Free Electron-hole Pairs Research Articles

Numerical simulation of the impurity photovoltaic effect in silicon solar cells

Recently, the impurity photovoltaic effect (IPV) was proposed to improve the solar cell performance. Free electron–hole pairs can be generated in a two-step process involving an impurity level in the energy gap and two lower-energy photons: first electrons are optically excited from the valence band to the defect level and then from the defect level to the conduction band. The IPV effect will thus enhance the long-wavelength response of the cell. A significant amount of theoretical work has been carried out on IPV effect in the literature, particularly on silicon solar cells with indium impurities as defect. However, the lack of an easily available solar cell simulator including the IPV effect is a handicap. In this work, the numerical solar cell simulator SCAPS of the ELIS group was extended to include IPV in collaboration between the ELIS and the LPDS groups. Also, some special features are implemented, such as the calculation of electron and hole photoemission cross-sections of the impurity using the model of Lucovsky. The functionality of new SCAPS version was checked against existing results in the literature. Also, new results are presented such as the evolution of solar cell parameters with the indium density. We find that increasing indium concentration can improve silicon solar cell parameters, especially the short-circuit current and the efficiency, without drastically decreasing the open-circuit voltage. This is possible if a suitable structure for the cell is chosen. The optimum indium density should be equal around the base region density to obtain a positive benefit from the IPV effect. Light trapping, which is related to the internal reflectance at the front and the back of the cell, is very important in the IPV study. Reflectivity at the front and the back should exceed 99.9% to obtain a real efficiency increase. We calculate an improvement of about 6 mA/cm 2 in the photocurrent, and about 2% for the efficiency, which is due to the enhancement of long-wavelength absorption by the IPV effect.

Theoretical treatment of the second-order Raman scattering

The theoretical analysis for the Raman scattering by 2LO phonons is carried out. The scattering process is considered, taking into account the Wannier excitons and the free electron-hole pairs as intermediate states.

Nonresonant electrical injection of excitons in an InGaAs quantum well

The authors report on electroluminescence measurements combined with photoluminescence excitation spectroscopy on a single InGaAs quantum well placed in the intrinsic region of a p-i-n photodiode. They show that at low current density, the spectra are dominated by the spectrally narrow excitonic emission. Moreover when increasing carrier injection, they observe the progressive transition from excitons into free electron-hole pairs. This structure meets all criteria to be integrated in a semiconductor microcavity and the present demonstration of exciton electroluminescence is the first step toward the achievement of the strong coupling regime under electrical injection.

Direct Observation of the Mott Transition in an Optically Excited Semiconductor Quantum Well

We have studied density-dependent time-resolved photoluminescence from a 80 A InGaAs/GaAs single quantum well excited by picosecond pulses. We succeed in giving evidence for the transition from an exciton-dominated population to an unbound electron-hole pair population as the pair density increases. For pair densities below this excitonic Mott transition we observe a spectrally separate emission from free electron-hole pairs in addition to excitonic luminescence, thereby proving the coexistence of both species. Exciton binding energy and band gap remain unchanged even near the upper bound of this coexistence region. Above the Mott density we observe a purely exponential high energy tail of the photoluminescence and a redshift of the band gap with pair density. The transition occurs gradually between 1 x 10(10) and 1 x 10(11) cm(-2) at the carrier temperatures of our experiment.

Nonperturbative Coulomb correlations generated by simultaneous excitation of excitonic and band-to-band continuum transitions

We investigate theoretically the nonlinear optical response of a semiconductor for excitation conditions where simultaneously all kinds of correlated two-pair transitions contribute to the dynamics. This includes transitions to biexcitons, exciton-exciton scattering states, two free electron-hole pairs as well as two-pair states involving an exciton and a free electron-hole pair. For certain excitation conditions two-pair correlations give rise to a complex line shape of four-wave-mixing spectra with an emission spread over the whole range between the exciton line and the band-edge. Even a strong suppression of the signal at the exciton line can be achieved while the emission is still concentrated between the exciton and the band-edge. The dependence of these features on the excitation conditions and on the strength of the Coulomb interaction is discussed. Comparing a nonperturbative treatment of the Coulomb interaction with the Born approximation and the mean-field theory clearly reveals the importance of nonperturbative Coulomb correlations even for excitations involving the continuum of free electron-hole pairs.

Efficient silicon light emitting diodes by boron implantation: the mechanism

Experiments and theoretical modeling are presented on the origin of efficient electroluminescence from boron implanted Si-LEDs. At low lattice temperatures two bound exciton traps created by high dose boron implantation were observed in the most efficient LEDs with external power efficiency above 0.12%. The temperature dependence of the correlation between the EL intensity from free and bound excitons is analyzed by a rate equation model. This analysis reveals that the bound excitons have a unique characteristic of a low recombination rate. The enhancement of EL from free electron-hole pairs with increasing temperature is due to the thermal activation of carriers from bound exciton traps.

