Thin Crystalline Silicon Solar Cells Research Articles

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.

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.

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.

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.

Flexibility-Assisted Heat Removal in Thin Crystalline Silicon Solar Cells

Flexibility-Assisted Heat Removal in Thin Crystalline Silicon Solar Cells

Experimental demonstration of broadband solar absorption beyond the lambertian limit in certain thin silicon photonic crystals

The tantalizing possibility of 31% solar-to-electric power conversion efficiency in thin film crystalline silicon solar cell architectures relies essentially on solar absorption well beyond the Lambertian light trapping limit (Bhattacharya and John in Nat Sci Rep 9:12482, 2019). Up to now, no solar cell architecture has exhibited above-Lambertian solar absorption, integrated over the broad solar spectrum. In this work, we experimentally demonstrate two types of photonic crystal (PhC) solar cells architectures that exceed Lambertian light absorption, integrated over the entire 300–1,200 nm wavelength band. These measurements confirm theoretically predicted wave-interference-based optical resonances associated with long lifetime, slow-light modes and parallel-to-interface refraction. These phenomena are beyond the realm of ray optics. Using two types of 10-μm thick PhC’s, first an Inverted Pyramid PhC with lattice constant a = 2,500 nm and second a Teepee PhC with a = 1,200 nm, we observe solar absorption well beyond the Lambertian limit over λ = 950–1,200 nm. Our absorption measurements correspond to the maximum-achievable-photocurrent-density (MAPD), under AM1.5G illumination at 4-degree incident angle, 41.29 and 41.52 mA/cm2 for the Inverted Pyramid and Teepee PhC, respectively, in agreement with wave-optics, numerical simulations. Both of these values exceed the MAPD (= 39.63 mA/cm2) corresponding to the Lambertian limit for a 10-μm thick silicon for solar absorption over the 300–1,200 nm band.

Optimization of back ITO layer as the sandwiched reflector for exploiting longer wavelength lights in thin and flexible (30 µm) single junction c-Si solar cells

Fabrication of thin and flexible crystalline silicon solar cells based on single junction concept is reported with detailed investigations on each step of the production flow chain. With the aim of minimizing material use/wastage as per the international technology roadmap for photovoltaic (ITRPV), which is also directly related to the device cost, efforts have been made to introduce thin (~30 µm) c-Si wafers instead of a conventional 180 µm wafer to fabricate single-junction solar cells. Due to the introduction of thin (~30 µm) c-Si wafer, the device becomes flexible, which is also an additional benefit towards the development of future roll-to-roll electronics. In order to address better carrier collection in thin silicon as well as light management, measures have been taken by introducing an indium tin oxide (ITO) layer both on top and at the bottom. The influence of this ITO layer along with back Al contact toward the cell efficiency has been discussed. X-ray diffraction (XRD) analysis has been carried out to investigate microstrain and dislocation density related changes in the thin wafer, which are known to have influence on the photoconversion efficiency. Under 1 Sun illumination, current – voltage characteristics and external quantum efficiency were measured and found to be promising.

Fabrication of a Silicon Nanowire Solar Cell on a Silicon-on-Insulator Substrate

This study proposes metal-assisted chemical etching (MAE) as a facile method to fabricate silicon nanowire (SiNW) array structures, with high optical confinement for thin crystalline silicon solar cells. Conventional SiNW arrays are generally fabricated on Si wafer substrates. However, tests on conventional SiNW-based solar cells cannot determine whether the photo-current is derived from SiNWs or from the Si wafer. Herein, SiNW arrays were fabricated on a silicon-on-insulator substrate with a 10-μm-thick silicon layer for measuring the photocurrent of the SiNW only. The 9 μm-long p-type SiNW arrays were applied to a solar cell structure fabricated using an n-type H-doped amorphous Si layer, thereby confirming the photovoltaic effect. However, the device exhibited a conversion efficiency of 0.0017% because of a low short-circuit current (Jsc) and a low open-circuit voltage (Voc). The low Jsc resulted from a high series resistance and high absorption loss from the amorphous Si layer, whereas the low Voc resulted from the high surface recombination velocity of the SiNW array structure. Therefore, reducing the surface recombination of SiNW-based solar cells can improve their conversion efficiency.

Optical absorption theoretical analysis for thin film crystalline silicon solar cells fabricated on the hexagonal close‐packed dome arrays

In this work, the low aspect ratio hexagonal close-packed dome arrays were designed to trap light for c-Si thin film solar cells. The optical performance of the designed solar cell was theoretically studied via simulation based on the rigorous coupled wave analysis in detail. The result of the simulation showed the model of the solar cell with arrays exhibited absorptivity enhancement in the longer wavelength range. Simultaneously, the integral for the AM 1.5 absorbance spectrum of each model showed that the absorption gains reach up to the highest point of 45.13% in the periodicity of 0.7 μm.

Absorption enhancement in thin film solar cells with bilayer silver nanoparticle arrays

In this paper, a systematic design and analysis of thin film crystalline silicon solar cells decorated with bilayer silver nanoparticles with different particle dimensions is presented. The particles are located on the rear of the solar cell. Using numerical simulations, we showed that the light absorption is enhanced when the particle radii of the upper layer Ag NPs is less than that of the lower. Moreover, our proposed structure showed a 9.97% increase in short-circuit current density and a 9.94% increase in intergraded quantum efficiency across the solar spectrum compared with the optimized counterpart decorated with uniform Ag NPs.

3D simulations of interdigitated back-contacted crystalline silicon solar cells on thin substrates

Interdigitated back contact technology is a promising candidate to be applied to thin crystalline silicon solar cells because of its simpler one-side interconnection while allowing a more flexible front surface treatment. This work explores the efficiency potential of IBC c-Si solar cells applied to thin c-Si substrates through 3D device simulations. In particular, we explore the impact of substrate thickness and front surface recombination velocity on cell performance with special attention to the different behavior in carrier collection of two different rear-surface doping structures. Firstly, the model is validated by comparing simulation results to a fabricated device on 280 µm-thick substrates with stripe-like p+ and n+ diffusions. It is revealed that efficiencies of 16–17% are reachable for substrates on the 10–15 µm range without changing the technology developed for thick ones. Next, the rear doping structure is modified leading to doped regions just under the metal contacts. This type of structure is expected in solar cells where high-temperature diffusions are replaced by point-like laser doped contacts, which is a feasible alternative to be applied to thin substrates. Simulation results show that diffusion length requirements for those locally-doped structures are more demanding due to the reduction of emitter regions. As a result, very well passivated front and rear surfaces are required to maintain short-circuit current densities to reasonable values. Finally, for both structures open-circuit voltage is kept almost constant with reduced thickness, despite the strong reduction in short-circuit current. Simulations show a reduction of dark saturation current density with substrate thinning due to the redistribution of dark current densities that flow parallel to the device surface.

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.

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.

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.

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.

Design rules for net absorption enhancement in pseudo-disordered photonic crystal for thin film solar cells.

The role of pseudo-disordered photonic crystals on the absorption efficiency of simplified thin film crystalline silicon solar cells is presented and discussed. The expected short circuit current can thus be further increased compared to a fully optimized square lattice of holes, thanks to carefully controlled positions of the nanoholes in the considered realistic simplified solar cell stack. In addition, the pseudo-disordered structures are less sensitive to the angle of incidence, especially in the long wavelength range.