Thin Silicon Solar Cells Research Articles

Broadband light trapping in nanotextured thin film photovoltaic devices

Substrates with engineered roughness are studied with the aim of achieving broadband and omnidirectional photon harvesting in thin film devices. Light coupling across the interfaces of a photonic device is induced by uniaxial pseudo-periodic gratings formed in a self-organised fashion via de-focused ion beam sputtering (IBS). The optical properties of the textured interfaces are assessed both experimentally and numerically using finite difference time domain (FDTD) algorithm, quantitatively demonstrating the optimal geometries which favour broadband diffuse scattering of radiation across the Vis-NIR spectral range. Thin film amorphous silicon solar cells based on the nanostructured patterns have been numerically studied via FDTD to assess absorption enhancement in comparison to flat reference devices, finding a 25% increase of short-circuit current, in good agreement with the experiment. Similar light trapping experiments performed on prototypical solar cells employing a PTB7:PC61BM organic absorber, allow to extend the general validity of the results to a relevant class of materials in the view of photovoltaic applications.

Tuneable and spectrally selective broadband reflector – Modulated photonic crystals and its application in solar cells

One dimensional photonic crystal has been developed as a broadband dielectric reflector, the stopband of which could be selectively tuned based on absorption spectrum of the absorber material. The design parameters and factors which contribute to the tunability of the photonic crystals are analyzed through simulations and experiments. The photonic crystal structures are fabricated using silicon rich silicon nitride and silicon oxynitride thin films deposited by PECVD at 200 °C. Modulated photonic crystal with a broad bandwidth, having an integrated reflectance of 97.6% in the wavelength range 580–1200 nm has been fabricated and applied in an amorphous silicon thin film solar cell as the back reflector. The optical performance of solar cells with these back reflectors has been studied in the longer wavelength as against the conventional metallic back reflector. The characterization of the thin film silicon solar cell with these photonic structures presented a short circuit current density of 14.77 mA/cm2. The angle dependent behaviour of the photonic crystal has been studied using angle dependent current-voltage measurement and a future prospectus of these structures as passivation layer for ultra thin crystalline silicon solar cells is also highlighted.

Progress in and potential of liquid phase crystallized silicon solar cells

Liquid phase crystallization of silicon (LPC-Si) offers great potential for high-quality Si films and a cost effective fabrication technique for thin crystalline silicon solar cells on glass. In this work, we report on the progress on LPC-silicon at HZB in the past years. Beginning with a brief description of the fabrication process, we summarize the work on the different contact systems developed for these absorbers before focusing on the interdigitated back contact architecture on which the highest efficiencies were reported. State-of-the art cells form the basis for a detailed discussion of the status of this technology. We investigate the current loss mechanisms and explore the potential for further improvement. Finally, based on this comprehensive quality assessment, we develop a roadmap to increase the cell efficiencies to wafer-equivalent values.

Metamaterial Mirror as Back Reflector for Thin Silicon Solar Cell Application

Metamaterial mirrors as back reflector is an innovative design for light trapping phenomenon in thin silicon solar cells. An optimized design for achieving maximum reflection in such metamaterial mirror is presented in this paper. In conventional metallic mirrors when light is reflected a phase reversal occurs and thus the intensity is reduced at the reflective surface. This effect is highly undesirable in thin solar cell applications where metal is used both as an electrical contact and an optical mirror. A mirror whose reflection phase can be varied from a perfect electric mirror (conventional metallic mirror) to that of a perfect magnetic mirror can be used to overcome this challenge in thin silicon solar cells. In a magnetic mirror no phase reversal of the incident electromagnetic wave occurs resulting in maximum electric field enhancement at the mirror surface. Such magnetic mirrors are classified as metamaterial mirrors. Simulations have been done with ComsolMultiphysics to obtain electric field and reflection for both TE (Transverse Electric) and TM (Transverse Magnetic) polarized light to obtain the optimized geometry. The generation rate and phase conservation has been observed from simulation when magnetic mirror is placed at the back of silicon substrate. The enhancement in the electric field is significantly increased which will lead to an enhanced absorption in the thin solar absorber resulting in high efficiencies.

