Rm Sc Research Articles

Efficiency Enhancement of Organic/GaAs Hybrid Photovoltaic Cells Using Transparent Graphene as Front Electrode

Large-area graphene of high quality and uniformity was successfully grown by chemical vapor deposition (CVD) using surface oxidation treatment of copper foil prior to the graphene growth step. The graphene was transferred to the polyethylene terephthalate (PET) substrate (G/PET) to act as a transparent front electrode of hybrid heterojunction photovoltaic (PV) cells; these cells were based on a structural motif of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, GaAs (1 0 0) as the inorganic n-type semiconductor, and a silver comb electrode atop of PEDOT:PSS. By using G/PET as a transparent front electrode, the power conversion efficiency (PCE), under simulated AM1.5G illumination conditions, was greatly enhanced by up to 26% (from 6.85% to 8.60%). All PV characteristics, including open-circuit voltage ( $V_{{\rm oc}}$ ), short-circuit current ( $J_{{\rm sc}}$ ), and fill factor (FF), contributed to this PCE enhancement. The reflectance, external quantum efficiency, and dark current were investigated to explain this observed PCE enhancement. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance in multilayer cells resulted in lower short-circuit current density, leading to lower PCE, in comparison with those with only one layer of graphene.

High-Efficiency Planar Thin-Film Si/PEDOT:PSS Hybrid Solar Cell

We study planar Si/PEDOT:PSS hybrid solar cells fabricated using silicon thin films of various thicknesses through a simple spin-coating process. The effects of the Si absorber thickness ( T ) on the performance of the hybrid cells are investigated. Average power conversion efficiencies (PCE) of 5.0%, 6.2%, 7.7%, and 8.2% have been achieved for T of 2.2, 5.5, 10.6, and 15.5 μm, respectively. It is found that the hybrid cell with T = 15.5 μm is able to generate a large short-circuit current density ( $J_{{\rm sc}}$ ) of 20.4 mA/cm2, which is about 78% of that measured for a reference bulk Si cell. Optical simulation is also performed to theoretically investigate the light harvesting characteristics of the hybrid cells. It is found that $J_{{\rm sc}}$ increases with T and saturates at about $T = 10{\rm -}15$ μm. By adopting a cell structure where the junction is situated at the back to minimize optical loss in PEDOT:PSS, a $J_{{\rm sc}}$ of 30.3 mA/cm2 can be achieved. It is predicted that the simple planar Si/PEDOT:PSS cell using only 15-μm-thick of Si thin film, without the incorporation of nanostructures for light trapping, can deliver a PCE of over 13%.

GaAs Solar Cells Grown by Hydride Vapor-Phase Epitaxy and the Development of GaInP Cladding Layers

The high cost of high-efficiency III–V photovoltaic devices currently limits them to niche markets. Hydride vapor-phase epitaxy (HVPE) growth of III–V materials recently reemerged as a low-cost, high-throughput alternative to conventional metal–organic vapor-phase epitaxy (MOVPE) growth of high-efficiency solar cells. Previously, we demonstrated unpassivated HVPE-grown GaAs p-n junctions with good quantum efficiency and high open-circuit voltage $(V_{{\rm oc}})$ . In this work, we demonstrate the growth of GaInP by HVPE for use as a high-quality surface passivation layer to GaAs solar cells. Solar cells grown with GaInP window layers show significantly improved quantum efficiency compared with unpassivated cells, increasing the short-circuit current $(J_{{\rm SC}})$ of these low-cost devices. These results show the potential of low-cost HVPE for the growth of high-quality III–V devices.

Ion-Implanted Screen-Printed n-Type Solar Cell With Tunnel Oxide Passivated Back Contact

This paper shows the results and the limitations of a 21% N-Cz 239-cm2 screen-printed cell with blanket p+ emitter and n+ back surface field. In addition, we show the properties and impact of tunnel oxide capped with doped n+ polysilicon and metal on the back side, which can overcome those limitations. Since both the doped n+ layer and the metal contact are outside the bulk silicon wafer, the Jo is dramatically reduced, resulting in much higher $V_{{\rm oc}}$ . Process optimization has resulted in high $iV_{{\rm oc}}$ of 728 mV on symmetric structures. The unmetallized cell structure with Al2O3/SiN passivated lightly doped p+ emitter and a tunnel oxide/n+ poly back also gave high $iV_{{\rm oc}}$ of 734 mV. The finished screen-printed 132-cm2 device gave a $V_{{\rm oc}}$ of 683 mV, $J_{{\rm sc}}$ of 39.4 mA/cm2, FF of 77.6%, and an efficiency of 20.9%. Cell analysis show that implementation of a selective emitter can give higher efficiency.

