Luminescent Coupling Effects Research Articles

Numerical Analysis of Radiative Recombination and Reabsorption in GaAs/Si Tandem

We demonstrate a numerical analysis of the device impact of photon reabsorption on single-junction GaAs and tandem GaAs/Si solar cells. A self-consistent optical–electrical model that considers nonideal losses within the devices is developed. For single-junction devices, we find that the impact of photon recycling on the voltage increases monotonically with the injection level. For record-level GaAs solar cells, the voltage boost is 33 mV under open-circuit conditions and 13 mV at the maximum power point. For tandem GaAs/Si solar cells, photon reabsorption moderates the sensitivity of tandem efficiency to both obvious parameters like absorber thickness and implicit parameters like shunt resistance ( $R_{{\rm sh}}$ ) and bulk lifetime. Considering luminescent coupling results in a GaAs top cell that is 9.5% thicker than without luminescent coupling. The tandem device is 50% more sensitive to $R_{{\rm sh}}$ changes in the GaAs cell than $R_{{\rm sh}}$ changes in the Si cell. The impact of the GaAs top-cell bulk lifetime on tandem efficiency is reduced by 61% if photon reabsorption is not considered. This integrated optoelectronic device model allows one quantification of the implicit effects of photon recycling and luminescent coupling on device parameters for GaAs/Si tandem, providing a valuable tool for high-performance device optimization.

Experimental characterization and self-consistent modeling of luminescence coupling effect in III-V multijunction solar cells

In this paper, we focused on developing an accurate model to describe the luminescent coupling (L-C) effect in multijunction solar cells (MJSC) under light concentration. We present here a transcend current-voltage (I-V) formula combined with a self-consistent simulation algorithm to derive the coupling yield γ dependence on light intensity by including the electrical parameters such as shunt resistance (Rsh) and series resistance (Rs), which were ignored in previous simulation models. The effects of both Rsh and Rs on γ were revealed, and the dependence of γ on the external voltage bias Vbias was investigated. Meanwhile, we have performed experiments to determine coupling yield γ by measuring the I-V curves of individual subcell of InGaP/GaAs/Ge triple junction solar cell under varied light intensity. We found that the measured results are only in good agreement with the simulated data obtained from the model where the resistance parameters were included. Based on these results, we calculated the conversion efficiency of MJSC and found that the efficiency increase due to L-C effect is 0.31% under 1 sun and 1.07% under 1000 suns. Thus the L-C analysis results presented here will work as an additional device optimization criteria for MJSC toward higher efficiency.

Voltage and light bias dependent quantum efficiency measurements of GaInP/GaInAs/Ge triple junction devices

III–V monolithic multi-junction solar cells reach efficiencies exceeding 40% and have applications in space and terrestrial concentrator systems. Due to the series connection of the junctions, the external quantum efficiency (EQE) measurement of multi-junction cells presents additional challenges compared to EQE measurement procedures for single-junction devices. Previous work has shown that optimization of bias light and voltage is necessary to minimize measurement artifacts that usually appear during the measurement of the Germanium (Ge) junctions with low shunt resistance. This paper aims to contribute to the improvement of multi-junction device characterization by reporting the effects of low and high shunt resistance junctions on EQE for various voltage and light bias conditions. Voltage and light bias are shown to have different effects on junctions with different shunt resistance. Therefore further investigation of junctions with different shunt resistance under variable light and voltage conditions is needed to provide better understanding of the precise test conditions for EQE measurements. Our results showed that in the high shunt resistance junctions the bias voltage technique is not necessary. However, in the case of junctions with low shunt resistance, a combination of voltage and light bias is required during EQE measurements. Furthermore, the luminescence coupling effect was observed during EQE measurements at very intense light bias conditions confirming previous indications. Finally, an alternative approach for measuring the EQE is proposed.

Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III-V multijunction solar cell. We describe a technique that uses a set of light-emitting diodes (LEDs) and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.

Elimination of Artifacts in External Quantum Efficiency Measurements for Multijunction Solar Cells Using a Pulsed Light Bias

A method using a pulsed light bias synchronized to the probing monochromatic light and superimposed on the conventional dc light and voltage biases is proposed to eliminate the artifacts in external quantum efficiency (EQE) measurements for multijunction solar cells. It is demonstrated experimentally that this method effectively eliminates the EQE measurement artifacts caused by the shunt effect, the luminescence coupling effect, or their combination.

Using Phase Effects to Understand Measurements of the Quantum Efficiency and Related Luminescent Coupling in a Multijunction Solar Cell

We analyze the quantum efficiency measurement of a series-connected multijunction solar cell by modeling the cell as an ac resistive-capacitive circuit and studying the complex current as a function of the light biasing. The photocurrent induced by directly absorbed photons is modeled as an independent current source, whereas the luminescent coupling current from higher bandgap to lower bandgap subcells is modeled as a dependent current source in the bottom subcell. We derive expressions for the equivalent impedance and the complex current in measurements of the top and bottom subcells of a two-junction device. High light biasing of the nonlimiting cell drives the magnitude and phase shift of the current toward well-defined limits, but insufficient light biasing yields a composite response that is not fully characteristic of either subcell. We recommend that the experimenter always monitor the phase shift of the signal for signs of insufficient light biasing, and to help identify genuine luminescent coupling effects.

Non-linear luminescent coupling in series-connected multijunction solar cells

The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-V multijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.