InGaN photovoltaics literature review/Photoconductivity in nanocrystalline GaN and amorphous GaON

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This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc.

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Photoconductivity in nanocrystalline GaN and amorphous GaON[1][1][1][1][1][1][11][11][edit | edit source]

Abstract: In this work Author(s) present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a‐GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a‐GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, Author(s) build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, Author(s) are able to characterize the density of states in the gap for the two materials.

• Polycrystalline GaN has recently been shown to be capable of blue light emission
• Morphous GaN can be stabilized into an amorphous form during growth by incorporating at least 12 at. % of O- accomplished by depositing in the presence of a carefully controlled partial pressure of water vapor
• The release of carriers during an anneal permits an estimate of the energy dependence of the trap depth distribution
• The temperature dependence during the sample cooling allows determination of the depth of the Fermi level in the dark configuration
• Covalent materials, even in a strongly disordered configuration can have a clear gap between valence and conduction bands. They commonly have a significant density of localized carrier states between the bands of extended states, inside what is called the mobility gap
• The interplay of trapping, thermal detrapping, and recombination from extended states and/or tail states ultimately determines the behavior of the photoconductivity
• Oxygen incorporation at high concentrations effectively changes the band structure of the material, and does not act as an impurity which introduces states in the gap
• The position of the Fermi level was determined from the temperature dependence of the dark conductivity and it lies at about 1 eV away from the mobility edge
• The distribution of localized states in the gap was found via spectral measurements: optical absorption, spectral photoconductivity, and TSC. Experimental results support that it is more likely that the Fermi level simply lies somewhere within the band tails
• At low energies the absorption coefficient in the GaON continues to drop away sharply, while in the nc-GaN film there is a clear change in slope at these low energies
• The recombination and trapping rates are proportional to the densities of carriers in the conduction band multiplied by the density of recombination centers and the density of unoccupied traps
• The trapping/detrapping rates are larger than the recombination rate, resulting in quasithermal equilibrium between the free and trapped electrons
• nc-GaN film exhibits a weak photoconductivity (approximately 50 fA at 9 V, corresponding to 10−7 A/W) which is fast, switching on and off
• GaON film has a response several orders of magnitude greater (several nanoamperes at 9 V, corresponding to 10−2 A/W) which is very slow, decaying nonexponentially over the course of hours, i.e., showing PPC (persistent photoconductivity)
• By examining intensity dependence of photoconductivity under 325 nm illumination, photoresponse was found to be linear in generation rate in nc-GaN films and had a squareroot dependence in GaON films.
• Spectral dependence of photoconductivity showed an exponential drop-off in combined densities of states of conduction- and valence-band tails with decreasing energy for both types of film.
• Author(s) developed a model which considers the effect of a large density of localized states in the gap to describe the photoconductive behavior observed in these two types of films

Theoretical possibilities of InxGa1−xN tandem PV structures[2][2][2][2][2][2][12][12][edit | edit source]

Abstract: Author(s) designed a model of InxGa1−xN tandem structure made of N successive p–n junctions going from two junctions for the less sophisticated structure to six junctions for the most sophisticated. Author(s) simulated the photocurrent density and the open-circuit voltage of each structure under AM 1.5 illumination in goal to optimize the number of successive junctions forming one structure.

For each value of N, Author(s) assumed that each junction absorbs the photons that are not absorbed by the preceding one. From the repartition of photons in the solar spectrum and starting from the energy gap of GaN, Author(s) fixed the gap of each junction that gives the same amount of photocurrent density in the structure. Then Author(s) calculated the current density accurately and optimized the thicknesses of p and n layers of each junction to make it give the same output current density. The evaluation of ni: the intrinsic concentration permitted to calculate the saturation current density and the open-circuit voltage of each junction. Assuming an overall fill factor of 80%, Author(s) divided the output peak power by the incident solar power and obtained the efficiency of each structure.

The numerical values for InxGa1−xN were taken from the relevant literature. The calculated efficiency goes from 27.49% for the two-junction tandem structure to 40.35% for a six-junction structure. The six-junction InxGa1−xN tandem structure has an open-circuit voltage of about 5.34 V and a short circuit current density of 9.1 mA/cm2.

