N-type Crystalline Silicon Solar Cells Research Articles

Approaching 23% efficient n-type crystalline silicon solar cells with a silicon oxide-based highly transparent passivating contact

The concept of passivating contacts is indispensable for realizing high-efficiency crystalline silicon (c-Si)-based solar cells, and its implementation and integration into production lines has become an essential research subject. A desirable transparent passivating contact should theoretically combine excellent electrical conductivity, distinguished surface passivation and high optical transparency. However, it is challenging to simultaneously achieve the optimization of all three requirements. Here, the application potential of the phosphorous-doped polycrystalline silicon-oxide (n-poly-SiOx) as an efficient hole-selective contact in tunnel oxide passivated contact (TOPCon) solar cells is highlighted. The oxygen content can be regulated by the carbon dioxide (CO2) gas flow during the plasma enhanced chemical vapor deposition (PECVD) process, and the optimal doping concentration is determined. While the wide band gap of n-poly-SiOx enables ideal optical performance, it also ensures excellent passivation and high conductivity, which eventually translates into an optimized short-circuit current density (41.53 mA/cm2), fill factor (79.21%), open-circuit voltage (687.8 mV) and efficiency of 22.62% for the proof-of-concept TOPCon solar cell. Additionally, this contact with the incorporation of oxygen elements can suppress the formation of hydrogen-induced blisters, which critically degrades passivation. Our work highlights a promising strategy to improve the performance of TOPCon solar cells by employing n-poly-SiOx based transparent passivating contact, and its passivation mechanism and working principle are also investigated.

Tunable work function of molybdenum oxynitride for electron-selective contact in crystalline silicon solar cells

Dopant-free carrier-selective contacts based on metal compounds have attracted considerable attention for high-efficiency crystalline silicon solar cells. In this work, the feasibility of using molybdenum oxynitride (MoOxNy) as an electron-selective contact layer in n-type crystalline silicon (n-Si) solar cells has been demonstrated. With the increase in the N2:Ar ratio during the sputtering process, the work function of a MoOxNy film decreases from 4.57 to 4.26 eV, which is advantageous for the MoOxNy film to be an electron transport layer. An efficiency of 18.0% has been achieved in n-Si based solar cells using a full-area MoOxNy contact for electron extraction, featuring a high fill factor of 84.6%.

An Investigation of Internal Quantum Efficiency of Bifacial Interdigitated Back Contact (IBC) Crystalline Silicon Solar Cell

In this article, we investigated the internal quantum efficiency (IQE) properties of n-type bifacial interdigitated back contact crystalline silicon solar cells using an IQE mapping system. In the cell structure, high and low IQE values were observed above the emitter and back surface field (BSF) regions. The IQE values above the BSF busbar were drastically reduced due to electrical shading loss. Line scan profiles at different wavelengths showed the detailed distribution of IQE values. The IQE values varied greatly depending not only on the difference between emitter and BSF regions but also on the rear side structure such as the electrode width and the distance between the emitter and BSF regions. On the other hand, the IQE spectra at over 950 nm improved by increasing the light absorbance ratio from the rear side. After module formation, the IQE spectra at short wavelengths were significantly reduced. The IQE properties were obtained from the front and rear sides. The difference in the short-circuit current between front side illumination and rear side illumination was mainly due to optical shading loss and carrier recombination loss at the BSF region. For a high cell efficiency, it is necessary to improve the passivation properties of the BSF region and optimize the electrode design.

Design, fabrication, and analysis of double-layer antireflection coatings (ARC) for industrial bifacial n-type crystalline silicon solar cells.

