Fundamental Characterization of Non-Conventional Semiconducting Materials

Using an array of specialized techniques (STEM, XRD, PDS, CMS, FTPS), our group studies the structure-property relationships of organic semiconductors and inorganic nanoparticles for application in next-generation electronic materials.

Scanning Transmission Electron Microscopy

Scanning Transmission Electron Microscopy (STEM) gives spatially resolved electron diffraction images.  We use this technique to study how crystallite quality and orientation vary across a polymer film, and what implications this has for processing and performance.

(a) Diffraction spots indicate the local direction of polymer chains (left), Comparing adjacent scans sheds light on microstructure (right). (b) Conjugated polymer (PBTTT, purple) on a lacy carbon support (orange).  Lines show the orientation of polymer aggregates, calculated from STEM diffraction patterns.



Synchroton X-Ray Diffraction

We employ radiation from synchrotron sources to probe the complex structural features of weakly scattering, semi-crystalline materials. With 2D grazing-incidence x-ray scattering, high-resolution grazing-incidence, and specular x-ray diffraction, we are able to characterize the crystal structure, crystalline disorder, film texture, and crystallinity of thin films, all of which affect semiconductor operation and performance. Crystallographic refinement calculations (by Monte-Carlo optimization) based on measured diffracted peak intensities also enable us to precisely determine molecular packing and crystal structure of polymer and small molecule crystals.

2D grazing-incidence X-ray scattering pattern of a polymer thin film and corresponding schematics of molecular packing.


Transverse Photothermal Deflection Spectroscopy

Transverse Photothermal Deflection Spectroscopy (PDS) probes the optical purity of crystalline materials. Using PDS to measure the optical absorption in lead sulfide nanocrystals (PbS), we demonstrate that we can measure surface traps with unprecedented sensitivity. We are also able to decouple the effect of particle polydispersity from that of surface defects in highly faceted nanomaterials, as seen by the size dependent increase in Urbach disorder energy (Eu). PDS has also been employed to investigate electronic traps in organic single crystals and polymer thin films.

PDS is a simple technique that is immune to standard optical pitfalls such as light scattering, reflection interference and sample non-uniformity. PDS has an absorption window ranging from 0.5 eV to 4.0 eV with step sizes as low as 0.002 eV. This extremely sensitive bulk material technique can measure normalized absorbance as low as 10-5.

Left: Typical PDS spectra of PbS nanocrystals grown from lead chloride precursors, while the particles exhibit near identical polydispersity their Urbach disorder energy increases with the crystals surface area to volume ratio. Right: Single crystals of fully istopically enriched 13C as well as normal 12C rubrene were measured showing similar optical purity over 5 decades of signal with slight variations in oxygen induced trap density as well as a 2% shift in vibronic harmonic peaks consistent with the isotopic enrichment.


Charge Modulation Spectroscopy

Charge Modulation Spectroscopy (CMS) is a sensitive technique able to capture the optical absorption of field-induced polarons under device-relevant conditions. Combined with a complete theoretical model, CMS enables the first quantifiable measurement of polaron size in organic semiconductors, allowing the molecular and crystalline structure to be directly correlated to carrier delocalization and charge transport.

CMS measurement chamber.


Fourier Transform Photocurrent Spectroscopy

Fourier Transform Photocurrent Spectroscopy (FTPS) is a highly sensitive way to measure EQE allowing for up to 10 decades of dynamic range. Amongst other applications, we have used FTPS to probe perovskite solar cell stability (in collaboration with the McGehee group). As an example of the potential of this technique,we measured pristine MAPbI3 cells along with sputtered ITO and AZO selective contacts that also encapsulate the device. The cells were cycled and aged for 1000+ hours in 85oC with 85% R. The sub-bandgap photocurrent from the perovskite, which is indicative of defects, remains identical after exposure, showing that the significant degradation observed is due to the contacts and not to the absorber.

Probing the stability of perovskite solar cells with FTPS.