@article{hagar_sayed_colter_bedair_2020, title={Multi-junction solar cells by Intermetallic Bonding and interconnect of Dissimilar Materials: GaAs/Si}, volume={215}, ISSN={["1879-3398"]}, DOI={10.1016/j.solmat.2020.110653}, abstractNote={We present a novel, low temperature approach to multijunction solar cell fabrication combining the high efficiency multi-junction concept with the low cost of thin film technology in one solar cell structure. The intermetallic bonding approach presented is based on joining indium metal which has been deposited on the metal contact grid of the respective solar cells. This approach avoids the problems of lattice mismatch and tunnel junction limitations, connecting solar cells of potentially any material with patterned contacts. No measurable increase in resistance has been measured between bonded materials. This method allows the independent development of each cell technology for use in multijunction solar cells. This technique can be applied to any commercial off-the-shelf solar cells, if available. A GaAs/Si multijunction solar cell bonded using this approach is demonstrated. The silicon cell is off-the-shelf with textured surface and commercial metal contacts. This is integrated with an in-house grown thin film GaAs cell. The GaAs/Si device is demonstrated in both two and three terminal configurations.}, journal={SOLAR ENERGY MATERIALS AND SOLAR CELLS}, author={Hagar, Brandon and Sayed, Islam and Colter, Peter C. and Bedair, S. M.}, year={2020}, month={Sep} } @article{sayed_bedair_2019, title={Quantum Well Solar Cells: Principles, Recent Progress, and Potential}, volume={9}, ISSN={["2156-3381"]}, DOI={10.1109/JPHOTOV.2019.2892079}, abstractNote={Quantum well solar cells, as a promising approach for next-generation photovoltaic technology, have received great attention in the last few years. Recent developments in materials growth and device structures of quantum wells have opened up new avenues for the incorporation of quantum well structures in next-generation III/V multi-junction solar cells. In this paper, the advantages and challenges of growing quantum wells in the unintentionally doped (i) region of p-i-n solar cells are reviewed. We focus on the recent progress in 1.1–1.3 eV strain-balanced InGaAs/GaAsP, 1.6–1.8 eV strain-balanced and lattice-matched InGaAsP/InGaP, and >2.1 eV strained InGaN/GaN quantum well solar cells, including optimization of the quantum well growth conditions and improving the solar cell structure. For each material system, the challenges associated with materials growth and device performance such as critical layer thickness constraints, strain balance, bandgap tunability, and carrier transport limitations, are discussed. The performance of each quantum well solar cell is compared with bulk absorber operating in the same bandgap range, with the advantages of each being highlighted. The effect of the unintentional background doping on carrier collection (by drift) is presented through modeling and recent experimental results. The recent strategies to enhance the electric field distribution across the quantum well region are reviewed. The potential of incorporating quantum well structures in next-generation multi-junction devices is also discussed.}, number={2}, journal={IEEE JOURNAL OF PHOTOVOLTAICS}, author={Sayed, Islam and Bedair, S. M.}, year={2019}, month={Mar}, pages={402–423} } @article{sayed_jain_steiner_geisz_bedair_2017, title={100-period InGaAsP/InGaP superlattice solar cell with sub-bandgap quantum efficiency approaching 80%}, volume={111}, ISSN={["1077-3118"]}, DOI={10.1063/1.4993888}, abstractNote={InGaAsP/InGaP quantum well (QW) structures are promising materials for next generation photovoltaic devices because of their tunable bandgap (1.50–1.80 eV) and being aluminum-free. However, the strain-balance limitations have previously limited light absorption in the QW region and constrained the external quantum efficiency (EQE) values beyond the In0.49Ga0.51P band-edge to less than 25%. In this work, we show that implementing a hundred period lattice matched InGaAsP/InGaP superlattice solar cell with more than 65% absorbing InGaAsP well resulted in more than 2× improvement in EQE values than previously reported strain balanced approaches. In addition, processing the devices with a rear optical reflector resulted in strong Fabry-Perot resonance oscillations and the EQE values were highly improved in the vicinity of these peaks, resulting in a short circuit current improvement of 10% relative to devices with a rear optical filter. These enhancements have resulted in an InGaAsP/InGaP superlattice solar cell with improved peak sub-bandgap EQE values exceeding 75% at 700 nm, an improvement in the short circuit current of 26% relative to standard InGaP devices, and an enhanced bandgap-voltage offset (Woc) of 0.4 V.