Boron-rich Layer Research Articles

Optimization of boron depletion for boron-doped emitter of N-type TOPCon solar cells

During the preparation of boron-doped emitters for TOPCon solar cells, boron atoms accumulate, forming a boron-rich layer (BRL). Oxidation, during the boron diffusion process, can eliminate the BRL. Prolonged oxidation which forms a SiO2 layer can remove the BRL, also act as a protective mask for the front surface. Nevertheless, during high-temperature thermal oxidation, boron will be redistributed. Boron reaches a higher equilibrium concentration in SiO2 than in the silicon wafer, and the SiO2/Si interface shifts inward during SiO2 formation. Simultaneously, the high temperature during oxidation prompts the boron inside the silicon wafer to diffuse deeper. The dual effects of oxidation and diffusion cause boron to be depleted near the surface of the silicon wafer. Boron depletion leads to a reverse bending of the energy band, which hampers carrier collection and impedes further enhancement of cell power conversion efficiency. In this paper, the oxidation process was mainly adjusted to reduce surface boron depletion, resulting in approximately 0.09%abs. increase in power conversion efficiency compared to pre-optimization.

Formation of emitter by boron spin-on doping from SiO2 nanosphere and properties of the related n-PERT solar cells

In this paper, spin-on doping (SOD) process using commercial boron dopant source was applied to form front p-type emitter on n-type mono-crystalline CZ substrate. Results showed that the thickness of boron rich layer (BRL) decreased from 180 nm to 20 nm with SiO2 nanosphere assisted SOD and the BRL can be removed entirely by further in-situ oxidation. Furthermore, the hole concentration of the emitter reduced from 5.5 × 1019 cm3 to 2.8 × 1019 cm3 by integrating these two processes. In a symmetric sample without BRL, a very low emitter saturation current density of 32 fA/cm2 with implied-Voc over 700 mV was obtained. The minority carrier lifetime got improved more than 30% after diffusion. Finally, n-PERT solar cell with a conversion efficiency of 20.26% was achieved using optimized boron emitter, showing an enhanced conversion efficiency of 0.54%.

Promotion of boron diffusion and gettering by employing thin Al2O3 layer as a reaction barrier

The formation of a boron-rich layer (BRL) is a common phenomenon during the fabrication of boron (B) emitter due to different solubility of B in the silicon oxide layer and the silicon substrate. In our work, we employ alumina oxide (Al2O3) layers as reaction barriers to avoid the production of the silicon oxide layer and eliminate the BRL. The B diffusion in silicon becomes much more accessible that the diffusion concentration and depth are increased dramatically with the absence of BRL. The Al2O3 layer can also improve the uniformity of surface sheet resistance due to its hydrophilic, bringing superior quality of liquid boron source spin coating. We investigate the influence of Al2O3 with different thicknesses and refractive indexes on the B diffusion. The diffusion profiles of B in the silicon capped with Al2O3 become broader as the thicknesses and refractive indexes of Al2O3 increase. Besides, iron is employed as a model impurity to study the gettering effects during the B diffusion. The increase of the maximum B concentration and the total amount of B doping contribute to a superior gettering effect with the interstitial iron concentration decrease from 3.91e12 cm−3 to a minimal value of 1.65e11 cm−3, and the iron concentration even dropped to 7.24e10 cm−3 after attaching a second deposition. We analyze the mechanisms of gettering systematically during the B diffusion. The present work will help deepen the understanding of B doping and the fabrication of B emitter.

