Photocarrier Collection Research Articles

A simulation study of the micro-grooved electrode structure for back-contact back-junction silicon solar cell

A micro-grooved electrode structure is investigated to illustrate its advantages when applied to the back-contact back-junction (BC–BJ) silicon solar cell. The finite element analysis shows that the micro-grooved electrodes enhances the photo-carrier collection and weakens the dependence of collection ability on pitch distance. The geometries of micro-groove are found to have little impact on the cell performance. These advantages open possibilities for the implementation of low cost fabrication methods. As a demonstration, a process involving laser doping and screen printing techniques are proposed and analyzed. The simulation results show that the laser induced lattice damage causes negligible deterioration of device electrical properties and the presence of parasitic metal insulator semiconductor structure near the screen printed electrodes actually leads to a performance improvement rather than degradation. Our preliminary results indicate that the micro-grooved electrode structure is promising for fabricating low cost high efficiency BC–BJ silicon solar cells.

A First Principles Study on Electronic and Magnetic Properties of Defects in ZnO/GaN Core-shell Nanowire Heterostructures

To date semiconductor nanowire (NW) heterostructures (HS) have attracted extensive attention as important components of electronic and optoelectronic nanodevices. Further NWs also show promising potency to enhance the solar energy harvesting, e.g. improving both light trapping, photo-carrier collection, and contacting surface area. In this work we show theoretically that the \(d^{0}\)-ferromagnetism and NW HS bandgap can be turned by engineering the HS interfaces in non-magnetic ZnO/GaN core/shell NW HS. In that NW HS the incorporation of one compound into the other leads to the bandgap narrowing in the nonisovalent alloy because of the type II band alignment betwwen ZnO and GaN. The \(d^{0}\)-ferromagnetic interface can be developed by creating \(p\)-type defect with \(N\) and/or \(n\)-type defect with Zn in Ga--O interface bonds due to the defect-induced polar discontinuity. It's noted that the GaN/ZnO NW HS itself without defect or with same number defects of both types are not ferromagnetic. So that the induced magnetic moment is suggested to be related to the missing charge introduced at these defects. In our study we focused on the effects of GaN/ZnO interfaces on the electronic and magnetic properties, e.g. interface states within the bandgap and interface-induced ferromagnetism and impact of surface reconstruction and quantum confinement. The origin of this \(d^{0}\)-FM is revealed by analyses of spin-polarized bandstructure indicated by the asymmetrical spin-up and spin-down states near the Fermi level, the projected densities of states (PDOSs) and the spin-polarized mulliken charge differences, indicated that most spin-polarized states are dominated by the interface defect site N$p$ electrons. The calculated GaN/ZnO interface magnetism, have been compared with FM at the LaAlO\(-SrTiO\(_{3}\) interface which are theoretically predicted [30] and experimentally confirmed [31], where the magnetic moments also arise from the polar discontinuity.

High-responsivity mid-infrared graphene detectors with antenna-enhanced photocarrier generation and collection.

Graphene is an attractive photoconductive material for optical detection due to its broad absorption spectrum and ultrashort response time. However, it remains a great challenge to achieve high responsivity in graphene detectors because of graphene's weak optical absorption (only 2.3% in the monolayer graphene sheet) and short photocarrier lifetime (<1 ps). Here we show that metallic antenna structures can be designed to simultaneously improve both light absorption and photocarrier collection in graphene detectors. The coupled antennas concentrate free space light into the nanoscale deep-subwavelength antenna gaps, where the graphene light interaction is greatly enhanced as a result of the ultrahigh electric field intensity inside the gap. Meanwhile, the metallic antennas are designed to serve as electrodes that collect the generated photocarriers very efficiently. We also elucidate the mechanism of photoconductive gain in the graphene detectors and demonstrate mid-infrared (mid-IR) antenna-assisted graphene detectors at room temperature with more than 200 times enhancement of responsivity (∼0.4 V/W at λ0 = 4.45 μm) compared to devices without antennas (<2 mV/W).

Fabrication and enhanced light-trapping properties of three-dimensional silicon nanostructures for photovoltaic applications

AbstractIn order to make photovoltaics an economically viable energy solution, next-generation solar cells with higher energy conversion efficiencies and lower costs are urgently desired. Among many possible solutions, three-dimensional (3D) silicon nanostructures with excellent light-trapping properties are one of the promising candidates and have recently attracted considerable attention for cost-effective photovoltaic applications. This is because their enhanced light-trapping characteristics and high carrier collection efficiencies can enable the use of cheaper and thinner silicon materials. In this review, recent developments in the controllable fabrication of 3D silicon nanostructures are summarized, followed by the investigation of optical properties on a number of different nanostructures, including nanowires, nanopillars, nanocones, nanopencils, and nanopyramids, etc. Even though nanostructures with radial p-n junction demonstrate excellent photon management properties and enhanced photo-carrier collection efficiencies, the photovoltaic performance of nanostructure-based solar cells is still significantly limited due to the high surface recombination effect, which is induced by high-density surface defects as well as the large surface area in high-aspect-ratio nanostructures. In this regard, various approaches in reducing the surface recombination are discussed and an overall geometrical consideration of both light-trapping and recombination effects to yield the best photovoltaic properties are emphasized.

