Glenn Alers

High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature

Efficient charge transport is demonstrated in TiO2/PbS quantum dot solar cells where the PbS absorber (∼1.1 eV band gap) is deposited by dip coating and ethanedithiol ligand exchange, with power efficiencies above 3% at AM1.5. An increase in power efficiency occurs as the device temperature is lowered to 170 K, with a open-circuit voltage of 0.66 V, short-circuit current density of 28.6 mA/cm2 and fill factor of 42.4%. This remarkable temperature dependence is due to a large increase in charge transport between the PbS quantum dots with decreasing temperature.

Resistivity dominated by surface scattering in sub-50 nm Cu wires

Electron scattering mechanisms in copper lines were investigated to understand the extendibility of copper interconnects when linewidth or thickness is less than the mean free path. Electron-beam lithography and a dual hard mask were used to produce interconnects with linewidths between 25 and 45 nm. Electron backscatter diffraction characterized grain structure. Temperature dependence of the line resistance determined resistivity, which was consistent with existing models for completely diffused surface scattering and line-edge roughness, with little contribution from grain boundary scattering. A simple analytical model was developed that describes resistivity from diffuse surface scattering and line-edge roughness.

CdTe Schottky diodes from colloidal nanocrystals

We have fabricated ultrathin photovoltaic cadmium telluride (CdTe) film solar cells from colloidal nanorod solutions with a power conversion efficiency of 5.0% and internal quantum efficiency approaching unity near the band edge. Sintering of the CdTe nanorod films was necessary to facilitate grain growth and enhanced optical absorption. By analyzing electrode dependence, capacitance-voltage, temperature dependence, and current-voltage characteristics, the device performance is shown to be dominated by the formation of a p-CdTe/Al Schottky junction. The reduced need for material and cheaper processing make this an attractive technology for solar power generation.

Quantum dot PbS0.9Se0.1/TiO2 heterojunction solar cells

We report on photovoltaic cells based on ternary PbS(0.9)Se(0.1) quantum dots utilizing a heterojunction type device configuration. The best device shows an AM 1.5 power conversion efficiency of 4.25%. Furthermore, this ternary PbS(x)Se(1-x) quantum dot heterojunction device has a peak external quantum efficiency above 100% at 2.76 eV, approximately 2.7× the bandgap energy. The ternary quantum dots combine the higher short circuit currents of the binary PbSe system with the higher open circuit voltages of the binary PbS system.

Air stability of TiO2/PbS colloidal nanoparticle solar cells and its impact on power efficiency

The short-term (less than 1 hour) exposure of TiO2/PbS quantum dot photovoltaics to air increases the open circuit voltage (Voc) and fill factor (FF) while slightly decreasing the short circuit current density (Jsc), leading to a power conversion efficiency above 4% and a peak external quantum efficiency over 80% for 1.1 eV PbS. The resulting Jsc, Voc, and FF under 100 mW/cm2 AM1.5 are 18.6 mA/cm2, 0.517 V, and 42% for 1.1 eV PbS and 8.03 mA/cm2, 0.655 V, and 35% for 1.7 eV PbS, respectively. Long-term air exposures result in much lower conductivities. Furthermore, short-term air exposure effects are fully reversible upon removal from air, and longer-term effects are mostly reversible through soaking in 1,2-ethanedithiol.

Mid-gap trap states in CdTe nanoparticle solar cells

Thin film solar cells comprised of quantum-confined CdTe nanoparticles are shown to have a low intrinsic density of mid-gap trap states relative to their equivalent bulk film, indicating that the ligands are effective at electrically passivating surface states. Sintering the nanoparticles into a poly-crystalline thin film increases device performance but also increases the density of mid-gap trap states due to doping from the CdCl treatment and the formation of long range disorder such as grain boundaries and dislocations. Long term aging under illumination increases the density of mid-gap traps in the unsintered films due to degradation of the ligands.

Power generation study of luminescent solar concentrator greenhouse

A Luminescent Solar Concentrator (LSC) greenhouse and an identical control greenhouse were constructed with photovoltaic (PV) cells attached to the roof panels of both structures. The placement and types of PV cells used in the LSC panels were varied for performance comparisons. Solar power generation was monitored continuously for one year, with leading LSC panels exhibiting a 37% increase in power production compared to the reference. The 22.3 m2 greenhouse was projected to generate a total of 1342 kWh per year, or 57.4 kWh/m2 if it were composed solely of the leading panel of Criss Cross panel design. The LSC panels showed no signs of degradation throughout the trial demonstrating the materials robustness in field conditions.

Thermoreflectance imaging of defects in thin-film solar cells

We have identified and characterized various defects in thin-film a-Si and CIGS solar cells with sub-micron spatial resolution using thermoreflectance imaging. A megapixel silicon-based CCD was used to obtain noncontact thermal images simultaneously with visible electroluminescence (EL) images. EL can be indicative of pre-breakdown sites due to trap assisted tunneling and stress induced leakage currents. Physical defects appear at reverse bias voltages of 8V in a-Si samples. Linear and nonlinear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as 4.5V. Pre-breakdown sites with electroluminescence are investigated.

Wavelength‐Selective Solar Photovoltaic Systems: Powering Greenhouses for Plant Growth at the Food‐Energy‐Water Nexus

Global renewable electricity generation capacity has rapidly increased in the past decade. Increasing the sustainability of electricity generation and the market share of solar photovoltaics (PV) will require continued cost reductions or higher efficiencies. Wavelength-Selective Photovoltaic Systems (WSPVs) combine luminescent solar cell technology with conventional silicon-based PV, thereby increasing efficiency and lowering the cost of electricity generation. WSPVs absorb some of the blue and green wavelengths of the solar spectrum but transmit the remaining wavelengths that can be utilized by photosynthesis for plants growing below. WSPVs are ideal for integrating electricity generation with glasshouse production, but it is not clear how they may affect plant development and physiological processes. The effects of tomato photosynthesis under WSPVs showed a small decrease in water use, whereas there were minimal effects on the number and fresh weight of fruit for a number of commercial species. Although more research is required on the impacts of WSPVs, they are a promising technology for greater integration of distributed electricity generation with food production operations, for reducing water loss in crops grown in controlled environments, as building-integrated solar facilities, or as alternatives to high-impact PV for energy generation over agricultural or natural ecosystems.

Thermal imaging for reliability characterization of copper vias

Microelectronic integrated circuits experience nonuniform high temperatures during normal operation. Thermal expansion mismatch among the different materials comprising the device lead to a large tensile stress after high temperature cycles. Voiding and open-circuit failure from cracking of interconnects are often observed during isothermal aging and thermal fatigue tests with or without electric current. Thermoreflectance microscopy as a high resolution, non-contact imaging technique is applied for thermal profiling and reliability analysis of 500nm diameter copper interconnects under temperature stress tests. In addition to external electrical measurements which can show the aggregate change in materials or devices electrical properties, we are able to detect local temperature rise at each via. While techniques such as scanning electron microscopy can be used to locate opened circuits; thermal imaging can detect the local change in vias resistance and in the thermal resistance of the surrounding material before the complete failure. We discuss how the thermal profile could be used to identify the location of the failure and the time-to-failure of a given via in a chain.