Mapping Orientational Order in a Bulk Heterojunction Solar Cell with Polarization-Dependent Photoconductive Atomic Force Microscopy

New methods connecting molecular structure, self-organization, and optoelectronic performance are important for understanding the current generation of organic photovoltaic (OPV) materials. In high power conversion efficiency (PCE) OPVs, light-harvesting small-molecules or polymers are typically blended with fullerene derivatives and deposited in thin films, forming a bulk heterojunction (BHJ), a self-assembled three-dimensional nanostructure of electron donors and acceptors that separates and transports charges. Recent data suggest micrometer-scale orientational order of donor domains exists within this complex nanomorphology, but the link to the optoelectronic properties is yet unexplored. Here we introduce polarization-dependent, photoconductive atomic force microscopy (pd-pcAFM) as a combined probe of orientational order and nanoscale optoelectronic properties (∼20 nm resolution). Using the donor 7,7′-(4,4-bis(2-ethylhexyl)-4<i>H</i>-silolo[3,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl[2,2′-bithiophen]-5-yl)benzo[<i>c</i>][1,2,5]thiadiazole), p-DTS(FBTTh₂)₂, we show significant spatial dependence of the nanoscale photocurrent with polarized light in both pristine and BHJ blends (up to 7.0% PCE) due to the local alignment of the molecular transition dipoles. By mapping the polarization dependence of the nanoscale photocurrent, we estimate the molecular orientation and orientational order parameter. Liquid crystalline disclinations are observed in all films, in agreement with complementary electron microscopy experiments, and the order parameter exceeds 0.3. The results demonstrate the utility of pd-pcAFM to investigate the optical/structural anisotropy that exists within a well-performing BHJ system and its relationship to optoelectronic properties on both the nanometer and micrometer length scales

Ultrafast Charge Generation in an Organic Bilayer Film

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT­(FBTTh₂)₂ (see Figure ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C₆₀ layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface

Silaindacenodithiophene-Based Molecular Donor: Morphological Features and Use in the Fabrication of Compositionally Tolerant, High-Efficiency Bulk Heterojunction Solar Cells

A novel solution-processable small molecule, namely, benzo­[1,2-<i>b</i>:4,5-<i>b</i>]­bis­(4,4′-dihexyl-4<i>H</i>-silolo­[3,2-<i>b</i>]­thiophene-2,2′-diyl)­bis­(6-fluoro-4-(5′-hexyl-[2,2′-bithiophene]-5-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole (p-SIDT­(FBTTh₂)₂), was designed and synthesized by utilizing the silaindacenodithiophene (SIDT) framework as the central D² donor unit within the D¹AD²AD¹ chromophore configuration. Relative to the widely studied 7,7′-[4,4-bis­(2-ethylhexyl)-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl]­bis­[6-fluoro-4-(5′-hexyl-[2,2′-bithiophene]-5-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole] (p-DTS­(FBTTh₂)₂), which contains the stronger donor fragment dithienosilole (DTS) as D², one finds that p-SIDT­(FBTTh₂)₂ exhibits a wider band gap and can be used to fabricate bulk heterojunction solar cells with higher open circuit voltage (0.91 V). Most remarkably, thin films comprising p-SIDT­(FBTTh₂)₂ can achieve exceptional levels of self-organization directly via solution deposition. For example, high-resolution transmission electron microscopy analysis shows that p-SIDT­(FBTTh₂)₂ spin-cast from chlorobenzene organizes into crystalline domains with lattice planes that extend over length scales on the order of hundreds of nanometers. Such features suggest liquid crystalline properties during the evolution of the film. Moreover, grazing incidence wide-angle X-ray scattering analysis shows a strong tendency for the molecules to exist with a strong “face-on” orientation relative to the substrate plane. Similar structural features, albeit of more restricted dimensions, can be observed within p-SIDT­(FBTTh₂)₂:PC₇₁BM bulk heterojunction thin films when the films are processed with 0.4% diiodooctane (DIO) solvent additive. DIO use also increases the solar cell power conversion efficiencies (PCEs) from 1.7% to 6.4%. Of significance from a practical device fabrication perspective is that, for p-SIDT­(FBTTh₂)₂:PC₇₁BM blends, there is a wide range of compositions (from 20:80 to 70:30 p-SIDT­(FBTTh₂)₂:PC₇₁BM) that provide good photovoltaic response, i.e., PCE = 4–6%, indicating a robust tendency to form the necessary continuous phases for charge carrier collection. Light intensity photocurrent measurements, charge selective diode fabrication, and internal quantum efficiency determinations were carried out to obtain insight into the mechanism of device operation. Inclusion of DIO in the casting solution results in films that exhibit much lower photocurrent dependence on voltage and a concomitant increase in fill factor. At the optimum blend ratio, devices show high charge carrier mobilities, while mismatched hole and electron mobilities in blends with high or low donor content result in reduced fill factors and device performance