Defect Pool Research Articles

Modelling of transport and recombination of photocarriers in un-doped hydrogenated amorphous silicon (a-Si:H)

In this paper, we report on the simulation of steady state photoconductivity in un-doped a- Si:H at temperatures from 30 to 500 K. The model is based on recombination at dangling bond states and band tail states. It takes also into account the hopping transitions in the conduction hand tail states to describe the conduction in localized states at low temperatures. At high temperatures, the multiple trapping process is considered to describe the conduction in extended states. The density of states includes the exponential density of conduction band tail states and valence band tail slates and the density of dangling bond states. This later is determined by the Defect Pool Model ‘DPM’. The experimental features observed on the temperature dependence of the photoconductivity are generally the thermal quenching, the low activated region and the temperature independent photoconductivity at very low temperatures. All these observations are well reproduced by the model in un-doped a-Si:H. By the examination of the relative contributions of two processes of conduction: (i) the multiple trapping and (ii) the multiple trapping associated with the hopping, the model results show that the multiple trapping process of electrons where the conduction is assured by free carriers in the thermal quenching region above 140 K is important while the hopping process of electrons is negligible. At 140 K and below, the hopping transport of electrons in the conduction band tail states makes an important contribution in the photoconductivity. It explains successfully the low activated region and the temperature independent photoconductivity at very low temperatures.

Light soaking effect on defect states distribution of Hydrogenated amorphous silicon investigated By means of constant photocurrent technique

In the present paper we have investigated, by using the constant photocurrent method in dc-mode (dc-CPM), the effect of the light soaking (LS) on the deep defect density (Nd) and the slope of the Urbach tail (E0) of a slightly phosphorus-doped hydrogenated amorphous silicon (a-Si:H) film prepared by Plasma-Enhanced Chemical Vapour Deposition (PECVD). By applying the derivative method, we have converted the measured data into a density of states (DOS) distribution in the lower part of the energy gap. The evolution of the sub-band-gap absorption coefficient and the CPMdetermined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by the defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing.

Defect Pool Numerical Model in Amorphous Semiconductor Device Modeling Program

Defect Pool Numerical Model in Amorphous Semiconductor Device Modeling Program

In situ X-ray imaging of defect and molten pool dynamics in laser additive manufacturing

The laser–matter interaction and solidification phenomena associated with laser additive manufacturing (LAM) remain unclear, slowing its process development and optimisation. Here, through in situ and operando high-speed synchrotron X-ray imaging, we reveal the underlying physical phenomena during the deposition of the first and second layer melt tracks. We show that the laser-induced gas/vapour jet promotes the formation of melt tracks and denuded zones via spattering (at a velocity of 1 m s−1). We also uncover mechanisms of pore migration by Marangoni-driven flow (recirculating at a velocity of 0.4 m s−1), pore dissolution and dispersion by laser re-melting. We develop a mechanism map for predicting the evolution of melt features, changes in melt track morphology from a continuous hemi-cylindrical track to disconnected beads with decreasing linear energy density and improved molten pool wetting with increasing laser power. Our results clarify aspects of the physics behind LAM, which are critical for its development.

Amorphous silicon passivation for 23.3% laser processed back contact solar cells

This paper presents amorphous silicon deposited at temperatures below 200 °C, leading to an excellent passivation layer for boron doped emitter and phosphorus doped back surface field areas in interdigitated back contact solar cells. A higher deposition temperature degrades the passivation of the boron emitter by an increased hydrogen effusion due to lower silicon hydrogen bond energy, proved by hydrogen effusion measurements. The high boron surface doping in crystalline silicon causes a band bending in the amorphous silicon. Under these conditions, at the interface, the intentionally undoped amorphous silicon becomes p-type conducting, with the consequence of an increased dangling bond defect density. For bulk amorphous silicon this effect is described by the defect pool model. We demonstrate, that the defect pool model is also applicable to the interface between amorphous and crystalline silicon. Our simulation shows the shift of the Fermi energy towards the valence band edge to be more pronounced for high temperature deposited amorphous silicon having a small bandgap. Application of optimized amorphous silicon as passivation layer for the boron doped emitter and phosphorus doped back surface field on the rear side of laser processed back contact solar cells, fabricated using four laser processing steps, yields an efficiency of 23.3%.

