| 1. |
- Anttu, Nicklas, et al.
(författare)
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Tailored emission to boost open-circuit voltage in solar cells
- 2019
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 3:5
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Tidskriftsartikel (refereegranskat)abstract
- Recently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage Voc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency ηint, non-radiative recombination dominates, and management of emitted photons plays negligible role for Voc . Similarly, for an external luminescence efficiency ηext<10%, externally emitted photons play negligible role, and Voc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher ηext, the Voc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops ηext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.
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| 2. |
- Chen, I. Ju, et al.
(författare)
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Conduction Band Offset and Polarization Effects in InAs Nanowire Polytype Junctions
- 2017
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:2, s. 902-908
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Tidskriftsartikel (refereegranskat)abstract
- Although zinc-blende (ZB) and wurtzite (WZ) structures differ only in the atomic stacking sequence, mixing of crystal phases can strongly affect the electronic properties, a problem particularly common to bottom up-grown nanostructures. A lack of understanding of the nature of electronic transport at crystal phase junctions thus severely limits our ability to develop functional nanowire devices. In this work we investigated electron transport in InAs nanowires with designed mixing of crystal structures, ZB/WZ/ZB, by temperature-dependent electrical measurements. The WZ inclusion gives rise to an energy barrier in the conduction band. Interpreting the experimental result in terms of thermionic emission and using a drift-diffusion model, we extracted values for the WZ/ZB band offset, 135 ± 10 meV, and interface sheet polarization charge density on the order of 10-3 C/m2. The extracted polarization charge density is 1-2 orders of magnitude smaller than previous experimental results, but in good agreement with first principle calculation of spontaneous polarization in WZ InAs. When the WZ length is reduced below 20 nm, an effective barrier lowering is observed, indicating the increasing importance of tunneling transport. Finally, we found that band-bending at ZB/WZ junctions can lead to bound electron states within an enclosed WZ segment of sufficient length, evidenced by our observation of Coulomb blockade at low temperature. These findings provide critical input for modeling and designing the electronic properties of novel functional devices, such as nanowire transistors, where crystal polytypes are commonly found.
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| 3. |
- Chen, Yang, et al.
(författare)
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One-dimensional electrical modeling of axial p-i-n junction InP nanowire array solar cells
- 2017
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Ingår i: 17th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2017. - 9781509053230 ; , s. 23-24
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Konferensbidrag (refereegranskat)abstract
- We demonstrate one-dimensional (1D) electrical modeling of InP nanowire array solar cells. This 1D modeling gives accurate description of the current voltage response even at high surface recombination velocity. The 1D electrical model decreases the simulation time by 3 orders of magnitude compared to a full three-dimensional (3D) model.
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| 4. |
- Chen, Yang, et al.
(författare)
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Optimization of the short-circuit current in an InP nanowire array solar cell through opto-electronic modeling
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:43
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Tidskriftsartikel (refereegranskat)abstract
- InP nanowire arrays with axial p-i-n junctions are promising devices for next-generation photovoltaics, with a demonstrated efficiency of 13.8%. However, the short-circuit current in such arrays does not match their absorption performance. Here, through combined optical and electrical modeling, we study how the absorption of photons and separation of the resulting photogenerated electron-hole pairs define and limit the short-circuit current in the nanowires. We identify how photogenerated minority carriers in the top n segment (i.e. holes) diffuse to the ohmic top contact where they recombine without contributing to the short-circuit current. In our modeling, such contact recombination can lead to a 60% drop in the short-circuit current. To hinder such hole diffusion, we include a gradient doping profile in the n segment to create a front surface barrier. This approach leads to a modest 5% increase in the short-circuit current, limited by Auger recombination with increased doping. A more efficient approach is to switch the n segment to a material with a higher band gap, like GaP. Then, a much smaller number of holes is photogenerated in the n segment, strongly limiting the amount that can diffuse and disappear into the top contact. For a 500 nm long top segment, the GaP approach leads to a 50% higher short-circuit current than with an InP top segment. Such a long top segment could facilitate the fabrication and contacting of nanowire array solar cells. Such design schemes for managing minority carriers could open the door to higher performance in single- and multi-junction nanowire-based solar cells.
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| 5. |
- Chen, Yang, et al.
