Multiple Quantum Well Solar Cells Research Articles

Absorption enhancement through Fabry-Pérot resonant modes in a 430 nm thick InGaAs/GaAsP multiple quantum wells solar cell

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 pairs of InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the external quantum efficiency below GaAs bandgap by a factor of four through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 10% conversion efficiency. We demonstrate numerically that this promising result can be further improved by anti-reflection layers. This study paves the way to very thin MQWs solar cells.

Absorption threshold extended to 1.15 eV using InGaAs/GaAsP quantum wells for over‐50%‐efficient lattice‐matched quad‐junction solar cells

AbstractBandgap engineering of strain‐balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high‐quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs‐based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15 eV. The bandgap‐voltage‐offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap‐tunable property. Towards the realization of fully lattice‐matched quad‐junction devices, we demonstrate a 70‐period, 1.15‐eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99‐eV quad‐junction cells, whose practical efficiency has a potential to achieve over 50%. With such a large period number of MQWs, the reverse‐biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15‐eV subcell; this is achieved with only a‐few‐percent drop at short‐circuit condition. The device presented here reaches the target open‐circuit voltage and over 75% of the current density required for realizing a 1.15‐eV subcell in a 50%‐efficient quad‐junction solar cell. We believe that future devices which exploit light‐trapping structures and enhanced carrier extraction will be able to reach the desired target. Copyright © 2015 John Wiley & Sons, Ltd.

Carrier Time-of-Flight Measurement Using a Probe Structure for Direct Evaluation of Carrier Transport in Multiple Quantum Well Solar Cells

Carrier transport across multiple quantum well (MQW) structures inserted in the i-region of a p-i-n diode is an important mechanism that determines the performance of MQW solar cells. We have employed a carrier time-of-flight measurement technique using a quantum-well probe to investigate the electron transport time across MQW structures. Delay of the carrier arrival time, defined as time-of-flight, caused by MQWs shows almost linear increment to the number of wells. Tunneling transport in InGaAs/GaAsP MQW structures is studied by varying the GaAsP barrier thickness. Barrier thickness of 2 nm results in extremely small electron time-of-flight, less than a hundred picoseconds per well, whereas MQW with 8-nm-thick barriers results in approximately 20 times slower transport. Furthermore, the fast time-of-flight in thin barriers shows no degradation after the insertion of GaAs interlayers that form the multistep potential. This suggests that the fast thermally assisted tunneling transport dominates the electron escape dynamics in thin-barrier InGaAs/GaAsP MQWs. The rapid carrier transport results in the significant suppression of current drop at the cell maximum power point of MQW solar cells.

Growth of InGaAs/GaAsP multiple quantum well solar cells on mis-orientated GaAs substrates

The effects of growth temperature on the properties of InGaAs/GaAsP multiple quantum well (MQW) solar cells on various mis-orientated GaAs substrates were studied using metalorganic vapor phase epitaxy. Thickness modulation effect caused by mismatch strain of InGaAs/GaAsP could be suppressed by low growth temperature. Consequently, abrupt MQWs with strong light absorption could be deposited on mis-oriented substrates. However, degradation in crystal quality and impurity incorporation are the main drawbacks with low temperature growth because they tend to strongly degraded carrier transport and collection efficiency. MQW solar cells grown at optimized temperature showed the better conversion efficiency. The further investigation should focus on improvement of crystal quality and background impurities.

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well solar cells

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that the strain relaxation by forming defects occurs when the number of quantum wells (QWs) increases to 30. It results in the low luminescence intensity and external quantum efficiency (EQE) due to the poor crystal quality of MQW region and the high non-radiative recombination rate in MQW region. However, the EQE of 25-period InGaN/GaN MQW solar cell with relatively high crystal quality of MQW region does not increase at all wavelengths compared with 15-period one. This may be attributed to the lower carrier collection efficiency resulting from the lower electric field of i-MQW region.