Surface passivation of heavily boron or phosphorus doped crystalline silicon utilizing amorphous silicon

Excellent surface passivation of heavily boron or phosphorus doped crystalline silicon is presented utilizing undoped hydrogenated amorphous silicon (a-Si:H). For passivating boron doped crystalline silicon surfaces, amorphous silicon needs to be deposited at low temperatures 150 °C≤Tdep≤200 °C, leading to a high bandgap. In contrast, low bandgap amorphous silicon causes an inferior surface passivation of highly boron doped crystalline silicon. Boron doping in crystalline silicon leads to a shift of the Fermi energy towards the valence band maximum in the undoped a-Si:H. A simulation, implementing dangling bond defects according to the defect pool model, shows this shift in the undoped a-Si:H passivation to be more pronounced if the a-Si:H has a lower bandgap. Hence, the inferior passivation of boron doped surfaces with low bandgap amorphous silicon stems from a lower silicon-hydrogen bond energy due to this shift of the Fermi energy. Hydrogen effusion and ellipsometry measurements support our interpretation.

Simplified method for the evaluation of the reverse dark current–voltage characteristic of thin film devices

An algorithm that simplifies the evaluation of the reverse dark current–voltage (J–V) characteristic of semiconductor thin film devices is presented. This algorithm, recognized with the symbols “0KRDA”, is an approximation of the SRH formalism that can be used when the dangling bond density is modeled with either the Uniform Density Model or with the Defect Pool Model. The 0KRDA is designed to replace the 0K‐Simmons–Taylor approximation (0KSTA) in reversed biased junctions operating under dark conditions. The dependence of the current density J with respect to the applied voltage V predicted with SRH formalism is well replicated by the 0KRDA. The small differences obtained in the calculated reverse dark currents can be removed by neglecting the contribution of gap states with energies closer than kT/5 to the intrinsic trap level. The transport physic controlling the shape of reverse dark J–V curves of thin film devices can be more easily visualized with the 0KRDA.

Towards modelling realistic ageing rates of amorphous silicon devices in operational environment

This paper presents a method to de-convolute the annealing and degradation processes of amorphous silicon devices. This will allow modelling realistic outdoor operation. Six devices underwent 14000 h indoor light exposure at different and varying thermal conditions. The devices are exposed to light at variable temperatures between 25 and 85 °C under illumination as well as annealed in the dark. The temperature set-points were altered several times during the test to gain insights on how the balance between light induced degradation and thermal annealing is shifted. Measurements show that the degradation level is largely determined by the device’s operating temperature and both processes balance themselves out without consideration of device history. This is explained by the proposed modelling approach based on the defect pool model. The key parameters in the proposed modelling approach are the effective activation energies for degradation and annealing.

Evaluation of the characteristic curves of a-Si:H based devices with the Simmons-Taylor approximation when the defect pool model is used

The performance of a-Si:H devices is highly sensitive to the density of gap states: tail states are distributed in two exponentials and defect states are generated by dangling bonds (DB). The density of DB in a-Si:H can be evaluated with the defect pool model (DPM). Charge trapping and recombination of electron–hole pairs through tail states are described by the Shockley–Read–Hall (SRH) formalism while defect states behave as amphoteric. Equations derived with the SRH formalism can be simplified with the Simmons–Taylor's approximation (STA), especially with the “0 K” approximation (0KSTA). Amphoteric-like defect states were approximated by donor- and acceptor-like decoupled states (DSA). The accuracy of STA was tested in a-Si:H based devices when the density of DB is evaluated with the DPM for different illumination conditions, voltages, temperatures, and some key electrical parameters. Our code was modified to include both the STA and the DSA. Our results indicate that the STA is very accurate under illuminated conditions. Under dark conditions, the STA is acceptable for forward voltages but overestimates the dark current at reverse voltages. The 0KSTA can be used under illuminated conditions for any applied voltage and under dark conditions for forward voltages.

