Ahsan Ashraf

Sub-50-nm self-assembled nanotextures for enhanced broadband antireflection in silicon solar cells

Materials providing broadband light antireflection have applications as highly transparent window coatings, military camouflage, and coatings for efficiently coupling light into solar cells and out of light-emitting diodes. In this work, densely packed silicon nanotextures with feature sizes smaller than 50 nm enhance the broadband antireflection compared with that predicted by their geometry alone. A significant fraction of the nanotexture volume comprises a surface layer whose optical properties differ substantially from those of the bulk, providing the key to improved performance. The nanotexture reflectivity is quantitatively well-modelled after accounting for both its profile and changes in refractive index at the surface. We employ block copolymer self-assembly for precise and tunable nanotexture design in the range of ~10-70 nm across macroscopic solar cell areas. Implementing this efficient antireflection approach in crystalline silicon solar cells significantly betters the performance gain compared with an optimized, planar antireflection coating.

Spontaneous and strong multi-layer graphene n-doping on soda-lime glass and its application in graphene-semiconductor junctions.

Scalable and low-cost doping of graphene could improve technologies in a wide range of fields such as microelectronics, optoelectronics, and energy storage. While achieving strong p-doping is relatively straightforward, non-electrostatic approaches to n-dope graphene, such as chemical doping, have yielded electron densities of 9.5 × 1012 e/cm2 or below. Furthermore, chemical doping is susceptible to degradation and can adversely affect intrinsic graphene’s properties. Here we demonstrate strong (1.33 × 1013 e/cm2), robust, and spontaneous graphene n-doping on a soda-lime-glass substrate via surface-transfer doping from Na without any external chemical, high-temperature, or vacuum processes. Remarkably, the n-doping reaches 2.11 × 1013 e/cm2 when graphene is transferred onto a p-type copper indium gallium diselenide (CIGS) semiconductor that itself has been deposited onto soda-lime-glass, via surface-transfer doping from Na atoms that diffuse to the CIGS surface. Using this effect, we demonstrate an n-graphene/p-semiconductor Schottky junction with ideality factor of 1.21 and strong photo-response. The ability to achieve strong and persistent graphene n-doping on low-cost, industry-standard materials paves the way toward an entirely new class of graphene-based devices such as photodetectors, photovoltaics, sensors, batteries, and supercapacitors.

Confinement-Induced Reduction in Phase Segregation and Interchain Disorder in Bulk Heterojunction Films

The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donor-acceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk heterojunction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (0-0 and 0-1) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics.

Hyperspectral laser beam induced current system for solar cell characterization

We introduce a novel hyperspectral laser beam induced current (LBIC) system that uses a supercontinuum laser that can be tuned from 400nm - 1200nm with diffracted limited spot size. The solar cell is light biased while simultaneously being illuminated by a chopped laser beam at a given wavelength. Current-voltage measurements performed by measuring the current perturbation due to the laser using a lock-in amplifier allow us to extract performance metrics at a specific lateral position and depth (by tuning the wavelength of the laser) while the device is at operating conditions. These parameters are simultaneously compared to material deformations as determined from the doping density, and the built-in voltage. Concurrently we also probe lateral recombination variation by measuring the activation energy thereby providing a comprehensive and unique analysis.

Measuring charge carrier mobility in photovoltaic devices with micron-scale resolution

We present a charge-extraction technique, micron-scale charge extraction by linearly increasing voltage, which enables simultaneous spatially resolved measurements of charge carrier mobility and photocurrent in thin-film photovoltaic devices with micron-scale resolution. An intensity-modulated laser with beam diameter near the optical diffraction limit is scanned over the device, while a linear voltage ramp in reverse bias is applied at each position of illumination. We calculate the majority carrier mobility, photocurrent, and number of photogenerated charge carriers from the resulting current transient. We demonstrate this technique on an organic photovoltaic device, but it is applicable to a wide range of photovoltaic materials.

Hyperspectral guided-mode quantum efficiency: A novel characterization technique for thin-film photovoltaics

We present a novel and comprehensive technique to measure the internal quantum efficiency of light absorbed as a guided-mode in a thin-film photovoltaic (TPV) over a broad range of wavelengths. Evanescent prism coupling is utilized to selectively excite individual guided-modes at a given wavelength and polarization across a broad spectrum. The Guided-Mode Quantum Efficiency (GIQE) provides a direct quantitative method to design TPVs which can optimize light trapping through guided-modes for higher power conversion efficiency. Furthermore, using theoretical photocarrier generation profile and GIQE of a specific guided-mode, we can probe the spatially dependent charge extraction in the active layer in a direction normal to the substrate. Therefore this technique enables the identification of regions of poor performance due to recombination and trapping within the active layer in a TPV device.