Thin Silicon Solar Cells Research Articles

From Rigid to Flexible: Progress, Challenges and Prospects of Thin c‐Si Solar Energy Devices

AbstractThe increasing adoption of solar energy as a renewable power source marks a significant shift toward clean, sustainable alternatives to conventional energy forms. A notable development in this field is the advancement of thin monocrystalline silicon (c‐Si) solar cells. Characterized by their lightweight, flexible nature, these solar cells promise to transform the renewable energy landscape with enhanced durability, adaptability, and portability. Amidst the growing demand for sustainable energy solutions, refining and evolving thin c‐Si solar cell technologies is crucial. This review comprehensively examines the latest progress in thin c‐Si solar energy conversion device technologies, offering an extensive overview of current methodologies for producing thin c‐Si films, advanced light trapping techniques, surface passivation strategies, and methods for managing thin wafers. It also explores the wide‐ranging applications of thin c‐Si solar cells. Furthermore, this article sheds light on the techno‐economic aspects, highlighting the intertwined challenges of commercializing these innovative cells. Through an in‐depth analysis of the latest advancements, this review serves as a valuable resource for researchers and industry experts, keeping them abreast of current trends and catalyzing further advancements in thin c‐Si solar cell technology.

Free-standing ultrathin silicon wafers and solar cells through edges reinforcement

Crystalline silicon solar cells with regular rigidity characteristics dominate the photovoltaic market, while lightweight and flexible thin crystalline silicon solar cells with significant market potential have not yet been widely developed. This is mainly caused by the brittleness of silicon wafers and the lack of a solution that can well address the high breakage rate during thin solar cells fabrication. Here, we present a thin silicon with reinforced ring (TSRR) structure, which is successfully used to prepare free-standing 4.7-μm 4-inch silicon wafers. Experiments and simulations of mechanical properties for both TSRR and conventional thin silicon structures confirm the supporting role of reinforced ring, which can share stress throughout the solar cell preparation and thus suppressing breakage rate. Furthermore, with the help of TSRR structure, an efficiency of 20.33% (certified 20.05%) is achieved on 28-μm silicon solar cell with a breakage rate of ~0%. Combining the simulations of optoelectrical properties for TSRR solar cell, the results indicate high efficiency can be realized by TSRR structure with a suitable width of the ring. Finally, we prepare 50 ~ 60-μm textured 182 × 182 mm2 TSRR wafers and perform key manufacturing processes, confirming the industrial compatibility of the TSRR method.

Influence of Photonics and Plasmonics Effect on Ultrathin Amorphous Silicon Thin Film Solar Cell

Abstract Our recent surge in silicon derived materials has major demand in thin film photovoltaic (PV) modules and enhancing the significant numbers. The rigorous coupled wave analysis method is a simple and fast method also known as developed to determine the light absorption and cell efficiency. The optics of thin film solar cells (amorphous silicon) garnering crucial role in the photovoltaic market. In this work, an ultrathin thin film amorphous silicon solar cell PV performance investigated by periodically textured surfaces at the nanoscale level. This analysis of periodic textured substrate was deriving optimal surface textures. The nanogratings lead to light scattering mechanism with the higher order diffraction angle and enhanced the light absorption of the incidence spectrum. The influence of grating period and height, the collection of the charge carriers increased at various wavelengths from ultraviolet, visible and infrared spectral regions are discussed with the assistance of photonic and plasmonic modes. Finally, nanoscale engineering mechanism the optimized thin film amorphous silicon solar cell yielded the highest current densities (22.6 and 23.8 mA/cm2) in both polarization modes.

Lithography‐Free Method to Synthesize Quasiperiodic Silicon Inverted‐Pyramid Arrays: A Broadband Light Trapper for High‐Efficiency Thin Silicon Solar Cells