Carrier Escape Time and Temperature-Dependent Carrier Collection Efficiency of Tunneling-Enhanced Multiple Quantum Well Solar Cells

Tunneling enhancement of cell performance in InGaAs/GaAsP multiple quantum well (MQW) solar cells has been studied to investigate the potential in overcoming the carrier collection problem, which hinders the maximum performance of quantum structure solar cells. To accurately investigate the effects of the tunneling effect, the study was carried out in samples with different GaAsP barrier thickness, controlled absorption edge, and constant built-in field. The tunneling effect has been confirmed by evaluating carrier escape times using the time-resolved photoluminescence technique and measuring carrier collection efficiency at various temperatures. The collection efficiencies at low temperature are found to be remarkably improved when barrier thickness was below 3 nm, which can be regarded as the critical thickness for efficiently facilitating tunneling enhancement. It can also be concluded that the carrier transport model based on thermal and tunneling processes is practical enough to describe most of the carrier sweep-out dynamics in MQW solar cells.

Effect of Piezoelectric polarization on Open Circuit Voltage of GaN/InGaN MQW Solar Cell

The piezoelectric polarization effect is dominant at the every GaN/InGaN interface in the MQW structure. In this paper we show that, how this piezoelectric polarization effect is useful in improving the open circuit voltage of GaN/InGaN MQW solar cell. For this purpose we have design a mathematical model of GaN/InGaN MQW solar cell and calculate the values of open circuit voltage with and without piezoelectric polarization effect. For simplicity of the model we are neglecting the current clouding effect, composition fluctuation and electro migration effect present in the device. Here we also assuming that, all the layers of GaN and InGaN material are terminated by Ga-face. Results indicate that the open circuit voltage of MQW solar cell improves by ~6.28% (at indium fraction in quantum well is 0.2 and quantum well thickness is 1 nm) and by ~16.77% (at indium fraction in quantum well is 0.2 and quantum well thickness is 3 nm) by using piezoelectric polarization effect.

Enhanced performance of InGaN/GaN multiple quantum well solar cells with double indium content

The performance of a multiple quantum well (MQW) InGaN solar cell with double indium content is investigated. It is found that the adoption of a double indium structure can effectively broaden the spectral response of the external quantum efficiencies and optimize the overall performance of the solar cell. Under AM1.5G illumination, the short-circuit current density (Jsc) and conversion efficiency of the solar cell are enhanced by 65% and 13% compared with those of a normal single-indium-content MQW solar cell. These improvements are mainly attributed to the expansion of the absorption spectrum and better extraction efficiency of the photon-generated carriers induced by higher polarization.

Modeling For High Efficiency GaN/InGaN Solar Cell

GaN/InGaN heterostructure contains a unique property of piezoelectric polarization charges at the interface due to different thermal expansion coefficients. In this paper, we report a simple mathematical model for Ga-face GaN/InGaN heterostructure solar cell. The results obtained from the given model indicates that the piezoelectric polarization charges at the interface of Ga-face GaN/InGaN heterostructure improves the efficiency of a single GaN/InGaN heterostructure photovoltaic solar cell in comparison to the non-polar solar cells by 45% for AM 1.5. This structure can also provide a fundamental solar cell unit for developing very high efficiency MQW solar cell and a MJ solar cell using Ga-face GaN/InGaN structure.

Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

Effects of Background Zn Doping on the Performance of InGaAs/GaAsP Multiple Quantum Well Solar Cells Grown by a Planetary Metal Organic Vapor Phase Epitaxy Reactor

The effects of background Zn doping on the performance of p–i–n GaAs solar cells with InGaAs/GaAsP multiple quantum wells (MQWs) in i-GaAs layer have been studied. The crystal growth was done by a planetary metalorganic vapor phase epitaxy (MOVPE) reactor. The background Zn doping, in an order of 1017 cm-3, degraded the solar cell efficiency by modifying the energy band diagram in a way that obstructed carrier transports. It was shown by calculation that the carrier transports across the MQWs region suffered from decrease in built-in electric field in absorber layers, leading to an efficiency loss by radiative and nonradiative recombinations. Consequently, the external quantum efficiency and the current density of a Zn-contaminated MQW solar cell were terribly poor. Reactor baking at 850 °C for 20 min seems to remove Zn residues effectively without noticeable effects on the succeeding growth of MQW solar cells. The InGaAs/GaAsP MQWs fabricated in the thermally cleaned reactor have shown a potential to extend the absorption edge of GaAs solar cells and to improve the efficiency of multi-junction solar cells by current matching. Therefore, the growth of InGaAs/GaAsP MQWs by planetary MOVPE reactors requires a careful treatment regarding the background doping issue.

