Germanium nanoparticles with non-diamond core structures for solar energy conversion

Márton Vörös,Gergely T Zimanyi,Giulia Galli,Stefan Wippermann,Adam Gali,Bálint Somogyi,Dario Rocca

doi:10.1039/c4ta01543f

Abstract

Multiple Exciton Generation (MEG) in nanoparticle-based solar cells promises to increase the cell-efficiency above the Shockley–Queisser limit. However, utilizing MEG is hampered by the Quantum Confinement Dilemma (QCD): quantum confinement advantageously increases the effective Coulomb interaction, but at the same time disadvantageously increases the electronic gap. Using ab initio calculations we showed that germanium nanoparticles with core structures of high pressure phases of bulk Ge can transcend the QCD, by simultaneously lowering gaps and increasing the MEG rates above those of NPs with a cubic diamond core. Synthesis routes to obtain Ge colloidal ST12 core structures are available and hence we propose that exploring ST12 Ge NPs for MEG solar cells is a promising research effort.

Highlights

Efficient conversion of solar energy is one of the greatest challenges of our times
Using ab initio calculations we showed that germanium nanoparticles with core structures of high pressure phases of bulk Ge can transcend the Quantum Confinement Dilemma (QCD), by simultaneously lowering gaps and increasing the Multiple Exciton Generation (MEG) rates above those of NPs with a cubic diamond core
We focus on Ge because (1) it is environmentally friendly, (2) Earth-abundant, (3) the MEG was shown to be more efficient in bulk Ge than in Si, and (4) Ge has a lower bulk gap of 0.66 eV at room temperature (RT) than bulk Si (1.11 eV at RT)

Summary

Introduction

Efficient conversion of solar energy is one of the greatest challenges of our times. Single junction solar cells are limited by the theoretical bound of about 33%, which was outlined in the seminal work by Shockley and Queisser (SQ).[1]. In our previous work on Si,[39] we determined that hydrogen terminated nanoparticles with a BC8/Si-III core-structure exhibited a gap lower than that of the Si NPs with a cubic-diamond core by 20–40% This gap reduction has the potential to greatly enhance the utility of MEG for solar applications. The rest of the paper is organized as follows: in Section 2 we describe our theoretical and computational approach to calculate the optical and impact ionization properties of nanoparticles; in Section 3 we report our results on germanium nanoparticles with high pressure core structures.

Methods

We also determined the low-energy part of the TDDFT spectrum

Electronic structure

Absorption spectrum

MEG transition rates

Summary and conclusions