The energy of photons, that is, the wavelength of light, can be upgraded through interactions with materials in a process called photon upconversion¹. Although upconversion in organic solids is important for various applications, such as photovoltaics and bioimaging, conventional upconversion systems, based on intersystem crossing (ISC), suffer from low efficiency^2,3,4,5,6. Here we report a novel upconversion system with heterojunctions of organic semiconductors. The upconversion occurs through charge separation and recombination, which mediate charge transfer states at the interface. This process can efficiently convert the incident photons to triplets without relying on ISC, which is typically facilitated by the heavy-atom effect¹. As a result, a solid-state upconversion system is achieved with an external efficiency that is two orders of magnitude higher than those demonstrated by conventional systems⁶. Using this result, efficient upconversion, from near-infrared to visible light, can be realized on flexible organic thin films under a weak light-emitting-diode-induced excitation, observable by naked eyes.

光子的能量，即光的波长，可以通过在称为光子上转换¹的过程中与材料的相互作用来升级。尽管有机固体中的上转换对于各种应用很重要，例如光伏和生物成像，但基于系统间交叉 (ISC) 的传统上转换系统效率低^2,3,4,5,6。在这里，我们报告了一种具有有机半导体异质结的新型上转换系统。上转换通过电荷分离和复合发生，介导界面处的电荷转移状态。这个过程可以有效地将入射光子转换为三重态，而不依赖于 ISC，这通常由重原子效应^1促进. 因此，固态上变频系统的外部效率比传统系统⁶所展示的效率高两个数量级。利用这一结果，可以在柔性有机薄膜上实现从近红外光到可见光的高效上转换，在弱发光二极管诱导激发下，肉眼可观察到。