Screen-printed Ag Contacts Research Articles

Evolutionary PERC+ solar cell efficiency projection towards 24% evaluating shadow-mask-deposited poly-Si fingers below the Ag front contact as next improvement step

Monofacial PERC and bifacial PERC + solar cells have become the mainstream solar cell technology exhibiting conversion efficiencies around 22.5% in mass production. We determine a specific saturation current density J0,Ag = 1400 fA/cm2 of the screen-printed Ag front contact. When weighted with the contact area fraction of 3.0% the Ag metal contacts contribute 42 fA/cm2 to the total J0,total = 130 fA/cm2 thereby being a main limitation of the Voc. We investigate carrier selective poly-Si on oxide (POLO) fingers below the screen-printed Ag contacts of PERC + solar cells in order to minimize contact recombination. We name this solar cell PERC + POLO. Numerical simulations reveal that PERC + POLO cells exhibit an efficiency potential up to 24.1% which is 0.3%abs. higher compared to PERC + solar cells. In order to enable low-cost manufacturing of poly-Si fingers, we investigate for the first time the deposition of suitable a-Si fingers by plasma-enhanced chemical vapour deposition (PECVD) through a shadow mask in a vacuum chamber. We demonstrate a-Si fingers as narrow as 70 μm and as high as 250 nm. The parasitic deposition below the mask increases the a-Si finger width by less than 30 μm compared to the mask opening width. First test wafers demonstrate an implied Voc up to 716 mV of PECVD a-Si layers which are crystalized and doped in a subsequent POCl3 diffusion. Applying this process sequence, PERC + POLO cells could be manufactured with the established industrial PERC + process only adding the PECVD deposition of a-Si fingers through a shadow mask.

Mechanism underlying the beneficial effect of forming gas annealing on screen-printed Ag contacts of crystalline Si solar cells

Forming gas annealing (FGA) has been known to be beneficial for improving the contact resistance of crystalline Si (c-Si) solar cells containing overfired front-side Ag contacts. This study examines the microstructural changes responsible for the beneficial effect of FGA through the use of ultrahigh-resolution scanning electron microscopy. The FGA process at 400°C under N2+10% H2 led to the formation of a layer of fine (<300nm) Ag particles on the glass surface of the interfacial pores, which were connected to the open channels in the porously sintered Ag bulk. Aside from the pore surfaces, FGA had little influence on the contact microstructure, and there was no noticeable formation of Ag or other metallic particles within the interfacial glass layer. The present results demonstrate that the Ag+ ions dissolved into the lead borosilicate glass during the firing process, and subsequently diffused out only to be reduced in the vicinity of the pore surfaces during FGA. This result suggests that the permeation rate of hydrogen into the lead borosilicate glass should be negligible, compared to the out-diffusion rate of the Ag+ ions during FGA. Furthermore, the conductivity measurements indicated that the pore surfaces that were decorated with a dense layer of fine Ag particles after FGA could act as an additional path for current transport across the contact interface, thereby lowering the contact resistance.

Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ c = 2.15 × 10−6 Ω-cm2 indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ c = 8.34 × 10−5 Ω-cm2. These results indicate that the ρ c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.

Transmission Electron Microscope Study of Screen-Printed Ag Contacts on Crystalline Si Solar Cells

Microstructural and chemical properties of screen-printed Ag contacts on an emitter surface in crystalline Si solar cells are investigated using a transmission electron microscope. The Pb-based glass layer, where many Ag precipitates are randomly distributed, is formed between a Ag thick film and textured Si. For both textured and nontextured Si surfaces, the Ag crystallites are epitaxially grown on Si with an abrupt interface along the {111} atomic plane. Based on high resolution electron microscopy images combined with fast Fourier transform patterns, the registry of Ag on Si driven by a geometrical matching condition leads to minimization of the effective lattice mismatch between Ag and Si, resulting in the formation of a Ag/Si epitaxial superlattice near the interface region.

SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si3N4) deposited over the n+-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si3N4 layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n+-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n+-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρc (in Ω-cm2) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρc = 2.53 × 10−5 Ω-cm2 is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρc is two orders of magnitude lower than the typical value of ρc = 3 × 10−3 Ω-cm2 reported previously for the Ag contacts of the solar cell. Such low value of ρc for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρc versus impurity doping level (Ns) shows that measured value of the ρc follows a linear relationship with the Ns as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n+-Si surface of the silicon solar cell.

Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρc) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρc of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10−5 Ω−cm2 is measured for the Ag metal contacts formed on the n+ silicon surface. This value is much lower than the ρc data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρc of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

TEM Characterization of Ag Precipitates in Screen Printed Ag Contacts for Si Solar Cells

TEM Characterization of Ag Precipitates in Screen Printed Ag Contacts for Si Solar Cells

Development of Screen-Printed Silicon Solar Cells With High Fill Factors on 100&amp;lt;tex&amp;gt;$Omega$&amp;lt;/tex&amp;gt;/sq Emitters

High-quality DuPont screen-printed Ag contacts were achieved on high sheet-resistance emitters (100 /spl Omega//sq) by rapid alloying of PV168 Ag paste. Excellent specific contact resistance (/spl sim/1 m/spl Omega/-cm/sup 2/) in conjunction with high fill factor (FF) (0.775) were obtained on 100 /spl Omega//sq emitters by a 900/spl deg/C spike firing of the PV168 paste in a belt furnace. The combination of the alloying characteristics of the PV168 Ag paste and optimized single-step rapid low-thermal budget firing resulted in a cost-effective manufacturable process for high-efficiency Si solar cells. In addition, the co-fired 100 /spl Omega//sq cell showed a noticeable improvement of /spl sim/0.5% in absolute efficiency over a conventional co-fired 45 /spl Omega//sq emitter cell. Lighter doping in the 100 /spl Omega//sq emitter cell resulted in better blue-response compared to the conventional cell, contributing to /spl sim/1.3 mA/cm/sup 2/ improvement in short-circuit current. Improved surface passivation on 100 /spl Omega//sq emitter cell resulted in additional 0.6 mA/cm/sup 2/ increase in J/sub sc/, 15-mV higher V/sub oc/, and a 0.6% increase in absolute cell efficiency. Front grid design optimization resulted in a FF of 0.780, and a further improvement in cell efficiency to reach 17.4%.