Monocrystalline silicon-based, n-type front and back contact (nFAB) solar cells are gradually attracting more interest from the photovoltaic industry, due to their good bifaciality and high efficiency potentials. To further improve the conversion efficiency, nFAB solar cells need to make better use of the solar spectrum. Conventional single-layer SiNx antireflection coating (ARC) tends to have a high reflection loss for ultraviolet photons. Thus, in this work, we prepare a double-layer ARC structure made of SiNx/SiOx stack, deposited by the plasma-enhanced chemical vapor deposition method. We investigate the effects of double-layer ARC through simulation and experimental studies by fabricating bifacial nFAB cells. The results show that the implementation of a double-layer ARC helps to greatly reduce front reflection in short wavelength, and thus allow for improvement of the photocurrent by up to 0.3 mA/cm2. As a result, the average cell efficiency of nFAB solar cells increases by absolute ∼0.2%.

Rapid and accurate characterization of silver-paste metallization on crystalline silicon solar cells by contact-end voltage measurement

Contact-end voltage measurement was applied to characterize the contact-formation process of silver paste metallization on p- and n-type crystalline silicon solar cells under different temperatures with well-designed fixtures and test patterns based on the circular transmission line model. The contact-end voltage values were found to be sensitive to sintering temperature, and the current density and contact end voltage curves of both contacts were linear, stating that the contacts were ohmic contacts. Their symbols on the n-type emitter reversed from negative to positive under the established connection mode, which indicated conductive-path changes manifested in the form of macroscopic electrical properties under insufficient, optimal, and over-fired conditions. We inferred that the conductive channel variations were mainly caused by the silver crystallites that precipitated on the emitter surface from the combination of the cross-sectional and interface morphology analyses. No similar phenomenon was observed on the p-type emitter for the few silver crystallites or silver-aluminum alloy without conductive-path alternation. Their values were much greater than those of the n-emitter, which agreed with the present industrial n-type cell characteristics. The measurement improved our understanding of the contact formation process, and can be used as a flexible approach for researchers to optimize the silver-paste formula and sintering processes for high-efficiency solar cells.

Comparison of potential-induced degradation (PID) of n-type and p-type silicon solar cells

Potential-induced degradation (PID) of photovoltaic (PV) modules is one of the most severe types of degradation, where power losses on system level may even exceed 30%. The PID process depends on the strength of the electric field, the temperature, the relative humidity, conductive soiling, time and the PV module materials. For p-type cells, it has been established that the decrease of the shunt resistance, due to migration of sodium ions across the n/p junction is the root cause of the degradation. On the other hand, it has recently been confirmed for n-type cells that the PID occurs due to an increase in recombination as charges are driven to the anti-reflection (AR) coating/emitter interface. In this paper, we present the comparison between PID of p-type and n-type crystalline silicon (c-Si) solar cells and their progression of PID. The time evolution of PID is studied by light and dark I-V curve measurements, electroluminescence images and progressions of the one- and two-diode equivalent model parameters, viz. photocurrent, 1st and 2nd diode reverse saturation currents, 1st and 2nd diode ideality factors, shunt resistance and series resistance.

Radiative recombination coefficient in crystalline silicon at low temperatures < 77 K by combined photoluminescence measurements

Spectral photoluminescence (sPL) and modulated photoluminescence (MPL) measurements were applied to determine the band-to-band radiative recombination coefficient, Brad, in crystalline silicon. We used precursors of n-type crystalline silicon solar cells consisting of two different wafers passivated with aluminum oxide stacks or intrinsic hydrogenated amorphous silicon, respectively. So far values for Brad can be found in the literature only above 77 K. In this high-temperature range the temperature dependence of Brad obtained using our combined sPL/MPL method is in good agreement with the available literature data for both samples. Interestingly, we have extended the measured range down to a temperature of 20 K and observed a strong increase of Brad by three orders of magnitude with decreasing temperature from 77 K to 20 K.