Lowest optical excitations in molecular crystals: bound excitons versus free electron-hole pairs in anthracene.

By solving the Bethe-Salpeter equation for the electron-hole Green function for crystalline anthracene we find the lowest absorption peak generated by strongly bound excitons or by a free electron-hole pair, depending on the polarization direction being parallel to the short or the long molecular axis, respectively. Both excitations are shifted to lower energies by pressure. The physical difference of these excitations is apparent from the electron-hole wave functions. Our findings are a major contribution to solve the long-standing puzzle about the nature of the lowest optical excitations in organic materials.

Broadband emission and low absorption in microdisks with AlGaAs quantum wells

In microphotoluminescence measurements at room temperature on Al0.2Ga0.8As/Al0.4Ga0.6As multiple quantum well microdisks, we observe a large number of whispering gallery modes in a remarkably broad energy range of about 250 meV. This is in contrast to microdisks containing InGaAs or GaAs quantum wells, where typically only one or two whispering gallery modes are observed, but similar to the behavior of microdisks containing self-organized InAs quantum dots. We conclude that impurity levels below the recombination energy of free electron-hole pairs inside the alloy quantum wells lead to an efficient internal broadband emitter with low absorption.

Origin of anomalous temperature dependence and high efficiency of silicon light-emitting diodes

Efficient electroluminescence with power efficiency up to 0.12% is observed from silicon pn diodes prepared by boron implantation with boron concentrations above the solubility limit at the postimplantation annealing temperature. The electroluminescence spectra exhibit a transition from two bound-exciton bands towards the free electron-hole pair recombination with an anomalous increase in the total intensity with increasing temperature. The implantation dose and temperature dependences of the relative peak intensities provide evidence for the relevance of excitonic traps as a supply for free electron-hole pairs and thus for the origin of the enhanced electroluminescence at elevated temperatures.

Dynamics of two‐pair correlations induced by simultaneous excitation of excitonic and band‐to‐band continuum transitions

AbstractThe impact of all kinds of two‐pair correlations (biexcitons, exciton–exciton scattering states, two free electron–hole pairs) on four‐wave‐mixing (FWM) signals generated by ultra‐short optical pulses that simultaneously excite excitons and free electron–hole pairs is analyzed. Signatures of two‐pair correlations are identified by comparing the full theory with mean‐field (MF) results. For systems with strong exciton binding two‐pair correlations suppress changes of the line width of the excitonic emission that are predicted by the MF theory when the central excitation energy is varied above the band edge. For systems with weakly bound excitons the presence of two‐pair correlations leads to an emission in between the exciton and the band edge with a complex line shape which strongly depends on the excitation energy.

Excitons, polarons, and laser action in poly(p-phenylene vinylene) films

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)−E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogeneration consists of two contributions; the intersystem crossing and, at higher energies, singlet fission. From the onset of the singlet fission process at ESF=2ET, we could estimate the energy of the lowest-lying triplet exciton, 1 3Bu, at 1.55 eV, with a singlet–triplet splitting as large as 0.9 eV. From photoinduced absorption spectroscopy we measured the triplet–triplet transition, T→T*, to be 1.45 eV. The quantum efficiency spectrum for polaron photogeneration also consists of two contributions: one extrinsic and the other intrinsic. The latter shows a monotonically increasing function of energy with an energy onset at 3.3 eV. The intrinsic photogeneration process is analyzed with a model of free-electron–hole pair photogeneration, which separate more effectively at higher energy. The carrier generation quantum yield at 3.65 eV is estimated to be 0.5%. The quantum efficiency for photoluminescence, on the other hand, shows one single step-function process, with an onset at 2.4 eV. The photoluminescence spectrum is well structured, showing five phonon side bands with 190 meV separation. We have also studied laser action in PPV thin films and microcavities such as microrings and microdisks. The effective gain spectrum is calculated and the estimated threshold excitation intensity for laser action for the 0-1 transition is found to be in good agreement with the data, with an estimated exciton density of 2×1018 cm−3. Lasing from microring devices shows several narrow waveguide modes, with intermode spacing of 0.45 nm that corresponds to an effective mode refractive index, neff=1.45. The spectral width of the laser modes is resolution limited and gives a lower estimate of the cavity quality factor, Q. For microrings we found Q>5000, which is limited by self-absorption in the polymer film.

FEMTOSECOND DYNAMICS OF PHOTOEXCITED HIGH DENSITY CARRIERS IN ZnO EPITAXIAL THIN FILMS

Time-resolved luminescence and optical gain spectra due to an electron-hole plasma (EHP) in ZnO epitaxial thin films are studied through optical Kerr gate luminescence spectroscopy. Above the critical Mott density, free electron-hole pairs are created instantaneously by intense pump pulses and bandgap is reduced without any detectable delay time. Cooling of the hot carriers toward the bottom of the renormalized bandgap and subsequent buildup of the quasi-thermalized EHP state take place. Then the recovery of the renormalized bandgap toward higher energy with decreasing the carrier density follows. From the lineshape analysis of the time-resolved optical gain spectra, we find that the decrease of the carrier density is governed by a bimolecular recombination process which represents radiative recombination of electrons with holes in the EHP state. The effective temperature as well as the carrier density of the EHP are also determined.