Thin silicon solar cells can be a low‐cost effective photo‐conversion device, if the device can efficiently absorb the solar spectrum. Herein, a new lithography‐free technique is developed for the fabrication of quasiperiodic silicon inverted‐pyramids arrays (SiIPAs), which show a high‐light‐trapping phenomenon in the ultraviolet–visible—near‐infrared (300–2000 nm). Fabricated SiIPA samples show a significant reduction of reflectance (3%) in the silicon absorption band (300–1000 nm). A unique additional absorption of 33–44% compared to the planar silicon is observed in the sub‐bandgap region of silicon (1100–2000 nm) for these samples. Photocurrent response measurement confirms the generation of additional electron–hole pairs in the sub‐bandgap region of silicon for the SiIPA samples in comparison with planar sample. In these results, the effect of field confinement and the creation of optical resonance modes within these structures, qualitatively supported by numerical simulation, are signified. The estimated short‐circuit current density using the experimental absorption spectrum of SiIPAs is 55.46 mA cm−2, which is far higher than the Lambertian limit of ≈43 mA cm−2. The theoretical efficiency of the solar cell can be achieved up to 35.22%, which surpasses the Shockley–Queisser limit of 33.7%.

Smaller texture improves flexibility of crystalline silicon solar cells

Flexible solar cells are widely researched for their potential usage in photovoltaics integrated into buildings, cars, unmanned arial vehicle and wearable electronics. The thin crystalline silicon solar cell (60–90 μm) is prone to crack due to surface texture when it is under bending. Here we investigated the effect of pyramid size on optical reflectivity and mechanical properties of silicon wafers. We find that smaller and uniform pyramids are beneficial for obtaining efficient and flexible silicon solar cells.

Investigation on significant efficiency enhancement of thin crystalline silicon solar cells

Investigation on significant efficiency enhancement of thin crystalline silicon solar cells

Multi-Resonant Mie Resonator Arrays for Broadband Light Trapping in Ultrathin c-Si Solar Cells.

Effective photon management is critical to realize high power conversion efficiencies for thin crystalline silicon (c-Si) solar cells. Standard few-100-µm-thick bulk cells achieve light trapping with macroscopic surface textures covered by thin, continuous antireflection coatings. Such sizeable textures are challenging to implement on ultrathin cells. Here, it is illustrated how nanoscale Mie-resonator-arrays with a bimodal size distribution support multiple resonances that can work in concert to achieve simultaneous antireflection and light-trapping across the broad solar spectrum. The effectiveness of these light-trapping antireflection coatings is experimentally demonstrated on a 2.8µm-thick c-Si solar cell. The measured short-circuit current and corresponding power conversion efficiency are notably improved, achieving efficiencies as high as 11.2%. Measurements of the saturation current density on completed cells indicate that thermal oxides can effectively limit surface recombination. The presented design principles are applicable to a wide range of solar cells.

Dual metallic-based plasmonic back reflector to broaden the absorption spectra of a thin film amorphous silicon solar cell

Here, to obtain high efficiency in a thin film amorphous silicon solar cell, a dual metallic plasmonic back reflector was investigated. The designed dual metal plasmonic back reflector can improve the photocurrent and hence the efficiency. At first, a double layer SiO 2/Si3N4 as the antireflection (AR) was analyzed. The efficiencies of 8.74%, 11.38%, 12.53%, and 13.04% are obtained for a cell without AR, with SiO2, Si3N4, and SiO2/SiN 4 AR layers, respectively. A disk-ring and dual rings-based Ni and Ti-based plasmonic back reflector were designed. The study was carried out by a 3D finite difference time domain method and the optimized parameters were calculated to obtain higher efficiencies. Finally, a relatively higher photocurrent and conversion efficiency of 19.86mA/cm2 and 15.65% were achieved for the optimized structure with a dual Ni rings back reflector, respectively. Because of the importance of amorphous silicon solar cells due to the cost problem of crystalline silicon, here, the simulation steps present a valuable road map to design a novel plasmonic thin-film amorphous silicon solar cell.

Optical and Electrical Analyses of Solar Cells with a Radial PN Junction and Incorporating an Innovative NW Design That Mimics ARC Layers.

The implementation of a texturing pattern on the surface of a solar cell is well known for reducing reflection, thus increasing the absorption of sunlight by the solar cell. Nanowires (NWs) that are large in their height have been widely used for this purpose. Through rigorous numerical simulations, this work explores the benefits of short but index-matched NWs and how these designs are also affected by surface recombination. Additionally, this work further optimized power conversion efficiency (PCE) by placing two or three NWs of different heights and diameters on top of each other to mimic the performance of two-NW and three-NW ARC designs with PCEs of 16.8% and 17.55%, respectively, when a radial pn junction is considered. These are the highest reported so far for such a thin silicon solar cell. Furthermore, we also show how these designs were impacted by surface recombination velocity and compare these findings to simple NWs of different heights and diameters.

