Micro characterization of laser structured solar cells with plated Ni–Ag contacts

https://doi.org/10.1016/j.solmat.2013.05.036Get rights and content

Highlights

  • Fabrication of Al back surface field Si solar cells (156×156 mm²) with plated front side contacts.

  • Cell design shows high cell efficiencies of up to 19% but low adhesion of plated contacts.

  • Cell performance analysis of tempering silicidation process of Ni contacts.

  • Macroscopic and microscopic characterization of tempering induced losses.

  • Laser doping process has to be optimized in terms of pn-junction depth homogeneity.

Abstract

An evolutionary approach to increase the cell efficiency of the industrially widely used Al-BSF Si solar cell design is the implementation of a lightly doped emitter and plated Ni–Ag front side contacts. This work shows that this approach can give a significant advantage to fully screen printed solar cells with cell efficiencies up to 19%. However, the adhesion of the plated contacts strongly depends on the tempering induced silicidation of the Ni layer. The application of macroscopic and microscopic characterization techniques indicates that the tempering induces local non-ohmic shunts due to a silicide spiking in the space charge region. The application of an additional selective emitter doping in this experiment could decrease the shunting probability but could not prevent it completely due to inhomogeneous doping depths on the textured surface.

Introduction

One evolutionary approach towards an industrially feasible high efficiency solar cell concept is the implementation of a selective emitter and plated Ni–Ag or Ni–Cu front side contacts in the conventional Al back surface field (Al-BSF) solar cell design. In comparison to solar cells with screen printed Ag front side contacts the plated Al-BSF solar cells can bring cell efficiency benefits due to reduced shading of the narrow plated metal fingers, decreased recombination losses due to an optimized emitter profile in the passivated regions and good contact resistances of the Ni seed layer even on lightly doped emitters. The good contact resistivity of Ni even on lowly doped surfaces enables also the introduction of lightly doped emitters. One advantage to screen printed selective emitter solar cells is also the possibility of very narrow finger width in the range of 20–50 µm, which allows a small finger pitch on lightly doped emitters for an optimized tradeoff between series resistance, recombination and optical losses.

Recent publications showed very high cell efficiencies with this approach [1], [2]. However, the adhesion of the plated metal contacts can be quite challenging. Past publications showed [3], [4], [5] that the application of a tempering step after the Ni plating in order to form a Ni silicide can bring a significant improvement of the metal contact adhesion. Fig. 1 shows peel force data from Mondon et al. [5] of the improved adhesion of plated contacts after tempering. Though, the silicidation process comes along with the risk of silicide or metal spiking induced shunts [6]. To reduce this risk a deep selective emitter doping under the Ni contacts can be implemented.

This paper will focus on the correlation of tempering induced macroscopic problems in the IV characteristics of plated selective emitter solar cells to process related inhomogeneities on the micro scale. In this context different macroscopic and microscopic characterization techniques will be used to identify the source of the problem and give an opportunity for further process developments.

Section snippets

Device fabrication and cell results

Two different types of test structures were fabricated in order to evaluate challenges in the implementation of laser doped selective emitters in combination with plated front side contacts in an industrial type Al-BSF solar cell design.

First of all large area (156×156 mm²) Al-BSF silicon solar cells were fabricated in the High Eta lab and the PV technology evaluation center (PV-TEC) at Fraunhofer ISE using commercial grade 1–3 Ω cm p-type Cz-Si. The textured Cz-Si wafers were provided by

Cell results

The light IV characteristic results of the processed solar cells shown in Table 1 are measured after the first tempering step of the plated contacts at 350 °C for 2 min in a nitrogen atmosphere.

The plated solar cells show a higher cell efficiency level compared to the screen printed reference solar cells. The main benefit is visible in the open circuit voltage Voc and the short circuit current density Jsc due to a better saturation current density of the lowly doped emitter and a better blue

Conclusion

This work showed that high cell efficiencies of up to 19% are possible for Al-BSF solar cells with an industrially feasible lightly doped emitter (150 Ω), which could be contacted by applying plated Ni–Ag front side contacts. However, the adhesion of the plated contacts is quite challenging and tempering induced silicidation processes in order to improve the adhesion come along with the risk of a significant degradation of the IV characteristic of the solar cells.

The use of macroscopic

Acknowledgments

This work was funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety in the frame of the project SONNE (FKZ 0325277E). The authors would like to thank the project partners at SolarWorld Innovations GmbH and RENA GmbH for the valuable discussions and the financial support in the frame of the project. Furthermore, the processing support from the whole PV-TEC team at Fraunhofer ISE and the plating and UV laser process support from Norbert Bay and Armin

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