Investigation of p+ emitter formation for n-type silicon solar cells application

This work reports on the study of boron diffusion in n-type silicon from preform source for p+ emitter formation. Using quartz tube furnace, three main parameters are investigated that are: drive-in temperature, drive-in time and temperature ramp-up time. It is found that the sheet resistance as measured by the four point probe method (4-pp) decreases from 93 Ω/⎕ to 24 Ω/⎕ as the drive-in temperature increases from 850 °C to 950 °C, likewise, it decreases from 57 Ω/⎕ to 38 Ω/⎕ as the drive-in time increases from 20 min to 40 min. The electrically active boron dopant profiles as measured by the electrochemical capacitance voltage technique (ECV) exhibit surface concentrations below the solid-solubility limit of boron in silicon for all the studied emitters, the maximum surface and peak boron concentrations of 1.30 × 1020 and 1.58 × 1020 atoms cm−3 were measured respectively on the emitter which is formed at a drive-in temperature of 950 °C. Furthermore, it is revealed from the secondary ion mass spectroscopy measurements (SIMS) the presence of the boron rich layer (BRL) on its surface. The Hall Effect measurement method is used for measuring sheet resistance and sheet charge carrier concentration.

Boron–oxygen complex yields n-type surface layer in semiconducting diamond

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 1021 cm-3) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1-1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B-O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B3O and B4O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior.

Different methods for removing boron-rich layer and their impacts on the efficiency of interdigitated back contact solar cells

In this paper, three commonly used methods for boron-rich layer (BRL) (BRL: abbreviation of boron-rich layer, a layer constituted by compound of boron and silicon.) removal and their impacts on the surface passivation and the efficiency of the final interdigitated back contact solar cells are investigated. The method of in-situ oxidation can remove BRL completely, but will cause serious degradation of the bulk lifetime. The method of treatment in boil nitric acid cannot remove BRL completely resulting in a poor passivation quality of the substrate surface. The method of chemical etch treatment (CET) (CET: Abbreviation of chemical etch treatment.) can remove BRL completely and would not cause any degradation of the bulk lifetime. Finally, interdigitated back contact solar cells have been fabricated using the three methods mentioned above for BRL removal and the highest efficiency of 21.17% is achieved with the BRL removed by the method of CET.

Boron-rich layer removal and surface passivation of boron-doped p–n silicon solar cells * * Project supported by the Beijing Municipal Science and Technology Commission, China (No. Z151100003515003), the Beijing Natural Science Foundation (No. 4173077, 2184112), the Fundamental Research Funds for the Central Universities, China (Nos. FRF-BR-16-018A, FRF-TP-17-022A1, 06400071), the National Natural Science Foundation of China (Nos. 110751402347, 61274134, 51402064, 61274059,

In boron-doped p+–n crystalline silicon (Si) solar cells, p-type boron doping control and surface passivation play a vital role in the realization of high-efficiency and low cost pursuit. In this study, boron-doped p+-emitters are formed by boron diffusion in an open-tube furnace using borontribromide (BBr3) as precursor. The formed emitters are characterized in detail in terms of shape of the doping profile, surface doping concentration, junction depth, sheet resistance and removal of the boron-rich layer (BRL). In the aspect of BRL removal, three different methods were adopted to investigate their influence on device performance. The results demonstrate that our proposed chemical etch treatment (CET) with the proper etching time could be an effective way to remove the BRL. After removal of the BRL, Al2O3/SiNx stacks are deposited by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) to passivate the cell surface. It was found that a reasonably-high implied Voc of 680 mV has been achieved for the fabricated n-type Si solar cells.

Avoidance of boron rich layer formation in the industrial boron spin-on dopant diffused n-type silicon solar cell without additional oxidation process

In the present work, the properties of boron rich layer (BRL) formed in the boron spin-on dopant diffusion as well as the avoidance of BRL formation without additional oxidation process are studied. The boron concentration distributions characterized by SIMS exhibit the formation and avoidance of BRL with the variation of oxygen content in the boron diffusion process. The existence of BRL can degrade the minority carrier lifetime due to the enhancement of recombination as well as weaken the anti-reflection effect due to the unmatched refractive index of BRL and SiO2/SiNx stack layers. The decreased minority carrier lifetime together with the increased surface reflectance eventually deteriorates the electrical behavior of solar cell with a maximum power conversion efficiency loss of 0.55%. Significantly, the formation of BRL is found to be avoided due to the complete oxidation of excess elemental boron atoms when the oxygen content increases to 10%, contributing to the recovery of solar cells’ electrical performance with an average efficiency of 19.47%. The present work will be helpful to deepen the understanding of BRL and simplify the boron diffusion process for massive cell production.

