Bulk Lifetime Research Articles

An Analytical Model for the Electrical Characteristics of Passivated Carrier- Selective Contact (CSC) Solar Cell

In this paper, we have developed a physics-based analytical model for the electrical characteristics of passivated silicon carrier-selective contact (CSC) solar cells and validated it with numerical simulations. The model comprises analytical solutions of: 1) the Poisson equation, accurately capturing the level of inversion and accumulation at the crystalline silicon (c-Si) layer interfaces, and 2) the recombination current capturing the various recombination mechanisms at work in the solar cell. The calculations establish that CSC solar cells have the potential to achieve an efficiency greater than 26% even when both c-Si interfaces have realistic SRV values of 100 cm/s. The model helps illustrate the physics underlying the performance of CSC solar cells for variation in key device parameters such as surface recombination velocities, bulk lifetime, contact layer doping, and amorphous silicon (a-Si) thickness amongst others. By circumventing the need for resource-intensive numerical simulations, the analytical model described in this paper can assist in the design and development of high-performance CSC solar cells.

Effect of dopant concentration and annealing of Yttrium doped CuO nanocrystallites studied by positron annihilation spectroscopy

In this paper, we report the solution combustion synthesized Yttrium doped CuO nanocrystallites studied by positron annihilation lifetime (PAL) and Doppler broadening (DB) spectroscopy. The lattice defect in Y doped CuO nanocrystallites is presented by the effect of dopant concentration and annealing temperature. X-ray diffraction measurements indicated monoclinic CuO with impurity phase (Y2O3) of Y at elevated temperature. Scanning Electron Microscopy observations showed the particles are more agglomerated upon annealing. The PAL study exhibited three-lifetime components (τ1, τ2, and τ3) wherein, as-prepared CuO exhibited an increased lifetime (τ1) of 212 ps compared to the reported bulk lifetime of 169 ps. The analysis confirmed the mixed contribution of the annihilations of trapped positrons in CuO bulk, point, and extended defects. The τ2 and τ3 are ascribed to the vacancy clusters and voids. Defects modification with doping concentration and annealing temperature are explained in detail. The average positron lifetime (τav) of all the samples increases with doping concentration and decreases with annealing temperature. Grain growth hindrance due to Y ion migration is presented effectively, signifying the higher average lifetime. Comparative analysis of the average lifetime with the extracted from the valance electron annihilations associated S parameter from DB and on the crystallite size variation with annealing is presented on the basis of defects formation and its curing mechanism. The defect structure change owing to charge compensation of Y in CuO is explained. The defect modification of doped and annealed CuO is explained well from the change in crystallite growth and surface defects.

Density Functional Theory Study on Defect Behavior Related to the Bulk Lifetime of Silicon Crystals for Power Device Application

Among the insulated‐gate bipolar transistors (IGBTs) and PIN junction diodes, there are devices that the recombination centers, namely lifetime‐control defects, are introduced into phosphorus (P) doped n‐type silicon (Si) crystals by electron beam irradiation. The lifetime‐control defects are considered as they form deep levels such as the vacancy–vacancy (V–V) pair and vacancy–phosphorus (V–P) pair. In the case of using Czochralski Si wafers, it is considered that some pairs of carbon (C), oxygen (O), and their complexes are believed to have a negative impact to the lifetime‐control defects. In this study, density functional theory (DFT) calculations are performed to understand the formation behavior of lifetime‐control defects and the formation behavior of complexes composed of C and O atoms believed to interact with lifetime‐control defects. On the basis of these results, DFT calculations related to the structural change of lifetime‐control defects are performed. The most important result is that instead of the interstitial carbon–substitutional carbon (Ci–Cs) pair, the interstitial Ci atom and interstitial carbon–interstitial oxygen (Ci–Oi) pair are the strong candidates to affect controllability of lifetime by interacting with V composed of lifetime‐control defects.

