Excitonic energy transfer (ET) offers exciting opportunities for advances in optoelectronic devic... more Excitonic energy transfer (ET) offers exciting opportunities for advances in optoelectronic devices such as solar cells. While recent experimental attempts have demonstrated its potential in both organic and inorganic photovoltaics (PVs), what remains to be addressed is quantitative understanding of how different ET modes contribute to PV performance and how ET contribution is differentiated from the classical optical coupling (OC) effects. In this study, we implement an ET scheme using a PV device platform, comprising CdSe/ZnS nanocrystal energy donor and 500 nm-thick ultrathin Si acceptor layers, and present the quantitative mechanistic description of how different ET modes, distinguished from the OC effects, increase the light absorption and PV efficiency. We find that nanocrystal sensitization enhances the short circuit current of ultrathin Si solar cells by up to 35%, of which the efficient ET, primarily driven by a long-range radiative mode, contributes to 38% of the total cur...
ABSTRACT We present a novel and comprehensive technique to measure the internal quantum efficienc... more ABSTRACT 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.
We investigate the effect of confinement on the crystalline microstructure of the polymer compone... more We investigate the effect of confinement on the crystalline microstructure of the polymer component of polymer : fullerene bulk heterojunction thin films using grazing incidence wide angle X-ray scattering. We find that the polymer crystallite size decreases and the alignment of the molecules along the surface normal increases, as the thin-film thickness is reduced from 920 nm to <20 nm and approaches the thin-film confinement regime. Furthermore, we find that the polymer crystallite size near the surface (air interface) is lower than the crystallite size in the bulk or the bottom (substrate interface) of bulk heterojunction films thicker than the confinement regime. Variation in polymer crystallite size can cause changes in charge carrier mobility and recombination rates, which in turn affect the performance of bulk heterojunction thin film devices such as photovoltaics and photodetectors.
We present a charge-extraction technique, micron-scale charge extraction by linearly increasing v... more 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.
Materials providing broadband light antireflection have applications as highly transparent window... more 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.
The effects of thin-film confinement on the material properties of ultrathin polymer (electron do... more 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, donorÀacceptor 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 (0À0 and 0À1) 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.
Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to accepto... more Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to acceptor volume fraction, morphology, and molecular orientation critically influence light absorption, exciton dissociation, charge transport, and recombination, all of which are crucial device properties in organic photovoltaics (OPV). Spatial variation of BHJ properties normal to the substrate, caused by phase segregation, can thereby create corresponding spatial variations in the OPVs optoelectronic properties. Here, normally incident and wave-guided optical modes are used to selectively excite localized regions within an inverted poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester BHJ OPV and corresponding internal quantum efficiencies are measured to study the spatial-dependent charge carrier collection probability within the BHJ. An electron-limited charge collection profile is observed for a thick (920 nm) BHJ due to fullerene-poor regions as a result of phase segregation. As the thickness of the BHJ is reduced (100 nm), charge transport is seen to be unaffected by the phase segregation. This has the potential to be a versatile non-destructive characterization technique for measuring the spatially varying charge collection probability in thin film photovoltaics and will help enable optimum device design and characterization.
We demonstrate a novel, multi-purpose optoelectronic characterization technique to quantify light... more We demonstrate a novel, multi-purpose optoelectronic characterization technique to quantify light trapping and photoinduced charge generation and extraction in photovoltaics and other multilayer thin-film optoelectronic devices. The technique measures the photogenerated current created via the selective evanescent coupling of incident light into each of the guided modes of an optoelectronic device. In analogy to the internal quantum efficiency commonly used to characterize photovoltaics (the ratio of photogenerated electrons extracted from the device to photons absorbed by the device for normally incident light), we define the guided-mode internal quantum efficiency (GIQE) as the ratio of photogenerated electrons extracted from the device to the photons absorbed by the device for a specific guided mode. We complement the measurement of GIQE with computational modeling to calculate the electromagnetic field distribution within the various layers of the device, enabling us to separate the contribution to the GIQE of the absorption in the photoactive layer from parasitic absorption in other layers. By separately quantifying the quantum efficiency of each guided mode, this technique enables improved optimization and design of optoelectronic devices, including photovoltaics that utilize waveguiding and light-trapping. Additionally, since the electromagnetic field of each guided mode has a unique spatial distribution within the photoactive layer, this technique also provides insight into the spatial distributions of charge-carrier extraction, regions of disorder, trap states, and defects within the photoactive layer.
We will outline a formalism that treats time and space on equal footing as in special relativity.... more We will outline a formalism that treats time and space on equal footing as in special relativity. We will define an extended classical theory that treats time as a dynamical variable like the spacial coordinates. We will use canonical quantization to replace Poisson brackets with commutators and show how a generator of time translations can be defined using the commutation relationship in quantum mechanics. Using this new formalism, we will derive Schrtidinger's equation, Ehrenfest theorem, the propagator and expectation values of time for a simple harmonic oscillator potential.
This project will attempt an in-depth study of algebraic coding theory. We will study the two bas... more This project will attempt an in-depth study of algebraic coding theory. We will study the two basic kinds of codes: Block codes and trellis codes. Specifically, we will look at linear block codes, cyclic codes, Hamming codes, and convolutional codes.