Fabrication of light trapping structures specialized for near-infrared light by nanoimprinting for the application to thin crystalline silicon solar cells

Vehicle-integrated photovoltaics (VIPV) are gaining attention to realize a decarbonized society in the future, and the specifications for solar cells used in VIPV are predicated on a low cost, high efficiency, and the ability to be applied to curved surfaces. One way to meet these requirements is to make the silicon substrate thinner. However, thinner substrates result in lower near-infrared light absorption and lower efficiency. To increase light absorption, light trapping structures (LTSs) can be implemented. However, conventional alkali etched pyramid textures are not specialized for near-infrared light and are insufficient to improve near-infrared light absorption. Therefore, in this study, as an alternative to alkaline etching, we employed a nanoimprinting method that can easily fabricate submicron-sized LTSs on solar cells over a large area. In addition, as a master mold fabrication method with submicron-sized patterns, silica colloidal lithography was adopted. As a result, by controlling silica coverage, diameter of silica particles (D), and etching time (tet), the density, height, and size of LTSs could be controlled. At the silica coverage of 40%, D = 800 nm, and tet = 5 min, the reduction of reflectance below 65% at 1100 nm and the theoretical short-circuit current gain of 1.55 mA/cm2 was achieved.

Comparison of random upright pyramids and inverted pyramid photonic crystals in thin crystalline silicon solar cells: An optical and morphological study

The potential of enabling a wide range of applications with lightweight, flexible, thin silicon photovoltaic (PV) devices has led to research interest in various light-trapping technologies for thin silicon absorbers. Herein, using both experiments and photonic modelling, we present a detailed comparison between random pyramid texturing and inverted pyramid photonic crystal patterning in thin silicon vis-à-vis light trapping. In particular, we investigate the potential of uniform single-dimension periodic structures in contrast to conventional multi-dimensional random structures for high broadband absorption efficiency in ultra-thin silicon. We find that inverted pyramid photonic crystals cause a large increase in optical path length especially for long wavelength photons due to strong wave interference producing exceptionally high absorption and so does random pyramids though with slightly lower absorption than that of photonic crystals. On the other hand, the random pyramids, fabricated via a one-step etching process with feature sizes ranging from sub-micron to 4 µm, have higher absorption at short wavelengths due to their multi-dimensional size distribution. Overall, we find that random textured pyramids appropriately fabricated can yield a comparable ideal photocurrent density to that by photonic crystals for 10 µm thick silicon.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Optimization of distributed Bragg’s reflectors in thin film solar cells

Distributed Bragg reflectors (DBRs) or one-dimensional photonic crystals (1DPCs) play key role in advanced opto-electronic and photonic devices. The term photonics is derived from photons in which particle indicates a quantum of light and photon carries energy proportional to the radiation frequency with zero rest mass. In this work, we present the designing 1DPCs by using Si/SiO2 layers using a plane wave expansion method (PWE). For that, various parameters are optimized such as thicknesses (d) of each layer, center wavelength (λc), refractive index (nH/nL) of individual layer and the number of DBR/period to analyze the photonic band gap (PBG). The transverse electric (TE) and magnetic (TM) field distribution at different wavelength is discussed and helpful to understand the light harvesting mechanism within the PBG. Finally, this optimized DBR layers were integrated into a thin film amorphous silicon solar cell as a back-end reflector. Due to the tuned PBG, the proposed and designed amorphous silicon thin film solar cell has yielded an improved of light absorption, which is useful for enhancing the photovoltaic performance.

Effect of sawing damage on flexibility of crystalline silicon wafers for thin flexible silicon solar cells

The cost of solar cell production can be reduced by wafer thinning. A thinner wafer provides flexibility, and crystalline silicon solar cells are promising as flexible solar cells due to their flexibility. However, as wafers become thinner, production yield decreases due to wafer breakage caused by sawing damage; thus, to further reduce wafer thickness, it is necessary to suppress sawing damage. We investigated the flexibility of wafers under various slice conditions by conducting biaxial bending tests and clarified the dominant factor causing sawing damage to further reduce the wafer thickness for crystalline silicon solar cells. The results of damage observation by scanning electron microscopy and evaluation of the crystal structure by Raman spectroscopy confirm that the damage structure changes significantly depending on wire specifications. The results from the biaxial bending tests indicate that the three-dimensional flexibility of a wafer is determined by wire specifications.

