Radial Junction Solar Cells Research Articles

High-performance radial junction solar cells on ZnO coated stainless steel with excellent flexibility and durability

High flexibility and industrial compatibility are the key factors for developing large-scale lightweight, wearable, and portable thin film flexible solar cells. Here, a rather flexible and mechanically durable three-dimensional (3D) radial junction (RJ) solar cell has been directly constructed over discrete standing Si nanowires (SiNWs), upon flexible ZnO-coated stainless steel (SS) foil substrates. The ZnO layer deposited on the SS surface can optimize the density of SiNWs to improve the optoelectronic performance. A power conversion efficiency of 6.01% has been accomplished, with open-circuit voltage and photo-current density of 0.76 V and 12.87 mA/cm2, respectively. Remarkably, the outstanding mechanical stability of these RJ devices has been recorded, which can sustain > 4000 cycles of large bending to a radius of 2.5 mm, with only a 7% efficiency decrease. Note that the one-pump-down fabrication procedure of this 3D nanostructure RJ cells upon flexible ZnO-coated SS substrates is compatible with the industrial-mainstream Roll-to-Roll technology, holding thus a strong promise to establishing a simple and low-cost strategy for high-performance and durable flexible photovoltaics.

Wafer‐Scale Radial Junction Solar Cells with 21.1% Efficiency Using c‐Si Microwires

AbstractMicrowire (MW)‐based radial junction crystalline silicon (c‐Si) solar cells have great potential as an emerging energy device with an efficiency of over 20%. However, the competitive efficiency of MW‐based c‐Si solar cells in realizing a wafer‐scale device is limiting its commercialization. In this study, the aim is to demonstrate that conventional fabrication techniques can be applied to MW‐based solar cells while not only increasing the size from the lab‐scale to the wafer‐scale but also retaining an efficiency of >20%. Surprisingly, an improvement in open‐circuit voltage and fill factor is observed with an increase in device size, due to the reduction of recombination loss at the device edge. Finally, a successful demonstration of 21.1% efficiency at 4‐inch wafer‐scale (25 cm2) in c‐Si MW solar cell is observed, while an efficiency of 20.6% at a lab‐scale size (1 cm2) is observed.

Triple Radial Junction Hydrogenated Amorphous Silicon Solar Cells with >2 V Open‐Circuit Voltage

When solar cells are used as the photoanode for direct water splitting, the output voltage required typically exceeds that of a single‐junction photovoltaic device. Toward this application, in this work, triple radial junction silicon nanowire (3RJ SiNW) solar cells are fabricated via a plasma‐assisted vapor‐liquid‐solid method using hydrogenated amorphous silicon (a‐Si:H) for all the absorber layers, as well as for the doped ones. A high open‐circuit voltage (VOC) of 2.05 V, short‐circuit current density (JSC) of 3.8 mA cm−2, and power conversion efficiency of 4.4% are obtained for solar cells with areas of 0.03 cm2 by optimizing the density of SiNWs grown on ZnO:Al/Ag/Corning glass substrates. For lower‐efficiency devices, however, VOC values as high as 2.2 V are consistently achieved. At these higher voltages, large variations in JSC are observed, attributed to small local variations in SiNW density. Herein, for the first time, the excellent potential of 3D radial junction solar cells for applications requiring high voltages and high surface areas, such as water splitting is demonstrated.

Tin reduction from fluorine doped tin oxide for silicon nanowire-based solar energy harvesting and storage.

