Fabrication of Asymmetrical Micro/Nanohole Arrays for Translucent Solar Cells

An array of 3D micro/nanohole structures fabricated on semiconductor materials offers potential in translucent photovoltaics (TLPV) due to their exceptional light absorption and photocarrier collection capabilities. Unlike traditional opaque solar panels typically installed on rooftops, these structures provide dual functionality, which is attractive for various applications, including building-integrated photovoltaic (BIPV) systems, urban green spaces, and portable electronics. Such devices are realized through advanced fabrication techniques such as deep-reactive ion etching (DRIE), focused ion beam (FIB) etching, and metal-assisted chemical etching. Although these methods achieve precise feature definition on semiconductors, they are generally restricted to small sample sizes (<a few 100 mm²), posing challenges for scaling up to large-area PV module production.

In this study, we introduce a rapid fabrication method using a UV laser beam (355 nm at a 20 ns pulse) to create an array of nano/microholes on Si P-N junction solar cells. To prepare perforated arrays, we initially conducted thermal oxidization of Si wafers (200 um thick) at 1,000 °C for 120 minutes, forming a 400 nm thick SiO₂ layer on both sides. The device area (10 mm × 10 mm) was defined using lithography and subsequent oxide etching in a diluted hydrofluoric acid (10 %). After wet cleaning (RCA 1 and 2), n-type dopants (phosphorous) were infused into the wafer for 60 minutes at 1200 °C. A second thermal oxidation process followed to drive the dopants deeper on the front side and to develop a thicker oxide layer on the back side (total 500 nm SiO₂), which acted as a protective layer to minimize potential beam damage to Si during UV laser drilling. The laser output averaged 1.6 W at a repetition rate of 30 kHz, with a nominal hole diameter on the back-side of the device of about 20 µm. In contrast, following the Gaussian laser beam profile, the front-side diameter varied depending on the number of laser passes (100’s nm to a few um). Additionally, the asymmetrical micro/nanohole dimensions on the front and back could be adjusted by modifying the wet etching duration in the HNA solution (a mixture of HF, HNO₃, and CH₃COOH).

We evaluated the effects of the laser beam on PV performance by comparing the primary PV parameters (Voc, Jsc, and FF) across our prototype cells fabricated under identical conditions but varying hole spacings from 60 µm to 300 µm. The initial fill factor (FF) for a planar control was 0.65, reduced to 0.47 in cells with a 60 µm spacing. The open-circuit voltage (Voc) remained above 90 % of its initial value for cells with spacings over 180 µm, but dropped to 0.48 V, a 20 % decrease, in cells with a 60 µm spacing. This reduction is likely due to increased non-radiative defects near the P-N junction caused by the laser processing unless HNA etching is optimized. Despite notable declines in FF and Voc, the short-circuit current (Isc) remained high, at over 90 % of the baseline value (26 mA/cm²) recorded for the initial planar cell. Our perforated Si solar cells exhibited notable translucency, attributed to significant sunlight scattering within the tapered micro/nanoholes across the cell thickness. This unique micro/nanohole geometry seems to enhance multiple light scattering events and absorption, which may partially explain the high photocurrent observed in our perforated cells. We will discuss how further optimization of micro/nanohole design and process refinements could enhance these optical benefits.