• short-circuit current density of a tandem cell Jsc is given by the least of the photocurrent densities produced by the junctions of the tandem cell.
• The open-circuit voltage of a tandem cell is is calculated to be equal to the sum of the open-circuit voltages of the tandem junctions.
• Simulations show that six-junctions InxGa1−xN tandem cell could reach an efficiency of more than 40% with a short-circuit current density of 9.1 mA/cm2 and an open-circuit voltage of 5.3 V.
• output power saturates when the number of junctions increases. More than six junctions would not give more power proportional to the complication of adding more cells.
• achievable short-circuit current density decreases as the tandem cell contains more layers, however, the increase of the open-circuit voltage as a function of the number of junctions is almost linear; hence, it compensates the decrease of the short-circuit current density, which is a function of the inverse of the same variable; this explains the increase of the output maximum power and the cell efficiency with the number of junctions.
• The used materials in tandem solar cells should have some similar properties like the thermal expansion coefficient, the electron affinity and the lattice mismatch
• Assuming a perfect quantum response of the materials and equal photocurrent densities for all the junctions of the tandem cell the energy gaps of the In_xGa_1-xN alloys were identified
• The bowing parameter is composition dependent
• The short-circuit current density is given by the least of the photocurrent densities produced by the junctions of the tandem cell
• A method for calculating the open circuit voltage and short circuit current density of individual and therefore all the junctions of tandem solar cells has been proposed
• The open-circuit voltage produced by the junctions of the tandem cell decrease almost linearly from the top to the bottom and is mainly produced by the first three junctions- can be attributed to the increase of saturation current density
• The logarithm of the saturation current density increases almost linearly from the top to the bottom
• The output power saturates when the number of junctions increases

The impact of piezoelectric polarization and nonradiative recombination on the performance of (0001) face GaN/InGaN photovoltaic devices[3][3][3][3][3][3][13][13][edit | edit source]

Abstract: The impact of piezoelectric polarization and nonradiative recombination on the short-circuit current densities (Jsc) of (0001) face GaN/InGaN photovoltaic devices is demonstrated. P-i-n diodes consisting of 170 nm thick intrinsic In0.09Ga0.91N layers sandwiched by GaN layers exhibit low Jsc ~ 40 μA/cm2. The piezoelectric polarization at the GaN/InGaN heterointerfaces creates drift currents opposite in direction needed for efficient carrier collection. Also, nonradiative recombination centers produce short carrier lifetimes, limiting Jsc. Alternative structures with intrinsic InGaN layers sandwiched by n-type InGaN or graded InyGa1−yN (y = 0–0.09) layer and a p-type In0.015Ga0.985N layer have favorable potentials, longer carrier lifetimes, and improve Jsc to ~ 0.40 mA/cm2.

• NRCs or Hall–Shockley–Read recombination centers can also affect the performance of PV devices, with an increase in NRCs reducing the carrier lifetimes.
• structural differences such as doping, layer thickness and composition, and growth conditions affect piezoelectric polarization and NRC formation within the structure.
• structures consisting of n-type and p-type InGaN layers sandwiching intrinsic InGaN layer are used to control the piezoelectric polarization in InGaN/GaN structures but other methods could be used such as delta doping to reduce polarization fields, growth on non-(0001) substrates, or growth of low dimensional materials such as nanowires
• InGaN is typically grown strained on GaN and the lattice-mismatch strain results in the formation of defects in the InGaN layer that may act as nonradiative recombination centers
• InGaN layer growth is typically performed at low growth temperatures to incorporate sufficient indium, resulting in increased incorporation impurities or other point and V-defect related NRCs
• Trimethylgallium, trimethylindium, and ammonia, are used for the Ga, In, and N sources in H2 and N2 carrier gas, and SiH4 and bis-methylcyclopentadienylmagnesium are used for n-type (Si) and p-type (Mg) doping
• Structural differences such as doping, layer thickness and composition, and growth conditions affect the piezoelectric polarization and NRC formation within the structure
• Piezoelectric polarization and nonradiative recombination affect the performance of (0001) face GaN/InGaN heterojunction p-i-n PV devices
• Placing InGaN layers above and below the intrinsic InGaN layer reduces the piezoelectric polarization and nonradiative recombination centers and increases J_sc

Superior radiation resistance of In1−xGaxN alloys: Full-solar-spectrum photovoltaic material system[4][4][4][4][4][4][14][14][edit | edit source]

Abstract: High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently author(s) have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in In1−xGaxN alloys.

