Organic PhotoVoltaic cells offer an attractive approach to low-cost solar energy conversion due to a combination of factors such as the availability of materials, flexibility, low-temperature and solution-based fabrication processes. However, due to the short exciton diffusion length and to the low charge mobility[i] of organic semiconductors, thin photoactive films are required, resulting in a very small absorption of light and therefore leading to a limited Power Conversion Efficiency. Nowadays, constructing optically thick but physically very thin photovoltaic absorbers is a challenge.The emerging area of plasmonics offers new paths for reaching this aim and improving the OPV efficiency. Usually, plasmonic structures integrated in current solar devices consist in[ii]:    Metallic nanoparticles used to excite localized surface plasmons that improve scattering and absorption cross-sections of the active medium;    Patterned metallic back-contact that can couple sunlight into surface plasmonic polariton modes that propagate at the semiconductor/metal interface.In any case, a significant portion of the sunlight spectrum still cannot be collected. However, the possibility of engineering the absorption band of the active layer to achieve the desired absorption performances can be accomplished by means of structures known as “Hyperbolic MetaMaterials”[iii](HMMs). Consisting of metal/dielectric multilayers with subwavelength dimensions, HMMs manifest, as a main feature, a hyperbolic dispersion. The optical response of HMMs can be easily designed in the framework of the Effective Medium Theory, so that new photonic absorption bands can be engineered in the visible-Near InfraRed range.Here we develop a new concept device, embedding subwavelength sized layers of the well-studied Bulk HeteroJunction P3HT/PCBM system as a dielectric and of Al as a metal. The HMM system shows enhanced photogeneration performances with respect to the control device, realized with equivalent thickness for the BHJ layer. The beneficial contribution of plasmonic designed thin layers becomes evident even with a single Al/BHJ bilayer. The relevance of the results goes well beyond the use of the particular system chosen in this work, since it is possible to replace it with a plethora of promising metal/dielectric combinations.

[i]Alan J. Heeger, Adv. Mater., 2014, 26, 10-28;

[ii]Chuan F. Guo, Tianyi Sun, Feng Cao, Qian Liu and Zhifeng Ren, Light: Science & Applications, 2014, 3, e161;

[iii]Alexander Poddubny, Ivan Iorsh, Pavel Belov and Yuri Kivshar, Nature Photonics, 2013, 7, 958-967;