Bulk Lifetime Research Articles

Simulating the Effect of Lifetime Non-uniformity on Solar Cell Performance Using cmd-PC1D 6 and Griddler 2

In this paper we present a novel approach to investigate the influence of non-uniformity in the minority carrier lifetime of c-Si wafers on solar cell efficiency. By using a combination of one-dimensional device modelling (cmd-PC1D 6) and finite element circuit modelling (Griddler 2) we are able to simulate the total IV characteristics of a solar cell based on input from minority carrier lifetime images. With suitable input regarding the optical properties, doping profiles, recombination behaviour and sheet resistance of the passivated and metallized surfaces involved this combined model can be used to predict the total cell efficiency of mc-Si solar cells and other cells with significant lateral variations in wafer quality. Several artificial test cases are investigated in order to determine the sensitivity of the method and the magnitude of the effect of lifetime distribution. We find that using the weighted average of the two-diode parameters J01, J02 and Jlight in most cases provides a good description of total cell performance, correctly describing the IV parameters within an error of ∼0.1 - 0.2% relative as compared to simulations taking the complete lateral distribution over the wafer into account. The deviation was however observed to be up to 1.5% for extreme, artificial cases. By instead assuming an average carrier lifetime over the wafer larger errors are observed, due to the non-linear relation between the two-diode parameters and the bulk lifetime.

Influence of Al2O3 and SiNx Passivation Layers on LeTID

In this work the carrier lifetime evolution of different passivation layers under illumination and at elevated temperatures are investigated. Multicrystalline silicon lifetime samples were treated by implementing typical industrial processing steps. The degradation was found to depend strongly on surface passivation type, but is independent of the surface doping and oxide charge. The influence of the passivation layer on the silicon bulk lifetime degradation is investigated. After reaching a degradation maximum, the lifetime samples feature a regeneration phase. Capacitance-voltage measurements show that the oxide charge is not influenced by the degradation, but a high decrease in carrier lifetime were measured. Firing experiments show that lifetime samples passivated after firing procedure are not prone to degradation.

Simulation of 20.96% Efficiency n-type Czochralski UMG Silicon Solar Cell

In this paper, we present the 3D simulation of>20% efficiency solar cells using n-type 100% Upgraded-Metallurgical Grade (UMG) Czochralski (CZ) silicon and Electronic Grade (EG) Float Zone (FZ) fabricated using the same process. The cells have a passivated emitter rear locally diffused (PERL) structure, with an etch-back approach on the rear to maintain high bulk lifetime in the cells via phosphorus gettering. Simulation ofthe power losses of both devices are analysed as a function of measured material and cell parameters, including minority carrier lifetime, reflectance, contact resistivity and recombination parameters of the diffused and non-diffused surfaces.

Solar Cells with 20% Efficiency and Lifetime Evaluation of Epitaxial Wafers

We present n-type epitaxially grown wafers deposited in a reactor that allows a process transfer to inline high-throughput reactors. Those wafers exhibit an effective lifetime of up to 1720 μs locally for a phosphorous concentration of 2·1015 cm-3 and a wafer thickness of about 100 μm. In these wafers the most detrimental defects are stacking faults with polycrystalline silicon inclusions. Comparing two samples with stacking faults densities differing by one order of magnitude revealed a difference in average effective minority carrier lifetime of also one order of magnitude (reduction from more than 1500 μs down to 111 μs for the defective sample).A solar cell fabricated from a 200 μm thick epitaxial wafer of low stacking fault density and a phosphorous concentration of 3·1016 cm-3 reaches an independently confirmed efficiency of 20%, an open circuit voltage of up to 658 mV, a short circuit current density of up to 39.6 mA/cm2 and a fill factor of up to 76.9%. Differences between this cell and FZ references can be attributed to a reduced bulk lifetime caused by the high doping concentration and most probably additional recombination due to polycrystalline silicon inclusions in stacking faults, although their amount is comparably low. A second solar cell made of an epitaxial wafer with a high stacking fault density exhibits an efficiency reduction of 0.5% absolute compared to the cell made of the high quality epitaxial wafer. This result underlines the importance of minimizing the stacking fault density in epitaxial wafers, in particular the density of those with polysilicon inclusions.

