Following in Situ the Deposition of Gold Electrodes on Low Band Gap Polymer Films

Metal top electrodes such as gold are widely used in organic solar cells. The active layer can be optimized by modifications of the polymer band gap via side-chain engineering, and low band gap polymers based on benzodithiophene units such as PTB7 and PTB7-Th are successfully used. The growth of gold contacts on PTB7 and PTB7-Th films is investigated with in situ grazing incidence small-angle X-ray scattering (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) during the sputter deposition of gold. From GIWAXS, the crystal structure of the gold film is determined. Independent of the type of side chain, gold crystals form in the very early stages and improve in quality during the sputter deposition until the late stages. From GISAXS, the nanoscale structure is determined. Differences in terms of gold cluster size and growth phase limits for the two polymers are caused by the side-chain modification and result in a different surface coverage in the early phases. The changes in the diffusion and coalescence behavior of the forming gold nanoparticles cause differences in the morphology of the gold contact in the fully percolated regime, which is attributed to the different amount of thiophene rings of the side chains acting as nucleation sites.