Flexible Ultra-Thin Silicon Solar-Cell Implemented with Energy-Down-Shift Via Cd0.5Zn0.5s/ZnS Core/Shell Quantum Dots

For the past few decades, flexible or bendable silicon, dye-sensitized, organic, and hybrid solar-cells have been intensively researched to improve power-conversion-efficiency (PCE) and to minimize the fabrication cost because of their attractive applications such as building-integrated photovoltaic (BIPV), and portable electronics power. Although flexible or bendable dye-sensitized, organic, and hybrid solar-cells have demonstrated a relevant PCE and solar-cell fabrication cost for a real photovoltaic application, their PCE and solar-cell fabrication cost was still quite higher than that of solid silicon solar-cells. Recently, S. Jeong et al. reported ultra-thin silicon (~10-um in thickness) solar-cell showing a maximum PCE of 13.7 %. However, it was fabricated with typical high-cost semiconductor processes such as SOI wafer, photolithograph, and deep reactive-ion etching (RIE). In addition, it could not demonstrate flexible or bendable solar-cell characteristics.

We, therefore, developed a flexible ultra-thin silicon solar-cell implemented with energy-down-shift layer via Cd_0.5Zn_0.5S/ZnS core/shell quantum-dots (QDs). In particular, in order to achieve a low cost solar-cell fabrication, we introduced ultra-thinning process (~30-um in thickness) via KOH etching, back-surface-filed (BSF) formation via thin Al-film evaporation and rapid-thermal annealing (RTA), top electrode patterning via thin film Ag evaporation, and spin-coating of core/shell QDs. In addition, in order to enhance an external quantum efficiency (EQE) at ultra-violet (UV) wavelength region, we implemented energy-down-shift layer by coating Cd_0.5Zn_0.5S/ZnS core/shell QDs on flexible ultra-thin silicon solar-cells.

It was observed that flexible ultra-thin (~30-um in thickness) silicon solar-cells implemented with energy-down-shift layer showed stable flexible and twistable characteristics as shown in Fig. 1. In addition, Fig. 2 presents the EQE as function of wavelength and current density vs. voltage characteristics. It was confirmed that the Cd_0.5Zn_0.5S/ZnS core/shell QDs on the flexible ultra-thin solar-cells enhanced the EQE of about 17 %_P at the UV light region (300 – 400 nm) as shown in Fig. 2(a). In particular, the QDs enhanced the PCE   to 0.77%_P by the energy-down-shift effect that enhanced the EQE, which resulting in that the PCE of the ultra-thin silicon solar-cells was about 12.37 %.

In the conference, we will present in detail physical, optical electrical, and flexible properties of flexible ultra-thin silicon solar-cells implemented with energy-down-shift layer. In particular, we will report how their PCE can be sustained after the bending cycles of 5,000.

* This work was financially supported by the Brain Korea 21 Plus Program in 2015.

Reference

[1] DH Lee, JY Kwon, and S Maldonado Nano letters 14(4), 1961-1967(2014)

[2] S Wang, BD Weil, and Y Li, Nano letters, 13 (9), 4393-4398 (2013)