Spin-dependent transport in fully processed silicon solar cells studied by pulsed Multifrequency Electrically Detected Magnetic Resonance below 600 MHz/20 mT and at 263 GHz/9.4 T

• Applicant: Professor Dr. Klaus Lips
• Subject Area: Analytical Chemistry
• Term: from 2012 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 221263527

The objective is to study spin dependent transport mechanisms to identify function determining paramagnetic states in fully processed thin film and wafer-based silicon solar cells with state of the art efficiencies by cw and pulsed electrically detected magnetic experiments (pEDMR) in an extreme frequency/field regime from < 600 MHz/20 mT up to 263 GHz/ 9.4 using the full set of EPR spectrometers available and newly developed at BeJEL with particular emphasis on the 263 GHz and X- and Q-band spectrometer located at HZB.In wafer-based c-Si, as well as amorphous and polycrystalline silicon thin film Si solar cells (a-Si:H and poly-Si, respectively), paramagnetic defects at the boundary between different silicon phases such as amorphous/crystalline silicon interfaces and grain boundaries of polycrystalline silicon as well as at localized states induced through disorder (a-Si:H) influence the solar conversion efficiency. Due to the low overall defect concentration in state of the art silicon solar cells, with interface/boundary regions in the nm range, ultra-sensitive indirect EPR detection schemes like EDMR are required to lift the sensitivity limit of microwave detected techniques. In the present study EDMR will be particularly useful due to its capability to connect device limiting charge transport and loss mechanisms with structural information on transport limiting paramagnetic states. We will further improve the 263 GHz cw and pEDMR experiment, particularly focusing on achieving higher B1 field strength to achieve pi/2 times in the 100 ns regime. In addition we will further develop dedicated thin-film EPR and EDMR probe heads and a complementary very low frequency (10-600 MHz) EDMR and ENDOR excitation scheme. Recently employed pEDMR detection schemes will be modified for operation at 263 GHz and 600 MHz respectively for the assignment of yet unresolved paramagnetic states and spin dependent transport path ways. These experiments will be conducted on different miniaturized state-of the art solar cell architectures manufactured at HZB. The work packages are:1. Growth of a-Si/c-Si and thin film Si solar cells with appropriate isotope enrichment (28Si, 29Si, 2D). Development of a new EDMR contact design that will result in a strong B1 field enhancement.2. Optimizing the design of the 263 GHz EPR/EDMR probe head for enhanced microwave power.3. Implementation of pEDMR methods at 263 GHz and 10-600 MHz in the same setup.4. Multi frequency cw and pEDMR experiments on c-Si and thin-film Si solar cells.5. Implementation of a complete cw V-band EPR and EDMR detection setup on a single chip.6. Combined conductive AFM and EPR experiments on a-Si/c-Si interfaces.7. Modeling of EPR parameters of the interface between different silicon phases and their sub-interface region by DFT methods taking excitonic states and transport into account.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1601: New Frontiers in Sensitivity for EPR Spectroscopy: from Biological Cells to Nano Materials
• International Connection: USA
• Co-Investigator: Dr. Alexander Schnegg
• Cooperation Partner: Professor Dr. Christoph Boehme