Quasi-steady State Photoconductance Measurements Research Articles

Contactless quasi-steady-state photoconductance (QSSPC) characterization of metal halide perovskite thin films

We apply the contactless quasi-steady-state photoconductance (QSSPC) method to co-evaporated methyl ammonium lead iodide (MAPbI3) perovskite thin-films. Using an adapted calibration for ultralow photoconductances, we extract the injection-dependent carrier lifetime of the MAPbI3 layer. The lifetime is found to be limited by radiative recombination at the high injection densities applied during the QSSPC measurement, enabling the extraction of the electron and hole mobility sum in the MAPbI3 using the known coefficient of radiative recombination of MAPbI3. Combining the QSSPC measurement with transient photoluminescence measurements, performed at much lower injection densities, we obtain the injection-dependent lifetime curve over several orders of magnitude. From the resulting lifetime curve, we determine the achievable open-circuit voltage of the examined MAPbI3 layer.

Correlation between in Situ Diagnostics of the Hydrogen Plasma and the Interface Passivation Quality of Hydrogen Plasma Post-Treated a-Si:H in Silicon Heterojunction Solar Cells.

Passivation of the interface defect states is crucial to mitigate the recombination losses in silicon solar cells. In this work, we have investigated the role of hydrogen plasma treatment (HPT) to passivate the interfacial defects between crystalline (c-Si) and hydrogenated amorphous silicon (a-Si:H) in silicon heterojunction (SHJ) solar cells. For the first time, we have found a correlation between the dynamic properties of hydrogen plasma and passivation quality of the films by using in situ optical emission spectroscopy and quasi-steady state photoconductance measurement. The optimum condition for saturation of the dangling bonds by HPT has been studied in detail by tuning the excited hydrogen (H) species and ion bombardment energies by controlling physical parameters like plasma current and chamber pressure. We have investigated the role of annealing after HPT to redistribute the H in the post-treated a-Si:H film and have obtained an i Voc of 755 mV, minority carrier lifetime (τeff) of 4.6 ms, and SRV of 1.5 cm/s on test structures having only an 10 nm intrinsic a-Si:H layer on textured silicon wafers. The H bond configuration at the interface of a-Si:H and c-Si has been investigated by Fourier transform infrared spectroscopy, which demonstrates improved monohydride bonding in the films after HPT derived from the analysis of microstructure parameter and H concentration values. Raman spectroscopy shows the absence of the nanocrystalline fraction after HPT and verifies reduced coordination defects due to annealing after HPT. The proof of concept has been validated by fabricated SHJ solar cells having a Voc of 729 mV and efficiency of 18.7% after HPT, with the best cell efficiency reaching 20.2% after doped layer optimization. The decrease in reverse saturation current and ideality factor after HPT verifies that the improvement in performance is from reduced recombination losses at the interface due to passivation of defects in midgap states.

Determination of the Recombination Velocity on Unequal Surfaces of Silicon Wafers from Two-Spectrum Excited Quasi-Steady-State Photoconductance

The minority charge carrier lifetime measured using the quasi-steady-state photoconductance (QSSPC) technique for a silicon wafer provides quantitative information about surface and bulk recombination. However, conventional methods used to determine the surface recombination velocity from a measured effective lifetime are mostly limited by assuming the identical passivation quality of both surfaces and a constant lifetime throughout a sample. In order to further separate the surface recombination velocities of the two unequal surfaces of a silicon wafer, we present an approach to analyze the decay curves of excess minority charge carriers illuminated with two different spectra. This approach is based on a numerical calculation of the one-dimensional continuity equation using the Crank–Nicholson finite difference method. By performing QSSPC measurements with LG700 and SP850 filters, the surface recombination velocities of the front and back surfaces of a set of passivated and bare silicon wafers, S1 and S2, were determined via our analysis method. The excess charge carrier density and the bulk lifetime distributions for wafers of different resistivities are theoretically demonstrated.

Optimized Temperature in Phosphorous Diffusion Gettering Setup of Chromium Transition Metal in Solar Grade Multicrystalline p-Type Silicon Wafer

