Silicon quantum dot based solar cells: addressing the issues of doping, voltage and current transport

Abstract

Silicon quantum dot (Si QD) solar cells offer the potential to tune the effective band gap through quantum confinement and hence allow fabrication of optimised tandem devices in one growth run in a thin film process. Previous work in our group has shown how such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. Doping multilayers with P and B allows formation of a rectifying junction with an effective band gap of 1.8 eV, which can give an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have big but opposite effects on QD crystallisation, with P(B) forming larger (smaller) QDs than for undoped material. Two possible models for the doping mechanisms in these materials are explored: one relying on doping of a sub‐oxide region around the Si QDs and the other based on the differing nucleation effects of P and B. In addition initial results on hetero‐interfaces in the QD superstructure are assessed as a means to improve vertical current transport, and the effects of hydrogenation on improving VOC by passivating defects. Routes to incorporating these explanations for doping and other structural improvements are discussed as means of optimising the performance of these Si QD cells. Copyright © 2010 John Wiley & Sons, Ltd.

Energetics and carrier transport in doped Si/SiO2quantum dots

“…40,59 Therefore, to compensate such deviations, we have applied a correction of 2.0 eV to VBO and HB values, and of -0.8 eV to CBO and EB values, while leaving E g unchanged. Since our QD size range is small, we apply the same correction for all the samples.…”

“…Most of the available studies on single-and two-QDs have been performed by using nonequilibrium Green functions formalism (NEGFF) with constant transition rates between QDs and one energy level per QD [31][32][33][34][35][36] , and by using tunneling transmission coefficients with planar Si/SiO 2 values for the barrier height, and bulk-Si band gap. [37][38][39][40] Here we make use of a different approach, 41-43 based on the transfer Hamiltonian formalism and non-coherent rate equations to describe the current, that takes into account the local potential due to the QD charge, com-V FIG. 1.…”

High Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping for next generation Si quantum dot photovoltaics

“…Impurity doping of Si QDs is still not well understood as the doping mechanism is more complex than in c-Si [8]. Doping can drastically alter the structural, electrical and optical properties of the Si QDs themselves.…”

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

“…Experimental investigations have shown that the material possesses better nanocrystal growth and carrier transport properties [8]. However, few studies have been conducted to comprehensively examine the new material's optical characteristics, which are essential in the understanding of device operation.…”