Emitter Saturation Current Research Articles

Model Parameters and Degradation Mechanism Analysis of Indium Phosphide Hetero-Junction Bipolar Transistors Exposed to Proton Irradiation

The degradation properties of Indium phosphide hetero-junction bipolar transistors (InP HBTs) under proton irradiation are studied and modelled using a compact model for pre-irradiation, post-irradiation, and post-annealing. The variation rates of the model parameters, such as the base–emitter saturation current (ISE) and ideality factor in the ideal region (NE) in the forward Gummel characteristics, the zero-biased capacitance (Cje) and the grading factor (Mjer) in the BE junction capacitance, and the transit time parameter in the base region (Tfb), are analysed to delve into the degradation mechanism induced by proton irradiation. The displacement damage, induced by proton irradiation in the space charge region of the base–emitter junction and in the quasi-neutral bulk base region, is found to be responsible for the decrease in current gain and cut-off frequency. After annealing, the variation rates of the parameters decrease significantly compared to post-irradiation. This suggests that the recombination of unstable defects leads to a slight recovery in the degradation characteristics of InP HBTs after a period of annealing.

Unlocking the potential of boronsilicate glass passivation for industrial tunnel oxide passivated contact solar cells

AbstractIn this work, we present a breakthrough in boronsilicate glass (BSG) passivated industrial tunnel oxide passivated contact (i‐TOPCon) solar cells. We find that a high‐temperature firing process significantly improves the front side BSG passivation quality; however, the use of such high‐temperatures is undesirable for metallization as it could lead to more junction damage by the metal paste spikes. In this study, we present a simple and industrially viable method to resolve this dilemma. With a high‐temperature industrial firing activation step to maximize the potential of BSG passivation, a low emitter saturation current (J0e) of 34 fA/cm2 has been achieved, demonstrating excellent boron emitter passivation that is comparable to state‐of‐the‐art SiO2 and Al2O3‐based passivation methods on similar structures and boron emitters. Applying this solution to cell device, the open‐circuit voltage (Voc) is improved by about 6 mV, corresponding to an absolute cell efficiency improvement of about 0.2%. Furthermore, after activating the BSG passivation, a lower temperature paste could be used at the rear side which further improves the Voc by around 3 mV. Combined together, an overall improvement of Voc close to 10 mV is achieved, propelling the cell Voc into the 690‐mV era. The effectiveness of this solution was also verified in a mass production line, with average cell efficiencies of around 23.2% (0.5% more than the baseline) and a maximum cell efficiency and Voc of 23.4% and 693 mV, respectively. This work opens new routes for further improving conventional solar cell efficiencies, in particular for BSG‐passivated TOPCon solar cells.

Toward Exotic Silicon Doping with a Low Thermal Budget and Flexible Profile Control by Liquid-Phase Epitaxy.

We investigate semiconductor p-n junction formation by liquid-phase epitaxy (LPE) using metallic pastes incorporating traditional and nontraditional dopants. The LPE technique enables us to control the shape of doping profiles with a low thermal budget through the choice of solvent, total amount of solvent deposited, and process temperature. We focus here on the Al-B, Zn-P, and Sn-Ga chemistries to dope silicon regions using the chemicophysical properties of a low-eutectic-temperature metallic solvent acting as a matrix for the dissolution of a high concentration of a dopant. Additionally, we developed a capping method enabling doping across a large surface area wafer with a tunable thickness well below 1 μm without film dewetting. In good agreement with thermodynamic simulation of the LPE process, we demonstrate B- and Al-doped regions with a sheet resistance ranging from less than 10 to 300 Ω/sq between 650 and 800 °C, which is significantly lower than the typical temperatures of gas-phase doping processes. Comprehensive electrical simulations suggest that LPE p-n junctions with a low carrier recombination activity can be fabricated via the reduction of surface doping concentration and improved surface recombination velocity. Our investigation of exotic LPE chemistries suggests that emitter saturation currents below 50 fA/cm2 could be achieved at doping concentrations relevant to solar cells.

