J. Bullot

The quenching effect of an external electric field on the fluorescence of TMPD in 2,2,4-trimethylpentane excited above the photoionization threshold

Abstract It is shown that, when an external electric field is applied to a dilute solution of TMPD in 2,2,4-trimethylpentane during excitation in the photoionization region, the fluorescence intensity level is decreased by an amount Δ F attaining about 4% for the strongest field used (45 kV cm −1 . This effect is interpreted in the framework of the Onsager model and shows that geminate recombination fluorescence contributes significantly to the total fluorescence at photoionization energies.

Dual-beam-modulated photoconductivity in hydrogenated amorphous silicon response-time and drift-mobility measurements

Abstract Dual-beam-modulated photoconductivity in a-Si:H is studied at room temperature. The complex modulation signal is analysed in terms of three independent relaxing components. It is shown that the fast-relaxing term is due to electron detrapping from states in the conduction-band tail and that its relaxation time is the response time τR of the photoconductivity. The product μτ is measured by steady-state photoconductivity, and we proceed by investigating the dependence of the ratio μτ/τR = μ/(1 +n t/n) on the photogeneration rate f where n t and n are the trapped and free electron densities respectively. The f dependence of the ratio n t/n is discussed. A model based on the existence of an exponential distribution of tail states allows us to show that n t/n∝ f 1−2γ (σp∝fγ), in agreement with the experimental data. This implies that the response time of the photoconductivity follows a τR∝ fγ dependence. The link between μτ/τR and the drift mobility μD measured by time of flight is discussed. It is fo...

On the ionization potential of a solute molecule in hydrocarbon solutions: Effect of temperature and phase

It is shown that the ionization potential I of N,N,N′,N′-tetramethylparaphenylenediamine (TMPD) dissolved in several hydrocarbons is constant in the solid phase from 77 K up to the melting point TM and suffers an abrupt decrease at this temperature ranging from 0.65 eV for tetramethylsilane to 0.42 eV for n-hexane. The meaning of the I jump at TM is discussed. From the invariance of I, it is concluded that the energy of the excess electron, V0, is constant in the solid phase.

Theoretical description of the isothermal recombination luminescence following the photoionization of a rigid organic solution

Abstract A model of isothermal recombination luminescence in a photo-ionized non-polar amorphous solid is presented. Considering only one type of trap and the potential barrier distribution engendered from it by the cation Coulomb field, an equation is obtained whose solution is numerically computed. After proper adjustment, only one set of four parameters is capable of generating a theoretical decay in agreement with experimental data. The changes in the parameter values induced by isothermal relaxation of the rigid matrix are briefly described.

Light intensity dependence of the photoconductivity of hydrogenated amorphous silicon

Abstract The physical meaning of the exponent γ characterizing the light intensity dependence of the photoconductiyity σ p in hydrogenated amorphous silicon is investigated on the basis of a dual-beam modulated photoconductiyity (DBMP) analysis. The frequency-dependent spectrum can be resolyed into three components which at high temperatures are due to thermal re-emission of electrons from D− centres, thermal re-emission of electrons from tail states and electron-hole recombination. It is found that each component follows its own generation rate dependence with an exponent γ i ≠ γ. On this basis a model is established which allows firstly σ p to be related to the components separated by DBMP, secondly each component to be characterized by its relative weight a i such that Σa i γ i,= 1, thirdly the dependence of γ on the position of the quasi-Fermi level in the gap to be related to the variations in Σa i and fourthly the weights a i and the subgap density of states to be quantitatively correlated. Experime...

Photoconductivity in hydrogenated amorphous silicon. I. Thermal emission and hopping of trapped charges

Abstract The photoconductivity of hydrogenated amorphous silicon is found to be the result of three different processes that have been identified using the dual-beam-modulated photoconductivity technique: the release of electrons trapped either in D− states or in the conduction-band tail, and a quenching process related to recombination. Each process follows its own generation rate dependence. A model has been established in which the above three processes add up to give the observed steady-state photoconductivity. The relative contributions of these processes have been determined over large temperature and light flux ranges. This allowed us to show firstly that the thermal release of electrons from gap states, either from D− or from the band tail, is still an important process at 90 K, secondly that the relative contributions of D− and tail states change drastically at about 165 K, that is when the electron quasi-Fermi level is located at about 0.3 eV below the mobility edge and thirdly that at low tempe...

Trapping and recombination in amorphous hydrogenated silicon studied by dual-beam-modulated photoconductivity

Abstract The frequency-dependent spectrum of dual-beam-modulated photoconductivity reveals that various processes contribute to steady-state photoconductivity in undoped a-Si: H. These are 1. electron thermal emission from doubly-occupied dangling bonds 2. electron thermal emission from localized states of the conduction-band tail, and 3. quenching of photoconductivity which proceeds by reducing the density of electrons trapped on dangling bonds. These results are obtained on the basis of a model which in particular allows the coefficient γ characterizing the generation rate dependence of photoconductivity (σpαfy p) to be related to the coefficients γi(i = 1—3) characterizing processes 1—3: γ = a 1γ1 + a 2γ2 — a 3γ3, where a i is a dimensionless parameter describing the relative contribution of process i. The temperature and the photogeneration rate dependences of these processes studied in the annealed and in the light-soaked states are used to identify the nature of the quenching process.

Photoconductivity in hydrogenated amorphous silicon. II. Relaxation of trapped charge carriers

Abstract Dual-beam modulated photoconductivity (DBMP) experiments allow measurement of three relaxation times that characterize the trapping-detrapping and the recombination processes participating in the steady-state photoconductivity of hydrogenated amorphous silicon. The shortest times t 1 and t 2 associated with electron thermal emission from doubly occupied dangling bonds and from band-tail states respectively are shown to be related to the response time τR of the initial decay of photoconductivity measured in a transient experiment. To discover the relationship existing betweent 1, t 2 and τR, we first analyse the temperature dependence of the steady-state mobility μ ss = μ n τR, /τr where μ n and τn are the free electron mobility and lifetime. We reach the conclusion that μ ssis the weighted average of the mobilities characterizing the electron traffic between D− centres and the conducting states on the one hand, and that between tail states and the conduction band on the other hand. The model allo...

Trapping in hydrogenated amorphous silicon studied by dual-beam modulated photoconductivity

Abstract The steady-state photoconductivity of hydrogenated amorphous silicon (a-Si: H) has been studied by means of the dual beam modulated photoconductivity (DBMP) method. The frequency-dependent spectrum of DBMP reveals the existence of two independent densities of trapping centres in the gap, which are the doubly occupied dangling-bond distribution and the localized states of the conduction-band tail. Steady-state photoconductivity is due to thermal emission from these two distributions of trapped electrons. The relative contribution of each is strongly dependent on temperature. This interpretation is supported by the details of the temperature dependence studies. In particular, the variations in the γ exponent of photoconductivity (σp ∞ f p γ, f p being the photogeneration rate) as a function of temperature are found to be correlated to the variations in the partial photoconductivities.

Space charge limited currents under illumination in a-Si:H

Space charge limited currents in a-Si:H have been measured under increasing light fluxes. The results show that N(E) increases above E F o but differ significantly from those obtained by SCLC or photoconductivity.