Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination

Abstract

Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of β-NaYF₄:20% Er³⁺ to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07±0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative.

Introduction

Silicon solar cells lose about 20% of the energy incident from the sun because photons with energies below the band-gap are not absorbed. Upconversion (UC) of such low-energy photons, meaning the generation of one high-energy photon out of at least two low-energy photons [1], pushes the theoretical efficiency limit of a silicon solar cell illuminated by non-concentrated light from close to 30% [2] up to 40.2% [3].

For the application on silicon solar cells, the highest UC efficiencies so far have been achieved with hexagonal β-NaYF₄ doped with trivalent erbium [4], [5]. Er³⁺ features conveniently spaced energy levels for UC. The β-NaYF₄ host lattice has low phonon energies of less than 450 cm⁻¹ and, accordingly, non-radiative losses are strongly reduced compared to, e.g. oxide lattices [6]. The application of upconverters to harvest solar energy implies certain requirements: UC has to take place under non-coherent illumination and ideally should work over a wide spectral range. In contrast, the reported UC investigations mostly relied on coherent laser excitation [4], [5], [7], [8], [9]. The only UC experiments with broad spectrum illumination were restricted to the visible range of the spectrum [10], and are therefore of minor significance for application on silicon solar cells. In contrast, in this paper we investigate the behavior of Er-based upconverters both with coherent laser excitation and additionally with broad spectrum illumination and compare the two approaches.