Effects of Background Zn Doping on the Performance of InGaAs/GaAsP Multiple Quantum Well Solar Cells Grown by a Planetary Metal Organic Vapor Phase Epitaxy Reactor

The effects of background Zn doping on the performance of p–i–n GaAs solar cells with InGaAs/GaAsP multiple quantum wells (MQWs) in i-GaAs layer have been studied. The crystal growth was done by a planetary metalorganic vapor phase epitaxy (MOVPE) reactor. The background Zn doping, in an order of 1017 cm-3, degraded the solar cell efficiency by modifying the energy band diagram in a way that obstructed carrier transports. It was shown by calculation that the carrier transports across the MQWs region suffered from decrease in built-in electric field in absorber layers, leading to an efficiency loss by radiative and nonradiative recombinations. Consequently, the external quantum efficiency and the current density of a Zn-contaminated MQW solar cell were terribly poor. Reactor baking at 850 °C for 20 min seems to remove Zn residues effectively without noticeable effects on the succeeding growth of MQW solar cells. The InGaAs/GaAsP MQWs fabricated in the thermally cleaned reactor have shown a potential to extend the absorption edge of GaAs solar cells and to improve the efficiency of multi-junction solar cells by current matching. Therefore, the growth of InGaAs/GaAsP MQWs by planetary MOVPE reactors requires a careful treatment regarding the background doping issue.

Impact of Strain Accumulation on InGaAs/GaAsP Multiple-Quantum-Well Solar Cells: Direct Correlation between In situ Strain Measurement and Cell Performances

The effects of accumulating strain inside InGaAs/GaAsP multiple-quantum-well (MQW) solar cells were investigated and their correlation with in situ wafer curvature measurement was examined. The p–i–n GaAs solar cells, containing 20-period InGaAs/GaAsP MQWs in an i-GaAs layer, were fabricated by metalorganic vapor phase epitaxy. The strain inside MQWs was varied by changing In content in an InGaAs well, while maintaining other parameters. As evidenced by curvature transience, the excessive strain led to lattice relaxation, resulting in defects, dislocations, and poor crystal quality. Consequently, short circuit current density and open circuit voltage deteriorated, and solar cell performance degraded. The highest conversion efficiency was obtained in a strain-balanced MQW solar cell. InGaAs/GaAsP MQWs have a great potential for extending the absorption edge of GaAs cells and for enhancing the efficiency of III/V multijunction solar cells by current matching. Hence, the growth of InGaAs/GaAsP MQWs for photovoltaic application requires a strain monitoring system and careful control such that the accumulating strain is minimized.

An approach to high efficiencies using GaAs/GaInNAs multiple quantum well and superlattice solar cell

A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice (SL) are inserted in the i-region of a GaAs p-i-n solar cell (SC) is presented. The results suggest the device can reach record efficiencies for single-junction solar cells. A theoretical model is developed to study the performance of this device. The conversion efficiency as a function of wells width and depth is modeled for MQW solar cells. It is shown that the MQW solar cells reach high conversion efficiency values. A study of the SL solar cell viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficient for SL structure are calculated in order to determinate the J-V characteristic. The influence of superlattice length on the conversion efficiency is researched, showing a better performance when width and cluster numbers are increased. The SL solar cell conversion efficiency is compared with the maximum conversion efficiency obtained for the MQW solar cell and shows an efficiency enhancement.

InGaN/GaN Multiple Quantum Well Solar Cells with Good Open-Circuit Voltage and Concentrator Action

The photovoltaic properties of large-chip-size (2.5×2.5 mm2) InGaN/GaN multiple quantum well (MQW) solar cells grown by metal organic chemical vapor deposition were studied under concentrated AM1.5G sun irradiation. We demonstrate a high open-circuit voltage of 2.31 V for blue-light-emitting InGaN/GaN MQW solar cells under 1 sun. The higher open-circuit voltage is mainly ascribed to the extremely low reversed saturation current density of approximately 10-19 mA/cm2. The open-circuit voltage and short-circuit current density were found to increase as sunlight intensity increases, with a peak value of 2.50 V observed at 190 suns, showing a great potential for concentrator applications.

