Matthew Beard | National Renewable Energy Laboratory (original) (raw)
Papers by Matthew Beard
Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-syn... more Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications. S ilicon is ubiquitous in modern technology, being the dominant semiconductor material for the microelectronics and photovoltaic industries. However, its poor optical properties resulting from the indirect nature of its lowest energy optical transition have always hindered its use in photonic and optoelectronic applications. In bulk crystalline Si (c-Si) the conduction band minimum (CBM) occurs at the six equivalent Δ-valleys, located in the Brillouin zone along the [100] directions about 85% from the Γ-point out toward the X-point zone boundary, whereas the valence band maximum (VBM) occurs at the Γ-point. Thus, the fundamental band gap transition is momentum forbidden (indirect) so the zero-phonon transitions are dipole forbidden. A key challenge in silicon-based photonics is to control the intrinsic material properties to yield stronger optical transitions at the bandgap. 1 Indeed, the observation of bright photoluminescence (PL) first from highly porous Si 2−4 and then from Si nanocrystals (NCs) 2 launched an explosion of research aimed at understanding how the emission and absorption can be enhanced to the benefit of silicon-based photonics. 5−10 In the current paper we focus on a concerted theoretical and experimental effort to understand and articulate the design principles that enhance interband transitions in Si NCs. We first clarify the (often confusing) roles of momenta ("band folding") versus the role of state mixing in the creation of strong absorption in NCs relative to the bulk solid, identifying the factors that need to be controlled to achieve such a change in absorption. We next measured the absolute absorption cross section on a per Si atom basis of plasma-synthesized and ligand-passivated Si NCs, comparing the results with the absorption of bulk c-Si, scaled with the local field factor correction for small particles. While the absorption of Si NCs has been measured previously, 11−14 ours is the first study to precisely determine the absorption on a per Si atom basis and compare with bulk c-Si. We demonstrate that above an energy of ∼2.2 eV the NC absorbs up to 5 times stronger than the bulk. Underlying Physics of Zero Phonon Absorption Strength in NCs Made of Indirect Band Gap Solids. The literature on absorption in Si nanostructures 11,12,15,16 lists many possible factors controlling the conversion of momen
Journal of Physical Chemistry A, 2004
We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr eff... more We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr effect (OHD-RIKES) spectroscopic techniques as probes of molecular and vibrational dynamics in the frequency range of 0.1 to 6 THz (3 to 200 cm-1). The spectra of ...
Acs Nano, 2008
We present a comprehensive study of the optical and electrical properties of transparent conducti... more We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes.
Nano Letters, 2008
Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dyna... more Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dynamics in transparent films of single-walled carbon nanotubes (SWNTs). Two films were investigated: a film with 94% semiconducting-type tubes (s-SWNTs) and a film with only 7% s-SWNT and 93% metal-type tubes (m-SWNTs). We conclude that charge-carriers are generated with >60% yields at low light intensities in both films. Free-carriers are generated by a linear exciton dissociation process that occurs within -1 ps and is independent of excitation wavelength or tube type.
Physical Review A, 2001
We present time-resolved measurements of the ballistic trajectory of ultrashort optical wave pack... more We present time-resolved measurements of the ballistic trajectory of ultrashort optical wave packets circulating near the surface of a dielectric microcavity. The wave-packet dynamics are determined by using twocolor two-photon induced fluorescence of Coumarin 510 within the microcavity as a correlator. Ballistic motion of the wave packets in whispering gallery modes and rainbow trajectories has been deduced from the time delay between collisions of the two wave packets. The cavity ringdown time is measured for each of the wave packets, which retain their integrity even after several round trips.
Nano Letters, 2007
We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (... more We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at ∼4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.
Journal of Physical Chemistry B, 2006
We report on the ultrafast photoinduced charge separation processes in varying compositions of po... more We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time-and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (W PCBM ) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.