Back scattering involving embedded silicon nitride (SiN) nanoparticles for c-Si solar cells

A novel material, structure and method of synthesis for dielectric light trapping have been presented in this paper. First, the light scattering behaviour of silicon nitride nanoparticles have been theoretically studied in order to find the optimized size for dielectric back scattering by FDTD simulations from Lumerical Inc. The optical results have been used in electrical analysis and thereby, estimate the effect of nanoparticles on efficiency of the solar cells depending on substrate thickness. Experimentally, silicon nitride (SiN) nanoparticles have been formed using hydrogen plasma treatment on SiN layer deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). The size and area coverage of the nanoparticles were controlled by varying the working pressure, power density and treatment duration. The nanoparticles were integrated with partial rear contact c-Si solar cells as dielectric back reflector structures for the light trapping in thin silicon solar cells. Experimental results revealed the increases of current density by 2.7% in presence of SiN nanoparticles.

Improving the Efficiency of Thin Film Amorphous Silicon Solar Cell by Changing the Location and Material of Plasmonic Metallic Nanostructures

In this work we present a 3D-finite difference time domain (FDTD) simulation of ultra thin amorphous silicon solar cell. The optical generation rate and plasmonic resonance enhance absorption in thin film cell in which silver nanoparticles are imbedded inside the depletion region of p-n junction. Here, the operation of thin film solar cell has been analyzed. Here, by inserting the metal nanoparticles within the active layer in the depletion region of the p-n junction, a new structure has been introduced. We have utilized three different materials, gold, silver and aluminium and the efficiencies of 5.27%, 5.3% and 3.74% have been obtained respectively. The results indicate a significant increase in the efficiency of proposed solar cell with silver nanoparticles.

The devisable reflection-enhanced lumpy silver particle and its application in thin film amorphous silicon solar cell

A tailored large-size plasmonic core-protuberances silver particle with unique optical properties with promising applications in thin-film solar cells was numerically and experimentally studied. The designed particle possesses an extremely enhanced electromagnetic near-field excited by localized surface plasmons resonance and its far field angular plots of light scattering power indicated an enhanced back-scattering ability. When embedded the lumpy particles into the rear of thin-film amorphous silicon solar cells, light trapping in broad wavelength range and ideal short circuit density of the cells increased. A simple one-step reaction was established to acquire the structured particles. The strengthening backward scattering capacity in a broadband light region was also verified experimentally.

Thin silicon solar cells: Pathway to cost-effective and defect-tolerant cell design

Thinner silicon wafers are a pathway to lower cost without compromising the efficiency of solar cells. In this work, we study the recombination mechanism for thin and thick silicon heterojunction solar cells, and we discuss the potential of using more defective material to manufacture high performance thin solar cells. Modelling the performance of silicon heterojunction solar cells indicates that at open-circuit voltage the recombination is dominated by Auger and surface, representing nearly 90% of the total recombination. At maximum power point, the surface is responsible for 50 to 80% of the overall recombination, and its contribution increases inversely with the wafer thickness. The experimental results show that for lower quality CZ material with 1 ms bulk lifetime, 60 µm-thick cells perform better than 170 µm-thick cells. The potential efficiency gain is 1% absolute. The gains in voltage of using thinner wafers are significantly higher for the lower quality CZ material, 25 mV, than for standard CZ material, 10 mV.

A new modeling approach for amorphous silicon passivated front contact for thin silicon solar cells

Thermal oxide (SiO2) and hydrogen-rich amorphous silicon (a-Si:H) interlayer at the front metal-silicon contact of thin silicon solar cells has been optimized based on a theoretical model. An analytical solution to the complete set of equations has been provided to highlighted the potential use and implications of the a-Si:H/SiO2 as a passivation coating at the front surface in crystalline silicon solar cells. The simple analytical expressions of the emitter reverse saturation current density and the photocurrent density were also obtained taking into account bulk recombination velocity and non-uniform doping profile. An optimum a-Si:H/SiO2 interlayer thicknesses were noted to enhance the collection of light-generated free carriers, which improves the efficiency of the short wavelength quantum. This is achieved by a drastic reduction in the effective recombination at the emitter upper boundary, a properly primarily responsible for the decrease in the emitter saturation current density. The findings indicated that the emitter region should be treated as an active layer because an optimum a-Si:H/SiO2 interlayer thicknesses at the front contact (W n,1 = 5 nm and δ = 14 A) give an improvement that can reach 3.6 mA/cm2 for the photocurrent density, 63 mV for the open-circuit voltage, and 3.95% for the cell efficiency.

