In the last decades a great amount of effort has been devoted to the fabrication of organic fi eld-effect transistors (OFET) featuring high performance as key elements for new organicbased logic applications. The endeavor has been focused on the improvement of each of the main contributing components of the devices. This includes design and synthesis of new semiconductors (p-and n-type), which show improved air stability, self-assembly behavior, and electrical characteristics, and, more recently, the development of novel high-performance gate dielectrics and novel processing techniques. [ 1 ] In addition to the optimization of the device architecture, the engineering of the interface semiconductor-dielectric [ 2 ] and semiconductormetallic contacts have attracted much interest due to the importance of the interfacial morphology and the hole-electron injection barrier on the device performance. In particular, selfassembled monolayers (SAMs) are widely used to tune the wettability and work function of the metal-organic junctions. [ 3 ] In contrast to the extensively employed chemisorbed alkane-and arene-thiol SAMs, the use of SAMs based on molecules that respond to external stimuli has still not been explored. Among responsive systems, azobenzenes are known to undergo reversible photoinduced isomerization between trans and cis forms, which can exhibit different optical and electrical properties. [ 4 ] Such unique photoisomerization process in SAMs based on azobenzene derivatives has been exploited for applications, such as the generation of light-triggered dynamic surfaces showing reversibly switchable wettability [ 5 ] and controlled DNA delivery, [ 6 ] and are an interesting platform to study switchable electronic characteristics at the nanoscale. [ 7 ] Although most photochromic molecules exhibit a poor switching capacity (i.e., yield) at the ensemble level when chemisorbed at surfaces, we recently found that the terminally thiol-functionalized biphenyl azobenzene (AZO, Scheme 1 ) [ 8 a] forms highly ordered and tightly packed SAMs on Au(111), which display isomerization yields exceeding 96%. The unexpectedly effi cient isomerization reaction in the monolayer most likely emerges from the light-induced collective isomerization over entire crystalline domains. [ 8 b] Here such a SAM is exploited once it is chemisorbed on the Au source and drain electrodes of an OFET to optically modulate for the fi rst time the charge injection at the electrodesemiconductor interface.As a test bed OFET devices were fabricated in a bottom-gate bottom-contact confi guration using an air-stable n-type system, i.e., N , N '-1H,1H-perfl uorobutyl dicyanoperylenecarboxydiimide (PDIF-CN 2 ), [ 9 ] as organic semiconductor (SC). By employing such a geometry, the illumination has to be performed from the top of the device, implying that the SC can act as a light-fi ltering and absorbing layer. To overcome this issue, two critical aspects had to be considered when choosing the ideal semiconductor. First, the absorption spectra of the SC...