Revisiting Solvent Additives for the Fabrication of Polymer:Fullerene Solar Cells: Exploring a Series of Benzaldehydes

Abstract

The power conversion efficiencies of organic solar cells delicately depend on the morphology of the light‐harvesting bulk heterojunctions (BHJ). Upon deposition from solution, the formation of tailored bicontinuous networks of polymers and fullerenes is often achieved using combinations of solvents and solvent additives. Common wisdom infers that best solar cell performances are achieved when the solvent additives exhibit excellent fullerene solubility. Herein, this concept is revisited based on the investigation of a series of structurally similar, substituted benzaldehydes. It is concluded that the solvent additives do not only have to feature the commonly accepted good fullerene solubility, but must also exhibit lowest polymer solubility to suppress liquid–liquid demixing and hence achieve best solar cell performance. Thus, this study adds an important item to the list of selection criteria of solvent additives toward the production of polymer:fullerene solar cells with optimized power conversion efficiencies. The microscopic picture of the resulting domain configurations within the light‐harvesting layers is developed around comprehensive multiscale investigations of the BHJ morphology, using atomic force microscopy, scanning transmission electron microscopy, and nano‐infrared microscopy. The latter is operated in two complementary modes, one of which is more bulk sensitive, whereas the other mode is surface sensitive.

Highly Selective Cu Staining of Sulfur-Containing Polymers Facilitates 3D Nanomorphology Reconstruction of Polymer:Fullerene Blends in Organic Solar Cells by FIB-SEM Tomography

“…We will demonstrate in the following that selective Cu infiltration can be applied as a convenient staining method to analyze the size and interconnectivity of PTB7 and PC 71 BM domains in BHJ layers. For this purpose, we analyzed different PTB7:PC 71 BM layers with nanomorphologies that were tuned by using different processing additives (AA, SA, and DIO) during layer deposition . It will be shown that Cu staining can be exploited not only for STEM imaging but also for SE-SEM imaging as a prerequisite for the 3D reconstruction of the BHJ domain structure by FIB-SEM tomography.…”

“…Only upon using DIO, an additional drying step was applied at an elevated temperature (50 °C, 20 min) on the next day to remove DIO residues. More details on the absorber-layer fabrication are described in the literature …”

“…After thermal treatment, highly selective Cu infiltration of the conjugated polymers (P3HT and PTB7) leads to a strongly enhanced electron scattering, whereas electron scattering and the corresponding STEM intensity of the fullerene phases remain unchanged. The new staining approach was applied to visualize the phase distribution of PTB7:PC 71 BM BHJ layers processed from neat o -xylene with different processing additives m -salicylaldehyde (SA), p -anisaldehyde (AA), and 1,8-diiodooctance (DIO) that are used to control the domain sizes . Contrast enhancement between polymer and fullerene phases is not only observed in composition-sensitive high-angle annular dark-field (HAADF-)­STEM images but can be also exploited for secondary-electron scanning electron microscopy (SE-SEM) imaging.…”

“…The new staining approach was applied to visualize the phase distribution of PTB7:PC 71 BM BHJ layers processed from neat o-xylene with different processing additives m-salicylaldehyde (SA), p-anisaldehyde (AA), and 1,8-diiodooctance (DIO) that are used to control the domain sizes. 26 Contrast enhancement between polymer and fullerene phases is not only observed in composition-sensitive high-angle annular dark-field (HAADF-)STEM images but can be also exploited for secondary-electron scanning electron microscopy (SE-SEM) imaging. This is the prerequisite for the 3D reconstruction of the domain structure in PTB7:PC 71 BM BHJ layers by focused-ion- beam/scanning-electron-microscopy (FIB-SEM) tomography.…”

High‐Efficiency Sequential‐Cast Organic Solar Cells Enabled by Dual Solvent‐Controlled Polymer Aggregation

“…[18,19] Additive strategy, thermal annealing, and solvent annealing are common methods to modulate the morphology of BHJ OSCs, but the introduction of the former is often accompanied by a decrease in device stability, and the post-treatment means exacerbate the complexity of production preparation, which is not conducive to the future commercial production. [20][21][22][23][24][25][26] In contrast to BHJ OSCs, the layer-by-layer solution spin-coating method, also known as the two-step method, allows for separate regulation of the donor and acceptor layer to form the desired vertical structure, which facilitates charge transport and collection, does not rely on the ratio of the donor to acceptor, and is less dependent on the use of additives, solvent concentration, etc. [27][28][29] It is a promising production method to prepare high-efficiency devices.…”