Bulk Minority Carrier Lifetime Research Articles

Charge Carrier Lifetime Determination in Graded Absorber Solar Cells Using Time‐Resolved Photoluminescence Simulations and Measurements

Thin‐film photovoltaic device efficiencies are limited by carrier recombination, thus understanding recombination mechanisms is critical for performance improvements. Bulk minority carrier lifetime (τ bulk) is a critical parameter for solar cells but is difficult to determine in P–N junction devices, especially for high doping. As doping ≥1016 cm−3 is required for efficient drift‐charge‐carrier‐collection devices, a method for τ bulk determination in doped P–N junction devices is necessary. This work utilizes time‐resolved photoluminescence (TRPL) simulations to quantify bulk and interface recombination properties in highly doped, graded absorber CdSeTe structures. The two methods developed here for τ bulk determination include utilization of an instantaneous lifetime representation to guide TRPL fitting and direct comparison between measured and simulated decays. Simulations verified that both methods are valid for state‐of‐the‐art device architectures which include graded bandgap absorbers, graded doping, and graded lifetimes. Shifts in the dominant recombination mechanism are identified for sufficiently long τ bulk, where front and back interface quality plays a more prominent role. Evaluation of surface recombination velocities and conduction band offset illustrate electro‐optical advantages of a positive conduction band offset and highlight the necessity of improved interfaces as bulk quality in photovoltaic devices improves.

Effect of Diode Field on Time-Resolved Photoluminescence of CdTe-Based Solar Cells

Time-resolved photoluminescence (TRPL) measurements with externally applied voltage bias were performed on a variety of thin-film CdTe-based solar cells. These measurements revealed a substantial variation in luminescence decays with the application of external voltage bias in many of the samples. Experimental results were replicated with a MATLAB-based modeling tool. Decay variations are attributed to the changing strength of internal p-n junction electric fields as bias is modulated; this exercise, therefore, illustrates whether or not field effects influence a particular TRPL measurement and how. It is found via modeling that the field influence on TRPL decays is dependent on the cell's bulk minority carrier lifetime, mobility, and carrier concentration, as well as the laser excitation power during a measurement. While nearly insignificant in poorer quality legacy cells, the field effects are substantial in state-of-the-art CdTe cells with high lifetime, mobility, and doping, complicating determination of bulk lifetime. It is found that forward bias is effective in eliminating junction fields. It is also shown that fields can be effectively suppressed by increasing excitation power, but doing so is imprecise and can also introduce additional errors unrelated to fields. It is therefore concluded that lifetime is most accurately measured in completed cells under forward bias at low excitation intensity.

Performance Comparison of CdTe:Na, CdTe:As, and CdTe:P Single Crystals for Solar Cell Applications.

We compared thermal stability, open-circuit voltage, short-circuit current, and fill factor values of single-crystal Cadmium telluride (CdTe) grown using the vertical Bridgman (VB) technique and doped with group V elements (phosphorus and arsenic), and group Ⅰ element (sodium), followed by an annealing process. The sodium-doped CdTe maintained a hole density of 1016 cm−3 or higher; after annealing for a long time, this decreased to 1015 cm−3 or less. The arsenic-doped CdTe maintained a hole density of approximately 1016 cm−3 even after the annealing process; however its bulk minority carrier lifetime decreased by approximately 10%. The phosphorus-doped CdTe maintained its properties after the annealing process, ultimately achieving a hole density of ~1016 cm−3 and a minority carrier lifetime of ~40 ns. The characteristics of a single-crystal solar cell were evaluated using a solar cell device that contained single-crystal CdTe with various dopants. The sodium-doped sample exhibited poor interfacial properties, and its performance decreased rapidly during annealing. The samples doped with group V elements exhibited stable characteristics even during long-term annealing. We concluded, therefore, that group V elements dopants are more suitable for CdTe single-crystal-based solar cell applications involving thermal stress conditions, such as space missions or extreme fabrication temperature environments.