• primary cause for the degradation of solar cells in space are due to bombardment by protons and electrons in the energy range of electron volts to hundreds of million electron volts.
• solar cell degradation in a space radiation environment can be successfully modeled using displacement damage dose methodology.
• PL intensities of GaAs and Ga0.51In0.49P are drastically suppressed by the irradiation whereas the PL signal of InN does not show any reduction in intensity after being subjected to a similar radiation dose.
• in general, nitrides are less sensitive to the irradiation as compared to GaAs and GaInP. GaN and Ga-rich InGaN materials maintain excellent optical properties even in the presence of high densities of structural defects.
• No strong reduction in PL intensity is observed in InN and In1–xGaxN with small Ga fractions. However, it is interesting to note the trend that as the Ga fraction increases, the irradiation-induced degradation in optical emission becomes increasingly severe.
• radiation sensitivity of GaAs and GaInP is far above that of InN, GaN, and InGaN alloys.
• insensitivity of optical properties of InGaN alloys to radiation damage is a good indicator of expected radiation hardness of photovoltaic devices made with these n-type alloys.
• Compared with conventional solar cell materials (GaAs and GaInP), InN and In1–xGaxN alloys show much less deterioration in their optical and transport properties. The different behaviors are explained in terms of the different energy configurations of the irradiation-induced defect states in the band diagram of the host materials.
• superior resistance against irradiation damage In1–xGaxN alloys present a great potential for applications in photovoltaic and other optoelectronic devices
• Authors investigated the effects of high-energy proton irradiation on the optical and electrical properties of In-rich In_1-xGa_xN alloys as a function of the displacement damage dose
• InGaN can withstand extraordinarily high dose of radiation and nitrides are less sensitive to the irradiation
• With Ga fraction increament, the irradiation-induced degradation in optical emission becomes increasingly severe in In_1-xGa_xN
• Defect levels located close to the midgap position are the most effective nonradiative recombination centers
• Nonradiative recombination rates decrease in semiconductors in which E_FS lies closer to the conduction band edge
• Radiation hardness of InN and InGaN alloys is an intrinsic property of these materials, rather than an extrinsic insensitivity caused by the high density of defects that already exist in the starting materials
• The irradiation-generated defect states are efficient electron traps in GaAs and GaInP, while in InN and In-rich InGaN they are not

Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells[5][5][5][5][5][5][15][15][edit | edit source]

Abstract: Author(s) demonstrate enhanced external quantum efficiency and current-voltage characteristics due to scattering by 100 nm silver nanoparticles in a single 2.5 nm thick InGaN quantum well photovoltaic device. Nanoparticle arrays were fabricated on the surface of the device using an anodic alumina template masking process. The Ag nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency by up to 54%. Additionally, the short-circuit current in cells with 200 nm p-GaN emitter regions is increased by 6% under AM 1.5 illumination. AFORS-Het simulation software results were used to predict cell performance and optimize emitter layer thickness.

