Efficient Polythiophene/Polyfluorene Copolymer Bulk Heterojunction Photovoltaic Devices: Device Physics and Annealing Effects

Abstract

Here the influence of annealing on the operational efficiency of all‐polymer solar cells based on blends of the polymers poly(3‐hexylthiophene) (P3HT) and poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthiophen‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (F8TBT) is investigated. Annealing of completed devices is found to result in an increase in power conversion efficiency from 0.14 to 1.20%, while annealing of films prior to top electrode deposition increases device efficiency to only 0.19% due to a lowering of the open‐circuit voltage and short‐circuit current. By studying the dependence of photocurrent on intensity and effective applied bias, annealing is found to increase charge generation efficiency through an increase in the efficiency of the separation of bound electron‐hole pairs following charge transfer. However, unlike many other all‐polymer blends, this increase in charge separation efficiency is not only due to an increase in the degree of phase separation that assists in the spatial separation of electron‐hole pairs, but also due to an order of magnitude increase in the hole mobility of the P3HT phase. The increase in hole mobility with annealing is attributed to the ordering of P3HT chains evidenced by the red‐shifting of P3HT optical absorption in the blend. We also use X‐ray photoelectron spectroscopy (XPS) to study the influence of annealing protocol on film interface composition. Surprisingly both top and bottom electrode/blend interfaces are enriched with P3HT, with the blend/top electrode interface consisting of more than 95% P3HT for as‐spun films and films annealed without a top electrode. Films annealed following top electrode deposition, however, show an increase in F8TBT composition to ∼15%. The implications of interfacial composition and the origin of open‐circuit voltage in these devices are also discussed.

Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells

“…Despite extensive washing, it is clear that PEDOT:PSS is still present at this interface, indicating that either the P3HT and PEDOT:PSS layers are intermixed or that there is a thin intractable PEDOT:PSS film overlaying the P3HT. The presence of irremovable PEDOT:PSS at this interface is consistent with the findings of other groups who also observed a PEDOT:PSS residue upon delamination. ,,, The lack of dichroism in Figure a indicates that there is no alignment of the components in the unannealed film.…”

The Quantitative Effect of Surface Wetting Layers on the Performance of Organic Bulk Heterojunction Photovoltaic Devices

“…However, when ϕ B is in excess of 0.85, substantial reductions in OPV performance are predicted. Indeed, for wetting layers having ϕ B > 0.95, as has been reported for a range of OPVs, , our data predict losses in photocurrent by up to a half. This suggests that OPV interfaces should be engineered to avoid surface wetting of predominantly one component where possible.…”

Interfacial Chemical Composition and Molecular Order in Organic Photovoltaic Blend Thin Films Probed by Surface-Enhanced Raman Spectroscopy

“…Preannealing increased the concentration of low-surface-energy P3HT at that top interface, whereas postannealing raised the concentration of high-surface-energy F8TBT. Consequently, postannealing is expected to improve electron extraction from F8TBT at the top interface and make the blend better suited to conventional device architectures, in good agreement with the reported improvement in device performance for P3HT:F8TBT solar cells after postannealing compared to preannealing …”