Climate change and its many associated impacts are among the most serious and pressing global challenges. Photovoltaics (PV) is instrumental in the mitigation of CO2 through the generation of low carbon electricity. However, the goal of limiting global warming to 1.5°C requires additional approaches. This paper presents how PV surfaces can reverse the Earth’s radiative imbalance caused by increasing atmospheric greenhouse gases and thereby stabilize global temperatures. The benefits are realized by: (1) high effective albedo between 30 – 40% (2) maximizing thermally emitted radiation; and (3) active infrared emission averaging 300 W/m2 through atmospheric wavelength windows, e.g., at 1.5-1.75 micron. With such PV surfaces, we show that 25 TW of PV can reverse or mitigate global warming.
Improved photovoltaic efficiencies by incorporating an internal front reflector in addition to an ideal back reflector is limited to devices with absorber bandgaps below ~1.34 eV due to the availability of AM1.5G photons above the reflector energy-cutoff, i.e. reflector bandgap. However, energy-selective front reflectors especially benefit photovoltaic devices under monochromatic illumination, i.e. laser power converters (LPCs). The efficiency improvements with and without a front reflector as a function of solar cell bandgap are compared under AM1.5G versus monochromatic light at the reflector bandgap. For a GaAs solar cell, where front reflectance enhancements are negligible, over 5% absolute efficiency increase is predicted in a GaAs LPC with a front reflector (605 nm illumination). Bandgap reduction further increases the efficiency enhancement under monochromatic illumination, where up to 60% relative efficiency increase can be achieved at Eg =0.5 eV with E_laser = 1.467 eV.
Silicon solar cells benefit from an internal Lambertian light distribution achieved through texturing, while the performance of direct-bandgap materials can be lower with an internal Lambertian light distribution than the light distribution of a planar cell. A novel analytic expression is derived for the emittance of cells with a Lambertian light distribution and partial rear reflectance. This expression enables comparison of Si, GaAs, CdTe, and CIS cells under planar and Lambertian light distributions with varying rear reflectance in the Auger limit. A Lambertian light distribution is shown to be particularly beneficial to thinner material with higher rear reflectance due to absorptance enhancement. It is found that a Lambertian light distribution increases radiative recombination in most absorbers but can reduce radiative recombination in some CIS material.
Daniel Law, J. Boisvert, E. Rehder, P. Chiu, S. Mesropian, R. Woo, X. Liu, W. Hong, C. Fetzer, S. Singer, D. Bhusari, K. Edmondson, A. Zakaria, B. Jun, D. Krut, R. King, S. Sharma, N. Karam
Recent progress in III-V multijunction space solar cell has led to Spectrolab’s GaInP/GaAs/Ge triple-junction, XTJ, cells with average 1-sun efficiency of 29% (AM0, 28°C) for cell size ranging from 59 to 72-cm2. High-efficiency inverted metamorphic (IMM) multijunction cells are developed as the next space solar cell architecture. Spectrolab’s large-area IMM3J and IMM4J cells have achieved 33% and 34% 1-sun, AM0 efficiencies, respectively. The IMM3J and the IMM4J cells have both demonstrated normalized power retention of 0.86 at 5x1014 e-/cm2 fluence and 0.83 and 0.82 at 1x1015 e-/cm2 fluence post 1-MeV electron radiation, respectively. The IMM cells were further assembled into coverglass-interconnect-cell (CIC) strings and affixed to typical rigid aluminum honeycomb panels for thermal cycling characterization. Preliminary temperature cycling data of two coupons populated with IMM cell strings showed no performance degradation. Spectrolab has also developed semiconductor bonded technology (SBT) where highperformance component subcells were grown on GaAs and InP substrates separately then bonded directly to form the final multijunction cells. Large-area SBT 5-junction cells have achieved a 35.1% efficiency under 1-sun, AM0 condition.
Low cost germanium photodetectors for sensing applications in the 900-1600 nm spectral region have been developed. By varying the Ge substrate resistivity as well as device area, photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Low leakage current devices of various sizes up to 1 cm2 have been fabricated and have consistently exhibited exceptionally high shunt resistances and excellent linearity. Over 5000 hours of active stress testing have left the ultra-low leakage currents unchanged. These data were measured in accordance with Telcordia 468-CORE requirements at 85°C, 125°C and 175°C. The results indicate that these mesa photodetectors meet telecommunication industry requirements for reliability. These devices are comparable to commercially available Ge photodetectors, and can be readily substituted for more complex InGaAs photo-detectors in applications such as laser monitor diodes.
Germanium (Ge) photodetectors are fabricated by growing epitaxial III-V compounds on Ge substrates and by in-situ formation of the PN junction by MOVPE. After material growth, Ge photodetectors are mesa-etched using conventional optoelectronic device processing techniques. By varying the Ge substrate resistivity and the device area, Ge photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Such devices have demonstrated leakage currents below 50(mu) A/cm2 at -0.1 V bias. For optoelectronic applications that require high temperature operation, high shunt resistance detectors exhibit leakage currents below (mu) A/cm2 at 80 degree(s)C. Low capacitance devices have measured as little as 275 pF at 0V bias for a 1 mm diameter detector. High shunt resistance devices are a low cost alternative to conventional InGaAs photodiodes in applications such as laser monitor diodes.
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