NDLI: Short-circuit current enhancement in p-i-n GaAs/Ge solar cells with tuned superlattices, embedded in the mid-region

Content Provider	IEEE Xplore Digital Library
Author	Varonides, A.C.
Copyright Year	2011
Description	Author affiliation: Physics & Electrical Engineering Department, University of Scranton, USA (Varonides, A.C.)
Abstract	High efficiency in crystalline solar cells is currently of great importance and can be achieved by a combination of high and low band gaps in heterostructure designs. Such devices absorb in both short and long wavelengths, thus covering wider absorption areas from the solar spectrum. On the other hand, p-i-n cells provide space for excess absorption at specific wavelengths through layers grown in the mid-region of the cell. In this communication, the host device is a GaAs-GaAs-Ge solar cell with the mid-layer (GaAs) hosting a short-period GaAs/Ge superlattice. Under illumination, the device generates two current density components (a) bulk current density of $18mA/cm^{2}$ and (b) excess current density of $12mA/cm^{2},$ due to thermionic escape mechanisms in quantum wells, with a total current in the neighborhood of $30mA/cm^{2}.$ We calculate from first principles thermionic currents from quantum traps under one-sun illumination, with quantum wells tuned to 1eV incident solar photons. We predict $12mA/cm^{2}$ from a structure with the following geometry: twenty-period superlattice with 20nm-width of Ge-layers (0.66eV as low gap material) and 200nm barriers (1.42eV, GaAs as wide-gap medium, with negligible tunneling). The repeat distance of the superlattice is 220nm which for 20 layers translates to 4.4 μm of total superlattice thickness. For an 80μm, bulk GaAs-GaAs-Ge solar cell, this means that the excess carrier contribution would come from a superlattice layer which is under 6% of total device thickness. We predict that the total short circuit current density is in excess of $30mA/cm^{2},$ which along with open-circuit voltage increase, makes the device desirable for high current cells and subsequently for high efficiency cells as well. By appropriate n-doping of the germanium layers we show that it is possible to increase total thermal currents right off the quantum wells in GaAs-Ge layers. The figure below depicts a short superlattice in the middle of the p-i-n PV device: green stands for Ge layer and the rest stands for GaAs. As seen from the second figure (next page), collection efficiency is depicted in excess of 28% with open-circuit voltage ∼1.00V, FF factor 90%, short circuit current 30 $ma?cm^{2}$ under nominal $100mW/cm^{2}$ solar radiation. Ideal applications of such designs would apply for tandem devices, where open circuit voltage increase due to the series connection would increase overall OC voltage (and hence efficiency) by a factor of 2.
Starting Page	1
Ending Page	2
File Size	166231
Page Count	2
File Format	PDF
ISBN	9781457717550
e-ISBN	9781457717567
DOI	10.1109/ISDRS.2011.6135394
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2011-12-07
Publisher Place	USA
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Superlattices Gallium arsenide Photovoltaic cells Current density PIN photodiodes Lighting Educational institutions
Content Type	Text
Resource Type	Article

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