• Plasmonic nanoparticle scattering offers a unique way to circumvent the inherent trade-off between absorption and carrier collection in the design of solar cells. Optically thick cells can absorb all nonreflected incident light but incomplete carrier collection can limit cell internal quantum efficiency. The thickness reduction of high efficiency, low cost, thin film solar cells is limited by the absorption properties of the active layer.
• In case of thin film cells, there is an additional benefit: metal nanostructures can couple incident light into guided modes that propagate through the active region, thereby increasing absorption and photocurrent.
• The cell with a 50 nm p-GaN emitter layer shows a 6% overall increase in EQE with the addition of nanoparticles.
• notable difference in EQE between the two cells (author(s) prototype) suggests that light absorption is most strongly enhanced in region closest to Ag nanoparticles. photocurrent enhancement may result from a combination of scattering, local field enhancement, and antireflection coating effects.
• presence of nanoparticle arrays on the cells increases the short circuit current appreciably, increase in short circuit current (Jsc) is attributed to absorption enhancement evident from spectral response measurements.
• enhanced Jsc could also be attributed to an increase in carrier collection efficiency at surface of the cell
• Metal nanoparticles can increase optical path length in very thin cells, where surface texturing is not possible, due to their large scattering crosssections and enhanced local fields
• The metal nanostructures in thin film cells can couple incident light into guided modes that propagate through the active region, thereby increasing absorption and photocurrent
• Metal layers improve the contact to both p-GaN and p-AlGaN layers causing an increase in the depletion width at the surface of the device
• The single QW system is a useful platform for the exploration of absorption enhancement from plasmonic scatterers on ultrathin cells designed for optimum photovoltaic response

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells[6][6][6][6][6][6][16][16][edit | edit source]

Abstract: Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.

• It has been reported that the composition fluctuation in InxGa1−xN MQWs tends to be severe for x>0.15. The fluctuation of In content is mainly due to the low miscibility of InN in GaN.
• for high In content (30%), severe composition fluctuation occurred in the InxGa1−xN layers, leading to various quantum wells with different barrier heights. The low EQE may result from the fact that many shallow quantum wells, containing band gap energies close to that of barriers, were formed by In fluctuation, and made most of the incident optical energies insufficient to excite carriers, leading to limited photocurrents.
• poor device performance is caused by compromised crystal quality in active region due to high In content. As lattice strain is introduced in InxGa1−xN/GaN structures, undesired defects are likely to form with increasing x, these defects reduce the electrical field across the intrinsic regions and hinder photogenerated carriers escaping from MQWs, leading to low FF and η. The suspected composition fluctuation could also reduce FF and η by preventing the carriers from being optically excited.
• HAADF STEM image revealed that the In distribution inside quantum wells is not completely homogeneous. A depletion of In is observed in some quantum well regions. The EDS attached to the STEM was utilized to characterize the distribution of In composition further. For the EDS analysis, the electron beam focused down to a diameter of 1.5 nm shows the intensity profile of In( Kα) measured by EDS scan, indicating the severe fluctuation of In atom distribution. The depletion regions of quantum wells with the lower In intensities imply that quantum wells were formed with shallow potential depths.
• The increase in FF and η with temperature indicates that additional photocurrents contributed by thermally activated carriers from shallow wells outweighed the reduction in Voc.
• The critical thicknesses (Tc) of In_xGa_1−xN grown on GaN are generally less than 100 nm for x>0.1, and decrease very rapidly with increasing x
• The composition fluctuation in In_xGa_1−xN MQWs tends to be severe for x>0.15
• For InGaN/GaN MQW solar cells the peak wavelength of EL (522 nm) is longer than that of EQE (375 nm)- can be attributed to the fact that in emission process carriers are relaxed to lower energy states before being recombined radiatively, whereas in absorption process electrons (holes) can be excited to higher states as long as the absorbed photon energy matches the energy level separation
• The low EQE is due to many shallow quantum wells, containing the band gap energies close to that of barriers- were formed by the In fluctuation
• Defects reduce the electrical field across the intrinsic regions and hinder the photogenerated carriers escaping from MQWs, leading to low FF and efficiency
• The depletion regions of quantum wells with the lower In intensities imply that quantum wells were formed with shallow potential depths
• Thermal activation increases photocurrent and hence J_sc
• Increasing saturation currents and recombinations causes V_oc to drop

Temperature dependences of InxGa1−xN multiple quantum well solar cells[7][7][7][7][7][7][17][17][edit | edit source]

Abstract: In this work, high open circuit voltages (Voc) of In0.2Ga0.8N and In0.28Ga0.72N multiple quantum well solar cells (MQWSCs) are experimentally obtained (2.2 V and 1.8 V, respectively). The Voc of In0.28Ga0.72N MQWSCs is lower than the expected value due to serious indium segregation problems causing more defects in In0.28Ga0.72N films, which is consistent with the observation of a high ideality factor in dark current measurement. The temperature dependence of the Voc and the short circuit current (Jsc) in In0.2Ga0.8N MQWSCs is found to be larger than the corresponding values in In0.28Ga0.72N MQWSCs. It is also observed that higher quantum well energy barrier exhibits a low fill factor of 0.52 due possibly to the loss of electric field and the higher energy barrier. This obtained efficiency increases with temperatures up to 100 °C and then decreases due to competing results between the reduction in Voc and an increase in Jsc.