Nano Letters, 2009
We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (... more We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry. * Corresponding authors: matt_beard@nrel.gov, anozik@nrel.gov. † These authors contributed equally to this work.
Journal of Physical Chemistry B, 2005
We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3... more We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3.45 nm. We find excellent agreement between the measured results and calculated values based on a model of smallparticle light absorption. The absorption cross-section per dot is 6.2 × 10 -16 R 3 cm 2 at 2.76 eV and 3.15 × 10 -16 R 1.28 cm 2 at the first-exciton absorption peak, with the dot radius R in nm. We find that the per-quantumdot particle oscillator strength of the first-exciton transition is constant for all sizes studied. The radiative lifetime of the first exciton calculated from the oscillator strength increases with dot size and ranges from 4 ns for the smallest dots to 14 ns for the largest ones.
Journal of Physical Chemistry A, 2002
We report the direct measurement of intramolecular electron transfer by detecting the electromagn... more We report the direct measurement of intramolecular electron transfer by detecting the electromagnetic (EM) waveform that is emitted during this process. It is detected in the time domain via free-space electro-optic sampling and then related to the dynamics of the charge-transfer event. Electromagnetic pulse generation from two systems, Betaine-30 in chloroform and DMANS in toluene, are studied to illustrate this technique. A finite-difference time-domain calculation with a time-dependent polarization is used to model the EM generation and propagation through the solution. This method is very general, since the movement of charge itself generates the EM waveform, and is sensitive to charge transfer occurring on a 0.1-10 ps time scale.
The finite-difference time-domain ͑FDTD͒ method has been applied to time-resolved THz spectroscop... more The finite-difference time-domain ͑FDTD͒ method has been applied to time-resolved THz spectroscopy ͑TRTS͒ experiments. Time-resolved THz spectroscopy utilizes an optical pump pulse to excite the sample, followed by a far-infrared ͑FIR͒ probe pulse with frequency components that span from 10 to 100 cm Ϫ1 . The subpicosecond evolution of the FIR spectrum is obtained as a function of time after the visible photoexcitation event. Significant challenges arise in interpreting these experimental results due to the very different frequencies of the pump and probe pulses. Therefore, it is essential to simulate the experiment. The method described entails numerically propagating both the THz probe pulse and the visible pump pulse simultaneously, keeping track of the transiently induced polarization from absorption of the visible pulse. Group velocity mismatch between the visible and THz pulse and a transiently changing response function are completely accounted for in the calculation. Furthermore, a spatially varying polarization can be included to account for a nonuniform excited region of the sample under investigation. The response function of the material is described as a multimode Brownian oscillator that can describe dispersive media in a very general sense. In particular, the overdamped, underdamped, and critically damped cases are all included, as well as special cases such as a Debye or Drude response. As a specific example, we present results of modeling a TRTS experiment of photoexcitation of a dye in solution, namely, 2,11,20,29-tetra-tert-butyl-2,3-napthalocyanine, dissolved in toluene. We carry out a nonlinear least squares fit of a parameterized model to the measured data to show that the FDTD-TRTS method is able to accurately reproduce the features observed in the measured data set.
Nano Letters, 2003
Arrays and lattices formed from nanoparticles (NPs) present unique opportunities for new optoelec... more Arrays and lattices formed from nanoparticles (NPs) present unique opportunities for new optoelectronic materials whose properties can be tuned by controlling the size of the individual NPs and their interparticle separation to effect strong inter-NP electronic coupling. Characterization of the interdot coupling as a function of interdot distance is essential. Using time-resolved THz spectroscopy, we report a six-fold increase in the transient photoconductivity in disordered arrays of 3.2 nm diameter InP NPs separated by 0.9 nm compared to arrays with 1.8 nm separation. Photoconductivity in the arrays is compared to that of isolated NPs and InP epilayers. The epilayer samples exhibit bulk transport behavior while the NP samples do not.