Characterization of Glass Frit in Conductive Paste for N-Type Crystalline Silicon Solar Cells

Contacting silver paste for an emitter of silicon solar cells has been pointed out to create shunting and to increase carrier recombination due to silver-crystallites at the emitter. However, since these observed electrical losses come of not only the crystallites but also multiple effects of constituents in the paste, whether the crystallites mainly cause these losses has still remained unknown. In this study, how glass frit “itself,” which is most important constituent in the paste, affects the electrical losses is investigated by applying the “floating contact method.” Furthermore, the contact interface between the frit and silicon surface is also investigated to elucidate the mechanism of the losses. The metal oxide glass frit itself is found to drastically cause the shunting and the recombination in n-type solar cells, resulting in loss in open-circuit voltage, and creates metal lead crystallites through redox reaction at silicon surface during firing. The shunting originates from the metallic lead, and the recombination is induced by diffused impurities or defects into the silicon emitter through the reaction. The electrical losses in the solar cells are led to not only by the silver-crystallites, but also by the glass frit itself.

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 laser induced forward transfer process for selective boron emitters

In this work, we present a novel technological approach to form highly boron-doped selective emitters. The selective emitters are formed by using a Laser Induced Forward Transfer Doping (DLIFT) process, which allows precise adjustment of the doping profile by tuning the laser parameters and choosing an appropriate doping source. Surface dopant concentrations of up to Ns≈1×1021cm−3 were achieved by using DLIFT. Subsequent BBr3 tube furnace diffusion was performed to form a low surface concentration (Ns≈1×1019cm−3) homogeneous emitter. In comparison to the conventionally applied BBr3 tube diffusion, an increase in open circuit voltage of up to 6mV is predicted based on the photoluminescence measurements performed after passivation, screen-printing and firing processes. Simulations suggest that this voltage gain is most possibly due to a lower emitter saturation current density (j0e) of the highly doped DLIFT samples compared to the BBr3 diffusion samples. Moreover, it is observed that the laser-induced defects are reduced successfully after a subsequent boron tribromide (BBr3) tube diffusion, which is used to form the homogeneous emitter after the DLIFT process. The micro Raman spectroscopy results indicated that the compressive stress of −200MPa in the silicon crystal lattice after DLIFT process is released by the subsequent BBr3 tube diffusion. These results demonstrate that the DLIFT process in combination with the conventionally applied BBr3 tube diffusion can be an effective approach to form selective boron emitters in high efficiency n-type crystalline silicon (c-Si) solar cells.

Effects of Aluminum in Metallization Paste on the Electrical Losses in Bifacial N-type Crystalline Silicon Solar Cells

Silver/aluminum (Ag/Al) paste has been used as metallization for p+ emitter of n-type solar cells. Nevertheless, the Ag/Al paste induces junction current leakage or shunting in the solar cells, resulting loss in open circuit voltage (Voc). However, the details still are not known about how glass frit and aluminum in the paste affect the p+ emitter, and result in the electrical losses, respectively. Furthermore, it is not still clear whether and how the aluminum addition induces the electrical losses. In this study, the “floating contact method” proposed by R. Hoenig was applied for the measurement to investigate the respective effect of glass frit and aluminum on the electrical losses. Conductive paste with the glass frit for the p+ emitter induces loss in Voc of the cells even if the paste contains no aluminum. The glass frit in the Ag/Al paste induces carrier recombination and significant shunting of p-n junction, but the aluminum in the paste mitigates these electrical losses because of weakening emergence of Ag-crystallites on the p+ emitter.

Impact of a boron rich layer on minority carrier lifetime degradation in boron spin-on dopant diffused n-type crystalline silicon solar cells

In the production of n-type crystalline silicon solar cells with boron diffused emitters, the formation of a boron rich layer (BRL) is a common phenomenon and is largely responsible for bulk lifetime degradation. The phenomenon of BRL formation during diffusion of boron spin-on dopant and its impact on bulk lifetime degradation are investigated in this work. The BRL formed beneath the borosilicate glass layer has thicknesses varying from 10 nm–150 nm depending on the diffusion conditions. The effective and bulk minority carrier lifetimes, measured with Al2O3 deposited layers and a quinhydron–methanol solution, show that carrier lifetime degradation is proportional to the BRL thicknesses and their surface recombination velocities. The controlled diffusion processes and different oxidation techniques used in this work can partially reduce the BRL thickness and improve carrier lifetime by more than 10%. But for BRL thicknesses higher than 50 nm, different etching techniques further lower the carrier lifetime and the degradation in the device cannot be recovered.