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Sayed, Islam E. H. and Jain, Nikhil and Steiner, Myles A. and Geisz, John F. and Bedair, S. M.}, year={2017}, month={Aug} } @article{sayed_jain_steiner_geisz_dippo_kuciauskas_colter_2017, title={In-situ curvature monitoring and X-ray diffraction study of InGaAsP/InGaP quantum wells}, volume={475}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2017.06.019}, abstractNote={The use of InGaAsP/InGaP quantum well structures is a promising approach for subcells in next generation multi-junction devices due to their tunable bandgap (1.50–1.80 eV) and for being aluminum-free. Despite these potentials, the accumulation of stress during the growth of these structures and high background doping in the quantum well region have previously limited the maximum number of quantum wells and barriers that can be included in the intrinsic region and the sub-bandgap external quantum efficiency to less than 30.0%. In this paper, we report on the use of in-situ curvature monitoring by multi-beam optical stress (MOS) sensor measurements during the growth of this quantum well structure to monitor the stress evolution in these thin films. A series of In0.32Ga0.68AsP/In0.49Ga0.51P quantum wells with various arsine to phosphine ratios have been analyzed by in-situ curvature monitoring and X-ray diffraction (XRD) to obtain nearly strain-free lattice matched structures. Sharp interfaces, as indicated by the XRD fringes, have been achieved by using triethyl-gallium and trimethyl-gallium as gallium precursors in InGaAsP and InGaP, respectively, with constant flows of trimethyl-indium and phosphine through the entire quantum well structure. The effect of the substrate miscut on quantum well growth was compared and analyzed using XRD, photoluminescence and time resolved photoluminescence. A 100 period quantum well device was successfully grown with minimal stress and approximately flat in-situ curvature.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Sayed, Islam E. H. and Jain, Nikhil and Steiner, Myles A. and Geisz, John F. and Dippo, Pat and Kuciauskas, Darius and Colter, Peter C.}, year={2017}, month={Oct}, pages={171–177} } @inproceedings{bedair_harmon_carlin_sayed_colter_2016, title={Annealed high band gap tunnel junctions with peak current densities above 800 A/cm(2)}, DOI={10.1109/pvsc.2016.7750052}, abstractNote={The development of high-performance high band gap tunnel junctions is critical for producing efficient multijunction photovoltaic cells that can operate at high solar concentrations. The n-InGaP/GaAs/p-AlGaAs TJ has been demonstrated to produce peak tunneling currents (Jpk) above 1000 A/cm2 with minimal absorption losses due to the use of thin (<50 Â) GaAs layer. We will report on the growth and device modeling of these structures as well as the effect of high temperature annealing on Jpk. A method to grow TJ structures resistant to annealing will be described, which has resulted in thermally annealed TJ with Jpk above 800 A/cm2. This is the highest value ever reported for an annealed high band gap TJ. Device modeling has been used to investigate the source of the high tunneling current, as well as the behavior of the annealed TJ.}, booktitle={2016 ieee 43rd photovoltaic specialists conference (pvsc)}, author={Bedair, S. M. and Harmon, J. L. and Carlin, C. Z. and Sayed, I. E. H. and Colter, P. C.}, year={2016}, pages={2320–2322} } @inproceedings{sayed_hagar_carlin_colter_bedair_2016, title={Extending the absorption threshold of InGaP solar cells to 1.60 eV using quantum wells: experimental and modeling results}, DOI={10.1109/pvsc.2016.7750063}, abstractNote={Strain balanced multiple quantum wells (SBMQWs) lattice matched to GaAs consisting of InGaAsP wells balanced with InGaP barriers have been used to extend the absorption of In0.49Ga0.51P subcells to longer wavelengths for use in five and six junction photovoltaic devices. Thin layers of InGaAsP quantum wells that absorb beyond 760 nm, have been grown with compositions within the miscibility gap of InGaAsP while maintaining thermodynamic stability. External quantum efficiency and current-voltage measurements reveal that InGaAsP/InGaP SBMQWs extend absorption beyond the InGaP band-edge and improve the short circuit current with minimal degradation of open circuit voltage. We study the effect of barrier height on the carrier transport through altering the Indium percentage in the InGaP barrier. Three samples of different barrier heights are fabricated and compared with each other. Results indicate that with proper design of the layers thicknesses and compositions, the absorption threshold of InGaP can be extended up to 780 nm (∼1.59 eV). The promising results of InGaAsP/InGaP SBMQWs in this work offer tremendous potential to alleviate current matching restrictions in next generation and current photovoltaic devices.