Ebeam fabrication of silicon nanodome photovoltaic devices without metal catalyst contamination

In this paper, the fabrication of silicon nanodome solar cells on crystalline wafers is reported. Crystalline silicon was patterned by ebeam lithography to define the silicon nano pillars with diameter of 100 nm, 1 μm and 5 μm. Unlike conventional bottom up growth of silicon nanowire from gold (Au), our method is free from contaminant. Consequently, it is a valuable method to fully evaluate the effect of nanostructures on solar cell performances. The fabricated devices were characterized through scanning electron microscopy, absorption measurements, illuminated solar cell I–V characteristics and monochromatic incident photon-to-electron conversion efficiency.     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

50 μm thin solar cells with 17.0% efficiency

Monocrystalline silicon solar cells with thicknesses below 50 μm manufactured by the transfer layer process at ipe reach efficiencies as high as 17.0%. We present a thin film solar cell, which is not attached to a glass superstrate, opening new process opportunities, as for example the usage of flexible superstrates. We show a free-standing 47 μm thin solar cell with a record conversion efficiency η=17.0%.     

Effective conversion efficiency enhancement of solar cell using ZnO/PS antireflection coating layers

Zinc oxide (ZnO) film was deposited on a porous silicon (PS) layer using a radio frequency sputtering system while the PS layer was prepared by a photoelectrochemical etching method. The ZnO/PS layers were found to be an excellent antireflection coating (ARC), exhibiting exceptional light trapping at wavelengths ranging from 400 to 1000 nm because of their lowest effective reflectance. This, in turn, leads to increase the efficiency of solar cell to 18.15%. The ZnO film was highly oriented with the c-axis perpendicular to the PS layer. The average crystallite size of the PS and ZnO/PS layers were 17.06 and 17.94 nm, respectively. Photoluminescence emission peaks proved the nanocrystalline characteristic of the PS layer and the ZnO film. Raman measurements of the ZnO/PS layers were determined at room temperature and indicate that a high-quality ZnO nanocrystalline film was formed. In the current paper, ZnO/PS ARC layers are attractive and offer a promising technique to produce high-efficiency, low-cost solar cells.     

Reduction in surface recombination through hydrogen and 1-heptene passivated silicon nanocrystals film on silicon solar cells

We demonstrate reduction in surface recombination by integrating silicon (Si) nanocrystal layer on single crystalline Si solar cell. Si nanocrystals (NCs) are grown by electrochemical etching of (1 0 0) oriented p-type Si wafer. The substructures on the substrate are extracted and passivated it with hydrogen and 1-heptene molecules. Colloidal dispersion of Si NCs was spin casted on solar cell at room temperature. Apart from the I–V curve depicting the efficiency of solar cell, diffuse reflectance, measurement of short circuit current as a function of wavelength and current–voltage characteristics of solar cell were recorded with and without NCs layer. The analysis showed 9.4% increase in Si solar cell efficiency due to the surface passivation effect offered by Si NCs. Measurements of surface recombination time confirms the improved passivation by NCs.     

Improving thin-film crystalline silicon solar cell efficiency with back surface field layer and blaze diffractive grating

Both 2D electromagnetic and electrical semiconductor simulations are performed sequentially in this study in order to better understand the structural principles of thin-film crystalline solar cells with back surface field and blaze diffractive grating. In the absence of adequate approximations for blazed gratings, we simulate silicon solar cells electromagnetically and electrically in order to deal with the geometrical complexity produced by the blazed grating with a BSF on top of it. Thin-film crystalline silicon solar cells (TF-c-Si SCs) typically exhibit poor quantum efficiency both at shorter wavelengths and longer wavelengths with sharp drops in spectral response. Longer wavelength spectral response (from 0.6 μm to 1.2 μm) is addressed here first by considering the influence of blaze gratings on the enhancement of effective optical absorption in thin-film crystalline silicon (TF-c-Si) solar cells. The effect of the back surface field layer (BSF) in terms of improving minority carrier collection is also taken into account. In the 2D electromagnetic simulation, polarization dependent two-dimensional (2D) numerical simulations based on rigorous coupled wave analysis (RCWA) and finite element method (FEM) are implemented for the optimization of optical absorption of the solar cell structure. A rather large tolerance in design parameters of the optimized blaze grating structure was found. The optimized blaze grating structures help in improving the cell efficiency, especially for weak absorption thin cell structures. The enhancement of equivalent optical path length reveals the efficient light trapping effect caused by the diffractions of the blaze grating structures, especially in the longer wavelength range. In the electrical semiconductor simulation, the BSF, which arises from the heavy acceptor doping that creates the concentration gradient, is set atop the blaze grating in order to provide an extra small drift field for the collection of minority electrons. Incorporating the optimized antireflection coating along with a BSF layer and a blaze-grating in the 2 μm cell doubles cell efficiency. The use of blazed gratings in thin-film solar cells, which can be performed upon silicon by means of lithography and ion-beam etching, is promising for low cost and high-efficient solar cell applications.     