Microscopic origin of the aluminium assisted spiking effects in n-type silicon solar cells

Contact formation with silver (Ag) thick film pastes on boron emitters of n-type crystalline silicon (Si) solar cells is a nontrivial technological task. Low contact resistances are up to present only achieved with the addition of aluminium (Al) to the paste. During contact formation, Al assisted spiking from the paste into the silicon emitter and bulk occurs, thus leading to a low contact resistance but also to a deterioration of other cell parameters. Both effects are coupled and can be adjusted by choosing proper Al contents of the paste and temperatures for contact formation. In this work the microscopic electric properties of single spikes are presented. These microscopic results, i.e. alterations of the local emitter doping density, the pronounced local recombination activity at the interface between spikes and Si and its influence on the charge collection efficiency, are used to explain the observed dependencies of global cell parameters on the Al content of contact pastes.

Improved Hydrogen Capping Effect in n-Type Crystalline Silicon Solar Cells by SiN(Si-Rich)/SiN(N-Rich) Stacked Passivation

The effect of hydrogen capping of SiN(Si-rich)/SiN(N-rich) stacks for n-type c-Si solar cells was investigated. Use of a passivation layer consisting of Si-rich SiN with a refractive index (n) of 2.7 and N-rich SiN with a refractive index of 2.1 improved the thermal stability. A single SiN passivation layer with a refractive index of 2.05 resulted in an initial lifetime of 200 μs whereas the layer with a refractive index of 2.7 resulted in a high initial lifetime of 2 ms, but the layer degraded rapidly after firing. A stacked passivation layer with refractive indices of 2.1 and 2.7 had a stable lifetime of 1.5 ms with an implied open-circuit voltage (iVoc) of 720 mV after firing. The thermally stable passivation mechanism with changing amounts of Si–N and Si–H bonding was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Incorporation of the SiNx stack layer (2.7 + 2.1) into the passivated rear of n-type Cz silicon screen-printed solar cells resulted in energy conversion efficiency of 19.69%. Improved internal quantum efficiency in the long-wavelength range above 900 nm, with Voc of 630 mV, is mainly because of superior passivation of the rear surface compared with conventional solar cells.

Boron Emitters from Doped PECVD Layers for n-type Crystalline Silicon Solar Cells with LCO

The intensified research into n-type silicon solar cells over the last few years let the application of boron doped emitters in suitable cell concepts become the preferred method to form the necessary p-n-junction. In this study an alternative process to fabricate a boron doped emitter via diffusion from a PECV-deposited doping source is presented and optimized for n-type crystalline silicon solar cell concepts. Doping profiles with a high surface concentration in combination with low emitter saturation current density values are achieved for improved contact and passivation characteristics. The boron emitter profile is compatible with various contacting techniques i.e. screen printing and vapour deposited Al, allowing for low-resistant contacting with Ag/Al pastes or sputtered Al. The comparably low emitter saturation current density j0E of 44 fA/cm2 allows for a VOC of 666mV, and thereby a cell efficiency of 19.7% is demonstrated on a large area (156.25 cm2) solar cell.

Passivated rear contacts for high-efficiency n-type Si solar cells providing high interface passivation quality and excellent transport characteristics

In this work passivated rear contacts are used to replace point contact passivation schemes for high-efficiency n-type crystalline silicon solar cells. Our structure is based on an ultra-thin tunnel oxide (SiO2) and a phosphorus-doped silicon layer, which significantly reduce the surface recombination at the metal–semiconductor interface. The passivation and transport mechanisms of this passivated contact will be addressed within this paper. Particular consideration will be given to the tunnel oxide's impact on interface passivation and on the I–V characteristics of n-type Si solar cells featuring a boron-diffused emitter. It will be shown that the tunnel oxide is a vital part of this passivated contact and that it is required to achieve excellent interface passivation for both open-circuit and maximum power point (MPP) conditions (implied open-circuit voltage iVoc>710mV and implied fill factor iFF>84%). It will also become clear that the transport barrier arising from the tunnel oxide does not constrict the majority carrier flow. Thus, a low series resistance is obtained, which in conjunction with the high iFF enables FFs well above 82%. Investigations on cell levels lead to an independently confirmed conversion efficiency of 23.0% for n-type cells with a boron-diffused emitter and the herein developed passivated rear contact, in which the efficiency is not limited by the electrical properties of our passivated contacts.

