Morphology-Dependent Trap Formation in Bulk Heterojunction Photodiodes

Abstract

We show that local structural variation affects the rate of aging in nanostructured polymer solar cells by comparing time-resolved electrostatic force microscopy (trEFM) and conventional device measurements on model polymer blends. Specifically, we study photovoltaic devices made from 1:1 blends of the polyfluorene copolymers poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). We photooxidize these films in situ using 365, 405, and 455 nm illumination under ambient conditions, with the wavelengths chosen to preferentially excite the different components. During photooxidation, we observe a faster loss of photocurrent generation from F8BT-rich domains, leaving the PFB-rich phases to show higher photoresponse even at wavelengths absorbed predominantly by F8BT. We propose that this effect is due to the more rapid degradation of PFB hole-transport pathways in the F8BT-rich regions, resulting in a loss of percolation pathways for hole transport in the F8BT-rich phase.

Photodecomposition and Morphology Evolution of Organometal Halide Perovskite Solar Cells

“…Importantly, we also note that the stability of the semiconductor active layer out to nearly 3000 J/cm 2 under illumination in an air environment compares favorably to similar studies of the degradation of all-organic semiconductor active layers, which can show performance degradation at doses of ~400 J/cm 2 . [32][33][34][35][36] These data further show the importance of material engineering of interfacial layers and electrodes to increase efficiencies and stability. 13,37 Figure 2A shows the current density-voltage (J-V) curves of the pre-degraded devices, which were made from films degraded with 0, 1500, 3000, 3400, 3600 and 3800 J/cm 2 photon doses.…”

Probing of Local Multifield Coupling Phenomena of Advanced Materials by Scanning Probe Microscopy Techniques

“…Charge generation and separation were typically fast processes that could not be probed by this type of trEFM, but it was suitable to track release of trapped charges, which might take hours depending on the material system. trEFM has also been used to study photo‐oxidation or degradation of organic solar cell materials . The fastest time resolution of trEFM was about 10 ns, with acquisition of the full data set of cantilever oscillation by digitizing the oscillation signal up to 50 MHz .…”

Functional Scanning Force Microscopy for Energy Nanodevices

“…But this still falls orders of magnitude slower than many fundamental processes in optoelectronic solar energy conversion devices. An important breakthrough in this direction is the time‐resolved EFM (tr‐EFM), which probes charge carrier dynamics in photovoltaic devices by tracking the cantilever oscillation frequency shift after a photoexcitation event . The frequency shift evolution is shown to be sensitive to photogenerated potential change and detectable time constant can be as fast as ≈10 ns .…”