Boussairi Bouzazi

Five-volt vertically-stacked, single-cell GaAs photonic power converter

The high-efficiency conversion of photonic power into electrical power is of broad-range applicability to many industries due to its electrical isolation from the surrounding environment and immunity to electromagnetic interference which affects the performance and reliability of sensitive electronics. A photonic power converter, or phototransducer, can absorb several watts of infrared laser power transmitted through a multimode fiber and convert this to electrical power for remote use. To convert this power into a useful voltage, we have designed, simulated, and fabricated a photovoltaic phototransducer that generates >5 V using a monolithic, lattice-matched, vertically-stacked, single-cell device that eliminates complex fabrication and assembly steps. Experimental measurements have demonstrated a conversion efficiency of up to 60.1% under illumination of ~11 W/cm2 at a wavelength of 835 nm, while simulations indicate that efficiencies reaching 70% should be realistically achievable using this novel design.

Advances with vertical epitaxial heterostructure architecture (VEHSA) phototransducers for optical to electrical power conversion efficiencies exceeding 50 percent

A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for and has achieved conversion efficiencies exceeding 50%. The III-V heterostructure was grown by MOCVD, based on the vertical stacking of a number of partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered in the 830 nm to 850 nm range. The device architecture allows for improved open-circuit voltage in the individual base segments due to efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no need for complicated fabrication processes or reflecting layers. Progress for device outputs achieving in excess of 12 V is reviewed in this study.

Enhanced photocarrier extraction mechanisms in ultra-thin photovoltaic GaAs n/p junctions

PV devices with active areas of ~3:4 mm2 were fabricated and tested with top electrodes having different emitter gridline spacings with active area shadowing values between 0% and 1.8%. As expected, the thicker n/p junctions exhibit hindered photocarrier extraction, with low fill factor (FF) values, for devices prepared with sparse gridline designs. However, this study clearly demonstrates that for thin n/p junctions photocarrier extraction can still be efficient (FF > 80%) even for devices with no gridlines, which we explain using a TCAD model. The electric field profiles of devices with and without hindered photocarrier extraction are also discussed.

Through cell vias contacts for multijunction solar cells

The efficiency of multijunction solar cells used in concentrated photovoltaic systems is limited by shading from the grid line top electrode and electrical losses in the top epilayers. We propose to use through cell vias contacts to suppress the top electrode. Simulations show that the combination of through cell vias contacts with thin fingers has a potential absolute efficiency gain of 2 to 3% for concentration factors between 500 and 2000x. In addition, bus bars suppression improves by more than 20% the power extracted from a 6” wafer. Such an architecture requires additional technological steps. We discuss the challenges associated with via etching and report promising etching results for III-V heterostructures and germanium.

Plasma etching applications in concentrated photovoltaic cell fabrication

Photovoltaic cells are conventionally electrically isolated (isolation) and then separated from the wafer (singulation) by saw dicing at the end of the fabrication process. However, saw dicing presents limitations in terms of cell shapes and causes excessive material losses. We propose isolation and singulation by plasma etching as an alternative to saw dicing. The etching process proposed also allows via hole etching for through cell via contacts (TCVC) [1]. This technology uses isolated metallized vias to carry front-side generated carriers to the backside. This new architecture could increase the efficiency and increase the energy production per wafer for concentrated photovoltaic (CPV) cells. In this paper, those two plasma etching applications for CPV cell fabrication are discussed. More precisely, triple junction solar cells have been fabricated with either a plasma singulation or with via holes and compared with reference cells (without via hole and saw dicing singulation). One sun IV measurements,...

Impact of Via Hole Integration on Multijunction Solar Cells for Through Cell Via Contacts and Associated Passivation Treatment

The impact of via etching on triple junction solar cell performance has been investigated for through cell via contact architectures. Triple junction solar cells with standard top and back contacts have been fabricated and vias have been etched through the subcells to investigate the new geometry proposed. The external quantum efficiency, the open-circuit voltage (Voc), the fill factor (FF), and the ideality factor have been measured and compared to those of standard triple junction solar cells without vias. In this way, we evaluate the losses attributable to via etching. Small performance losses from via integration are observed, but performance can be partially restored with an ammonium sulfide passivation treatment. Furthermore, the results show that the V oc losses are almost absent at the high sun concentration that the new architecture is designed for. The source of the performance degradation is correlated with a larger total surface recombination at the edges of the device. The passivation treatment allows an effective surface passivation of the via holes.

Thermal behaviors and ageing of GaAs and InGaP solar cells for thermal-CPV hybrid energy systems

The main parameters of InGaP and GaAs thin solar cells (SCs) at average light concentration ratios (X) of ~54 and ~93 suns were investigated in the temperature range 25-250 °C. The main parameters of the two devices showed quasi-linear behaviors with increasing the operating temperature. The conversion efficiencies were found to drop ~27 and ~40 % in InGaP and GaAs, respectively independently of the two concentration ratios. Furthermore, the two devices showed a decrease in output power averagely ranging from 25 to 27 %, which yields to a difference less than 2%. In term of thermal reliability, the two devices did not show significant degradation after approximately 4 months of heat dumping. Hence, these results imply that GaAs still deliver more output power at 250 °C.

Microtechnologies for high efficiency solar cells

We present the various process steps that are required to build a high efficiency solar cell. We show then the advantage of plasma etching compared to saw dicing to singulate the cells.

Design optimizations of InGaAsN(Sb) subcells for concentrator photovoltaic systems

The InGaAsN(Sb) material system is an attractive candidate for use in lattice-matched four-junction (4J) solar cells based on germanium substrates. Design optimizations for an InGaAsN(Sb) subcell are proposed for optimal power conversion efficiency within a 4J solar cell under a highly concentrated AM1.5D solar spectrum. The performance of the subcell is modeled using drift-diffusion simulations using Crosslight Apsys. An InGaAsN(Sb) test subcell was fabricated to obtain realistic materials parameters for the optimization of subcell performance. A thin InGaAsN(Sb) subcell is suggested for operation at 1000 Sun illumination intensities at low carrier lifetimes and mobilities.