Excitonic energy transfer (ET) offers exciting opportunities for advances in optoelectronic devic... more Excitonic energy transfer (ET) offers exciting opportunities for advances in optoelectronic devices such as solar cells. While recent experimental attempts have demonstrated its potential in both organic and inorganic photovoltaics (PVs), what remains to be addressed is quantitative understanding of how different ET modes contribute to PV performance and how ET contribution is differentiated from the classical optical coupling (OC) effects. In this study, we implement an ET scheme using a PV device platform, comprising CdSe/ZnS nanocrystal energy donor and 500 nm-thick ultrathin Si acceptor layers, and present the quantitative mechanistic description of how different ET modes, distinguished from the OC effects, increase the light absorption and PV efficiency. We find that nanocrystal sensitization enhances the short circuit current of ultrathin Si solar cells by up to 35%, of which the efficient ET, primarily driven by a long-range radiative mode, contributes to 38% of the total cur...
ABSTRACT We present a novel and comprehensive technique to measure the internal quantum efficienc... more ABSTRACT 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.
We investigate the effect of confinement on the crystalline microstructure of the polymer compone... more We investigate the effect of confinement on the crystalline microstructure of the polymer component of polymer : fullerene bulk heterojunction thin films using grazing incidence wide angle X-ray scattering. We find that the polymer crystallite size decreases and the alignment of the molecules along the surface normal increases, as the thin-film thickness is reduced from 920 nm to <20 nm and approaches the thin-film confinement regime. Furthermore, we find that the polymer crystallite size near the surface (air interface) is lower than the crystallite size in the bulk or the bottom (substrate interface) of bulk heterojunction films thicker than the confinement regime. Variation in polymer crystallite size can cause changes in charge carrier mobility and recombination rates, which in turn affect the performance of bulk heterojunction thin film devices such as photovoltaics and photodetectors.
We present a charge-extraction technique, micron-scale charge extraction by linearly increasing v... more 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.
Materials providing broadband light antireflection have applications as highly transparent window... more 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.
The effects of thin-film confinement on the material properties of ultrathin polymer (electron do... more 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, donorÀacceptor 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 (0À0 and 0À1) 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.
Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to accepto... more Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to acceptor volume fraction, morphology, and molecular orientation critically influence light absorption, exciton dissociation, charge transport, and recombination, all of which are crucial device properties in organic photovoltaics (OPV). Spatial variation of BHJ properties normal to the substrate, caused by phase segregation, can thereby create corresponding spatial variations in the OPVs optoelectronic properties. Here, normally incident and wave-guided optical modes are used to selectively excite localized regions within an inverted poly(3-hexythiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester BHJ OPV and corresponding internal quantum efficiencies are measured to study the spatial-dependent charge carrier collection probability within the BHJ. An electron-limited charge collection profile is observed for a thick (920 nm) BHJ due to fullerene-poor regions as a result of phase segregation. As the thickness of the BHJ is reduced (100 nm), charge transport is seen to be unaffected by the phase segregation. This has the potential to be a versatile non-destructive characterization technique for measuring the spatially varying charge collection probability in thin film photovoltaics and will help enable optimum device design and characterization.
We demonstrate a novel, multi-purpose optoelectronic characterization technique to quantify light... more We demonstrate a novel, multi-purpose optoelectronic characterization technique to quantify light trapping and photoinduced charge generation and extraction in photovoltaics and other multilayer thin-film optoelectronic devices. The technique measures the photogenerated current created via the selective evanescent coupling of incident light into each of the guided modes of an optoelectronic device. In analogy to the internal quantum efficiency commonly used to characterize photovoltaics (the ratio of photogenerated electrons extracted from the device to photons absorbed by the device for normally incident light), we define the guided-mode internal quantum efficiency (GIQE) as the ratio of photogenerated electrons extracted from the device to the photons absorbed by the device for a specific guided mode. We complement the measurement of GIQE with computational modeling to calculate the electromagnetic field distribution within the various layers of the device, enabling us to separate the contribution to the GIQE of the absorption in the photoactive layer from parasitic absorption in other layers. By separately quantifying the quantum efficiency of each guided mode, this technique enables improved optimization and design of optoelectronic devices, including photovoltaics that utilize waveguiding and light-trapping. Additionally, since the electromagnetic field of each guided mode has a unique spatial distribution within the photoactive layer, this technique also provides insight into the spatial distributions of charge-carrier extraction, regions of disorder, trap states, and defects within the photoactive layer.
We will outline a formalism that treats time and space on equal footing as in special relativity.... more We will outline a formalism that treats time and space on equal footing as in special relativity. We will define an extended classical theory that treats time as a dynamical variable like the spacial coordinates. We will use canonical quantization to replace Poisson brackets with commutators and show how a generator of time translations can be defined using the commutation relationship in quantum mechanics. Using this new formalism, we will derive Schrtidinger's equation, Ehrenfest theorem, the propagator and expectation values of time for a simple harmonic oscillator potential.
This project will attempt an in-depth study of algebraic coding theory. We will study the two bas... more This project will attempt an in-depth study of algebraic coding theory. We will study the two basic kinds of codes: Block codes and trellis codes. Specifically, we will look at linear block codes, cyclic codes, Hamming codes, and convolutional codes.
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Papers by Ahsan Ashraf