Staffan Hellstroem

Effect of electric field on carrier escape mechanisms in quantum dot intermediate band solar cells

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k·p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate...

Investigation of optical transitions in InAs/GaAs(Sb)/AlAsSb quantum dots using modulation spectroscopy

InAs quantum dots (QDs) were grown in an AlAs0.56Sb0.44/GaAs matrix in the unintentionally doped (uid) region of an In0.52Al0.48As solar cell, establishing a variety of optical transitions both into and out of the QDs. The ultimate goal is to demonstrate sequential absorption, where one photon is absorbed, promoting an electron from the valence band into the QD, and a second photon is absorbed in order to promote the trapped electron from a QD state into the host conduction band. In this study, we directly investigate the optical properties of the solar cell using photoreflectance and evaluate the possibility of sequential absorption by measuring spectral responsivity with broadband infrared illumination.

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.

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.

Intermediate band solar cell design using InAs quantum dots in AlAsSb cladding

We present simulations of InAs QDs embedded in AlAsSb, which may be a promising candidate system for realizing intermediate band solar cells as it features bandgaps close to the ideal and a nearly flat type-II valence band lineup. We have also experimentally investigated InAs quantum dots (QDs) grown in an AlAs0.56Sb0.44 matrix in the unintentionally doped (uid) region of an In0.52Al0.48As solar cell. Optical and electrical properties of the solar cell were investigated to evaluate the possibility of photon assisted absorption from the QD states. As well, potential designs for a AlAs0.56Sb0.44 heterojunction QD solar cell were evaluated.

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.

Experimental examination of an InAs/GaAs(Sb)/AlAsSb quantum dot approach to the intermediate band solar cell

In this study work towards an InAs/AlAs_0.56Sb_0.44 quantum dot (QD) prototype intermediate band solar cell(IBSC) grown on InP is presented. InAs QDs are suspended in an AlAs_0.56Sb_0.44/GaAs matrix in the unintentionally doped (uid) region of an In_0.52Al_0.48As solar cell are compared to a bulk In_0.52Al_0.48As solar cell. While addition of the InAs/AlAsSb matrix results in a drastic drop in AM1.5G performance, electroluminescence shows a peak shift from 855nm to 1623nm and spectral responsivity shows a similar absorption edge shift. A proposed design improvement uses an AlAs_0.56Sb_0.44 emitter to remove the injection barrier created by the use of AlAs_0.56Sb_0.44 in the uid-region.