}, booktitle={2016 ieee 43rd photovoltaic specialists conference (pvsc)}, author={Sayed, I. E. H. and Hagar, B. G. and Carlin, C. Z. and Colter, P. C. and Bedair, S. M.}, year={2016}, pages={2366–2370} } @article{bedair_carlin_harmon_sayed_colter_2016, title={High Performance Tunnel Junction with Resistance to Thermal Annealing}, volume={1766}, ISSN={["0094-243X"]}, DOI={10.1063/1.4962071}, abstractNote={The availability of high band gap (>1.9 eV) tunnel junctions (TJ) with large peak current densities (Jpk) is crucial for the development of multijunction photovoltaic cells that can operate at concentrations above 1000 suns. Existing TJ designs include thick GaAs layers which reduce the overall efficiency due to absorption. We have developed an n-InGaP/GaAs/p-AlGaAs structure with a GaAs layer that is 50 A or thinner that has an as-grown Jpk above 2000 A/cm2 an annealed Jpk above 1000 A/cm2. Due to the memory effect of the Te n-type dopant, modifications to the shut off time of the DETe precursor produced the high Jpk that was observed.}, journal={12TH INTERNATIONAL CONFERENCE ON CONCENTRATOR PHOTOVOLTAIC SYSTEMS (CPV-12)}, author={Bedair, S. M. and Carlin, C. Zachary and Harmon, Jeffrey L. and Sayed, Islam E. Hashem and Colter, P. C.}, year={2016} } @article{bedair_harmon_carlin_sayed_colter_2016, title={High performance as-grown and annealed high band gap tunnel junctions: Te behavior at the interface}, volume={108}, ISSN={["1077-3118"]}, DOI={10.1063/1.4951690}, abstractNote={The performance of n+-InGaP(Te)/p+-AlGaAs(C) high band gap tunnel junctions (TJ) is critical for achieving high efficiency in multijunction photovoltaics. Several limitations for as grown and annealed TJ can be attributed to the Te doping of InGaP and its behavior at the junction interface. Te atoms in InGaP tend to get attached at step edges, resulting in a Te memory effect. In this work, we use the peak tunneling current (Jpk) in this TJ as a diagnostic tool to study the behavior of the Te dopant at the TJ interface. Additionally, we used our understanding of Te behavior at the interface, guided by device modeling, to modify the Te source shut-off procedure and the growth rate. These modifications lead to a record performance for both the as-grown (2000 A/cm2) and annealed (1000 A/cm2) high band gap tunnel junction.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Bedair, S. M. and Harmon, Jeffrey L. and Carlin, C. Zachary and Sayed, Islam E. Hashem and Colter, P. C.}, year={2016}, month={May} } @inproceedings{sayed_hagar_carlin_colter_bedair_2016, title={InGaP-based quantum well solar cells}, DOI={10.1109/pvsc.2016.7749566}, abstractNote={Quantum well structures hold tremendous potential in taking next step beyond current photovoltaic structures in achieving solar conversion efficiencies beyond 50%. In this paper we investigate p-i-n InGaP solar cells incorporating InGaAsP/InGaP strain balanced multiple quantum wells (SBMQWs) to tune the absorption threshold beyond the In0.49Ga0.51P cut-off (∼ 1.85 eV). The effects of quantum well number and thickness on the optoelectronic properties of InGaAsP/InGaP SBMQWs are investigated. Specifically, we investigate the bandgap tunability of these SBMQW devices by varying well and barrier thickness. Spectral response measurements reveal that longer excitonic absorption with efficient carrier transport can be realized if proper materials compositions and thicknesses are realized. In addition, InGaP pi-n solar cells including various numbers of InGaAsP/InGaP SBMQWs with an effective bandgap of 1.65 eV in the intrinsic (i) layer were fabricated and characterized. With up to 30 quantum wells, spectral response and light I-V measurements reveal an improvement in the excitonic absorption and short circuit current in comparison to the standard device. The promising results in this work provide an alternative path for realizing 1.5–1.8 eV subcells in next-generation multi-junction solar cells.}, booktitle={2016 ieee 43rd photovoltaic specialists conference (pvsc)}, author={Sayed, I. E. H. and Hagar, B. G. and Carlin, C. Z. and Colter, P. C. and Bedair, S. M.}, year={2016}, pages={147–150} } @article{sayed_carlin_hagar_colter_bedair_2016, title={Strain-Balanced InGaAsP/GaInP Multiple Quantum Well Solar Cells With a Tunable Bandgap (1.65-1.82 eV)}, volume={6}, ISSN={["2156-3381"]}, DOI={10.1109/jphotov.2016.2549745}, abstractNote={Currently available materials for III–V multijunction solar cells lattice matched to GaAs covering the spectral range from 1.65 to 1.82 eV are composed of either immiscible quaternary alloys or contain aluminum. We report the fabrication of a novel aluminum-free In

