InGaN Solar Cells Research Articles

Enhanced performance of InGaN solar cell by using a super-thin AlN interlayer

A super-thin AlN layer is inserted between the intrinsic InGaN and p-InGaN in the InGaN solar cell structure to improve the photovoltaic property. The dark current is markedly decreased by more than two orders of magnitude and the short-circuit current density is increased from 0.77 mA/cm2 to 1.25 mA/cm2, leading to a doubled conversion efficiency compared to the conventional structure. Electrical transport analysis reveals that the forward electrical property is greatly improved in the range of open circuit voltage and the leakage current mechanism changes from defect related Poole-Frenkel emission to interface tunneling emission. The improvement on the electrical and photovoltaic properties is ascribed to insertion of the AlN interlayer, which not only provides a barrier to reduce tunneling for electrons, but also suppresses the nonradiative recombination.

Modeling of polarization effects in InGaN PIN solar cells

In this paper, we study the effect of polarization on the performance of InGaN solar cells. By using the APSYS software, we show that the performance of common device designs is adversely affected by the interface charges between the contact layers and absorber. An improved design based on graded layers in the $${[0 0 0\overline{1}]}$$ or N-face growth direction is shown to be almost immune to these effects.

Effects of InGaN/GaN superlattice absorption layers on the structural and optical properties of InGaN solar cells

This article studies metal-organic vapor phase epitaxy-grown InGaN p-i-n solar cells with superlattice (SL) absorption layers on c-plane sapphire for the influence of InGaN/GaN SLs on the structural and optical properties of the solar cells. Numerical simulations indicate that conventional p-i-n solar cells with a 200-nm-thick In0.06Ga0.94N absorption layer provide absorption rates as high as 65%. However, experimentally, the optical properties of such an epistructure are deteriorated by the formation of structural defects and result in the fabricated devices having worse photovoltaic characteristics. On the other hand, high-resolution x-ray diffraction and photoluminescence analyses show that solar cells with a SL have improved crystalline quality and can accommodate more indium content than those using an InGaN layer. It was also found that the degree of the exciton localization effect does not rise considerably with increases in the indium content and the SL pair numbers. This could be due to variations in dislocation density, interface roughness, and well width during the SL growth. Using SLs of a reasonable crystalline quality, these fabricated solar cells exhibit improved photovoltaic characteristics.

Effects of polarization charge on the photovoltaic properties of InGaN solar cells

AbstractThe effects of interface polarization charge on the photovoltaic characteristics of GaN/InGaN solar cells have been analyzed in detail using 2D drift‐diffusion simulations. The polarization charge at the GaN/InGaN interface creates an electric field that forces carriers generated by light to drift in opposite direction needed for efficient collection and substantially reduces the short circuit current (Isc) and open circuit voltage (Voc). The polarization charge plays an important role in the photovoltaic properties of InGaN solar cells comparable to that of defects. For small interface charge, the potential barrier could increase the Voc.

Theoretical simulations of the effects of the indium content, thickness, and defect density of the i-layer on the performance of p-i-n InGaN single homojunction solar cells

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 (χ), open circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF). 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, Jsc and absorption increase while χ, Voc, and FF decrease. The combined effects of the absorption, χ, Voc, Jsc, and FF lead to a higher conversion efficiency in the high-indium-content solar cell. A high-quality In0.75Ga0.25N solar cell with a 4 μm i-layer thickness can exhibit as high a conversion efficiency as ∼23%. In addition, the similar trend of conversion efficiency to that of Jsc shows that Jsc 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 Conversion Efficiency of Textured InGaN Solar Cells With Interdigitated Imbedded Electrodes

Textured n-GaN/i-InGaN/p-GaN solar cells with interdigitated imbedded electrodes (IIEs) eliminating the electrode-shading loss have been investigated. In the absence of the electrode-shading effect, the optimized textured solar cell exhibits a conversion efficiency of 1.03%, which is 78% and 47% higher than those of the conventional structure and the structure with mirror coated on silicon substrate with electrode shading, respectively. The short-circuit current density of this textured HE device is about 0.65 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is 71% and 44% higher than those of the two compared structures, respectively.