Random piezoelectric field in real [001]-oriented strain-relaxed semiconductor heterostructures.

We work out a theory of piezoelectricity in an actual semiconductor heterostructure which is composed of a lattice-mismatched zinc-blende layer grown on a [001]-oriented substrate. In contrast to earlier theories, we predict a large density of fixed bulk piezoelectric charges, which are induced by strain fluctuations connected with interface roughness. The piezoelectric charges create a high electric field. The random piezoelectric field presents a conceptually new important scattering mechanism. The system of charge carriers in such a heterostructure becomes strongly disordered and includes generally both free electron-hole pairs near the interface and excitons far from it.

Impurity-induced resonance Raman scattering at the (e,A 0) threshold in lightly carbon-doped p-type GaAs at [formula omitted

In lightly carbon-doped p-type GaAs the impurity-induced resonance Raman scattering (RRS) by one longitudinal-optic (LO) phonon has been studied at low temperature by using a tunable Ti–sapphire laser. In spite of involvement of strong luminescence in the spectral region of the Raman scattering, a pronounced scattering resonance at the (e,A 0) threshold has been well identified. Both the extrinsic LO-Raman scattering and the luminescence of (e,A 0) LO phonon replicas have been analyzed in a quantitative way. We find that carbon-acceptor-induced extrinsic scattering is fairly intense compared to the intrinsic Raman scattering. It is interpreted in terms of the large oscillator strength of the (e,A 0) transition. Compared to the intrinsic scattering due to free electron–hole pairs, the large (e,A 0) oscillator strength leads to about two orders of magnitude enhancement in the Raman intensity.

Longitudinal optical phonon assisted relaxation of excitons due to intersubband transitions in quantum wells

Longitudinal optical phonon assisted relaxation of excitons involving intersubband transitions is studied in quantum wells. Considering the relaxation of an exciton from a higher subband to a free electron–hole pair in lower subbands by longitudinal optical phonon emission, we calculate the rate of relaxation as a function of exciton density, temperature, and wave vector K∥ of the center-of-mass of exciton in quantum wells. The theory is finally applied to GaAs quantum wells and results are compared with experiment.

Signatures of Two-Pair Correlations after Simultaneous Excitation of Excitons and Free Electron-Hole Pairs in a Quantum Wire

The impact of all kinds of two-pair correlations (biexcitons, exciton–exciton scattering states, two free electron–hole pairs) on four-wave-mixing (FWM) signals generated by ultra-short optical pulses that simultaneously excite excitons and free electron–hole pairs is analyzed. Calculations accounting non-perturbatively for the Coulomb interaction are compared with the Born approximation (BA) and mean-field (MF) results. Thereby signatures of two-pair correlations are identified as well as qualitative differences between microscopic scattering theories and phenomenological treatments of pair relaxations.

Intensity correlated nonlinear photoluminescence spectra in undoped andp-doped GaAs and GaAs quantum wells

We study the line shapes and excitation density dependence of the intensity correlated nonlinear (NL) photoluminescence (PL) spectra due to excitons and free electron-hole pairs in molecular beam epitaxy grown undoped and p-doped GaAs and GaAs quantum wells. The laser excitation intensity (I) is varied by nearly three orders of magnitude. The NL PL spectra show negative peaks at energies corresponding to free and bound excitons as I increases. These peaks do not diminish, but continue to increase with I up to the highest value of I used.

Charge-induced dephasing in thin polythiophene films

The third-order nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$ normalized to the absorption coefficient \ensuremath{\alpha} displays a dramatic drop at 0.4 eV above the ${S}_{0}\ensuremath{-}{S}_{1}$ transition in polythiophene derivatives. Steady-state photoconductivity data show the intrinsic photocrent starting exactly where the decrease in ${\ensuremath{\chi}}^{(3)}/\ensuremath{\alpha}$ is seen. These results point to a change from bound excitons to free-electron hole pairs at 0.4 eV above the ${S}_{0}\ensuremath{-}{S}_{1}$ transition, and support the description of such disordered conjugated polymers using a molecular approach rather than as infinite one-dimensional semiconductors.

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K||.

Ultrafast relaxation processes of solitons, polarons, and self-trapped excitons in MX-chain

Ultrafast relaxation processes of photoexcited states of a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2Cl 2](PF 6) 4 have been investigated by femtosecond transient absorption spectra measurements. It has been found that a considerable part of the neutral soliton pairs is generated directly from photogenerated charge transfer excitons. Charged soliton pairs have been found to be generated from free electron–hole pairs. The rise time and the decay time of the charged soliton absorption band are 2.2 and 6.5 ps, respectively. The polaron pairs are generated from free electron–hole pairs within time resolution.