Yushuai Dai

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.

InAs nanowires grown by metal-organic vapor-phase epitaxy (MOVPE) employing PS/PMMA diblock copolymer nanopatterning.

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 × 10(10) cm(2). 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.

Effects of electric field on thermal and tunneling carrier escape in InAs/GaAs quantum dot solar cells

The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in a p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electric field intensity is below 40kV/cm, thermal escape is dominant in all confined states in QDs; when electric field intensity is above 40kV/cm, tunneling is dominant in shallow confined states and thermal escape is dominant in the ground state of QDs.

Optimization in wide-band-gap quantum dot solar cells

Quantum dots (QDs) have been under extensive study as a promising material to realize the concept of the intermediate band solar cell (IBSC). Because thermal escape is the dominant mechanism of carrier escape at room temperature, wide-band-gap (WBG) semiconductor can be used to suppress thermal escape by increasing barrier height for InAs quantum dots. Embedded InAs QD in the wide-bandgap matrix (InGaP and AlGaAs) is demonstrated with increased sub-band-gap carrier collection. The deeper confinement and larger activation energy is a move towards realizing an IBSC Additionally, activation energy extracted from temperature dependent external quantum efficiency (TDEQE) of InAs/AlGaAs is 324 meV, which is closer to the transition between IB to CB in an ideal IBSC.

The effect of barrier composition on quantum dot solar cell performance

The use of nanostructures such as quantum dots (QD) offers tremendous potential to realize high-efficiency photovoltaic (PV) cells. The optimization of the electronic structure of the layers within the QD region should lead to improved PV performance. This includes the QD layer itself, but also the surrounding barrier and/or strain balancing layers that comprise the QD active region. In this paper, the effect of the barrier layer composition (i.e. the cladding layers grown following QD capping layer) on the optoelectronic properties of InAs QDs was investigated. Specifically, the composition is changed from GaAs (1.42 eV) to InGaP (1.85 eV) and the effect of carrier collection and 1-sun efficiency is observed. The examination of the effects of these layers will contribute to understanding the dominant factor in carrier collection within QD-enhanced photovoltaics. In addition, the direct growth of InAs QD on InGaP surfaces is presented and compared to GaAs. The direct proximity of the InGaP surface greatly enlarges the InAs QD.

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.

Carrier collection in quantum dots solar cells with barrier modification

InAs quantum dot (QD) has been attractive in high conversion efficiency solar cell applications, due to its extended absorption in the infrared spectrum and as a promising material for the intermediate band solar cell (IBSC). To enhance the sequential absorption process towards the concept of IBSC, modified barriers of InGaP were applied to suppress thermal escape and tunneling process in InAs quantum dots solar cells (QDSCs). Despite improved spectral response from QD absorption, InAs QDSC with InGaP barrier is associated with degradation in the bulk spectral response at room temperature; the carrier collection can be optimized via adjusting operation condition and solar cell design.

Electric field effect on carrier escape from InAs/GaAs quantum dots solar cells

The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electric field intensity is below 40kV/cm, thermal escape is dominant in all confined state in QDs; when electric field intensity is above 40kV/cm, tunneling is dominant in shallow confined states and thermal escape is dominant in ground state of QDs.

Optical study of carrier transport in InAs/GaAs quantum dots solar cell

Optical transitions and carrier transport in InAs/GaAs self-assembled quantum dots solar cells (QDSC) were investigated via temperature dependent photoluminescence (TDPL) spectroscopy, charge collection efficiency calculated from bias dependent spectral response, and low temperature photo-current measurements. Band structure simulations were used to calculate the tunneling rate and thermal escape rate of electrons in quantum dots under different bias conditions. It is found that, tunneling rate and thermal escape rate of electrons in the quantum dots increase along p-n direction. From TDPL, compared to test structures, ground state electron activation energy decrease 60 meV in QDSC. TDPL also indicates that electrons are the main carriers escaping from the QD confinement if temperature higher than 120K, while holes are the rate limiting carrier if temperature lower than 120K. Charge collection efficiency is greatly improved above the bulk GaAs absorption edge (870nm) under large reverse bias (-4.5V). Low temperature (20K) bias photocurrent shows that ground state carrier can be collected even at 0.7V forward bias. This indicates that hole tunneling may be dominant at low temperature, or electron experiences tunneling then thermal escape.

Spectroscopic analysis of InAs quantum dot solar cells

The operation of solar cells incorporating multiple repeat units of InAs quantum dot structures, as well as those with and without δ-doping of 4 and 8 electrons per quantum dot, were studied. Room temperature measurements of these samples revealed high quality devices, but insignificant differences between δ-doped samples and undoped samples. An IR-pumped quantum efficiency measurement was performed at 6 K to probe the extraction of quantum confined carriers through a two-photon process while shutting off phonon-assisted extraction. No two-photon signal rose above the noise, but additional sub-bandgap illuminated IV curves revealed current generation in the quantum dot devices, suggesting the dominant carrier removal mechanism is through tunneling. Finally, dark-diode data was taken and fit to determine ideality factor as a function of temperature. Control devices had an overall larger ideality, while QD devices exhibited variations as a function of temperature, which were attributed to kinetic barriers in the first QD layers, as well as possible Auger recombination at very low temperature.