POF III / Topic 1 Solar cells of next generation
Subtopic 3 - Silicon solar cells
Subtopic 3 “Silicon Solar Cells” addresses the next generation of a PV technology that has proven its suitability for mass production but needs a substantial research effort to fit into a future PV scenario with respect to cost and efficiency. The topic aims at efficient and versatile silicon based solar cell devices overcoming present efficiency restrictions. To this end, new material and new design options are combined with a continuous improvement of state-of-the-art technologies. In addition, the versatility of the technology is used to develop specific solutions for integrating photovoltaic components into chemical or electrochemical storage systems.
Silicon-based multi-junction devices (task 3.1, task 3.3)
In the past years equipment and processes for tandem solar cells and modules based on amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) have been successfully implemented and developed at PVcomB (see Fig. 1) and a baseline process for tandem modules has been established. The core of this line is the silicon deposition cluster tool (AKT1600) based on plasma-enhanced chemical vapor deposition (PECVD). With the focus on the development of the a-Si:H/µc-Si:H tandem cell, a number of innovative materials such as µc-SiOx alloys for doped layer  and ZnO:Al TCO films with unique light scattering abilities and excellent optical and electrical properties (e.g, electron mobilities as high as 67 cm2/Vs [2, 3] have been developed at PVcomB. As a result high-efficiency tandem solar cells reach 12.1 % (measured at ISE CalLab) after stabilization upon 1000 h of light soaking (4). Research results on materials, devices and process technology have been successfully transferred to production together with partners in industry .
Besides tandem cells this technology offers the possibility to design a variety of single, tandem, triple and even “quadruple”  junction devices with various and adjustable current-to-voltage ratios. In combination with other materials and devices, this allows for hybride devices. Two applications have been successfully demonstrated with solar cell devices from PVcomB:
- The combination of an a-Si:H single junction with an organic upconverter .
- The combination of an a-Si:H/a-Si:H tandem junction with an organic solar cell reaching a conversion efficiency of >11 % .
- The integration of an a-Si:H/a-Si:H/µc-Si:H triple junction in a device for photocatalytic hydrogen splitting device . Here, in particular the high output voltage (Vmpp) > 1.4 V, needed for water splitting, is an advantage.
The objectives in this task are:
- Maintaining an in-house TF-Si baseline as a state-of-the-art reference line for thin film Si PV modules, in order to monitor equipment and processes and to support the other tasks.
- Support of the existing industry partners as well as offering a technology basis for future industrial applications in hybrid applications (task 3.3).
- Development of a technology basis to provide tailor-made devices for task 3.3. The key challenge in order to develop these novel multi-junctions is to tailor the silicon device and process in order to fulfill the specific requirements, such as: specific voltage/current combinations, e.g. a restricted process temperature window, an adapted contacting scheme , a specific choice of substrate, or design of interfaces and surfaces e.g. for chemical resistivity to certain electrolytes.
 S Kirner, O Gabriel, B Stannowski, B Rech, R Schlatmann, The growth of microcrystalline silicon oxide thin films studied by in situ plasma diagnostics, Applied Physics Letters 102 (5), 051906-051906-4.
 F. Ruske, M. Roczen, K. Lee, M. Wimmer, S. Gall, J. Hüpkes, D. Hrunski, B. Rech, Improved electrical transport in Al-doped zinc oxide by thermal treatment, J. Appl. Phys. 107, 013708 (2010).
 S. Neubert, M. Wimmer, F. Ruske, S. Calnan, O. Gabriel, B. Stannowski, R. Schlatmann, B. Rech, Improved conversion efficiency of a-Si:H/µc-Si:H thin-film solar cells by using annealed Al-doped zinc oxide as front electrode material, Prog. Photovolt: Res. Appl. (2013).
 B. Stannowski, O. Gabriel, S. Calnan, T. Frijnts, A. Heidelberg, S. Neubert, S. Kirner, S. Ring, M. Zelt, B. Rau, J.-H. Zollondz, H. Bloess, R. Schlatmann, B. Rech, Achievements and challenges in thin film silicon module production, Solar Energy Materials and Solar Cells 119 (2013) 196–203.
 S. Kirner, S. Neubert, C. Schultz, O. Gabriel, B. Stannowski, B. Rech, and R. Schlatmann Quadruple-junction solar cells and modules based on amorphous and microcrystalline silicon with high stable efficiencies, Japanese Journal of Applied Physics 54, 08KB03 (2015).
 Y.Y. Cheng et al., Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion, Energy Environ. Sci. 5, 6953 (2012).
 S. Roland et al., Hybrid Organic/Inorganic Thin‐Film Multijunction Solar Cells Exceeding 11% Power Conversion Efficiency, Advanced Materials 27 (7), 1262-1267.
 P. Bogdanoff, D. Stellmach, O. Gabriel, B. Stannowski, R. Schlatmann, R. v.d. Krol, S. Fiechter, Artificial Leaf for Water Splitting Based on a Triple‐Junction Thin‐Film Silicon Solar Cell and a PEDOT:PSS/Catalyst Blend, Energy Technology 4 (1), 230-241 (2016).
Silicon Heterojunction (SHJ) solar cells (Task 3.2)
A way for the silicon wafer technology to face future challenges in efficiency increase and cost reduction is by developing high efficient silicon heterojunction (SHJ) solar cells. Therefore, a SHJ Baseline process for industrial cells up to 6-inch size has been established at PVcomB. At PVcomB, research is focused on using process know-how from the a-Si:H/µc-Si:H technology to SHJ cell technology by implementing advanced silicon and silicon alloy materials and processes, such as nc-SiOx:H, from the AKT tool.
Currently SHJ cell efficiencies of 22.5 %, certified by ISFH CalTeC, are obtained. The research is strongly supported and embedded in industrial collaborations and receives significant external funding from national and European sources. Cell efficiencies of 24.5 % for double-side and 25 % efficient all-rear side contacted (IBC) SHJs are target until 2018 (PV 1.3.7)
Solar cells and modules based on liquid-phase crystallized silicon on glass (LPCSG) (Task 3.4)
For solar cells based on liquid-phase crystallized silicon (LPC-Si), as developed in task 3.4, an excellent passivation of the contacts is needed in order to benefit from the high quality of the LPC-Si absorber. This is addressed in close cooperation with the SHJ work in task 3.2, with the deposition of low-resistive and passivated contacts carried out in the AKT tool.