(Invited, Digital Presentation) Probing Triaxial Stress States Using Curvature-Based Methodology for Enabling Highly Robust and Reliable Integrated 3D Systems-on-Package in Advanced Microelectronics Industries

With the nascent of the systems-on-package in advanced 3D microelectronics, mechanical stress states in the system get much more complicated than in the past. Stress states are increasingly triaxial (as opposed to mostly biaxial stress states in the planar semiconductor devices), and with that, the roles of shear stresses in causing relatively new and unique mechanical damages and failures in the 3D stacks of chips and devices (memory, logic, sensors, actuators, data storage, etc.) cannot be undermined. The difficult stress management in a Through-Silicon Via (TSV) as the vertical interconnection between chips on stack has been an early indication of what more to come to the emerging, advanced microelectronics industries in the near, immediate future. Not only the mechanical damages that come into the picture, but also the delicate interrelation between the mechanical straining of the silicon and the corresponding electronic mobility affecting the very working of the semiconductor devices. The methods of stress determination used in the semiconductor industries so far (X-ray Diffraction, Raman Spectroscopy, etc.) have mostly worked under the assumptions of biaxial stress states which were mostly true in the planar semiconductor chip design. Such assumptions may no longer hold true for enabling highly robust and reliable integrated 3D Systems-on-Package in today’s and tomorrow’s microelectronics industries. In addition, once the 3D stacks of devices are fully integrated, it becomes next to impossible to manage the highly convoluted stress states originating from many silicon devices with mostly surface-based Raman techniques and low-penetration depth of conventional laboratory XRD. In this study, we propose to use the curvature-based approaches that would allow determination of the complete 3D stress states in every level of the stacking of silicon devices, and thus by following the stress state evolution during the stacking, the individual and overall resultant of the stress states in the highly integrated 3D systems may be analyzed. The approach originated from our previous work – on TSV for semiconductor chips as well as on thin silicon solar cells for the emerging photovoltaics industry – using synchrotron-based X-ray microdiffraction to determine the origin and evolution of the stress states in the devices (during thermal cycling or subsequent manufacturing steps). However, during those investigations, we realized that the diffraction science is not necessarily needed for highly accurate stress state determination, in fact, just the detection of the local curvature of the surface of the monocrystalline silicon in the devices would suffice to provide for equally highly accurate stress state determination. We have demonstrated the efficacy of this approach since in our work in silicon-based photovoltaics (Tippabhotla et al. Progress in Photovoltaics 2017). As long as we have the silicon surface from each level of devices (individually and after the stacking), a laser-based system could detect the change in local curvature in high spatial resolution and hence could provide with highly accurate stress state determination. No synchrotron radiation would be needed.

Gross fracture pits at the intersection of two single scratches during grinding of silicon

Infeed grinding is used in the industry for grinding thick silicon wafers. The grinding process being a series of overlapping scratches, leaves behind the surface and sub-surface damages in the silicon. Continuous in-feed grinding of a thick wafer when used to produce thin wafer leads to breakage upon release from the fixture. This happens because of not only damage induced by a single scratch, but also due to cumulative damage induced as the grinding proceeds with several overlapping and intersecting scratches. In a single scratch, depths below a critical depth can cause material removal without gross fracture and pits; however, it is not clear if this applies to multiple intersecting scratches while grinding silicon. In this study, the grinding operation is performed for a very short period of time, gradually removing the material over only a partial region of the wafer surface while leaving the remaining portion unground. This produces multiple intersecting grinding scratches of increasing depth. Scanning electron microscopy and surface profile measurements of these scratches show that while in a single scratch, material is removed without gross fracture and pitting; when two such scratch intersects, it results in gross fracture at the site of their intersection. Such gross fractures at intersections can happen even at a very shallow depth of scratch as low as 28 nm. This gross fracture at the intersections act as local sites to initiate wafer damage and material removal. Based on the results, we attempted to redefine the critical depth of cut for grinding operation. The study paves the way to manufacture thin damage-free wafers needed for the next generation thin silicon solar cells.

