Enhancing near-infrared solar cell response using upconverting transparentceramics

Abstract

In order to demonstrate efficiency improvement of solar cells through utilization of frequency-shifted sub-band-gap photons, we prepared Y₃Al₅O₁₂ transparent ceramics co-doped with 3.0 at% Yb³⁺ and 0.5 at% Er³⁺ (YAG:3.0Yb/0.5Er) by solid state reaction. The material’s upconversion luminescence was investigated and it was found that near-infrared light could be effectively converted to green light centered at 563 nm, where a dye-sensitized solar cell normally possessed high spectral response. Using a thin disk of YAG:3.0Yb/0.5Er transparent ceramics attached to the rear of the dye-sensitized solar cell, the near-infrared response of the dye-sensitized solar cell was enhanced, due to the frequency upconversion.

Introduction

Transmission of the sub-band-gap light is one of the major loss mechanisms in conventional solar cells. One possibility to reduce such transmission loss and to improve the solar cell’s efficiency is to upconvert the otherwise transmitted low-energy photons from the solar spectrum to highly responsive high-energy photons. An apparent benefit from this was testified by Trupke et al. [1], who determined that the upper limit of photovoltaic conversion efficiency of a single junction solar cell coupled to an ideal upconversion device can be as high as 47.6% for non-concentrated sunlight and 63.2% for concentrated sunlight. Additionally, this “upconversion” technique does not require the use of complicated structures such as those used for multiple-junction solar cells [2] or quantum well solar cells [3]. As early in 1983, Saxena [4] described the use of terbium doped lanthanum fluoride and thulium doped calcium tungstate materials for photovoltaic devices; however, actual measurement on solar cells was not reported. In 1996, Gibart et al. [5] reported an upconversion-enhanced GaAs solar cell using a vitroceramic co-doped with trivalent erbium and trivalent ytterbium under the excitation of 1 W Ti:sapphire infrared laser light at an energy of 1.391 eV. Shalav et al. [6] further observed a quantum efficiency increase in a silicon solar cell by 2.5±0.2% under 5.1 mW 1523 nm laser excitation using an erbium-doped sodium yttrium fluoride (NaYF₄:Er³⁺) upconversion phosphor.

Applying upconversion to Si solar cells is technically arguable, even though a few recent investigations on Si solar cells were performed [6], [7]. According to Ref. [8], it is suggested that materials with band gap above 1.25 eV would be better suitable for upconversion, and otherwise for downconversion. In fact, the absorption band of Yb³⁺ ions, one of the most effective sensitizers for NIR (near-infrared) to visible upconversion, is inside the absorption band of Si (E_g=1.12 eV and λ=1100 nm). Adding Yb³⁺ ions could be less helpful to the expected efficiency enhancement in Si solar cells. Therefore, it would be very interesting to study the upconversion efficiency-enhancing mechanism in new active solar cell materials, other than both Si and GaAs investigated before.

In this work, we report a 980 nm laser driven photovoltaic cell by adhering a bulk YAG:3.0Yb/0.5Er transparent ceramic to the back side of a dye-sensitized solar cell (DSSC). This work differentiates itself from the previous works in two aspects: (1) use of a DSSC, instead of Si and GaAs, gives a much wider band gap (the dye’s band gap is about 1.6 eV) and (2) use of a bulk YAG:3.0Yb/0.5Er ceramic, instead of those normally used nanoparticles [6], can potentially deliver higher upconversion efficiency. In this manuscript, we first introduced the upconversion properties of YAG:3.0Yb/0.5Er transparent ceramics. We then discuss the improvement of the near-infrared response of a DSSC attached to a YAG:3.0Yb/0.5Er ceramicswafer.