InP Solar Cells Research Articles

Enhanced Near‐Bandgap Response in InP Nanopillar Solar Cells

The effect of nanopillar texturing on the performance of InP solar cells is investigated. Maskless, lithography‐free reactive ion etching of InP nanopillars improves the open‐circuit voltage, reduces reflectance over a broad spectral range, and enhances the near‐bandgap response compared to a flat, non‐textured cell with comparable reflectance in the infrared. Electron‐beam induced current measurements indicate an increased effective minority carrier collection length. The response at short wavelengths decreases due to the formation of a defective surface layer with strong non‐radiative recombination. Plasma oxidation and wet etching partially restore the blue response resulting in a power conversion efficiency of 14.4%.

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

AbstractGaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the

First principle analyses of direct bandgap solar cells with absorbing substrates versus mirrors

Direct bandgap InP, GaAs, CdTe, and Ga0.5In0.5P solar cells containing backside mirrors as well as parasitically absorbing substrates are analyzed for their limiting open circuit voltage and power conversion efficiency with comparison to record solar cells. From the principle of detailed balance, it is shown quantitatively that mirror solar cells have greater voltage and power conversion efficiency than their substrate counterparts. Next, the radiative recombination coefficient and maximum radiative lifetime of GaAs mirror and substrate solar cells are calculated and compared to the nonradiative Auger and Shockley-Read-Hall (SRH) lifetimes. Mirror solar cells have greater radiative lifetime than their substrate variants. Auger lifetime exceeds radiative lifetime for both substrate and mirror cells while SRH lifetime may be less or greater than radiative lifetime depending on trap concentration and capture cross section. Finally, the change in free energy of the photogenerated carriers is analyzed in a comparison between InP, GaAs, CdTe, and Ga0.5In0.5P mirror and substrate solar cells in order to characterize the relationship between solar photon quality and free energy management in solar cells with differing bandgaps. Wider bandgap visible threshold Ga0.5In0.5P solar cells make better use of the available change in free energy of the photogenerated charge carriers, even when normalized to the bandgap energy, than narrower bandgap near-IR threshold InP, GaAs, and CdTe solar cells.

Numerical Simulation of Nc-Silicon PIN Solar Cells with AMPS-1D

P+a-SiC/ I nc-Si/N+a-Si structure solar cells is simulated by AMPS-1D program package to characterize the new thin film solar cell. In order to analyze the characteristics of the device, the thickness of layer are considered. The results show that the thickness of layer and the value of layer have a great effect on the conversion efficiency. Our results suggest a high performance P a-SiC/ I nc-Si/N a-Si structure solar cells with high efficiency of 14.411% and fill factor of 0.738. The simulation result is potentially valuable in exploring gradual bandgap P+a-SiC/I nc-Si/N+a-Si structure solar cells with high performance.

Real-time transmission Mueller polarimetry on hydrogenated polymorphous silicon under current injection

We report on the use of an innovative optical characterization technique—real-time Mueller polarimetric imaging in transmission—for the characterization of thin-film silicon solar cells. In this work, we used this technique to monitor the evolution of optical retardance induced by the mechanical stresses in hydrogenated amorphous and polymorphous silicon (a-Si : H and pm-Si : H) p–i–n (PIN) solar cells. Under current injection of 200 mA cm−2, the retardance of the pm-Si : H PIN solar cells decreased, while that of the a-Si : H PIN solar cells showed no significant change. After the current injection, the pm-Si : H PIN solar cells showed dramatic macroscopic changes on a scale of tens of micrometres, such as local peel-off and delamination from the substrate. Our results demonstrate that current injection introduces local stress relaxation, which can be efficiently monitored prior to irreversible damage from a decrease in the retardance of the pm-Si : H PIN solar cells.