Effective Surface Recombination of p+ Layers Doped Using Ion Implantation or Surface Deposited B Sources

Two key factors need to be fulfilled in order to enable high efficiency bifacial p-type PERT cell. High bulk lifetime values retention during production processes, and minimized effective surface recombination, Seff in the p+ layer. The influence of boron doped layer parameters on the effective surface recombination in the p+ layer of an n+-p-p+ bifacial PERT solar cell is evaluated. Back IQE data of n+-p-p+ cells with over-doped p+ layer were used for evaluation of Seff values in the p+ layer. Simulation was made for several doping profiles with the surface doping concentration, Bs, in the range 1018 - 1020 cm-3. According to simulation, the dominating parameter controlling the effect of the built-in charge in the passivation layer on the effective surface recombination is the Bs level. For Bs values above ∼1019 cm-3 this charge has no significant influence. p+ layer doping was made by B ion implantation as well as by thermal diffusion from deposited B source. Measurements were performed on symmetrically B doped n-Si wafers with different passivation. Good agreement between simulation and measurements was found.

N-Type polysilicon passivating contact for industrial bifacial n-type solar cells

We present a high-performance bifacial n-type solar cell with LPCVD n+ polysilicon (polySi) back side passivating contacts and fire-through screen-printed metallization, processed on full area 6″ Cz wafers. The cells were manufactured with low-cost industrial process steps yielding a best efficiency of 20.7%, and an average Voc of 674mV. We analysed effects of variation of doping level, thickness, and oxide properties of the n-type polySi/SiOx layers, as well as hydrogenation from a PECVD SiNx:H coating, which led to recombination current densities down to ~2fA/cm2 and ~4fA/cm2 on planar and textured surface, respectively. Analysis shows that the wafer bulk lifetime in the cell is high and that the Voc of the cell is limited by the Jo of the uniform diffused boron emitter and its contacts. Ways to improve the efficiency of the cell to >22% are indicated.

Dislocation-limited performance of advanced solar cells determined by TCAD modeling

In recent years, low-cost silicon for photovoltaic applications has trended toward smaller grain sizes with lower dislocation densities. These changes, coupled with advanced device architectures like heterojunction and passivated emitter and rear cells, have enabled higher efficiencies, including new world records. To establish material requirements for further improvement, we model dislocations with different recombination strengths in Sentaurus Device using mid-gap defects spatially localized at the nanoscale in three dimensions. This approach accounts for the charging of dislocations and differentiates between dislocations and grain boundaries or metal impurities. Simulations that model dislocations as macroscopic variations in effective bulk lifetime or diode parameters do not address these issues. We validate our model by simulating the world record multicrystalline silicon solar cell. We find that three parameters influence cell efficiency: the area fraction of the cell containing dislocations, the dislocation density within defect clusters, and the concentration of recombination centers at these dislocations. We provide targets for these parameters and show how our results can be used to determine the potential gains from reducing dislocations in experimental devices and materials. We further predict that device architectures that lead to higher injection, such as silicon heterojunction cells, are more robust to the presence of dislocations.

P-Type Indium-Doped Passivated Emitter Rear Solar Cells (PERC) on Czochralski Silicon Without Light-Induced Degradation