Decay of excess carriers in a two-defect model semiconductor: A time-resolved photoluminescence study

The study of charge carrier decay dynamics in semiconductors plays an important role in determining various device properties, for example, the bulk minority carrier lifetime and the potential drop at interfaces. Time-resolved photoluminescence spectroscopy is an efficient technique to extract carrier decay time constants, with the simplest case being that of a mono-exponential decay controlled by a single type of recombination center. However, many modern materials often display multi-exponential decay behavior influenced by a broad distribution of defects. Utilizing CuIn1-xGaxSe2 as a reference system, we present a comprehensive and systematic theoretical study of the decay kinetics in a non-interacting two-defect-level system by using both the classical analytical approach developed by Hornbeck and Haynes and a homogeneous kinetic rate equation model solved numerically. We show how the trapping level's carrier capture cross-section symmetry factor and the initial injection level controls the parameter range for the agreement between these two approaches, analyzing the validity of multi-exponential fits for extracting the carrier lifetime. Further, we analytically and numerically demonstrate that information on the carrier recombination lifetime is contained in the fast time constant, whereas the agreement of the subsequent slow time constant with the analytical model depends on the energetic position of the defect level within the bandgap.

Phosphorous Diffusion Gettering of Trapping Centers in Upgraded Metallurgical‐Grade Solar Silicon

Experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers is shown, as observed by means of inductively coupled photoconductance (PC) decay and lifetime measurements carried out on upgraded metallurgical‐grade silicon (UMG‐Si) wafers. The presence of trapping species dominating the long time range of the PC decay of UMG material (slow traps), which is effectively removed after a PDG conducted at 780 °C, is detected. Notwithstanding, a second trapping mechanism, characterized by a shorter time constant, still governs the response at very low injection levels after the gettering. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18‐fold, up to the range of 70 μs. Thereby, the way for developing gettering strategies capable of successfully removing trap centers and improving the bulk lifetime of unconventional Si material is paved.

Nickel and gold identification in p-type silicon through TDLS: a modeling study

In Silicon, impurities introduce recombination centers and degrade the minority carrier lifetime. It is therefore important to identify the nature of these impurities through their characteristics: the capture cross sectionσand the defect levelEt. For this purpose, a study of the bulk lifetime of minority carriers can be carried out. The temperature dependence of the lifetime based on the Shockley-Read-Hall (SRH) statistic and related to recombination through defects is studied. Nickel and gold in p-type Si have been selected for the SRH lifetime modeling. The objective of the analysis is to carry out a study to evaluate gold and nickel identification prior to temperature-dependent lifetime measurements using the microwave phase-shift (μW-PS) technique. The μW-PS is derived from the PCD technique and is sensitive to lower impurity concentrations. It has been shown that both gold and nickel can be unambiguously identified from the calculated TDLS curves.

Investigation of bulk and surface minority carrier lifetimes in metamorphic InAsSb grown on GaAs and Si

Monolithic integration of III–V-based optoelectronic devices onto Si wafers provides enormous benefits to many device manufacturing technologies. Therefore, it is essential to understand the effect of limiting factors such as dislocations on the material properties. In this paper, we study the minority carrier lifetimes in mid-wave infrared InAsSb alloys grown on lattice-matched GaSb and lattice-mismatched semi-insulating GaAs and Ge/Si substrates. Time-resolved microwave reflection measurement has been performed to study the carrier dynamics and different recombination mechanisms over the temperature range of 20–300 K at various optical injection levels. The sample on GaAs is found to have a lower lifetime over the entire temperature region than the sample on the Ge/Si substrate. The threading dislocation density values estimated from the lifetime analysis were found to be 2.9±0.2 times larger in the sample on GaAs when averaged over the temperature range of 70–200 K. Furthermore, we studied the contribution of various recombination mechanisms, and it was shown that the lifetime in the sample on GaAs is dominated by the Shockley–Read–Hall recombination up to 140 K, above which the Auger recombination is the limiting factor. This transition temperature is 80 K and 100 K for the samples on GaSb and Ge/Si, respectively. We have also investigated the effect of the surface recombination on the total lifetime. The extracted bulk lifetime was found to be up to 2× higher when the surface effect was excluded.

High Optical Performance of Pseudo-Single-Crystal Semiconductors Formed on Glass Using Layer-Exchanged Ge Seed Layer

