Research

ITO-free flexible polymer solar cells: From small model devices to roll-to-roll processed large modules

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Abstract

Manufacturing of flexible ITO-free polymer solar cell modules by roll-to-roll methods (R2R) is described. Inverted devices with top illumination were built on a Kapton foil and an Aluminum/Chromium bi-layer system was used as electron contact. The layer structure was Kapton/Al/Cr/P3HT:PCBM/PEDOT:PSS/Ag (printed) and devices were encapsulated. Small area cells (3 cm2 active area) were first carefully optimized investigating the influence of a number of discrete parameters on performance. A maximum power conversion efficiency of 1.4% was achieved under 1 sun illumination (AM 1.5G, 1000 W m−2). Optimized lab-scale single devices were then transferred to a full R2R process combining slot-die coating and screen printing. All the layers were processed from solution under ambient conditions. Two different concepts were explored: (i) serially connected stripe modules (to reduce the Ohmic losses) and (ii) monolithic modules (to achieve high geometric fill factor and increase the flexibility of the process). For this second concept, the only layer that needs to be patterned is the silver grid electrode and the grid pattern design can then be readily tuned. As an example, four different patterns were used and the resultant performances compared. Modules comprising 16 serially connected cells gave total area efficiencies up to 0.5% (235 cm2 – 1% on the active area) while the best monolithic ones gave 0.35% (100 cm2 – 0.4% on the active area). The freshly prepared devices consistently showed an inflection point in the IV curve indicative of a rather poor photovoltaic behavior. Upon light exposure and repeated IV scans the inflection point partially disappeared, and performance significantly increased.