InGaN Solar Cells Research Articles

Nonpolar and semipolar InGaN/GaN multiple-quantum-well solar cells with improved carrier collection efficiency

We demonstrate the nonpolar and semipolar InGaN/GaN multiple-quantum-well (MQW) solar cells grown on the nonpolar m-plane and semipolar (202¯1) plane bulk GaN substrates. The optical properties and photovoltaic performance of the nonpolar and semipolar InGaN solar cells were systematically studied, and the results were compared to the conventional polar c-plane devices. The absorption spectra, current density–voltage (J–V) characteristics, external quantum efficiency (EQE), and internal quantum efficiency (IQE) were measured for nonpolar m-plane, semipolar (202¯1) plane, and polar c-plane InGaN/GaN MQW solar cells. Nonpolar m-plane InGaN/GaN MQW solar cells showed the best performance across all devices, with a high open-circuit voltage of 2.32 V, a low bandgap-voltage offset of 0.59 V, and the highest EQE and IQE. In contrast, the polar c-plane device showed the lowest EQE despite the highest absorption spectra. This huge difference is attributed to the better carrier transport and collection on nonpolar m-plane devices due to the reduced polarization effects, which were further confirmed by bias-dependent EQE measurements and energy band diagram simulations. This study demonstrates the high potential of nonpolar and semipolar InGaN solar cells and can serve as guidance for the future design and fabrication of high efficiency III-nitride solar cells.

Increase of photogenerated carriers in thick quantum wells in InGaN solar cells verified by laser-assisted capacitance-voltage measurement

For the InGaN/GaN multiple quantum well (MQW) solar cells, it is known that in the thicker InGaN QWs the collection of photogenerated carriers suffer from the impact of both stronger piezoelectric field and enhanced non-radiative recombination process, which may lead to an inferior performance for thick-well solar cells. However, we find that the photovoltaic intensities of our MQW solar cells with different well thickness are almost the same. Combined with the profiles of apparent carrier concentration obtained from the capacitance-voltage measurement with additional laser illumination, we conclude that the increase of photogenerated carriers in the thick QWs may partially counteract the adverse effects of both polarization effect and deteriorated material quality on the photocurrent for the MQW solar cells with thick InGaN well layers, compared with the thin-well ones.

Microstructural Analysis with Graded and Non-Graded Indium in InGaN Solar Cell

Microstructural Analysis with Graded and Non-Graded Indium in InGaN Solar Cell

Quantum confinement effect on the electronic and optical features of InGaN-based solar cells with InGaN/GaN superlattices as the absorption layers

The short period group III-V nitrides superlattices (SLs), have turned out contemporary in the technology of optoelectronics and solar cell applications. Our theoretical simulation is carried out by means of first-principles full potential linearized augmented plane waves (FP-LAPW) methodology within generalized gradient approximations (GGA) in conjunction with the modified Becke–Johnson (mBJ) potential. In this respect, a comprehensive study for the electronic structures and optical aspects of (InxGa1−xN)n/(GaN)n (001) zinc-blende superlattices (SLs) (x=0.5 and n=3–4), is carried out. Specifically, the electronic band structure calculations and their related features, like the absorption coefficient, reflectivity, refractive index and electron energy loss function spectra of these systems are computed over a wide photon energy scale up to 25eV. The effect of periodicity layer numbers and In composition (x=50%) on the band gaps of (InxGa1−xN)n/(GaN)n SLs is examined. The tailoring of the underlying energy band gap relies on the In content and the periodicity of the superlattices. All these ultrathin superlattices (n=3–4) possess a direct energy band gap. The InGaN layers have immense prominence in ascertaining the underlying energy gap of these superlattice because of the distinctive quantum confinement impact. Furthermore, the flexible energy gap of these ultrathin-period SLs leads to the alteration of the absorption coefficients and static refractive indices. It is viable to attain InGaN solar cells possessing high efficiencies since the energy gap covers all the spectrum optical regime. Predominantly, it is feasible to tune the optical characteristics of these short-period SLs and provide plausible results for the optoelectronic devices applications.

