Organic heterojunctions are widely used in organic electronics and they are composed of semiconductors interfaced together. Good ordering in the molecular packing inside the heterojunctions is highly desired but it is still challenging to interface organic single crystals to form single crystalline heterojunctions. Here, we describe how organic heterojunctions are formed by interfacing two single crystals from a droplet of a mixed solution containing two semiconductors. Based on crystallization of six organic semiconductors from a droplet on a substrate, two distinct crystallization mechanisms have been recognized in the sense that crystals form at either the top interface between the air and solution or the bottom interface between the substrate and solution. The preference for one interface rather than the other depends on the semiconductor-substrate pair and, for a given semiconductor, it can be switched by changing the substrate, suggesting that the preference is associated with the semiconductor substrate molecular interaction. Furthermore, simultaneous crystallization of two semiconductors at two different interfaces to reduce their mutual disturbance results in the formation of bilayer single crystals interfaced together for organic heterojunctions. These single-crystalline heterojunctions exhibit ambipolar charge transport in field-effect transistors, with the highest electron mobility of 1.90 cm(2) V-1 s(-1) and the highest hole mobility of 1.02 cm(2) V-1 s(-1). Hence, by elucidating the interfacial crystallization events, this work should greatly harvest the solution-grown organic single-crystalline heterojunctions.