A Kelvin Probe Force Microscopy Study of the Photogeneration of Surface Charges in All‐Thiophene Photovoltaic Blends

Abstract

Light‐induced generation of charges into an electron acceptor–donor phase‐segregated blend is studied. The blend is made of highly ordered nanoscopic crystals of 3″‐methyl‐4″‐hexyl‐2,2′:5′,2″:5″,2‴:5‴,2″″‐quinquethiophene‐1″,1″‐dioxide embedded into a regioregular poly(3‐hexylthiophene) matrix, acting as acceptor and donor materials, respectively. Kelvin probe force microscopy investigations reveal a tendency for the acceptor nanocrystals to capture the generated electrons whereas the donor matrix becomes more positively charged. The presence of particular positively charged defects, i.e., nanocrystals, is also observed within the film. The charging and discharging of both materials is studied in real time, as well as the effect of different acceptor–donor ratios. Upon prolonged thermal annealing at high temperatures the chemical structure of the blend is altered, leading to the disappearance of charge separation upon light irradiation. The obtained results allow a better understanding of the correlation between the nanoscopic structure of the photoactive material and solar‐cell performance.

Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging

“…As suggested above, these results appear to contradict previous reports of a correlation between surface photovoltage and open-circuit voltage 41 and/or local charge separation efficiency, [42][43][44][45][46][47][48][49] as our data are not consistent with either of these hypotheses, at least as a function of photooxidation in PFB/ F8BT blends. On the other hand, the trEFM charging rate measurements, which are well correlated with the macroscopic quantum efficiency, do demonstrate a clear decline in local charging rate with increasing photochemical degradation.…”

“…These contradictions can be explained, in part, by the differences in the material systems and experimental configurations but nevertheless highlight the difficulty in using basic SKPM data to draw conclusions about device performance. For instance, Palermo and co-workers have shown that, at least on insulating substrates, the surface photovoltages of neighboring donors and acceptors shift as one would expect: the acceptor becomes more negative, whereas the donor becomes more positive, consistent with electron and hole accumulation in each material. − However, studies on active device structures incorporating a conductive substrate (as is the case for the measurements presented herein) typically show a concerted shift in the potential of the entire surface, sometimes irrespective of the local structure. − On the basis of this observation in conjunction with detailed numerical simulations, Kemerink and co-workers have argued convincingly that the surface photovoltage in device-like organic photodiode structures is dominated by the difference in transport rates for electrons and holes and the injection barrier at the bottom contact, where an imbalance leads to an excess of a single carrier throughout the film. The steady-state concentration of each carrier under illumination is determined by a competition between charge generation, recombination, transport, and trapping.…”

The Relationship between Nanoscale Architecture and Function in Photovoltaic Multichromophoric Arrays as Visualized by Kelvin Probe Force Microscopy

“…The average potential shift, calculated for different measurements performed with different tips, amounted to 90 ± 10 mV. This negative shift of the SP has been previously observed in KPFM of photoactive blends − and can be attributed to trapping of electrons in the blend under steady-state conditions while holes can move to the underlying anode. Although a significant potential shift was observed by KPFM upon exposure to white light, no clear distinction between the two applied materials could be found: both polymers are likely to form complex three-dimensional interpenetrated networks of fiberlike structures.…”

Acridine orange as a biosensitive photovoltaic material

“…(4), we obtain C ¼ 9.8 6 1.2 Â 10 19 m À2 s À1 , which leads to an internal quantum yield of Y ¼ C/U ¼ 3.4% in AO. This value compares well with C ¼ 1.0 Â 10 20 m À2 s À1 and Y ¼ 3.4% in P3HT, 17 which indicates that AO is capable of generating sufficient amounts of carriers in photovoltaic devices. In addition, the conduction band energy level of AO can be determined from its energy band gap and the work function obtained from KPFM measurements.…”