$_x$

$_{1-x}$

$_{1-z}$

$_z$

/Ga

$_{1-y}$

$_y$

P (x > y ) strain-balanced multiple quantum-well (SBMQW) p-i-n solar cell structure lattice matched to GaAs, grown by metal–organic chemical vapor deposition. SBMQWs consist of alternating layers of In

$_x$

$_{1-x}$

$_{1-z}$

$_z$

wells and Ga

$_{1-y}$

$_y$

P barriers (x > y) under compressive and tensile strain, respectively. When compared with standard GaInP devices, SBMQW structures exhibit longer photoluminescence wavelength (680–780 nm) emission and enhanced light absorption with improved short-circuit current density. In this study, the SBMQW emission and absorption wavelength is controlled by adjusting the layer thickness of InGaAsP wells, while the arsenic and indium compositions are fixed. We show that carriers generated in QWs are extracted via thermionic emission. The proposed SBMQWs allow more flexibility in the design of current multijunction solar cells and future cells with more than four junctions. InGaAsP/GaInP SBMQWs may also be used in applications other than solar cells, such as light-emitting diodes (LEDs) and lasers, with the advantages of tuning the emission and absorption processes.}, number={4}, journal={IEEE JOURNAL OF PHOTOVOLTAICS}, author={Sayed, Islam E. Hashem and Carlin, Conrad Zachary and Hagar, Brandon G. and Colter, Peter C. and Bedair, S. M.}, year={2016}, month={Jul}, pages={997–1003} } @inproceedings{sayed_carlin_hagar_colter_bedair_2015, title={Tunable GaInP solar cell lattice matched to GaAs}, DOI={10.1109/pvsc.2015.7356081}, abstractNote={A new strain-balanced multiple quantum well (MQW) approach to tune the Ga_0.51In_0.49P bandgap is demonstrated. This approach is based on Ga_1-xIn_xP/Ga_1-yIn_yP (x > y) or Ga_1-xIn_xAs_zP_1-z/Ga_1-yIn_yP (x > y) structures, strain balanced and lattice matched to GaAs in a p-i-n solar cell structure. A red shift in the absorption edge and an increase in the short circuit current were observed. Carriers generated in quantum wells due to transitions between the quantum levels are transported across the barriers via thermionic emission. The proposed structure allows more flexibility in the design of current multi-junction solar cells and future cells with more than four junctions.}, booktitle={2015 ieee 42nd photovoltaic specialist conference (pvsc)}, author={Sayed, I. E. H. and Carlin, C. Z. and Hagar, B. and Colter, P. C. and Bedair, S. M.}, year={2015} }