
Charge transport in SHJ cells
In bulk semi-conductors, charge transport mostly stems from drift-diffusion mechanisms3. However when considering potential barriers due to perturbations in the band structure of the system, such as a P-N junction, a heterojunction or a contact with a metal, some additional transport mechanisms are crucial. The SHJ cell structure uses several layers in between the absorber and the metallic electrodes at both sides, with radically different properties, creating hetero-interfaces involving complex transport mechanisms in the transverse direction. These mechanisms are not yet fully understood but are generally described as a combination of phenomena such as tunneling (intra-band tunneling, band-to-band tunneling, trap-assisted tunneling), thermionic emission and hopping [82]. In addition, one has to consider lateral transport of charges (enhanced by the TCO), and device inhomogeneities (process, handling, etc.) which will affect the charge carrier transport. In this paragraph, we will address contact formation and mechanisms for charge transport over potential barriers at contacts and interfaces. Then we will examine the different contacts and interfaces of the SHJ cell, how they can be experimentally characterized, and what is known about the phenomena at play. We will also give insights on lateral transport, and inhomogeneities.
Transport mechanisms at the metal/TCO contact
The mechanisms of transport at metal/ITO contacts seem to be dominated by Field-Emission and Thermionic-Field-Emission, as TCO doping are on the order of > 1019 −3. The metal/semi-conductor contact theory predicts that should decrease with increasing TCO carrier density, but it is not always the case experimentally [65], [67]. Barraud et al. reported that hydrogenated indium oxides (IO:H) feature very different contact resistivities when water partial pressure is varied, while the charge density stays quite constant, and hypothesized that this may be due to silver oxides forming at the TCO/metal contact [90]. Schube et al. also observed deviations from the expected trend when high temperatures are used for the curing (T=350°C), and they attribute this to cavities forming at the TCO/metal contact due to differing surface energies between the ITO and the silver particles [89].
Measuring the Ag/ITO contact resistance
The easiest way to extract the contact resistance between a layer and a metal is to deposit this layer on an insulating substrate, prior to the TLM electrodes deposition. This method has often been used to measure the Ag/ITO contact by depositing ITO on glass substrates e.g. in [91]. This allows a straight-forward measurement as all the current goes only into the ITO, however it lacks representativeness to the final device, as ITO may grow differently on a glass substrate and a textured c-Si surface [57]. For classical solar cells, contact resistance measurements are carried out at the front emitter side, by screen-printing TLM electrodes instead of a conventional solar cell design. As the PN junction confines the current in the thin emitter, the emitter/electrode contact resistance as well as the emitter sheet resistance can be extracted by TLM [87]. Similarly, in rear emitter SHJ cells, the Ag/TCO at the rear emitter side can be measured by TLM [89]. The c-Si(n)/a-Si:H(p) heterojunction confines the current in the TCO layer- as the a-Si:H(p) emitter has a very large resistivity it does not participate in lateral transport- (see Figure 26). In the following, we label these samples such as “p-side ITO/Ag TLM samples”.
Table of contents :
1 General introduction
1.1 Photovoltaics in the energy production
1.2 Photovoltaic solar cells
1.2.1 Photovoltaic cells’ working principle
1.2.2 Basics of PV solar cells
1.2.3 Efficiency limiting factors
1.2.4 Photovoltaic solar cell technologies
1.3 Objectives of this work
2 State-of-the-art
2.1 Silicon heterojunction solar cells
2.2 Measuring series resistance
2.3 Measuring contact resistance
2.3.1 The transfer length method (TLM)
2.3.2 Transfer length model for a two-layer system
2.4 Resistive power loss analysis
2.5 Charge transport in SHJ cells
2.5.1 TCO/Ag contact
2.5.2 Transport through the interfaces of SHJ cells
2.5.3 Lateral transport in SHJ cells
2.5.4 Cell inhomogeneity and impact on transport
2.6 Chapter outlook
3 Characterization & fabrication processes
3.1 Fabrication of SHJ cells at CEA industrial pilot line
3.2 Effective lifetime measurements
3.3 Luminescence techniques for imaging
3.4 Ellipsometry
3.5 I-V measurements
3.6 Review of the different 𝑅𝑆 measurement methods
3.7 Numerical simulation on Silvaco Atlas
3.7.1 Simulation parameters
3.7.2 Simulating solar cell performance
3.7.3 Simulating TLM samples
3.8 Contact resistance measurement
3.8.1 Improving the measurement precision of contact resistivity
3.8.2 Technical implementation of the TLM
3.9 Chapter outlook
4 Development of methods to measure contact resistance in SHJ cells
4.1 Measuring the ITO sheet resistance and ITO/Ag contact with high fidelity to SHJ structure
4.1.1 4-point probe measurement of ITO sheet resistance
4.1.2 Insulating the TCO layer from the c-Si to measure Ag/TCO contact resistance and sheet resistance of the TCO
4.1.3 Simulation of Ag/ITO TLM samples
4.2 Measuring the electron and hole contact layers in SHJ structures
4.2.1 Development of a process for the fabrication of structures for electron and hole contact resistivity measurement
4.2.2 Measurement of the electron and hole contact resistivities
4.2.3 Discussion of the approach
4.3 Chapter outlook
5 Impact of varying the fabrication process on SHJ cells and on the electron contact
5.1 Influence of the c-Si substrate doping
5.1.1 Influence of c-Si doping on J-V parameters
5.1.2 Influence of c-Si doping on effective lifetime
5.1.3 Influence of c-Si doping on the electron contact properties
5.1.4 Analysis of the 𝑅𝑆 variation with c-Si doping
5.2 Integrating alternative TCOs
5.3 Varying the thickness of the front stack layers
5.3.1 ITO thickness
5.3.2 Varying the a-Si:H(i) layer thickness
5.3.3 Varying the a-Si:H(n) layer thickness
5.3.4 Breakdown of the electron contact
5.4 Chapter outlook
6 Impact of varying measurement conditions on SHJ cells and contacts
6.1 Effect of measurement conditions on the determination of the Ag/ITO contact resistance
6.1.1 Temperature
6.1.2 Illumination
6.2 Effect of measurement conditions on electron and hole contact resistance
6.2.1 Dependence of c-Si resistivity versus temperature and illumination
6.2.2 Variation of the electron and hole contact resistance with temperature
6.2.3 Variation of the electron and hole contact resistance with illumination
6.3 Chapter outlook
7 Resistive power loss analysis for bifacial SHJ cells
7.1 Lateral transport in SHJ cells
7.1.1 Two-layer TLM with interface and contact resistances
7.1.2 Resistive power loss due to lateral transport
7.2 Comparison of the models with experimental data
7.3 Resistive loss breakdown for a standard CEA SHJ cell
7.4 Impact of the electron and hole contacts on 𝑅𝑆
7.5 Chapter outlook
General conclusion and perspectives
Appendices
