Suqiong Zhou

Large-Area Nanosphere Self-Assembly by a Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing

A high throughput surface texturing process for optical and optoelectric devices based on a large-area self-assembly of nanospheres via a low-cost micropropulsive injection (MPI) method is presented. The novel MPI process enables the formation of a well-organized monolayer of hexagonally arranged nanosphere arrays (NAs) with tunable periodicity directly on the water surface, which is then transferred onto the preset substrates. This process can readily reach a throughput of 3000 wafers/h, which is compatible with the high volume photovoltaic manufacturing, thereby presenting a highly versatile platform for the fabrication of periodic nanotexturing on device surfaces. Specifically, a double-sided grating texturing with top-sided nanopencils and bottom-sided inverted-nanopyramids is realized in a thin film of crystalline silicon (28 μm in thickness) using chemical etching on the mask of NAs to significantly enhance antireflection and light trapping, resulting in absorptions nearly approaching the Lambertian limit over a broad wavelength range of 375-1000 nm and even surpassing this limit beyond 1000 nm. In addition, it is demonstrated that the NAs can serve as templates for replicas of three-dimensional conformal amorphous silicon films with significantly enhanced light harvesting. The MPI induced self-assembly process may provide a universal and cost-effective solution for boosting light utilization, a problem of crucial importance for ultrathin solar cells.

Realization of 13.6% Efficiency on 20 μm Thick Si/Organic Hybrid Heterojunction Solar Cells via Advanced Nanotexturing and Surface Recombination Suppression

Hybrid silicon/polymer solar cells promise to be an economically feasible alternative energy solution for various applications if ultrathin flexible crystalline silicon (c-Si) substrates are used. However, utilization of ultrathin c-Si encounters problems in light harvesting and electronic losses at surfaces, which severely degrade the performance of solar cells. Here, we developed a metal-assisted chemical etching method to deliver front-side surface texturing of hierarchically bowl-like nanopores on 20 μm c-Si, enabling an omnidirectional light harvesting over the entire solar spectrum as well as an enlarged contact area with the polymer. In addition, a back surface field was introduced on the back side of the thin c-Si to minimize the series resistance losses as well as to suppress the surface recombination by the built high-low junction. Through these improvements, a power conversion efficiency (PCE) up to 13.6% was achieved under an air mass 1.5 G irradiation for silicon/organic hybrid solar cells with the c-Si thickness of only about 20 μm. This PCE is as high as the record currently reported in hybrid solar cells constructed from bulk c-Si, suggesting a design rule for efficient silicon/organic solar cells with thinner absorbers.

Improved optical absorption in visible wavelength range for silicon solar cells via texturing with nanopyramid arrays

Surface-texture with silicon (Si) nanopyramid arrays has been considered as a promising choice for extremely high performance solar cells due to their excellent anti-reflective effects and inherent low parasitic surface areas. However, the current techniques of fabricating Si nanopyramid arrays are always complicated and cost-ineffective. Here, a high throughput nanosphere patterning method is developed to form periodic upright nanopyramid (UNP) arrays in wafer-scale. A direct comparison with the state-of-the-art texture of random pyramids is demonstrated in optical and electronic properties. In combination with the antireflection effect of a SiNx coating layer, the periodic UNP arrays help to provide a remarkable improvement in short-wavelength response over the random pyramids, attributing to a short-current density gain of 1.35 mA/cm2. The advanced texture of periodic UNP arrays provided in this work shows a huge potential to be integrated into the mass production of high-efficiency Si solar cells.

Colloidal transfer printing method for periodically textured thin films in flexible media with greatly enhanced solar energy harvesting

The successful fabrication of high-performance flexible thin film solar cells (TFSCs) directly on diverse substrates is intrinsically limited by the processing temperature and substrate property. In this work, a colloidal transfer-printing (CTP) method is developed to fabricate large-area flexible thin-film absorbers with an antireflection coating and periodic configurations. Compared with a planar film, such structures exhibit much lower reflectance due to the antireflection introduced by the textured polydimethylsiloxane and the enhanced scattering introduced by the periodic back-scattering reflector. Optical simulation using the finite-element method indicates the structural periodicity for maximum light absorption is of 300 nm for an ultrathin amorphous silicon (a-Si) film with a thickness of 160 nm. The patterned a-Si film yields an overall absorption of 64.8%, which is much larger than the planar counterpart of 38.5%. This new approach to thin-film transfer can be readily extended to other material systems and device structures, opening up an effective alternative to traditional fabrication of the low-cost and high-performance optoelectronic devices.

High performance organic/20 μm crystalline-silicon heterojunction solar cells

Si/organic hybrid solar cells have attracted considerable attention as a promise long-range photovoltaic technique with low process cost and high power conversion efficiency. However, the consumption of a whole bulk silicon wafer in this kind of cells is obviously not cost-effective. Here, we report a flexible poly (3, 4-ethylene-dioxythiophene):polystyrenesulfonate/crystalline silicon (c-Si) heterojunction solar cell with substrate thickness of sub-20 μm. The c-Si substrate has a nanopore surface texturing fabricated by a sample metal-assisted chemical etching process. In comparison to the non-textured thin c-Si cell, the nanopore-textured cell shows a 19.5% increase in JSC and a 40.2% increase in the efficiency. The inherent advantages of absorption improvement, p–n junction area increase, and carrier collection capability enhancement endow this nanopore-textured thin c-Si hybrid solar cell to approach an efficiency of 8.7%, as high as its bulk counterpart.

Advanced optoelectronic simulations for ultrathin crystalline silicon solar cells with rationally designed nanopatternings

With the rapid development of photovoltaic (PV) technology, ultrathin crystalline silicon (c-Si) solar cells (SCs) with the thickness of only 10 !! 20 μm have attracted tremendous attention due to the reduced material loss and a high photoelectric conversion efficiency (PCE). However, the shortened active layer decreases the optical-path of incident light substantially and thus lowers optical absorption efficiency. The surface textures are the optimal ways to improve the light-harvesting efficiencies by suppressing the reflection of the entire system and coupling incident light into the underlying absorber layer. In this report, we study the light-trapping performances of two typical surface nanostructures [i.e., inverted-nanopyramid (INP) and nanopencil (NP)] with the aid of the three-dimensional (3D) optoelectronic simulation that based on the finite-element method (FEM). We investigated theoretically the light-harvesting properties of 20 μmthick c-Si thin films structured by INPs with three typical periodicities (i.e., 300, 670, and 1400 nm) and their combined designs (i.e., front, rear and double-sided surface textures). As a result, the optimized design yields a photocurrent density (Jph) of 39.86mA/cm2, which is about 76% higher than the flat counterpart and is only 3% lower than the value of Lambertian limit. Besides, we have verified experimentally the results, which are well-matched with the simulated one. For NP, excellent light-trapping can be achieved by adjusting the configurations of the top portion (i.e., pitch, diameter and height, et al.). The broadband enhancement in optical performance was obtained when compared to flat, nanopillar and nanocone, and the mechanism behind was fully illustrated by analyzing the absorption profiles. Moreover, the NP arrays with rational design were successfully applied in hybrid SCs by employing organic hole-transporting poly (3,4-ethylene dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) on the top of the n-type c-Si, leading to simultaneously increase both in optical- and electro- properties. More results and explanations will be shown in a detailed way.

Low aspect-ratio reconstructed surface texturing with efficient light trapping for crystalline silicon solar cell applications

We report on the performances of reconstructed nanopore, black-silicon and nanopillar. Compared with the conventional structures, these nanostructures offer a lower integrated reflectance and much smaller surface recombination losses.