Hydrogen plasma reduction of fluorine doped tin oxide is a beneficial method to form tin nanodroplets on the sample surface directly in the plasma-enhanced chemical vapor deposition reactor. The formation of catalyst droplets is a crucial initial step for vapor-liquid-solid growth of silicon nanowires for radial junction solar cells and solar fuel cell technology. We present an original optical model which allows us to trace the formation process on fluorine doped tin oxide on soda-lime glass substrate from the in situ data and is in a good agreement with the spectroscopic ellipsometry data measured before and during the reduction process. The model reproduces well the phase shift introduced by a transition double layer in fluorine doped tin oxide which acts as a barrier against the sodium diffusion. Furthermore, we study the process of tin reduction from fluorine doped tin oxide in a real time and compare estimated amount of produced metallic tin with images from scanning electron microscopy.The proposed approach is very important for in situ real-time monitoring of the one-pump-down fabrication process used to grow nanowires and form radial junction devices.

Silicon Microwire Arrays with Nanoscale Spacing for Radial Junction c-Si Solar Cells with an Efficiency of 20.5.

Structural optimization of microwire arrays is important for the successful demonstration of the practical feasibility of radial junction crystalline silicon (c-Si) solar cells. In this study, we investigated an optimized design of tapered microwire (TMW) arrays to maximize the light absorption of c-Si solar cells, while minimizing the surface recombination, for simultaneously improving the open-circuit voltage and short-circuit current density (Jsc). Finite-difference time-domain simulations confirmed that controlling the spacing between the TMWs at the nanometer scale is more effective for increasing the light absorption than increasing the TMW length. The photogenerated current of a c-Si TMW array with a 200 nm spacing was calculated to be 42.90 mA/cm2, which is close to the theoretical limit of 43.37 mA/cm2 in the 300-1100 nm wavelength range. To experimentally demonstrate the TMW arrays with a nanometer-scale spacing of 200 nm, which cannot be realized by conventional photolithography, we utilized a soft lithography method based on polystyrene beads for patterning a c-Si wafer. The solar cells based on optimized TMW arrays exhibited a Jsc of 42.5 mA/cm2 and power conversion efficiency of 20.5%, which exceed those of the previously reported microwire-based radial junction solar cells.

Silicon Nanowire Solar Cells with μc‐Si:H Absorbers for Radial Junction Devices

Silicon nanowire (SiNW) radial junction (RJ) solar cells using hydrogenated microcrystalline silicon (μc‐Si:H) as absorber material have been studied. Since 2013, the performance of such RJ devices has been limited by the low fill factor (FF) and open‐circuit voltage (VOC). Thanks to the use of n‐type hydrogenated microcrystalline silicon oxide (μc‐SiOx:H) as a bottom doped layer, the authors developed μc‐Si:H RJ solar cells with a FF of 69.7% and a VOC of 0.41 V yielding a power conversion efficiency of 4.1%, which is more than 40% higher than the previously published efficiency record of 2.9%. Herein, the role of n‐type μc‐SiOx:H in the improvement of FF is highlighted.

Field‐Induced Radial Junction for Dopant‐Free Crystalline Silicon Microwire Solar Cells with an Efficiency of Over 20%

AbstractRadial junctions on crystalline silicon (c‐Si) microwire structures considerably reduce the diffusion length of photoinduced minority carriers required for energy generation by decoupling light absorption and carrier separation in orthogonal spatial directions. Hence, radial junctions mitigate the need for high‐purity materials, and thus reduce the fabrication cost of c‐Si solar cells. In this study, the formation of dopant‐free radial junctions from atomic layer deposition (ALD) of Al2O3 on an n‐c‐Si microwire surface is reported. ALD‐Al2O3 generates a p+ inversion layer, which eventually forms the radial junction on the n‐c‐Si surface. The width of depletion region induced by the p+ inversion layer is calculated from PC1D simulation as 900 nm. The fabricated dopant‐free radial junction c‐Si solar cells exhibit a power conversion efficiency of 20.1%, which is higher than those of previously reported microwire‐based radial junction solar cells. Notably, internal quantum efficiencies of over 90% are obtained in the 300–980 nm wavelength region, thereby verifying the successful formation of radial junctions.