Solar cells fabricated on boron (B)-doped Czochralski (Cz) Si wafers in the photovoltaic industry are known to suffer from light-induced degradation (LID) in efficiency. This paper reports on promising LID-free large-area indium (In)-doped Cz Si solar cells. Two different commercial-grade B-doped Cz materials were included for comparison. To study the impact of LID on the cell structure, ion-implanted large-area (239 and 242.22 cm2) screen-printed full aluminum (Al) back-surface field (BSF) baseline cells, as well as higher performance passivated emitter rear cells (PERC) with oxide passivation and local Al BSF, were fabricated. In-doped PERC cells achieved 20.3% efficiency, while the B-doped cells gave efficiencies of 20.7% and 20.5% from low- (2 Ω⋅cm) and high-resistivity (6.2 Ω⋅cm) substrates, respectively. It was found that initial efficiency of In-doped PERC cells was ∼0.2% lower due to lower bulk lifetime and higher back-surface recombination velocity. However, In-doped PERC cells showed no LID and surpassed the B-doped PERC cell efficiency by 0.3–0.5% after 0.8-sun 48-h illumination at 37 °C.

Adequate Method for Decoupling Bulk Lifetime and Surface Recombination Velocity in Silicon Wafers

Adequate Method for Decoupling Bulk Lifetime and Surface Recombination Velocity in Silicon Wafers N. Khelifatia,b,∗, D. Bouhafs, A. Mebarek-Azzem, S. El-Hak Abaidia, B. Palahouane and Y. Kouhlane Centre de Recherche en Technologie des Semi-conducteurs pour l’Energetique (CRTSE), DDCS, Algiers, Algeria Universite M’hamed Bougara (UMBB), Faculte des Sciences, Boumerdes, Algeria Universite de Setif, Faculte des sciences-Departement de physique, Setif, Algeria

Optimization of the Phosphorus Doped BSF Doping Profile and Formation Method for N-type Bifacial Solar Cells

n-type PERT (passivated emitter, rear totally diffused) bifacial solar cells with boron and phosphorus diffusion as p+ emitter and n+ BSF (back surface field) have attracted significant research interest recently. In this work, the influences of wafer thickness, bulk lifetime, emitter, BSF on the photovoltaic characteristics of solar cells are discussed. The performance of the solar cell is determined by using one-dimensional solar cell simulation software PC1D. The simulation results show that the key role of the BSF is to decrease the surface doping concentration reducing the recombination and thus, increasing the cell efficiency. A lightly phosphorus doped BSF (LD BSF) was experimentally optimized to get low surface dopant concentration for n type bifacial solar cells. Pre-oxidation combined with a multi-plateau drive-in, using limited source diffusion was carried out before pre-deposition. It could reduce the surface dopant concentration with minimal impact on the sheet resistance.

Permanent annihilation of thermally activated defects which limit the lifetime of float‐zone silicon

We have observed very large changes in the minority carrier lifetime when high purity float‐zone (FZ) silicon wafers are subject to heat‐treatments in the range of 200–1100 °C. Recombination centres were found to become activated upon annealing at 450–700 °C, causing significant reductions in the bulk lifetime, detrimental for high efficiency solar cells and stable high powered devices. Photoluminescence imaging of wafers annealed at 500 °C revealed concentric circular patterns, with lower lifetimes occurring in the centre, and higher lifetimes around the periphery. Deep level transient spectroscopy measurements on samples extracted from the centre of an n‐type FZ silicon wafer annealed at 500 °C revealed a large variety of defects with activation energies ranging between 0.16–0.36 eV. Our measurements indicate that vacancy related defects are causing the severe degradation in lifetime when FZ wafers are annealed at 450–700 °C. Upon annealing FZ silicon at temperatures >800 °C, the lifetime is completely recovered, whereby the defect‐rich regions vanish and do not reappear (permanently annihilated). Our results indicate that, in general, as‐grown FZ silicon should not be assumed to be defect lean, nor can it be assumed that the bulk lifetime will remain stable during thermal processing, unless annealed at temperatures >1000 °C.

