Crystalline rubrene via a novel process and realization of a pyro-phototronic device with a rubrene-based film†
In this report, a recently invented novel pyro-phototronic effect is explored in an optoelectronic device containing a fully organic thin film. A unique plasma-based process is implemented for the synthesis of rubrene crystal directly grown in the form of thin film using a single-step approach. Applicability of the as-prepared crystalline rubrene thin film is demonstrated as a light-responsive device working under a photovoltaic (self-powered) mode. In particular, the detailed transient photoresponse of the device highlights the coupling of pyro-phototronic and photovoltaic effects. Thus the two important aspects of this study are: report of a novel one-step process for synthesis of crystalline rubrene containing thin film and demonstration of an optoelectronic device with crystalline rubrene, where the key governing mechanism is a coupled effect of pyro-phototronic and photovoltaic processes. This work also delivers a new experimental approach for realizing the pyro-phototronic effect based on a fully organic crystalline medium with rubrene, which has not been explored yet. Moreover, a symmetry-imposed limitation on pyroelectric materials, which states that the crystal has to be non-centrosymmetric in order to show pyroelectric behavior, is circumvented in this particular study. The present study shows that the occurrence of pyro-phototronic effect in a film containing a centrosymmetric rubrene crystal is solely due to the presence of a thin amorphous oxide layer formed over the crystalline film thereby inducing the surface polarization effect. Hence the existence of pyro-phototronic effect coupled with photovoltaic effect in a crystalline rubrene film indicates the huge potential of the two effects in improving the performance of optoelectronic devices. Moreover, generation of pyroelectricity due to surface polarization in rubrene crystals can be used as a possible analytical tool for probing the organization and dynamics of different biological systems.