Development of silicon nanowires with optimized characteristics and fabrication of radial junction solar cells with < 100 nm amorphous silicon absorber layer

We report the growth and characterization of Silicon nanowires (SiNWs) via the VLS mechanism with two different tin (Sn) catalyst thicknesses of 2 and 4 nm at 400 and 500 °C by PECVD. The effect of catalyst thickness, plasma power density and growth temperature on the morphology of nanowires is investigated. The structural morphology of the grown NWs is analyzed using FESEM and HRTEM. It has been inferred the density, diameter and length of NWs can be effectively controlled by PECVD process parameters. HRTEM analysis demonstrates a highly crystalline silicon core with (111) plane as preferred orientation. Grazing incidence X-ray diffraction spectra of the samples prepared at two different temperatures confirmed the (111), (220) and (311) plane orientation of the grown NWs. Raman spectra of the SiNWs grown on glass substrate shown broadened spectrum centering at 508 cm-1 confirming the crystalline nature of NWs core surrounded with dense layer of a-Si. Optimized parameters are used to fabricate radial junction solar cells and the effect of catalyst thickness on the performance is explored. Short circuit current density of 13.53 mA/cm2 and open circuit voltage of 0.82 V for 0.25 cm2 solar cell having power conversion efficiency of 6.66% is obtained.

Nanoscale Photovoltaic Responses in 3D Radial Junction Solar Cells Revealed by High Spatial Resolution Laser Excitation Photoelectric Microscopy.

The actual light absorption photovoltaic responses realized in three-dimensional (3D) radial junction (RJ) units can be rather different from their planar counterparts and remain largely unexplored. We here adopt a laser excitation photoelectric microscope (LEPM) technology to probe the local light harvesting and photoelectric signals of 3D hydrogenated amorphous silicon (a-Si:H) RJ thin film solar cells constructed over a Si nanowire (SiNW) matrix, with a high spatial resolution of 600 nm thanks to the use of a high numerical aperture objective. The LEPM scan can help to resolve clearly the impacts of local structural damages, which are invisible to optical and SEM observations. More importantly, the high-resolution photoelectric mapping establishes a straightforward link between the local 3D geometry of RJ units and their light conversion performance. Surprisingly, it is found that the maximal photoelectric signals are usually recorded in the void locations among the standing SiNW RJs, instead of the overhead positions above the RJs. This phenomenon can be well explained and reproduced by finite element simulation analysis, which highlights unambiguously the dominant contribution of inter-RJ-unit scattering against direct mode incoupling in the 3D solar cell architecture. This LEPM mapping technology and the results help to achieve a straightforward and high-resolution evaluation of the local photovoltaic responses among the 3D RJ units, providing a solid basis for further structural optimization and performance improvement.

Shape-Controlled Silicon Microwire Arrays from Au–Ag-Catalyzed Metal-Assisted Chemical Etching for Radial Junction Solar Cells

Metal-assisted chemical etching (MaCE) has been extensively studied as a cost-effective way to produce silicon nano/micro structure arrays. However, it is hard to keep the resultant morphologies of...

Design Principles for Fabrication of InP-Based Radial Junction Nanowire Solar Cells Using an Electron Selective Contact

Nanowire solar cells hold several advantages over planar solar cells, such as reduced reflection, facile strain relaxation, extreme light trapping, increased defect tolerance, etc. However, because of their large surface-to-volume ratio, nanowires tend to have very low effective minority carrier lifetime. To overcome this issue, a radial junction solar cell was proposed. However, in experimental realization, the efficiency of a radial junction solar cell remains significantly lower than its axial counterpart. This is mainly because of the inability to simultaneously control the doping in both the core and the shell while maintaining low defect density at the interface. To overcome the above-mentioned issues, we propose and simulate a core–shell heterojunction solar cell using p-InP as a core material and ITO/ZnO as a shell material. Using finite-difference time-domain simulations, we show that use of an oxide coating over InP core can significantly increase the absorption in InP nanowire arrays, and for an optimized thickness of oxide layer, InP consumption can be reduced by as much as four folds without sacrificing the ideal short circuit current. In addition, our device simulation results show that even for a core minority carrier lifetime of 50 ps, an efficiency of 23% can be obtained if both core and shell can be heavily doped while maintaining an interface recombination velocity of less than 104 cm/s. Finally, we discuss how the proposed device structure can reduce the fabrication complexities related to epitaxial homojunction/heterojunction core–shell solar cell structure while achieving a high efficiency under optimized conditions.

