**1. Introduction**

InGaN alloys materials are extensively utilized in light emitting diodes (LEDs) and laser diodes (LDs). In recent years, InGaN alloys materials have also been considered to use in solar cells because of their favorable photovoltaic properties, including a direct band gap, a higher absorption coefficient at the band edge, superior radiation resistance, high carrier mobility, thermal stability and so on. Especially, the band gap of the InGaN alloys materials can even vary from 0.7 eV to 3.4 eV, which covers almost the sun spectrum. [1] Additionally, fourjunction tandem solar cells with a theoretical conversion efficiency of over 60% have been designed, but these designs require some junctions that have band gap of greater than 2.4 eV. Very few materials have a band gap of over 2.4 eV but InGaN alloys. Therefore, InGaN alloys are the candidate of using in highly efficient tandem solar cells.

Many challenges must be overcome before InGaN alloys can be used widely photovoltaic devices. A large lattice mismatch between GaN and InN atoms limits InGaN alloys using in photovoltaic devices in which these alloys must incorporate a thick absorb layer with high indium composition for absorbing incident light. Generally, the critical thickness of In0.1Ga0.9N is approximately 100 nm, and this thickness falls rapidly as the indium composition increasing. When the thickness of InGaN alloys layer exceeds a critical value, defects are formed as recombination centers. These recombination centers increase the rate of consump‐ tion of photogenerated electron-hole pairs, degrading photovoltaic performance. Owing to the need for high crystalline quality, the thickness of absorb layers in InGaN alloys photovoltaic devices is limited by challenges related to epitaxial deposition such that a compromise of multiple quantum well (MQW) structure is used for the absorb layers in InGaN alloys photovoltaic devices, which results in insufficient light absorption.

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Previous studies have utilized several methods for improving the collecting of light in InGaN/ GaN MQWs solar cells, such as the use of a ZnO or SiO2 sub-wavelength structure to realize a graded refractive index interface to reduce Fresnel reflection and simultaneously to increase the light scattering effect. Silver nanoparticles have also been used to exploit the surface plasmon effect to promote the scattering of light. However, because the light absorption is not sufficient, the lower external quantum efficiency and the higher absorption coefficient in the ultraviolet region have not been solved yet. [2]A back reflector that reflects the unused light back to the absorr layer provides a solution and has an important role in thin film solar cells. For this purpose, distributed Bragg reflectors (DBRs) are good candidates for InGaN/GaN MQWs solar cells.

Due to InGaN alloys materials have tunable band gaps and superior photovoltaic character‐ istics, they would be emerged as promising solar cells materials. [3] The band gap of InGaN alloys materials changed from 0.7 eV to 3.4 eV and these band gaps can be tuned by changing the contents of indium in the InGaN alloys materials. InGaN alloys mateials also have direct band gaps in the entire alloys range. For achieving higher photoelectric conversion efficiency, the multiple quantum wells solar cell structure has been proposed, which exhibits stronger absorption properties than bulk materials. [3] The quantum wells layers of the solar cells extend the absorption spectrum into the infrared region, in the case of radiant energy levels in the semiconductor band gap. The quantum wells similar to the *p–i–n* diode structure may extend the solar cells absorption spectrum, and then results in a higher short circuit current density (*Jsc*). The recombination rate of these quantum wells has been improved, which result in a higher open circuit voltage (*Voc*). The finally result is that the photoelectric conversion efficiency (*η*) of the solar cells has been increased.

Group III–Nitride alloys materials solar cells are the better candidates for multiple quantum wells solar cells, because these alloys have direct band gaps and also have the better absorption of the solar spectrum. Recently, multi-junctions solar cells have been used to obtain higher conversion efficiency than single-junction solar cells. [3]However, the properties of latticematched quantum wells solar cells with different In content significantly complicate the fabrication process and design of the solar cells device. The average strain is chief when designing strained quantum wells solar cells. In order to maximize the absorption of quantum wells, it is better to maximize the carries concentration of quantum wells, the width and the number of quantum wells. In fact, all these factors increase the average strain, so the solar cells eventually become limited by the critical thickness. So the relative contributions of tunneling and thermionic emission currents of the multiple quantum wells structure are functions of the operation temperature and should provide guidance for the optimized design of multiple quantum wells solar cells tailored for operation in specific temperature ranges.

