High performance A–D–A oligothiophene-based organic solar cells employing two-step annealing and solution-processable copper thiocyanate (CuSCN) as an interfacial hole transporting layer
Two acceptor–donor–acceptor (A–D–A)-type oligomers 1 and 2 comprising dithieno[3,2-b:2′,3′-d]pyrrole as a central unit and dicyanovinylene (DCV) or 1,3-indanedione as terminal acceptor groups have been synthesized for use as a donor in a solution-processed bulk-heterojunction (BHJ) solar cell in combination with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor. By employing PEDOT:PSS as a HTL and two-step annealing treatment i.e. thermal annealing followed by solvent vapor annealing (TSA) of the active layer the power conversion efficiencies (PCEs) were significantly improved from 3.72% to 6.95% for 1 and 4.18% to 7.54% for 2. The improvement of the PCE was mainly due to the increase in JSC and FF values which can be attributed to the re-organization of the donor material within the blend leading to balanced charge transport and improved crystallinity of the donor in the active layer. The influence of TSA on the degree of crystallinity and blend morphology was investigated by UV-Vis absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). When PEDOT:PSS was replaced by CuSCN as the HTL, a further enhancement of the photovoltaic parameters is realized leading to an increase in the PCE to 7.63% for 1 and 8.22% for 2. These results not only provide important insight into the influence of the two-step annealing process on molecular packing and charge transport properties, but also reveal the significance of CuSCN as the HTL on the device performance of tailored molecular materials.