Boron Implanted Junction with In Situ Oxide Passivation and Application to p‐PERT Bifacial Silicon Solar Cell

Boron junction and its passivation is an active topic in photovoltaic research due to its importance to passivated emitter and rear totally‐diffused (PERT) bifacial Si solar cell. In this paper, a systematic study on boron‐implanted junction, its passivation and ohmic contact formation, as well as application to p‐PERT bifacial cells has been presented. More specifically, the impact of junction profile and surface passivation on boron junction quality, which can be influenced by implantation and in situ oxidation anneal parameters is studied. Good‐quality boron emitter and metal/p+‐Si ohmic contact are achieved, as demonstrated by emitter saturation current of 5–30 fA cm−2 and specific contact resistance of 3–6 mΩ cm−1. A roadmap for a p‐PERT bifacial cell using fully ion implanted (boron and phosphorus) technology, based on which p‐PERT bifacial cells demonstrats front side efficiency of 20.6% (open circuit voltage of 658 mV), rear side efficiency of ≈17% and bifaciality factor of 0.82–0.87 is also presented. Effective cell efficiency of ≈22% is achieved with a Halm IV‐tester with white transporting belts that enhance reflection about 10% from the cell rear.

Effective passivation of p+ and n+ emitters using SiO2/Al2O3/SiNx stacks: Surface passivation mechanisms and application to industrial p-PERT bifacial Si solar cells

In this paper, we present an effective emitter passivation scheme using SiO2/Al2O3/SiNx stacks. Our study shows that SiO2/Al2O3/SiNx stacks can well passivate both p+ and n+ emitters due to an excellent chemical passivation combined with a weak field-effect passivation. Good quality boron and phosphorus emitters were achieved over a broad emitter-doping range, as demonstrated by post-fired emitter saturation current of 20 and 30 fA cm−2, respectively. Based on the results obtained with SiO2/Al2O3/SiNx emitter passivation, we present an industrial roadmap for a p-PERT bifacial cell structure. Using this roadmap, we demonstrate industrial p-PERT bifacial cells with front side efficiency of 20.5%, rear side efficiency of 19.8% (bifaciality factor BF = 0.98) for rear textured cells and 17.5% (BF = 0.85) for rear planar cells. In particular, the cells with bifacial SiO2/Al2O3/SiNx passivation on both p+ and n+ emitters also demonstrate promising performance and a simplified cell process. The results show that SiO2/Al2O3/SiNx emitter passivation scheme is a promising candidate for photovoltaic industry.

Effects of diffusion process on potential induced degradation of silicon solar cells.

Potential induced degradation (PID) has recently been identified as one of the most important degradation mechanisms for silicon solar cells. It is widely considered that PID is closely related with the manufacture and application period of solar modules. In this study, the effects of diffusion sheet resistance on PID were verified and explained by testing the emitter doping profile, the minority carrier lifetime, the emitter saturation current, the electrical performance of different cells, and the PID process. With increasing sheet resistance of cells, the depth and saturation current density of the emitter both decreased, and the cell efficiency increased, whereas the PID phenomenon became serious. It was found that higher sheet resistance or thinner P-N junction could lead to higher PID sensitivity. Therefore, more attention should be paid to PID phenomenon as the photovoltaic industry develops in the direction of high sheet resistance.

Recombination processes in passivated boron-implanted black silicon emitters

In this paper, we study the recombination mechanisms in ion-implanted black silicon (bSi) emitters and discuss their advantages over diffused emitters. In the case of diffusion, the large bSi surface area increases emitter doping and consequently Auger recombination compared to a planar surface. The total doping dose is on the contrary independent of the surface area in implanted emitters, and as a result, we show that ion implantation allows control of emitter doping without compromise in the surface aspect ratio. The possibility to control surface doping via implantation anneal becomes highly advantageous in bSi emitters, where surface passivation becomes critical due to the increased surface area. We extract fundamental surface recombination velocities Sn through numerical simulations and obtain the lowest values at the highest anneal temperatures. With these conditions, an excellent emitter saturation current (J0e) is obtained in implanted bSi emitters, reaching 20 fA/cm2 ± 5 fA/cm2 at a sheet resistance of 170 Ω/sq. Finally, we identify the different regimes of recombination in planar and bSi emitters as a function of implantation anneal temperature. Based on experimental data and numerical simulations, we show that surface recombination can be reduced to a negligible contribution in implanted bSi emitters, which explains the low J0e obtained.