For the fabrication of InGaN alloys solar cells, there is considerable interest in the growth of GaN and InGaN alloys on Si substrates, because Si substrates have the advantages of low cost and large size. [4] Si substrates have a better thermal conductivity, whereas sapphire and SiC substrates have worse thermal conductivity. However, integration has been hampered by the large density of defects and cracks arising from larger lattice mismatch between GaN and Si. Some studies have grown GaN on Si (100) or Si (111) substrates by adding buffers to overcome the larger density of defects and cracks. [5]

Previous studies have utilized several methods for improving the collecting of light in InGaN/ GaN MQWs solar cells, such as the use of a ZnO or SiO2 sub-wavelength structure to realize a graded refractive index interface to reduce Fresnel reflection and simultaneously to increase the light scattering effect. Silver nanoparticles have also been used to exploit the surface plasmon effect to promote the scattering of light. However, because the light absorption is not sufficient, the lower external quantum efficiency and the higher absorption coefficient in the ultraviolet region have not been solved yet. [2]A back reflector that reflects the unused light back to the absorr layer provides a solution and has an important role in thin film solar cells. For this purpose, distributed Bragg reflectors (DBRs) are good candidates for InGaN/GaN

Due to InGaN alloys materials have tunable band gaps and superior photovoltaic character‐ istics, they would be emerged as promising solar cells materials. [3] The band gap of InGaN alloys materials changed from 0.7 eV to 3.4 eV and these band gaps can be tuned by changing the contents of indium in the InGaN alloys materials. InGaN alloys mateials also have direct band gaps in the entire alloys range. For achieving higher photoelectric conversion efficiency, the multiple quantum wells solar cell structure has been proposed, which exhibits stronger absorption properties than bulk materials. [3] The quantum wells layers of the solar cells extend the absorption spectrum into the infrared region, in the case of radiant energy levels in the semiconductor band gap. The quantum wells similar to the *p–i–n* diode structure may extend the solar cells absorption spectrum, and then results in a higher short circuit current density (*Jsc*). The recombination rate of these quantum wells has been improved, which result in a higher open circuit voltage (*Voc*). The finally result is that the photoelectric conversion efficiency

Group III–Nitride alloys materials solar cells are the better candidates for multiple quantum wells solar cells, because these alloys have direct band gaps and also have the better absorption of the solar spectrum. Recently, multi-junctions solar cells have been used to obtain higher conversion efficiency than single-junction solar cells. [3]However, the properties of latticematched quantum wells solar cells with different In content significantly complicate the fabrication process and design of the solar cells device. The average strain is chief when designing strained quantum wells solar cells. In order to maximize the absorption of quantum wells, it is better to maximize the carries concentration of quantum wells, the width and the number of quantum wells. In fact, all these factors increase the average strain, so the solar cells eventually become limited by the critical thickness. So the relative contributions of tunneling and thermionic emission currents of the multiple quantum wells structure are functions of the operation temperature and should provide guidance for the optimized design of multiple

quantum wells solar cells tailored for operation in specific temperature ranges.

For the fabrication of InGaN alloys solar cells, there is considerable interest in the growth of GaN and InGaN alloys on Si substrates, because Si substrates have the advantages of low cost and large size. [4] Si substrates have a better thermal conductivity, whereas sapphire and SiC substrates have worse thermal conductivity. However, integration has been hampered by the large density of defects and cracks arising from larger lattice mismatch between GaN and Si.

MQWs solar cells.

334 Solar Cells - New Approaches and Reviews

(*η*) of the solar cells has been increased.

