Susanne Fritz

Influence of hydrogen on interstitial iron concentration in multicrystalline silicon during annealing steps

The influence of in-diffusing atomic H into an annealed multicrystalline silicon (mc-Si) wafer on the concentration of interstitial iron [Fei] was investigated. Neighboring wafers with similar initial [Fei] were annealed with and without in-diffusing H. In-diffusion was realized by exposing the samples to a microwave induced remote hydrogen plasma at 400 °C. [Fei] was detected based on lifetime measurements before and after dissociating the FeB complex. Surface passivation was achieved by a quinhydrone-methanol solution at room temperature to avoid further temperature steps or in-diffusion of H during surface passivation. From [Fei] measurements before and after the annealing steps with and without H, the influence of H alone on [Fei] could be accessed. The results were compared to previous experiments where the same SiNx:H layers were used as surface passivation for multiple [Fei] measurements of mc-Si samples before and after several anneals at 400 °C. It could be shown that a H plasma atmosphere has a ...

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.

Crystalline Nature of Metal Spikes and Silicon Inclusions in Ag/Al Screen-Printing Metallization

For contacting boron emitters by screen-printing metal pastes, up to now, it has been necessary to add a small amount of Al to the Ag paste to facilitate a reasonable contact resistivity. With the addition of Al to the Ag paste, deep Ag/Al spikes appear, which can be deep enough to penetrate the emitter and, therefore, affect the emitter and space charge region, and, finally, affect the performance of the solar cell. In this paper, a transmission electron microscopy (TEM) analysis of these Ag/Al spikes is presented. The crystalline nature of the Ag/Al spikes is revealed for different surface structures of the crystalline Si wafer and different Al contents in the screen-printing paste. This result is confirmed by X-ray diffraction measurements of etched-back contacts. Additionally, TEM energy-dispersive X-ray spectroscopy facilitates the examination of the Si-rich inclusions found in the Ag/Al spikes. They prove to be multicrystalline Si precipitates with at least 99 at% Si. The observations help to understand the contact formation process of Al containing Ag screen-printing pastes and support the previously presented model.

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.

A Phenomenological Model for the Contact Formation of Ag/Al Screen-Printing Pastes through SiN x :H Layers

The process of contact formation of Al containing Ag screen-printing pastes to p emitters is investigated by an electron microscopic analysis of etched-back contacts (aqua regia and hydrofluoric acid) and polished contact cross-sections. Ag/Al contact spots grown into the Si surface are detected below inhomogeneously structured areas of the bulk contact that contain Al. In addition, Si accumulations are found in the Ag/Al phase in these parts of the contact after contact formation. It is concluded that during the firing process Ag and Al are transported from the bulk contact to the Si surface, where Ag/Al contact spots grow. The solved Si diffuses in the parts of the contact containing Al. Despite the diffusion of Si, Ag and Al, the SiNx:H layer below the inhomogeneous part of the contact and especially above the Ag/Al contact spots is not completely etched away, as is shown by EDX measurements using top view and cross-sectional samples. Based on the observations made by SEM and EDX analysis, a phenomenological model for the exchange of Si, Ag and Al through holes in the SiNx:H layer and the growth of Ag/Al contact spots below the SiNx:H layer is presented.

p+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μm thick p+-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p+-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p+-doped Si regio...

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.

Microscopic thick film contact formation mechanism to p(p+) Si and influence of the metal contact on the Si substrate - lessons learned from the KONSENS project

The German research project KONSENS investigated the contact formation of screen-printed contacts to crystalline silicon. Special focus was set on contact formation to boron emitters using Al containing Ag thick film pastes and on the formation of local Al contacts. The contact resistances to B emitters using Al containing Ag thick film pastes show dependence on surface topography, i.e. the surface texture. Regarding paste composition, we found that contact formation may be influenced by the amount of Al, as well as by the amount of glass frit. The SiNX:H layer serves as a mask during contact formation preserving the Si surface structure above the contact points. A small hole within the SiNX:H layer allows for material exchange and thereby facilitates contact formation. This means that the contact spots show the same surface morphology as the Si. Additionally, we find that the contact spots are crystalline and consist mainly of Ag. The pn-junction can be smeared out at points of direct metal-silicon contact. Our data suggest that the Al from the Ag thick film paste does not lead to local Al doping underneath the contact spots. Our findings can be explained by a model for contact formation of Al containing Ag thick film pastes to B emitters. The formation of Al rear contacts is influenced by the Al particle size in the Al thick film paste. Smaller Al particles allow for a more homogenous full area Al BSF. On PERC rear structures smaller Al particles in the Al thick film paste result in a thicker local Al BSF and less maximum spread distance of diffused Si in the Al thick film paste.

Contacting p+ emitters with Al containing Ag screen printing paste : a microscopic analysis

The process of contact formation of Al containing Ag screen-printing pastes to p emitters is investigated by an electron microscopic analysis of etched-back contacts (aqua regia and hydrofluoric acid) and polished contact cross-sections. Ag/Al contact spots grown into the Si surface are detected below inhomogeneously structured areas of the bulk contact that contain Al. In addition, Si accumulations are found in the Ag/Al phase in these parts of the contact after contact formation. It is concluded that during the firing process Ag and Al are transported from the bulk contact to the Si surface, where Ag/Al contact spots grow. The solved Si diffuses in the parts of the contact containing Al. Despite the diffusion of Si, Ag and Al, the SiNx:H layer below the inhomogeneous part of the contact and especially above the Ag/Al contact spots is not completely etched away, as is shown by EDX measurements using top view and cross-sectional samples. Based on the observations made by SEM and EDX analysis, a phenomenological model for the exchange of Si, Ag and Al through holes in the SiNx:H layer and the growth of Ag/Al contact spots below the SiNx:H layer is presented.

Kelvin probe force microscopy (KPFM) : investigation of local boron doped emitter regions formed by inkjet boron inks for industrial feasible IBC solar cells

Solar cell concepts like the IBC concept based on locally doped areas promise a high efficiency of more than 24%. This, however, imposes two major challenges: the capability to create these locally doped regions and – which is often forgotten – the capability to check the actual electronic quality of these sometimes microscopic regions (doping level etc.). In order to keep manufacturing costs small, the approach of local doping by ink jetting boron-containing inks is chosen for this contribution. Though liquid deposited doping sources exhibit inhomogeneous properties by itself, the spatial resolved quality control is of utmost importance. Therefore, Kelvin probe force microscopy (KPFM) is applied to map the surface potential and thus the local doping level. Using this technique, some special features of ink jetted boron-containing inks and driven-in emitters like edge sharpness, dopant distribution among other topics are discussed.