Introduction The multi-junction solar cell has continued to update the highest conversion efficiency of solar cells. However, the bulk Ge substrate, has been used as a bottom cell material for multi-junction solar cells, is too expensive for consumer application. One promising approach to reducing the fabrication cost is substituting the bulk Ge substrate with a Ge thin film on an inexpensive glass substrate.We formed a large-grained (> 100 µm) Ge layer on glass by using Al-induced layer exchange (ALILE) at a low temperature of 325 °C [1]. In addition, the resistivity of ALILE-Ge is low enough (< 103 cm-3) to be used as a bottom electrode due to highly Al doping [2]. Here, we adopted the Ge layer formed by ALILE as an epitaxial seed and grow Ge and GaAs as bottom and middle gap absorbing layers, respectively. We demonstrate the highest optical performance among the polycrystalline semiconductors formed on glass without transferring. Experimental Procedure First, the 50-nm-thick Ge seed layer on the glass substrate was prepared using ALILE (Fig. 1(a)). The upper Al layer is removed with HF (1.5%) treatment for 1 min. The resulting ALILE-Ge seed layer is highly (111)-oriented and large-grained (Figs. 1(b),(c)). Then, the Ge or GaAs layers were formed on the ALILE-Ge seed layer by solid phase epitaxy (SPE) or molecular beam epitaxy (MBE), respectively. For the SPE, amorphous-Ge layers (100-500 nm thick) were deposited at room temperature and then annealed at 350 °C in a N2 ambient (Fig. 2(a)). For the MBE, the 500-nm-thick GaAs layers were grown with heating the substrate at T g = 500‒570 °C (Fig. 3(a)). Results and Discussion Figure 2(b) shows that the SPE-Ge layers appear larger minority carrier lifetime, τ eff, than ALILE-Ge. In addition, τ eff increases with the increasing thickness of SPE-Ge, w. The SPE-Ge for w = 500 nm achieves τ eff of more than 1 µs. We calculated the bulk minority carrier lifetime of SPE-Ge, τ bulk, as expressed the equation in Fig. 2(c). From the intercept of the least-squares line in Fig. 2(c), τ bulk is determined to be 5.6 µs. Figure 2(d) shows that this value is close to that of a single-crystal Ge with hole concentration in the latter half of 1017 cm-3 [3]. SIMS measurement revealed that SPE-Ge has 6 × 1017 cm-3 of Al, which is diffused from ALILE-Ge. Assuming that the Al atoms in the SPE-Ge are fully activated as acceptors, τ bulk of the SPE-Ge is mostly limited by impurity scattering.Figure 3(b) shows that the EBSD image of the GaAs sample for T g = 520 °C was very similar to that of ALILE-Ge. Figures 3(c) and 3(d) show the stacked structure of ITO/GaAs/Ge/glass as intended. The SAED pattern in Fig. 3(e) indicates that the GaAs film is epitaxially (111) oriented and single crystalline in this area (800 nm in diameter).The composition of GaAs and the FWHMs of the TO and LO mode peaks were calculated from the EDX and Raman spectra, respectively. Figure 4(a) shows that the FWHMs initially decrease with increasing T g, indicating that the higher T g improves the crystallinity of the GaAs films. Conversely, the FWHMs increase for T g > 550 °C, due to the reduction of the As atomic ratio in the GaAs. Figure 4(b) shows clear photoresponse spectra rising near a wavelength of 900 nm, which corresponds to the bandgap of GaAs. Figure 4(c) shows that internal quantum efficiency (IQE) is maximum at wavelengths of 700–800 nm for all samples. Figure 4(d) shows that the photoresponsivity and IQE initially increase with increasing T g and decrease for T g > 550 °C. This behavior is consistent with the FWHM results. We note that the photoresponsivity and IQE of the ALILE-Ge samples are comparable to those of the single-crystal GaAs which was simultaneously grown on Ge(111) substrate. This is because GaAs on ALILE-Ge is large-grained and behaves like a pseudo-single crystal. Thus, the maximum photoresponsivity (470 mA/W) and IQE (90%) were obtained at T g = 550 °C.In conclusion, we demonstrate the great potential of ALILE-Ge as an epitaxial seed for the Ge and GaAs layers. Their optical performance is highest among the polycrystalline ones formed on glass. This achievement will directly lead to the fabrication of novel high-efficiency multi-junction solar cells based on inexpensive substrates.

Influence of Tabula Rasa on Process- and Light-Induced Degradation of Solar Cells Fabricated From Czochralski Silicon

