In this contribution, we use spectrally resolved and quantitative photoluminescence measurements to parse voltage losses in CdSeTe films and finished devices. We show that sub-bandgap features, in part due to arsenic doping, are responsible for a significant decrease in the thermodynamic voltage limit Voc,ideal. Nevertheless, thanks to excellent material quality and interface passivation, the internal voltage iVoc (i.e., quasi-Fermi-level splitting with QFLS=q×iVoc) of finished devices approaches 1000 mV, in agreement with the high minority-carrier lifetimes measured. The selectivity of the device, and in particular of the back hole contact, is thus the main limitation in these devices.
KEYWORDS: Solar cells, Silicon, Solar energy, Tandem solar cells, Electrical engineering, Energy efficiency, Computer engineering, Physics, Doping, Optics
KEYWORDS: Group III-V semiconductors, Luminescence, Tandem solar cells, Photovoltaics, Silicon solar cells, Silicon, Solar cells, Photons, Absorption, Diffusion, Performance modeling, Solar energy, Computer architecture
A model, derived from the detailed balance model from Shockley and Queisser, has been adapted to monolithically grown GaAsP/Si tandem dual junction solar cells. In this architecture, due to the difference of lattice parameters between the silicon bottom cell – acting as the substrate – and the GaAsP top cell, threading dislocations (TDs) arise at the IIIV/ Si interface and propagate in the top cell. These TDs act as non-radiative recombination centers, degrading the performances of the tandem cell. Our model takes into account the impact of TDs by integrating the NTT model developed by Yamaguchi et. al.. Two surface geometries have been investigated: flat and ideally textured. Finally the model considers the luminescent coupling (LC) between the cells due to reemitted photons from the top cell cascading to the bottom cell. Without dislocations, LC allows a greater flexibility in the cell design by rebalancing the currents between the two cells when the top cell presents a higher short-circuit current. However we show that, as the TD density (TDD) increases, nonradiative recombinations take over radiative recombinations in the top cell and the LC is quenched. As a result, nonoptimized tandem cells with higher short-circuit current in the top cell experience a very fast degradation of efficiency for TDDs over 104cm-2. On the other hand optimized cells with matching currents only experience a small efficiency drop for TDDs up to 105cm-2. High TDD cells therefore need to be current-matched for optimal performances as the flexibility due to LC is lost.
Lattice-mismatched 1.7eV Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on Si substrates using Solid Source Molecular Beam Epitaxy (SSMBE). As a consequence of the 4%-lattice-mismatch, threading dislocations (TDs) nucleate at the interface between the Si substrate and III-V epilayers and propagate to the active regions of the cell. There they act as recombination centers and degrade the performances of the cell. In our case, direct AlAs/GaAs superlattice growth coupled with InAlAs/AlAs strained layer superlattice (SLS) dislocation filter layers (DFLSs) have been used to reduce the TD density from 1×109cm-2 to 1(±0.2)×107cm-2. Lattice-matched Al0.2Ga0.8As cells have also been grown on GaAs as a reference. The best cell grown on silicon exhibits a Voc of 964mV, compared with a Voc of 1128mV on GaAs. Fill factors of respectively 77.6% and 80.2% have been calculated. Due to the lack of an anti-reflection coating and the non-optimized architecture of the devices, relatively low Jsc have been measured: 7.30mA.cm-2 on Si and 6.74mA.cm-2 on GaAs. The difference in short-circuit currents is believed to be caused by a difference of thickness between the samples due to discrepancies in the calibration of the MBE prior to each growth. The bandgap-voltage offset of the cells, defined as Eg/q-Voc, is relatively high on both substrates with 736mV measured on Si versus 572mV on GaAs. The non-negligible TD density partly explains this result on Si. On GaAs, non-ideal growth conditions are possibly responsible for these suboptimal performances.
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