Change in the electrical performance of GaAs solar cells with InGaAs quantum dot layers by electron irradiation

The radiation response of GaAs PiN solar cells with 50 stacked layers containing self-aligned In0.4Ga0.6As quantum dots (QDs) was compared to that of GaAs PiN solar cells with non QD layers. Those solar cells were radiated with 1MeV electrons up to 1×1016/cm2, and the change in their electrical characteristics under AM 0 was investigated using an in-situ measurement technique. After electron irradiation at 1×1016/cm2, the value of open circuit voltage (VOC) for the In0.4Ga0.6As 50 QD and the non QD solar cells remained 90 and 80% of the initial value, respectively. On the other hand, the values of short circuit current (ISC) and maximum power (PMAX) for the 50 QD solar cells became approximately 80 and 55% of the initial value by electron irradiation at 1×1016/cm2, respectively, although the non QD solar cells showed 95% for ISC and 63% for PMAX after the irradiation. The recovery of the electrical characteristics of both the InGaAs 50 QD and the non QD solar cells irradiated with electrons at 1×1016/cm2 were observed under AM0 light illumination at RT.

Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Texturing the glass surface is a promising method for improving the light trapping properties of superstrate thin-film silicon solar cells, as it enables thinner absorber layers and, possibly, higher cell efficiencies. In this paper we present the optical and morphological properties of borosilicate glass superstrates textured with the aluminium induced texture (AIT) method. High haze values are achieved without any reduction in the total optical transmission of the glass sheets after the AIT process. Scanning electron microscope and atomic force microscope (AFM) measurements reveal a laterally uniform surface morphology of the AIT texture. We demonstrate that the surface roughness and thus the transmission haze can be controlled by adjusting the AIT process parameters. From the AFM images, we extract histograms of the local height and angle distributions of the texture. Samples with a wide angle distribution are shown to produce the highest optical haze. The results of this analysis provide a better understanding of the correlation between the AIT process parameters and the resulting surface morphology. This analysis is further extended to an amorphous silicon pin solar cell deposited onto the textured glass substrate.

Influence of the electro-optical properties of an α-Si:H single layer on the performances of a pin solar cell

We analyze the results of an extensive characterization study involving electrical and optical measurements carried out on hydrogenated amorphous silicon (α-Si:H) thin film materials fabricated under a wide range of deposition conditions. By adjusting the synthesis parameters, we evidenced how conductivity, activation energy, electrical transport and optical absorption of an α-Si:H layer can be modified and optimized. We analyzed the activation energy and the pre-exponential factor of the dark conductivity by varying the dopant-to-silane gas flow ratio. Optical measurements allowed to extract the absorption spectra and the optical bandgap. Additionally, we report on the temperature dependence of the activation energy to satisfy the Meyer–Neldel rule. Finally, the influence of the individual films parameters upon the final performances of a single junction pin α-Si:H have been studied. The measurements show how a more than doubled enhancement in energy conversion efficiency can be obtained in an α-Si:H solar cell with a proper selection of synthesis conditions.

Boron-doped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) for application in thin-film silicon solar cells

We report on the development of p-type μc-SiOx:H material, in particular the relationship between the deposition parameters and the material properties like band gap, electrical conductivity, and crystalline volume fraction. The material was deposited from gas mixtures of silane, carbon dioxide and hydrogen by RF-PECVD. The gas flows were varied systematically to evaluate their influence on the material properties. An increase of the oxygen content in the material disturbs the crystalline growth. This can be counteracted by appropriate hydrogen dilutions. Materials with a combination of reasonably high conductivity of 4×10−6 S/cm at a high optical band gap E04 of 2.56eV and a refractive index of 1.95 are obtained. Applied in single junction μc-Si:H pin solar cells the improved properties of the μc-SiOx:H p-layers are reflected in higher quantum efficiency in the short wavelength range by 10% compare to cells without adding CO2 during p-layer deposition.

Light Induced Changes in PIN Solar Cells: Beyond the Staebler-Wronski Effect

ABSTRACTHydrogenated polymorphous silicon (pm-Si:H) is one of the most promising candidates for a stable top cell material in multi-junction thin film solar cells. Solar cells using pm-Si:H as their absorbing layer show very interesting degradation kinetics when compared to hydrogenated amorphous silicon (a-Si:H), summarized by macroscopic structural changes and irreversible changes in solar cell characteristics, while nevertheless preserving a higher stabilized efficiency. Notably, pm-Si:H solar cells, once degraded, respond to neither annealing nor further light-soaking. Such results suggest a device degradation mechanism including structural changes, active hydrogen motion, and interface delamination mediated by fast hydrogen diffusion and accumulation at the interface. Interestingly, a similar behavior was reported for a-Si:H solar cells under severe light soaking conditions (at 350 °C or under 50 suns) while pm-Si:H solar cells show such behavior under 1 sun at 40 °C.

