A Simple Model Describing the Symmetric <formula formulatype="inline"><tex Notation="TeX">$I\hbox{--}V$</tex></formula> Characteristics of <formula formulatype="inline"><tex Notation="TeX">$\hbox{p}$</tex></formula> Polycrystalline Si/ <formula formulatype="inline"><tex Notation="TeX">$\hbox{n}$</tex></formula> Monocrystalline Si, and <formula formulatype="inline"> <tex Notation="TeX">$\hbox{n}$</tex></formula>

Abstract

We present an analytical model for the current transport in polycrystalline (poly)Si/interfacial oxide/monocrystalline (c)-Si base junctions, which consistently describes the symmetrical behavior of an n + poly-Si emitter/p c-Si base and p + poly-Si emitter/n c-Si base configuration. Our model is focused on a regime within which the current transport is possibly dominated by a flow through oxide pinholes rather than by tunneling. For an emitter region assumed to form underneath the interfacial oxide by diffusion of dopants from the poly-Si into the c-Si, we calculate the minority charge carrier distribution and the resistance implied for majority charge carriers. With reasonable parameters, our model simultaneously reproduces the experimentally observed low emitter saturation current densities and low junction resistances values. Our model provides a plausible explanation for the high current gain observed in p-n-p and n-p-n bipolar transistors featuring a poly-Si emitter. In principle, the obtained correlation between recombination current and series resistance is analogous to the situation in a base region of a solar cell with local rear contacts. Thus, a poly-Si/c-Si junction can be explained within the framework of a classical p-n junction picture for a passivated, locally contacted emitter.

Modeling recombination and contact resistance of poly‐Si junctions

“…14 Since already the BJT community reports on an inconsistency of the tunnel model with respect to the description of electron collecting/emitting or hole collecting/emitting POLO junctions, 13 Peibst et al proposed an alternative picture, assuming that conduction through the interfacial oxide happens at places where the oxide is broken up locally (Pinholes). 15,16 This model symmetrically explains the recombination and contact resistance of n + /p and p + /n POLO junctions. It was suggested that both mechanisms possibly superimpose in real junctions, in particular for the case of interfacial oxides with thicknesses d ox < 1.5 nm formed by wet-chemical or ozone oxidation.…”

“…thus depends only on these two quantities. 1 For values of S > 10 15 , the solar cell efficiency is rather limited by the intrinsic recombination of crystalline Si (c-Si) than by the carrier selective contact. Such high contact selectivities were realized by a layer stack of doped, mostly poly-silicon (poly-Si) on a thin silicon oxide (SiO x ).…”

“…The pinhole model of Peibst et al 15,16 completely disregards tunneling through the regions where the interfacial oxide is still intact, while the tunneling model of Steinkemper et al 14 vice versa disregards pinhole transport. An analytical simplification of this tunneling model that describes the junction as a metal/SiO x /c-Si stack was proposed by Feldmann et al 23 and later applied to multiscale modelling of solar cells.…”

Contacting a single nanometer‐sized pinhole in the interfacial oxide of a poly‐silicon on oxide (POLO) solar cell junction

“…The principle of any carrier-selective contact is to maximize the blocking effect for minority carriers whilst minimizing the barrier of majority carriers. 1,2 One type of carrier selective junction is realized by a layer of highly doped poly-Si that is deposited on a thin interfacial oxide [3][4][5] called poly-Si on oxide (POLO) junction. This type of junction is utilized in a p-type Si solar cell with a record energy conversion efficiency of 26.1%.…”

Inline PECVD Deposition of Poly‐Si‐Based Tunnel Oxide Passivating Contacts

“…By the use of an interdigitated back contact structure Kaneka achieved an efficiency of 26.6% . The tunnel oxide passivating contact (TOPCon) and POLO structures represent another approach which is based on a thin intermediate oxide layer and a heavily doped nano/polycrystalline silicon layer above and leads to efficiencies of >25%. For some years now “silicon‐free” alternative thin films as metal oxide layers like WO x , MoO x have proved a very high potential .…”