Stretchable and colorless freestanding microwire arrays for transparent solar cells with flexibility

Transparent solar cells (TSCs) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, the rigidity and color-tinted transparent nature of those devices strongly limit the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n -silicon microwires (SiMWs). Flat-tip SiMWs with controllable spacing are fabricated via deep-reactive ion etching and embedded in a freestanding transparent polymer matrix. The light transmittance can be tuned from ~10 to 55% by adjusting the spacing between the microwires. For TSCs, a heterojunction is formed with a p-type polymer in the top portion of the n-type flat-tip SiMWs. Ohmic contact with an indium-doped ZnO film occurs at the bottom, and the side surface has an Al 2 O 3 passivation layer. Furthermore, slanted-tip SiMWs are developed by a novel solvent-assisted wet etching method to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip SiMWs helps light-matter interactions in adjacent microwires. The TSC based on the slanted-tip SiMWs demonstrates 8% efficiency at a visible transparency of 10% with flexibility. This efficiency is the highest among Si-based TSCs and comparable with that of state-of-the-art neutral-color TSCs based on organic–inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs. Transparent solar cells: Microwire-array shows potential A flexible, stretchable and fully transparent solar cell shows promise for harvesting sunlight as it hits windows. In designing transparent solar cells, there is a trade-off between efficiency and transparency – many existing cells are rigid and tinted because of the way they absorb certain wavelengths of light, and this limits their use in electronic devices and windows. Now, Kyoung Jin Choi at the Ulsan National Institute of Science and Technology, Ulsan, Korea, and co-workers have used a freestanding form of microwire-arrays to create a flexible, fully transparent solar cell. Their design comprises a crystalline silicon microwire array embedded in a transparent polymer matrix. The conducting junction occurs where the n-type-Si microwire tips meet the p-type po