George T. Nelson

Characterization of InAlAs solar cells grown by MOVPE

Epitaxial layers of InAlAs are prime candidates for the top cell in triple-junction photovoltaics (PV). Growth conditions during metalorganic vapor phase epitaxy (MOVPE) of InAlAs affect the material properties and subsequently the device characteristics of the epilayers. Impurity concentrations in InAlAs epilayers grown under various conditions are analyzed by secondary-ion mass spectrometry (SIMS) in order to assess impurity incorporation. Deep-level transient spectroscopy (DLTS) is used to assess the energy level and concentration of carrier traps. The effect of defects (traps) on the device characteristics are modeled with a Sentaurus simulation. Devices were fabricated and tested in a solar simulator before and after contact etch. Spectral response (SR) and electroluminescence (EL) are also measured. Final experimental results showed an efficiency of 9.74% without an antireflective coating.

GaSb solar cells grown on GaAs via interfacial misfit arrays for use in the III-Sb multi-junction cell

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shu...

Inverted growth evaluation for epitaxial lift off (ELO) quantum dot solar cell and enhanced absorption by back surface texturing

Thin ELO devices with light management allow for increased quantum dot (QD) absorption without the need for excessive strain management. A growth study was designed to evaluate challenges regarding inverted growth of ELO QDSCs. Two upright GaAs cell designs were investigated: a standard device and a “thick emitter” device. For each design, two QDSCs and a baseline without QDs were grown and fabricated. One standard QDSC was annealed for two hours while one thick emitter QDSC had an increased QD superlattice period. The standard baseline had a Jsc of 20.7 mA/cm² and a Voc of 1.01 V, while the standard QDSC had a J_SC of 21.2 mA/cm² and V_oc of 0.988 V. The thick emitter baseline had a J_SC of 21.4 mA/cm² and a V_oc of 1.04, while the thick emitter QDSC with increased period had a J_SC of 21.9 mA/cm² and V_oc of 0.99 V which is on par with the best reported QDSC Voc^s· Finally, inverted GaAs cells with and without QDs were fabricated with back surface reflectors (BSRs) and removed from the substrate by ELO. Devices with flat BSRs and textured BSRs are compared. The QDSC with the flat BSR had a sub-bandgap integrated spectral response equivalent to 0.38 mA/cm² while the textured BSR showed 0.57 mA/cm², which represents a 30% increase over the flat BSR.

GaSb on GaAs solar cells Grown using interfacial misfit arrays (Conference Presentation)

State of the art InGaP2/GaAs/In0.28Ga0.72As inverted metamorphic (IMM) solar cells have achieved impressive results, however, the thick metamorphic buffer needed between the lattice matched GaAs and lattice mismatched InGaAs requires significant effort and time to grow and retains a fairly high defect density. One approach to this problem is to replace the bottom InGaAs junction with an Sb-based material such as 0.73 eV GaSb or ~1.0 eV Al0.2Ga0.8Sb. By using interfacial misfit (IMF) arrays, the high degree of strain (7.8%) between GaAs and GaSb can be relaxed solely by laterally propagating 90° misfit dislocations that are confined to the GaAs-GaSb interface layer. We have used molecular beam epitaxy to grow GaSb single junction solar cells homoepitaxially on GaSb and heteroepitaxially on GaAs using IMF. Under 15-sun AM1.5 illumination, the control cell achieved 5% efficiency with a WOC of 366 mV, while the IMF cell was able to reach 2.1% with WOC of 546 mV. Shunting and high non-radiative dark current were main cause of FF and efficiency loss in the IMF devices. Threading dislocations or point defects were the expected source behind the losses, leading to minority carrier lifetimes less than 1ns. Deep level transient spectroscopy (DLTS) was used to search for defects electrically and two traps were found in IMF material that were not detected in the homoepitaxial GaSb device. One of these traps had a trap density of 7 × 1015 cm-3, about one order of magnitude higher than the control cell defect at 4 × 1016 cm-3.