Investigation of antimony diffusion for a local back surface field with laser-fired contacts in crystalline silicon solar cells

We report on a laser-doping process for the formation of a local back surface field with antimony for n-type crystalline silicon solar cells. The local back surface field was formed at low temperature with a laser-fired contact process. A 100 nm thick Sb layer with 44 mW laser power resulted in a junction depth of 500 nm, and a carrier concentration of 5 × 1020 cm−3 yielded the best electrical results, with a current density of 35 mA cm−2 and an efficiency of 17.16%.

Preparation of Al-doped hydrogenated nanocrystalline cubic silicon carbide by VHF-PECVD for heterojunction emitter of n-type crystalline silicon solar cells

Al-doped p-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films were successfully deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) using dimethylaluminum hydride (DMAH) as an Al source. A high dark conductivity of 6.88×10−4S/cm was obtained for a 27nm-thick film under high H2/MMS and plasma power conditions. We applied this optimized film to a heterojunction emitter of n-type c-Si solar cells. The solar cell showed high fill factor of 0.756. The internal quantum efficiency in the short wavelength region was improved to 0.87 with decreasing the thickness of a buffer layer.

Electrical behaviour of n-type silicon solar cells under reverse bias: Influence of the manufacturing process

This study focuses on the electrical behaviour under reverse bias of industrial n-type crystalline silicon (c-Si) solar cells and particularly on the way the cell manufacturing process impacts this reverse behaviour and breakdown dynamics. Different cell process flows were implemented on n-type Czochralski (Cz) silicon substrates. Corresponding n-type solar cells were then studied by means of reverse current–voltage (I–V) measurements and Reverse Bias Electroluminescence imaging. The breakdown dynamics of our cells was found to differ from one process to another. Three main breakdown behaviours, each assigned to a particular type of defect, have been clearly identified. The limiting type of defect (i.e. the one responsible for the actual cell hard breakdown) was found to depend on the cell manufacturing process. The suppression of the limiting defects was done by means of diamond pin cleavage and allowed us to significantly improve the reverse behaviour of the cells.

ELECTRICAL PROPERTIES OF P3HT (POLY [3-HEXYLTHIOPHENE])/n-TYPE CRYSTALLINE SILICON (n-c-Si) SOLAR CELLS

Au / P 3 HT (poly [3-hexylthiophene])/n-type crystalline silicon ( n - c - Si ) solar cells have been fabricated. The Aluminum back contact is obtained by evaporation on silicon substrate. An 80 nm P 3 HT layer thick was spin-coated on silicon substrate followed by thermal annealing. Finally golden contacts are deposited by sputtering. The best characteristics of this flawed solar cell are: V oc =0.47 V , I sc =7.42 mA / cm 2 and an efficiency of 1.29%. The area of this device is 0.07 cm2. In order to get a deep understanding of the electrical properties of the heterojunction, capacitance-voltage and current-voltage-temperature measurements have been made. A compact electrical equivalent circuit has been used to describe the dark current-voltage characteristics. It is based on the combination of two exponential mechanisms, shunt and series resistances and space-charge limited current terms. From the temperature dependence of the extracted parameters we can obtain the limiting conduction mechanism. We found that the polymeric layer limits the current not only at low voltages, through Multi-Tunneling Capture Emission, but also at high voltages, through series resistance and Space-Charge Limited Current. On the other hand, the Silicon wafer limits the current at medium voltages, through the diffusion mechanism. In addition, the model is useful to estimate the open circuit voltage and built in voltage of the solar cell using only dark current voltage measurements.