Monocrystalline Si solar cells are fabricated from Czochralski (Cz) Si, which contains 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> oxygen atoms. Cz Si undergoes degradation during high-temperature thermal processing steps, such as dopant diffusion to form the p-n junction. This degradation in the bulk minority carrier lifetime can be related to the formation of oxygen precipitates. We found that a high-temperature annealing process known as tabula rasa (TR) not only mitigates process-induced degradation via oxygen precipitate nuclei dissolution, but also modifies subsequent light-induced degradation. We report on the bulk carrier lifetime of n- and p-type Cz Si after TR, which homogenizes the interstitial oxygen in the bulk Si to its monoatomic form in either an N2 or O2 environment. A control sample, which was not subjected to a TR processing step, experienced severe process-induced degradation during a boron emitter thermal budget as oxygen precipitates were formed in the Si bulk. These precipitates could be imaged using photoluminescence. Additionally, samples that underwent a TR processing step prior to the boron emitter thermal budget show efficient gettering of metallic impurities compared to the control sample, which showed a decline in the implied open-circuit voltage after the gettering step. Furthermore, modification of the interstitial oxygen bonding by TR had a strong effect on the light-induced degradation kinetics. Instead of a typically observed monotonic decrease, minority carrier lifetime increases first, followed by a nonmonotonic decrease over a ~20 h period. We conclude that by modifying the interstitial oxygen bonding via TR pretreatment, p-type Cz Si wafers become substantially resistant to harsh solar cell processes and strongly modified light-induced degradation, which would open alternative ways to mitigate this degradation mechanism.

Kinetics of Bulk Lifetime Degradation in Float‐Zone Silicon: Fast Activation and Annihilation of Grown‐In Defects and the Role of Hydrogen versus Light

Float‐zone (FZ) silicon often has grown‐in defects that are thermally activated in a broad temperature window (≈300–800 °C). These defects cause efficient electron‐hole pair recombination, which deteriorates the bulk minority carrier lifetime and thereby possible photovoltaic conversion efficiencies. Little is known so far about these defects which are possibly Si‐vacancy/nitrogen‐related (VxNy). Herein, it is shown that the defect activation takes place on sub‐second timescales, as does the destruction of the defects at higher temperatures. Complete defect annihilation, however, is not achieved until nitrogen impurities are effused from the wafer, as confirmed by secondary ion mass spectrometry. Hydrogenation experiments reveal the temporary and only partial passivation of recombination centers. In combination with deep‐level transient spectroscopy, at least two possible defect states are revealed, only one of which interacts with H. With the help of density functional theory V1N1‐centers, which induce Si dangling bonds (DBs), are proposed as one possible defect candidate. Such DBs can be passivated by H. The associated formation energy, as well as their sensitivity to light‐induced free carriers, is consistent with the experimental results. These results are anticipated to contribute to a deeper understanding of bulk‐Si defects, which are pivotal for the mitigation of solar cell degradation processes.

Growth and Characterization of Arsenic-Doped CdTe1−xSex Single Crystals Grown by the Cd-Solvent Traveling Heater Method

The photovoltaic performance of CdTe solar cells is mainly limited by low doping and short minority carrier lifetime. Group-V element doping and Se-alloying have a significant impact on tuning these fundamental CdTe properties. In this paper, we report the growth of p-type As-doped, Cd-rich CdTe1−xSex single crystals using metallic Cd as the solvent in the traveling-heater method. The structural and electrical properties of CdTe1−xSex are examined for different Se concentrations. CdTe1−xSex single crystals (0 ≤ x ≤ 0.5) with zincblende structure indicate homogeneous composition. The 1017 cm−3 As-doping activation efficiency can be maintained at close to 50% for x ≤ 0.2. Se alloying leads to bulk minority carrier lifetime exceeding 30 ns for samples doped near 1017 cm−3. These results help us to overcome the current roadblocks in device performance.

Carrier recombination dynamics in Ga0.51In0.49P double-heterostructures up to 500 K

The bulk minority carrier lifetime and interface recombination velocity (IRV) in GaInP double-heterostructures (DHs) lattice matched to GaAs are extracted using time-resolved photoluminescence (PL) measured between 300 and 500 K. Effective lifetimes show a strong dependence on temperature for samples with insufficiently strong confinement potentials due to significant thermionic emission losses out of the DHs. An increased PL signal from the underlying GaAs layer relative to GaInP’s PL at high temperatures supports this hypothesis. The impact is a shorter recombination lifetime which can be wrongly interpreted as a high IRV of up to 4500 cm s−1 at 500 K. These effects are investigated experimentally using samples of different barrier heights based on the Al-content in (AlxGa1–x)0.51In0.49P. A larger barrier height is shown to inhibit thermionic emission out of the DH, thus revealing a more accurate IRV of 300 cm s−1 at 500 K. The results are then used to develop a correction procedure to extract a more accurate IRV at the barriers of the DH. Optoelectronic device simulations are used to gain insight into carrier dynamics as a function of temperature and the DH’s barrier height, and confirm a strong inhibition of the thermionic emission losses as the Al-content is maximized in the barrier.

