Effective Lifetime Values Research Articles

English

In this manuscript, we present a study on the effects of magnetic field on charge carrier effective lifetime within the base of a polycrystalline PV solar cell p-n junction. This study is elaborated in two forms as a theoretical study and an experimental study. Indeed, under various magnetic field values, PV solar cell is illuminated by using the light from stroboscopic. From a theoretical approach mainly use the open circuit voltage decay method.  Green’s functions and boundaries conditions are used to solve charge carrier’s diffusion equation. Open circuit transient voltage decay expression is found. The charge carriers’ effective lifetime values inversely proportional to the slope of open circuit voltage decay curve are deducted. A simple experimental set-up used open circuit voltage decay method mainly, these experimental measurements of charge carriers’ effective lifetime are done and the effects of magnetic field are then outlined. Globally, magnetic effects on effective charge carrier lifetime are then analyzed. These two types of results obtained present qualitatively the decay of charge carriers’ effective lifetime when magnetic field increase. In the first time, theoretical results and experimental results are in good agreement and in the second time these results are in good agreement with literature results. Key words: Effective lifetime, open circuit voltage decay, magnetic field.

Application of Quasi-Steady-State Photoconductance Technique to Lifetime Measurements on Crystalline Germanium Substrates

Similar to other high quality crystalline absorbers, an accurate knowledge of surface passivation of crystalline Germanium (c-Ge) substrates is crucial for a straightforward improvement of photovoltaic device performance. For crystalline silicon devices, this information is typically obtained by quasisteady state photoconductance (QSS-PC) technique using Sinton WCT-120 tool. In this work, we explore the conditions to adapt this measurement technique to c-Ge substrates. Based on PC-1D simulations, we deduce that a minimum effective lifetime is needed corresponding to an effective diffusion length equal to the substrate thickness. Apart from this, an accurate estimation of the total photogeneration inside the c-Ge sample is also mandatory. This condition implies that the light intensity that impinges onto the sample must be measured with a c-Ge sensor, although the integrated c-Si sensor can be used for high flash intensities. Additionally, the optical factor used to evaluate sample reflectance must be also known, which is determined by measuring robust effective lifetime values under photoconductance decay conditions. Finally, knowledge about carrier mobility in c-Ge is also necessary to translate the measured photoconductance to the corresponding excess carrier density values. Lifetime measurements of passivated c-Ge substrates done by QSS-PC technique are validated by comparing them with the ones obtained by microwave photoconductance technique.

Taking monocrystalline silicon to the ultimate lifetime limit

A central quantity to assess the high quality of monocrystalline silicon (on scales beyond mere purity) is the minority charge carrier lifetime. We demonstrate that the lifetime in high purity float zone material can be improved beyond existing observations, thanks to a deeper understanding of grown-in defects and how they can be permanently annihilated. In a first step we investigate the influence of several process sequences on the lifetime by applying a low temperature superacid passivation treatment. We find that a pre-treatment consisting of an oxidation at 1050 °C followed by a POCl3 diffusion at 900 °C can improve the lifetime by deactivating or eliminating grown-in defects. Then, pre-treated wafers of different float zone materials are passivated with three state-of-the-art layer stacks. Very high effective lifetime values are measured, thereby demonstrating the high quality of the surface passivation schemes and the pre-treated silicon wafers. The measured effective lifetimes exceed previous records, and we report an effective lifetime of 225 ms measured on a 200 µm thick 100 Ω cm n-type silicon wafer symmetrically passivated with a layer stack of a thin thermally grown oxide and a polycrystalline layer (the TOPCon layer stack).

Comparison of line-wise pl-imaging and area-wise pl-imaging

In this contribution we compare the results of a line-scanning camera and line-wise illumination based PL-imaging system (Line-PLI) to the results of a “classic” PL-imaging system featuring a matrix camera and a full area illumination of the wafer.We investigate the impact of the different injection conditions which result from the significantly higher irradiance user in the Line-PLI system. We derive a formula which can easily be used to predict how high the irradiance may be without causing the lifetime to be limited by intrinsic silicon recombination. We compare the image contrasts caused by regions of different recombination activity and we compare the robustness of the method, which calibrates the PL-images to local effective lifetime using photoconductance (QSSPC) measurements.As a result it can be said that contrasts between regions of high and low recombination activity are more pronounced using the investigated Line-PLI setup, which qualifies it excellently for the visualization of process inhomogeneities causing defects on solar cell precursor. We calculate that the irradiance of the investigated setup is still low enough to differentiate between regions of different Shockley-Read-Hall recombination if the effective sample lifetimes are roughly below 250 µs. Moreover we derive an uncertainty of 24 % on the absolute values of effective lifetime using the Area-PLI setup while we derive an uncertainty of only 8 % on the absolute values of effective lifetime using the Line-PLI setup for the investigated samples.

