Spatially Resolved Spectroscopic Mapping of Photocurrent and Photoluminescence in Polymer Blend Photovoltaic Devices

Combined confocal photoluminescence and photocurrent microscopy is used to study the interplay between blend morphology, polymer conformation, and photocurrent generation in polymer solar cells based on blends of the polymers PFB (poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine)) and F8BT (poly(9,9'-dioctylfluorene-co-benzodiathiazole)). Two-dimensional photoluminescence spectral maps of the blend are acqaired that allow for the composition of the domains to be identified, whereas the use of different laser sources allows for photocurrent imaging with selective excitation. Photoluminescence mapping additionally facilitates correlation between exciplex emission and photocurrent generation for each domain. We find that photocurrent is higher in F8BT-rich domains regardless of excitation source (375 nm, preferential absorption by PFB; 405 nm, equal absorption; 445 nm, preferential absorption by F8BT) though the discrepancy is less when PFB is preferentially excited. Prominent exciplex emission is observed in PFB-rich domains that suggests that, although PFB-rich domains have a higher degree of intermixing than the F8BT-rich domains (beneficial for exciton dissociation), this intermixing may be too fine, hindering charge separation and promoting exciplex formation. We also discuss the influence of surface and capping layers on device performance finding evidence for reduced charge collection efficiency for regions in the F8BT-rich phase covered by a PFB capping layer.