Molecular hydrogen produced via solar energy is emerging as a prominent way to convert and store the conspicuous, yet intermittent, amount of energy that the Sun daily irradiates on Earth. A plethora of different approaches  is building up, creating an heterogenous picture of viable technologies which can tackle the need of promoting sustainable Solar Fuel production.
Among many, Hybrid Organic Photoelectrochemical (HOPEC) water splitting is gaining momentum in this field, and various approaches are currently being developed to realize hybrid tandem systems to perform unbiased water splitting. By taking advantage of the organic semiconductors properties such as low cost, stability, tuneable electronic properties and ease of large area production, these materials can help overcoming the limitations of standard inorganic photoelectrochemical water splitting. The potential of hybrid organic systems has been proven by our previous works[1]. Indeed, excellent photocurrent performances[2] or extended operational lifetime[3] have been obtained through careful optimization of hybrid photocathodes (PC) architecture.
Through materials research and device engineering, we are working toward the realization of a robust and high performing architecture. To meet this goal, in depth characterization and optimization of each layer has been performed. To this extent, through Intensity Modulated Photocurrent/Photovoltage spectroscopy (IMPS/IMVS) and Electrochemical Impedance Spectroscopy (EIS) we collected valuable insights on the charge transport mechanisms both from the Bulk Heterojunction to the charge selective contacts and from the catalyst layer to the electrolyte.

Various materials have been tested to maximize the performances. Charge selective contacts potential candidates have been selected from the classes of metal oxides, Transition Metal Dichalcogenides (TMD) and Small Molecules. Furthermore, to maximize the photovoltage and photocurrent of the device it is of paramount importance to tune the properties of hybrid organic PC acting on their photoactive layer, taking advantage of the latest advancements in the field of organic photovoltaic (OPV). Promising materials from OPV are, among many, the high-performance photo-absorbers PCE11 and PCDTBT, and the non-fullerene acceptors IDTBR and IDFBR, which were found to be responsible of a sharp increase in the open circuit voltage in OPV devices[4]. Their improved electronic properties and optimized band gap are here exploited to realize hybrid PC specifically designed to be coupled in a tandem configuration with a high performing perovskite[5] , realizing a full water-splitting system with cheap, easily processable and suitable for large area production materials. By modifying the hybrid PC, it was possible to extend the absorption range of the stack. Taking advantage of the high V_oc of the perovskite and the additional photovoltage coming from the PC, the Perovskite-HOPEC system efficiently performs the full water splitting reaction without the application of any external bias. The results clearly indicate that the Solar To Hydrogen (STH) efficiency of the system increases sensitively when proper design of the tandem system is achieved, with STH above 2% for the best performing case. These results prove that a tandem hybrid organic perovskite-photocathode stack can be used to realize efficient photoelectrochemical systems for solar fuels production, relying on highly tuneable materials and architectures.