Thin film silicon n–i–p solar cells deposited by VHF PECVD at 100 °C substrate temperature

The applicability of the very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) technique to the fabrication of solar cells in an n–i–p configuration at 100 °C substrate temperature is being investigated. Amorphous and microcrystalline silicon cells are made with the absorber layers grown in conditions close to the amorphous-to-microcrystalline transition, which proved to give the best quality layers. It was observed that post-deposition annealing at 100 °C resulted in a relative increase of the efficiency of up to 50% for both amorphous and microcrystalline cells. For an amorphous solar cell deposited on stainless steel foil with a non-textured back reflector, an efficiency of 5.3% was achieved. A too rough substrate (textured back reflector), with an rms roughness higher than 80 nm, was found to give rise to shunting paths.     

Development of micromorph tandem solar cells on flexible low-cost plastic substrates

We report on the development of fully flexible microcrystalline and micromorph tandem solar cells directly on low-cost substrates like poly-ethylen-terephtalate (PET) and poly-ethylen-naphtalate (PEN). The cells are deposited in nip or nip/nip configuration on the plastic substrate coated with a highly reflecting Ag–ZnO back contact. Light trapping is achieved by combining a periodically textured substrate and a diffusing ZnO front contact. Single-junction microcrystalline cells with a stable efficiency of 8.7% are achieved with an i-layer thickness of 1.2 μm. In tandem devices we obtain an efficiency of 10.9% (initial) with an open circuit voltage of 1.35 V and a fill factor (FF) of 71.5%. These cells are slightly top limited with 11.26 and 11.46 mA/cm² in the amorphous (270 nm thick) and the microcrystalline (1.2 μm thick) sub-cells, respectively. We introduce an intermediate reflector (IR) between the bottom and the top cell because it allows increasing the top cell current without compromising the stability by a thicker absorber. The IRs consist of either an ex-situ ZnO or a low refractive index P-doped silicon–oxygen compound deposited in-situ with a plasma process that is fully compatible with solar cell processing. We demonstrate significant current improvement (up to 8% relative) using both kinds of IRs.     

Twenty-two percent efficiency HIT solar cell

We have achieved the world's highest solar cell conversion efficiency of 22.3% (V_oc: 0.725 V, I_sc: 3.909 A, FF: 0.791, total area: 100.5 cm², confirmed by AIST) by using a heterojunction with intrinsic thin layer (HIT) structure. This is the world's first practical-size (>100 cm²) silicon solar cell that exceeds a conversion efficiency of 22% as a confirmed value. This high efficiency has been achieved mainly due to improvements in a-Si:H/c-Si hetero-interface properties and optical confinement.The excellent a-Si:H/c-Si hetero-interface of the HIT structure enables a high V_oc of over 0.720 V and results in better temperature properties. In order to reduce the power-generating cost, we are now investigating numerous technologies to further improve the conversion efficiency, especially the V_oc, of HIT solar cells, with the aim of achieving 23% efficiency in the laboratory by 2010.     