Three-dimensional radial junction solar cell based on ordered silicon nanowires

Highly ordered silicon nanowires (SiNWs) were fabricated by nanoimprint lithography and Bosch etching methods. A polycrystalline silicon shell was grown to form a radial p–n junction. To enhance its anti-reflection properties and conductivity, a thin ITO layer was deposited on the SiNWs solar cell, then a micro-grid electrode was introduced to minimize the metal areas to maximize carrier collection. Finally, shorter nanowires were used to reduce surface recombination and achieve an efficiency of 10.5%. This work is expected to show some possible techniques to improve the performance of silicon nanostructure solar cell.

Comparison of influence of gold contamination on the performances of planar and three dimensional c-Si solar cells

Radial junction solar cell is proposed as an alternative device geometry to planar junction solar cell due to its superior electro-optical performance. In this geometry, densely-packed micropillars enable the collection of minority carriers in the radial direction. Thus, the distance that carriers should travel to reach the p-n junction is shortened, allowing the use of low quality materials with poor carrier lifetime. In this study, we have experimentally studied the advantage of radial junction approach in low quality Si. For this purpose, we fabricated planar and radial junction solar cells using Si wafers with Au impurities. Average efficiency values of 15.74% and 15.66% were obtained for planar and radial cells in uncontaminated samples, respectively, indicating that close efficiency values were obtained when using high-quality materials. However, in the case of contamination, the efficiency of the cells with radial junction degraded less compared to the planar junction with the values of 14.71% and 12.72%, respectively. This is consistent with the expectation that radial junction cells are less sensitive to the quality of the material used. Moreover, these results, together with future structural optimization, lay a solid foundation for lowering fabrication costs without reducing the performance of the solar cell.

Toward Efficient Radial Junction Silicon Nanowire‐Based Solar Mini‐Modules

In this work, laser scribing is used to obtain a monolithic series connection between adjacent silicon nanowire radial junction (SiNW RJ) solar cells on the same glass substrate. The SiNW RJ solar cells have been deposited in a low‐temperature plasma‐enhanced chemical vapor deposition (PECVD) reactor in a single pump‐down process. The crystalline SiNWs have been grown by plasma‐assisted vapor‐liquid‐solid method using tin (Sn) metal drops as a growth catalyst. A detailed study of the laser scribing step P2 is performed for better selective removal of the SiNW RJs. The laser scribed spots are characterized by scanning electron microscopy (SEM) and Raman spectroscopy mapping. In addition, a silver grid is optimized and inkjet printed on top of the mini‐modules for better device performance. Based on that, a very good performance of the SiNW RJ mini‐modules is achieved with an energy conversion efficiency over 4% and power generation in the active area of 10 cm2. This work presents a break‐through in the modularization of the SiNW RJ solar cells and demonstrates the potential of the silicon nanowire devices for industrial applications as well as their compatibility with industrial fabrication processes.

Effect of diffusion parameters on emitter formation in silicon solar cells by proximity rapid thermal diffusion

N-type emitters have been formed in p-type monocrystalline silicon with good uniformity and high peak doping concentration by proximity rapid thermal diffusion (PRTD). High diffusion rates are achievable for relatively low temperatures (< 1000°C) with the addition of O2 to the N2 diffusion atmosphere. Solar cells have been prepared from the diffused samples to assess their performance and efficiencies of up to 6.0% have been achieved. The devices possess high series resistance and high recombination rates amongst other factors which limit their performance. Reducing the junction depth improves Jsc and efficiency but is accompanied by degrading shunt resistance and FF for junction thicknesses below ~ 400nm. Further refinements of cell processing should improve efficiency and result in a diffusion process for forming shallow emitters for application to microstructured devices such as micropillar radial junction solar cells.

