Water‐Soluble Polyfluorenes as an Interfacial Layer Leading to Cathode‐Independent High Performance of Organic Solar Cells

Abstract

Novel poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl)‐9‐fluorene)) dibromide (WPF‐6‐oxy‐F) and poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐methoxyethoxy)ethyl)‐fluorene)] dibromide (WPF‐oxy‐F) compounds are developed and the use of these water‐soluble polymers as an interfacial layer for low‐cost poly(3‐hexylthiophene):phenyl‐C61 butyric acid methyl ester (P3HT:PCBM) organic solar cells (OSCs) is investigated. When WPF‐oxy‐F or WPF‐6‐oxy‐F is simply inserted between the active layer and the cathode as an interfacial dipole layer by spin‐coating water‐soluble polyfluorenes, the open‐circuit voltage (Voc), fill factor (FF), and power‐conversion efficiency (PCE) of photovoltaic cells with high work‐function metal cathodes, such as Al, Ag, Au, and Cu, dramatically increases. For example, when WPF‐6‐oxy‐F is used with Al, Ag, Au, or Cu, regardless of the work‐function of the metal cathode, the Voc is 0.64, 0.64, 0.58, and 0.63 V, respectively, approaching the original value of the P3HT:PCBM system because of the formation of large interfacial dipoles through a reduction of the metal work‐function. In particular, introducing WPF‐6‐oxy‐F into a low‐cost Cu cathode dramatically enhanced the device efficiency from 0.8% to 3.36%.

Nonconjugated Anionic Polyelectrolyte as an Interfacial Layer for the Organic Optoelectronic Devices

“…0.1 eV). This is a different result from the cases of the reported IFLs based on PEO, CPEs, − P-OH, and PVs, , for which the large increase of V oc was attributed to the enhancement of the PCE of the devices. Recently, Wang et al reported a similar observation in the device with poly­(vinyl­pyrroli­done) (PVP) as an IFL, which is a nonconjugated neutral polymer with a strong dipole moment.…”

“…Most of the increase in the device efficiency resulted from the 12% enhancement in the short-circuit current ( J sc ), while the fill factor (FF) was very close to that of the device without an IFL. This result is not usually observed in the previously reported results of the buffer layer based on PEO, CPEs, − P-OH, and PVs, , for which either the J sc and the FF increase simultaneously or only the FF increases along with the increase of the V oc .…”

Effects of Polyfluorene Polyelectrolyte Interfacial Layers on Selectivity and Recombination Measured Using the Interdigitated Back-Contact Solar Cell

“…The PFFC interfacial layer increases the electron selectivity (decreases the hole selectivity) relative to PEDOT–PSS by 13 orders of magnitude from log S h = 16 ± 1 to 2.7 ± 0.1. This is consistent with the wide use of cationically functionalized organic semiconductors as interlayers at the electron-collecting electrode of perovskite and organic photovoltaics. ,,− At the silicon interface, PFFC nearly shifts the PEDOT–PSS contact from a highly hole selective contact to one that is more electron- than hole-selective. In contrast, PFFA preserves much of the hole selectivity of PEDOT–PSS with the selectivity changing from log S h = 16 ± 1 to 10.5 ± 0.4.…”

“…Ionically functionalized polyfluorenes and related organic semiconductors have been most widely studied with bound cationic functionality and to improve electron selectivity (as observed here). ,,− The results of PFFA suggest that it could provide hole selectivity approaching that of PEDOT–PSS. To test this hypothesis outside of the silicon model system, we tested PFFA and PFFC as interfacial layers to modify the hole-collecting ITO contact in a P3HT/PCBM solar cell.…”

Development of Novel Conjugated Polyelectrolytes as Water-Processable Interlayer Materials for High-Performance Organic Photodiodes

“…The measured noise current was just above ∼2−3 times of the shot noise limit calculated from the dark current, implying that the shot noise and thus dark current are the main source of noise in the fabricated OPDs. As summarized in Figure 3(b), the calculated D* values of OPDs with EG40 reached over 10 13 Jones for the entire visible range (400−700 nm), which is comparable to Si-based inorganic photodiodes, 22 and these results are comparable to the results reported as highperformance OPDs (Table S1). 30−32 Fast response time against incident light is one of the most important performance criteria for OPDs.…”