A Solid‐State Plasmonic Solar Cell via Metal Nanoparticle Self‐Assembly

Finding new mechanisms for photo-induced electron-hole separation is an important challenge for solar energy conversion. In the last decade, metal particles have been proposed as potential sensitizers in photovoltaics. Metal nanoparticles (NP) have very high absorption and scattering cross sections, that can be tailored throughout the visible region to the near infrared by varying their size and shape. [ 1 , 2 ] At least three different ways to implement metal particles in photovoltaic systems have been explored. In the fi rst case, the local near fi eld enhancement associated with surface plasmon excitation is used to enhance charge carrier generation. [ 3 ] Near fi eld effects have been reported to increase photocurrents in dye sensitized solar cells [ 4–8 ] and polymer based organic solar cells. [ 9–11 ] A second approach uses the high scattering cross section of metal particles at the surface plasmon resonance wavelengths to re-direct light into a solar cell substrate. [ 3 , 12–15 ] The third possibility is to use metal particles as the actual light harvesting element inducing charge separation in a photovoltaic device, which so far has received far less attention. We report on the fabrication and performance analysis of such a solid-state plasmonic solar cell: Photocurrent is generated by the excitation of gold or silver nanoparticles adsorbed to TiO 2 , while holes are transported through a hole conducting material (Spiro-OMeTAD) to the counter electrode. The nanoparticle fi lms are created via a fast and scalable, electrostatic self-assembly method yielding well-defi ned interparticle distances. These optically homogeneous fi lms absorb up to 37% of the incident photons at the respective surface plasmon resonance wavelengths. The spectral photocurrent response of the solar cells closely follows the absorption spectrum of the metal nanoparticle layers. The photocurrents are sustainable for at least 20 hours demonstrating that the charge separation process is regenerative. We propose three mechanisms for surface plasmon based charge carrier separation.