Strengthening of the Restored Surface Layer of Steel Parts 45 by Laser Boring

By means of the proposed technologies, the relationship and strengthening of the optimal modes of recovery of the restoring layer and the optimal laser treatment modes with bat-retaining straps are chosen. The microstructural, X-ray diffraction studies of the restored surfaces of the cams of the distributing shaft of the car were determined, the wear resistance and microhardness of the working surfaces were determined. By means of the proposed technologies of restoration and strengthening, optimum modes of applying the restoring layer and optimal modes of laser treatment (pumping energy of 20 kJ without melting of the surface) with bat-retaining coatings (thickness 2 mm) were chosen. The unevenness of the pulsed laser treatment with the restriction of the diameter of the laser spot and the treatment of a boron-rich surface layer with overlapping of the corresponding zones leads not only to the quenching but also to the release of individual local microparticles. New phases such as boron carbide, carbides - borides, iron borides, and others are formed. The strengthened surface layer has a complex structure and generally contains a martensite base with thin layers of borides, сarbides. The microhardness of the processing zone rises 3-4 times (Нμ 6500-7000 MPa).

Role of Oxygen for Boron Diffusion From Borosilicate Glass Layers Deposited by Atmospheric Pressure Chemical Vapor Deposition

We investigate the impact of different oxygen concentrations during boron diffusion at 950°C using borosilicate glass (BSG) layers deposited by atmospheric pressure chemical vapor deposition. Therefore, we vary the boron concentration in the BSG layer and the oxygen concentration in the gaseous ambient of the high-temperature step. Characterization includes sheet resistance as well as electrochemical capacitance voltage measurements for the determination of the resulting boron doping profile. In addition, we perform quasi steady-state photoconductance measurements to investigate the influence of the oxygen concentration on the recombination parameters of the boron-diffused surface. Low recombination current densities J 0 of J 0 = 38 fA/cm² at a sheet resistance of 54 Ω/sq are observed. Finally, we show that the oxygen concentration is a crucial parameter to prevent the formation of a boron-rich layer at the silicon surface.

Co-diffusion of boron and phosphorus for ultra-thin crystalline silicon solar cells

This paper reports the fabrication of crystalline silicon passivated emitter rear totally diffused (c-Si PERT) solar cells with ultra-thin p-type wafers 50 µm in thickness. Co-diffusion of boron and phosphorus in a single rapid thermal processing cycle, and an Al spin-on glass post-curing process were developed to remove the boron rich layer which is detrimental to c-Si solar cells. Co-diffusion was carried out with spin-on diffusion sources using boric acid and a P spin on dopant for simple and cost-effective emitter and back surface field (BSF) formation processes. The fabricated ultra-thin c-Si PERT cell featured an open circuit voltage (Voc) of 0.575 V, a short circuit current density (Jsc) of 35.8 mA cm−2, a fill factor of 0.725, and a power conversion efficiency of 15.0%. The efficiency has improved by 2% compared with the standard structure cell with Al-BSF using thin evaporated Al 2 µm in thickness. Along with cell output parameters, the flexural strength and critical bending radius were measured by a four point bending test, and the results showed that the solar cells with thinner rear Al electrodes are more applicable for a flexible solar cell device.

A One Step Method to Produce Boron Emitters

A one‐step method to obtain boron emitters for n‐type solar cells based on the use of boron tribromide (BBr3) as a dopant source is developed. During this study several experimental parameters are varied. It is observed that besides the diffusion temperature, the nitrogen gas flux and the BBr3 bubbler temperature have a significant impact on the obtained emitter properties. Using the adequate experimental conditions a homogenous boron emitter without boron rich layer (BRL), a sheet resistance of 77 Ω sq−1 and a dark saturation current <100 fA cm−2 is obtained.