Dual interface gratings design for absorption enhancement in thin crystalline silicon solar cells

We numerically study and analyze the light absorption enhancement in thin crystalline silicon solar cell with dual interface gratings. The structure combines the front dielectric nanowalls and the sinusoidal plasmonic grating at back reflector. We show that having specific interfaces with well-chosen period, fill factor and height can allow more efficient dielectric and plasmonic modes coupling into active layer and can improve the solar cell performance. For 1µm active layer case, the optimal result for the proposed structure achieves short-circuit current of ~23.6mA/cm2, which performs over 50% better than flat solar cell structure, the short-circuit current of which is ~15.5mA/cm2. In addition, the active layer thickness and angular analysis show that the proposed structure maintains its advantage over flat structure.

Fabrication of honeycomb textured glass substrate and nanotexturing of zinc oxide front electrode for its application in high efficiency thin film amorphous silicon solar cell

A significant part of broad band sunlight remains unabsorbed in a simple structured amorphous silicon solar cell. This absorption can be enhanced by adopting a light-trapping scheme with the help of a textured front surface and back reflector. For this purpose, honeycomb-type texture was fabricated on a glass surface by chemical etching. A 3 μm×3 μm honeycomb patterned optical-mask was used to create an image-pattern of an etch-mask on the glass surfaces. We used 0.5% hydrofluoric acid (HF) as an etchant solution with an optimized etching time ranging between 25 and 28 min. This single textured glass shows an enhancement in diffused transmission of light. In solar cell application, a 630-nm-thick Al-doped ZnO (AZO) layer was deposited over the textured glass surface. An additional random etching was carried out on the AZO with 1% HF acid solution for 10 s. This results to a double textured AZO superstrate on which solar cells were fabricated. The solar cells show higher short circuit current density to 17.2 mA/cm2. Finally, we obtained photovoltaic conversion efficiency of an optimized solar cell as 10.75% (with the corresponding Jsc as 17.03 mA/cm2).

A Study on the Optimization of CP Based Low-temperature Tabbing Process for Fabrication of Thin c-Si Solar Cell Module

Thin crystalline silicon (C-Si) solar cell is expected to be a low price energy source by decreasing the consumption of Si. However, thin c-Si solar cell entails the bowing and crack issues in high temperature manufacturing process. Thus, the conventional tabbing process, based on high temperature soldering (> 250°C), has difficulties for applying to thin c-Si solar cell modules. In this paper, a conductive paste (CP) based interconnection process has been proposed to fabricate thin c-Si solar cell modules with high production yield, instead of existing soldering materials. To optimize the process condition for CP based interconnection, we compared the performance and stability of modules fabricated under various lamination temperature (120, 150, and 175°C). The power from CP based module is similar to that with conventional tabbing process, as modules are fabricated. However, the output of CP based module laminated at 120°C decreases significantly (14.1% for Damp heat and 6.1% for thermal cycle) in harsh condition, while the output drops only in 3% in the samples process at 150°C, 175°C. The peel test indicates that the unstable performance of sample laminated at 120°C is attributed to weak adhesion strength (1.7 N) between cell and ribbon compared to other cases (2.7 N). As a result, optimized lamination temperature for CP based module process is 150°C, considering stability and energy consumption during the fabrication.

