Photovoltaics at multi-terawatt scale: Waiting is not an option.

25% annual PV growth is possible over the next decade.

Microscopy Visualization of Carrier Transport in CdSeTe/CdTe Solar Cells.

Solar cells are essentially minority carrier devices, and it is therefore of central importance to understand the pertinent carrier transport processes. Here, we advanced a transport imaging technique to directly visualize the charge motion and collection in the direction of relevant carrier transport and to understand the cell operation and degradation in state-of-the-art cadmium telluride solar cells. We revealed complex carrier transport profiles in the inhomogeneous polycrystalline thin-film solar cell, with the influence of electric junction, interface, recombination, and material composition.

Carrier-Transport Study of Gallium Arsenide Hillock Defects.

Single-crystalline gallium arsenide (GaAs) grown by various techniques can exhibit hillock defects on the surface when sub-optimal growth conditions are employed. The defects act as nonradiative recombination centers and limit solar cell performance. In this paper, we applied near-field transport imaging to study hillock defects in a GaAs thin film.

Thermomechanical Lift-Off and Recontacting of CdTe Solar Cells.

Controlled delamination of thin-film photovoltaics (PV) post-growth can reveal interfaces that are critical to device performance yet are poorly understood because of their inaccessibility within the device stack. In this work, we demonstrate a technique to lift off thin-film solar cells from their glass substrates in a clean, reproducible manner by first laminating a polymeric backsheet to the device and then thermally shocking the system at low temperatures ( T ≤ -30 °C). To enable clean delamination of diverse thin-film architectures, a theoretical framework is developed and key process control parameters are identified.

Room temperature biological quantum random walk in phycocyanin nanowires.

Quantum nano-structures are likely to become primary elements of future devices. However, there are a number of significant scientific challenges to real world applications of quantum devices. These include de-coherence that erodes operation of a quantum device and control issues.

Optical attenuation coefficient in individual ZnO nanowires.

Attenuation coefficient measurements for the propagation of bandedge luminescence are made on individual ZnO nanowires by combining the localized excitation capability of a scanning electron microscope (SEM) with near-field scanning optical microscopy (NSOM) to record the distribution and intensity of wave-guided emission. Measurements were made for individual nanostructures with triangular cross-sections ranging in diameter from 680 to 2300 nm. The effective attenuation coefficient shows an inverse dependence on nanowire diameter (d(-1)), indicating scattering losses due to non-ideal waveguiding behavior.

Three-dimensional transport imaging for the spatially resolved determination of carrier diffusion length in bulk materials.

A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (μτ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Green's function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity.

Origin of the hook effect in extrinsic photoconductors.

The response of extrinsic photoconductors to a step change in incident photon flux has long been known to exhibit a sharp transient feature, particularly at higher signal levels, known as the hook effect. We demonstrate experimentally and theoretically that the hook effect can be due to reduced illumination adjacent to the injecting contact. This nonuniformity can be produced by the transverse illumination of the detector that is common for far-infrared Ge:Ga devices.

Transient behavior of infrared photoconductors: application of a numerical model.

A numerical model for the transient response of extrinsic photoconductors is applied to the behavior of Ge:Ga and GaAs:Te detectors. Photoconductors display a two-component response to changes in illumination. The characteristic time and magnitude for the slow component have been studied as a function of background flux, applied field, temperature, device length, and signal size.

Transient response in doped germanium photoconductors under very low background operation.

Doped germanium photoconductors are the most sensitive detectors for astronomy in the wavelength range 40-240 µm. Under the extremely low background conditions encountered in cooled satellite instruments, these devices exhibit a number of transient effects, such as slow relaxation after a step change in illumination or bias, and spontaneous spiking at high signal levels. Such behavior can degrade the excellent instantaneous sensitivity of these detectors and create calibration uncertainties.