Dark I–U–T measurements of single crystalline silicon solar cells

The effect of parasitic resistances on silicon solar cell performance was discussed. The current–voltage I–U characteristics of single crystalline silicon solar cells at different temperatures were measured in the dark. A one and two diodes equivalent model was used to describe the electronic properties of the solar cells. The diode ideality factors, the series and shunt resistance, that determine the fill factor and the efficiency of the solar cell, have been estimated. It was proved that the performance of the tested silicon solar cell can be described with enough accuracy by the one diode equivalent model with series resistance r_s equal to 0.1 Ω and an empirical ideality factor m_id equal to 1.4.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Characterization of PIII textured industrial multicrystalline silicon solar cells

Optimized textured structure is one of the most important elements for high efficiency multicrystalline silicon solar cells. In this paper, in order to incorporate low reflectance nanostructures into conventional industrial solar cells, structures with aspect ratios of about 1:1 and average reflectance of 8.0% have been generated using plasma immersion ion implantation. A sheet resistance of 56.9 Ω/sq has been obtained by adjusting the phosphorous diffusion conditions, while the thickness of the silicon nitride vary in 70–80 nm by extending the deposition time by 100 s. Under the conventional co-firing conditions, a solar cell with efficiency of 16.3% and short-circuit current density 34.23 mA/cm² has been fabricated.     

New organic photosensitizers incorporating carbazole and dimethylarylamine moieties for dye-sensitized solar cells

Four organic dyes (XS10–13) employing carbazole unit as electron donor and N,N-dimethylarylamine moieties as electron-donating groups were designed and synthesized for nanocrystalline TiO₂ dye-sensitized solar cells. The electron-donating groups of dimethylarylamine increase the electron density of donor moiety and enhance the molar extinction coefficient of dyes. For a typical device the maximum IPCE value could reach 86%, with a short-circuit photocurrent density (J_sc) of 9.8 mA cm^?2, an open-circuit photovoltage (V_oc) of 642 mV, and fill factor (FF) of 0.63, which corresponds to an overall conversion efficiency (η) of 4.0%. For a comparison, the N719-sensitized TiO₂ solar cell showed an efficiency of 6.4%.     

Microcrystalline silicon grown by VHF PECVD and the fabrication of solar cells

Intrinsic microcrystalline silicon has been deposited by very high frequency plasma enhanced chemical vapor deposition technique at frequency of 75 MHz. Different gas mixtures of silane and hydrogen were utilized, and the evolution of microstructure and phase in film were studied, while keeping the substrate temperature at 200 °C and the chamber pressure at 0.5 Torr. Optimised material was inserted in p–i–n solar cells: preliminary efficiency of 5.5% was reached for 1 μm-thick solar cells with the V_oc around 0.6 V.     

Monolithically series-interconnected transparent modules of dye-sensitized solar cells

We have realized a new type of dye-sensitized solar cell (DSC) modules. The monolithically series interconnected structure, which is similar to the structure of amorphous silicon solar cells (SCs), was employed so that the advantages of DSCs compared to conventional silicon SCs (low costs, low energy consumption in production processes) were fully exploited. To achieve other important features of DSCs (transparency and color choice) we have developed transparent counter electrodes (CEs) composed of Pt-loaded In₂O₃:Sn nanoparticles and separators composed of SiO₂ nanoparticles to replace conventional non-transparent ones used in the modules. The performance of the new CEs is significantly improved to be close to those of conventional ones during electric generation operations. In all 85% of the maximal conversion efficiency was maintained after 2000 h of a durability test under 1 sun light soaking at 60 °C.     

Thin film silicon solar cells by AIC on foreign substrates

This paper presents the fabrication of thin film crystalline silicon solar cells on foreign substrates like alumina, glass–ceramic (GC) and metallic foils (ferritic steel—FS) using seed layer approach, which employs aluminium induced crystallisation (AIC) of amorphous silicon. Effect of hydrogen content in a-Si:H precursor films on the AIC process has been studied and the results showed that defects in the AIC grown films increased with increase of hydrogen content. At the optimal thermal annealing conditions, the AIC grown poly-Si films showed an average grain size of 7.6, 26, and 8.1 μm for the films synthesised on alumina, GC, and FS, respectively. The grains were (1 0 0) oriented with a sharp Raman peak around 520 cm^?1. Similarly, n-type seed layers were also fabricated by over-doping of as-grown AIC layers using a highly phosphorus doped glass solution. The resistivity of as-grown films reduced from 8.4×10^?2 Ω cm (p-type) to 4.1×10^?4 Ω cm (n-type) after phosphorus diffusion. These seed layers of n-type/p-type were thickened to form an absorber layer by vapour phase epitaxy or solid phase epitaxy. The passivation step was applied before the heterojunction formation, while it was after in the case of homojunction. Open circuit voltage of the junctions showed a strong dependence on the hydrogenation temperature and microwave (μW) power of electron cyclotron resonance (ECR) plasma of hydrogen. Effective passivation was achieved at a μW power of 650 W and hydrogenation temperature of 400 °C. Higher values of solar conversion efficiencies of 5% and 2.9% were achieved for the n-type and p-type heterojunction cells, respectively fabricated on alumina substrates. The analysis of the results and limiting factors are discussed in detail.     

