Speaker
Description
Over the recent years an increased effort was devoted to nanophotonic engineering as a promising route to reach and overcome the detailed-balance limit.
In this project we focus on the optimization of the open-circuit voltage ($V_{oc}$) of a nanowire solar cell. The ultimate limit for the $V_{oc}$ can be achieved if the external radiative efficiency ($\eta_{ext}^{PL}$) and the ratio between solid angles of incident $ε_{in}$ and emitted light $ε_{out}$ are unity:
$V_{oc}=V_{OC}^{Ultimate}-\frac{k_b T}{q}ln(\frac{\epsilon_{in}}{\epsilon_{out}}) -\frac{k_b T}{q}ln(\eta_{ext}^{PL}) $
The external radiative efficiency $\eta_{ext}^{PL}$ can be estimated as the product of the internal radiative efficiency $\eta_{int}^{PL}$ and the photon escape probability ($P_{esc}$). Nanowires feature a fundamentally larger $P_{esc}$ than a planar layer since the light is emitted in a guided mode which adiabatically expands into air. To increase $\eta_{int}^{PL}$, both the surface recombination velocity and the number of non-radiative recombination centers should be decreased. The mismatch between the solid angles can be minimized by placing a nano-photonic lens on the top of the nanowire array.
Hereby, we present top-down fabricated GaAs nanowire solar cell. We demonstrate that inductively coupled plasma reactive ion etching (ICP-RIE) can be used to obtain GaAs nanowires of optimal geometry. Changing plasma chemistry allows us to control wires tapering angle that results in different Pesc. To decrease the surface recombination velocity, sidewalls of the NWs are passivated with lattice-matched InGaP. The latter passivation scheme greatly enhances the measured nanowires photoluminescence, decreasing the surface recombination velocity.
We show that the presence of the back-surface field facilitates charge separation, resulting in higher cell efficiency. Factors limiting solar cell efficiency are discussed.
| Speaker Country | Netherlands |
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