Back-surface recombination, electron reflectors, and paths to 28% efficiency for thin-film photovoltaics: A CdTe case study

As thin-film and silicon solar technologies mature, questions emerge about the upper bounds of thin-film solar performance and realistic experimental paths to reach them. Directions include increasing absorber hole density and bulk lifetime, improving the junction interface, reducing back-surface recombination, and implementing a back-surface electron reflector. Textbook solutions of idealized p-n junctions create a powerful conceptualization of solar cells as predominantly minority-carrier-driven devices. We demonstrate that thin films are distinct, and models often fail to capture the important role of majority-carrier lifetime, leading to contradictions with lifetime measurements and overestimates of potential device improvement from back-surface passivation and/or reflectors. Furthermore, we identify methods to probe majority-carrier lifetime and re-examine the degree to which back-surface passivation and electron reflectors can increase efficiency for a range of common thin-film interface and absorber properties, using current and emerging CdTe technology as an example. Results indicate that a practical approach is to focus first on improving front-interface recombination velocity and the absorber properties, and then on implementing the back-surface passivation or reflector, which can ultimately allow thin-film solar technology to reach 28% efficiency.

Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration

Quasi-steady-state photoconductance measurements on silicon ingots and blocks with different photo-generation profiles are simulated in this work. The results show that deeper generation profiles, achieved by using a long-pass optical filter with a longer cut-off wavelength, can reduce the impact of the high surface recombination velocity of the ingot surface. This results in higher measured effective lifetimes and reduces the reliance on transfer functions to convert the measured lifetimes into bulk lifetimes. However, there exists a trade-off between generating carriers further from the surface to reduce surface recombination and ensuring that the generated carriers are within the sensitivity range of the photoconductance sensing coil. The simulations are compared with experimental results measured on a monocrystalline silicon block using both quasi-steady-state and transient photoconductance decay, as the transient method is relatively less prone to the impact of surface recombination, and provides a lower bound on the bulk lifetime. The results confirm an increased accuracy in bulk lifetimes extracted from quasi-steady-state measurements when measured using deeper photo-generation profiles.

Исследование влияния радиации на рекомбинационные потери в гетеропереходных солнечных элементах на основе монокристаллического кремния

AbstractA method has been developed for numerically estimating the recombination loss in silicon heterojunction solar cells under irradiation. The calculations are based on an analysis of the experimental short-circuit currents. The suggested model makes it possible to evaluate the degree of degradation of semiconductor structures by calculating the decrease in the bulk lifetime and in the diffusion length of carriers. The results obtained are of practical importance for examining the possibility of using this type of solar cells in space conditions.

Degradation of Surface Passivation and Bulk in p-Type Monocrystalline Silicon Wafers at Elevated Temperature

The surface passivation quality and the bulk lifetime of boron-doped p-type Czochralski silicon wafers were studied in response to dark annealing at 175 °C, using in situ effective lifetime measurements. We investigated non-fired and fired silicon nitride (SiN x ), aluminum oxide (AlO x ) capped with SiN x , and thermally-grown silicon oxide (SiO2). Modulation in surface passivation quality and bulk lifetime was detected only in cases where hydrogen is assumed to be released into the silicon wafer from the dielectric (AlO x /SiN x stack and fired SiN x layer). Interestingly, the degradation of both the surface and the bulk were followed by a recovery. It is also interesting to note that the changes in the surface and the bulk seem to be related, as the surface degradation starts when the initial bulk degradation ends. This study indicates a possible involvement of hydrogen in both the degradation and the recovery processes. The evolution of the effective lifetime as a function of time is similar to that reported for carrier-induced degradation in multicrystalline wafers; however, occurring on a different time scale. Hence, these findings may also be valuable for investigation of other degradation mechanisms in different silicon materials.

Optimization of micron size passivated contact and doping level for high efficiency interdigitated back contact solar cells

The Interdigitated back contact (IBC) solar cell is one of the most efficient commercial solar cells in the PV market. In order to improve the IBC solar cell efficiency, the entire solar cell structure must be well designed. The first issue is the solar cell’s doping level, including the p+ and n+ surfaces. The second is that the contact and passivation must be optimized. In this paper, we analyzed the recombination parameter (Jo) and specific contact resistivity (ρc) which are influenced by doping level, contact size and passivation. We found that the Jo decreases regularly as the ratio of contact size decreases in the diffusion wafer. An industrial, cost effective IBC solar cell fabrication with laser process was developed. The IBC solar cell with micron size contact and special doping level gave Voc of 690.0 mV, Jsc of 42.0 mA/cm2, FF of 81.0%, and η of 23.5%. The software 3D Quokka@ was used to calculate the IBC solar cell efficiency through simulation using a device model. Through the energy loss analysis, the simulation indicated that IBC solar cell with 25.0% efficiency could achieve with optimized doping level, micron size passivated contact, low contact resistance in the high bulk lifetime wafer.