Recently most reported InGaN alloys solar cells have very low photovoltaic efficiency compared to Si-based solar cells. However, InGaN alloys materials superior resistance against irradiation damage makes themselves very suitable for the applications in photovoltaic devices, and motivates further development. In some investigations, GaN has usually been grown on sapphire or SiC substrates, which are expensive and difficult to integrate into the silicon industry. [3] Therefore, it is desirable to grow GaN on silicon substrate and integrate GaN with the mature silicon fabrication techniques.

The optoelectronic performance of InGaN solar cells devices are researched by preparing InGaN/GaN multiple quantum wells with In composition exceeding 0.3, attempting to alleviate to the phase separation phenomenon of InN and GaN materials at a certain degree by this InGaN/GaN multiple quantum wells structure. The InGaN/GaN multiple quantum wells solar cells have a better optoelectronic performance at wavelengths longer than 430 nm. The InGaN solar cells devices show better open circuit voltage (2.0 V) and external quantum efficiency (45%) and high fill factor (65%) because of the InGaN/GaN multiple quantum wells structure.

Recently, concentrator systems solar cells are becoming a main technology for the large scale electrical power by utilizing high conversion efficiency group III–V multi-junctions solar cells. Sharp's triple-junctions InGaP/GaAs/InGaAs concentrator solar cells have got a high conver‐ sion efficiency of 44.4% under about 300 suns. [6] The solar cells will probably to be four or more junction cells with higher conversion efficiency in the future. The ideal highest conver‐ sion efficiency is 55% for four-junctions solar cells, which could be achieved by utilizing an optimized band gap combination of 1.9/1.4/1.0/0.7 eV. [6]The four-junctions solar cells is a promising photovoltaic candidate devices because of the bandgap combination of InGaP/GaAs (1.9/1.4 eV) which matched to GaAs in their lattice and that of InGaAsP/ InGaAs (1.0/0.7 eV) which matched to InP in their lattice.

Currently, only a few contributions have been reported on InGaAsP materials solar cells grown by metal organic chemical vapor deposition (MOCVD). In fact, InGaAsP materials solar cells can also be grown by molecular beam epitaxy (MBE), which can give more precise growth control technology. Researcher ever think that the photovoltaic performance of solar cells grown by MBE method is not probably better than MOCVD growth methods, because the growth temperature of MBE methods is very lower, which results in more defect states and deep defect centers in bulk materials. Recently, a high efficiency GaInP/GaAs/GaInAsN triplejunctions solar cell was successfully grown by MBE methods. [6]The experimental results showed that the solar cells containing group III–V materials grown by MBE growth are as good as the MOCVD growth. It has been a challenge to grown a high quality InGaAsP materials which bandgap is only 1.0 eV on an InP substrate by MBE methods. Furthermore, a compre‐ hensive study on the carrier recombination dynamics of InGaAsP material grown by MBE has not been reported.

The solar cells of InP/InGaAsP double hetero-junction (DH) structure have been investigated and compared these solar cells to the InP control cells. [7] The InGaAsP has many band gap values from 0.75 eV to 1.35 eV which very matched to the InP in lattice. The InP/InGaAsP double hetero-junction structure solar cells whose light absorption layer is the InGaAsP has also been investigated. [7] The investigated results are that the InP/InGaAsP double heterojunction structure solar cells have a lower open-circuit voltage and the short-circuit current improves twice compared to the InP control cells. So the InP/InGaAsP double hetero-junction structure can greatly improve the conversion efficiency of the solar cells.

The photoelectric performance of InP/InGaAsP multiple quantum wells solar cells are improved by light scatter coming from deposited dielectric or metal nanoparticles. [8]The integration of dielectric or metal nanoparticles on the multiple quantum wells solar cells showed that incident light can enter the bulk by lateral optical propagation paths, and the refractive index can provide the optical confinement between the quantum well layers and surrounding materials layers. By the materials surface optimization of silica and Au nanopar‐ ticle, short-circuit current density could be increased by 12.9% and photoelectric conversion efficiency also could be increased by 17%, respectively.