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.

PC1Dmod 6.1 – state-of-the-art models in a well-known interface for improved simulation of Si solar cells

In this paper, we present a new, updated version of the commonly used semiconductor device simulator PC1D named PC1Dmod 6.1. The new program is based on the previously published command line version cmd-PC1D 6.0, which has implemented several new options related to the device physics, but now uses an updated version of the original PC1D graphical user interface. The program thus provides the possibility for using Fermi–Dirac statistics and a selection of state-of-the-art models for crystalline silicon, including injection-dependent band gap narrowing, carrier mobility and Auger recombination, in a familiar setting.Version 6.1 also has implemented the recently published band gap narrowing model by Yan and Cuevas, which is based on empirical studies of a large selection of both n+ and p+ emitters, in addition to Schenk's model. It has also implemented the mobility model for compensated material by Schindler et al. Finally, the maximum number of nodes, time steps and wavelengths have been increased in order to reduce unnecessary constraints on simulations and external files.The results from the PC1Dmod 6.1 simulations have been compared with those of other simulation tools and with previously published data to verify the correct implementation of the new models. Emitter saturation currents calculated using PC1Dmod 6.1 showed an excellent agreement with those obtained using the emitter recombination calculator EDNA 2, and the new program was able to successfully reproduce previously published experimental data and previous implementations of the models. Both PC1Dmod 6.1 and the command line version cmd-PC1D 6.1 are open source software, and are freely available for download.

Wet chemical treatment of boron doped emitters on n-type (1 0 0) c-Si prior to amorphous silicon passivation

The influence of the cleaning process on the amorphous silicon passivation of homojunction emitters is investigated. A significant variation in the passivation quality following different cleaning sequences is not observed, even though differences in cleaning performance are evident. These results point out the effectiveness of our cleaning treatment and provide a hydrogen termination for intrinsic amorphous silicon passivation. A post-deposition treatment improves the passivation level yielding emitter saturation currents determined by Auger recombination in the order of 70fA/cm2 and below.

Influence of Implantation Damage on Emitter Recombination

In this study the influence of implantation damage on emitter recombination is examined for both boron and phosphorus implanted emitters after thermal processing. Dominant defects are identified and used to describe observed changes in emitter saturation current, J0, as a function of implant fluence. Recombination through defects is identified as the cause of increased J0 compared with simulated values.For P-implanted samples J0n+ is shown to depend on annealing temperature for samples partially amorphised by a 1×1015 cm-2 P implantation at 40 keV. J0n+ of samples completely amorphised by a 3×1015 cm-2 phosphorus implant show much reduced dependence on annealing temperature. For boron implanted samples annealed at 1000°C, J0p+ was found to be below 25 fA.cm-2 for implant fluence less than 5×1014 cm-2 but to increase significantly for high fluence, where defects such as boron-interstitial clusters and dislocation loops are likely to dominate the observed recombination.

The Potential Advantage of Industrially Processed Boron Emitters Compared to Phosphorus Emitters

A boron emitter that is transferrable to mass production is compared to a phosphorus emitter of industrial standard and a phosphorus emitter with reduced phosphorus concentration, on the viewpoints of the doping profiles, emitter saturation current (J0E), and internal quantum efficiency at shorter wavelength (= blue IQE). The doping profiles are characterized considering active and inactive dopant, and including the doping source before HF treatment. We found boron atoms are segregated towards the doping source while phosphorus segregates towards the emitter, which could explain the absence of inactive dopant in boron emitters as opposed to a substantial amount of inactive dopant in phosphorus emitters. It was also found that inactive phosphorus is still present even down to 1×1019 cm-3 also for the lightly doped emitter. The boron emitter showed the highest blue IQE probably due to the absence of both inactive dopant and heavy doping above 1×1020 cm-3. On the other hand, 10mV Voc gain of the phosphorus emitters by decreasing J0E from 180 to 45 fA/cm2 implied the possibility of Voc improvement also for the boron emitter by reducing J0E which is 97 fA/cm2 at present.