Journal of Physical Chemistry B, 2002
The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-r... more The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-resolved terahertz spectroscopy, a noncontact electrical probe with sub-picosecond temporal resolution. The photoconductivity deviates strongly from Drude behavior and is ...
Nano Letters, 2002
The size-dependent transient photoconductivity in CdSe nanoparticles (NPs) has been investigated ... more The size-dependent transient photoconductivity in CdSe nanoparticles (NPs) has been investigated using time-resolved THz spectroscopy (TRTS). We find that the mobility increases with an r 4 dependence for NPs in the strong confinement regime and is governed by a restricted mean free path for NPs in the weak confinement regime. The photoexcited carriers undergo ballistic transport to the surface of the NPs. The average number of electron−hole pairs generated in the NPs falls into three distinct groupings based on the NP radius.
Physical Review B, 2000
The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal reso... more The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density (Ͻ2ϫ10 16 cm Ϫ3 ) with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz ͑far-infrared͒ pulses of several hundred femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model (6540 cm 2 V Ϫ1 s Ϫ1 at room temperature with Nϭ1.6ϫ10 16 cm Ϫ3 , and 13600 cm 2 V Ϫ1 s Ϫ1 at 70 K with Nϭ1.5ϫ10 16 cm Ϫ3 ) are in good agreement with literature values. There are, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the ⌫ valley due to LO-phonon-assisted intravalley absorption. At long delay times ͑5-900 ps͒, the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is 8.5ϫ10 5 cm/s.
Journal of The American Chemical Society, 2006
We report an alternative synthesis and the first optical characterization of colloidal PbTe nanoc... more We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 ( 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.
Nano Letters, 2007
Multiple exciton generation (MEG) is a process whereby multiple electron−hole pairs, or excitons,... more Multiple exciton generation (MEG) is a process whereby multiple electron−hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap ≡ E g ) 1.20 eV) to be 2.4 ± 0.1E g and find an excitonproduction quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4E g . While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.
Nano Letters, 2008
We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that prod... more We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport.
Nano Letters, 2005
We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in collo... more We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.
Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-syn... more Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications. S ilicon is ubiquitous in modern technology, being the dominant semiconductor material for the microelectronics and photovoltaic industries. However, its poor optical properties resulting from the indirect nature of its lowest energy optical transition have always hindered its use in photonic and optoelectronic applications. In bulk crystalline Si (c-Si) the conduction band minimum (CBM) occurs at the six equivalent Δ-valleys, located in the Brillouin zone along the [100] directions about 85% from the Γ-point out toward the X-point zone boundary, whereas the valence band maximum (VBM) occurs at the Γ-point. Thus, the fundamental band gap transition is momentum forbidden (indirect) so the zero-phonon transitions are dipole forbidden. A key challenge in silicon-based photonics is to control the intrinsic material properties to yield stronger optical transitions at the bandgap. 1 Indeed, the observation of bright photoluminescence (PL) first from highly porous Si 2−4 and then from Si nanocrystals (NCs) 2 launched an explosion of research aimed at understanding how the emission and absorption can be enhanced to the benefit of silicon-based photonics. 5−10 In the current paper we focus on a concerted theoretical and experimental effort to understand and articulate the design principles that enhance interband transitions in Si NCs. We first clarify the (often confusing) roles of momenta ("band folding") versus the role of state mixing in the creation of strong absorption in NCs relative to the bulk solid, identifying the factors that need to be controlled to achieve such a change in absorption. We next measured the absolute absorption cross section on a per Si atom basis of plasma-synthesized and ligand-passivated Si NCs, comparing the results with the absorption of bulk c-Si, scaled with the local field factor correction for small particles. While the absorption of Si NCs has been measured previously, 11−14 ours is the first study to precisely determine the absorption on a per Si atom basis and compare with bulk c-Si. We demonstrate that above an energy of ∼2.2 eV the NC absorbs up to 5 times stronger than the bulk. Underlying Physics of Zero Phonon Absorption Strength in NCs Made of Indirect Band Gap Solids. The literature on absorption in Si nanostructures 11,12,15,16 lists many possible factors controlling the conversion of momen
Journal of Physical Chemistry A, 2004
We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr eff... more We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr effect (OHD-RIKES) spectroscopic techniques as probes of molecular and vibrational dynamics in the frequency range of 0.1 to 6 THz (3 to 200 cm-1). The spectra of ...