We have investigated in this work the effect of the temperature profile during homogeneous phosphorous diffusion gettering (PDG) on multicrystalline (mc-Si) silicon p-type wafers destined for photovoltaic solar cells. Temperatures were varied from 800 ◦C to 950 ◦C with time cycle of 90 minutes. Phosphorous profile of np junction was measured by secondary ion mass spectroscopy (SIMS) from 0.45 μm to 2.4 μm. Chromium concentration profile measured on the same samples by SIMS shows a high accumulated concentration of Cr atoms in the gettering layer at 900 ◦C and 950 ◦C, compared to samples obtained at 800 ◦C and 850 ◦C. The effective lifetime (τeff) of minority charge carriers characterized by quasi-steady state photoconductance (QSSPC) is in correlation with these results. From the QSSPC measurements we have observed an amelioration of τeff from 7 μs before PDG to 26 μs in the samples after PDG, processed at 900 ◦C. This indicates the extraction of a non-negligible concentration (5× 10 cm−3 to 5× 10 cm−3) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of τeff (18 μs) is observed in the samples treated at 950 ◦C due probably to a partial dissolution of the metallic precipitates, especially at the grain boundaries and in the dislocations vicinity. The related τCr-Impurity lifetime value of about 8.5 μs is extracted, which is the result of interstitial Cri or CriBs pairs, proving their strongest recombination activity in silicon.

Out-of-plane Excess Carrier Density Variations in Point Contact Lattice-based test Structures for QSSPC Contact Recombination Current Measurements

We discuss the effect of out-of-plane excess carrier density variations on contact recombination currents extracted from quasi steady state photoconductance (QSSPC) measurements on a dedicated test structure. The test structure features lattices of point contacts. The contacts are thick and non-transparent. Therefore, asymmetric test structures are generally used to ensure a homogeneous in-plane generation rate. As a result, the QSSPC measurements are relatively prone to out-of-plane excess carrier density variations. Their effect on the extracted contact recombination characteristic is discussed theoretically and the theory is confirmed by experiment. The resulting framework contributes to the set of design rules and best practices for contact recombination current characterization using QSSPC measurements on point contact lattice based test structures.

Excess Carrier Density Variations in Test Structures for Photoconductance-Based Contact Recombination Current Measurements

We recently proposed a novel test structure for the extraction of recombination characteristics of metal-contacted areas on silicon wafers using quasi-steady state photoconductance measurements. In this test structure, photoconductance measurements are performed on several wafer areas with different contact fractions. Each area consists of a lattice of point contacts on an otherwise passivated wafer. The requirement of constant excess carrier densities throughout the quasi-neutral wafer bulk is absolutely essential for the simple extraction of figures of merit for contact recombination. In this study, we first discuss the requirements for constant injection levels during photoconductance measurements, without reference to a particular geometry. Then, we use a simple 1-D model to elucidate how injection levels vary in the test structure plane. We use the model to investigate limiting cases for which the requirement of constant injection levels is satisfied. In addition, we investigate the breakdown of the assumption of constant injection levels. We discuss the influence of nonconstant excess carrier concentrations on photoconductance measurements in the context of our test structure. Finally, we validate our model by comparison with experiments. Our analysis is particularly useful in the context of understanding the design rules for contact size and pitch in our test structure.

Experimental study on scan imaging of silicon wafer by laser-induced photocarrier radiometry

In this paper, the modulated laser-induced excess minority carrier density wave (CDW) of bipolar semiconductor is developed. An analytical expression of CDW in frequency domain is introduced. The numerical simulations are carried out to analyze the effects of minority carrier transport parameters (minority carrier lifetime, diffusion coefficient, and two surfaces recombination velocities) on response of laser-induced photocarrier radiometry (PCR) signal to frequency for semiconductor silicon wafer. The PCR amplitude increases with increasing the minority carrier lifetime, and decreased with increasing the carrier diffusion coefficient and surface recombination velocity. In contrast, the PCR phase lag decreases with increasing the minority carrier lifetime, and increases with increasing the carrier diffusion coefficient and surface recombination velocity. The silicon (Si) wafer with an artificial defect (mechanical scratch) on the surface is experimentally investigated by PCR scanning image system. The distribution maps of the minority carrier transport parameters are obtained by best-fitting method which is based on the carrier density wave analytical expression, and the influences of the artificial defect on carrier transport parameters are discussed in detail. The experimental results indicate that the surface recombination velocity and carrier diffusivity at artificial damaged location are dramatically increased compared with those in the healthy region. The carrier bulk lifetime of whole Si wafer is obtained to be about 38.33 μs by PCR scanning image measurements. Simultaneously, quasi steady-state photoconductance (QSSPC) method is used to measure the carrier effective lifetime of Si wafer, and it is about 33.85 μs. Therefore, the carrier bulk lifetime of Si wafer by PCR scanning image measurement is in good agreement with the QSSPC measurement. However, QSSPC measurement could obtain only the carrier effective lifetime of Si wafer. Furthermore, PCR scanning image measurement can be employed to measure the carrier transport parameters with high resolution in comparison with QSSPC measurement, and to evaluate the localized imperfection.

