20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction.

Bandgap gradient is a proven approach for improving the open-circuit voltages (Vs) in Cu(In,Ga)Se and Cu(Zn,Sn)Se thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers.

Reduced Recombination and Improved Performance of CdSe/CdTe Solar Cells due to Cu Migration Induced by Light Soaking.

The performance of CdTe solar cells has advanced impressively in recent years with the incorporation of Se. Instabilities associated with light soaking and copper reorganization have been extensively examined for the previous generation of CdS/CdTe solar cells, but instabilities in Cu-doped Se-alloyed CdTe devices remain relatively unexplored. In this work, we fabricated a range of CdSe/CdTe solar cells by sputtering CdSe layers with thicknesses of 100, 120, 150, 180, and 200 nm on transparent oxide-coated glass and then depositing CdTe by close-spaced sublimation.

Optical Properties of Magnesium-Zinc Oxide for Thin Film Photovoltaics.

Motivated by their utility in CdTe-based thin film photovoltaics (PV) devices, an investigation of thin films of the magnesium-zinc oxide (MgZnO or MZO) alloy system was undertaken applying spectroscopic ellipsometry (SE). Dominant wurtzite phase MZO thin films with Mg contents in the range 0 ≤ ≤ 0.42 were deposited on room temperature soda lime glass (SLG) substrates by magnetron co-sputtering of MgO and ZnO targets followed by annealing.

Effects of Cu Precursor on the Performance of Efficient CdTe Solar Cells.

Copper (Cu) incorporation is a key process for fabricating efficient CdTe-based thin-film solar cells and has been used in CdTe-based solar cell module manufacturing. Here, we investigate the effects of different Cu precursors on the performance of CdTe-based thin-film solar cells by incorporating Cu using a metallic Cu source (evaporated Cu) and ionic Cu sources (solution-processed cuprous chloride (CuCl) and copper chloride (CuCl)). We find that ionic Cu precursors offer much better control in Cu diffusion than the metallic Cu precursor, producing better front junction quality, lower back-barrier heights, and better bulk defect property.

Back-Surface Passivation of CdTe Solar Cells Using Solution-Processed Oxidized Aluminum.

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solution-based process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al species are deposited at the back surface of CdTe devices. The open circuit voltage () is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific.

CuSCN as the Back Contact for Efficient ZMO/CdTe Solar Cells.

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges.

Impact of Moisture on Photoexcited Charge Carrier Dynamics in Methylammonium Lead Halide Perovskites.

Organic-inorganic metal halide perovskites are notoriously unstable in humid environments. While many studies have revealed the morphology and crystal structure changes that accompany exposure to humidity, little is known about changes to the photophysics that accompany the degradation process. By combining in situ steady-state and time-resolved photoluminescence with Hall effect measurements, we examined the changes in the photoexcited carrier dynamics for methylammonium lead iodide (MAPbI) and bromide (MAPbBr) films exposed to nitrogen gas containing water vapor at 80% relative humidity.

Environmental Impacts from Photovoltaic Solar Cells Made with Single Walled Carbon Nanotubes.

An ex-ante life cycle inventory was developed for single walled carbon nanotube (SWCNT) PV cells, including a laboratory-made 1% efficient device and an aspirational 28% efficient four-cell tandem device. The environmental impact of unit energy generation from the mono-Si PV technology was used as a reference point. Compared to monocrystalline Si (mono-Si), the environmental impacts from 1% SWCNT was ∼18 times higher due mainly to the short lifetime of three years.

Enhanced Grain Size, Photoluminescence, and Photoconversion Efficiency with Cadmium Addition during the Two-Step Growth of CHNHPbI.

Control over grain size and crystallinity is important for preparation of methylammonium lead iodide (MAPbI) solar cells. We explore the effects of using small concentrations of Cd and unusually high concentrations of methylammonium iodide during the growth of MAPbI in the two-step solution process. In addition to improved crystallinity and an enhancement in the size of the grains, time-resolved photoluminescence measurements indicated a dramatic increase in the carrier lifetime.

Probing Photocurrent Nonuniformities in the Subcells of Monolithic Perovskite/Silicon Tandem Solar Cells.

Perovskite/silicon tandem solar cells with high power conversion efficiencies have the potential to become a commercially viable photovoltaic option in the near future. However, device design and optimization is challenging because conventional characterization methods do not give clear feedback on the localized chemical and physical factors that limit performance within individual subcells, especially when stability and degradation is a concern. In this study, we use light beam induced current (LBIC) to probe photocurrent collection nonuniformities in the individual subcells of perovskite/silicon tandems.

High speed, intermediate resolution, large area laser beam induced current imaging and laser scribing system for photovoltaic devices and modules.

We have developed a laser beam induced current imaging tool for photovoltaic devices and modules that utilizes diode pumped Q-switched lasers. Power densities on the order of one sun (100 mW/cm) can be produced in a ∼40 μm spot size by operating the lasers at low diode current and high repetition rate. Using galvanostatically controlled mirrors in an overhead configuration and high speed data acquisition, large areas can be scanned in short times.

Wiring-up carbon single wall nanotubes to polycrystalline inorganic semiconductor thin films: low-barrier, copper-free back contact to CdTe solar cells.

We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.