Theoretical Study and Simulations of an InGaN Dual-Junction Solar Cell

This study aims to determine the optimal configuration of the dual-junction InGaN solar cell. Several parameters of the dual-InGaN-junction solar cell have been investigated as the band gap combination and the thicknesses of the layers. Physical models and the optical properties of the InxGa1−xN according to the indium content have been used. The dual-junction solar cell has been designed and simulated for each chosen band gap combination. The current densities drawn from the sub-cells were matched by adjusting their emitter layers thicknesses. The best conversion efficiency obtained for the optimized dual-junction In0.49Ga0.51N/In0.74Ga0.26N solar cell, under standard conditions, was 34.93% which corresponds to the band gap combination of 1.73 eV/1.13 eV. The short-circuit current density and the open circuit voltage obtained from the tandem cell In0.49Ga0.51N/In0.74Ga0.26N are respectively, 21.3941 mA/cm2 and 1.9144 V. The current mismatch was 0.057%. The effects of the front and back layers thicknesses of the top and bottom cells on the efficiency were also studied. Furthermore, the electrical characteristics of the dual-junction solar cell and its sub-cells were also discussed.

Simulation study of InGaN intermediate-band solar cells

The performances of single-junction InGaN solar cells with various intermediate bands (IBs) have been simulated using the lifetime model of a 1D simulation program called Analysis of Microelectronic and Photonic Structures (AMPS-1D). It has been observed that the maximum efficiencies of the InGaN solar cells with one, two and three intermediate bands are 47.72%, 55.10% and 58.20%, respectively, which outperform the 25.96% efficiency of the conventional single-junction structure by far. This is primarily attributed to the outstanding capability of the light harvesting from the sub-bandgap absorption. At the optimized bandgap of 2.41 eV, two-IB InGaN solar cells with the IB positions located at 0.95–1.1 eV and 0.3–0.75 eV, respectively, may have an opportunity to realize over 50% efficiency.

Improving InGaN heterojunction solar cells efficiency using a semibulk absorber

We demonstrate enhanced short circuit current density and power conversion efficiency in InGaN heterojunction solar cells using a semibulk absorber (multi-layered InGaN/GaN structure), with 8% indium concentration. The semibulk absorber shows peak external quantum efficiency of 85% and short current density of 0.57mA/cm2 under AM 1.5G, i.e. almost four times higher than the typical InGaN solar cells based on single thick InGaN absorber. The power conversion efficiency is around 0.39% under AM 1.5G, almost three times higher than state of the art for 8% indium incorporation. The improvement in power conversion efficiency is attributed to the enhancement in structural quality of the InGaN absorber. Simulations and experimental results are presented for an in depth investigation of the parameters limiting the power conversion efficiency of the solar cell. The semibulk absorber is an elegant solution for the realization of highly efficient InGaN-based PIN heterojunction solar cells.

Role of V-pits in the performance improvement of InGaN solar cells

We study the influence of V-pits on the overall conversion efficiency of bulk In0.12Ga0.88N based heterojunction solar cells grown by MOVPE. We show that V-pits significantly enhances the extraction of the photogenerated carriers in the InGaN absorber, resulting in a peak external quantum efficiency of 79% and a short circuit current density (twice the state of the art) of 2.56 mA/cm−2 under AM 1.5G conditions.

Comparision between Ga- and N-polarity InGaN solar cells with gradient-In-composition intrinsic layers**Project supported by the National Natural Science Foundation of China (Grant Nos. 61306108, 61172131, and 61271377), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China (Grant No. 2013693), and the Anhui Polytechnic University Funds for Excellent Young Scientists, China (Grant No. 2014YQQ005).