Group III–V compound multi-junctions solar cells have the advantage for achieving photo‐ electric conversion efficiency exceed to 40%, and they are also a promising photovoltaic materials for the space and terrestrial solar cells devices. [9] Among the multi-junctions solar cells technologies, the double-junctions solar cells is the simplest structure and has attracted extensive interesting for further optimizing these solar cells device performance. One of the most trusted materials is the GaInP/GaAs alloys system whose band gap is 1.9 and 1.4 eV respectively. When GaInP materials are acted as the top cell, many problems of bulk defects and crystal quality found existing in other alloy materials such as AlGaAs can even be avoided.

As a result, the researches on the GaInP and GaAs materials property have been become very important, especially the study on the recombination dynamics of carriers in the active layers. For example, surface recombination velocity of the GaInP layer could be measured by the intensity of photoluminescence, and the effective lifetime of minority carriers of the GaInP or GaAs layer could be also measured by the intensity of photoluminescence. On the other hand, the characterization of carrier dynamics and the loss of carriers have been reported little from references. Despite the electroluminescence measurement has been used for modeling the irradiation-induced degradation of the multi-junctions solar cells structures in space condi‐ tions, compared with the photoluminescence technique, it is believed that the electrolumines‐ cence measurement is also competent in revealing detailed the kinetics of the recombination loss of carriers. InGaP solar cells which have about 1.9 eV band gap and lattice-matched to GaAs, have been used for the top cells of multi-junctions solar cells.

InGaP/GaAs/InGaAs multi-junctions solar cells have been achieved high conversion efficiency of 36.4% under AM1.5. Recently, the intermediate band solar cells have been extensively studied and providing a high conversion efficiency of over 60% under concentrated sunlight conditions. [10]The intermediate band is formed by the quantum dot superlattice in inter‐ mediate band solar cells, which located in the bandgap and used to absorb the sub-bandgap photons in the intermediate band state. To achieve a conversion efficiency of more than 60%, the materials with a bandgap of about 1.9 eV are needed. However, the quantum dot super‐ lattice solar cells have used GaAs semiconductor materials whose bandgap is only 1.4 eV. In order to realize intermediate band solar cells with a conversion efficiency of over 60%, a wide bandgap semiconductor material is needed. In fact, InGaP is a suitable intermediate band material with bandgap of 1.9 eV. [10]

The solar cells of InP/InGaAsP double hetero-junction (DH) structure have been investigated and compared these solar cells to the InP control cells. [7] The InGaAsP has many band gap values from 0.75 eV to 1.35 eV which very matched to the InP in lattice. The InP/InGaAsP double hetero-junction structure solar cells whose light absorption layer is the InGaAsP has also been investigated. [7] The investigated results are that the InP/InGaAsP double heterojunction structure solar cells have a lower open-circuit voltage and the short-circuit current improves twice compared to the InP control cells. So the InP/InGaAsP double hetero-junction

The photoelectric performance of InP/InGaAsP multiple quantum wells solar cells are improved by light scatter coming from deposited dielectric or metal nanoparticles. [8]The integration of dielectric or metal nanoparticles on the multiple quantum wells solar cells showed that incident light can enter the bulk by lateral optical propagation paths, and the refractive index can provide the optical confinement between the quantum well layers and surrounding materials layers. By the materials surface optimization of silica and Au nanopar‐ ticle, short-circuit current density could be increased by 12.9% and photoelectric conversion