Nonuniformity Sunlight-Irradiation Effect on Photovoltaic Performance of Concentrating Photovoltaic Using Microsolar Cells Without Secondary Optics

We propose a compact 20-mm-thick concentrating photovoltaic (CPV) module that is much thinner than conventional CPV modules. To achieve this structure, we assembled a CPV module by placing microsolar cells with an area smaller than 1 mm2 directly on the back of a lens array. Since our CPV module has no secondary optics, the incident sunlight is focused through the lens array onto the solar cell so that the light intensity is nonuniformly distributed due to chromatic and comatic aberration. In this paper, we analyze the dependence of the short current ( $I_{{\rm sc}}$ ) and the fill factor of our CPV module on the area, the position, and the intensity distribution of the irradiated sunlight on the solar cell. Our results indicate that the main factor that affects the PV performance of our CPV module is not the sunlight's nonuniform irradiation in the three-junction subcells due to chromatic and comatic aberration but the resistance loss between the emitter layer of the top cell and the grid electrodes.

PbS/ZnO Heterojunction Colloidal Quantum Dot Photovoltaic Devices by a Room Temperature Air-Spray Method

Colloidal quantum-dot-based photovoltaic devices (CQDPVs) were fabricated at room temperature in air atmosphere via a spraying technique. Lead sulfide colloidal quantum dots (CQDs) were utilized for this process and various fabrication conditions such as the spraying pressure, types of ligand molecules, duration of ligand exchange, and the band-gap of the CQDs were investigated in order to optimize the device performance. The power conversion efficiency reached 4.00% ( $V_{{\rm OC}}$ of 0.57 V, $J_{{\rm SC}}$ of 11.79 mA·cm−2, and FF of 0.60) when ∼145 nm thick sprayed CQD layers were utilized; this value is comparable to that achieved with the conventional spin-coated devices. The generality of the conditions used for fabrication of the sprayed CQDPVs was demonstrated in the fabrication of various CQDs having different band-gaps (1.34–1.61 eV). This technique provides an avenue for the application of a high-throughput process for CQDPV fabrication. Because the materials used herein for device fabrication are not completely optimized, there is further scope for improving device performance.

Development of Dye-Sensitized Solar Cells Based on Gold/Gelatin Gel Electrolyte: Effect of Different Aspect Ratio of Gold Nanocrystals

This study describes the application of gold/gelatin (Au/gelatin) gel electrolyte to a set of quasi-solid-state dye-sensitized solar cells (DSSCs). A set of DSSC based on Au/gelatin gel electrolyte was developed. Au nanocrystals of different aspect ratio were synthesized and immobilized in gelatin gel electrolyte. The structures of the synthesized nanocrystals and nanocomposite gel were confirmed by using Fourier transform infrared spectroscopy, a scanning electron microscopy energy-dispersive X-ray, and transmission electron microscopy analysis. The thermogravimetric analysis indicated that introduction of Au nanocrystals to the polymer matrix resulted in an increase in thermal stability. Fabricated DSSC was exposed to a $100$ -mW/cm2 solar simulator in ambient atmosphere. It was observed that the photoconversion efficiency (PCE) changes with different aspect ratio of Au. In addition, significantly high open-circuit voltage ( $V_{{\rm oc}}$ ), current density ( $J_{{\rm sc}}$ ), and PCE of 0.656 V, $4.94$ mA · cm−2, and 1.975%, respectively, are found for Au nanocrystals with maximum aspect ratio of 3.5 at an optimum concentration of 0.04%.

Quantification and Modeling of Spectral and Angular Losses of Naturally Soiled PV Modules

Spectral and angle of incidence (AOI) losses on naturally soiled crystalline silicon photovoltaic (PV) modules have been investigated in this study. The test modules designated as “moderately soiled (3 g/m2 )” and “heavily soiled (74.6 g/m2)” showed short-circuit current ( $I_{{\rm sc}}$ ) losses of about 10% and 41%, respectively. The spectral reflectance and quantum efficiency (QE) losses were also quantitatively determined. In the wavelength range of 350–1100 nm, the average reflectance of moderately and heavily soiled modules increased (as compared with the clean surface) by 58.4% and 87.2%, respectively. In the moderately soiled module, the 26.3% (average) reduction in QE is mainly because of 23% of absorption and 5.5% of reflection in the dust. In the highly soiled module, the 75.3% (average) reduction in QE is mainly because of 62% of absorption and 31% of reflection in the dust particles. It is also seen that the typical critical AOI of 57° for cleaned PV modules decreased to 38° for the moderately soiled module and 20° for the heavily soiled module. This influence is crucial for fixed tilt modules as they experience a wide range of AOI during daily operation, and a significant fraction of energy is generated at higher AOIs.

