All-Solution-Processed, Ambient Method for ITO-Free, Roll-Coated Tandem Polymer Solar Cells using Solution-Processed Metal Films

A solution-processed silver film is employed in the processing of top-illuminated indium-tin-oxide (ITO)-free polymer solar cells in single-and double-junction (tandem) structures. The nontransparent silver film fully covers the substrate and serves as the bottom electrode whereas a PEDOT:PSS/Ag grid forms the semitransparent top electrode. All layers are roll-coated/printed on a flexible substrate by using only two techniques: slot-die coating for up to 11 consecutive layers and flexo-printing for the last Ag grid layer. The slot-die coated Ag film is compared to an evaporated Ag film in terms of surface morphological and topographical properties and to ITO in terms of flexibility. The slot-die coated Ag film demonstrates extremely low roughness (a root-mean-square roughness of 3 nm was measured over 240 x 320 mu m(2) area), is highly conductive (< 1 Omega/square), highly flexible, and cost-effective in comparison to other reported metal films applied in polymer solar cells. Such properties result in high fill factors exceeding 50% in both single and tandem structures on large-area devices (1 cm(2)) and the corresponding efficiencies exceed 2%.