Impact of boron rich layer on performance degradation in boric acid diffused emitters for n-type crystalline Si solar cells

Boron rich layer (BRL) formed beneath the borosilicate glass layer during p-type emitter formation is an undesirable phenomenon. It influences different cell parameters and can degrade the device performance. In this work, the device degradation study is done for different BRL thicknesses produced with different concentrations of the boric acid dopant source. The bulk carrier lifetime reduces to more than 75% and emitter saturation current density becomes more than 10−12 mA cm−2 for 60 nm of BRL thickness. The observed Jsc and Voc values become zero for BRL thicknesses higher than 40 nm as seen in this work and the device properties could not be enhanced. So, higher thicknesses of BRL should be avoided.

Isotopic Analysis and Characterization of Boric Acid Diffused Emitters in n-type c-Si Solar Cells

Boric acid (BA) is one of the boron dopant sources used in recent years to fabricate emitters for n-type c- Si solar cells. Boric acid solution with concentrations less than 4% is sufficient enough to get doping concentrations with sheet resistance values 10 Ω/sq to 90 Ω/sq under different diffusion conditions. Isotopic analysis of BA diffused emitter, optimized for different sheet resistances have been studied in this work. Cs ions of 1KV 300x300 raster ions have been used for the emitters and it has resulted detection of 11B and 28Si isotopes in all the boron diffused samples. Isotopic analysis of borosilicate glass (BSG) and boron rich layer (BRL), corresponding to different dopant concentrations, also shows isotope detections and they vary according to natural abundance.

Influence of boron rich layer on quantum efficiencies in p+–n–n+ Si structures during diffusion of boron spin on dopant source

Boron spin on dopant source is used for p+ emitter formation in n-type crystalline Si and it results in formation of born rich layer (BRL) after removal of borosilicate glass layer. Various BRL thicknesses less than 40 nm are able to improve the quantum efficiencies in the blue wavelength range, thereby showing the possibilities of passivation of the emitter surface. However, the bulk silicon lifetime can degrade up to 9% due to 40 nm BRL thicknesses. So, a thin layer of BRL is helpful in the overall gain in the device performance if the degradation in the bulk properties is negotiated.

High efficiency screen-printed n-type silicon solar cell using co-diffusion of APCVD boron emitter and POCl3 back surface field

We present the fabrication and analysis of Passivated Emitter and Rear Totally Diffused (PERT) solar cells on n-type silicon using a co-diffusion process. In a single high temperature step, a BSG/SiOx stack deposited by APCVD and a POCl3 back surface field diffuse into the wafer to form the boron doped emitter and phosphorus doped back surface field. The SiOx layer on top of BSG acts as a masking layer to prevent cross-doping of phosphorus as well as a blocking layer for boron out-diffusion. This resulted in an initial sheet resistance of 76 Ω/□ with good uniformity and a final p+ emitter sheet resistance of 97 Ω/□ after boron rich layer removal. Additionally, bulk lifetime was investigated before and after the high temperature step that resulted in an increase from 1.2 ms to 1.5 ms due to a POCl3 gettering effect. A peak cell efficiency of 20.3% was achieved and each recombination component in terms of saturation current density was calculated and analyzed to understand the cell for further efficiency enhancement.

Influence of nitride and nitridation on the doping properties of PECVD-deposited BSG layers

High efficiency p-type passivated emitter and rear (PERC) silicon (Si) solar cells are becoming industrial standard with demonstrated efficiencies above 22.6 %. Alternatively to the PERC design there is the possibility to apply a full-area boron back-surface-field (B-BSF) to the solar cell rear side for implementation of passivated emitter and rear totally diffused solar cells (PERT). In this work we investigate plasma enhanced chemical vapor deposited (PECVD) boron silicate glass (BSG) layers employing the precursor gases nitrous oxide (N2O), silane (SiH4) and diborane (B2H6). We experimentally demonstrate the impact of nitride (N) content in the BSG on the boron (B) doping efficiency within a thermal drive-in step, i.e. a subsequent high temperature diffusion step post BSG-deposition. It is found that the N content of the BSG which is mainly controlled by the N2O:SiH4 precursor gas flow ratio has to be kept as low as possible to achieve optimal B doping results of the Si bulk material. In addition an in-situ nitridation process has been developed to prevent build-up of a boron rich layer (BRL) on the highly doped Si surface. PERT and PERC solar cells were processed from material with different base resistivities and equal solar cell efficiencies were achieved.