Recent advances in InN‐based solar cells: status and challenges in InGaN and InAlN solar cells

AbstractIn this paper, we review and discuss the recent advances in InN‐based solar cells. Before the discussion on InN‐based solar cells, two major losses in a solar cell, transparency loss and excess excitation loss, are explained and the effectiveness of a multi‐junction tandem cell in reducing the both losses simultaneously is described. Then, the status and issues for multi‐junction tandem solar cells using the conventional III‐V materials (InGaP/InGaAs systems) are reviewed briefly in order to understand the advantages of InN‐based material as a multi‐junction solar cell material. It is shown that main problem in the conventional III‐V multi‐junction solar cell is the current mismatching between sub‐cells, which reduces significantly the conversion efficiency of the tandem cells. The main advantage of InGaN and InAlN as a multi‐junction solar cell is that a wanted band‐gap between 0.7 and 2.5 eV is realized by changing only their composition.This means that current matching between sub‐cells will be easily achieved using InGaN and InAlN systems. After the status of InGaN solar cell research is summarized, the results on the MOVPE growth of InGaN with an In content up to 0.4, Mg doping using Cp2Mg, and the formation of an n+‐p homo‐junction are described. The insertion of step‐graded InxGa1‐xN interlayer between the GaN and InGaN epilayer is also shown to be effective to improve the epilayer quality. The study on InAlN solar cell is on the stage of the growth of In‐rich InAlN. Single‐crystalline InAlN films with an Al content from 0 (Eg = 0.7 eV) to 0.43 (2 eV) have been successfully grown by employing atmospheric‐pressure MOVPE. The study on Mg‐doping for InAlN will be the next step. Finally, we will summarize and discuss the challenges in InGaN and InAlN solar cells. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Investigation on the conversion efficiency of InGaN solar cells fabricated on GaN and ZnO substrates

AbstractBy the use of a device simulator that takes the quantum effect into account, we have investigated the characteristics of InGaN solar cells stacked coherently on GaN or ZnO substrates. We have found that it is necessary to fabricate non‐polar cells, N‐polar cells with p–i–n structures, or III‐polar cells with n–i–p structures in order to achieve high conversion efficiency. InGaN solar cells grown on GaN substrates and on ZnO substrates exhibit almost the same conver‐ sion efficiencies, in spite of the difference in the intensities of the strains in the InGaN films. Taking the smaller lattice mismatch between InGaN and ZnO into account, we conclude that the use of ZnO substrates, which makes the growth of thick InGaN films with high In concentrations simpler to achieve, is advantageous in the fabrication of high conversion efficiency InGaN solar cells. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Substrate-free large gap InGaN solar cells with bottom reflector

In this study, we report on the realization of the In 0.085Ga 0.915N p– i– n solar cell by low-pressure metalorganic vapor phase epitaxy (MOVPE). The w–2θ scans of (0 0 0 2) reflection observation indicate good suppression of phase separation for the p– i– n solar cells with a 150-nm-thick i-In 0.085Ga 0.915N epilayer. The sharp and narrow signals in the photoluminescence spectra of In 0.085Ga 0.915N provided evidence for high material quality, even through the epilayer thickness exceeded its critical value. Furthermore, the laser lift-off (LLO) technique is used to fabricate the substrate-free thin-film solar cells (TF-SCs) with a bottom reflector. Although the samples have suffered from thermal-gradient-induced damage during LLO process, the fabricated TF-SCs still exhibit a low forward voltage of 3.3 V at 20 mA along with an ideality factor of 3.1. Finally, since unabsorbed photons can be reflected by the bottom reflector, the TF-SCs show a 13.6% increase in short-circuit current density as compared to their counterparts without a bottom reflector.

