Printed contact on emitter with low dopant surface concentration

Energy Procedia, 2012

We present a macroscopic and microscopic analysis of the contact formation of two different commercially available silver screen printing pastes (A and B) for contacting lowly doped phosphorus emitters on crystalline silicon solar cells. The new paste B shows lower contact resistance than paste A on conventional emitters of sheet resistances R sh = 60 /sq and 100 /sq as well as for selective emitters with R sh,se = 20 /sq. Paste B achieves a contact resistance ρ c = 3 m cm 2 on high ohmic emitters with R sh = 100 /sq. Initial investigations demonstrate different contact formations for the two pastes. Paste B shows a higher density of silver crystallites grown in the silicon emitter compared to paste A for the lowly doped emitter with R sh = 100 /sq. Additionally, the glass layer at the interface between the front grid of paste B and the investigated emitters contains a high density of silver colloids. The high density of Ag-crystallites and Ag-colloids, the weaker glass frits aggressivity, as well as the different formation of glass layer accompany the better contacting features of paste B on high ohmic emitters.

Energy Procedia, 2011

In silicon solar cells, forming good ohmic contact between the emitter and the metal with minimum contact resistance is critical to achieve peak electrical performance [1]. In commercial solar cells, screen printable silver paste is commonly used to form contact. Factors related to paste chemistry, process conditions and the solar cell wafers influence the contact quality. In this paper, the effect of paste chemistry and emitter sheet resistance on contact quality is described. Several paste chemistries were tested for contact resistance with Transmission Line Model (TLM) measurements on wafers with sheet resistance between 45-100 Ω/□. The series resistance of the solar cells was recorded over 50°C firing window. The paste chemistry was further refined to form low resistance contacts on solar cells with emitter sheet resistance on 100 Ω/□.

2014

Screen printing of the metallization of phosphorus diffused emitters is a well-established process for industrial silicon wafer-based solar cells. Previously, screen printed silver pastes typically required a very high phosphorus surface doping concentration to ensure a low-resistance ohmic contact. Recently, paste manufacturers have focused on the development of silver pastes capable of contacting phosphorus emitters with progressively lower surface concentrations, to minimize surface recombination losses and enable higher cell conversion efficiencies. In this paper, we report on the progress of contacting inline-diffused phosphorus emitters, of which the surface concentrations have been reduced by an etch-back process, using two different pastes. Solar cells with emitter surface concentrations ranging from 4.0 × 10 20 to 1.7 × 10 20 phosphorus atoms/cm 3 were made using two different silver pastes. We present a microstructural analysis of the contact formation, which indicates the possible dominant current transport mechanisms for the two pastes. A high density of silver crystallites formed with a very narrow interfacial glass layer makes the Sol 9600 paste suitable for contacting lowly doped phosphorus emitters. Efficiency gains of 0.2%-0.3% (absolute) were achieved, reaching a maximum efficiency of 18.6% on 156 mm × 156 mm p-type pseudo-square Cz mono-crystalline silicon solar cells.

2003

We are developing inkjet printing as a low cost, high throughput approach to the deposition of front contacts for Si solar cells. High deposition rates of 1µm per printing pass were achieved with a new metalorganic ink composed of silver(trifluoroacetate) in ethylene glycol. The printing conditions were optimized to achieve a relatively high line resolution of 120 µm. The optimal parameters for the piezoelectric inkjet were a pulse frequency of 50 Hz and pulse amplitude of 25 V. The best resolution and the line quality were achieved at a substrate temperature of 180 °C and drop separation of 40 µm.

