Working principle of carrier selective poly-Si/c-Si junctions: Is tunnelling the whole story?
Introduction
Carrier selective junctions based on silicon rich, mostly polycrystalline (poly-) layers deposited on an interfacial oxide grown on a crystalline (c-) silicon absorber are currently attracting significant research interest. Excellent junction quality with recombination current densities J0 down to 0.7 fA/cm2 [1], as well as efficiencies up to 25.1% [2] have been demonstrated with this approach.
Regarding the physical working principle of poly-Si/c-Si junctions, different pictures exist [3], [4]. For interfacial oxide thicknesses of ~2 nm or less, it is safe to assume that tunnelling through the oxide takes place. Nevertheless, we present in this work strong support that, at least for our samples with initial interfacial oxide thicknesses >2 nm, additional effects are relevant, which could in particular explain the excellent performance of our p+ poly-Si/c-Si junctions.
Section snippets
Arguments for additional relevant effects
Our first argument for additional current transport mechanisms besides lateral homogeneous tunnelling is the experimental observation of low junction resistances ρc and low recombination current densities J0 for both n+ poly-Si/c-Si and p+ poly-Si/c-Si junctions. This might not be consistent with the tunnelling model, as explained in the following:
As indicated in Fig. 1, the poly-Si is inherently highly defective, i.e., a high density of energy states within the band gap of the poly-Si could
Alternative picture
The alternative picture presented in the following is a generalized version of our previous model according to Ref. [4]. A comparison of both approaches, as well as a summary of differences and similarities is given in the appendix.
As known from secondary ion mass spectrometry (SIMS) and electrochemical capacitance voltage (ECV) measurements [7], [16], dopants diffuse from the poly-Si into the c-Si during the junction formation process, especially for boron doped poly-Si layers (Fig. 4(a)).
Conclusions
We present arguments that laterally homogeneous tunnelling through an unaltered interfacial oxide is possibly not the only current mechanism present in poly-Si/c-Si junctions. First, excellent p+ poly-Si/c-Si junction properties (low recombination current, low junction resistance) have been achieved despite a “wrong tunnelling barrier height ratio” for electrons and holes, which seems inconsistent to the expectations of the tunnelling mode. Second, structural modifications of the interfacial
Acknowledgments
We would like to thank Susanne Mau, Bianca Gehring and Renate Winter for their help with sample processing, and Andrea Lissel for her help with the TEM sample preparation. We furthermore would like to thank Nils-Peter Harder and Bianca Lim for valuable discussions at his/her ISFH times. We furthermore would like to thank Uwe Höhne and Jan-Dirk Kähler from Centrotherm for the LPCVD (low-pressure chemical vapor deposition) poly-Si depositions.
This work is mainly performed in the framework of the
References (31)
- et al.
Tunnel oxide passivated carrier-selective contacts based on ultra-thin SiO2 layers
Sol. Energy Mater. Sol. Cells
(2015) - et al.
Phosphorus-diffused polysilicon contacts for solar cells
Sol. Energy Mater. Sol. Cells
(2015) - et al.
Tunnel oxide passivated contacts as an alternative to partial rear contacts
Sol. Energy Mater. Sol. Cells
(2014) - et al.
Recombination behaviour and contact resistance of n+ and p+ poly-crystalline Si/mono-crystalline Si junctions
Sol. Energy Mater. Sol. Cells
(2014) Polycrystalline Silicon/Monocrystalline Silicon Junctions and Their Application as Passivated Contacts for Si Solar Cells
(2016)- S.W. Glunz, F. Feldmann, A. Richter, M. Bivour, C. Reichel, H. Steinkemper, J. Benick, M. Hermle, The irresistible...
- et al.
Numerical simulation of carrier-selective electron contacts featuring tunnel oxides
IEEE J. Photovolt.
(2015) - et al.
A simple model describing the symmetric I–V characteristics of p polycrystalline Si/n monocrystalline Si, and n polycrystalline Si/p monocrystalline Si junctions
IEEE J. Photovolt.
(2014) - et al.
Lateral polysilicon p-n diodes
J. Electrochem. Soc.
(1978) Photoemission of electrons from silicon into silicon dioxide
Phys. Rev.
(1965)
Ion Implantation for Poly-Si Passivated Back-Junction Back-Contacted Solar Cells
IEEE J. Photovolt.
Polysilicon emitters for bipolar transistors: a review and re-evaluation of theory and experiment
IEEE Trans. Electron. Dev.
Cited by (188)
Photon management in silicon photovoltaic cells: A critical review
2024, Solar Energy Materials and Solar CellsMitigating parasitic absorption in Poly-Si contacts for TOPCon solar cells: A comprehensive review
2024, Solar Energy Materials and Solar CellsImprovement of passivation performance of silicon nanocrystal/silicon oxide compound layer by two-step hydrogen plasma treatment
2023, Solar Energy Materials and Solar CellsExploring hafnium oxide's potential for passivating contacts for silicon solar cells
2023, Solar Energy Materials and Solar Cells