A thermalization energy analysis of the threshold voltage shift in amorphous indium gallium zinc oxide thin film transistors under simultaneous negative gate bias and illumination

It has been previously observed that thin film transistors (TFTs) utilizing an amorphous indium gallium zinc oxide (a-IGZO) semiconducting channel suffer from a threshold voltage shift when subjected to a negative gate bias and light illumination simultaneously. In this work, a thermalization energy analysis has been applied to previously published data on negative bias under illumination stress (NBIS) in a-IGZO TFTs. A barrier to defect conversion of 0.65–0.75 eV is extracted, which is consistent with reported energies of oxygen vacancy migration. The attempt-to-escape frequency is extracted to be 106−107 s−1, which suggests a weak localization of carriers in band tail states over a 20–40 nm distance. Models for the NBIS mechanism based on charge trapping are reviewed and a defect pool model is proposed in which two distinct distributions of defect states exist in the a-IGZO band gap: these are associated with states that are formed as neutrally charged and 2+ charged oxygen vacancies at the time of film formation. In this model, threshold voltage shift is not due to a defect creation process, but to a change in the energy distribution of states in the band gap upon defect migration as this allows a state formed as a neutrally charged vacancy to be converted into one formed as a 2+ charged vacancy and vice versa. Carrier localization close to the defect migration site is necessary for the conversion process to take place, and such defect migration sites are associated with conduction and valence band tail states. Under negative gate bias stressing, the conduction band tail is depleted of carriers, but the bias is insufficient to accumulate holes in the valence band tail states, and so no threshold voltage shift results. It is only under illumination that the quasi Fermi level for holes is sufficiently lowered to allow occupation of valence band tail states. The resulting charge localization then allows a negative threshold voltage shift, but only under conditions of simultaneous negative gate bias and illumination, as observed experimentally as the NBIS effect.

Impact of Fermi-level dependent defect equilibration on Voc of amorphous/crystalline silicon heterojunction solar cells

Abstract We analyze the microscopic mechanisms limiting the open-circuit voltage Voc of high efficiency amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. The study is based on passivation experiments with undoped a-Si:H layers as well as device-relevant doped/undoped a-Si:H stack structures and extensive layer characterization. We conclude that the density of strained bonds in the a-Si:H and the Fermi level (EF) position at the heterointerface codetermine the passivation potential of a given structure. Thus, the commonly observed deterioration of the undoped a-Si:H/c-Si passivation upon deposition of a doped a-Si:H top layer can be interpreted as defect equilibration in the a-Si:H layer (i.e. the EF-dependent formation of dangling bond defects in a-Si:H from strained Si- Si bonds). Consistently, employing the well-established “defect pool model” which describes the EF-dependent defect formation, the observed recombination velocities of (p/n)a-Si:H/(i)a-Si:H/c-Si stack structures can be quantitatively explained without invoking any fit parameters. The impact of this effect on solar cell Voc is quantified.

Evidences of the defect pool model in the dark current-voltage characteristics of hydrogenated amorphous silicon based p-i-n devices

In the scientific literature the density of states of hydrogenated amorphous silicon has been assumed to be either uniform or spatially variable inside the intrinsic layer of p-i-n solar cells. The dependence of the dark current voltage characteristics of amorphous silicon based solar cells with respect to the intrinsic layer thickness and to the mobility gap of a thin interfacial layer grown at the p/i interface is explored with numerical techniques. Our results indicate that the reported experimental trends can be adequately explained with the defect pool model and not by assuming an uniform density of states in the intrinsic layer.

