Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of the boron nanostructures have been found to be metallic, despite their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importantly, the low-dimensional semiconducting boron allotropes are predicted to be an excellent solar-cell material with a power conversion efficiency of up to 22%, paving the way for their promising optoelectronic application.