Heat degradation of sputter-deposited Cu(In,Ga)Se2 solar cells and modules: Impact of processing conditions and bias

We report accelerated heat degradation studies on fully encapsulated Cu(In,Ga)Se2 modules as a function of film growth parameters, in particular back contact selenization (preSe), as well as the impact of bias (light/voltage) during heat degradation. We show that pre-Se conditions have a profound effect on the heat stability of the device, whereby reduced preSe, while increasing initial efficiency, results in strong heat degradation, driven by a combination of reduced space-charge region and reduced minority carrier lifetime (as evident from external quantum efficiency measurements) in the light-soaked state and resulting in strong degradation of short-circuit current. This is also accompanied by a stronger increase in the shallow acceptor concentration (as measured by capacitance-voltage profiling) in the degraded state, suggesting that the SeCu divacancy complex (VSe-VCu) is likely responsible. In this case, appearance of a high concentration of deep acceptor states accompanies increased shallow doping upon light-soaking, with the former reducing bulk lifetime and the latter further affecting electron collection due to narrow depletion width. This result suggests that bulk structural properties of the absorber film are strongly impacted by the back contact selenization conditions, making the film more susceptible to heat degradation. In the second part of this paper we show that electrical or light bias during heat exposure reduces degradation, in particular almost fully eliminating the above short-circuit current loss. This is a surprising result as usually the positive effects of bias are attributed to interfacial changes, while our results demonstrate that bulk properties can be improved as well.

Numerical exploration for structure design and free-energy loss analysis of the high-efficiency polysilicon passivated-contact p-type silicon solar cell

In this work, the application of the p-type and n-type polysilicon passivated contact on industrial-level p-type silicon solar cell is studied using numerical simulation. The effects of (i) the structure design, (ii) the bulk lifetime and resistivity of the p-type wafer, and (iii) the carrier selectivity of polysilicon passivated contact on cell performances are investigated. Furthermore, the corresponding energy-loss pathways are classified by using free energy loss analysis (FELA). In essence, the rear-junction solar cell with the n-type polysilicon passivated-contact generates more internal power because of the better surface passivation and less front metallization shading, but the efficiency potential is limited by the low lifetime of the state-of-the-art p-type Czochralski (Cz) wafer. Thus, the p-type polysilicon passivated contact serving as the back-surface field would be more favorable if the lifetime of the p-type Cz silicon were less than 350 μs. Over the long term, the lifetime of the p-type wafer possibly becomes the bottleneck of the high-efficiency polysilicon passivated-contact solar cells. Finally, we present the roadmap toward the 23% industrial p-type silicon solar cell with the p-type or n-type polysilicon passivated contact.

Comparison of light-induced degradation and regeneration in P-type monocrystalline full aluminum back surface field and passivated emitter rear cells

This paper reports on a systematic and quantitative assessment of light induced degradation (LID) and regeneration in full Al-BSF and passivated emitter rear contact cells (PERC) along with the fundamental understanding of the difference between the two. After LID, PERC cells showed a much greater loss in cell efficiency than full Al-BSF cells (∼0.9% vs ∼0.6%) because the degradation in bulk lifetime also erodes the benefit of superior BSRV in PERC cells. Three main regeneration conditions involving the combination of heat and light (75 °C/1 Sun/48 h, 130 °C/2 Suns/1.5 h and 200 °C/3 Suns/30 s) were implemented to eliminate LID loss due to BO defects. Low temperature/long time (75 °C/48 h) and high temperature/short time (200 °C/30s) regeneration process was unable to reach 100% stabilization. The intermediate temperature/time (130 °C/1.5 h) generation achieved nearly full recovery and stabilization (over 99%) for both full Al-BSF and PERC cells. We discussed the effect of temperature, time and suns in regeneration mechanism for two cells.

A simplified formulation for calculation of minority-carrier effective lifetime

Abstract An improved simple equation to calculate the minority-carrier effective lifetime in photoconductance measurements is proposed by the perturbation method. When the given constraints are satisfied, the given formulation leads to a fast and accurate determination of effective lifetime from the bulk lifetime, surface recombination velocity, the thickness of the wafer, and diffusion coefficient.