Current Enhancement of CdTe-Based Solar Cells

We report on the realization of CdTe solar cell photocurrent enhancement using an n-type CdSe heterojunction partner sputtered on commercial SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :F coated soda-lime glass substrates. With high-temperature close-space sublimation CdTe deposition followed by CdCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> activation, this thin-film stack allows for substantial interdiffusion at the CdSe/CdTe interface facilitating a CdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> alloy formation. The bowing effect causes a reduced optical bandgap of the alloyed absorber layer and, therefore, leads to current enhancement in the long-wavelength region and a decrease in open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ). To overcome the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> loss and maintain a high short-circuit current (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ), the CdTe cell configuration has been modified using combined CdS:O/CdSe window layers. The new device structure has demonstrated enhanced collection from both short-and long-wavelength regions as well as a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> improvement. With an optimized synthesis process, a small-area cell using CdS:O/CdSe window layer showed an efficiency of 15.2% with a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> of 831 mV, a J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> of 26.3 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and a fill factor of 69.5%, measured under an AM1.5 illumination without antireflection coating. The results provide new directions for further improvement of CdTe-based solar cells.

Honeycomb Structure on Multi-crystalline Silicon Al-BSF Solar Cell With 17.8% Efficiency

In this paper, large-area honeycomb (HC) texturing of multi-crystalline Si substrates with a newly developed process chain based on roller nanoimprint lithography and inline-capable plasma etching is demonstrated. The realized HC-textures show optical properties superior to random pyramids on c-Si, while the surface passivation quality is comparable. First, results on 156 × 156 mm2 multi-crystalline Si solar cells with an aluminum back-surface field show an efficiency of η = 17.8% compared with η = 17.3% on isotextured reference samples. This efficiency gain of 0.5% results from an increase in $J_{{\rm sc}}$ of up to 1.3 mA/cm2, which underlines the high optical and electrical quality of the texture. On a smaller area (50 × 50 mm2), the gain in $J_{{\rm sc}}$ of HC-textured cells compared with isotextured references was even higher with up to 2.5 mA/cm2.

Na Incorporation in Cu(In,Ga)(Se,S)&lt;sub&gt;2&lt;/sub&gt; Films Grown on Insulator-Coated Stainless Steel Foil Using a Metal Precursor Reaction

Cu(In,Ga)(Se,S)2 (CIGSS) films are formed on flexible 430 stainless steel foils through reacting Cu–In–Ga metal precursors in H2Se and H2S. The foils are coated using an electrically insulating silicone-based resin that can sustain temperatures as high as 600 °C. NaF films with various thicknesses are deposited between the metal precursor and Mo back contact in order to incorporate Na into the CIGSS films. The metal precursor reaction with the extrinsic Na incorporation results in greater Ga homogenization and S incorporation. The higher Na incorporation enhances $V_{{\rm OC}}$ and lowers $J_{{\rm SC}}$ . The temperature-dependent $V_{{\rm OC}}$ and drive-level capacitance profiling measurements reveal that the Na incorporation results in higher activation energy of the recombination and increased carrier density.

Efficient Small-Molecule-Based Inverted Organic Solar Cells With Conjugated Polyelectrolyte as a Cathode Interlayer

An efficient organic photovoltaics (OPVs) fabricated by using benzofuran-substituted diketopyrrolopyrrole (DPP)-oligothiophene-3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c] pyrrole-1,4-dione (DPP(TBFu)2) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the bulk heterojunction composite and a conjugated polyelectrolyte as the electron extraction layer in the inverted device structure is reported. Improved fill factor (FF) but reduced short-circuit current density were obtained from DPP(TBFu)2:PC61BM-based OPVs processed from chloroform solution with small percentage of solvent processing additive, 1,8-diiodooctane (DIO). The investigation by grazing incidence X-ray diffraction and resonant soft X-ray scattering indicated that the large phase separation in the thin film of DPP(TBFu) 2:PC61BM BHJ composite was induced by DIO, which results in decreased $J_{{\rm SC}}$ .

