Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends.

In this Review, we summarize recent work on modeling of organic/metal and organic/organic interfaces. Some of the models discussed have a semiempirical approach, that is, experimentally derived values are used in combination with theory, and others rely completely of calculations. The models are categorized according to the types of interfaces they apply to, and the strength of the interaction at the interface has been used as the main factor. We explain the basics of the models, their use, and give examples on how the models correlate with experimental results. We stress that given the complexity of organic/metal and organic/organic interface formation, it is crucial to know the exact way in which the interface was formed before choosing the model that is applicable, as none of the models presented covers the whole range of interface interaction strengths (weak physisorption to strong chemisorption).

Energy‐Level Alignment at Organic/Metal and Organic/Organic Interfaces

Polymer morphology has proven to be extremely important in determining the optoelectronic properties in polymer-based devices. The understanding and manipulation of polymer morphology has been the focus of electronic and optoelectronic polymer-device research. In this article, recent advances in the understanding and controlling of polymer morphology are reviewed with respect to the solvent selection and various annealing processes. We also review the mixed-solvent effects on the dynamics of film evolution in selected polymer-blend systems, which facilitate the formation of optimal percolation paths and therefore provide a simple approach to improve photovoltaic performance. Recently, the occurrence of vertical phase separation has been found in some polymer:fullerene bulk heterojunctions. [1-3] The origin and applications of this inhomogeneous distribution of the polymer donor and fullerene acceptor are addressed. The current status and device physics of the inverted structure solar cells is also reviewed, including the advantage of utilizing the spontaneous vertical phase separation, which provides a promising alternative to the conventional structure for obtaining higher device performance.

Recent Progress in Polymer Solar Cells: Manipulation of Polymer:Fullerene Morphology and the Formation of Efficient Inverted Polymer Solar Cells

New designs of donor polymers yield organic solar cells with fill factors approaching 80%, significantly higher than those of conventional cells. This enhanced performance is attributed to the close-packed and highly ordered structure of the polymers PTPD3T and PBT13T, which leads to efficient charge extraction and suppressed recombination.

Polymer solar cells with enhanced fill factors

The mixed solvent approach has been demonstrated as a promising method to modify nanomorphology in polymer solar cells. This work aims to understand the unique role of the additive in the mixture solvent and how the optimized nanoscale phase separation develops laterally and vertically during the non-equilibrium spin-coating process. We found the donor/acceptor components in the active layer can phase separate into an optimum morphology with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as solubility and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.

/pdf/effects-of-solvent-mixtures-on-the-nanoscale-phase-1p8kv4f56z.pdf

Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells

In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend—Effects of specific substrate interaction

Blends of the low-bandgap polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl- 2',1',3'-benzothiadiazole)] (APFO-3) and the fullerene deriv. [6,6]-phenyl-C61-butyric acid Me ester (PCBM) were spin-coated from chloroform soln. into thin films, which were examd. with dynamic secondary ion mass spectrometry. For blends with high PCBM content, the depth profiles show compn. waves that were caused by surface-directed phase sepn. during spin-coating. The formation of such multilayer structures by spontaneous self-stratification probably has implications for optimization strategies for the performance of org. solar cells. [on SciFinder (R)]

https://iopscience.iop.org/article/10.1088/0953-8984/17/50/L01/pdf

Multilayer formation in spin-coated thin films of low-bandgap polyfluorene: PCBM blends

Interactions between poly(sodium styrenesulfonate) (NaPSS) and sodium polyacrylate−acrylamide copolymers, NaP(AA/AM), in aqueous solutions have been studied. Equilibrium pseudoternary phase diagrams have been determined, for varying combinations of molecular weights and charge densities of the components and with varying concentrations of salt in solution. All the fully charged mixtures segregate at sufficiently high concentrations of polyelectrolytes (typically 6−8 wt %). Addition of salt and/or increasing the molecular weight results in a larger two-phase area in the diagrams. The main finding on reducing the charge density of one of the components P(AA/AM) is that the extent of the two-phase region changes relatively little until the charge density is reduced to about 20%, where, abruptly, complete miscibility occurs. The phase behavior is qualitatively reproduced by a modified Flory−Huggins model where dissociated counterions are included as a separate component contributing to the entropy of mixing. ...

Phase Behavior of Anionic Polyelectrolyte Mixtures in Aqueous Solution. Effects of Molecular Weights, Polymer Charge Density, and Ionic Strength of Solution

Svante Nilsson

Papers

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend—Effects of specific substrate interaction

Multilayer formation in spin-coated thin films of low-bandgap polyfluorene: PCBM blends

Phase Behavior of Anionic Polyelectrolyte Mixtures in Aqueous Solution. Effects of Molecular Weights, Polymer Charge Density, and Ionic Strength of Solution

Associative and segregative phase behaviour of a mixed aqueous cationic surfactant and anionic hydrophilic polymer system