0.4% absolute efficiency gain by novel back contact

Replacing the aluminum back contact of screen-printed multicrystalline silicon solar cells by a novel low-temperature layer sequence boosts the absolute power conversion efficiency η by Δη=0.4%. The optimized hydrogenated amorphous silicon (a-Si:H)-based back side junction provides efficient back side passivation and contacting at the same time. The improved passivation quality reduces the effective surface recombination velocity S_eff to S_eff<20 cm s^?1. Due to the optimized back side layer sequence, the open circuit voltage V_OC rises by ΔV_OC=15 mV up to V_OC=622 mV and the short circuit current increases by ΔJ_SC=0.8 mA cm^?2.     

Enhanced nanocrystalline silicon solar cell with a photonic crystal back-reflector

Nanocrystalline silicon solar cells were enhanced with a photonic crystal back-reflector. Rigorous scattering matrix simulations were used to optimize a photonic crystal back-reflector consisting of a triangular lattice of nano-holes, with a pitch near 800 nm. The photonic crystal back-reflector with a pitch of 800 nm was fabricated on the crystalline silicon substrate by photolithography and reactive-ion etching, and coated with silver and zinc oxide. Nanocrystalline silicon solar cells were grown on the patterned substrates. We observed ～7% enhancement of the absorption and photo-generated current relative to a Ag/ZnO substrate, with an enhancement ratio of 1.5 near the band edge. Significant enhancement occurred in photon absorption at near infrared wavelengths greater than 700 nm, due to diffraction resonances of the incoming light.     

Improvement of multicrystalline silicon solar cell performance via chemical vapor etching method-based porous silicon nanostructures

In this paper, we report on the effect of chemical vapor etching-based porous silicon (PS) on the performance of multicrystalline silicon solar cells performed via deep n⁺/p junction-type structures. Chemical vapor etching of silicon leads to the formation of porous silicon (PS) nanostructures that dramatically decrease the surface reflectivity from 30% to about 8%, and increase the minority carrier diffusion lengths from 90 μm to 170 μm. As a result, the short-circuit current density was improved by more than 20% and the fill factor (FF) by about a 10%. An enhancement of the photovoltaic conversion energy efficiency of the solar cells from 7% to 10% was observed. This low-cost PS formation process can be applied in the photovoltaic cell technology as a standard procedure.     

The effect of hybrid photovoltaic thermal device operating conditions on intrinsic layer thickness optimization of hydrogenated amorphous silicon solar cells

Historically, the design of hybrid solar photovoltaic thermal (PVT) systems has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV) devices to avoid temperature-related losses. This approach neglects the associated performance losses in the thermal system and leads to a decrease in the overall exergy of the system. Consequently, this paper explores the use of hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an effort to maintain higher and more favorable operating temperatures for the thermal system. Amorphous silicon not only has a smaller temperature coefficient than c-Si, but also can display improved PV performance over extended periods of higher temperatures by annealing out defect states from the Staebler–Wronski effect. In order to determine the potential improvements in a-Si:H PV performance associated with increased thicknesses of the i-layers made possible by higher operating temperatures, a-Si:H PV cells were tested under 1 sun illumination (AM1.5) at temperatures of 25 °C (STC), 50 °C (representative PV operating conditions), and 90 °C (representative PVT operating conditions). PV cells with an i-layer thicknesses of 420, 630 and 840 nm were evaluated at each temperature. Results show that operating a-Si:H-based PV at 90 °C, with thicker i-layers than the cells currently used in commercial production, provided a greater power output compared to the thinner cells operating at either PV or PVT operating temperatures. These results indicate that incorporating a-Si:H as the absorber material in a PVT system can improve the thermal performance, while simultaneously improving the electrical performance of a-Si:H-based PV.