Impact of the injection‐level‐dependent lifetime on Voc, FF, ideality m, J02, and the dim light response in a commercial PERC cell

AbstractThe injection‐level‐dependent (ILD) lifetime of the silicon wafer impacts many characteristics of the final photovoltaic cell. While efficiency is commonly understood to be impacted by the silicon bulk lifetime (at the maximum power point injection level), this work demonstrates the wide ranging impacts of ILD lifetime on the Voc, the fill factor (FF), the diode ideality factor m, and the dim light response. Instead of a two‐diode model, we utilize a boundary + ILD bulk lifetime model to analyze a commercial passivated emitter rear contact (PERC) cell featuring an AlOx dielectric rear passivation. The ILD lifetime is directly measured and used to calculate the bulk recombination current across injection levels. With this boundary + ILD lifetime model, we demonstrate the role of the ILD lifetime on many cell parameters in this PERC cell. For most high efficiency commercial p‐type monocrystalline solar cells, the typically lower bulk lifetime at the maximum power point versus the lifetime at the open circuit point reduces the measured FF and pseudo‐FF. This work illustrates that for a commercial PERC cell with AlOx rear passivation, the ILD lifetime is the primary mechanism behind reduced FF, ideality factors greater than 1, and the source of the J02 term in the two‐diode model. The crucial implications of this work are not only to better understand commercial PERC cell loss mechanisms but also to encourage a focus on different metrics in cell diagnostics. One such metric is the Voc at 0.1 or 0.05 suns. Copyright © 2016 John Wiley & Sons, Ltd.

Comprehensive simulation study of industrially relevant silicon solar cell architectures for an optimal material parameter choice

Solar cell production always requires a tradeoff between cell efficiency and production costs. This also concerns the choice of the silicon base material. In general, a long base lifetime is beneficial to achieve high conversion efficiency, but it strongly depends on the cell concept to which extent the cell performance is improved and whether a payback of the higher material costs can be expected. Therefore, in this comprehensive simulation study of various industrially relevant solar cell architectures, we present an investigation of the influence of the bulk lifetime and the resistivity of the base material on the cell performance. A consistent set of cell and simulation parameters is chosen to allow for a direct quantitative comparison of the different cell types. The parameters were chosen rather conservatively in order to describe realistic industrial cells and not record laboratory cells. The observed trends are analyzed using detailed loss breakdowns and are compared with experimental results. For various cell concepts, critical lifetimes τcrit can be observed for which the optimal material parameters change with increasing bulk lifetime when comparing materials of different base resistivity. The underlying physical reasons are explained in detail to accomplish two major aims: (i) generating a better understanding of limitations and challenges concerning different solar cell concepts and (ii) serving as a guide for an optimal material parameter choice for different cell architectures. Copyright © 2016 John Wiley & Sons, Ltd.

Emitter/absorber interface of CdTe solar cells

The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔEC ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se2 (CIGS) cells. The basic principle is that positive ΔEC, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔEC ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔEC < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result in a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔEC of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔEC. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.

Photoluminescence imaging for quality control in silicon solar cell manufacturing

ABSTRACTWe report on progress with PL imaging applications in silicon solar cell production, specifically focusing on the characterization of silicon bricks prior to wafer cutting. Silicon bricks represent an ideal opportunity to characterize and quantify the electronic material quality at an early stage of the PV value chain. Quantitative data on bulk lifetime can be obtained on bricks without any specific sample preparation, unlike unprocessed wafers. Spatially resolved bulk lifetime, interstitial iron concentration, and defect density measurements are demonstrated on bricks from different manufacturers including both high performance multicrystalline and older generation multicrystalline bricks. We find significant variability in bulk lifetime and iron concentration across the samples which is not related to its date of manufacture. However we do see a qualitative reduction in crystallographic defects in the newer high performance multicrystalline bricks. Data is parameterized in different ways to suggest possible paths to better predict solar cell efficiencies from an early stage of inspection.Brick level PL measurements were previously performed using a conventional area scanning PL imaging system, which is associated with light spreading artefacts of weakly absorbed light. To overcome these artefacts, a new line scanning photoluminescence imaging system is used. We show a reduction in contrast smearing between high- and low lifetime regions in the new setup leading to image quality suitable for defect detection and quantitative measurements without deconvolution correction.