Modelling the light induced metastable effects in amorphous silicon

AbstractWe present results of computer simulations of the light induced degradation of amorphous silicon solar cells. It is now well established that when amorphous silicon is illuminated the density of dangling bond states increases. Dangling bond states produce amphoteric electronic mid‐gap states which act as efficient charge trapping and recombination centres. The increase in dangling bond states causes a decrease in the performance of amorphous silicon solar cells. To show this effect, a modelling approach has been developed which uses the density of localised states with exponentially increasing band‐tails and dangling bond defect states distribution chosen according to the defect pool model. The calculation of the evolution of dangling bond state density during illumination has been achieved through a dynamic scaling relation derived from a defect creation model. The approach considers the amphoteric nature of the dangling bond state and thus accounts for the contributions of the different charge states of the dangling bond during the degradation process. This paper attempts to describe the simulation approach which calculates the defect density as a function of energy, position in the solar cell and illumination time. In excellent agreement with other workers, our simulation results show that the increase in the density of neutral dangling bond states during illumination is higher than of the charged states. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical simulation of the transient photoconductivity in hydrogenated amorphous siliconincluding localized-state electron hopping

The transient photoconductivity (TPC) in hydrogenated amorphous silicon is studied bynumerical simulation. In addition to normal free carrier capture (emission) transitions into(from) localized states by the multiple-trapping (MT) process, we include theprocess of electron hopping (EH) through the conduction band tail states. Thedistribution of the dangling bond density is calculated by the defect pool model, whileexponential distributions are assumed for the conduction and valence band tails.The simulation results are in good agreement with previous theoretical results: ahopping transport energy level identified as the peak of the energy distribution ofthe hopping photoconductivity is in excellent agreement with the theoreticalhopping transport energy of Monroe. The simulated TPC is studied as a functionof temperature in order to determine the relative contribution of MT and EHtransport mechanisms. A smooth transition around 130 K between high temperatureextended-state conduction via MT and low temperature localized-state EH isdetermined.

Metastable defect creation in tritiated hydrogenated amorphous silicon and the Staebler–Wronski effect

Defect creation through tritium decay in tritiated hydrogenated amorphous silicon provides a unique technique for the study of defect dynamics in hydrogenated amorphous silicon (a-Si:H). Isothermal Capacitance Transient Spectroscopy (ICTS) and Constant Photocurrent Method (CPM) were used to measure the positively charged, D+, and negatively charged, D−, states in the gap of a-Si:H:T, respectively. The samples were thermally annealed prior to the measurement of defect state densities. The ICTS experiments showed a decrease in the concentration of positively charged dangling bonds while the CPM measurements showed an increase in the density of D− states with time. This increase was much larger than the concentration of decayed tritium atoms. The CPM results also showed that the Urbach energy decreased with time. The decrease in Urbach energy indicates a decrease in the concentration of weak bonds in the valence band tail and suggests weak bond to dangling bond conversion. CPM experiments carried out under bias confirm the role of excess carriers in the defect creation process. The dynamic defect pool model was developed to quantitatively explain the experimental data. The defect evolution in a-Si:H:T is similar to the evolution of defect states in light exposure studies, supporting the interpretation of the Staebler–Wronski effect in terms of weak bond to dangling bond conversion.

Characteristic curves of hydrogenated amorphous silicon based solar cells modeled with the defect pool model

Using the improved expression of the defect pool model proposed by Powell and Deane we match the experimental current–voltage and the spectral response characteristic curves of hydrogenated amorphous silicon solar cells. We compare the electrical parameters resulting from using the different defect pool models published in the literature and from assuming a uniform density of dangling bond in every device layer. We discuss the applicability of the algorithm derived by Schumm for the stabilized state exploring its sensitivity to the sample history. Finally we propose an expression for stabilized cells adapting Schumm's ideas to the expression derived by Powell and Deane.