• Wider bandgap barrier materials determine the open circuit voltage
• The short circuit current is determined by the narrower bandgap well materials
• A single bandgap solar cell has the maximum efficiency at an energy bandgap of around 1.35–1.5 eV
• The quantum well with lower bandgap materials can absorb more light spectrum and contribute additional photocurrent
• The electric field across the intrinsic region is an important factor affecting the photogenerated carrier's escape from QWs
• At higher temperatures, short circuit current (Jsc) increases slightly and open circuit voltage (Voc) drops significantly in MQWSCs. solar cell efficiency is seriously degraded due to a large voltage drop at high temperature.
• large series resistance of solar cells may also reduce the fill factor
• loss of electric field can be attributed to higher background impurities in the intrinsic region and defects caused by lattice mismatch. In addition, it is noted that temperature effect on photocurrent in low indium content alloy MQWSCs is more obvious than that in low indium content alloy MQWSCs. It is possible that when temperature becomes higher, the photogenerated carriers with lower energy barriers have a larger probability to escape out of QW than those with higher energy barriers.
• large number of defects in high In composition film could result in higher background doping, which in turn effects the loss of electric field and thus seriously degrades the fill factor of solar cells. Furthermore, higher energy barrier can also degrade the fill factor of solar cells.
• large drop in fill factor of MQWSCs is observed with an increasing energy barrier in QWs.
• To achieve a high efficiency in MQWSCs, a compromise between Jsc and Voc is necessary. If the electric field can be maintained, addition of quantum well in solar cell will result in an increase in Jsc and a reduction in the Voc. The efficiency depends mainly on the compromise between Jsc and Voc.
• increase in Jsc is due to bandgap narrowing with increasing temperature. More carriers are generated in QWs and escape of these carriers is enhanced by thermal contribution.

Substantial photo-response of InGaN p–i–n homojunction solar cells[8][8][8][8][8][8][18][18][edit | edit source]

Abstract: 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 (Voc) 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.

• heterojunction may cause poor quality of InGaN for future practical application due to large latticemismatch betweenGaNand InGaN, especially for full-spectrum-response devices in which higher In content is necessary. Therefore, it is important to develop InGaN solar cells with homojunction structures.
• there is a 38% increase of the light power reaching absorption region for the devices with grid contact, compared with devices with semitransparent contact. However, the observed 26% increase in Jsc for the devices with grid contact is much lower, which is currently attributed to the larger series resistance of these devices. This larger resistance may be caused by the transverse flow of current in p-GaN to the grids with a spacing of 45 μm.
• devices with semitransparent contact have a high fill factor (FF) of 69.4% while the devices with grid contact have a smaller FF of 57.6%. This reduction of the FF value is currently attributed to large resistance which may deteriorate J–V performance of devices with grid contact
• devices with grid contact exhibit a higher maximum output power density (Pmax) of 2.32 mW cm−2, compared with 2.12 mW cm−2 of devices with semitransparent contact.
• for devices with grid contact, major loss mechanisms include reflection at the p-GaN surface and absorption of grid lines.
• InGaN alloys with a length of a few hundred nanometers can absorb a substantial fraction of the incident light
• Thomas Swan low-pressure MOCVD technique was used
• The barrier at the p-InGaN/p-GaN interface is of benefit in keeping the electrons generated in p-InGaN away from the p-type ohmic contact and contributes to the light-generated current
• The observed 26% increase in Jsc for the devices with grid contact is much lower- attributed to the larger series resistance of these devices
• Devices with grid contact showed a higher power density, which indicated that more incident light can be absorbed and then converted into electric energy, compared with devices with semitransparent contact

