Nanometer-Mesa Inverted-Pyramid Photonic Crystals for Thin Silicon Solar Cells

Abstract

The usage of ultrathin flexible silicon foil can further extend the functionality of silicon and emerging silicon-based tandem solar cells particularly in building and vehicle-integrated photovoltaics where high-efficiency, lightweight, and flexible solar panels are highly desired. However, silicon's relatively weak optical absorption coefficient especially in the near infrared (NIR) region limits its optoelectronic applications with a reduced wafer thickness. Herein, we seek to overcome this limitation by exploring the wave interference phenomenon for effective absorption of NIR light in ultrathin silicon. Particularly, inverted pyramid photonic crystals (PhCs) with nano−micrometer-scale feature sizes are carved directly on silicon. Detailed experimental and theoretical studies are presented by systematically examining the optical properties of PhC-integrated thin silicon substrates (down to a 10 μm thickness). The corresponding maximum photocurrent density for a thin absorber is projected and compared with that predicted by Lambertian's limit. In contrast to traditionally configured microscale inverse pyramids, we show that a small mesa width is critical to achieving high optical performance for a wave-interference-based absorption enhancement. Mesa widths as small as 35 nm are realized over a large wafer-scale fabrication using facile techniques. The optical performance of 10 μm silicon indicates that an ideal photocurrent density approaching 40 mA/cm 2 is feasible. This study indicates that photonic crystals provide strong wave interference in ultrathin silicon, and in particular, we observe high optical absorption even after removing more than 90% of the silicon from conventional "thick" Si wafers.

Innovative Strategies for Photons Management on Ultrathin Silicon Solar Cells

“…In the NIR region, the device presents increased light absorption of 10% compared to earlier results. [ 5 , 40 ] The authors compare the NIR absorption to a Lambertian reference (dashed black line in Figure 2c ) and observe that the inverted pyramid provides, on average, 30% higher absorption (red and green lines in Figure 2c ). This work uses the transfer matrix method to simulate parasitic Al absorption, which shows that the Al back reflector (dashed blue line in Figure 2c ) contributes 13% of the measured absorption at the wavelength of λ = 1100 nm.…”

Photonic Crystal Structures for Photovoltaic Applications

“…It is worth mentioning that some authors treat inverted pyramid structures as a type of texture [ 71 , 88 ], while others treat them as PCs [ 89 , 90 , 91 ], calling them surface PCs [ 91 ]. If the lattice spacing is comparable to the desired wavelength, light trapping should be enhanced by robust wave interference in the photonic structure [ 89 ] and resonant effects [ 91 ].…”

Morphological Relaxation of Strained Epitaxial Films for Stripe‐Geometry Devices

“…Several studies were aimed at using the finite size effects of channels (nanochannels) in research systems to predict the physical properties of industrial devices. [4][5][6][7][8][9] For example, the development of the fully-depleted complementary metal oxide semiconductors (FD-CMOS) technology [10] is based on ultra-thin silicon-on-insulator (SOI) [11] for which strain engineering in Si and SiGe layers of gate and source/drain areas respectively is a major technological challenge. [12,13] The unique features of nanogates can offer dramatic changes in growth modes, as well as matter and strain redistribution that are conceptually new as compared to bulk substrates given that finite-size effects come significantly into play.…”