Light induced electrical and macroscopic changes in hydrogenated polymorphous silicon solar cells

We report on light-induced electrical and macroscopic changes in hydrogenated polymorphous silicon (pm-Si:H) PIN solar cells. To explain the particular light-soaking behavior of such cells - namely an increase of the open circuit voltage (Voc ) and a rapid drop of the short circuit current density (Jsc )- we correlate these effects to changes in hydrogen incorporation and structural properties in the layers of the cells. Numerous techniques such as current-voltage characteristics, infrared spectroscopy, hydrogen exodiffusion, Raman spectroscopy, atomic force microscopy, scanning electron microscopy and spectroscopic ellipsometry are used to study the light-induced changes from microscopic to macroscopic scales (up to tens of microns). Such comprehensive use of complementary techniques lead us to suggest that light-soaking produces the diffusion of molecular hydrogen, hydrogen accumulation at p-layer/substrate interface and localized delamination of the interface. Based on these results we propose that light-induced degradation of PIN solar cells has to be addressed from not only as a material issue, but also a device point of view. In particular we bring experimental evidence that localized delamination at the interface between the p-layer and SnO2 substrate by light-induced hydrogen motion causes the rapid drop of Jsc.

An optimal solar cell for under water measurement

For the purpose of under water solar radiation measurement, a computational work has been performed to determine the optimal solar cell for this measurement. This work covers GaAs, CdTe, a-Si, InP, multi-Si, GaInP and mono-Si solar cells. The work involves the prediction of the total photocurrent and the efficiency of each cell at various depths below the water surface.

Electrical properties of stacking electrodes for flexible crystalline semiconductor photonic devices

We report here electrical properties of low-temperature-stacked electrodes for large-area flexible photonic devices, based on single-crystalline InP nanomembrane (NM) transfer and stacking processes. Au, Al and ITO electrodes were investigated. An excellent ohmic contact was demonstrated on the stacked InP NM–ITO electrode, with a measured contact resistivity of 0.45 Ω cm2. Two types of flexible InP solar cells were also fabricated and characterized, based on the stacked InP NM–ITO and InP NM–Al contacts, respectively. The efficiency of solar cells with ITO as back contact is five times higher than that with Al as back contact. Such low-temperature energy-efficient NM transfer and electrode-stacking techniques can be applied to a wide range of flexible thin film photonic devices.

Excimer laser annealing and chemical texturing of ZnO:Al sputtered at room temperature for photovoltaic applications

We demonstrate the effect of excimer (XeCl=308 nm) laser annealing on thin films of ZnO:Al deposited by RF sputtering at room temperature. The as-deposited films have good sheet resistance (<11 Ω/□) but poor transparency, and a subsequent chemical etching step using dilute HCl to texture the film surface results in a level of haze ineffective for light-trapping in thin film photovoltaic cells. Excimer laser annealing at the optimized fluence (single pulses of 0.5–0.7 J/cm 2) improves the film transparency, particularly through a blue-shift in the band-gap, without significantly impacting the conductivity. More importantly, chemical etching of these laser annealed films results in textured films with controllable spectral distributions of haze. We demonstrate the enhanced optical properties (transmission and haze) after laser annealing and etching the ZnO:Al films through the fabrication of hydrogenated microcrystalline silicon pin solar cells, and show a significant improvement in the photocurrent density (up to 2.2 mA/cm 2) for the optimally annealed substrates—particularly at wavelengths greater than 600 nm (up to 1.7 mA/cm 2) where light-trapping is important.