Performances of Ga- and N-polarity solar cells (SCs) adopting gradient-In-composition intrinsic layer (IL) are compared. It is found the gradient ILs can greatly weaken the negative influence from the polarization effects for the Ga- polarity case, and the highest conversion efficiency (η) of 2.18% can be obtained in the structure with a linear increase of In composition in the IL from bottom to top. This is mainly attributed to the adsorptions of more photons caused by the higher In composition in the IL closer to the p-GaN window layer. In contrast, for the N-polarity case, the SC structure with an InGaN IL adopting fixed In composition prevails over the ones adopting the gradient-In-composition IL, where the highest η of 9.28% can be obtained at x of 0.62. N-polarity SC structures are proven to have greater potential preparations in high-efficient InGaN SCs.

Ag nanoparticles-embedded surface plasmonic InGaN-based solar cells via scattering and localized field enhancement.

Ag nanoparticles are embedded in intentionally etched micro-circle p-GaN holes by means of a thermal agglomeration process to enhance the light absorption efficiency in InGaN/GaN multi-quantum-well (MQW) solar cells. The Ag nanoparticles are theoretically and experimentally verified to generate the plasmon light scattering and the localized field enhancement near the MQW absorption layer. The external quantum efficiency enhancement at a target wavelength region is demonstrated by matching the plasmon resonance of Ag nanoparticles, resulting in a Jsc improvement of 9.1%. Furthermore, the Ag-nanoparticle-embedded InGaN solar cell is effectively fabricated considering the carrier extraction that more than 70% of F.F. and 2.2 V of high Voc are simultaneously attained.

Analysis of loss mechanisms in InGaN solar cells using a semi-analytical model

InGaN semiconductors are promising candidates for high-efficiency next-generation thin film solar cells. In this work, we study the photovoltaic performance of single-junction and two-junction InGaN solar cells using a semi-analytical model. We analyze the major loss mechanisms in InGaN solar cell including transmission loss, thermalization loss, spatial relaxation loss, and recombination loss. We find that transmission loss plays a major role for InGaN solar cells due to the large bandgaps of III-nitride materials. Among the recombination losses, Shockley-Read-Hall recombination loss is the dominant process. Compared to other III-V photovoltaic materials, we discovered that the emittance of InGaN solar cells is strongly impacted by Urbach tail energy. For two- and multi-junction InGaN solar cells, we discover that the current matching condition results in a limited range of top-junction bandgaps. This theoretical work provides detailed guidance for the design of high-performance InGaN solar cells.

Structural and optical study of core–shell InGaN layers of nanorod arrays with multiple stacks of InGaN/GaN superlattices for absorption of longer solar spectrum

We report on the material and optical properties of core–shell InGaN layers grown on GaN nanorod arrays. The core–shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core–shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer.

Leak path passivation by in situ Al-N for InGaN solar cells operating at wavelengths up to 570 nm

A leak path passivation (LPP) technology for InGaN solar cells with photo-response up to 570 nm was developed by inserting in situ monolayers of Al-N into active layers. The InGaN layer in the passivated sample is partially relaxed and incorporates more than 23.5% In. By adopting in situ Al-N LPP, the open circuit voltage increases from 0.96 V to 1.35 V under one sun illumination (1.45–1.68 V under 72 suns), and the dark shunt resistance increases from 3.6 kΩ cm2 to 12.6 kΩ cm2, leading to an increase in power conversion efficiency by a factor of 2.0–2.26 (1–72 suns). This in situ Al-N LPP approach paves a way to exploit the full potential of InGaN for high efficiency solar cell application, accepting the reality of defective high-In-content thick and relaxed InGaN.

Theoretical analysis of the influence of band tail defects on PIN InGaN solar cells

AbstractInGaN solar cells have higher conversion efficiency than traditional Si based solar cells. However, efficiency of experimental InGaN solar cell is lower than expected. There are still several problems, such as the structure design of InGaN solar cells, the quality of InGaN materials, need to be discussed and solved. Thus, the researches of structure design and defect effects are important for the implications of InGaN solar cells. The performance of p‐GaN/i‐InxGa1–x/n‐GaN solar cells with different In content is simulated by AMPS‐1D. It is found that the optimum efficiency of p‐GaN/i‐InxGa1–xN/n‐GaN solar cells is 8.9% when the In content is 0.26. The p‐GaN/i‐In0.26Ga0.74/n‐GaN solar cells with band tail defects are simulated. The simulated results show the efficiency decreases with the increase of the E‐character. And the efficiency decreases sharply from 8.30% to 7.19% when the capture cross section is 10–14 cm–2. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Proposal of leak path passivation for InGaN solar cells to reduce the leakage current

