Ternary Blend Ink Formulations for Fabricating Organic Solar Cells via Inkjet Printing
Kraft, Thomas M.
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Two approaches were followed to achieve increased control over properties of the photo-active layer (PAL) in solution processed polymer solar cells (PSCs). This was accomplished by either (1) the addition of functionalized single-walled carbon nanotubes (SWCNTs) to improve the charge transport properties of the device or (2) the realization of dual donor polymer ternary blends to achieve colour-tuned devices. In the first component of the study, P3HT:PC61BM blends were doped with SWCNTs with the ambition to improve the morphology and charge transport within the PAL. The SWCNTs were functionalized with alkyl chains to increase their dispersive properties in solution, increase their interaction with the P3HT polymer matrix, and to disrupt the metallic characteristic of the tubes, which ensures that the incorporated SWCNTs are primarily semi-conducting. P3HT:PCBM:CNT composite films were characterized and prepared for use as the photoactive layer within the inverted solar cell. The CNT doping acts to increase order within the active layer and improve the active layer’s charge transport properties (conductivity) as well as showed some promise to increase the stability of the device. The goal is that improved charge transport will allow high level polymer solar cell (PSC) performance as the active layer thickness and area are increased, which is an important consideration for large-area inkjet printing. The use of ternary blends (two donor polymers with a fullerene acceptor) in bulk-heterojunction (BHJ) photovoltaic devices on plastic substrates was investigated as a future means to colour-tune ink-jet printed PSCs. The study involved the blending of two of the three chosen donor polymers [red (P3HT), blue (B1), and green (G1)] with PC61BM. Through external quantum efficiency (EQE) measurements, it was shown that even devices with blends exhibiting poor efficiencies, caused by traps, both polymers were able to contribute to the photovoltaic (PV) effect. However, traps were avoided to create a parallel-like BHJ (PBHJ) when two polymers were chosen with suitable physical compatibility (harmonious solid state mixing), and appropriate HOMO-HOMO energy band alignment. The parallel diode model was used to describe the PV circuit of devices with the B1:G1:PC61BM ternary blend.