Toward Efficient and Omnidirectional n-Type Si Solar Cells: Concurrent Improvement in Optical and Electrical Characteristics by Employing Microscale Hierarchical Structures

We demonstrated that hierarchical structures combining different scales (i.e., pyramids from 1.5 to 7.5 μm in width on grooves from 40 to 50 μm in diameter) exhibit excellent broadband and omnidirectional light-trapping characteristics. These microscaled hierarchical structures could not only improve light absorption but prevent poor electrical properties typically observed from nanostructures (e.g., ultra-high-density surface defects and nonconformal deposition of following layers, causing low open-circuit voltages and fill factors). The microscaled hierarchical Si heterojunction solar cells fabricated with hydrogenated amorphous Si layers on as-cut Czochralski n-type substrates show a high short-circuit current density of 36.4 mA/cm(2), an open-circuit voltage of 607 mV, and a conversion efficiency of 15.2% due to excellent antireflection and light-scattering characteristics without sacrificing minority carrier lifetimes. Compared to cells with grooved structures, hierarchical heterojunction solar cells exhibit a daily power density enhancement (69%) much higher than the power density enhancement at normal angle of incidence (49%), demonstrating omnidirectional photovoltaic characteristics of hierarchical structures. Such a concept of hierarchical structures simultaneously improving light absorption and photocarrier collection efficiency opens avenues for developing large-area and cost-effective solar energy devices in the industry.

Performance improvement of flexible bulk heterojunction solar cells using PTB7:PC71BM by optimizing spin coating and drying processes

Bulk-heterojunction solar cells were fabricated using a dichlorobenzene solution of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) on a flexible indium–tin-oxide-coated polyethylene terephthalate substrate. It was found that the performance of the solar cells could be markedly improved by minimizing the spin coating time of a blend of PTB7 and PC71BM to 10 s and maximizing the successive drying and solidification time up to 30 min in a confined Petri dish. As a result, a short-circuit current density of 14.5 mA/cm2, an open-circuit voltage of 0.62 V, and a power conversion efficiency of 3.67% were obtained. These improvements are attributed to the growth of favorable nanostructures during the slow drying process that increased the photocarrier collection efficiency while simultaneously increasing the performance fluctuations of each device.

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In1‐x,Gax)S2 (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.

Determining diode parameters of illuminated current density-voltage curves considering the voltage-dependent photo current density

Suitable procedures for the determination of diode parameters of illuminated and dark current density-voltage ( j - V ) curves of solar cells are investigated. Within these procedures we avoid misinterpretations by the commonly used shifting approximation which assumes a voltage-independent photo carrier collection. To this end, the voltage-dependent collection efficiency is determined by j - V measurements under incremental illumination. After that, illuminated curves can be converted into dark curves taking the voltage-dependent photo current density into account. The converted dark curve next is consigned into a program that numerically solves the implicit equation of the current density and derives the diode parameters by means of a weighted least-squares minimization. Only if fluctuations of the illumination are noticeably high this procedure cannot be used, since the converted dark curve also reveals a high noise level. Then, the determination of diode parameters is performed stepwise with the help of commonly used methods.

Probing photo-carrier collection efficiencies of individual silicon nanowire diodes on a wafer substrate.

Vertically aligned silicon nanowire (SiNW) diodes are promising candidates for the integration into various opto-electronic device concepts for e.g. sensing or solar energy conversion. Individual SiNW p-n diodes have intensively been studied, but to date an assessment of their device performance once integrated on a silicon substrate has not been made. We show that using a scanning electron microscope (SEM) equipped with a nano-manipulator and an optical fiber feed-through for tunable (wavelength, power using a tunable laser source) sample illumination, the dark and illuminated current-voltage (I-V) curve of individual SiNW diodes on the substrate wafer can be measured. Surprisingly, the I-V-curve of the serially coupled system composed of SiNW/wafers is accurately described by an equivalent circuit model of a single diode and diode parameters like series and shunting resistivity, diode ideality factor and photocurrent can be retrieved from a fit. We show that the photo-carrier collection efficiency (PCE) of the integrated diode illuminated with variable wavelength and intensity light directly gives insight into the quality of the device design at the nanoscale. We find that the PCE decreases for high light intensities and photocurrent densities, due to the fact that considerable amounts of photo-excited carriers generated within the substrate lead to a decrease in shunting resistivity of the SiNW diode and deteriorate its rectification. The PCE decreases systematically for smaller wavelengths of visible light, showing the possibility of monitoring the effectiveness of the SiNW device surface passivation using the shown measurement technique. The integrated device was pre-characterized using secondary ion mass spectrometry (SIMS), TCAD simulations and electron beam induced current (EBIC) measurements to validate the properties of the characterized material at the single SiNW diode level.