N-type 고효율 태양전지용 Boron Diffused Layer의 형성 방법 및 특성 분석

N-type crystalline silicon solar cells have high metal impurity tolerance and higher minority carrier lifetime that increases conversion efficiency. However, junction quality between the boron diffused layer and the n-type substrate is more important for increased efficiency. In this paper, the current status and prospects for boron diffused layers in N-type crystalline silicon solar cell applications are described. Boron diffused layer formation methods (thermal diffusion and co-diffusion using <TEX>$a-SiO_X:B$</TEX>), boron rich layer (BRL) and boron silicate glass (BSG) reactions, and analysis of the effects to improve junction characteristics are discussed. In-situ oxidation is performed to remove the boron rich layer. The oxidation process after diffusion shows a lower B-O peak than before the Oxidation process was changed into <TEX>$SiO_2$</TEX> phase by FTIR and BRL. The <TEX>$a-SiO_X:B$</TEX> layer is deposited by PECVD using <TEX>$SiH_4$</TEX>, <TEX>$B_2H_6$</TEX>, <TEX>$H_2$</TEX>, <TEX>$CO_2$</TEX> gases in N-type wafer and annealed by thermal tube furnace for performing the P+ layer. MCLT (minority carrier lifetime) is improved by increasing <TEX>$SiH_4$</TEX> and <TEX>$B_2H_6$</TEX>. When <TEX>$a-SiO_X:B$</TEX> is removed, the Si-O peak decreases and the B-H peak declines a little, but MCLT is improved by hydrogen passivated inactive boron atoms. In this paper, we focused on the boron emitter for N-type crystalline solar cells.

A novel strategy for preparing a selective back surface field of n-type emitter wrap through solar cells

In the present study, we have developed a novel mixed co-diffusion (MCD) process by which to prepare a selective back surface field (BSF) of n-type emitter wrap through (EWT) solar cells, which combines a plasma enhanced chemical vapor deposition (PECVD) phosphorus-doped n-type microcrystalline silicon as the dopant source, with a low temperature thermal oxidation (LTO) process. Through comparison between our MCD process and a standard co-diffusion (SCD) process, a BSF with a shallow doping depth of 0.56µm and high doping concentration of 1.9×1020at/cm3 is easily obtained by the MCD process under the low temperature of 750℃. Therefore, the MCD process is shown to reduce the number of high temperature processes, which cannot produce dopant redistribution, and can accurately control the doping concentrations and depths of the BSF and emitter. In addition, the novel method also eliminates the boron-rich layer, which induces misfit dislocations and bulk lifetime degradation, without extra chemical treatment. Therefore, the MCD process' open circuit voltage, short circuit current density, conversion efficiency and fill factor of the solar cells are respectively increased by 7mV, 6mA/cm2, 2% and 2%. These results indicate that the MCD process is a novel and potential agent for the SCD process.

Boron-rich layer properties formed by boron spin on dopant diffusion in n-type silicon

Starting with N-type base, a p-type emitter is formed using boron spin on dopant (BSoD) which results in formation of boron rich layer (BRL) on top of the emitter and can be used in selective emitter and FF improvements in solar cells. In this work, the morphologies of BRL for varying thicknesses, depending on the diffusion conditions, have been studied to know their impact on emitter formation. The characterizations show that BRL properties are dependent on its thickness and its boron concentration. BRL has amorphous phase with peak boron concentration over 1021atoms/cm3, thickness less than 100nm, refractive indices of 1.4–1.6 and contact resistance 1.0–6.0mΩ-cm2. The bond properties of the constituent elements of BRL vary depending on the thickness.