Acs Nano, 2008
We present a comprehensive study of the optical and electrical properties of transparent conducti... more We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes.
Nano Letters, 2008
Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dyna... more Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dynamics in transparent films of single-walled carbon nanotubes (SWNTs). Two films were investigated: a film with 94% semiconducting-type tubes (s-SWNTs) and a film with only 7% s-SWNT and 93% metal-type tubes (m-SWNTs). We conclude that charge-carriers are generated with &amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;60% yields at low light intensities in both films. Free-carriers are generated by a linear exciton dissociation process that occurs within -1 ps and is independent of excitation wavelength or tube type.
Physical Review A, 2001
We present time-resolved measurements of the ballistic trajectory of ultrashort optical wave pack... more We present time-resolved measurements of the ballistic trajectory of ultrashort optical wave packets circulating near the surface of a dielectric microcavity. The wave-packet dynamics are determined by using twocolor two-photon induced fluorescence of Coumarin 510 within the microcavity as a correlator. Ballistic motion of the wave packets in whispering gallery modes and rainbow trajectories has been deduced from the time delay between collisions of the two wave packets. The cavity ringdown time is measured for each of the wave packets, which retain their integrity even after several round trips.
Nano Letters, 2007
We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (... more We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at ∼4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.
Journal of Physical Chemistry B, 2006
We report on the ultrafast photoinduced charge separation processes in varying compositions of po... more We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time-and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (W PCBM ) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.
Nano Letters, 2009
We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (... more We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry. * Corresponding authors: matt_beard@nrel.gov, anozik@nrel.gov. † These authors contributed equally to this work.
Journal of Physical Chemistry B, 2005
We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3... more We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3.45 nm. We find excellent agreement between the measured results and calculated values based on a model of smallparticle light absorption. The absorption cross-section per dot is 6.2 × 10 -16 R 3 cm 2 at 2.76 eV and 3.15 × 10 -16 R 1.28 cm 2 at the first-exciton absorption peak, with the dot radius R in nm. We find that the per-quantumdot particle oscillator strength of the first-exciton transition is constant for all sizes studied. The radiative lifetime of the first exciton calculated from the oscillator strength increases with dot size and ranges from 4 ns for the smallest dots to 14 ns for the largest ones.
Journal of Physical Chemistry A, 2002
We report the direct measurement of intramolecular electron transfer by detecting the electromagn... more We report the direct measurement of intramolecular electron transfer by detecting the electromagnetic (EM) waveform that is emitted during this process. It is detected in the time domain via free-space electro-optic sampling and then related to the dynamics of the charge-transfer event. Electromagnetic pulse generation from two systems, Betaine-30 in chloroform and DMANS in toluene, are studied to illustrate this technique. A finite-difference time-domain calculation with a time-dependent polarization is used to model the EM generation and propagation through the solution. This method is very general, since the movement of charge itself generates the EM waveform, and is sensitive to charge transfer occurring on a 0.1-10 ps time scale.