Effects of AlN thickness on structural and transport properties of In-rich n-AlInN/AlN/p-Si(0 0 1) heterojunctions grown by magnetron sputtering

We have studied the effect of AlN buffer layers, having variable thicknesses in situ grown by rf-magnetron sputtering, on the structural and vibration dynamic properties of AlInN thin films grown by dc-magnetron sputtering on Si(0 0 1) substrates at 700 °C. X-ray diffraction patterns reveal that an increase in thickness of the AlN buffer layer can significantly promote the textured growth orientation of AlInN(0 0 0 2)//Si(0 0 1) and improve the grain sizes of AlInN. They also reveal that the Al composition in the resultant AlInN thin film increases with the thickness of AlN; however, phase separations, which are usually observed in metal-organic chemical vapour deposition (MOCVD) growth of AlInN with similar Al compositions, are not detectable. Instead, uniform element distribution profiles along the growth direction of AlInN are detected by secondary ion-mass spectroscopy. Micro-Raman scattering spectra collected at room temperature exhibit three optical phonon features from the AlInN thin films, which, in terms of their evolutions with Al composition, can be assigned to AlInN , InN-like A1(LO), and AlN-like A1(LO), respectively, consistent with that recently observed in MOCVD grown In-rich AlInN. The solar cell based on n-AlInN/AlN/p-Si exhibits a typical diode-like dark I–V curve; however, its photocarrier collection is apparently suppressed due to the polarization charges at the junction interfaces.

Relationship between the structural and electronic properties of n-GaAs layer grown on rough Si substrate by molecular beam epitaxy

The relationship between the electronic and structural quality of n–GaAs layer grown by Molecular Beam Epitaxy (MBE) on rough silicon surface substrate (RSi) was investigated by Reflection High–Energy Electron Diffraction (RHEED), Time–Resolved Photoluminescence (TRPL), High–Resolution X–Ray Diffractometer (HRXRD) and photocurrent spectroscopy. The results demonstrate a high correlation between the defect density and the minority carrier lifetime, showing that the defect density is a strong cause for trapping the minority carriers. Atomic Force Microscopy (AFM) and HRXRD analysis show a flattening of GaAs film on rough Si substrate. The minority carrier lifetime is enhanced when the GaAs layer is grown on RSi. This enhancement was explained by the minimisation of the defect density when using the rough surface. The improvement of minority carrier lifetime is at the origin of enhancement of photo–carrier collection.

Performance comparison of front- and back-illuminated AlGaN-based metal–semiconductor–metal solar-blind ultraviolet photodetectors

In this work, AlGaN-based metal–semiconductor–metal solar-blind ultraviolet photodetectors (PDs) with low dark current were fabricated on sapphire substrates. In both front- and back-illumination operation modes, the PDs exhibited sharp photoresponse cutoffs at ∼280 nm with solar-blind/ultraviolet rejection ratios of more than 103. The quantum efficiency of the back-illuminated PD was observed to generally be higher than that of the front-illuminated PD. Nevertheless, at very low bias range, the front-illuminated PD exhibited greater photoresponsivity. The observed performance differences of the PDs in the different illumination modes are discussed in terms of surface reflectivity and photocarrier collection efficiency.

Role of hot carriers in the interfacial transport in amorphous silicon/crystalline silicon heterostructure solar cells

AbstractThe transport of photogenerated minority carriers (photocarriers) across the heterointerface of amorphous silicon (a‐Si) and crystalline silicon (c‐Si) in a‐Si/c‐Si heterostructure solar cell is shown in this work to critically depend on the non‐Maxwellian energy distribution function (EDF) of those carriers impinging on the heterointerface. A theoretical model is presented that integrates the effect of the high electric field inversion region upon EDF of the impinging carriers with the transmission probability of those carriers across the heterointerface. The transport of the photocarriers across the high electric field inversion region is simulated by the full solution of the Boltzmann transport equation by Monte Carlo (MC) technique while the transmission probability of carriers across the heterointerface is calculated through the percolation path technique. The results are discussed under two different condition of band bending; strongly inverted and weakly inverted c‐Si surface. The results comparing different conditions of band bending show that the energy distribution of the carriers impinging on the heterointerface is non‐Maxwellian and the integrated photocarrier collection increases with the strength of the inversion field since the carrier population is weighted toward higher energy where the transmission probability through the barrier is higher. Thus, we demonstrate that hot carriers play an important role in heterostructure cell operation.