Deposition temperature independent excellent passivation of highly boron doped silicon emitters by thermal atomic layer deposited Al2O3

In this work, we demonstrate that by using H2O based thermal atomic layer deposited (ALD) Al2O3 films, excellent passivation (emitter saturation current density of ∼28 fA/cm2) on industrial highly boron p+-doped silicon emitters (sheet resistance of ∼62 Ω/sq) can be achieved. The surface passivation of the Al2O3 film is activated by a fast industrial high-temperature firing step identical to the one used for screen printed contact formation. Deposition temperatures in the range of 100-300 °C and peak firing temperatures of ∼800 °C (set temperature) are investigated, using commercial-grade 5″ Cz silicon wafers (∼5 Ω cm n-type). It is found that the level of surface passivation after activation is excellent for the whole investigated deposition temperature range. These results are explained by advanced computer simulations indicating that the obtained emitter saturation current densities are quite close to their intrinsic limit value where the emitter saturation current is solely ruled by Auger recombination. The process developed is industrially relevant and robust.

Surface recombination parameters of interdigitated-back-contact silicon solar cells obtained by modeling luminescence images

Current losses at surfaces play a crucial role in the optimization of high-efficiency silicon solar cells. We present a new approach to characterize the surface recombination activity of interdigitated-back-contact (IBC) silicon solar cells by comparing experimental and simulated photoluminescence images (PL). The different recombination properties of the p- and n-doped regions of IBC cells in combination with the operating condition lead to contrast profiles in the PL image that vary with bias voltage. We achieve a good matching of experimental and simulated data for the investigated cells enabling the analysis of how sensitive the simulated contrast patterns are to changes in the surface recombination at the emitter, back- and front-surface-field and if a better matching of the experimental PL images is possible. Using these PL images in combination with simulations around the open circuit voltage Voc the determination of surface recombination velocities S and emitter saturation currents J0 on finished cells is made possible for the first time. This approach also opens a new path towards loss analysis of finished PERL, PERC, MWT and other silicon solar cells.

Unified Lifetime Metrology and Photoluminescence Imaging of Passivation Defects for Silicon PV

We have recently introduced an improved QSS-μPCD lifetime measurement with a strict quality of decay control technique. This enabled unified, parameter free determination of the decay lifetime and the steady-state lifetime over a broad steady-state injection range (up to about 30suns). Excellent correlation was found with Sinton QSSPC measurement. The advantage of spatially resolved μPCD was applied for wafer mapping. Emitter saturation current, J0, mapping was obtained directly from small perturbation decay lifetime using the Basore-Hansen procedure. The maps revealed commonly present passivation defects manifested as high J0 areas.In this work we report further improvement of the approach, especially regarding speed and resolution. Our research involved photoluminescence measurements on passivated test wafers in sequence with the QSS-μPCD unified lifetime measurement including J0. On selected sites the value of J0 was intentionally varied with corona charge induced field-effect. This enabled a unique PL calibration to J0 and other recombination parameters. Quantitative characteristics enabled us to compare PL procedures for the determination of J0 and to select a practical version for J0 imaging that uses direct J0 determination from the ratio of generation rate G to PL intensity. We have found that this J0 procedure correlates well with the commonly used Kane-Swanson method. In addition, it carries the practical advantages of needing only a single PL image with a straightforward calibration and relaxed requirements regarding injection range restriction.

Emitter Saturation Current Densities Determined by Self-consistent Calibration: Impact of Laterally Non-uniform Lifetime Distribution on Calibration Accuracy

The impact of laterally non-uniform carrier lifetime on the determination of the emitter saturation current(Joe) from photoconductance-based measurements, based on the self-consistent method proposed by Trupke and Bardos, is investigated using a model assuming two adjacent regions with considerably difference Joe. It is shown that the method can result in an underestimation of the mean Joe if the distribution of the Joe across the sensed area is not uniform. From simulation it is verified that t he error can be eliminated through independent measurement of the sample optical properties. The self-consistent calibration mechanism is also studied by the model. Experimental measurements confirm the model predictions.

