Above-average correlative materials characterization

When it comes to assessing novel materials for optoelectronic uses, optical characterization is typically the initial approach. Researchers depend heavily on steady-state photoluminescence (PL) for this purpose, as it offers a non-destructive and highly sensitive means of scrutinizing a material's electronic structure, optical transitions, and emission traits.

However, relying solely on PL data can be misleading, as it fails to reveal the intricate microscopic properties that ultimately govern a material's real-world performance. To attain a more comprehensive understanding, a more precise and correlated methodology is warranted.

The principal hurdle arises from the fact that steady-state PL offers a time-integrated perspective of the emission, and depending on the measurement setup, it may also average out spatial disparities. This static equilibrium measurement reflects a balance between charge carrier generation and recombination, which means that it does not directly disclose how these excited charge-carriers evolve and ultimately steer a device's energy and light manipulation.

For instance, two semiconductor thin films might present identical steady-state PL emission, yet they could operate quite differently in a practical device. One film might exhibit lengthy carrier lifetimes, indicative of low non-radiative losses – advantageous for solar cells – while the other could harbor high defect densities, masked by robust light absorption or intense excitation light.

Conversely, weak PL emission does not necessarily imply inferior material quality. A layered solar cell example illustrates this point, where diminished luminescence often signals swift, advantageous charge extraction across an interface instead of detrimental defect recombination.

Time-resolved photoluminescence (TRPL) is a pivotal technique to uncover these concealed dynamics. Unlike steady-state PL, which perpetually irradiates a sample, TRPL deploys a brief laser pulse and gauges the photoluminescence intensity in the aftermath. This technique can unveil carrier dynamics spanning from picoseconds to microseconds.

TRPL becomes particularly crucial in materials where localized variations significantly impact device behavior. For example, it can expose the divergence between areas with prolonged emission, suggestive of efficient radiative recombination, and regions with rapid decay resulting from, for example, defect recombination.

However, steady-state PL averages over time and space, necessitating TRPL to address only part of the issue. Spatially resolved TRPL tackles the missing spatial dimension by employing a high-precision, motorized XY stage in concert with TRPL's exact timing. This arrangement yields a 2D map that correlates localized structural features, such as grain boundaries and defects, with variations in carrier kinetics across the surface.

Complementary to these methods is time-resolved emission spectroscopy (TRES), which incorporates spectral selection with time-resolved detection to gauge how emission dynamics differ across the spectrum. By documenting wavelength-dependent decay behavior, TRES reveals how various spectral components evolve following excitation, enabling researchers to monitor changes in photon energy distribution over time, thereby providing insight into the underlying mechanisms governing material behavior.

Achieving a more coherent and dependable physical insight becomes possible when temporal, spatial, and spectral factors are correlated on the exact same sample region. This requirement for correlation is the driving force behind PicoQuant’s Solira, an integrated workflow platform encompassing steady-state PL, TRPL, spatially resolved TRPL, TRES (with the additional FlexLambda Kit), and supplementary imaging techniques like hyperspectral imaging for diverse scientific inquiries.

By amalgamating these techniques within a single, customizable microscope system, materials scientists can derive a more informed interpretation of material behavior. For example, they can pinpoint pertinent emission bands at chosen points of interest using steady-state PL, subsequently employ TRPL to measure the associated decay kinetics, and potentially apply TRES to discern how these kinetics vary across the emission spectrum.