Josh Engelhardt

Passivating boron silicate glasses for co-diffused high-efficiency n-type silicon solar cell application

Doping layers commonly have but one function: supplying the dopants to form a doped region within a substrate. This work presents B doping layers/stacks, which at the same time supply dopant atoms, passivate the B-doped crystalline Si surface sufficiently well (j0E < 50 fA/cm2), and show optical properties suitable for anti-reflective coating. Furthermore, these boron silicate glasses can act as a barrier against parasitic P in-diffusion during a co-diffusion step. The boron emitters diffused from the inductively coupled plasma plasma-enhanced chemical vapor-deposited B containing SiOx layers are investigated and optimized concerning passivation quality and contact properties for high-efficiency n-type solar Si cell designs. It is shown that even 10 nm thin SiOx:B films already allow for suitable emitter sheet resistance for screen-printed contacts. Furthermore, SiOx:B layers presented here allow for iVOC values of 675 mV and contact resistivity of 1 mΩcm2 for commercial Ag instead of Ag/Al pastes on the ...

High Speed Regeneration of BO-Defects: Improving Long-Term Solar Cell Performance within Seconds

Boron-oxygen related defects typically limit the efficiency of solar cells made from silicon containing high concentrations of boron as well as oxygen. The detrimental effect of these defects can be eliminated by applying a Regeneration procedure that needs carrier injection at slightly elevated temperatures. The kinetics of this process is influenced by different processing steps like thermal treatment and was found to rely on a high enough hydrogen concentration in the silicon bulk. It is shown here that neither emitter formation nor the use of Al2O3/SiNx:H or SiO2/SiNx:H passivation stacks affect Regeneration in a fundamental way. By contrast, the thickness of a SiNx:H layer acting as hydrogen source during a high temperature firing step has direct influence on Regeneration confirming that better hydrogenation results in faster Regeneration reactions. Condensing different process steps that all accelerate Regeneration allows the application of a high-speed Regeneration process consisting of a combination of relatively high temperature and high carrier injection, resulting in complete Regeneration of BO defects in less than 10 s. This makes Regeneration feasible as an in-line process in solar cell production and thus solves the problem of the boron-oxygen defects. Even further acceleration is achieved by laser induced Regeneration where the substrate is heated and illuminated simultaneously.

N-type bi-facial solar cells with boron emitters from doped PECVD layers

This work is mainly focused on an alternative method for emitter formation by means of boron diffusion from a boron-doped plasma-enhanced chemical vapor deposition (PECVD) doping source. With this approach only one high temperature process is necessary for emitter and BSF/FSF formation (co-diffusion), without depletion of surface doping concentration. This enables time and cost-efficient fabrication of solar cells with high conversion efficiencies, as shown in this work, on large area (156.25 cm) bi-facial devices with conversion efficiencies up to 19.7% measured with white back sheet. Furthermore, the contact formation with screen-printing of silver/aluminum (Ag/Al) pastes and its emitter shunting behavior due to Ag/Al spikes, varying with the firing conditions in a belt furnace, are of major interest. Low contact resistance values below 4 mΩcm can be realized with screen-printed Ag/Al contacts on 55 –70 Ω/sq PECVD boron emitters. In addition, Ag/Al spikes with a depth of around 1 – 3 μm could be detected with SEM measurements.

Contact Formation on p-Doped Si by Screen-Printing Pure Ag Pastes for Bifacial n-Type Si Solar Cells

n-type solar cell concepts increasingly utilize emitter formation by diffusion from boron doped sources. Combining the advantage of n-type silicon material and bifacial cell architecture enables high-efficiency and versatile photovoltaics. In case of boron emitters, it was standard until now to form a metal-semiconductor contact by screenprinting Al containing Ag pastes. Instead of utilizing Al to enable Ag to form a sufficient contact with the risk in loss of VOC and FF, different glass compositions for pure Ag pastes were developed to form a contact with low impact on cell efficiency. In direct comparison this method in the first try already surpasses the performance of commercial Al containing Ag pastes in direct comparison. The experimental pastes show a distinctive gain in solar cell characteristics in contrast to commercial pure Ag pastes. In this case we reached an overall efficiency of 18.6% using pure Ag pastes. Contact resistivity values thereby range below 1-2 mcm2 comparable to pure Ag pastes on n-type emitters and Ag/Al pastes on p-type emitters.