Design of Ultrathin InP Solar Cell Using Carrier Selective Contacts

Most recently, III–V based ultrathin solar cells have attracted considerable attention for their inherent advantages, such as increased tolerance to defect recombination, efficient charge carrier separation, photon recycling, flexibility, and reduced material consumption. However, so far, almost all reported devices make use of conventional doped p-i-n kind of structures with a wide-bandgap III–V lattice-matched epitaxial window layer, for passivation and reduced contact recombination. Here, we show that a high-efficiency device can be obtained utilizing an InP thin film of thickness as low as 280 nm, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide-bandgap electron and hole selective contacts for electrons and holes transport, respectively. Under ideal conditions [assuming interface recombination velocity (IRV) = 103 cm/s and bulk lifetime = 1 μs], the proposed solar cell structure can achieve efficiency as high as 28%. Although, in the presence of bulk and interface Shockley–Read–Hall recombination, the efficiency reduces, still for bulk minority carrier lifetime as low as 2 ns and an IRV as high as 105 cm/s, an efficiency of ∼22% can be achieved with InP thickness as low as 280 nm. The proposed device structure will be beneficial in cases where the growth of controlled p-n homojunction and window layer can be tedious as in case of low-cost deposition techniques, such as thin-film vapour–liquid–solid and close-spaced vapour transport.

Iodine–Ethanol Surface Passivation for Measurement of Millisecond Carrier Lifetimes in Silicon Wafers with Different Crystallographic Orientations

To improve silicon device fabrication processes, it is necessary to monitor bulk minority carrier lifetimes accurately, and this requires surface recombination to be well controlled and, ideally, minimized. Good surface passivation can result from thermal oxidation or by deposition of dielectrics (e.g., Al2O3, SiNx, and amorphous Si), but these forms of passivation can modify the lifetime of the material under investigation. Various schemes can passivate surfaces on a temporary basis without modifying the bulk, and, in this article, the virtues of the iodine–ethanol (I–E) temporary surface passivation scheme are explored. A procedure for preparing the wafer surfaces prior to passivation is developed. For the optimized pretreatment, a series of experiments on 3–5 Ω cm float‐zone wafers cut from the same ingot with different thicknesses is conducted. This enables the material's bulk lifetime to be measured at 1015 cm−3 injection as 46 ms, with the surface recombination velocity being 6.5 ± 0.3 cm s−1. I–E passivation is then compared to a recently developed superacid‐derived temporary passivation scheme. Although the latter is superior on (100)‐orientation substrates, I–E performs much better on (111)‐orientation substrates, making it a better choice for (111)‐orientation wafers, such as those used for power devices.

Time-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence

ABSTRACT Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley–Read–Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation.

Minority carrier lifetime of Ge film epitaxial grown on a large-grain seed layer on glass

To develop high-efficiency multijunction solar cells onto inexpensive substrates, an innovative technique for growing a large-grained Ge layer on glass is strongly desired. We investigated the epitaxial growth of a light absorbing Ge film (500-nm thickness) on a large-grained (>100 μm) Ge seed layer (50-nm thickness) formed on glass by Al-induced layer exchange. After examining molecular beam epitaxy (MBE) and solid-phase epitaxy (SPE) for both methods, Ge layers with a low Al concentration were epitaxial grown at 350 °C. Microwave photoconductivity decay revealed that the MBE-Ge layer exhibited a short minority carrier lifetime owing to the rough surface. Conversely, the SPE-Ge layer was relatively flat and exhibited a long bulk minority carrier lifetime (5.6 μs), which is close to that of a single-crystal Ge. Therefore, the seed layer concept that combines SPE with Al-induced layer exchange is a promising way for fabricating ideal bottom cells for high-efficiency multijunction solar cells based on inexpensive substrates.

Gettering of transition metals in high-performance multicrystalline silicon by silicon nitride films and phosphorus diffusion

High-performance multicrystalline silicon (HP mc-Si) from directional solidification has become the mainstream industrial material for fabricating mc-Si based solar cells for photovoltaic applications. Transition metal impurities are inherently contained in HP mc-Si during ingot growth, and they are one of the major efficiency-limiting drawbacks. In this work, we investigate the gettering of transition metals (Cu, Ni, Fe, and Cr) in HP mc-Si wafers along an industrial-standard p-type HP mc-Si ingot, via examining the metal concentration and distribution in the near-surface gettering layers using secondary ion mass spectrometry. We applied both conventional phosphorus diffusion gettering and the recently developed silicon nitride (from plasma-enhanced chemical vapour deposition) gettering techniques. Both techniques are shown to remove significant quantities of metals from the silicon wafer bulk to the surface gettering layers. Improvements in the bulk minority carrier lifetimes throughout the ingot height are also observed by lifetime measurements and spatially-resolved photoluminescence imaging. The gettered Cu and Ni concentrations, as well as the as-grown dissolved Fe concentrations in the silicon wafer bulk, along the HP mc-Si ingot height are shown to follow a similar concentration profile as the metals in conventional mc-Si ingots.