Comparison of POCl3 diffusion and phosphorus ion-implantation induced gettering in crystalline Si solar cells

Most p-type Si solar cells involves phosphorus-doped emitter by POCl3 diffusion or phosphorus ion-implantation. Although the formation of the phosphorus emitter is known to getter impurities like Fe, the difference in the impact of these two gettering techniques on cell performance is not well quantified. Therefore, this paper compares the gettering efficiency of POCl3 diffusion and phosphorus ion-implantation on Czochralski(Cz) and cast quasi-mono Si wafers. Cz-Si wafers were used to measure bulk lifetime and iron concentration before and after POCl3 diffusion and phosphorus implantation with different doses. Increase in phosphorus implantation dose improved the gettering efficiency by increasing bulk lifetime and decreasing iron concentration but remained inferior to POCl3 diffusion partly due to single side gettering as opposed to double side gettering with higher phosphorus surface concentration during POCl3 diffusion. Moreover, large-area solar cells were fabricated on cast quasi-mono and Cz-Si wafers to quantify the impact of these emitters on cell parameters. POCl3 diffused cast quasi-mono cells showed ~0.4% higher efficiency due to higher bulk lifetime compared to phosphorus implanted emitter. However, phosphorus implanted Cz cells gave ~0.3% higher efficiency due to lower emitter saturation current, resulting from the benefit of in-situ oxide surface passivation during the implant anneal.

Analysis of short circuit current loss in rear emitter crystalline Si solar cell

Short circuit current (Jsc) loss in rear emitter crystalline Si solar cell is analyzed in detail by a 2D device simulation and compared with the experimental results. There is a significant loss in Jsc for the rear emitter n-Si solar cell with an n-type doped front surface field (FSF) when the base substrate resistivity is low. It is due to an increase in recombination in the FSF region led by a less barrier height for minority carriers with a lower substrate resistivity. The barrier height less than 0.1 eV causes large loss in Jsc. To achieve higher Jsc for the cells with FSF, the control of the doping concentration in FSF, the substrate thickness, and the barrier height for the minority carriers are important. A rear emitter heterojunction Si solar cell with an amorphous Si passivation layer shows no substrate resistivity dependence on Jsc since an amorphous Si possess a higher barrier height and a long bulk lifetime of more than a few milliseconds.

Lifetime Measurement in n-Type 4H-SiC by Mean of the Microwave Phase-Shift

During the past years, our team developed an original lifetime measurement method for semiconductors wafers, called microwave phase-shift (µW-PS). It was successfully employed to determinate bulk lifetime and surface recombination velocities on silicon. We recently adapted the method to silicon carbide (SiC), using a continuous UV laser and acousto-optic modulator. The theory is presented, using the standard continuity equation in n-type 4H-SiC. The proposed measurement setup firstly allows to estimate a bulk lifetime ranging from 2.3 to 6.7 µs.

Modelling of Effective Minority Carrier Lifetime in 4H-SiC n-Type Epilayers

We present an extended model for the simulation of the effective minority carrier lifetime in 4H-SiC epiwafers after optical excitation. This multilayer model uses measured values (such as doping profile, point defect concentration and capture cross sections, epilayer thickness) as input parameters. The bulk lifetime and the diffusion constant are calculated from the actual time dependent excess carrier profiles, resulting in more realistic transients having different decay regimes than in other models. This enables a better understanding of optical lifetime measurements.

Improved empirical method for calculating short circuit current density images of silicon solar cells from saturation current density images and vice versa

An empirical dependence of the short circuit current density Jsc as a function of the dark saturation current density J01 is proposed, which describes this dependence down to a bulk lifetime of 1ns. This method avoids artifacts, which appear when applying the previously proposed quadratic dependence. The parameters of the new dependence are fitted to PC1D simulations and to experimental LBIC results for various wavelengths and AM 1.5 for a typical industrial BSF-type solar cell and a PERC cell. This dependence can also be used to calculate J01 images from LBIC-based Jsc images. It turns out that this method is more reliable in BSF than in PERC cells.