Digital inkjet printing (DIP) is a precise and promising technology to create fine gridlines for silicon solar cell. It is based on drop on demand (DoD) technology, which drops only when it is properly aligned to avoid wide gridlines. The gridline spreading is controlled by heating the chuck (wafer holder) to 200C at the time of the ink droplets. This eliminates drying step that is common to the screen-printing technology and eradicate gridline spreading. The screen-printed technology (SPT) on the other hand cannot exclude the drying step but relies on the silver loading and organics to balance the paste viscosity and rheology, which are used to control the gridline spreading. The difference between the inkjet and screen-printed inks is the particle size of the paste or ink constituents. While the DIP inks are in the nano-particle range, the SPT counterparts are in the micro-regime. Therefore, understanding the sintering of the nano-particle sizes is needed to achieve low contact resistance and hence high fill factor (FF), which is influenced by the total series resistance of the device. In this paper the microstructural analysis of the inkjet contact system was used to optimize the peak firing temperature for DIP gridlines. The optimized temperature profile found was similar to the SPT. This led to FF of ~79.2% for mono crystalline cell and efficiency of ~19.3% and ~17.4% for multi-crystalline with FF of ~78.5%. It was also found that the series resistance of these cells was not dominated by contact but emitter resistance

Energy Procedia, 2013

The etching mechanism of silver pastes with glass frit through the SiNx anti reflection coating of a standard silicon solar cell during the fast firing step is well understood. In this paper we investigate the firing through behavior for different alternative dielectric layers or stacks that are of great interest for new high efficiency solar cell concepts. On n-doped surface SiO 2 and TiO 2 were compared with SiN x . On p-doped surface TiO 2 and SiN x as single layers and as capping layers on a thin Al 2 O 3 layer were compared. The contact formation was analyzed using contact resistance measurement and scanning electron microscopy. Generally, it turns out that SiN x is the most stable passivation layer during a firing process when compared with all other tested materials. However, stacked on Al 2 O 3 the contact resistance is decreased. Contact resistances as low as c = 3 m cm² were obtained on lowly doped boron surface with a surface concentration of N A = 2.3×10 19 cm -3 .

A comparison of the loss mechanisms in screen-printed solar cells relative to buried contact cells and cells with photolithography-defined contacts is presented in this paper. Model calculations show that emitter recombination accounts for about 0.5% absolute efficiency loss in conventional screen-printed cells with low-sheet-resistance emitters. Ohmic contact to highsheet-resistance emitters by screen-printing has been investigated to regain this efficiency loss.

VEYSEL UNSUR. Understanding the solar cell contact formation by digital inkjet printing (Under the direction of DR. ABASIFREKE EBONG)

IEEE Electron Device Letters, 2011

Homogeneous high-sheet-resistance emitter (HHSE), excellent surface passivation, and high-quality contacts, along with narrow gridlines, are needed for high-efficiency solar cells. However, HHSE in conjunction with screen-printed (SP) contacts often gives low fill factor (FF) because of high contact resistance. We capitalized on the glass-etching property of light-induced plating of silver to decrease the contact resistance and formed high-quality contacts to 100-110 Ω/sq HHSE. This led to the achievement of 78.5% FF, 38.3 mA/cm 2 short-circuit current density (J SC) due to narrow line widths (65 μm), and efficiency of 18.7%.

Energy Procedia, 2013

Local rear metal contacting through passivating dielectric layers has the ability to increase silicon solar cell efficiencies to over 20%. To-date most contact schemes have involved the formation of localised aluminium-alloyed regions through patterned AlO x or SiN x passivating layers. Recently electrochemically-formed anodic aluminium oxide (AAO) layers have been shown to enhance minority carrier lifetimes of phosphorus-diffused p-type CZ wafers when formed over an intervening layer of SiO 2 or SiN x , suggesting that these layers may find applications as passivation layers for cells. We report here on the inkjet patterning of AAO layers formed over a thermally-grown thin oxide layer on p-type silicon surfaces. The process, which involves the inkjet printing of 50% (w/w) phosphoric acid, was used to form well-resolved arrays of holes with a diameter as small as 20-40 μm in the dielectric stack. Alloying of aluminium, which was evaporated over the patterned dielectric stack, resulted in the formation of localised back surface field (BSF) regions having a thickness up to 8 μm. Future work will focus on adapting this process for use in local rear metal contacting of silicon solar cells.