J. J. H. Pijpers

Carrier multiplication in bulk indium nitride

Carrier multiplication (CM) is the process of generating multiple electron-hole pairs from one absorbed photon. Narrow-gap InN is a material that has been proposed for achieving efficient CM. We quantify the CM efficiency in bulk InN using terahertz time-domain spectroscopy. While the CM onset occurs at relatively low photon energies in InN (1.7 ± 0.2 eV), corresponding to 2.7 ± 0.3 times its bandgap, the excitation efficiency above the onset increases linearly with a slope of only ∼13%/Eg. Based on these numbers, the efficiency increase of an InN based photovoltaic device owing to CM is limited to maximum 1% point.

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.

CHAPTER 11:Carrier Dynamics in Photovoltaic Structures and Materials Studied by Time-Resolved Terahertz Spectroscopy

Terahertz (THz) radiation, covering the frequency range from a few to about a hundred milli-electron volts (meV), is particularly sensitive to the response of charge motion in materials. Using a pump-probe scheme, time resolved terahertz spectroscopy (TRTS) allows, in a contact-free way, the study of charge carrier dynamics with ultrafast time resolution. In this chapter, we review how TRTS has been successfully employed for the characterization of photovoltaic materials and architectures. Particularly, we focus on TRTS studies that interrogate key processes occurring after photo-excitation in nano-structured semiconductor systems, including charge generation, separation and transport. Understanding the fundamental properties of charge carriers in these novel geometries is important for the development of future solar energy conversion technologies.

Ultrafast intraband relaxation in colloidal quantum dots

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots using time-resolved luminescence and time-resolved terahertz spectroscopy. The rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes a direct proof of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 plusmn 0.15 ps time scale.