Precise structural control is essential for high-performance semiconductors. In organic electronics, traditional methods for tuning the dimensions of device structures often rely on cumbersome, limited-resolution processes such as shadow mask patterning, printing, or viscosity tuning. Here, we report ultraviolet (UV) irradiation in ambient conditions as a transformative approach for tuning structural parameters of organic small molecule hole transport layers (HTLs) in vertical and lateral directions. The method preserves HTL conductivity while facilitating uniform thickness reduction through synergistic photo-induced oligomerization and photo-oxidative layer shrinking. Controlled thinning applies to various organic materials. In cavity architectures, UV-treated organic photodetectors show narrowband detection from 900 to 1200 nm with a full width at half maximum down to 25 nm, and UV-treated organic light-emitting diodes exhibit 75 nm peak tunability. Moreover, this strategy permits micrometer-scale lateral patterning of HTLs. Our work opens new opportunities for precise and practical engineering for organic electronic devices.