n-type emitters on p-type wafers

Boron-doped wafers are commonly used in the PV industry. Many excellent surface passivation techniques are available, facilitating the examination of emitters. Today, n-type emitters are commonly formed by POCl₃ diffusion [94].

As described in Section 5.1.1 many advantages justify the epitaxial deposition of emitters not only for epitaxial wafer equivalents, but also for silicon wafer solar cells. Figure 5-10: Homogeneity of the VOC over the left wafer of the deposition process.

5.3.1 Phosphine flow during cooling

The preliminary tests were performed on p-type <100> Fz wafers with resistivities of 2 to 7 Ωcm. The wafers were chemically treated in order to remove the saw damage, followed by an RCA cleaning. Our first investigation of a phosphorus-doped epitaxial emitter examines doping profiles with constant doping levels. Based on simulation results the doping level was first fixed to a carrier density of 4x10¹⁸ cm^-3 [97]. Figure 5-11 shows the SIMS-measured doping profiles of a sample cooled after the epitaxy in a H2 atmosphere (triangles) and a sample cooled in a H2/PH3 atmosphere (squares). The emitters have box-section profiles and result in sheet resistances of 47 and 34 Ω/sq., respectively. A decrease in the phosphorus concentration towards the surface is evident for the emitter cooled in pure hydrogen. This results from out-diffusion of phosphorus during the cooling process. Phosphorus out-diffusion in silicon at temperatures over 1000°C has already been reported [55, 112] and can be avoided when applying a PH₃/H₂ mixture during cooling. A flow of 35 sccm phosphine in hydrogen (7 ppm PH₃) increases the phosphorus concentration to more than 2x10¹⁹ cm^-3, as opposed to a decrease towards the surface to approximately 2x10¹⁸ cm^-3 when cooling without phosphine. The resultant emitter profile is similar to that of a blue-sensitive emitter obtained by POCl₃ diffusion. Usually, two diffusion steps are needed in order to form an emitter with a deeply-diffused, low carrier density layer and a higher surface concentration [96]. In our case, not only the out-diffusion of the phosphorus is prevented but also a diffusion with phosphine during cooling is achieved. This results in a two-layer system consisting of a grown epitaxial layer and a diffusion layer with a higher surface concentration. A low contact resistance between the n-type silicon surface and the evaporated metal contact is necessary to produce a solar cell with a high fill factor [80]. As the contact resistance increases with decreasing surface doping concentration, it is important to reach sufficiently high surface concentrations.

A first solar cell batch was produced by varying the phosphine concentration during cooling of the samples after the emitter deposition. The same solar cell process as described in Section 4.1.2 was applied. Figure 5-12 shows the fill factor and efficiency of the solar cells with epitaxial emitters on p-type wafers as a function of the phosphine concentration during the cooling step. Cooling with 7 ppm phosphine concentration exhibits a dramatic increase of the fill factor of approximately 10% absolute compared to no phospine gas flow. The best result

was obtained for a 2 µm thick epitaxial emitter with a V_OC of 593 mV, a fill factor of 75%, a J_SC of 28 mA/cm² and a cell efficiency of 12.6%. Further increases of the phosphine flow during the cooling step were investigated for EpiWE cells and are presented in Section 5.4.2.

0 1 2 3 4

10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸ 10¹⁹

0.00 0.02 0.04 0.06 0.08 0.10 10¹⁸

10¹⁹

Cooling with PH₃ without PH₃

Phosphorus [cm-3 ]

Depth [µm]

Figure 5-11: SIMS measurement of epitaxial emitters cooled with PH3 or without.

7 1 none 7 8 9 10 11 12 13

7 1 none 50

55 60 65 70 75

80 FF [%] η [%]

Phosphine concentration during cooling [ppm]

Figure 5-12: Fill factor FF and efficiency η dependence on the PH3 flow during cooling.

5.3.2 Solar cells with texturing

An enhancement of the short circuit current density JSC and therefore the efficiency can be achieved with front-side texturing. A new batch of wafer solar cells with texture and epitaxial emitters was fabricated on 0.5-1.7 Ωcm Cz wafers with thicknesses of approximately 210 µm. Firstly, the damage was chemically removed while simultaneously etching random pyramids. Figure 5-13-A shows scanning electron microscopy (SEM) measurements of a

cross-section with pyramids of a height of up to 15 µm. After the RCA cleaning, an optimised n-type epitaxial emitter was deposited, as will be described in Section 5.4.2. This emitter has a thickness of 1 µm with a doping of 5x10¹⁸ cm^-3 and received then a 119 ppm PH3 diffusion during cooling. The layers were deposited either at the standard deposition temperature of 1150°C or at a lower temperature of 1000°C to reduce the thermal stress to the wafer.

Figure 5-13-B shows the random pyramids after the emitter deposition. The shape of the pyramids has changed and the tip of the pyramid is less sharp. This indicates that the emitter is thinner at the pyramids cone point. However, the path length enhancement of the texture should only be affected marginally, as the inclination is similar.

Solar cells of these structures were fabricated. The best cell result was achieved for a cell with an epitaxial emitter grown at 1000°C. The cell efficiency reached 16.5% with an open circuit voltage VOC of 607 mV, a short circuit current density JSC of 34.4 mA/cm² and a fill factor of 79.0%. This sample exhibits a very low series resistance and low dark saturation currents.

Unfortunately, problems with the edge isolation occurred during this solar cell batch and the large inhomogeneity within the batch complicates a comparison of the results. New solar cells should be fabricated, including e.g. an optimised rear side. The passivated emitter, rear locally diffused (PERL) structure [110] and laser-fired rear contacts (LFC) [111] are the most promising approaches.

A

B

Figure 5-13: Cross-sectional SEM picture of random pyramids (A) and of random pyramids overgrown with an epitaxial emitter (B).

5.4