Christopher Kerestes

Strain Effects on Radiation Tolerance of Triple-Junction Solar Cells With InAs Quantum Dots in the GaAs Junction

A comparison of quantum dot (QD) triple-junction solar cells (TJSCs) with the QD superlattice under tensile strain are compared with those under compressive strain and baseline devices to examine the effects of strain induced by the InAs QD layers in the middle junction. Theoretical results show samples with tensile-strained InAs QDs have lower defect formation energy while compressive-strained QDs have the greatest. Experimentally, it is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1-MeV electrons, TJSCs with tensile strain exhibit a faster degradation in Isc of the QD samples and slower degradation in Voc but overall faster degradation in efficiency compared with baseline TJSCs, regardless of the magnitude of tensile strain. Compressively strained QD TJSCs have similar degradation in Isc and slower degradation in Voc compared with baseline TJSCs. From this study, it is determined that a slightly compressive strain in the QD superlattice allows for the best performance pre- and postirradiation for QD TJSCs based upon AM0 IV and quantum efficiency measurements and analysis. Fabricating devices with improvements determined from samples with varying strain leads to QD TJSCs with better radiation tolerance in terms of power output for 5, 10, 15, and 20 layers of QDs.

Investigation of carrier escape mechanism in InAs/GaAs quantum dot solar cells

In order to enhance understanding of the short circuit improvement in InAs/GaAs quantum dot (QD) solar cells, the thermally assisted and tunneling mechanisms of carrier escape from the QD quantum confinement are investigated. The dependence of voltage biased spectral responsivity for QD solar cells at room temperature is studied to analyze carrier extraction through tunneling. Photoexcited carrier confinement and escape were also studied by means of temperature dependent spectral response (TDSR) and temperature dependent photoluminescence (TDPL). Energy required to move a carrier from the ground state to the first excited state, thermal activation energy (Ea), in a quantum dot is calculated from TDPL to be 114 meV. It is found that at room temperature carrier escape from the quantum dot confinement is affected by both thermal assisted escape and tunneling while at low temperature tunneling is the dominant in carrier escape from both wetting layer and QDs. For all temperature ranges, carrier exchange between ground states and excited states and carrier escape from ground states (GS) is first thermal escape to excited states (ES) then tunneling.

Investigation of quantum dot enhanced triple junction solar cells

InAs quantum dots have been incorporated into the middle junction of an InGaP/(In)GaAs/Ge triple junction solar cell (TJSC) on four inch wafers, in aims of band gap engineering a high efficiency solar cell to even higher limits. Results of QD growth on 4” diameter Ge templates gave densities near 1×1011 cm−3 and QD height between 2–5 nm. Arrays of 10 layers of InAs QDs have been grown between the base and emitter in the middle cell of a full triple junction solar cell. Control triple junction cells that received growth interrupts without QD growth showed similar results (within 5 mV open circuit voltage) to standard triple junction cells without an interrupt. Integrated current of the (In)GaAs junction with 10 layers of strain balanced InAs QD layers shows a gain of 0.37 mA/cm2 beyond the band edge. One sun AM0 current-voltage measurements of QD TJSC show an efficiency of 26.9% with a Voc of 2.57 V.

Strain effects on radiation tolerance of quantum dot solar cells

A comparison of quantum dot (QD) triple junction solar cells (TJSCs) under tensile strain are compared to those under compressive strain and baseline devices to examine the effects of strain induced by the QD layers. It is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1 MeV electrons triple junction solar cells with tensile strain exhibit a faster degradation in Isc of the QD samples and slower degradation in Voc but overall faster degradation in efficiency compared to baseline TJSCs, regardless of the magnitude of tensile strain. Compressively strain QD TJSCs have similar degradation in Isc and slower degradation in Voc compared to baseline TJSCs. From this study it is determined a strain of +400 ppm in the QD superlattice allows for the best performance pre- and post- irradiation for QD TJSCs based upon AM0 IV and quantum efficiency.

Radiation effects on quantum dot enhanced solar cells

Radiation tolerance of quantum dot (QD) enhanced solar cells has been measured and modeled. GaAs solar cells enhanced with 10, 20, 40, 60, and 100X layers of strain compensated QDs are compared to baseline devices without QDs. Radiation resistance of the QD layers is higher than the bulk material. Increasing the number of QD layers does not lead to a systematic decrease in QD response throughout the course of radiation exposure. Additionally, InGaP/(In)GaAs/Ge triple junction solar cells with and without 10 layers of strain compensated QDs in the (In)GaAs triple junction solar cells are analyzed. Triple junction solar cells with QDs have a better resistance to Voc degradation but these samples have a degradation in Isc that leads to lower radiation resistance for power output.

Manufacturing improvement of IMM solar cells and flex string arrays on the AFRL Mantech program

The manufacturing improvement of IMM solar cells and flex string arrays (FSAs) is a focus of the AFRL-funded IMM Mantech program. Significant manufacturability improvements have been achieved to date, with focus on reduction in materials cost and touch labor, process automation, and demonstration of large area IMM cells (≥ 60cm2) on 4” and 6” wafers. Confidence testing of FSAs encompassing multiple roll/unroll cycles, hot and cold rolled soak, vibration testing, thermal vacuum (TVAC) and hot GEO thermal cycling has shown negligible degradation of power output.

Investigation of carrier removal from QD TJSCs

Quantum dot triple junction solar cells (QD TJSCs) have potential for higher efficiency for space and terrestrial applications. Extended absorption in the QD layers can increase efficiency by increasing the short circuit current density of the device, as long as carrier extraction remains efficient and quality of the bulk material remains high. Experimental studies have been conducted to quantify the carrier extraction probability from quantum confined levels and bulk material. One studies present insight to the carrier extraction mechanisms from the quantum confined states through the use of temperature dependent measurements. A second study analyses the loss in carrier collection probability in the bulk material by investigating the change in minority carrier lifetimes and surface recombination velocity throughout the device. Recent studies for space applications have shown response from quantum structures to have increased radiation tolerance. The role strain and bonding strength within the quantum structures play in improving the radiation tolerance is investigated. The combination of sufficiently good bulk material and device enhancement from the quantum confinement leads to temperature dependent measurements that show TJSCs outperform baseline TJSCs near and above 60°C. Insight into the physical mechanisms behind this phenomenon is presented.