For solar cells composed of direct bandgap semiconductors such as GaAs, the performance can be significantly improved by utilizing photon recycling and luminescence coupling effects. Accurate modeling with those effects may offer insightful guidance in designing such devices. Previous research has demonstrated different numerical models on photon recycling and luminescent coupling. However, most of those works are based on complicated theoretical derivation and idealized assumptions, which made them hard to implement. In addition, very few works provide method to model both photon recycling and luminescent coupling effects. In this paper, we demonstrate an easy-to-implement but accurate numerical model to simulate those effects in multijunction solar cells. Our numerical model can be incorporated into commonly used equivalent circuit model with high accuracy. The simulation results were compared with experimental data and exhibit good consistency. Our numerical simulation is based on a self-consistent optical-electrical model that includes non-ideal losses in both the single junction and the tandem device. Based on the numerical analysis, we modified the two-diode circuit model by introducing additional current-control-current sources to represent the effects of both photon recycling and luminescence coupling. The effects of photon recycling on the diode equation have been investigated based on detailed-balanced model, accounting for internal optical losses. We also showed the practical limit of performance enhancement of photon recycling and luminescent coupling effects. This work will potentially facilitate the accurate simulation of solar cell with non-ideal effects, and provide more efficient tools for multijunction solar cell design and optimization.
KEYWORDS: Thin film solar cells, Thin films, Solar cells, Oxides, Titanium, Crystals, Silicon solar cells, Silicon, Energy conversion efficiency, Oxygen, Atomic layer deposition
In crystalline silicon (c-Si) solar cells, carrier selective contacts are among the remaining issues to be addressed in order to reach the theoretical efficiency limit. Especially in ultra-thin-film c-Si solar cells with small volumes and higher carrier concentrations, contact recombination is more critical to the overall performance. In this paper, the advantages of using TiOX as electron-selective layers for contact passivation in c-Si solar cells are analyzed. We characterize the metal/TiOX/n-Si electron-selective contact with the contact recombination factor J0c and the contact resistivity ρc for the first time. Experimental results show that both J0c and ρc decrease after the insertion of TiOX. In addition, the effect of post-deposition rapid-thermal-annealing (RTA) at different temperatures is also evaluated. The best J0c of 5.5 pA/cm2 and the lowest ρc of 13.6 mΩ·cm2 are achieved after the RTA process. This work reveals the potential of TiOX as an electron-selective layer for contact passivation to enable high-efficiency ultra-thin c-Si solar cells with a low cost.
In this paper, a novel electro-absorption modulation mechanism based on coupled-quantum-wells (CQWs) is proposed and demonstrated. Compared to a quantum-confined-stark-effect (QCSE) modulator with multiple fully decoupled single-QWs, the newly designed CQW modulator has two sub-quantum-wells partially coupled with a small barrier in between. Modulation is based on the change of electron and hole wave-function overlap in the CQWs, which requires a small bias electric field of <10 kV/cm) compared to the operation of a typical QCSE modulator which requires >50 kV/cm bias electrical field. Theoretically, the power consumption of this new CQW modulator can be lower than 20 fJ/bit and the speed can be higher than 10 Gbps, which outperforms the best Ge/SiGe QCSE modulator that has been previously demonstrated. A proof-of-concept Ge/SiGe CQW modulator based on this novel modulation mechanism was designed and fabricated. Instead of a traditional PIN diode structure, the new CQW modulator uses a PIP structure.
State-of-the-art III-V cells have reached the highest energy conversion efficiency among all types of solar cells. However, these cells are not applicable to widespread terrestrial solar energy system yet due to the high cost of epitaxial growth. Ultra-thin film absorbers with advanced light management is one of the most promising solutions to drive down the cost. In this paper, we present an ultra-thin film nano-window gallium arsenide (GaAs) solar cell design. This ultrathin cell consists of a nano-structured Al0.8Ga0.2As window layer on the front side to reduce the reflection and to trap the light, and a metal reflector on the back side to further increase the light path. The 300 nm thick GaAs cell with Al0.8Ga0.2As nano-window shows a broad band absorption enhancement from visible to near infrared (NIR), achieving a spectrally averaged absorption of 94% under normal incidence. In addition, this cell shows excellent angular absorption properties, achieving over 85% spectral averaged absorption at up to 60 degree off normal incidence. Meanwhile, this structure with planar junction and nano-window has solved the issue of low fill factor and low open-circuit voltage in nano-structured GaAs solar cell. A nano-window cell with a 3 μm thick GaAs junction demonstrated an open circuit voltage of 0.9V.
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