Although methods for harvesting subbandgap solar photons have been demonstrated, present approaches still face substantial challenges. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot (QD) intermediate band photovoltaic (PV) device using photocurrent measurements under subbandgap illumination. We show that subbandgap photons can generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent regardless of whether the dominant carrier escape mechanism is optical, tunneling, or thermal. We introduce a new design for an InAs QD-based nanostructured material that can efficiently upconvert two low-energy photons into one high-energy photon. Efficiency is enhanced by intentionally sacrificing a small amount of photon energy to minimize radiative and nonradiative loss. Upconversion PV devices based on this approach separate the absorption of subbandgap photons from the current-harvesting junction, circumventing the carrier-trapping problems.
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.
The effects of delta-doping InAs quantum-dot (QD)-enhanced GaAs solar cells were studied both through modeling and device experimentation. Delta doping of two, four, and eight electrons per QD, as well as nine holes per QD, was used in this study. It was observed that QD doping reduced Shockley-Read-Hallrecombination in the QDs, which results in a reduced dark current and an improved open-circuit voltage over undoped QD devices. A voltage recovery of 121 mV was observed for the eight-electron sample compared with the undoped sample. QD doping had no positive effects on subbandgap photon collection but actually degraded bulk and QD response as doping levels were increased by limiting minority carrier collection through the QD region. Despite this, an absolute AM0 efficiency improvement of 1.41% was observed for the four-electron sample over the undoped QD device while maintaining a current enhancement.
Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked II-IV solar cells with low cost. In this work, we describe the development of InGaAs solar cells, designed to harvest long wavelength photons when stacked in tandem with a high efficiency InGaP/GaAs/InGaAsNSb triple junction solar cell. High performance InGaAs solar cells, grown on InP by MOCVD, were achieved through a combination of detailed modeling, material development and device characterization. The transfer printing apparatus of Semprius Inc. was used to create a four-terminal device with an uncertified conversion efficiency of 44.1% at 690 suns.
The effect of the position of InAs quantum dots (QD) within the intrinsic region of pin-GaAs solar cells is reported. Simulations suggest placing the QDs in regions of reduced recombination enables a recovery of open-circuit voltage (VOC). Devices with the QDs placed in the center and near the doped regions of a pin-GaAs solar cell were experimentally investigated. While the VOC of the emitter-shifted device was degraded, the center and base-shifted devices exhibited VOC comparable to the baseline structure. This asymmetry is attributed to background doping which modifies the recombination profile and must be considered when optimizing QD placement.
Baseline and quantum dot (QD) GaAs pn-junction diodes were characterized by deep level transient spectroscopy before and after both 1MeV electron irradiation and 140 keV proton irradiation. Prior to irradiation, the addition of quantum dots appeared to have introduced a higher density of defects at EC-0.75 eV. After 1 MeV electron irradiation the well-known electron defects E3, E4 and E5 were observed in the baseline sample. In the quantum dot sample after 1 MeV electron irradiation, defects near E3, E4 and EC-0.75 eV were also observed. Compared to the irradiated baseline, the QD sample shows a higher density of more complex E4 defect and a lower density of the simple E3 defect, while the EC-0.75 eV defect seemed to be unaffected by electron irradiation. As well, after proton irradiation, well known proton defects PR1, PR2, PR4′ are observed. The QD sample shows a lower density PR4′ defects and a similar density of PR2 defects, when compared to the proton irradiated baseline sample.
During quantum dot (QD) growth, substrate misorientation has been shown to play a role in the QD growth mechanism, changing their size, shape and density. Since various misorientation angles are used in production of solar cells, this work investigates QD enhanced GaAs p-i-n solar cells grown using the Stranski–Krastanov (SK) growth method on substrates misoriented either 2° or 6° off the (100) in the direction. Results of this work show that 2° misoriented samples have a lower critical thickness for InAs QD formation as compared to the 6° misorientation: ∼1.7 monolayers (ML) versus∼1.8 ML, respectively. In addition, the 6° substrates showed a more uniform QD density and size distribution of QDs without significant QD coalescence. Photoluminescence of both substrate types shows that the QD ground state transitions are similar in wavelength. Results of the solar cells under one sun illumination show that QD cells grown on both 2° and 6° substrates have higher short-circuit current density than comparable cells without QD and maintain a high open-circuit voltage. The QD contributed short-circuit current density was normalized for QD size and density for both the 2° and 6° samples. Values of 360 A/cm2 per cm3 of InAs and 400 A/cm2 per cm3 of InAs were found for the 2° and 6° samples, respectively. For both substrate types, the reduced number of coalesced QDs promoted effective strain balancing, while the increased QD density lead to strong QD absorption.
InAs/GaAs strain-balanced quantum dot (QD) n-i-p solar cells were fabricated by epitaxial lift-off (ELO), creating thin and flexible devices that exhibit an enhanced sub-GaAs bandgap current collection extending into the near infrared. Materials and optical analysis indicates that QD quality after ELO processing is preserved, which is supported by transmission electron microscopy images of the QD superlattice post-ELO. Spectral responsivity measurements depict a broadband resonant cavity enhancement past the GaAs bandedge, which is due to the thinning of the device. Integrated external quantum efficiency shows a QD contribution to the short circuit current density of 0.23 mA/cm2.
GaSb/InGaAs quantum dot–well (QDW) hybrid active regions with type-II band alignment are explored for increasing the infrared absorption in GaAs solar cells. Analyzed GaAs p–i–n structures comprise five layers of either GaSb quantum dot (QD), InGaAs quantum well (QW) or GaSb/InGaAs QDW layers in the i-region. It is found that the QDW solar cells outperform the QW and QD solar cells beyond GaAs band edge. In QDW solar cells an increase in efficiency is observed over QD solar cells due to additional QW absorption. An analysis of bulk response degradation in QDW solar cell is also presented. Improved photoresponse in QDW solar cells over QW and QD solar cells proves the potential for QDW hybrid structures in achieving high efficiency intermediate band solar cells.
Dense arrays of indium arsenide (InAs) nanowire materials have been grown by selective-area metal–organic vapor-phase epitaxy (SA-MOVPE) using polystyrene-b-poly(methyl methacrylate) (PS/PMMA) diblock copolymer (DBC) nanopatterning technique, which is a catalyst-free approach. Nanoscale openings were defined in a thin (∼10 nm) SiNx layer deposited on a (111)B-oriented GaAs substrate using the DBC process and CF4 reactive ion etching (RIE), which served as a hard mask for the nanowire growth. InAs nanowires with diameters down to ∼20 nm and micrometer-scale lengths were achieved with a density of ∼5 × 1010 cm2. The nanowire structures were characterized by scanning electron microscopy and transmission
electron microscopy, which indicate twin defects in a primary zincblende crystal structure and the absence of threading dislocation within the imaged regions.