Lifetime Imaging on Silicon Bricks Using the Ratio of Photoluminescence Images With Different Excitation Wavelengths

The efficiency of silicon solar cells is eventually limited by the bulk lifetime of minority carriers in the wafer. The characterization of the bulk quality, measured on silicon bricks early in the production process, provides an immediate indication of the efficiency potential of the material, the feedstock purity, and the quality of the crystallization process. Previously, the intensity ratio of two photoluminescence images that are acquired with different spectral filters (i.e., the two-filter method) has been used to measure the bulk lifetime of silicon bricks. In this paper, we report on a similar approach, which uses the ratio of two photoluminescence images each acquired with a different illumination wavelength (915 and 1064 nm); one of which is within the photoluminescence emission band of silicon. We demonstrate with modeling that this two-laser method is more robust against uncertainties that affect the spectrum of photoluminescence emission and its detection, while the two-filter method is more robust against uncertainties in the carrier density profile and the illumination system properties. Experimentally, both the two-laser and two-filter methods show good agreement across a monocrystalline and multicrystalline silicon brick. However, challenges using the two-laser method arose when a brick with strongly injection-dependent lifetime was measured.

High p-type doping, mobility, and photocarrier lifetime in arsenic-doped CdTe single crystals

Group-V element doping is promising for simultaneously maximizing the hole concentration and minority carrier lifetime in CdTe for thin film solar cells, but there are roadblocks concerning point defects including the possibility of self-compensation by AX metastability. Herein, we report on doping, lifetime, and mobility of CdTe single crystals doped with As between 1016 and 1020 cm−3 grown from the Cd solvent by the travelling heater method. Evidence consistent with AX instability as a major contributor to compensation in samples doped below 1017 cm−3 is presented, while for higher-doped samples, precipitation of a second phase on planar structural defects is also observed and may explain spatial variation in properties such as lifetime. Rapid cooling after crystal growth increases doping efficiency and mobility for times up to 20–30 days at room temperature with the highest efficiencies observed close to 45% and a hole mobility of 70 cm2/Vs at room temperature. A doping limit in the low 1017/cm3 range is observed for samples quenched at 200–300 °C/h. Bulk minority carrier lifetimes exceeding 20 ns are observed for samples doped near 1016 cm−3 relaxed in the dark and for unintentionally doped samples, while a lifetime of nearly 5 ns is observed for 1018 cm−3 As doping. These results help us to establish limits on properties expected for group-V doped CdTe polycrystalline thin films for use in photovoltaics.

Superacid-derived surface passivation for measurement of ultra-long lifetimes in silicon photovoltaic materials

Accurate measurements of bulk minority carrier lifetime are essential in order to determine the true limit of silicon's performance and to improve solar cell production processes. The thin film which forms when silicon wafers are dipped in solutions containing superacids such as bis(trifluoromethane)sulfonimide (TFSI) has recently been found to be effective at electronically passivating the silicon surface. In this paper we first study the role of the solvent in which TFSI is dissolved for the passivation process. We study ten solvents with a wide range of relative polarities, finding TFSI dissolved in hexane provides improved temporal stability, marginally better passivation and improved solution longevity compared to dichloroethane which has been used previously. Sample storage conditions, particularly humidity, can strongly influence the passivation stability. The optimised TFSI-hexane passivation scheme is then applied to a set of 3 Ω cm n-type wafers cut from the same float-zone ingot to have different thicknesses. This enables the reproducibility of the scheme to be systematically evaluated. At 1015 cm−3 injection the best case effective surface recombination velocity is 0.69 ± 0.04 cm/s, with bulk lifetimes measured up to the intrinsic lifetime limit at high injection and >43 ms at lower injection. Immersion of silicon in superacid-based ionic solutions therefore provides excellent surface passivation, and, as it is applied at room temperature, the effects on true bulk lifetime are minimal.