Influence of rear side coating on emitter formation during POCL 3 diffusion process

The influence of a SiNx coating of a Si wafer on sheet resistance (RSh) of a neighbouring wafer during POCl3 diffusion process is investigated. Wafers facing the SiNx layer of the neighboring wafer in the next slot of the quartz boat show a lower RSh compared to those facing a bare Si wafer, e.g. a reduction from 61 Ω/sq to 52 Ω/sq and a thicker PSG layer are determined. The active doping profile, measured by ECV, shows a deeper plateau region while the tail region is unchanged. Accordingly, the emitter saturation current density rises from 130 fA cm to 193 fA cm. We propose that the thicker PSG layer originates from a lower consumption of the reactive gases (POCl3-N2 and O2) at the SiNx coated surface and thus a higher availability of them at the bare Si surface. On the other hand, we also investigate the influence of the thickness of the SiNx rear coating on the emitter at the non-coated Si surface. Already a very thin layer of 20 nm SiNx causes a significant change in RSh and the emitter profile, while there was no difference observed for the SiNx thickness in the range from 20 nm to 160 nm. Facing a rear coated neighbor wafer during diffusion seems to improve the uniformity of RSh on 6-inch large area wafers.

>20% efficient 80 µm thin industrial-type large-area solar cells from 100 µm Sawn c-Si Wafers

Reducing the thickness of crystalline Si wafers to be processed into solar cells yields several significant benefits: PV module manufacturing cost can be reduced and the required diffusion length of minority carriers is smaller. The latter in turn enables a higher efficiency potential and a larger spread of Si materials to be employed for rear junction solar cell concepts which are advantageous for n-type devices. Industrial-type 80 μm thin large-area rear junction solar cells manufactured from 100 μm wire-sawn wafers exhibit an independently certified efficiency of 20.1% with VOC of 672 mV.

Contact formation on boron doped silicon substrates from passivating PECV-deposited dielectric doping layers with anti-reflective properties by screen-printing Ag pastes for high-efficiency n-type silicon solar cells

n-type silicon solar cell designs for high-efficiency commonly incorporate boron emitter formation. PECV-deposited boron diffusion sources are an alternative to primarily used boron gas diffusion sources. CVD layers are multi-functional allowing for diffusion of boron, surface passivation and contact formation by one single layer. In this case, these BSG layers are applied in a co-diffused cell design for screen-printed contacts. Reaching iVOC values of over 660 mV as ARC layers, BSG layers are shown to be capable to support the contact formation of commercial Ag pastes to boron emitters at standard firing conditions. An analysis of the contact resistivity as well as the contact formation by SEM is conducted to verify the high quality of the screen-printed contact.

Surface Texturing for Silicon Solar Cell Application Using ICP-PECVD Plasma Technique

This work is focused on plasma-induced texturing using a single chamber inductively coupled plasma – plasma-enhanced chemical vapour deposition (ICP-PECVD) lab-tool. Plasma treatment using pure NF3 under low pressure conditions leads to high etch rates (1.2–1.4 μm/min) enabling fast surface texturing. 6 min plasma treatment on wafers with saw damage decreases the effective reflectivity Reff from 31% to 13.7% without anti-reflection coating (ARC), which is lower than the value for a common alkaline surface texture. For planar (KOH pre-treated) samples Reff can be reduced from 38% to 17.8% after 4 min plasma treatment. Lifetime measurements reveal a postprocess lifetime of 250 μs without any further damage removal etching (DRE). This refers to a low density of plasma-induced defects. Furthermore, it is demonstrated that plasma textured surfaces enable excellent contact formation for screen-printing of Ag/Al pastes on boron emitters leading to low specific contact resistances below 5 mΩcm even for low set peak firing temperatures (TSet < 800°C). These values are much lower than values realized with a common isotropic wet acidic etch and slightly lower than values realized with a common alkaline surface texture, leading to reduced power losses due to lowered series resistance. In addition, the specific contact resistance on POCl3 emitters by screen-printing of commercial available Ag pastes is determined to be below 1 mΩcm, nearly independent of the set peak firing temperature.