Impact of Texture Roughness on the Front-Side Metallization of Stencil-Printed Silicon Solar Cells

Realizing narrow contact fingers with low lateral resistance is a major goal for the front-side metallization of silicon solar cells. The formation of screen- or stencil-printed contact fingers is governed by a variety of influencing factors. One of these factors is the surface roughness of the textured silicon wafer. However, only a few investigations have been carried out to investigate this impact in detail. In this study, the influence of arithmetical mean roughness $R_{{\rm a}}$ of four differently textured wafer surfaces on contact finger geometry and lateral finger resistance, as well as optical and electrical losses, has been investigated. It will be shown that texture roughness has a considerable impact on the properties of the front-side grid. Narrower contact fingers could be realized on the smoothest texture, leading to a current density gain of $\Delta j_{{\rm sc}} = +0.27$ mA/cm2. On the other hand, increasing texture roughness has affected the amount of transferred paste and, thus, has led to a lower lateral finger resistance R L. Thus, contact fingers on the roughest texture have benefited from a fill factor gain of Δ FF = +0.24 %abs. A sensitivity analysis of both impacts has shown that the current density gain has overcompensated the fill factor loss. Thus, textures with a small roughness are beneficial with respect to the formation and electrical properties of stencil-printed front-side grids.

Dye-Sensitized Solar Cells Based on Porous Hollow Tin Oxide Nanofibers

Porous hollow tin oxide (SnO2) nanofibers and their composite with titanium dioxide (TiO2) particles (Degussa P25) were investigated as a photoanode for dye-sensitized solar cells. Incorporation of TiO2 particles in porous hollow SnO2 fibers enhanced the power conversion efficiency ( $\eta )$ from 4.06% to 5.72% under 100-mW/cm $^{2}$ light intensity. The enhancement of efficiency was mainly attributed to increase in current density ( $J_{\rm sc}$ ) and improvement in fill factor (FF). Increase in $J_{\rm sc}$ was caused by higher dye loading as indicated by UV–Vis absorption spectra and the improvement in FF was attributed to faster charge transport time as obtained from transient analysis. The microstructure of SnO2 fibers was studied using transmission electron microscope, scanning electron microscope, and X-ray diffraction. The electron transfer and recombination life times were studied using transient analysis, whereas interfacial charge transfer was studied using electrochemical impedance spectroscopy.

Superconducting and charge-density-wave orders in the spin-fermion model: A comparative analysis

We present comparative analysis of superconducting and charge-density-wave orders in the spin-fluctuation scenario for the cuprates. That spin-fluctuation exchange gives rise to d-wave superconductivity is well known. Several groups recently argued that the same spin-mediated interaction may also account for charge-density-wave order with momenta $(Q,0)$ or $(0,Q)$, detected in underdoped cuprates. This has been questioned on the basis that charge-density-wave channel mixes fermions from both nested and anti-nested regions on the Fermi surface, and fermions in the anti-nested region do not have a natural tendency to form a bound state, even if the interaction is attractive. We show that anti-nesting is not an obstacle for charge order, but to see this one needs to go beyond the conventional Eliashberg approximation. We show that in the prefect nesting/antinesting case, when the velocities of hot fermions are either parallel or antiparallel, the onset temperatures in superconducting and charge-density-wave channels are of comparable strength for any magnetic correlation length $\xi$. The superconducting $T_{\rm sc}$ is larger than $T_{\rm cdw}$, but only numerically. When the velocities of hot fermions are not strictly parallel/antiparallel, $T_{\rm cdw}$ progressively decreases as $\xi$ decreases and vanishes at some critical $\xi$.

Non-Lambertian Reflective Back Scattering and Its Impact on Device Performance of Ultrathin GaAs Single-Junction Solar Cells

This paper studies non-Lambertian scattering and its impacts on the optical properties and device performance of the ultrathin GaAs single-junction solar cell with a reflective back scattering layer. The Phong distribution is used to quantify the scattering effectiveness of the textured back surface, as well as its impacts on device absorptance, emittance, photon extraction and recycling factor, short-circuit current density $(J_{{\rm sc}})$ , external quantum efficiency (EQE), and power conversion efficiency. Both a general GaAs cell design and the ultrathin cell design are carefully investigated. A Phong exponent m of ∼12 is determined by fitting both simulated $J_{{\rm sc}}$ and EQE to their experimental values, with a more accurate averaged reflectivity of the textured Al0.52In 0.48P/Au interface taken into account. Additionally, the measured open-circuit voltage ( $V_{{\rm oc}}$ ) is lower than the best achievable value due to the nonradiative recombination in the device, and a limited lifetime of ∼130 ns is determined by fitting the simulated and measured $V_{{\rm oc}}$ ; a specific series resistivity of 1.2 Ω·cm2 is determined to account for the 77.8% fill factor.