Group III–V compound multi-junctions solar cells have the advantage for achieving photo‐ electric conversion efficiency exceed to 40%, and they are also a promising photovoltaic materials for the space and terrestrial solar cells devices. [9] Among the multi-junctions solar cells technologies, the double-junctions solar cells is the simplest structure and has attracted extensive interesting for further optimizing these solar cells device performance. One of the most trusted materials is the GaInP/GaAs alloys system whose band gap is 1.9 and 1.4 eV respectively. When GaInP materials are acted as the top cell, many problems of bulk defects and crystal quality found existing in other alloy materials such as AlGaAs can even be avoided. As a result, the researches on the GaInP and GaAs materials property have been become very important, especially the study on the recombination dynamics of carriers in the active layers. For example, surface recombination velocity of the GaInP layer could be measured by the intensity of photoluminescence, and the effective lifetime of minority carriers of the GaInP or GaAs layer could be also measured by the intensity of photoluminescence. On the other hand, the characterization of carrier dynamics and the loss of carriers have been reported little from references. Despite the electroluminescence measurement has been used for modeling the irradiation-induced degradation of the multi-junctions solar cells structures in space condi‐ tions, compared with the photoluminescence technique, it is believed that the electrolumines‐ cence measurement is also competent in revealing detailed the kinetics of the recombination loss of carriers. InGaP solar cells which have about 1.9 eV band gap and lattice-matched to

InGaP/GaAs/InGaAs multi-junctions solar cells have been achieved high conversion efficiency of 36.4% under AM1.5. Recently, the intermediate band solar cells have been extensively studied and providing a high conversion efficiency of over 60% under concentrated sunlight conditions. [10]The intermediate band is formed by the quantum dot superlattice in inter‐ mediate band solar cells, which located in the bandgap and used to absorb the sub-bandgap photons in the intermediate band state. To achieve a conversion efficiency of more than 60%,

structure can greatly improve the conversion efficiency of the solar cells.

efficiency also could be increased by 17%, respectively.

336 Solar Cells - New Approaches and Reviews

GaAs, have been used for the top cells of multi-junctions solar cells.

InGaP material is very difficult to grow by the solid-source molecular beam epitaxy technique which is suitable for growing a high quality quantum dot structure. The InGaP epitaxial layers are required to obtain sufficient light absorption in the solar cells devices, a large-scale phosphorus source are needed in the molecular beam epitaxy chamber. In fact, all of most InGaP solar cells are grown by metal organic chemical vapor deposition.

An InGaP/GaAs tandem solar cell of 4 cm2 larger area is realized, which got a new conversion efficiency record of over 30% under AM 1.5. [11]Those tandem solar cells performances were improved by utilizing InGaP tunnel junction and a double hetero-junctions structure, where the InGaP layers are surrounded by AlInP barrier layers. The double hetero-junctions structure increased the peak current value of InGaP tunnel junction. The AlInP barrier layer takes the part of a back surface field; it is found that the AlInP barrier layer can effectively reflect minority carriers in the InGaP top cells. The AlInP barrier layer has a high potential and can prevent the diffusion of zinc from a doped tunnel junction toward the top cells during growth. Further‐ more, the InGaP tunnel junction can also reduce the light absorption loss and increase the photogenerated current in the GaAs bottom cell. [11]

The photoelectric conversion efficiency of InGaP/GaAs/Ge multi-junctions solar cells has been technically improved up to 32% under AM1.5. The InGaP material is on first hetero-junction growth layer, combined with Ge bottom layer and double hetero-junction tunnel junctions, in which Ge substrate is precisely matched with the conventional GaAs in their lattice structure. If the AlInGaP material whose band gap is 1.95 eV is employed in the top cells layer, the conversion efficiency of these solar cells should be improved further. Furthermore, thin film structure of InGaP/GaAs solar cells on metal film has been reported. The thin film solar cells of InGaP/GaAs are founded lightweight, high flexibility, high radiation resistance and high efficiency.

The use of multiple quantum wells is very advantageous in the conventional tandem solar cells, because multiple quantum wells can independently tailor the absorption edge of each cell, which has no such problem of lattice mismatch and relaxation. [12]The InGaP/GaAs solar cells was improved by using strain balanced multiple quantum wells; the multiple quantum wells structure of tandem solar cells has achieved the conversion efficiency of over 30% under AM1.5. This conversion efficiency is a new record for currently photovoltaic devices. The conversion efficiency of over 34% could be realized under AM1.5 by optimizing the solar cells device structure. The possibility and gains are currently researched by introducing multiple quantum wells structure into top cells and bottom cells layer of an InGaP/GaAs solar cells device.