Analytical model for the optical functions of indium gallium nitride with application to thin film solar photovoltaic cells[9][9][9][9][9][9][19][19][edit | edit source]

Abstract: This paper presents the preliminary results of optical characterization using spectroscopic ellipsometry of wurtzite indium gallium nitride (InxGa1−xN) thin films with medium indium content (0.38 < x < 0.68) that were deposited on silicon dioxide using plasma-enhanced evaporation. A Kramers–Kronig consistent parametric analytical model using Gaussian oscillators to describe the absorption spectra has been developed to extract the real and imaginary components of the dielectric function (ɛ1, ɛ2) of InxGa1−xN films. Scanning electron microscope (SEM) images are presented to examine film microstructure and verify film thicknesses determined from ellipsometry modeling. This fitting procedure, model, and parameters can be employed in the future to extract physical parameters from ellipsometric data from other InxGa1−xN films.

• film composition was determined by the Tauc method using absorption data obtained from the spectroscopic ellipsometry models.
• Ellipsometric measurements of the InxGa1−xN films yield two parameters for each wavelength and angle of incidence: Ψ and Δ, representing the change in amplitude ratio and change in phase shift, respectively, between the p- and s-components of the light beam's electric field. Through this method important information such as film thickness, surface roughness, optical constants (dielectric constants: ɛ1 and ɛ2; index of refraction: η and extinction coefficient: κ) and absorption coefficients, α, can be obtained.
• 204 nm-thick In0.64Ga0.36N and 221 nm-thick In0.68Ga0.32N films show single-phase isolated platelet/nanocolumnar grains with average diameters of 85 nm and 105 nm, respectively. The InxGa1−xN films display a clear relationship of larger grain or nanocolumn sizes with increasing indium content, which has been reported previously by author(s).
• Typically, transmission and reflection (T&R) measurements are performed to obtain absorption information. However, spectroscopic ellipsometry is a good alternative if T&R is not possible due to opaque substrates such as the Si/SiO2 used by author(s). Additionally, ellipsometry can be used to determine the optical band gap of a thin film if photoluminescence and spectrophotometry are not options.
• Kramers–Kronig consistent parametric model has been developed for the optical functions of wurtzite InxGa1−xN alloy films of medium indium contents (0.38 < x < 0.68) deposited by a novel peed system. This model employing simple Gaussian oscillators is used to fit spectroscopic ellipsometric data over the 0.8–4.5 eV range to obtain film thicknesses, dielectric functions and absorption coefficients.

InGaN-Based p–i–n Solar Cells with Graphene Electrodes[10][10][10][10][10][10][20][20][edit | edit source]

Abstract:InGaN-based p–i–n solar cells with graphene electrodes were fabricated and compared with solar cells using indium tin oxide (ITO) electrodes. In particular, authors analyzed the properties of graphene film by means of high-resolution transmission electron microscopic (HRTEM) and Raman spectroscopy, also comparing optical properties with those of ITO, conventionally used as transparent electrodes. The solar cells using graphene revealed a short circuit current density of 0.83 mA/cm², an open circuit voltage of 2.0 V, a fill factor of 75.2%, and conversion efficiency of 1.2%, comparable to the performance of solar cells using ITO.

• ITO also has limitations such as increasing cost due to indium scarcity, limited transparency at near UV and IR regions, and a sensitivity to acidic and base chemical sources. Furthermore, ITO can easily crack and break when deposited on bending substrates, making it difficult to use for applications such as flexible displays and touch screens
• Graphene has outstanding optical and electrical properties, i.e., high mobility, transparency, flexibility, and mechanical and chemical stability
• Graphene, having 4.5 eV work function, reveals slightly lower Schottky barrier with p-GaN than ITO which shows 4.3 eV work function
• The approximately 10% increase in the Jsc of the graphene sample could be explained by the overlapping EQE region of the graphene sample with the solar spectrum, which had a larger area than that of the ITO sample
• ITO transmittance varies with thickness and wavelength, while the thin-layered graphene maintains almost the same transmittance

References[edit | edit source]