Water-based reactive silver ink for flexible polymer solar cells and its interaction with the polymeric hole-transporting layer

Polymer solar cells (PSCs) have attracted considerable attention in the last few decades because of their intrinsic lightweight, their potential transparency, the possibility to fabricate them on flexible substrates, and the low-cost production potentially achievable by means of all solution and large-scale roll-to-roll (R2R) processing [1]. PSCs are characterized by a multilayer architecture where the photoactive layer is sandwiched between a semitransparent front electrode and a metal back electrode [2]. The solution-based deposition of the metal back electrode in inverted PSCs is considered one of the crucial issues towards the upscaling of PSC technology. In this work, a water-based solution-processable silver ink formulation with low sintering temperature was successfully employed as precursor for the fabrication of the metallic back-electrode in flexible inverted PSC devices fabricated using R2R-compatible printing techniques. In order to investigate the effect of the deposition of such reactive silver ink on the underlying PSC layers, different back-electrode architectures were investigated and thoroughly characterized. In addition, the influence of the thickness of the hole-transporting poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer on the functional performance of the PSC devices was also investigated. Results revealed that devices incorporating a silver electrode with multi-finger grid structure outperformed those equipped with a full-coverage electrode configuration. This effect was correlated with the amount of silver ink that can penetrate through the PEDOT:PSS layer into the underlying PSC layers and cause shunting effects. Moreover the thickness of the PEDOT:PSS layer represents a crucial parameter in order to ensure correct functioning of the PSC devices.