Near surface inversion layer recombination in Al2O3 passivated n-type silicon

On n-type silicon, negatively charged surface passivation layers create a near surface recombination channel, which could significantly reduce the effective carrier lifetime at low injection levels (Δn < 1014 cm−3). This effect is described by Shockley Read Hall recombination at homogeneously distributed defects in the silicon wafer. In the near surface region, fixed charges in the dielectric layer significantly change the carrier concentrations and the recombination rate of defects. Sentaurus device simulations show that the contribution of the near surface recombination to the effective carrier lifetime depends on the properties of the involved defects. The lifetime reduction is strongest when the involved defects have an energy level in the lower half of the band gap and a very high electron to hole capture cross section ratio. For the simulation, a very low defect density in the order of 108 cm−3 is assumed, which is a realistic value in highly pure float zone silicon. Quasi-steady state photoconductance measurements on n-type silicon with Al2O3 passivation are done and fitted with the recombination model. Very good correlation between simulation and experiment is achieved when the involved recombination centers have an electron to hole capture cross section ratio of 107 and an energy level of −0.2 eV w.r.t. the intrinsic level. The simulated defect properties are discussed in respect of transition metal and doping related defects reported in literature.

Improved Diffused-region Recombination-current Pre-factor Analysis

Photoconductance (PC) measurements of the diffused-region recombination-current pre-factor J0d make the approximation of excess-carrier Δn density uniformity. That is, from the front illuminated surface to the rear the Δn is constant. Kane and Swanson outlined acceptable sample parameters such that the error caused by this approximation is small when performing transient J0d measurements. For quasi-steady state (QSS) PC measurements, where there is generation, the uniform carrier-density approximation leads to a larger J0d under-estimation compared to the transient approach. We avoid this approximation by numerically solving the J0d accounting for depthwise generation, recombination and carrier diffusion in the quasi-neutral bulk that occurs during a QSS PC measurement. We demonstrate the application of the technique to samples with surface-diffusion sheet resistances 10–140Ω/sq., formed by etch back, and a sample where the surface passivation degrades with damp heat exposure. We find, for the samples tested, the direct application of the Kane and Swanson method: i) under-estimates the J0d by 2% to 8% when the sample bulk is lightly doped (boron doping of 1.3×1014 cm-3), and ii) under-estimates the J0d by 10% to 80% when the wafer is moderately doped (phosphorus doping of 1.5×1015 cm-3), for samples with J0d 350 to 2200 fA/cm2; compared with an experimental uncertainty of 6%. For both samples sets the under-estimation increases as J0d increases.

Direct monitoring of minority carrier density during light induced degradation in Czochralski silicon by photoluminescence imaging

In this paper, we present a new method for studying the light induced degradation process, in which the minority carrier density is monitored directly during light soaking by photoluminescence imaging. We show experimentally that above a certain minority carrier concentration limit, Δnlim, the boron oxygen (B-O) defect generation rate is fully independent of the injected carrier concentration. By simulation, we determine Δnlim for a range of p-type Czochralski silicon samples with different boron concentrations. The normalized defect concentrations, Nt*, are determined for the same samples by time-resolved Quasi Steady State Photoconductance measurements. After 10 min of light degradation, no correlation between Δnlim, and Nt* is observed. These results indicate that the role of the excess carriers during the rapid decay is to first change the charge state of the defects by shifting the electron quasi-Fermi level across the energy level of the defect centre in its passive state (Elat = EV + (635 ± 18) meV) and that, subsequently, another rate-determining step proceeds before the defect centre becomes recombination active.

Influence of different excitation spectra on the measured carrier lifetimes in quasi-steady-state photoconductance measurements

The interpretation of lifetime measurements on unpassivated silicon wafers by means of the quasi-steady-state photoconductance (QSSPC) technique is a practically highly relevant, but challenging task. We investigate QSSPC measurements on damage-etched samples with five different resistivities from 0.25 Ω cm p-type to 100 Ω cm n-type float-zone material with four different thicknesses each, ranging from 105 to 250 μm. The influence of different excitation spectra, realized by adding different long pass filters to the standard QSSPC Xenon flash spectrum, on the injection-dependent effective excess carrier lifetime is studied in detail. We observed that the measured effective lifetime is significantly dependent upon the excitation spectrum in use and is augmented by increasing the long pass filter cut-off wavelength, the latter being equivalent to more symmetrical excess carrier density profiles. Furthermore, the size and the injection dependency of the dominating surface recombination channels are investigated. Finally, two different approaches to accurately calculate the excess carrier generation rate within the sample are presented and compared.