Tandem quantum well solar cells

Abstract

Quantum wells offer advantages in conventional bulk tandem solar cells since they allow the independent tailoring of the absorption edge of either cell with no lattice mismatch and subsequent relaxation. We describe progress in the band gap engineering of InGaP/GaAs solar cells using strain balanced quantum wells and present a tandem quantum well structure which has achieved 30.6% efficiency under 54 suns AM1.5g. This is a record for photovoltaic nanostructured devices. We predict realistic efficiencies of 34% under 600suns, AM1.5d low AOD for optimized devices. Finally, the possibility and potential gains of introducing quantum wells into both cells of an InGaP/GaAs device are discussed.

Recent results for single‐junction and tandem quantum well solar cells

“…Laboratory measurements at ENE on a MQW tandem found an efficiency of 30.6 ± 1.2% under a flashed xenon light source at 54 suns 47and 24 ± 1°C 47. The light intensity was adjusted by aperture filters and measured using a calibrated GaAs reference cell.…”

“…It is therefore important to make the correct choice of substrate orientation since the 6°control top cell is current matched to the QW bottom cell. The control top cell/tandem bottom cell combination from Figure 14 could lead to efficiencies of up to 34% 49 accounting for the altered spectral filtering of the bottom cell.…”

“…The most efficient design to achieve this aim would contain a high QE region of shallow QWs. Further efficiency gains may also be made by incorporating QWs into the top cell, as this can optimize both sub‐cell band gaps to a given terrestrial spectrum and unlock efficiency gains from the higher J SC of deeper bottom cell QWs 46, 47.…”

Promises of advanced multi-junction solar cells for the use in CPV systems

“…The inverted metamorphic growth is another way to match the current densities by growing a higher bandgap Besides the growth of lattice-mismatched structures, strain-balanced quantum wells can be used to extend the absorption of the Ga0.99In0.01As middle cell towards longer wavelengths and match the current densities in a triplejunction device. Thin alternating layers of GaAsyP1-y and GaxIn1-xAs are typically used in this approach [10,21]. The current density achievable for the GaInAs middle cell depends on the lowest transition energy in the quantum wells.…”

Strain balanced quantum posts for intermediate band solar cells

“…Strain balanced multi-quantum-wells (SB-MQW) have been successfully implemented as the bottom GaAs cell of a current matched tandem configuration to tailor the absorption edge and increase the overall efficiency. [1] In the case of quantum dots (QDs), there are different approaches that may lead to a better conversion of the sunlight into electricity, being one of them the intermediate band solar cell concept (IBSC). [2,3] Recent detailed balance calculations, based in more realistic assumptions than previous results and including an intermediate band with multiple levels thermally interconnected, seem to indicate that even InAs/GaAs QDs, with bandgaps different from the optimum ones, can exhibit higher efficiency than a single gap cell at 1000 suns.…”