Enhanced Carrier Escape in MSQW Solar Cell and Its Impact on Photovoltaics Performance

A multiple stepped quantum wells (MSQWs) solar cell, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and improvements in short-circuit current and fill factor have been demonstrated. We have studied carrier recombination dynamics and carrier escape kinetics from the wells, comparing the MSQW with the normal multiple quantum well (MQW). Carrier recombination rate was examined by time-resolved photoluminescence (PL) and a longer carrier lifetime was observed for MSQWs than for MQWs. Carrier escape from the wells was also investigated in terms of temperature-dependent PL. Smaller thermal activation energy was observed for MSQWs than for MQWs. Carrier radiative recombination loss was investigated by bias-dependent PL, and it was found to be smaller for the MSQW than for the normal MQW. Such advantages of the MSQW allowed us to stack a sufficient number of quantum wells to increase short-circuit current without degrading fill factor. For normal MQW solar cells, degradation in fill factor is unavoidable.

GaAs/GaInNAs quantum well and superlattice solar cell

A theoretical study of GaAs/GaInNAs solar cells based on multiple-quantum well solar cells (MQWSCs) and superlattice solar cell (SLSC) configuration is presented. The conversion efficiency as a function of the quantum well width and depth is modeled for MQWSC, reaching high values. A study of the SLSC viability is also presented. The influence of the cluster width on the conversion efficiency is researched showing a better performance when width and the cluster number are increased. The SLSC conversion efficiency is compared with the maximum conversion efficiency obtained for the MQWSC showing that it is reached an amazing increment of 4%.

Increasing Solar Efficiency of InGaN/GaN Multiple Quantum Well Solar Cells with a Reflective Aluminum Layer or a Flip-Chip Structure

Multiple quantum well solar cells (MQWSC) have the potential to be a highly efficient alternative to tandem or cascade systems. However, in conventional multiple quantum well solar cells, the quantum well layer does not usually receive and absorb enough light. To increase the absorption in the quantum well layer of an InGaN/GaN multiple quantum well solar cell, a reflective aluminum layer has been proposed and investigated here. We also studied the use of a flip-chip structure to increase the effectiveness of the heat sink and to prevent the shading loss that occurs in top contact metal of InGaN/GaN MQWSCs. We found that In0.2Ga0.8N/GaN MQWSCs with a reflective aluminum layer are 9.8% more efficient than those without a reflective aluminum layer. Meanwhile, In0.28Ga0.72N/GaN MQWSCs with a reflective aluminum layer are 4% more efficient than those without a reflective layer. In0.28Ga0.72N/GaN MQWSCs were expected by their lower well bandgap to be more efficient than In0.2Ga0.8N/GaN MQWSCs, but they are not. The lower efficiency enhancement in In0.28Ga0.72N/GaN MQWSCs is attributable to the greater number of defects or recombination centers in the film with higher indium content. In0.2Ga0.8N/GaN and In0.28Ga0.72N/GaN MQWSCs with a flip-chip structure exhibit efficiency enhancements of 1.4% and 1.1%, respectively, over those without a flip-chip structure.

New Results in Optical Modelling of Quantum Well Solar Cells

This project brought further advancements to the quantum well solar cell concept proposed by Keith Barnham. In this paper, the optical modelling of MQW solar cells was analyzed and we focussed on the following topics: (i) simulation of the refraction index and the reflectance, (ii) simulation of the absorption coefficient, (iii) simulation of the quantum efficiency for the absorption process, (iv) discussion and modelling of the quantum confinement effect, and (v) evaluation of datasheet parameters of the MQW cell.

Solar energy harvesting scheme using syringe-like ZnO nanorod arrays for InGaN/GaN multiple quantum well solar cells

Syringe-like ZnO nanorod arrays (NRAs) synthesized by a hydrothermal method were applied as the light-harvesting layer on InGaN-based multiple quantum well (MQW) solar cells. Theoretical calculations show that the NRAs with an abrupt shrinkage of tip diameter can further suppress surface reflectance in comparison with the flat NRAs. InGaN-based MQW solar cells with the syringe-like NRAs exhibit greatly improved conversion efficiencies by 36%. These results are attributed to the improved flatness of the refractive index profile at the air/device interface, which results in enhanced light trapping effect on the device surface.