Introduction

The silicon heterojunction solar cell (SHJ) represents a new generation of silicon wafer solar cells with which high levels of efficiency can be achieved. A cross-sectional structure of a SHJ solar cell is shown in Fig. 1. The IEK-5 deals with material, process, cell, module and system development for the SHJ solar cell and its applications, which covers the entire value chain of SHJ technology. Cell technology ranges from the wet chemical pretreatment of crystalline silicon wafers to the deposition of functional thin layers using various deposition processes to the final thermal treatment after deposition.

The focus of process development is on the development of cost-effective processes for the production of industrial SHJ solar cells on large-scale systems. Material development focuses on the production and characterization of silicon alloys and their implementation in SHJ solar cells. Module development specializes in lightweight and flexible SHJ modules for special applications such as vehicle-integrated photovoltaics. System research deals with life cycle assessment and cost analyzes for SHJ based PV systems such as the integration of SHJ modules in road traffic.

Further fields of work are the development of perovskite silicon tandem solar cells, SHJ solar cells in the back-contacted configuration (Interdigitated Back-Contacted: IBC), the 3D optical-electrical Simulation of SHJ solar cells, and the production and characterization of passivated SiO₂ / poly-Si Contacts.

Solar Cell Development

We operate a baseline that covers the entire process technology for the industrial production of high-efficiency SHJ solar cells on M2 wafer size: from wet chemical pretreatment to thin film deposition and metallization. In addition, various measurement facilities are available for characterizing the finished solar cells and the respective layers. We achieved a champion efficiency of 24.51% on M2 size (156.75 mm x 156.75 mm) wafer, which was independently certified by ISFH CalTeC (see Fig. 2).

Advanced research development based on SHJ solar cells to further increase the efficiency is also being conducted, with both scientific and industrial application perspectives. The topics are:

• Highly transparent and conductive hydrogenated nanocrystalline silicon oxide (nc-SiO_x:H) as a low-parasitic absorbing window layer in SHJ solar cells
• Development of high-quality TCOs for SHJ solar cells, as alternative to indium tin oxide (ITO), with high carrier mobility and relatively low carrier density, such as ITiO and IWO, or sputtered indium-free AZO and PECVD processed doped-ZnO films, with lower costs
• Light soaking mechanism for SHJ solar cells and modules
• Long-term stability based on TCO optimization for SHJ solar cells and modules
• Catalytic doping of silicon thin films for the application in SHJ solar cells
• SHJ solar cells for the application as bottom cells in high-efficiency multi-junction solar cells, e.g. perovskite/SHJ tandem solar cells

Silicon carbide based transparent passivating contacts solar cells

The classical triple challenge for front contacts of crystalline silicon (c-Si) solar cells - high conductivity, excellent surface passivation, and high optical transparency - are often in conflict and require compromises. For example, separating the metal contacts from the c-Si absorber with different types of passivating front contact schemes enables excellent surface passivation and accordingly high open-circuit voltages (V_oc), but the solar cells often suffer from significant parasitic light absorption in the amorphous or polycrystalline silicon contact layers, leading to a significant reduction in short-circuit current density (J_sc).

A highly transparent passivating contact (TPC) offers the opportunity to solve this challenge. It consists of a wet-chemically grown thin silicon oxide followed by a bilayer stack of hydrogenated nanocrystalline silicon carbide (nc-SiC:H(n)) deposited by hot-wire chemical vapor deposition (HWCVD) and a sputtered indium tin oxide (c-Si(n)/SiO₂/nc-SiC:H(n)/ITO), which combines high optical transparency and low surface recombination with a much lower need for compromise. Additionally, this contact avoids the need for additional hydrogenation or high-temperature post-deposition annealing steps. The efficient solar cell yields a high J_sc > 40 mA/cm², and a V_oc > 725 mV. While the high bandgap of nc-SiC:H(n) provides high optical transparency, the double layer structure is the key innovation leading to a combination of good passivation and high conductivity, as evidenced by the high fill factor (FF > 80%) leading to a certified efficiency of 24%.

Further tasks include upscaling the current 20 mm x 20 mm size to M2 size, developing TPC-based TCO-free solar cells, and the development of high-temperature stable TPC solar cells for industrial application.

Module Development

One of the critical processes in PV manufacturing is module lamination. The work in our module team can be divided into two parts. One part is the interconnection of the solar cells. The cells are first interconnected into strings in a variety of ways, from five busbars to multi-wires. Here, the solar cells can be full size, half size or third size. Then the solar cell clusters are assembled with ribbons to produce a PV module. The other part is vacuum lamination, which achieves the longevity of the solar module to withstand different weather conditions. Both crosslinking and thermoplastic polymer encapsulants are evaluated in this process, including EVA, POE and TPO. Various sizes of modules have been encapsulated, from single module to mini module to commercial module sizes with structures such as double glass, glass/back sheet, front/back sheet. Energy conversion efficiencies beyond 23% on active module area of 243.36 cm² are achieved in 2020.

Another topic is the lightweight, flexible module. By eliminating the need for a glass module structure, a significant weight reduction can be achieved while maintaining compatible performance. It expands the variety of applications that are unsuitable for heavy silicon panels and without additional load bears during installation, such as integration of PV on the curved surface of vehicles (VIPV) and of buildings (BIPV), as well as the application of floating PV.

Device Simulation

Even though silicon heterojunction (SHJ) solar cells have demonstrated high energy conversion efficiencies (>25%), such devices are still suffering from losses. Those can be separated between optical losses and electrical power losses. Optical losses represent incident light that is not absorbed in the absorber and therefore not generating charge carriers. Electrical power losses describe any loss that occur after the generation of charge carriers. Those can be resistive losses, recombination, energy losses at interfaces, etc. The optimization of SHJ solar cells to improve their efficiency requires a deeper knowledge of material and interface properties and their impact with solar cell device performance. Therefore, detailed device simulation is necessary which considers all relevant physical mechanisms of SHJ solar cells.

Another option to improve the efficiency of SHJ solar cells is the implementation as a bottom cell in perovskite/silicon tandem solar cells. Here, due to the larger number of interfaces and involved materials, device simulations are even more relevant. Such tandem devices have the potential to show very high efficiencies >34%.

Analysis of Photovoltaic Systems for Road Transport

The decarbonization of the road transport sector leads to an increased electrification of any type of vehicle. To fulfill the targets for the reduction of CO₂ emissions, this electric energy has to come from renewable energy sources. Therefore, photovoltaic (PV) will play a major role to decrease greenhouse gas emissions in the road transport sector. There are basically two options for PV in the road transport sector; (i) the direct use of PV-generated electricity for charging of batteries and (ii) the production of so-called “green hydrogen” which can be used as fuel for the vehicles. Both options can be applied in any type of vehicle, like passenger cars, light-duty vehicles, trucks and busses. Furthermore, the direct use of PV-generated electricity allows integrating PV into the vehicle to take advantage of the vehicle’s area for generation of electricity.

The analysis of the different applications for PV in the road transport sector covers (i) the whole life-cycle of all components, including their transport to the user, (ii) total cost of ownership, (iii) different use-case scenarios of the vehicles and (iv) modeling of energy yield of PV generation from solar irradiance.

Team Members