Comparative study of free-volume nanostructurization in glassy and crystalline As2S3 re-examined with annihilating positrons

Evolution of free-volume atomic scale structure under crystal-to-glass transition was re-examined in arsenic trisulfide As2S3 in glassy and crystalline state, employing mutually correlated positron annihilation lifetime studies treated in a framework of two-state simple positron trapping model. In crystalline state represented by mineral orpiment As2S3 supplied from Wanshan mine (China), the trapping rate is restricted to ~1.14 ns−1, associated with 0.240 ns defect-free bulk and 0.402 ns defect-specific lifetimes due to free volumes stabilized preferentially within inter-layer spaces. The positron trapping rate is notably enhanced to ~1.51 ns−1 in glassy state presented by melt-quenched As2S3, the positron traps being smaller with decreased bulk (0.278 ns) and defect (0.380 ns) lifetimes due to nanostructurized free volumes stabilized within corrugated quasi-layers. In general, vitrification in As2S3 produces depressing and time-enhancing trend in the recorded PAL spectra peaks. The ratio of bulk positron lifetimes for glassy and crystalline states serving as criterion of vitrification completeness reaches 1.16, meaning that some additional volume is available for annihilation in a glass. The ratio of mean positron lifetimes approaching 1.12 reflects a degree of realized crystal-to-glass transformation. Vitrification-related changes in As2S3 can be imagined as fragmentation of existing free-volume voids due to crumpling of intrinsic inter-layer spaces.

Tabula Rasa for n‐Cz silicon‐based photovoltaics

AbstractHigh‐temperature annealing, known as Tabula Rasa (TR), proves to be an effective method for dissolving oxygen precipitate nuclei in n‐Cz silicon and makes this material resistant to temperature‐induced and process‐induced lifetime degradation. Tabula Rasa is especially effective in n‐Cz wafers with oxygen concentration >15 ppma. Vacancies, self‐interstitials, and their aggregates result from TR as a metastable side effect. Temperature‐dependent lifetime spectroscopy reveals that these metastable defects have shallow energy levels ~0.12 eV. Their concentrations strongly depend on the ambient gases during TR because of an offset of the thermal equilibrium between vacancies and self‐interstitials. However, these metastable defects anneal out at typical cell processing temperatures ≥850°C and have little effect on the bulk lifetime of the processed cell structures. Without dissolving built‐in oxygen precipitate nuclei, high‐temperature solar cell processing severely degrades the minority carrier lifetimes to below 0.1 millisecond, while TR‐treated n‐Cz wafers after the cell processing steps exhibit carrier lifetimes above 2.2 milliseconds.

Graphene oxide films for field effect surface passivation of silicon for solar cells

In recent years it has been shown that graphene oxide (GO) can be used to passivate silicon surfaces resulting in increased photocurrents in metal-insulator-semiconductor (MIS) tunneling diodes, and in improved efficiencies in Schottky-barrier solar cells with either metal or graphene barriers, however, the source of this passivation is still unclear. The suggested mechanisms responsible for the enhanced device performance include the dangling bond saturation at the surface by the diverse functional groups decorating the GO sheets which reduce the recombination sites, or field effect passivation produced by intrinsic negative surface charge of GO. In this work through a series of measurements of minority carrier lifetime with the microwave photo-conductance decay (µPCD) technique, infrared absorption spectra, and surface potential with Kelvin probe force microscopy (KPFM) we show that there is no evidence of significant chemical passivation coming from the GO films but rather negative field effect passivation. We also discuss the stability of GO's passivation and the flexibility of this material for its application as temporary passivation layer for bulk lifetime measurements, or as a potential cheap alternative to current passivation materials used in solar cell fabrication.