Growth of InAs quantum dots in a metamorphic InGaAs bottom cell of an inverse metamorphic solar cell

State-of-the-art triple junction inverse metamorphic solar cells have been known to have bottom InGaAs junction that are limiting to the overall radiation performance. It has been proposed that the addition of quantum dots to the bottom junction would improve radiation tolerance of the bottom cell, and thus enhance the triple junctions overall performance in high dose applications. A significant amount of development has been made on quantum dots in metamorphic InGaAs for laser applications, however to date little work has been accomplished for photovoltaic devices. For this study we evaluate the growth of InAs quantum dots on Ino.31 GaAs grown metamorphically on GaAs. Simulations show that the addition of 50 layers of quantum dots to the bottom junction increases the expected efficiency by 0.2% at a dose of 1×1012 protons/cm2, however this would be a larger increase at a higher dose or if the bottom cell were current limiting. Evaluating the formation by atomic force microscopy with a height of 2.5 nm and density of approximately 7.8×1010 dots/cm2. Temperature dependent photoluminescence measurements were completed to extract an activation energy of 84.6 meV, which corresponds with the calculated electron confinement in the quantum dot.

Novel InAs/GaAs QD subcell design for radiation hard 3-J ELO IMM solar cell

InAs/GaAs quantum dots (QDs) have been investigated as a potential method of engineering the bandgap of the middle junction in triple junction solar cells to better match current generation and radiation tolerance to that of the InGaP top cell. While it is possible to successfully include QDs without inducing middle junction voltage degradation, they can lead to a slight reduction of minority carrier diffusion lengths in films grown after the QDs. Switching to an inverted metamorphic structure allows for the top cell to be grown first, but leads to challenges in maintaining current collection in the middle junction. In this work, a novel InAs/GaAs QD middle junction cell design is proposed for improving the efficiency of triple junction inverted metamorphic solar cells. Conventionally designed thin emitter devices as well as the proposed thick emitter devices were grown with and without InAs QDs in the uid region of the middle junction. The conventionally designed QDSC exhibited 1.8% relative increase in Jsc over the control device with no loss in VoC resulting in a 1.8% relative increase in efficiency over the control device.

InAlAs solar cells grown by alternative MOVPE precursors

Epitaxial layers of InAlAs are potential candidates for the top cell in triple-junction photovoltaics on InP. Growth conditions during metalorganic vapor phase epitaxy (MOVPE) of InAlAs affect the material properties and subsequently the device characteristics of the epilayers. Material development of InAlAs lattice-matched to InP using trimethylaluminum (TMAl) and tritertiarybutyl aluminum (TTBAl) is compared. Furthermore, InAlAs development using TTBAl is compared in conjunction with either arsine or tertiarybutylarsine (TBAs). Identical devices were grown from each aluminum source and are fabricated in parallel. The TTBAl/TBAs-InAlAs may have degraded during growth due to gas phase pre-reactions and resulted in a 6% efficient cell. However, using arsine, the TMAl-InAlAs devices showed maximum efficiency of 11.5% under 1-sun AM1.5, while TTBAl-InAlAs devices showed 12% (both without anti-reflective coatings). Dark current-voltage measurements of TMAl-InAlAs indicate higher material quality, though TTBAl-InAlAs has greater spectral response.

Study of deep levels in InAlAsSb grown via organometallic vapor phase epitaxy

Triple junction solar cells lattice matched to InP have the potential to exceed 50% efficiency under AM1.5 500× illumination with a bandgap stack of 1.74 / 1.1 / 0.7 eV. A top cell having 1.74 eV requires development of high-quality InAlAsSb, which currently has very little development effort reported. Preliminary deep-level transient spectroscopy results in p-type InAlAsSb indicate the presence of deep-levels 0.20 eV and 0.44 eV above the valence band with concentration ~1015 cm-3. Experiments such as spectral response and dark IV analysis reveal poor solar cell metrics, which may be a direct consequence of the detected defects. The InAlAsSb deep-levels are compared to the better-understood levels found in InAlAs, however, no evidence of a correlation is found.