Nanoscale Charge Transport and Internal Structure of Bulk Heterojunction Conjugated Polymer/Fullerene Solar Cells by Scanning Probe Microscopy

Abstract

Tapping mode atomic force microscopy (AFM) was used to probe the surface and internal structures of bulk heterojunction poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films. AFM images obtained from the surface of the P3HT:PCBM film and from the cross section reveal for the first time the nanoscale three-dimensional P3HT and PCBM networks. Two different measurement modes of conducting atomic force microscopy (C-AFM) were used to study the nanoscale charge transport in P3HT:PCBM films. In C-AFM, the local current map at a constant bias and the surface topography are obtained simultaneously. Hole and electron current images can be used to determine the composition of the phase segregation domains. Alternatively, local current−voltage curves can be measured that can be used to calculate charge carrier mobilities. Hole current images and mobilities were obtained using Pt-coated silicon probes, whereas Mg-coated silicon probes were used to measure nanoscale electron mobilities. Variations in electrical properties and surface topography were examined. Comparison against films of neat P3HT shows that blending P3HT with PCBM decreases the polymer hole mobility. In all cases, nanoscale hole and electron mobilities increase upon annealing, consistent with increased internal order. A direct correlation between the C-AFM-determined nanoscale properties and solar cell power conversion efficiencies is observed.

Room to Improve Conjugated Polymer-Based Solar Cells: Understanding How Thermal Annealing Affects the Fullerene Component of a Bulk Heterojunction Photovoltaic Device

“…The changes in the phase-separated regions observed here strongly resemble the changes in film morphology that P3HT:PCBM blends have been observed to undergo upon thermal annealing: for both PS:PCBM and P3HT: PCBM blend films, the domains become similarly more distinct upon annealing, and the size of the domains increases in a comparable way upon annealing. 1,13,25,26 Thus, based on these results, we conclude that even though the nanometer-scale domain structures are not identical, PS:PCBM blends should provide a good platform to extract trends regarding charge transport of the fullerene component of P3HT:PCBM solar cells. Now that we have evidence that the fullerene network morphology is similar when PCBM is blended with P3HT or PS, we can use the behavior of diodes based on PS:PCBM blend films to study how thermal annealing affects electron transport on the PCBM fullerene network in a BHJ-like geometry.…”

“…2,12,13 Moreover, recent work examining the cross-sectional topography of P3HT:PCBM films suggests that the surface topography of such films correlates relatively well with the bulk network morphology. 13 Figures 1A shows an AFM phase image of an as-cast 1:0.67 PS:PCBM blend film. The image shows nanometer-scale features on top of a nearly smooth film background.…”

“…1,[9][10][11] It is less clear, however, how annealing affects the fullerene component of a BHJ polymer solar cell: the way in which the fullerene molecules are distributed in the polymer matrix and the details of the BHJ morphology have been the subject of much debate. 1,2,9,10,[12][13][14][15] In this work, we directly address the question of what happens to the fullerene component of a polymer BHJ solar cell upon thermal annealing. By fabricating "fullerene-only" BHJ halfdevices (that use electronically insulating polystyrene (PS) rather than a semiconducting polymer like P3HT), we show that thermal annealing causes a decrease in the ability of the fullerene network to conduct electrons as a result of increased phase segregation of the blended polymer and fullerene components.…”

Connecting physical properties of spin-casting solvents with morphology, nanoscale charge transport, and device performance of poly(3-hexylthiophene):phenyl-C₆₁-butyric acid methyl ester bulk heterojunction solar cells

“…However, hole current measurements were specifically conducted here since lower hole mobility usually determines carrier transport in polymer solar cells and limits final device performance. 36 The C-AFM images of the blends prepared with CB and 1,2-DCB solvents are shown in Figs. 4(c) and 4(d), respectively.…”

“…A variety of scanning probe microscopy (SPM)-based techniques such as atomic force microscopy (AFM), 31 scanning tunneling microscopy, 32 electrostatic force microscopy, 33 Kelvin probe force microscopy, 34,35 conductive atomic force microscopy (C-AFM), 33,36 and near-field scanning optical microscopy) 37,38 have been used in recent years to study the morphology and nanoscale optoelectronic properties of the blend. Thus, measurement of nanoscale structural, electrical, and optical properties of the active layer with high spatial resolution using SPMbased methods can be used in conjunction with device scale measurements to correlate physical properties of the solvent to blend morphology, nanoscale charge transport processes, and device performance.…”