(författare)
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Optimized efficiency in InP nanowire solar cells with accurate 1D analysis
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:4
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor nanowire arrays are a promising candidate for next generation solar cells due to enhanced absorption and reduced material consumption. However, to optimize their performance, time consuming three-dimensional (3D) opto-electronics modeling is usually performed. Here, we develop an accurate one-dimensional (1D) modeling method for the analysis. The 1D modeling is about 400 times faster than 3D modeling and allows direct application of concepts from planar pn-junctions on the analysis of nanowire solar cells. We show that the superposition principle can break down in InP nanowires due to strong surface recombination in the depletion region, giving rise to an IV-behavior similar to that with low shunt resistance. Importantly, we find that the open-circuit voltage of nanowire solar cells is typically limited by contact leakage. Therefore, to increase the efficiency, we have investigated the effect of high-bandgap GaP carrier-selective contact segments at the top and bottom of the InP nanowire and we find that GaP contact segments improve the solar cell efficiency. Next, we discuss the merit of p-i-n and p-n junction concepts in nanowire solar cells. With GaP carrier selective top and bottom contact segments in the InP nanowire array, we find that a p-n junction design is superior to a p-i-n junction design. We predict a best efficiency of 25% for a surface recombination velocity of 4500 cm s-1, corresponding to a non-radiative lifetime of 1 ns in p-n junction cells. The developed 1D model can be used for general modeling of axial p-n and p-i-n junctions in semiconductor nanowires. This includes also LED applications and we expect faster progress in device modeling using our method.
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| 6. |
- Kivisaari, Pyry, et al.
(författare)
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Bipolar Monte Carlo simulation of hot carriers in III-N LEDs
- 2015
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Ingår i: 2015 International Conference on Simulation of Semiconductor Processes and Devices, SISPAD 2015. - 9781467378581 ; , s. 393-396
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Konferensbidrag (refereegranskat)abstract
- We carry out bipolar Monte Carlo (MC) simulations of electron and hole transport in a multi-quantum well light-emitting diode with an electron-blocking layer. The MC simulation accounts for the most important interband recombination and intraband scattering processes and solves self-consistently for the non-quasiequilibrium transport. The fully bipolar MC simulator results in better convergence than our previous Monte Carlo-drift-diffusion (MCDD) model and also shows clear signatures of hot holes. Accounting for both hot electron and hot hole effects increases the total current and decreases the efficiency especially at high bias voltages. We also present our in-house full band structure calculations for GaN to be coupled later with the MC simulation in order to enable even more detailed predictions of device operation.
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| 7. |
- Kivisaari, Pyry, et al.
(författare)
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Bipolar Monte Carlo simulation of hot carriers in III-N LEDs
- 2015
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Ingår i: 15th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2015. - 9781479983797 ; 2015-May, s. 11-12
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Konferensbidrag (refereegranskat)abstract
- We perform fully bipolar Monte Carlo simulations of electrons and holes in III-Nitride multi-quantum well light-emitting diodes (LEDs) to investigate the effects of hot carriers. Our results show how accounting for hot carriers affects the current-voltage characteristics and device efficiency. We also discuss the effects of bandstructure details on the simulation results. Further simulations with versatile QW and EBL configurations are needed to confirm the relationship between hot carrier effects and current-voltage characteristics.
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| 8. |
- Kivisaari, Pyry, et al.
(författare)
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Diffusion-driven current transport to near-surface nanostructures
- 2015
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Ingår i: 15th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2015. - 9781479983797 ; , s. 117-118
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Konferensbidrag (refereegranskat)abstract
- Diffusion-driven current transport (DDCT) has recently been proposed as a new way to organize the current injection in nanoscale optoelectronic devices. The very recent first proof-of-principle experiments have also shown that DDCT works as predicted theoretically. In this work we perform simulations on DDCT-based III-Nitride devices and demonstrate how the optimization of DDCT differs significantly from the optimization of conventional double heterostructure based devices.
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| 9. |
- Kivisaari, Pyry, et al.
(författare)
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Elimination of Lateral Resistance and Current Crowding in Large-Area LEDs by Composition Grading and Diffusion-Driven Charge Transport
- 2017
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Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X. ; 3:6
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Tidskriftsartikel (refereegranskat)abstract
- Gallium nitride based light-emitting diodes (LEDs) are presently fundamentally transforming the lighting industry, but limitations in the materials and fabrication methods of LEDs introduce substantial challenges to their future development. Among the remaining key bottlenecks of GaN LEDs are the resistive losses and current crowding that strongly increase the heat generation at high powers. In this work the authors show how a new design paradigm based on diffusion-driven charge transport (DDCT) and selective-area growth (SAG) of GaN can be used to reduce the resistive losses of LEDs below the level achievable with presently available structures. The authors carry out full device simulations and demonstrate SAG of both n- and p-doped GaN on device templates with InGaN quantum wells that can be excited using DDCT. The results indicate that especially when combined with material composition grading, the new approach offers the possibility to substantially reduce the resistive heating in high-power LEDs.
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| 10. |
- Kivisaari, Pyry, et al.
(författare)
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Elimination of resistive losses in large-area LEDs by new diffusion-driven devices
- 2017
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Ingår i: Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XXI. - : SPIE. - 9781510606890 ; 10124
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Konferensbidrag (refereegranskat)abstract
- High-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2μm distance between n- and p-GaN regions.
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