Radial junction solar cells prepared on single crystalline silicon wafers by metal‐assisted etching (Phys. Status Solidi RRL 5/2017)

Radial junction solar cells prepared on single crystalline silicon wafers by metal‐assisted etching (Phys. Status Solidi RRL 5/2017)

Radial junction solar cells prepared on single crystalline silicon wafers by metal‐assisted etching

Radial junction solar cells have been proposed as an alternative device geometry to conventional planar solar cells with its remarkable electrical and optical performance. In this geometry, densely packed nano/micropillars allow minority carrier collection in the radial direction and shorten carrier diffusion length to p–n junction. Besides, reduced reflection from surface and increased light trapping in nano/micropillars enhance solar cell efficiency. In this study, photolithography and metal‐assisted etching (MAE) techniques are used to form well‐ordered silicon micropillar arrays; standard doping, passivation, and metallization steps are followed to form radial junction solar cells. The effect of micropillar length on optical and electrical performance of the solar cells have been investigated. We observed that optical reflection from solar cells surface decreased with increasing micropillar length, hence solar cell short circuit current (Jsc) and efficiency (ɳ) increased. Our best solar cell efficiency is 15.6% and this is one of the highest reported values obtained from the radial junction solar cells prepared by MAE technique.

Rapid thermal annealing treated spin-on doped antireflective radial junction Si nanopillar solar cell.

Radial junction nanopillar Si solar cells are interesting for cost effective efficiency improvement. Here, we address a convenient top-down fabrication of Si nanopillar solar cells using spin-on doping and rapid thermal annealing (RTA) for conformal PN junction formation. Broadband suppressed reflection as low as an average of 5% in the 300-1100 nm wavelength range and un-optimized cell efficiency of 7.3% are achieved. The solar cell performance can be improved by optimization of spin-on-doping and suitable surface passivation. Overall, the all RTA processed, spin-on doped nanopillar radial junction solar cell shows a very promising route for low cost and high efficiency thin film solar cell perspectives.

Ge nanopillar solar cells epitaxially grown by metalorganic chemical vapor deposition

Radial junction solar cells with vertically aligned wire arrays have been widely studied to improve the power conversion efficiency. In this work, we report the first Ge nanopillar solar cell. Nanopillar arrays are selectively patterned on p-type Ge (100) substrates using nanosphere lithography and deep reactive ion etching processes. Nanoscale radial and planar junctions are realized by an n-type Ge emitter layer which is epitaxially grown by MOCVD using isobutylgermane. In situ epitaxial surface passivation is employed using an InGaP layer to avoid high surface recombination rates and Fermi level pinning. High quality n-ohmic contact is realized by protecting the top contact area during the nanopillar patterning. The short circuit current density and the power conversion efficiency of the Ge nanopillar solar cell are demonstrated to be improved up to 18 and 30%, respectively, compared to those of the Ge solar cell with a planar surface.

In-situ Mueller matrix ellipsometry of silicon nanowires grown by plasma-enhanced vapor-liquid-solid method for radial junction solar cells

In-situ Mueller matrix spectroscopic ellipsometry was applied for monitoring the silicon nanowire growth by plasma-enhanced vapor-liquid-solid method. The technique is proposed as a real-time, non-destructive, and non-invasive characterization of the deposition process in a plasma-enhanced chemical vapor deposition reactor. The data have been taken by spectrally resolved Mueller matrix ellipsometer every 1min during the 8–10min long nanowire growth process. We have developed an easy-to-apply optical model to fit the experimental data, which enables to study the evolution of the parameters of the structure during initial stages of the growth. The first results provide information about the effective deposition rate determined from the linear increase of the deposited silicon volume with the deposition time.