Experimental Facilities

Our methodology to assess photovoltaic materials and solar cells involves a combination of optical spectroscopy, to study the fundamental properties of the semiconductor materials under investigation, coupled with parallel investigations of semiconductor device physics, to correlate the optical properties of the systems to the optoelectronic behavior and transport processes. Through these parallel and complementary investigations we aim to develop a real understanding of the mechanisms facilitating and/or inhibiting the operation of the solar cells in the systems studied.

The group, also, has the additional advantage of access to semiconductor fabrication facilities developed in the Department of Physics & Astronomy through NSF MRSEC funding, and excellent materials characterization capabilities through the Samuel Roberts Noble Electron Microscopy Laboratory, both of which are utilized directly by students within the Photovoltaics Materials Group. In addition to these capabilities, our group has a strong collaboration with the Santos Group giving us access to high quality III-V materials.

Optical Spectroscopy

• Photoluminescence: The lab is equipped with a Princeton Instruments spectrometer fitted with a Roper Scientific InGaAs linear array (operating between 900 - 1600 nm) and a Princeton Instruments PIXIS-eXcelon silicon CCD (350 - 950nm). This system is fitted with both UV and IR gratings, producing the total accessible spectral width from 300 nm to 1700 nm. Also included in the laboratory are a Thorlabs HeNe laser (model: HNL210L-JP) and a Kimmon Electric HeCd dual-wavelength (325 and 442 nm) laser (model: IK552R-F) for optical excitation in both the UV and IR spectral regions. The laboratory also includes a Janis Model SHI-4-5 optical closed-cycle cryostat system operating between 4.2 and 300 K for optical and electro-optical spectroscopy.


• Micro-magneto photoluminescence: A Janis ST-500 superconducting magnet system for micro-PL under high magnetic fields is also housed in the Laboratory. This system is used to investigate the fundamental nature of localized centers and QD physics in photovoltaic materials at temperatures ranging from 4.2 K to ambient and magnetic fields from 0 - 7 Tesla.

• Reflectivity/photo-modulation: In tandem with the device characterization capabilities described below [quantum efficiency (QE) measurements], those systems are also utilized for additional optical spectroscopy: specifically photo-modulation, electro-reflectance spectroscopy, and resonant PL to facilitate the evaluation of system band gaps and radiative transitions energies.

Semiconductor Device Physics

• Solar Cell Analysis: The laboratory includes a Newport Corporation quantum efficiency system for internal and external quantum efficiency (I/EQE) measurements. This QE system, coupled with a Newport class-ABA solar simulator, gives the laboratory full solar cell characterization capability. A Linkam micro-cryostat with full temperature control (77K to 500K) is also available in this laboratory, which is utilized to investigate carrier transport and parasitic losses in solar cells.

• Impedance Spectroscopy: Capacitance-Voltage and impedance spectroscopy are investigated using a HP4291A LCR unit. These techniques enable the evaluation of minority carrier dynamics, recombination lifetimes, the doping concentrations, and space charge regions within the device, as wells as, the parasitic electrical properties of our solar cells. The measurements are routinely performed as a function of temperature and under solar illumination.