Simulation and Characterization of Epitaxial n- and p-Type Emitters on Silicon Wafer Solar Cells

Epitaxial emitters deposited by atmospheric pressure CVD have been studied using different characterization methods such as spreading resistance profiling (SRP) or electrochemical capacitance voltage profiling (ECV). Comparing simulations with measured data, very low minority carrier lifetimes of τeff = 20μs have been determined for FZ-material after a standard epitaxy process. LBIC measurements showed very low and inhomogeneous distributed diffusion lengths in the range of 100-250μm before process optimization. Therefore an emitter epitaxy process working at lower temperatures was developed [1] leading to diffusion lengths of 750μm. The effective minority carrier lifetimes and emitter saturation currents were determined using QSSPC measurements. For samples with an emitter, deposited at optimized temperature, a best value for the effective minority carrier lifetime of τeff = 212μs was measured and an emitter saturation current could be determined to J0e= 46 fA/cm2. In order to investigate the stress in the epitaxial layers μRaman and μPL measurements were performed showing no increased values at the interface. The doping density in boron doped emitters was determined with μPL and in good agreement with measured ECV-profiles. The Shockley-Read-Hall lifetime in the emitter bulk showed no increased values at the interface between substrate and epitaxial layer.

Formation of High Efficiency Epitaxial Emitters by APCVD

Improvements in emitter passivation as well as back contact passivation lead to several high efficiency solar cell concepts. PERC (Passivated Emitter Rear Contact) solar cells on monocrystalline wafers above 21% [1] have been achieved, but the diffusion processes with POCL3 and BBr3 have emerged as one of the limiting factors. The formation of high efficiency emitters by APCVD (Atmospheric Pressure Chemical Vapour Deposition) has several advantages compared to the standard diffusion. Epitaxial emitters can be optimised to match the passivation and metallisation of high efficiency concepts as well as industrial approaches like nickel plating. Simulations by PC1D show the potential of epitaxial emitters by featuring low contact resistance in combination with high shunt resistance, a good blue response and low emitter saturation current (J0e) [2,3]. The emitter profiles can be designed in depth and in doping in the range from 1x1017cm-3 up to 1x1020cm-3 for p-type and n-type emitters. Simple reference solar cells with J0e= 46 fA/cm2 and efficiencies of η=17.5% on float zone (FZ) and η=16.1% on multicrystalline (mc) wafers have been processed. Furthermore an epitaxial selective emitter has been developed. The deposition process itself has been enhanced to improve the emitter profiles as well as material quality.

Improving the efficiency of crystalline silicon solar cells by an intersected selective laser doping

The selective emitter technology is the most effective method to improve the silicon solar cell performance. However, the strict alignment requirements between the Ag gridlines and the selective emitters should be addressed. In the present study, a novel front metal contact patterning scheme for crystalline silicon (c-Si) solar cells is developed. An intersected selective laser doping process is applied. The control experiment indicates that the best efficiency increases for ∼23.8% from ∼14.39% to ∼17.74% after receiving the laser doping process. Moreover, our laser doping process does not affect the open-circuit voltage and short-circuit current significantly and the emitter properties have not been degraded in terms of the emitter saturation current. The improved front metal contact results in the improved energy conversion efficiency. The developed process offers an elegant and reliable approach for the low-cost, high throughput industrial-scale production process.

Novel noncontact approach to monitoring the field-effect passivation of emitters

Abstract We report a comprehensive noncontact approach to measurement of the field-effect passivation of emitters for p and n-type base silicon solar cells. The corona charge-voltage technique, extensively used in silicon IC fabrication lines, is utilized in an automated sequence with two complementary measurements that monitor charge induced changes of recombination in the emitter. Quasi-steady-state microwave photoconductance decay, QSS-μPCD measures decay lifetime τeff, the injection level, Δn and the emitter saturation current, J0. UV and blue ac-surface photovoltage (UV-SPV) gives a passivation indicator analogous to short wavelength quantum efficiency. Field-effect characteristics of τeff, Seff, J0, and UV-SPV are presented for symmetrical n + and p + emitter test wafers that quantify the effect of charge, polarity and differences between n + and p + emitters. New findings include field effect hysteresis and charging induced emitter degradation phenomenon analogous to stress induced degradation in MOS devices.