The finite-difference time-domain ͑FDTD͒ method has been applied to time-resolved THz spectroscop... more The finite-difference time-domain ͑FDTD͒ method has been applied to time-resolved THz spectroscopy ͑TRTS͒ experiments. Time-resolved THz spectroscopy utilizes an optical pump pulse to excite the sample, followed by a far-infrared ͑FIR͒ probe pulse with frequency components that span from 10 to 100 cm Ϫ1 . The subpicosecond evolution of the FIR spectrum is obtained as a function of time after the visible photoexcitation event. Significant challenges arise in interpreting these experimental results due to the very different frequencies of the pump and probe pulses. Therefore, it is essential to simulate the experiment. The method described entails numerically propagating both the THz probe pulse and the visible pump pulse simultaneously, keeping track of the transiently induced polarization from absorption of the visible pulse. Group velocity mismatch between the visible and THz pulse and a transiently changing response function are completely accounted for in the calculation. Furthermore, a spatially varying polarization can be included to account for a nonuniform excited region of the sample under investigation. The response function of the material is described as a multimode Brownian oscillator that can describe dispersive media in a very general sense. In particular, the overdamped, underdamped, and critically damped cases are all included, as well as special cases such as a Debye or Drude response. As a specific example, we present results of modeling a TRTS experiment of photoexcitation of a dye in solution, namely, 2,11,20,29-tetra-tert-butyl-2,3-napthalocyanine, dissolved in toluene. We carry out a nonlinear least squares fit of a parameterized model to the measured data to show that the FDTD-TRTS method is able to accurately reproduce the features observed in the measured data set.
Nano Letters, 2003
Arrays and lattices formed from nanoparticles (NPs) present unique opportunities for new optoelec... more Arrays and lattices formed from nanoparticles (NPs) present unique opportunities for new optoelectronic materials whose properties can be tuned by controlling the size of the individual NPs and their interparticle separation to effect strong inter-NP electronic coupling. Characterization of the interdot coupling as a function of interdot distance is essential. Using time-resolved THz spectroscopy, we report a six-fold increase in the transient photoconductivity in disordered arrays of 3.2 nm diameter InP NPs separated by 0.9 nm compared to arrays with 1.8 nm separation. Photoconductivity in the arrays is compared to that of isolated NPs and InP epilayers. The epilayer samples exhibit bulk transport behavior while the NP samples do not.
Journal of Physical Chemistry B, 2002
The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-r... more The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-resolved terahertz spectroscopy, a noncontact electrical probe with sub-picosecond temporal resolution. The photoconductivity deviates strongly from Drude behavior and is ...
Nano Letters, 2002
The size-dependent transient photoconductivity in CdSe nanoparticles (NPs) has been investigated ... more The size-dependent transient photoconductivity in CdSe nanoparticles (NPs) has been investigated using time-resolved THz spectroscopy (TRTS). We find that the mobility increases with an r 4 dependence for NPs in the strong confinement regime and is governed by a restricted mean free path for NPs in the weak confinement regime. The photoexcited carriers undergo ballistic transport to the surface of the NPs. The average number of electron−hole pairs generated in the NPs falls into three distinct groupings based on the NP radius.
Physical Review B, 2000
The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal reso... more The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density (Ͻ2ϫ10 16 cm Ϫ3 ) with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz ͑far-infrared͒ pulses of several hundred femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model (6540 cm 2 V Ϫ1 s Ϫ1 at room temperature with Nϭ1.6ϫ10 16 cm Ϫ3 , and 13600 cm 2 V Ϫ1 s Ϫ1 at 70 K with Nϭ1.5ϫ10 16 cm Ϫ3 ) are in good agreement with literature values. There are, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the ⌫ valley due to LO-phonon-assisted intravalley absorption. At long delay times ͑5-900 ps͒, the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is 8.5ϫ10 5 cm/s.
Journal of The American Chemical Society, 2006
We report an alternative synthesis and the first optical characterization of colloidal PbTe nanoc... more We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 ( 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.
Nano Letters, 2007
Multiple exciton generation (MEG) is a process whereby multiple electron−hole pairs, or excitons,... more Multiple exciton generation (MEG) is a process whereby multiple electron−hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap ≡ E g ) 1.20 eV) to be 2.4 ± 0.1E g and find an excitonproduction quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4E g . While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.
Nano Letters, 2008
We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that prod... more We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport.
Nano Letters, 2005
We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in collo... more We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.