We propose some general ways to passivate the leak paths in InGaN solar cells and report some experimental evidences of its effectiveness. By adopting an AlOx passivation process, the photovoltaic performances of GaN pn-junctions and InGaN solar cells, grown by molecular beam epitaxy, have been significantly improved. The open circuit voltage under 1 sun illumination increases from 1.46 to 2.26 V for a GaN pn junction, and from 0.95 to 1.27 V for an InGaN solar cell, demonstrating evidence of leak path passivation (LPP) by AlOx. The proposed LPP is expected to be a realistic way to exploit the potential of thick and relaxed but defective InGaN for solar cell applications.

Simulation and optimization of current matching multi-junction InGaN solar cells

In this work, theoretical investigation of the InGaN multi-junction solar cells is carried out. Calculations were performed under 1-sun AM1.5 using the one diode ideal model and respecting rigorously the current matching condition. Tandem cells comprising two, three and four junctions were simulated. Optimum compositions of the sub-cells have been identified and conversion efficiencies of 25.9, 34.7, 39.7 and 42.2 % for one, two, three and four junctions, respectively are obtained.

Improvement of strained InGaN solar cell performance with a heavily doped n+‐GaN substrate

We simulated III–V nitride heterojunction solar cells with a Mg‐doped GaN/intrinsic InGaN/Si‐doped GaN structure. We investigated the threshold dislocation density, the carrier lifetime, and the strain in the InGaN active layer. The conversion efficiency was sensitive to carrier lifetime limited by Shockely–Read recombination rather than the dislocation density. The electric field induced by the piezoelectric effect in the InGaN layer on +c polarity GaN was opposite to that in the depletion region. The recombination of photo‐generated carriers in the p‐GaN/i‐InGaN/ n‐GaN solar cell structure took place dominantly at the i/n interface even at 0 V, and the performance was remarkably degraded. It was found that this negative influence of the piezoelectric effect was reduced by a heavy doping of more than 3 × 1019 cm−3 into the n+‐GaN layer at the interface.

Characterization of InGaN Solar Cells

The III-V materials are extensively studied for optoelectronic applications in the blue and UV spectral regions. InGaN ternary alloy is considered for its wide spectral coverage, good electrical characteristics and appreciable resistance to high electrical currents. For this purpose, the operation of InGaN photovoltaic cells was studied by 2D numerical simulation under AM1.5 spectrum illumination, using the software Silvaco and the two environments Athena/Atlas.

Conversion efficiency in a solar splitting system

In this paper we report on concentrator photovoltaic system made by splitting the solar system based on separate Si, GaAs, and InGaN solar cells. The SSCPV module was fabricated and conversion efficiency up to 24.8% was achieved for the concentration factor of 12.8 that is in correlation with theoretical predictions.

Indium rich InGaN solar cells grown by MOCVD

This study focuses on both epitaxial growths of InxGa1−xN epilayers with graded In content, and the performance of solar cells structures grown on sapphire substrate by using metal organic chemical vapor deposition. The high resolution X-ray and Hall Effect characterization were carried out after epitaxial InGaN solar cell structures growth. The In content of the graded InGaN layer was calculated from the X-ray reciprocal space mapping measurements. Indium contents of the graded InGaN epilayers change from 8.8 to 7.1 % in Sample A, 15.7–7.1 % in Sample B, and 26.6–15.1 % in Sample C. The current voltage measurements of the solar cell devices were carried out after a standard micro fabrication procedure. Sample B exhibits better performance with a short-circuit current density of 6 mA/cm2, open-circuit voltage of 0.25 V, fill factor of 39.13 %, and the best efficiency measured under a standard solar simulator with one-sun air mass 1.5 global light sources (100 mW/cm2) at room temperature for finished devices was 0.66 %.