Finite element simulations of compositionally graded InGaN solar cells

The solar power conversion efficiency of compositionally graded In x Ga 1− x N solar cells was simulated using a finite element approach. Incorporating a compositionally graded region on the InGaN side of a p-GaN/n-In x Ga 1− x N heterojunction removes a barrier for hole transport into GaN and increases the cell efficiency. The design also avoids many of the problems found to date in homojunction cells as no p-type high-In content region is required. Simulations predict 28.9% efficiency for a p-GaN/n-In x Ga 1− x N/n-In 0.5Ga 0.5N/p-Si/n-Si tandem structure using realistic material parameters. The thickness and doping concentration of the graded region was found to substantially affect the performance of the cells.

Correlation of crystalline defects with photoluminescence of InGaN layers

We report structural studies of InGaN epilayers of various thicknesses by x-ray diffraction, showing a strong dependence of the type and spatial distribution of extended crystalline defects on layer thickness. The photoluminescence intensity for the samples was observed to increase with thickness up to 200 nm and decrease for higher thicknesses, a result attributed to creation of dislocation loops within the epilayer. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN solar cells, with controlled types and dislocation densities in the InGaN epilayers, a key requirement for realizing high photocurrent generation in InGaN.

InGaN/GaN multiple quantum well solar cells with long operating wavelengths

We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (&amp;gt;420 nm). The fabricated solar cells based on In0.3Ga0.7N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).

Crystalline Perfection of Epitaxial Structure: Correlations with Composition, Thickness, and Elastic Strain of Epitaxial Layers

AbstractCrystalline perfection of InGaN epi-layers is the missing design parameter for InGaN solar cells. Structural deterioration of InGaN epi-layers depends on the thickness, composition and growth conditions as well. Increasing the InGaN epi-layer thickness beyond a critical point introduces extended crystalline defects that hinder the optical absorption and electrical properties. Increasing the InGaN composition further reduces this critical layer thickness. The optical absorption band edge is sharp for III-nitride direct band gap materials. The band edge profile is deteriorated by creation of extended crystalline defects in the InGaN epitaxial material. The design of InGaN solar cells requires the growth of epi-layers where a trade off between crystalline perfection and optical absorption properties is reached.

Utilizing Polarization Induced Band Bending for InGaN Solar Cell Design

AbstractStrong polarization effects observed in III-nitride materials can invert the surface carrier type. The corresponding band bending can be used to design InGaN solar cells. Similar surface inversion was observed in the past with silicon-based Schottky-barrier solar cells, but was limited by Fermi level pinning. The formation of two-dimensional electron gas by polarization fields in III-nitrides has been reported. Using a similar idea, the growth of a thin AlN capping layer on p-InGaN has resulted in band bending, hence depletion region, under the surface that can be used separate any generated photo-carriers. Hall measurements at different depths on these structures confirm the inversion of surface carrier type. Solar cells based on this concept have resulted in an open circuit voltage of 2.15 V and short circuit current of 21.8 μA.

Characteristics of InGaN designed for photovoltaic applications

AbstractThis work addresses the required properties and device structures for an InGaN solar cell. Homojunction InGaN solar cells with a bandgap greater than 2.0 eV are specifically targeted due to material limitations. These devices are attractive because over half the available power in the solar spectrum is above 2.0 eV. Using high growth rates, InGaN films with indium compositions ranging from 1 to 32% have been grown by Molecular Beam Epitaxy with negligible phase separation according to X‐ray diffraction analysis, and better than 190 arc‐sec ω‐2θ FWHM for ∼0.6 μm thick In0.32Ga0.68N film. Using measured transmission data, the adsorption coefficient of InGaN at 2.4 eV was calculated as α ≅ 2×105 cm–1 near the band edge. This results in the optimal solar cell thickness of less than a micron and may lead to high open circuit voltage while reducing the constraints on limited minority carrier lifetimes. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth, fabrication, and characterization of InGaN solar cells

AbstractThe InGaN alloy system offers a unique opportunity to develop high efficiency multi‐junction solar cells. In this study, single junction solar cells made of Inx Ga1–x N are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si‐doped InGaN layer, an intrinsic layer and an Mg‐doped InGaN layer on the top. The I –V curves indicate that the cell made of all‐GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo‐responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Design and characterization of GaN∕InGaN solar cells

We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.