Si and Other Group IV Quantum Dot Based Materials for Tandem Solar Cells

Abstract

Quantum dot materials, in which Si QDs are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin film process. 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. For approx. 2nm diameter QDs these result in an effective band gap of 1.8eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open-circuit voltage of almost 500mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have large but opposite effects on QD crystallisation, with P (B) doped material forming larger (smaller) QDs than for undoped material. Alternative materials for quantum dots are Ge and Sn. These allow lower processing temperatures to be used, more compatible with underlying layers. Alternative matrices to SiO2 such as SiNx or SiC offer higher tunnelling probability and hence lower resistance. These alternative matrix materials can also be used as hetero interlayers to improve the transport in the growth direction whilst maintaining quantum confinement. Group IV alloys can also be used to modify band gap. GeC in particular looks to have useful band gap and sputtering properties. Such alloy materials could be used in hetero-junction or homojunction devices in combination with SiQD based materials to fabricate all thin film tandem cells.