Electronic quality improvement of crystalline silicon by stain etching-based PS nanostructures for solar cells application

In the present work, the impact of stain etching-based Porous Silicon (PS) nanostructures on both the effective lifetime and photoluminescence were examined. Results showed that stain etching-based PS nanostructures increase the effective lifetime from 3μs (un-etched samples) to 48μs (following 8min etching time) at a minority carrier density (Δn) of 1015cm−3 and drastically decrease the silicon surface reflectivity from about 28% to approximately 7%. These results allowed to correlate between the rise of the effective lifetime values and the photoluminescence intensity; which in turn depends on the effects of hydrogen and oxygen passivation. Consequently, the electronic quality of the multi-crystalline silicon improved significantly leading to high photovoltaic quantum efficiency response. This low-cost stain etching-based PS nanostructures process could potentially be applied in the photovoltaic cell technology and applications.

CVD diamond as a position sensitive detector using charge carrier transition time

A concept of position sensitive single crystal chemical vapour deposition (scCVD) diamond detector is proposed and tested using the lateral time resolved ion beam induced current (TRIBIC) using 4.5MeV proton microbeam. Simulation of charge transients confirmed that it is possible to correlate the length of the signal pulse to the position of the proton impact. The simulation also shows that the position sensitivity of such device is improved when the mobility and the effective lifetime values of the electrons and the holes are significantly different. A proof of concept was confirmed using lateral TRIBIC performed on a 1×1×0.5mm scCVD diamond detector.

Al2O3 Passivation on c-si Surfaces for Low Temperature Solar Cell Applications

Functional passivation of high resistivity p-type c-Si wafer surfaces was achieved using 10nm Al2O3 layers and low temperatures for both the thermal ALD process and post-deposition anneal. Effective lifetime values higher than 1ms were measured at excess carrier density Δn=1015cm-3. This result was reached in combination with temperatures of 100°C and 200°C for the Al2O3 layer deposition and anneal, respectively. The Al2O3/c-Si interface was characterized using conductance-voltage and capacitance-voltage measurements. In particular, significantly reduced interface density of the electrically active defects Dit ∼ 2 x 1010eV-1cm-2 was detected, which enabled excellent chemical passivation. The measured density of fixed charges at the interface, Qf, after anneal were in the range +1 x 1012 to -1 x 1012cm-2 indicating that both inversion and accumulation conditions result in relevant field-effect passivation using Al2O3 layers and low temperature processes. Numerical simulations on representative test structures show that the uniform Qf effect can be understood in terms of a surface damage region (SDR) present near the interface in combination with asymmetry in the lifetime of holes and electrons in the SDR. The combination of low processing temperatures, thin layers and good passivation properties facilitate a technology for future low temperature solar cell applications.

Plasmas for texturing, cleaning, and deposition: towards a one pump down process for heterojunction solar cells

AbstractLow temperature plasma deposition of a‐Si:H thin films has emerged as a promising alternative for high efficiency hetero junction (HJ) solar cells. In this work we study plasma processes for texturing and cleaning c‐Si wafers pursuing a low cost dry fabrication process of HJ solar cells. We have studied two independent plasma processes: i) Texturing of c‐Si wafers using SF6 ‐ O2 plasmas in a RIE system, in order to reduce the surface reflectance and therefore improve the light trapping. The effects of the RF power and gas ratio on the c‐Si surface texture have been studied in detail. Highly textured surfaces, with very low reflectance values (around 6% in the range of 300 – 1000 nm) have been achieved. ii) Etching of the native oxide and passivation of the c‐Si surface by plasma, in a standard RF PECVD system. We used SiF4 plasma with optimized conditions for an efficient native oxide removal, and without breaking the vacuum, 40 nm of a‐Si:H were deposited in order to passivate the c‐Si surface. High effective lifetime values were obtained (τeff ≈ 1.5 ms), providing high implicit open circuit voltages (Voc ≈ 0.713 V) and low surface recombination velocities (Seff < 9 cm s‐1). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystalline silicon surface passivation by high-frequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications

