Improving InGaN heterojunction solar cells efficiency using a semibulk absorber (original) (raw)
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Optimization of semibulk InGaN-based solar cell using realistic modeling
Solar Energy, 2017
Due to its high absorption coefficient and variable bandgap, InGaN is being intensively studied for photovoltaic applications. Growth of thick homogenous InGaN absorbers is challenging due to relaxation, clustering, and transition from 2D to 3D growth. These issues can be avoided by a semibulk multilayer structure. In this work, we analyze InGaN-based semibulk-structured solar cells in detail. We show that for indium content lower than 15%, GaN interlayers' thickness has no influence on carrier transport due to the low barrier height. A conversion efficiency of 2.4% can be expected for this indium content. However, for higher indium content (15-30%), we show that the thinner the GaN interlayers, the better the conversion efficiency. Beyond 30% of indium, the conversion efficiency is hindered by the barriers' important height even for very thin thicknesses of GaN interlayers. We show also that, for semibulk structure, both growth direction (N-face and metal-face) have similar impact on efficiency. This theoretical study gives the guidelines for the fabrication of InGaN-based solar cells that can be used as a wide-bandgap top cell in multijunction solar cells.
Journal of Applied Physics, 2010
In this study, we conducted numerical simulations with the consideration of microelectronic and photonic structures to determine the feasibility of and to design the device structure for the optimized performance of InGaN p-i-n single homojunction solar cells. Operation mechanisms of InGaN p-i-n single homojunction solar cells were explored through the calculation of the characteristic parameters such as the absorption, collection efficiency ͑͒, open circuit voltage ͑V oc ͒, short circuit current density ͑J sc ͒, and fill factor ͑FF͒. Simulation results show that the characteristic parameters of InGaN solar cells strongly depend on the indium content, thickness, and defect density of the i-layer. As the indium content in the cell increases, J sc and absorption increase while , V oc , and FF decrease. The combined effects of the absorption, , V oc , J sc , and FF lead to a higher conversion efficiency in the high-indium-content solar cell. A high-quality In 0.75 Ga 0.25 N solar cell with a 4 m i-layer thickness can exhibit as high a conversion efficiency as ϳ23%. In addition, the similar trend of conversion efficiency to that of J sc shows that J sc is a dominant factor to determine the performance of p-i-n InGaN solar cells. Furthermore, compared with the previous simulation results without the consideration of defect density, the lower calculated conversion efficiency verifies that the sample quality has a great effect on the performance of a solar cell and a high-quality InGaN alloy is necessary for the device fabrication. Simulation results help us to better understand the electro-optical characteristics of InGaN solar cells and can be utilized for efficiency enhancement through optimization of the device structure.
Improved Efficiency by Using Transparent Contact Layers in InGaN-Based p-i-n Solar Cells
IEEE Electron Device Letters , 2010
InGaN/GaN p-i-n solar cells were fabricated either without a current spreading layer or with ITO or Ni/Au spreading layers. A 10.8% indium composition was confirmed within an i-InGaN layer using X-ray diffraction. I-V characteristics were measured at AM1.5 conditions, with solar cell parameters being obtained based on I-V curves in all cases. Current spreading layers produced strong effects on efficiency. The solar cell with the ITO current spreading layer showed the best results, i.e., a short circuit current density of 0.644 mA/cm 2 , an open circuit voltage of 2.0 V, a fill factor of 79.5%, a peak external quantum efficiency of 74.1%, and a conversion efficiency of 1.0%.
High Performance InGaN-Based Solar Cells
2012
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Design and Realization of Wide-Band-Gap ($\sim$2.67 eV) InGaN p-n Junction Solar Cell
IEEE Electron Device Letters, 2010
The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%, which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV, respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm 2 are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure. Index Terms-Fabrication, InGaN solar cell, solar cell. I. INTRODUCTION W IDE-BAND-GAP (> 2.4 eV) materials are required in multiple-junction solar cells to achieve ultrahigh (> 50%) efficiency [1]. InGaN alloy is one of the few materials with band gap > 2.4 eV, and the band gap range of this alloy is tunable from 0.65 to 3.4 eV. III-nitride materials are direct band gap semiconductors and have strong absorption coefficients of ∼10 5 cm −1 at the band edge while simultaneously possessing other desirable characteristics such as radiation hardness and high peak and saturation velocities [2], [3]. InGaN solar cells with record high open-circuit voltage of 2.0 V [4] and high internal quantum efficiency [5], [6] have been demonstrated. These results were achieved with InGaN/GaN doubleheterojunction p-in structures with intrinsic InGaN layers. The InGaN composition in these devices was < 12%.
Acta Materialia, 2013
We report a comparison of the morphological, structural and optical properties of both InGaN single-layer and multilayered structures, the latter consisting of periodic thin GaN interlayers inserted during InGaN growth. It is shown that such a structure suppresses the In concentration fluctuations and corresponding different states of strain relaxation with depth, both detrimental to solar cell applications. Measurements performed by X-ray diffraction, cathodoluminescence and photoluminescence demonstrate that this multilayer growth is a promising approach to increase both the InGaN layer total thickness and In content in InGaN epilayers. As an example, single-phase 120 nm thick InGaN with 14.3% In content is obtained and found to possess high structural quality.
Micromachines
A solar cell structure with a graded bandgap absorber layer based on InGaN has been proposed to overcome early predicted efficiency. Technological issues such as carrier concentration in the p- and n-type are based on the data available in the literature. The influence of carrier concentration-dependent mobility on the absorber layer has been studied, obtaining considerable improvements in efficiency and photocurrent density. Efficiency over the tandem solar cell theoretical limit has been reached. A current density of 52.95 mA/cm2, with an efficiency of over 85%, is determined for a PiN structure with an InGaN step-graded bandgap absorption layer and 65.44% of power conversion efficiency for the same structure considering piezoelectric polarization of fully-strained layers and interfaces with electron and hole surface recombination velocities of 10−3 cm/s.
Characterization and analysis of InGaN photovoltaic devices
Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005., 2005
The InGaN material system is investigated to achieve high efficiency solar cells, using tandem and quantum-well structures to implement high efficiency concepts. Here InGaN p-i-n and quantum-well solar cells are designed, grown by MOCVD and fabricated into mesa devices. They are electrically characterized by I-V response under dark, white light and UV illumination and internal quantum efficiency (IQE). Material characterization is done by Xray diffraction, photoluminescence and photoemission. InGaN solar cells with high In compositions are grown in two configurations, one incorporating it into the i-region of a p-i-n solar cells, and the other incorporating as the wellregion of a quantum-well device. A QE of 8% was measured from these quantum-wells. Solar cells with In lean In 0.07 Ga 0.93 N p-i-n device structures show an IQE of 19% as well as photoemission at 500nm, confirming the suitability of the material for photovoltaic applications.
High internal and external quantum efficiency InGaN/GaN solar cells
Applied Physics Letters, 2011
High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/ cm 2 , and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions.
Substantial photo-response of InGaN p–i–n homojunction solar cells
Semiconductor Science and Technology, 2009
InGaN p-i-n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (V oc ) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p-i-n solar cells.