Improvement of photon management in partial rear contact solar cells using a combination of DBR and Mie scatterers

In this article, we present systematic simulations and numerical analysis of a novel light trapping scheme in a partial rear contact (PRC) solar cell involving a combined effect of rear located Distributed Bragg Reflectors (DBRs) and Mie scatterers comprising of dielectric nanoparticles (DNP), thereby, enhancing the efficiency of the device. We have studied the effect of three different types of DBRs in combination with embedded silica (SiO2) DNPs which scatter light into silicon substrate of PRC c-Si solar cell. The materials for DBRs are chosen in such a way that they may serve the dual purpose of reflecting more than 90% of incident light at the rear surface and passivating it as well. The internal reflection from the rear surface, absorption enhancement ratio and average scattering angle have been computed from 3-dimensional finite difference time domain (FDTD) simulations and performing numerical analysis later on. Further, these results are used in the analysis of basic solar cell to extract the parameters like short circuit current density, open circuit voltage, fill factor, reverse saturation current density and efficiency of solar cell. It has been observed that significant increase in efficiency can be achieved for solar cells having 10–100µm thick substrates by incorporating this light trapping scheme. Beyond 100µm thickness, the conversion efficiency approaches a saturation value. Moreover, a combination of DBR with silica nanoparticles results in maximum efficiency near 50µm thickness of solar cell thereby improving the baseline efficiency from ~20.3% to an absolute value of 22.9%. This study opens up a new perspective of light management using the advantages of highly reflective DBRs and highly scattering DNPs which can be incorporated in a rather simple and inexpensive way for thin (<100µm) silicon solar cells.

Plasmon enhancement of optical absorption in ultra-thin film solar cells by rear located aluminum nanodisk arrays

In this work, in order to enhance the light absorption in one micron thick crystalline silicon solar cells, a back reflecting and rear located plasmonic nanodisk scheme is proposed. We investigate the scattering properties of aluminum nanostructures located at the back side and optimize them for enhancing absorption in the silicon layer by using finite difference time domain simulations. The results indicate that the period and diameters of nanodisks, thickness of spacer layer have a strong impact on short circuit current enhancements. The optimized Al nanoparticle arrays embedded in rear located SiO2 layer enhance Jsc with an increase of 47% from the non-plasmonic case of 18.9 to 27.8 mA/cm2 when comparing with a typical stack with a planar aluminum back reflector and a back reflector with plasmonic nanoparticles. This finding could lead to improved light trapping within a thin silicon solar cell device.

Probing stress and fracture mechanism in encapsulated thin silicon solar cells by synchrotron X-ray microdiffraction

Thin (<150µm) silicon solar cell technology is attractive due to the significant cost reduction associated with it. Consequently, fracture mechanisms in the thin silicon solar cells during soldering and lamination need to be fully understood quantitatively in order to enable photovoltaics (PV) systems implementation in both manufacturing and field operations. Synchrotron X-ray Microdiffraction (µSXRD) has proven to be a very effective means to quantitatively probe the mechanical stress which is the driving force of the fracture mechanisms (initiation, propagation, and propensity) in the thin silicon solar cells, especially when they are already encapsulated. In this article, we present the first ever stress examination in encapsulated thin silicon solar cells and show how nominally the same silicon solar cells encapsulated by different polymer encapsulants could have very different residual stresses after the lamination process. It is then not difficult to see how the earlier observation, as reported by Sander et al. (2013) [1], of very different fracture rates within the same silicon solar cells encapsulated by different Ethylene Vinyl Acetate (EVA) materials could come about. The complete second degree tensor components of the residual stress of the silicon solar cells after lamination process are also reported in this paper signifying the full and unique capabilities of the Synchrotron X-Ray Microdiffraction technique not only for measuring residual stress but also for measuring other potential mechanical damage within thin silicon solar cells.

Light trapping enhancement induced by bimetallic non-alloyed nanoparticles on a disordered subwavelength flexible thin film crystalline silicon substrate using metal-assisted chemical etching.

We report the application of gold and silver (AuAg) bimetallic non-alloyed nanoparticles (BNNPs) on disordered subwavelength structures (d-SWSs). The combined advantages of the plasmonic structures and d-SWSs improved the light trapping performance of flexible thin film crystalline silicon (c-Si) solar cells. Antireflective d-SWSs were fabricated using spin-coated Ag ink and subsequent metal-assisted chemical etching, which reduced the ion-induced surface damage produced by the dry etching process. The dimensions of the d-SWSs were finely tuned by adjusting the Ag ink ratio. Au8Ag8 BNNPs were employed on optimized d-SWSs to achieve low reflectance at broadband wavelengths. The Au8Ag8 BNNPs on the d-SWSs showed 180% and 145% enhanced absorption compared to bare c-Si film and Au8Ag8 BNNPs on c-Si film, respectively, in the wavelength range of 300-1100 nm. After 200 cycles of bending the antireflection of the structures remained similar to the original level. This study introduces new approaches for light management in flexible thin film c-Si solar cells over the broadband wavelength range.

