Lothar A. Reichertz

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates

Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

Photovoltaic effects in BiFeO3

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.

Demonstration of a III–Nitride/Silicon Tandem Solar Cell

We report on the proof of principle of a III–nitride/silicon tandem solar cell. Photovoltaic activity is demonstrated in a 0.25 cm2 dual junction solar cell, made of p- and n-type GaN layers which were grown by molecular beam epitaxy (MBE) on a standard n-type Si wafer with an Al doped p-type surface. An open circuit voltage (Voc) of 2.4 V was measured under 1× sun AM1.5G condition with additional UV laser illumination of the GaN junction. Experiments under various illumination conditions were performed to verify that both junctions are active and working in series.

Demonstration of homojunction ZnTe solar cells

We report on the proof of photovoltaic activity of homojunction ZnTe solar cells in which n-ZnTe layers are fabricated by thermal diffusion of Al into p-ZnTe at several diffusion times to control the junction depth. An open circuit voltage of approximately 0.9 V was obtained under 1× sun AM1.5G condition in all solar cells, independent of diffusion times, while a short circuit current dropped down with increasing the diffusion time due to an increased light absorption in heavily defective Al-diffused layer. These fundamental results provide a basis for future development of intermediate band solar cells based on ZnTe materials.

DOUBLE ACCEPTOR DOPED GE : A NEW MEDIUM FOR INTER-VALENCE-BAND LASERS

We report on intervalence‐band laser emission from Be‐ and Zn‐doped germanium crystals. The duty cycle of 10−3 at a repetition rate of 1 kHz is one order of magnitude larger than the highest duty cycle reported for p‐Ge lasers doped by group II acceptors. This improvement is due to the much larger hole binding energy of double acceptors Be and Zn which results in a strong reduction of the internal absorption of the generated far‐infrared radiation. Laser action has been achieved with crystal volumes as small as 0.04 cm−3, and a laser pulse length of 25 μs has been reached. Germanium crystals doped with these acceptors may offer an opportunity for achieving the ultimate goal of continuous wave operation.

Demonstration of ZnTe1-xOx Intermediate Band Solar Cell

We report the optical properties of ZnTe1-xOx (ZnTeO) and evidence for the photovoltaic (PV) activity of a ZnTeO intermediate band solar cell (IBSC). By photomodulated reflectance measurements, electron transitions from the intermediate band to the conduction band were demonstrated. The optical absorption coefficients for the electron transition from the valence band to the intermediate band exceeds 2×104 cm-1, suitable for thin-film PV device applications. The ZnTeO IBSC exhibits an enhanced spectral response below the band edge of ZnTe, and all results are consistent with the proposed conversion mechanism of IBSC.

Stimulated far-infrared emission from copper-doped germanium crystals

We have detected stimulated far-infrared emission from copper-doped germanium single crystals. By varying the magnetic field between 1 and 2.3 T, we have achieved emission in the range of 70–120 cm−1. Laser action was observed for crystals with a copper acceptor concentration as high as 1.5×1015 cm−3, a doping level that is considerably higher than that of any single or double acceptor doped Ge laser. Stimulated emission from Ge crystals with such a high Cu concentration is possible because only a small fraction of the copper acceptors is ionized during operation.

First Results on GaAs blocked impurity band (BIB) structures for far-infrared detector arrays

We are developing a GaAs photoconductive detector for far-infrared (FIR) astronomy. A detector based on GaAs in the blocked impurity band (BIB) con.guration is expected to extend the long wavelegth limit of currently available stressed Ge:Ga photoconductors up to about 330 microns. Without the need of uniaxial stress applied to the crystal, this would furthermore allow the fabrication of single chip arrays with a large number of pixels. We are reporting results of the characterization of preliminary GaAs BIB test structures. The experimental work is supported by numerical modeling that includes all contact and space charge effects.

Far-infrared absorption in GaAs:Te liquid phase epitaxial films

The far-infrared absorption spectrum of n-type GaAs is of interest for applications such as GaAs photoconductors and blocked impurity band detectors. The linear optical absorption coefficients α for three n-type GaAs films of varying doping concentrations have been measured in the range of 10 to 100 cm−1 using Fourier transform infrared spectrometry. These results show α having maximum values of between 46 cm−1 at 1×1015 cm−3 and approximately 800 cm−1 at 2.1×1016 cm−3. The formation and widening of a donor impurity band with increasing impurity concentration is clearly demonstrated.

In-rich InGaN thin films: Progress on growth, compositional uniformity, and doping for device applications

A number of In-rich InGaN films with In contents in the 20–40% range have been grown at moderately low temperatures on sapphire and silicon substrates at high growth rates using a versatile molecular beam epitaxy-type technology that utilizes an energetic beam of N atoms called energetic neutral atom beam lithography and epitaxy to overcome reaction barriers in the group III-nitride system. Extensive characterization results on the crystalline, optical, and electrical properties of the In-rich InGaN materials are reported. It was found that N-rich growth conditions are required to produce materials that have excellent crystallinity, uniform compositions, and bright band edge photoluminescence. For In-rich InGaN growth on sapphire, electrical transport measurements show reasonably low carrier concentrations and high mobilities. Successful p-type doping of In-rich InGaN with ∼20% and ∼40% In contents is demonstrated, and preliminary results on the formation of a p–n junction are reported. For In-rich InGaN ...