Argyrios C. Varonides

Thermionic escape of net photogenerated carriers and current densities from illuminated lightly doped single quantum wells

Photogenerated carriers captured in the intrinsic region of p/i/n GaAs–AlGaAs quantum well systems and contributing to current densities after thermionic escape, is the purpose of this communication. Illuminated intrinsic regions of solar cells with multiple quantum wells are expected to contribute an increase to overall collected currents when compared to bulk p/i/n solar cells. Carriers of the order of 1012cm−2 are found to be generated in single quantum wells of the i-region, and under 1017/cm2/s1 steady illumination flux levels (under Auger and radiative recombination mechanisms taken into account). Next, the contribution from each quantum well is evaluated in the form of thermionic current density values, and in terms of temperature and individual quasi-Fermi level position relative to the fundamental miniband in each well. Based on analytic results (for 30% Al fraction, and for 3–10nm lightly doped GaAs layers), computations show that (a) current density may vary from 1 to 9mA/cm2/10nm quantum well, as a function of background doping level in the neighborhood of 1011–1012cm−3, and at three representative temperatures −10°C,27°C, 40°C (b) current density may vary from 0.1 to 0.9mA/cm2 for a range of widths from 3 to 10nm, at the same temperatures, and for similar background doping (c) lightly doped 10nm GaAs quantum wells may generate current densities varying from 0.1 to 0.8mA/cm2 under photo-carrier levels in the order of 1012cm−2, and room temperature.

a-Si:H (alloy) p-i-n superlattice solar cell contacts

A means of increasing the open-circuit voltage of p-i-n amorphous silicon-based solar cells without degrading their other properties is proposed. This is accomplished through the use of superlattices that replace the n and p contact regions. The proposed structure is a-Si:H/a-Si/sub 0.8/N/sub 0.2/:H for the p contact and a-Si:H/a-Si/sub 0.5/C/sub 0.5/:H for the n contact. Using reasonable values for relevant parameters, the proposed structure may have an open-circuit voltage higher than that of present cells by as much as 0.20 V. >

A NEW P(A-SiN:H/A-Si:H)-I(A-Si:H)-N(A-Sic:H/A-Si:H) Superlattice Solar Cell

The objective of this work is (1) the evaluation and control of the Fermi level position in an amorphous hydrogenated silicon p-i-n Solar Cell through its carbon (or nitrogen) alloy n-and p-contact regions. The Fermi levels in both p- and n-regions of a P(a-Si:H/a-Si0.8N0.2:H)- i - N(a-Si:H./a-Si0.5C0.5:H) superlattice solar cell are computed. (2) prediction of open-circuit voltage values for this novel device. The p- and n- regions are replaced by superlattices, in which the Fermi levels in the wide gap barrier regions are, on an absolute scale, closer to the valence band or conduction band edges of the low band gap material (a-Si:H). In order for the p- and n- superlattices to yield a larger built-in voltage, the density of states in the doped wide band gap material, must be much greater, and the Fermi level of the wider band gap layer has to be closer to the corresponding band edge than that of the undoped low gap quantum well. Hence, the wide gap material must be heavily doped and have effective tails wider than that of the undoped low band gap material. As a result, the final Fermi level will lie between the corresponding bulk Fermi levels of the layers of the superlattice.

High Efficiency Multijunction Solar Cells with Finely-Tuned Quantum Wells

The field of high efficiency (inorganic) photovoltaics (PV) is rapidly maturing in both efficiency goals and cover all cost reduction of fabrication. On one hand, know-how from space industry in new solar cell design configurations and on the other, fabrication cost reduction challenges for terrestrial uses of solar energy, have paved the way to a new generation of PV devices, capable of capturing most of the solar spectrum. For quite a while now, the goal of inorganic solar cell design has been the total (if possible) capture-absorption of the solar spectrum from a single solar cell, designed in such a way that a multiple of incident wavelengths could be simultaneously absorbed. Multi-absorption in device physics indicates parallel existence of different materials that absorb solar photons of different energies. Bulk solid state devices absorb at specific energy thresholds, depending on their respective energy gap (EG). More than one energy gaps would on principle offer new ways of photon absorption: if such a structure could be fabricated, two or more groups of photons could be absorbed simultaneously. The point became then what lattice-matched semiconductor materials could offer such multiple levels of absorption without much recombination losses. It was soon realized that such layer multiplicity combined with quantum size effects could lead to higher efficiency collection of photo-excited carriers. At the moment, the main reason that slows down quantum effect solar cell production is high fabrication cost, since it involves primarily expensive methods of multilayer growth. Existing multi-layer cells are fabricated in the bulk, with three (mostly) layers of lattice-matched and non-lattice-matched (pseudo-morphic) semiconductor materials (GaInP/InGaN etc), where photo-carrier collection occurs in the bulk of the base (coming from the emitter which lies right under the window layer). These carriers are given excess to conduction via tunnel junction (grown between at each interface and connecting the layers in series). This basic idea of a design has proven very successful in recent years, leading to solar cells of efficiency levels well above 30% (Fraunhofer Institute’s multi-gap solar cell at 40.8%, and NREL’s device at 40.2% respectively). Successful alloys have demonstrated high performance, such as InxGa1 − xP alloys (x (%) of gallium phosphide and (1 − x) (%) of indium phosphide). Other successful candidates, in current use and perpetual cell design consideration, are the lattice-matched GaAs/AlGaAs and InP/GaAs pairs or AlAs/GaAs/GaAs triple layers and alloys, which are heavily used in both solar and the electronics industry.

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

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/cm2 and (b) excess current density of 12mA/cm2, due to thermionic escape mechanisms in quantum wells, with a total current in the neighborhood of 30mA/cm2. 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/cm2 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/cm2, 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?cm2 under nominal 100mW/cm2 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.