(Invited) Ionic Surface Passivation to Extend the Performance of Crystalline Silicon

High efficiency silicon solar cells require substrates with carrier lifetimes well into the millisecond range. High bulk lifetimes can be extremely difficult to measure accurately because of recombination at the sample’s surfaces. Although it is possible, in principle, to passivate the surface well by oxidation or by deposition of a dielectric, such surface passivation approaches often change the bulk lifetime which is being measured. This can be because: (i) the passivation occurs at elevated temperature hence creating or destroying bulk defects; (ii) hydrogen enters the bulk and passivates recombination centres; or (iii) impurities are gettered to the passivating film at the sample’s surface. It has recently been discovered that the thin film which forms on the silicon surface when samples are immersed in ionic solutions containing superacids can provide exceptional surface passivation [1-3]. As this passivating layer is formed at room temperature, the bulk lifetime of the silicon is not modified and hence the artefacts identified above are avoided. In this talk it will be shown how surface recombination velocities of 0.7 cm/s can be achieved using room temperature ionic passivation, and it will be demonstrated how this can be used to measure extremely long bulk lifetimes (>43 ms) [3]. Results of experiments which aim to establish the mechanism by which passivation occurs will be reported, and factors such as the solvent polarity, humidity and solution storage conditions will be reviewed. The talk will then highlight three applications of the ionic passivation for improving the performance of silicon materials. The first will be in the development of high efficiency interdigitated back contact (IBC) solar cells, in which ionic surface passivation is used to understand process-induced bulk lifetime degradation [4]. The second will be in the understanding of light-induced degradation which has recently been found to occur in high performance p-type float-zone silicon [5]. The final example will involve using the superacid-based passivation to measure bulk lifetimes up to and beyond the current intrinsic lifetime limit [6]. It will be shown that the true limit of the bulk lifetime of silicon must be higher than previously assumed, and results from a study which aims to re-evaluate the fundamental limit of silicon’s performance will be presented.

Crystal growth and evaluation of ultra-long carrier lifetime Czochralski silicon

Impact of the ultralow-carbon concentration of below 1 × 1015 atoms/cm3 and thermal history on the bulk lifetime of phosphorus (P)-doped magnetic-field-applied Czochralski (MCZ) silicon was investigated. In order to accurately measure the long bulk lifetime, a direct-current photoconductive decay method was applied to a rectangular sample with a sectional area larger than 400 mm2. The measurement of an ultra-long bulk lifetime of longer than 20 ms was demonstrated using the P-doped MCZ silicon with a carbon concentration of approximately 5 × 1014 atoms/cm3. Furthermore, the bulk lifetime of MCZ silicon with shorter exposure time at 300–600 °C was extremely short; we believe that the formation behavior of the carbon-related defects in the crystal growth process affect the bulk lifetime. Therefore, not only reducing the carbon concentration but also improving the thermal history at low temperature is effective for increasing the bulk lifetime of as-grown P-doped MCZ silicon.

Methods to Improve Bulk Lifetime in n-Type Czochralski-Grown Upgraded Metallurgical-Grade Silicon Wafers

This paper investigates the potential of three different methods— tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation, for improving the carrier lifetime in n-type Czochralski-grown upgraded metallurgical-grade (UMG) silicon samples. Our results show that the lifetimes in the UMG wafers used in this study were affected by both mobile metallic impurities and as-grown oxygen precipitate nuclei. Thus, the dissolution of grown-in oxygen precipitate nuclei via TR and the removal of mobile impurities via PDG step were found to significantly improve the electronic quality of the UMG wafers. Finally, we report bulk lifetimes and 1-sun implied open-circuit voltages of the UMG wafers after boron and phosphorus diffusions, as typically applied in n-type cell fabrication.

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.

Hydrogen induced degradation: A possible mechanism for light- and elevated temperature- induced degradation in n-type silicon

In this work, we demonstrate a form of minority carrier degradation on n-type Cz silicon that affects both the bulk and surface related lifetimes. We identify three key behaviors of the degradation mechanism; 1) a firing dependence for the extent of degradation, 2) the appearance of bulk degradation when wafers are fired in the presence of a diffused emitter and 3) a firing related apparent surface degradation when wafers are fired in the absence of an emitter. We further report a defect capture cross-section ratio of σn/σp = 0.028 ± 0.003 for the defect in n-type. Utilizing our understanding of light and elevated temperature induced degradation (LeTID) in p-type silicon, we demonstrate that the degradation behaviors in both n-type and p-type silicon are closely correlated. In light of numerous reports on the involvement of hydrogen, the potential role of a hydrogen-induced degradation mechanism is discussed in both p- and n-type silicon, particularly in relation to the diffusion of hydrogen and influence of hydrogen-dopant interactions.