A novel trapezoidal profile of optimized diffraction grating for light trapping in thin silicon solar cells

A novel trapezoidal profile of optimized diffraction grating for light trapping in thin silicon solar cells

Light-Harvesting Properties of Embedded Tin Oxide Nanoparticles for Partial Rear Contact Silicon Solar Cells

In this paper, a novel method of preparation and systematic study of application of low-loss tin oxide (SnO2) nanospheres located at the rear side of crystalline silicon solar cells having partial rear contact have been presented. The improvement in efficiency due to light harvesting through optical scattering of tin oxide nanospheres is significant for thin silicon solar cells. Finite-difference time-domain (FDTD) simulations reveal that embedding of the rear-located nanospheres is necessary for back scattering of light from the rear surface. An analytical electrical model has been developed utilizing the results of optical simulations to estimate the solar cell parameters and efficiency enhancement of solar cells. The model shows that an absolute efficiency enhancement of ∼19% can be achieved for 16% efficient 10-μm thin silicon solar cell with partial rear contact. The enhancement is lower (∼6%) for thicker (180 μm) partial rear contact cells. Experimentally, SnO2 nanospheres have been synthesized and applied at the rear side of partial rear contact solar cell as a proof of experiment to validate the potential of this approach. A relative enhancement of short-circuit current by 2.3% and open-circuit voltage by 2.5% has been achieved experimentally for 180-μm silicon solar cells leading to 5.2% higher efficiency with respect to baseline efficiency validating this concept.

Thin Film Polycrystalline Silicon Solar Cells Studied by Transient Terahertz Probe Spectroscopy

Abstract Thin film polycrystalline silicon (poly-Si) solar cells were annealed in hydrogen plasma or in water vapour to suppress electrical activity of defects and impurities (passivation). The hydrogen plasma passivation procedure was optimized and a modification of several hydrogenation parameters was suggested, e.g. higher hydrogen pressure of 300 Pa and keeping bias voltage Vbias constant during the whole passivation process until the plasma was switched off. Since the plasma hydrogenation process was run as a closed system (without hydrogen flow) and the achieved passivation results (up to 497 mV) were close to the limit for poly-Si (500 mV), the generally accepted necessity to operate the process with a hydrogen flow was called into question. Samples passivated in a hydrogen plasma at different processing conditions were analyzed by transient terahertz probe spectroscopy and Suns-VOC method to measure a solar cell's charge carrier lifetime and its open-circuit voltage VOC, respectively. A correlation of these measured parameters was observed even though both these techniques characterize solar cells in completely different states. The transient terahertz probe spectroscopy appears to be a useful tool for a contactless investigation of materials where ultrafast processes of ns-long lifetimes play a crucial role, e.g. nanostructured objects and materials.

High efficiency thin‐film amorphous silicon solar cells

AbstractEnhancing light absorption within thin film amorphous silicon (a‐Si) solar cells should lead to higher efficiency. This improvement is typically done using various light trapping techniques such as utilizing textured back reflectors for pronounced light scattering within the cell thus achieving higher absorption. It is believed that embedding metallic nanoparticles (MNPs) inside the structure could increase light scattering. However, embedding MNPs can also cause significant structure defects and pronounced efficiency drop as well – it has been indicated by many experiments that disproved this belief. In search of ways to improve efficiency, we have investigated the impact of MNP's size, and location within the solar cell, in addition to the effect of defects, and doping levels on the overall efficiency. On the basis of our 3D multiphysics (optical‐electric) modeling, we developed a design guideline for embedding these MNPs and reducing the impact of defects created in the embedding process. The results of simulations were compared to relevant measured data, and it showed a good agreement. Subsequently, models were used to predict performance, and over 30% improvement in solar cell efficiency (~13% is predicted); which is beyond the state of the art. This was predicted by optimizing the size and location of the MNPs and tailoring the doping levels to have better forward light trapping and absorption.