Abstract
We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage $V_{{{\rm OC}}}$ in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), $V_{{\rm{OC}}}$ degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), $V_{{\rm{OC}}}$ does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved $V_{{\rm{OC}}}$ of 0.23 V, which is beyond the detailed balance limit of standard solar cells. $V_{{\rm{OC}}}$ dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of $V_{{{\rm OC}}}$ .
Highlights
P HOTOVOLTAIC technologies have made great advances over the past decades, and laboratory-record solar cells are approaching their theoretical conversion efficiency limits
The determined N concentrations of the GaNAs quantum wells (QWs) cover the range of 0–3.1%
The XRD measurements confirm that QW thickness, AlGaAs barrier thickness, and AlGaAs composition are constant in the sample series with only slight variation between samples
Summary
P HOTOVOLTAIC technologies have made great advances over the past decades, and laboratory-record solar cells are approaching their theoretical conversion efficiency limits. A fundamental tradeoff exists between open-circuit voltage VOC and short-circuit current ISC of a solar cell device, which both depend on the bandgap of the material. To overcome these limitations, novel solar cell concepts have been proposed, such as the intermediate band solar cell (IBSC) [1]. To realize the theoretically anticipated function, solar cells with embedded nanostructures have been widely used, in which confined electronic states formed by the nanostructures are utilized as intermediate energy levels. To understand the relationship between achievable VOC and energetic position of the intermediate energy states, a systematic study of confinement depth is indispensable Such a study is, difficult due to the lack of suitable material systems, from which quantum structures with varying confinement depth can be grown epitaxially. AlGaAs reference GaAs QWs 0% N GaNAs QWs 0.5% N GaNAs QWs 1.3% N GaNAs QWs 1.9% N GaNAs QWs 3.1% N
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