Dilute Nitride Quantum Engineered Solar Cells: For Next Generation of Multispectral Ultra-High Efficiency Si and III-V Photovoltaics
Kharel, Khim Lal 1982-
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Each year, the global photovoltaic markets continue to rapidly grow. However, one of the major challenges faced by photovoltaic manufacturing remains to be the cost of solar cell fabrication. The improvement of a cell’s photo-conversion efficiency is a critical factor in driving down the levelized cost of a solar cell. To improve the efficiency beyond the single junction limit, we must focus on the transmission and thermalization losses because they represent the largest contributions to the efficiency limit. In this research work, we investigate, from a theoretical perspective, a quantum engineered solar cell in tandem with an inexpensive silicon technology, to minimize the major losses. A GaAsyP1-x-yNx/GaP (where GaAsyP1-x-yNx has a lattice constant matched with silicon at y=4.7*x-0.1) based symmetric or asymmetric MQWs solar cell, designed to minimize the effects of the degraded minority carrier transport properties of bulk dilute nitride layers is proposed. Similarly, MBE grown GaAsN/GaAs MQWs solar cell device design has been adopted to lower the thermalization loss which enables a significant sub-GaAs-bandgap photocurrent generation while maintaining a world record-setting open-circuit voltage (Voc) approaching the ideal radiative limit (i.e. Woc=Eg-Voc ~ 0.4 V). Using a drift-diffusion approach, the tandem efficiency of the purposed p-i-n GaAsPN/GaP RTT MQWs solar cell in conjunction with an existing 25.6% HIT silicon device has been simulated under 1 sun and AM 1.5 G spectrum and the results show the possibility of achieving an efficiency of above 33% with this type of device. To gain a better understanding of the carrier transport mechanism in the fabricated devices, the optical and electrical properties were measured and analyzed. Bias-dependent EQE analysis shows 30x faster carrier escape in RTT devices compare to thick barrier MQWs cells. Similarly, extracted barrier heights of carriers from PL measurements in RTT devices are lower than in thick barrier MQWs devices. This suggests the improvement in the carrier-escape mechanisms in RTT devices. Finally, electronic temperatures of carriers were extracted from PL measurements which shows a significantly high (up to 1000 K for 300 K lattice temperature) and unusual carrier temperatures which suggests the presence of a significant hot carrier effect.