A passivation scheme, featuring nanocomposite amorphous silicon suboxides (a-SiOx:H) is investigated and analyzed in this work. The a-SiOx:H films are deposited by high-frequency plasma-enhanced chemical-vapor deposition via decomposition of silane (SiH4), carbon dioxide (CO2), and hydrogen (H2) as source gases. The plasma deposition parameters of a-SiOx:H films are optimized in terms of effective lifetime, while the oxygen content and the resulting optical band gap EG of the a-SiOx:H films are controlled by varying the CO2 partial pressure χO=[CO2]/([CO2]+[SiH4]). Postannealing at low temperatures of those films shows a beneficial effect in form of a drastic increase of the effective lifetime. This improvement of the passivation quality by low temperature annealing for the a-SiOx:H likely originates from defect reduction of the film close to the interface. Raman spectra reveal the existence of Si–(OH)x and Si–O–Si bonds after thermal annealing of the layers, leading to a higher effective lifetime, as it reduces the defect absorption of the suboxides. The surface passivation quality of a-SiOx:H within both n-type and p-type silicon has been studied as a function of injection level. Record high effective lifetime values of 4.7 ms on 1 Ω cm n-type float zone (FZ) wafers and 14.2 ms on 130 Ω cm p-type FZ wafers prove the applicability for a surface passivation of silicon wafers applicable to any kind of silicon-based solar cells. The effective lifetime values achieved on a highly doped crystalline wafer (1 Ω cm resistivity) appears to be the highest value ever reported. Samples prepared in this way feature a high quality passivation yielding effective lifetime values exceeding those of record SiO2 and SiNx passivation schemes.

Dry fabrication process for heterojunction solar cells through in-situ plasma cleaning and passivation

We have studied low temperature plasma processes ( T sub≤200 °C) for the efficient cleaning and passivation of c-Si wafers, aiming for fully dry fabrication of heterojunction solar cells. We have experimented with H 2–SiF 4 plasmas in a standard RF PECVD reactor in order to etch the native oxide from the c-Si wafer and a thin a-Si:H layer was deposited from SiH 4 to passivate the c-Si surface. In-situ ellipsometry was used to optimize the process conditions for an efficient surface cleaning. Various plasma treatments were performed before a-Si:H deposition in order to reduce the surface recombination. Optimized process conditions resulted in high effective lifetime values ( τ eff≈1.55 ms), low effective surface recombination velocities ( S eff≤9 cm s −1) and high implicit open circuit voltages ( V oc≈0.713 V).

Electrical properties of n-type multicrystalline silicon for photovoltaic application—Impact of high temperature boron diffusion

The effects of boron diffusion were investigated on electronic grade n-type single- and multicrystalline silicon wafers for solar cells application. Doped p +-layers were formed using specific spin-on dopant (SOD) source annealed in a tube furnace. Homogeneous diffused layers with sheet resistance values of 70 Ω/sq were achieved on large area silicon wafers. Bulk electrical properties were investigated measuring the effective carrier lifetime after the diffusion step. A significant degradation of the bulk electrical properties was observed using the SOD technique. Unlike for the single-crystalline silicon, it was determined that this effect was not due to an interstitial iron contamination originating from the SOD, but rather by the thermal degradation of the material. Boron diffusion was followed by a phosphorous diffusion sequence, necessary in the fabrication process for the formation of the back surface field. This additional step leads to an improvement of effective lifetime values due to an efficient gettering of the impurities. Finally, bulk carrier lifetimes from 100 μs to over 300 μs were deduced on n-type multicrystalline silicon after boron and phosphorus diffusion. This underlines the potential of this material for photovoltaic application and its stability after high temperature treatments provided that a gettering step is included after the boron diffusion step.

Formation of Boron‐doped region using spin‐on dopant: investigation on the impact of metallic impurities

AbstractInvestigation on the electrical properties of p+‐doped regions formed by spin‐on‐dopant (SOD) technique was achieved. Using this technique, boron‐diffused regions were formed on both p‐type and n‐type float zone wafers. Homogeneous sheet resistances were obtained for both types of wafers. Bulk properties were investigated by measuring effective carrier lifetime. An iron contamination was observed after the boron diffusion step and interstitial iron concentrations were deduced from lifetime measurements. More investigations proved that the iron was initially present within the SOD film. A phosphorus diffusion allows to remove this bulk contamination, leading to an improvement of effective lifetime values. Nevertheless, the corresponding emitter saturation current density was estimated on n‐type wafers and presented a high value. It is likely that this poor electrical quality is the consequence of a high iron concentration which remains in the diffused region. Some possibilities are suggested to avoid or to limit this contamination. Copyright © 2008 John Wiley & Sons, Ltd.

Evaluation of CdTe by Nuclear Particle Measurements

The properties of CdTe have been evaluated by investigating the pulse-height response of surface-barrier devices to monoenergetic alpha particles incident normal to the surface. The energy ε required to form a hole-electron pair was found to be 4.65±0.2 eV. The count rate efficiency and energy resolution that have been obtained indicate that crystals can be grown without major fluctuations in the densities of trapping or recombination centers. The effective lifetime values within the high field regions range from less than 10−9 sec to 4×10−8 sec for holes and electrons. These values represent a marked increase over the previously established value of 10−10 sec.