Ray Tracing of Thin PERC Silicon Solar Cells with Cone Textures

Thinning of crystalline silicon (c-Si) wafer is a promising approach to reduce the technology cost of passivated emitter rear cell (PERC) solar cell. However, reducing the wafer thickness compromises light absorption, hence short-circuit current density (Jsc) in the solar cell. This necessitates effective light trapping in the device. In this work, upright cone textures are incorporated on the surface of 50 μm PERC monocrystalline silicon solar cell. SunSolve ray tracer is used to simulate the optical and electrical properties of the solar cell within 300-1200 nm wavelength region. Besides, the solar cell is also simulated with a front silicon nitride (SiNx) anti-reflective coating (ARC) on the cone textures. From the results, the thin PERC solar cell with cone textures and SiNx ARC demonstrates Jsc of up to 38.8 mA/cm2 and conversion efficiency of 20.4%. This is a significant performance improvement when compared to the planar cell, with Jsc of 25.1 mA/cm2 and efficiency of 13.1%. The improvement is attributed to the enhanced broadband light absorption and increased external quantum efficiency in the device.

Study on Electrical Characteristics of Thin Flexible Crystalline Silicon Solar Cells

Flexibility makes output voltage-current curve of thin crystalline silicon solar cell’s photovoltaic (PV) array stair-stepping and corresponding power-voltage curve possessing non-uniform illumination characteristics, so existing conventional PV array model is no more suitable and it is necessary to build a new mathematical model of PV array that is suitable for flexibility condition. Based on engineering model of PV array and combining with series-parallel connection of electric circuit, the PV array under flexibility is analyzed in detail and it is derived theoretically that the mathematical model of PV array under flexibility can be expressed by piecewis function.

Tuning the optoelectronic properties of AZO thin films for silicon thin film solar cell applications

Al-doped ZnO (AZO) thin films are deposited using dc magnetron sputtering and the process conditions are optimized to obtain transparent conducting electrode (TCE) with desirable properties suitable for photovoltaic applications. In the course, the effects of deposition parameters such as growth temperature, deposition time and plasma power density on the structural and optoelectronic properties were investigated using suitable characterization techniques. XRD analysis of the deposited films at different process conditions showed a strong c-axis preferred orientation. The surface roughness of the deposited films was examined using AFM analysis. Elemental analysis was carried out using XPS. The resistivity and sheet resistance of the thin films decreased with increase in temperature, deposition time and power density. The optimized films deposited at 250 °C resulted in electrical resistivity of 6.23 × 10−4 Ωcm, sheet resistance of 9.2 Ω/□ and exhibited an optical transmittance of >85% in the visible range. Figure of merit (FOM) calculations were carried out to analyze the suitability of deposited thin films for thin film amorphous silicon solar cell applications. The photogain of optimized intrinsic a-Si:H layer was in the range of 104, whereas no photogain was observed in doped a-Si:H layers. The thin film solar cell fabricated using the optimized AZO film as TCE exhibited power conversion efficiency of 6.24% when measured at AM 1.5 condition.

Design investigation on 100 μm-thickness thin silicon PERC solar cells with assistance of machine learning

Thin crystalline silicon passivated emitter and rear cell (PERC) solar cells are a very prospective technology for next-phase photovoltaic development due to the potential of high cost effectiveness. The reduction of silicon wafer thickness can significantly save the costs, but there is a loss of cell efficiency if cell design is not conducted. For the thinned 100 μm-thickness PERC solar cells without design, the efficiency loss is pronounced from commercial 180 μm-thickness. In this paper, we have designed and optimized SiO2/SiNx/SiNx/SiOx thin films (here two SiNx layers have different refractive index) on the front surface and SiNx/SiOx thin films on the back surface for the standard front single-sided textured PERC cells. Based on this, we further design and investigate the case of double-sided textured PERC solar cells. Compared with the reference cell, the present designs can lead to the short-circuit current density increase by 0.6 mA/cm2 and the open-circuit voltage enhancement by 10 mV for the front textured case, which causes the efficiency gain of 0.7% from 21.6% to 22.3%. For the double-sided textured cells, the efficiency has an extra increase of 0.6% from 22.3% to 22.9%. Finally, we have constructed the efficiency prediction model by using the multilayer perceptron algorithm in machine learning. It is found from the SHAP values that a significant effect of the front SiNx thickness is observed to predict the performance of the PERC cells.