Study of the effects of different fractions of large grains of μc-Si:H:F films on the infrared absorption on thin film solar cells

We have studied microcrystalline silicon films (μc-Si:H:F) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) at low temperature (∼200°C) from SiF4, H2 and Ar gas mixtures. We found that the H2/SiF4 ratio plays an important role on the crystalline fraction of the films, specifically on the films grain size, deduced from UV–vis spectroscopic ellipsometry. We observed that higher crystallinity (and larger fraction of large-grains) in the films provides a higher absorption on the infrared (IR) region, which is of particular interest for tandem solar cells. PIN solar cells were fabricated with intrinsic μc-Si:H:F films of different crystallinity (and different fraction of large grains) in order to observe their effect on the solar cells IR absorption and on the cells performance characteristics.

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.

Improved power conversion efficiency of InP solar cells using organic window layers

We employ the organic semiconductor 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) as a nanometer thick window layer for p-InP/indium tin oxide (ITO) Schottky barrier diode solar cells. The power conversion efficiency is enhanced compared to ITO/InP cells lacking the PTCDA window layer, primarily due to neutralizing InP surface state charges via hole injection from the PTCDA. This leads to an increased ITO/p-InP Schottky barrier height, and hence to an increased open circuit voltage. The power conversion efficiency of the cells increases from 13.2±0.5% for the ITO/InP cell to 15.4±0.4% for the ITO/4 nm PTCDA/p-InP cell under 1 sun, AM1.5G simulated solar illumination. The PTCDA window layer is also shown to contribute to the photocurrent by light absorption followed by exciton dissociation at the organic/inorganic semiconductor interface.

Numerical Simulations for the Efficiency Improvement of Hybrid Dye-microcrystalline Silicon pin-solar cells

ABSTRACTIn hybrid solar cells consisting of dye sensitizers incorporated in the i-layer of a microcrystalline silicon (μc-Si:H) pin solar cell the dye sensitizer molecules are embedded in the matrix and enhance the overall absorption of the dye-matrix system due to their high absorption coefficient in the spectral range interesting for photovoltaic applications. This contribution investigates the efficiency improvement of hybrid dye-μc-Si:H solar cells compared to pure μc-Si:H solar cells by simulation. The results indicate that, under optimum conditions, the efficiency can be improved by more than a factor of 1.2 compared to a pure μc-Si:H cell. The thickness reduction for the hybrid system can be as large as 50 % for the same efficiency. However, the efficiency improvement also depends on the amount of additionally induced defects in the matrix by the embedded dye molecules. Therefore, the simulations investigate the performance of the hybrid solar cell for different absorption enhancements and defect densities.

Performance of amorphous and microcrystalline silicon pin solar cells under variable light intensity

AbstractWe have studied the solar cell behaviour under variable light intensity (VLI) of a standard a‐Si:H pin solar cell with a wide band gap a‐SiC:H emitter layer, and microcrystalline (μc)‐Si:H solar cells of different degrees of crystallinity, using experiments in conjunction with detailed electrical‐optical modelling. Both experiments and modelling reveal that whereas the fill factor (FF) of the a‐Si:H pin cell decreases with increasing light intensity, starting from a low applied light bias, that of the μc‐Si:H cells increases with light intensity over a major part of this range. This fact enables one to attain the maximum of the open‐circuit voltage ‐ fill factor product (Voc x FF) at 1 to 2 suns intensity for the latter case; however this is not achieved for the a‐Si:H cell. Using modelling we try to understand this difference in behaviour of the FF under VLI for the two types of cells. We also predict under what conditions it would be possible to shift the (Voc x FF)max for the a‐Si:H cell towards one sun intensity. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Temperature dependence of pin solar cell parameters with intrinsic layers made of pm-Si:H and low crystalline volume fraction μc-Si:H

A comparison of the temperature dependence of the IV characteristics parameters of hydrogenated silicon pin solar cells with intrinsic layers made of polymorphous silicon (pm-Si:H) and of μc-Si:H with low crystalline volume fraction has been performed. When using pm-Si:H, higher efficiency and higher filling factors are achieved over a wide temperature range. Diode quality factors of both types of cells show similar temperature dependence. Recombination processes over the whole intrinsic layer dominates the forward current. A change of the cell parameters under illumination is also observed. The transport mechanism of both cells is similar in the temperature range that is important for most applications. Due to its optical and transport properties, pm-Si:H poses a very interesting alternative to μc-Si:H and a-Si:H in the temperature range of normal terrestrial applications.