Multi-Quantum well solar cells (MQWSC) have been shown to present several advantages, among which are low dark currents and tunable bandgaps. They are especially suited for implementation in multi-junction cells, and are highly promising for absorbers in Hot Carrier Solar Cells (HCSC). Such applications require high concentration ratio, which arises the issue of collection efficiency. Whereas it is usually considered that collection in MQW is very close to unity at one sun, it has been shown to not be the case under high concentration at the maximum power point. We propose in this work to take advantage of the luminescence spectral variation to investigate the depth collection efficiency. In order to validate the model, a series of strain compensated InGaAs/GaAsP MQW solar cells with intentional variation of the MQW doping concentration are grown. This has the effect of switching the space charge region position and width as well as the electric field intensity. Recording the luminescence spectra at various illumination intensities and applied voltages, we show that the in-depth quasi-Fermi level splitting and thus collection properties can be probed. Other measurements (EQE, luminescence intensity variation) are shown to be consistent with these results. Regarding their use as HCSC, the luminescence of MQW solar cells has been mainly used so far for investigating the quasi-Fermi level splitting and the temperature. Our results improve our understanding by adding information on carrier transport.
The present paper proposes Carrier Collection Efficiency (CCE) as a useful evaluation measure to investigate the carrier transport in quantum well solar cells. CCE is defined as the ratio of the carriers extracted as photocurrent to the total number of the carriers that are photo-excited in the p-n junction area, and can be easily calculated by normalizing the collected current, i.e. the difference between the current under light irradiation and that in the dark, to its saturation value at reverse bias. By measuring CCE as a function of the irradiation wavelength and the applied bias, we can directly and quantitatively evaluate the efficiency of the carrier extraction under operation of the cell, and clarify the underlying problem of the carrier transport. The proposed derivation procedure of CCE is based on the assumption that the saturation of the collected current at reverse bias indicates 100% collection of the photo-excited carriers. We validated this hypothesis by studying the balance between the number of the photo-excited carriers that can be collected at a sufficiently large reverse bias and the number of the photons absorbed in the wells. As a result, the absorption fraction in the MQW region well agreed with the saturated external quantum efficiency as we predicted, indicating CCE defined in this study is an appropriate approximation for the collection efficiency of the carrier generated in the active region of a solar cell device.
A quantum-well suparlattice cell, in which In0.13Ga0.86As (4.7 nm) / GaAs0.57P0.43 (3.1 nm) strain-balanced quantum
wells are inserted in the intrinsic region of a GaAs pin cell, has been implemented by metalorganic vapor-phase epitaxy
(MOVPE) and has exhibited an enhanced short-circuit current density, with an increment of 3.0 mA/cm2 and a minimal
drop in open-circuit voltage (0.03 V) compared to a pin cell without the superlattice. The collection efficiency of photocarriers,
which are generated in a cell upon the irradiation of monochromatic light, to an external circuit has been
evaluated for both the superlattice cell and a conventional quantum-well cell with thicker wells and barriers. This carrier
collection efficiency is was above 0.95 for the superlattice cell, regardless of a wavelength and an external bias, while the
value for the quantum-well cell degraded to be below 0.8 at a large forward bias, which evidenced superior carrier
transport with the help of tunneling through the thin barriers. With such a fast electron-hole separation in the
superlattice, photo-current generation by two-step photon absorption has been observed, using the electron ground state
of the superlattice as an intermediate band.
Hot carrier solar cells have a fundamental efficiency limit well in excess of single junction devices. Developing a
hot carrier absorber material, which exhibits sufficiently slow carrier cooling to maintain a hot carrier population
under realistic levels of solar concentration is a key challenge in developing real-world hot carrier devices.
We propose strain-balanced In0.25GaAs/GaAsP0.33 quantum wells as a suitable absorber material and present
continuous-wave photoluminescence spectroscopy of this structure. Samples were optimised with deep wells and
the GaAs surface buffer layer was reduced in thickness to maximise photon absorption in the well region. The
effect of well thickness on carrier distribution temperature was also investigated. An enhanced hot carrier effect
was observed in the optimised structures and a hot carrier distribution temperature was measured in the thick
well (14 nm) sample under photon flux density equivalent to 1000 Suns concentration.