Experimental demonstration of energy-transfer ratchet intermediate-band solar cell

Tomah Sogabe,Ned Ekins-Daukes,Ryo Tamaki,Koichi Yamaguchi,Chao-Yu Hung,Stanko Tomić,Yoshitaka Okada

doi:10.1038/s42005-021-00543-1

Abstract

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediate-band solar cells (IBSCs) under maximum sunlight concentration. However, an actual IBSC device with an efficiency larger than the Shockley-Queisser (SQ) limit has so far not been reported. The main difficulties lie in realizing an efficient sequential two-photon absorption (STPA) which requires a sufficiently long lifetime intermediate state or intermediate band. In this article, we propose the concept of a ratchet type IBSC, utilizing a long lifetime of rare-earth ion luminescence centers in Erbium-doped GaAs. The temperature dependent differential external quantum efficiency reveals a significant STPA contribution originating from the Er3+ luminescence center. All the results were modeled and interpreted by integrating the ratchet effect with up-conversion along with a density functional theory (DFT) simulation. Our work demonstrates that the long lifetime energy-transfer mechanism in Er3+ centers contributes directly to the formation of a ratchet type IB.

Highlights

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediateband solar cells (IBSCs) under maximum sunlight concentration
The conventional intermediate band (IB), which has both transitions from the valence band (VB) to the IB and from the IB to the conduction band (CB), is replaced by an IB that is only coupled to the VB, and simultaneously a non-emissive ratchet band that is only coupled to the CB is introduced at an energy ΔE below the IB (Fig. 1a)
The optical characterization of Er-doped GaAs grown by molecular beam epitaxy (MBE) showed that the energy level at 1540 nm exhibits an atomic-like PL signal whose peak energy is almost independent of the temperature

Summary

Introduction

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediateband solar cells (IBSCs) under maximum sunlight concentration. A detailed balance calculation reveals that the theoretical efficiency limit of an ideal solar cell that employs sequential two-photon absorption (STPA) and intermediate levels is 63.2% under maximum sunlight concentration[1,2], which is almost one and a half times higher than the Shockley–Queisser (SQ) limit of ≈42% for conventional single-junction solar cells under the same condition[3,4] This high efficiency limit has attracted considerable attention and has encouraged many scientists to develop and realize such high-efficiency intermediate-band solar cells (IBSCs). If the energy relaxation from the IB to the ratchet state is sufficiently fast, most of the carriers excited to the IB will be transferred to ratchet band This transfer can significantly suppress the recombination between the IB and the VB and can lead to a higher efficiency than that of an IBSC with a single IB only, especially at low or moderate light concentrations[7]. Rare-earth-ion-doped materials have been considered for use as spectrum converters in solar cells, because they provide an efficient two-photon up-conversion mechanism or impurity band, which improves overall light absorption[21,22,23,24,25,26]

Methods

Results

Discussion

Conclusion