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New Morphological Paradigm Uncovered in Organic Solar Cells Print

Organic solar cells are made of light, flexible, renewable materials; they require simple and inexpensive processing steps and could produce an economically competitive and environmentally friendly energy source. Understanding the fundamentals of organic solar cell function is therefore vital to uncovering their maximum potential. Models describing critical device functions such as charge separation and transport often depend on simplistic morphological assumptions, including discrete interfaces between pure electron donor and acceptor materials. In contrast, recent spectroscopy and scattering studies conducted by North Carolina State University and Cambridge University researchers at ALS Beamlines 5.3.2 and 7.3.3 found a substantial amount of molecular mixing between model materials (polymers and fullerenes) currently used in bulk heterojunction (BHJ) organic solar cells. This suggests that the amorphous portions of these devices do not have pure domains, and the paradigm of device operation may need to be refined to accommodate this newly discovered complexity.

Improving Solar Cell Models

Organic photovoltaics (OPVs), or solar cells, have the potential to provide a low-cost and renewable source of environmentally friendly energy. Scientific research is currently aimed at optimizing this potential, understanding how OPVs materials and function can be manipulated and improved to achieve scientific and economic viability for widespread use by consumers.

The research discussed here is an example of how probing the fundamental material properties of such devices can yield more promising results than can be achieved with purely phenomenological approaches. Using advanced x-ray scattering, spectroscopy, and microscopy at the ALS, researchers discovered that the traditional 2-domain model was incorrect; a “heterogeneous soup” with 3 domains is a better morphological model of an OPV’s absorbing layer. This improved understanding will guide the future development and optimization of organic solar cells by reducing laborious trial-and-error development and forcing other presupposed models to be surveyed for accuracy in the hopes of finding crucial answers.

 

Schematic of two morphological pictures for organic solar cells. Top is the traditional model of a BHJ, while the bottom depicts a more likely morphology based on the current study.

 

The absorption of a photon in organic photovoltaics (OPVs) results in a bound exciton (electron/hole pair) that must be separated into constituent charges quickly and subsequently transported to each electrode. An interface between donor and acceptor materials must, therefore, be near the site of absorption, and pathways to transport charge out of the device must be present.

The highest-performing devices were traditionally assumed to have a BHJ morphology, where each material is pure and confined to an interpenetrating network on a size scale small enough for charge separation but large and pure enough for efficient charge transport. However, current research endeavors to discover the potential of OPVs are turning to fundamental measurements to optimize the nanoscale morphology of the absorbing layer, comprising a donor and an acceptor material.

This study utilized poly(3-hexyl thiophene) (P3HT) as a donor polymer material, and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) as an acceptor fullerene. It revealed an absorbing layer containing an amorphous mixed phase in a significant volume fraction of the material (about one third), implying a very different morphological picture—and device function—than was previously believed. The film’s morphology is likely composed of nanocrystals of pure polymer and relatively pure fullerene nano-agglomerates that are surrounded by a mixed liquid-like phase containing both materials, yielding a heterogeneous mixture of three phases rather than the two typically envisioned. The fact that quantum efficiency (electron out per incident photon) is high for this material system suggests that the actual morphology is advantageous for device function.

Many factors come into play in determining a solar cell’s final morphology. Only by carefully considering each of their influences individually can these factors be properly exploited. One such fundamental property is the thermodynamic miscibility of the materials.

NEXAFS spectra of amorphous portions of a polymer–fullerene blend before (top) and after (bottom) being brought to thermodynamic equilibrium.

 

The thermodynamic miscibility of typical polymer–fullerene pair materials used in BHJ solar cells was quantitatively measured using a combination of near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission x-ray microscopy (STXM).

To make OPVs, films are spincast from a blend solution and annealed above the glass transition temperature (when amorphous polymers transition from a solid- to a liquid-like state) to allow for phase separation of the constituents. For this study, films were annealed at these temperatures for extended times and their state of thermodynamic equilibrium was confirmed via STXM imaging at key resonance energies individually sensitive to the two components. Subsequently, the quantitative composition was measured by acquiring NEXAFS spectra from locations in between large fullerene crystals and in thermodynamic equilibrium with the rest of the sample, which were then fit to spectra of the pure components. The crystallinity of the equilibrated samples was additionally studied using grazing-incidence wide-angle x-ray scattering (GIWAXS).

Thermodynamic phase diagram for the P3HT (polymer) – PCBM (fullerene) system where the polymer regioregularity (RR) has been varied. Of note is that the fullerene is always somewhat miscible in the polymer. The two RR grades both contain approximately 50 wt % crystalline phases, which were shown to be pure via GIWAXS. Because the spectroscopy measurement averages the PCBM composition over both amorphous and crystalline polymer regions, the miscibility of the RR grades are approximately double the values displayed here.

 

The surprising result is a large degree of mixing at the molecular level in the amorphous phases of the devices—a size scale smaller than was previously thought proper for obtaining good charge transport and high device efficiencies. Good molecular mixing may create an environment allowing both close proximity to a molecule of opposing species for charge separation, as well as connection to molecules of like species for charge transport to crystalline phases, and finally transport out of the device. The miscibility level measured here would allow for such a mode of device operation, and is now the potential target morphology for systems based on newly developed materials.

 

Researchers Brian Collins and Harald Ade.


 

Research conducted by B.A. Collins, E. Gann, L. Guignard, and H. Ade (North Carolina State University) and X. He and C.R. McNeill (University of Cambridge).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), and the Engineering and Physical Sciences Research Council (UK). Operation of the ALS is supported by DOE BES.

Publication about this research: B.A. Collins, E. Gann, L. Guignard, X. He, C.R. McNeill, and H. Ade, "Molecular miscibility of polymer-fullerene blends," J. Phys. Chem. Lett. 1, 3160 (2010).

ALS Science Highlight #225

 

ALSNews Vol. 319