Conversion Efficiency Improvement of InGaN/GaN Multiple-Quantum-Well Solar Cells With &lt;italic&gt;Ex Situ&lt;/italic&gt; AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN solar cells (SCs) were improved using ex situ AlN nucleation layer. Replacing the in situ GaN nucleation layer with the sputtered ex situ AlN nucleation layer reduced the total dislocation density of GaN layer from $3.7 \times 10^{8}$ to $2.2 \times 10^{8}$ cm $^{-2}$ . The dislocation density reduction of GaN with sputtered ex situ AlN nucleation could suppress the reverse leakage current and the forward recombination current in low forward voltage range of SCs, and thus can increase short-circuit current density ( $J_{\rm sc})$ and open-circuit voltage ( $V_{\rm oc})$ of the SCs. A 1-sun power conversion efficiency ( $\boldsymbol \eta $ %) of SCs with ex situ AlN nucleation (1.89%) showed an enhancement of 26% compared with that of conventional SC (1.50%). Furthermore, the 100-sun $\boldsymbol \eta $ % of SCs with ex situ AlN nucleation (1.97%) showed 18% improvement compared with that of conventional SC (1.67%).

Degradation of CdTe Solar Cells: Simulation and Experiment

Time-dependent numerical modeling is employed in conjunction with experimental data to investigate degradation mechanisms in cadmium telluride (CdTe) solar cells. Two mechanisms are tested against the data: 1) defect generation in the junction region caused by excess charge carriers and reactant defects and 2) back barrier increase. Junction effects result in stable $J_{{\rm sc}}$ with significant losses in $V_{{\rm oc}}$ and FF, in accordance with typical data for the type of light-soak stress conditions considered here. The back barrier increase causes additional FF loss. The results suggest that both mid-gap recombination centers and shallow acceptor-type defects form near the main junction as degradation proceeds. Our data reaffirm that the inclusion of copper in the back contact is associated with better initial performance and more rapid degradation. Correlations were observed between degradation rates and apparent doping hysteresis obtained from bidirectional capacitance–voltage (C–V) scans. Our time-resolved photoluminescence (PL) results show no correlation between light-soak-induced $V_{{\rm oc}}$ loss and PL lifetime.

CuIn1−xAlxSe2 Thin Films Grown by Co-Sputtering and Modified Selenization: Application in Flexible Solar Cells

Thin films of CuIn1−xAlxSe2 (CIAS) were grown on the flexible 10 micrometer thin stainless steel substrates, by dc co-sputtering from the elemental cathodes, followed by annealing with modified selenization. CuInAl alloyed precursor films were selenized both by noble gas assisted Se vapor transport in a tubular furnace and vacuum evaporation of Se in an evaporation chamber. CIAS thin films were optimized for better adhesion. X-ray diffraction, scanning electron microscopy, and UV-visible absorption spectroscopy were used to characterize the selenized films. The composition of CIAS films was varied by substituting In with Al in CuInSe2 (CIS) from $0\le \boldsymbol {x} \le 0.65$ ( $\boldsymbol {x}$ = Al/Al+In). Lattice parameters, average crystallite sizes, and compact density of the films, decreased when compared to CIS and (112) peak shifted to higher Bragg’s angle, upon Al incorporation. The dislocation density and strain were found to increase with Al doping. Solar cells with SS/Mo/CIAS/CdS/iZnO:AZnO/Al configuration were fabricated and were tested for current-voltage characteristics for various ‘ $\boldsymbol {x}$ ’ values, under Air Mass 1.5 Global one sun illumination. The best CIAS solar cell showed the efficiency of 6.8%, with $\boldsymbol {x} = 0.13$ , $\text{E}_{\rm g} = 1.17$ eV, fill factor 45.04, and short circuit current density $J_{\rm sc}~30$ mA/cm $^{2}$ .

Designing Bottom Silicon Solar Cells for Multijunction Devices

We report on efforts to design high-efficiency silicon homojunction subcells for use in multijunction stack devices. Both simulation and experimental works have been performed looking at a silicon solar cell under a truncated spectrum below 1.5 eV filtered by the upper layers in the multijunction stack. Good agreement is seen between the modeling and experimental results, identifying different emitter design requirements when the solar cell operates under a full or truncated spectrum. A well-passivated front surface, i.e., with low-interface surface recombination velocity, required a lightly doped emitter profile to maximize open-circuit voltage ( $V_{{\rm OC}}$ ), while a high-interface recombination surface requires a heavily doped for higher $V_{{\rm OC}}$ values. The impact on short-circuit current density ( $J_{{\rm SC}}$ ) is found to be minimal, even with large variations in the interface recombination and emitter profiles. In a tandem stack, an interface with low- and high-interface recombination velocities would require lightly doped and intermediate-doped emitters, respectively, for maximum conversion efficiency (η).