We propose and experimentally demonstrate a monolithic InP/InGaAsP mode- evolution-based polarization rotator. Using the adiabatic mode transition inside a tapered half-ridge waveguide, fabrication-tolerant conversion with an efficiency over 97.7% is obtained over the entire C-band.
Carrier transport inside InGaAs/GaAs/GaAsP multiple quantum well (MQW) solar cells was discussed under highconcentrated sunlight illumination up to 338 suns. Current-voltage (I-V) characteristic curves for a GaAs reference cell and MQW cells with GaAsP barrier thickness of 2, 4, and 6 nm were investigated under dark and high-concentration illumination. Carrier collection efficiency (CCE) was estimated by net photocurrent, which is the difference between illuminated current and dark current density at each bias voltage normalized by the value at the saturated point. For the 2-nm barrier, CCE exhibited almost no degradation compared with the GaAs reference cell. On the other hand, CCE for the 6-nm barrier degraded with forward biases as the sunlight concentration ratio increased. The degradation in CCE under a high-concentration ratio is shown to be the result of carrier accumulation in quantum wells. Thin barriers seemed to eliminate such accumulation with the help of the carrier tunneling effect through the barriers.
The conversion efficiency of a solar cell depends on the degree of non-radiative recombination, and thus efficiency improvements can also be achieved by reducing Shockley–Read–Hall (SRH) recombination losses. This type of loss depends not only on the crystal quality, but also on the device structure. A clear separation of the contributions of these factors would improve our understanding of the control of non-radiative recombination. In this work, we discuss the reduction of the integrated SRH recombination rate in the depletion region (USRHdep) by changing the emitter material, instead of fabricating a base layer with a higher crystal quality. First, we theoretically show that, by employing a suitable n-InGaP/p-GaAs heterojunction structure instead of a GaAs p–n homojunction, the integrated USRHdep can be reduced, because a significant part of the depletion region in the heterojunction is located in the wide-gap emitter material, which has a lower intrinsic carrier density. Then, the effective SRH recombination coefficient in the depletion region (Adep¯) is obtained from experiments, and the effect of the structural modification on USRHdep is analyzed. We are able to clearly assess the effect of the heterojunction structure on the non-radiative recombination because the grown samples exhibit the same radiative recombination loss. The analysis reveals that, in suitable heterojunction solar cells with an emitter layer containing a low intrinsic carrier concentration, Adep¯ (and thus also USRHdep) is effectively reduced.
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