All‐Inorganic Colloidal Quantum Dot Photovoltaics Employing Solution‐Phase Halide Passivation

A new solution-phase halide passivation strategy to improve the electronic properties of colloidal quantum dot films is reported. We prove experimentally that the approach leads to an order-of-magnitude increase in mobility and a notable reduction in trap state density. We build solar cells having the highest efficiency (6.6%) reported using all-inorganic colloidal quantum dots. The improved photocurrent results from increased efficiency of collection of infrared-generated photocarriers.

Compensation of Native Defect Acceptors in Microcrystalline Ge and Si1-xGex Thin Films by Oxygen Incorporation: Electrical Properties and Solar Cell Performance

Undoped hydrogenated microcrystalline Ge (µc-Ge:H) films grown by plasma-enhanced chemical vapor deposition reveal high concentration of free holes (>1018 cm-3) when the films exhibit a high crystalline volume fraction. ESR and Hall-effect experiments suggest that the acceptor states arise from the native dangling bond defects at Ge crystalline grain boundaries. It is demonstrated that an intentional oxygen incorporation during the µc-Ge:H deposition reduces the hole concentration by two orders of magnitude. Furthermore, µc-Si1-xGex:H (x=0.1–0.3) alloy p–i–n solar cells show marked improvements in photocarrier collection properties upon oxygen incorporation into the i-layer in the order of 5×1018–1020 cm-3. These findings are explained by the effect of the compensation of the negatively charged Ge dangling bonds by oxygen donors.

Photovoltaic properties of PSi impregnated with eumelanin.

A bulk heterojunction of porous silicon and eumelanin, where the columnar pores of porous silicon are filled with eumelanin, is proposed as a new organic-inorganic hybrid material for photovoltaic applications. The addition of eumelanin, whose absorption in the near infrared region is significantly higher than porous silicon, should greatly enhance the light absorption capabilities of the empty porous silicon matrix, which are very low in the low energy side of the visible spectral range (from about 600 nm downwards). The experimental results show that indeed the photocarrier collection efficiency at longer wavelengths in eumelanin-impregnated samples is clearly higher with respect to empty porous silicon matrices.

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3 months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.

Exploration of nano-element array architectures for substrate solar cells using an a-Si:H absorber

Architectures involving Ag and transparent conducting oxide (TCO) nano-element arrays for light and photocarrier collection management in substrate solar cells are numerically explored and compared. Some architectures with TCO nano-elements are shown to perform better than the best reported Ag arrays and (1) increase JSC at least 57% over that of a planar 200 nm a-Si:H control, (2) attain absorber utilization &amp;lt;7 mg/W, and (3) have only 224 nm as the longest collection length. Photonic effects are the cause of the light trapping enhancement in these devices. While the computations were done for a-Si:H, the insight provided is equally applicable to other absorbers.

Amorphous Si Rear Schottky Junction Solar Cell With a LiF/Al Back Electrode

Amorphous Si (a-Si) rear Schottky junction solar cells with a LiF/Al back electrode are proposed as an alternative prototype for high-efficiency thin-film photovoltaics. This device is free from absorption losses occurring at the rear n-type a-Si layer, and thus, overall power conversion efficiency was improved by 13% compared with a conventional p-i-n type solar cell. An ultrathin LiF layer between the absorber and the rear electrode reduces shunt leakage, as well as series resistance; this, in turn, suppresses degradation of the open-circuit voltage and the fill factor while enhancing photocarrier collection in the long-wavelength regime.

Incorporation of a light and carrier collection management nano-element array into superstrate a-Si:H solar cells

Superstrate a-Si:H solar cells incorporating a nano-column array for light and photocarrier collection have been fabricated and evaluated. It is found that the short circuit current density (JSC) is significantly increased while the open circuit voltage and fill factor are not detrimentally affected by this architecture. Numerical analysis of JSC matches experiment and shows that the enhanced JSC observed is due to both effective absorber thickness and photonic-plasmonic effects. Further analysis shows that this nano-column architecture can lead to a 42% increase in conversion efficiency over that of the planar control for a 200 nm absorber thickness cell.