Computer simulation of the a-Si:H p–i–n solar cell performance sensitivity tothe free carrier’s mobilities, the capture cross sections and the density of gapstates

This paper deals with the effects of some material properties on theJ–V characteristic of hydrogenated amorphous silicon (a-Si:H)-based p–i–n solar cell. Thefactors considered are the free carrier’s mobilities, the capture cross sections of the gapstates and the bulk density of states (i-layer DOS). Accurate investigation of the cellphoto-parameters’ sensitivity to these factors is carried out using a simulation program,previously developed by our group. The model is based on a complete set of Poisson andcarrier continuity equations taking into account the defect pool model for the a-Si:H gapdensity of states. Ours results reveal the important role of the hole mobility when it takeslow values, as given frequently in the literature, on the recombination rate at interfaceregions as well as in the bulk. This considerably affects the short-circuit current density(Jsc), the open-circuitvoltage (Voc), the fill factor (FF) and the conversion efficiency(η). Moreover, by changing separately the capture cross sections of theband’s tails and those of dangling bonds we found more precisely thatJsc,Voc,and η are more sensitive to the interface recombination while the FF seems to dependmore on the bulk recombination. Ultimately, when the i-layer DOS is higher than1016 cm−3, the recombination rate, which was first limited by the valence band tail’s states, becomeslimited by the dangling bond’s states. Then, any further increase significantly deterioratesthe solar cell performance.

Equivalent Circuit Description of Threshold Voltage Shift in a-Si:H TFTs From a Probabilistic Analysis of Carrier Population Dynamics

Amorphous hydrogenated silicon thin-film transistors (TFTs) are critical components in large area display and sensor systems, and the need for TFT circuits has been increasing. However, the intrinsic metastability associated with the TFT leads to a threshold voltage shift (VT shift) with time, under prolonged gate bias. For design of reliable TFT circuits, it is imperative to accurately predict this instability for time-varying analog gate bias. In this paper, the author model the threshold voltage variation using a probabilistic analysis of the electron population dynamics as prescribed by the defect pool and charge trapping mechanisms. The model is then extended for prediction of the effect of variable gate bias, and in particular the device history, on the VT shift. Based on this model, a passive equivalent circuit is synthesized to accurately predict the VT shift in TFTs for applications in active matrix organic light emitting diode displays and sensors

Numerical simulation of the steady state photoconductivity in hydrogenated amorphoussilicon including localized state electron hopping

Numerical simulation of the steady state photoconductivity in hydrogenated amorphoussilicon over a wide temperature range (25–500 K) is extended, to include previouslyneglected carrier transitions between localized states. In addition to free carrier capture(emission) transitions into (from) localized states, we include the process of electronhopping in conduction band tail states. Exponential distributions are assumed for bothconduction and valence band tail states, while the dangling bond defect distribution iscalculated in accordance with the defect pool model. Localized to extended statetransitions follow the Simmons and Taylor statistics, and localized to localized statetransitions involve electron hopping between nearest neighbour sites. Comparisonwith simulations in the absence of electron hopping reveals a smooth transitionaround 110 K, between regions of (high temperature) extended state conductionand (low temperature) hopping conduction. A hopping transport energy level isidentified as the peak of the energy distribution of the hopping photocarriers, andshows a temperature dependence in agreement with existing theoretical work.

Modeling a-Si:H p–i–n solar cells with the defect pool model

Using self-consistent computer modeling we find that the experimental current–voltage ( J– V) and the spectral response (SR) characteristic curves of a-Si:H p–i–n solar cells can be fitted by either assuming a uniform density of dangling bonds (DB) in each device layer (UDM) or by relying on the defect pool model (DPM). Fittings within the DPM were achieved using the algorithms proposed by Powell and Deane and Schumm. One Si–H bond mediating in the creation of dangling bonds in the first expressions proposed by Powell and Deane and Schumm are appropriate for modeling solar cells in the initial state. The applicability of the algorithm proposed by Schumm for a-Si:H in the stabilized state is also discussed in solar cells. Using DPM we have explored the optimum doping and band gap profile in the intrinsic layer leading us to the maximum efficiency of a-Si:H p–i–n cells.