Minimizing POCl3磷扩散死层的最小化

15th International Photovoltaic Science & Engineering Conference (PVSEC-15) Shanghai China 2005 11-1 Minimizing the Dead layer of Industrial Phospho-rous Emitters Formed by POCl3 DiffusionPatrick Choulat1, Filip Duerinckx, Emmanuel Van Kerschaver, Frederic Dross, Guy Beaucarne1IMEC vzw, Kapeldreef 75, B-3001 Heverlee, BelgiumTel : +32 16 288 311, Fax : +32 16 281 501, E-Mail: Patrick.Choulat@imec.beAbstract: In this work, an analysis of various types of homogeneous emitters using POCl3 gas as a precursor for diffusion is described. It is known that heavily doped emitters usually contain a dead layer near the top surface. In this region, there is an excess of non-electrically active phosphorous, which largely contributes to a high emitter saturation current density and results in poor quantum effi-ciency in the blue wavelength region. The study looks into the detrimental impact of the dead layer on solar cell performance, and in particular at how the dead layer can be minimized while keeping a low sheet resistance. We used an industrial POCl3 diffusion furnace operating at reduced pressure. Different P diffusions were carried out on multicrystalline wafers, each targeting a sheet resistance of about 50 ohm/sq. The investigated process parameters were POCl3/Oxygen gas ratio and partial doping time. The samples were char-acterised by Secondary Ion Mass Spectroscopy (SIMS) and Spreading Resistance Profiling (SRP) on polished Cz substrates. Bifacial test structures to measure Joe, as well as industrial textured multicrystalline solar cell were processed and analysed. A partial doping time of 40% of a 30 min diffusion and low POCl3/O2 ratio were found to be optimum to minimise the impact of the dead layer.Key Words: Multi-crystalline solar cell, Dead layer, Homogeneous emitter.1 IntroductionThe creation of an emitter by diffusion of phosphorous from a POCl3 ambient at high temperature is a standard process in industry. Typical emitters have a low sheet resistance of about 50 Ohm/sq. The present trend is towards shallower emit-ters with higher sheet resistances, reducing useless absorption in highly doped material. This however presents several challenges. The variability in the diffusion process increases and special Ag pastes have to be used for such shallow emitters [1,2], with a narrow firing window. From a manufacturing point of view, maintaining sheet resistance at a low value is preferable. There are however possibilities to significantly improve emitter per-formance without increasing the sheet resistance.2 ExperimentalAll experiments were carried out in an industrial quartz furnace tube in a low pressure environment of 200 mbar. The reduced pressure provides an increase of gas velocity for better uniformity and allows only reactant gases to be present inside the tube. The two precursor gases, POCl3 and O2 are injected from 2 separate lines to ensure a complete chemical reaction on the wafer surface and to ensure a good uniformity of protective oxidation from Chlorine during stabilization step before diffu-sion.During the diffusion process the decomposition of the pre-cursors gases will form a Phosphorous Silicate Glass, PSG, on top of the Silicon surface. If the glass in contact with the Silicon is rich enough with Phosphorous, it will act as an infinite source and the surface concentration will be determined by the solid solubility of Phosphorous in Silicon at the temperature of the diffusion.3 Partial doping timeThe first experiment aimed at defining the minimum depo-sition time required to saturate the emitter sheet resistance. We define the total diffusion time as being the sum of the partial doping time (deposition time) + the drive in time. Several diffu-sions of 30 min at 860o C were run with successively shorter pe-riod of deposition time. Results are shown in Fig.1. One can see that a 30 min diffusion with 30 min deposition of Phosphorous leads to the same sheet resistivity as a diffusion of 30 min where POCl3 had been turned off after 12 min and followed by a 18 min drive in. Beyond this point, the emitter sheet resistivity starts to increase dramatically, giving rise to a loss of process controllability when aiming at a specific sheet resisitivity. Therefore, 12 min of deposition is sufficient to contain enough Phosphorous in the glass to saturate the surface of Silicon. However, although the value of the emitter resistivity remains unchanged for shorter deposition time, it’s believed that the amount of non electrically active Phosphorous should be less than where the dopant is supplied during the full duration of the diffusion and therefore leads to a reduction of the dead layer. A similar behavior was already observed for different diffusion temperatures [3]. For demonstration, monocrystalline samples were characterised by SIMS and SRP. Bifacial test structures to measure the emitter saturation current on Cz substrates, as welltial doping time for a 30 minutes diffusionTotal diffusion time = [partial doping time (deposition time) + drive in time]Fig.2 shows SRP and SIMS profiles of two diffusions that had the same total process time but different ratios of deposition time/drive-in time. The amount of phosphorus in the dead layeris about 25 % higher for the longer deposition time. Yet, the SRP profile remains identical and the sheet resistivity was measured to be 48 Ohm/sq for both diffusions. In parallel, in-dustrial multi-crystalline industrial solar cell were processed for 3 different partial doping times. One can see in Tab.1, a clear increase in both Jsc and Voc as the deposition time is reduced, even though the sheet resistance remains essentially the same. Emitter saturation current measurements also corroborate the characterisation analysis and show a reduction in Joe as the par-tial doping time is decreased with respect to the drive in time for a given total diffusion time of 30 minutes. The improvement in Jsc and Voc translates an absolute efficiency increase of 0.3%. Furthermore, a reduction of the partial doping time is all the more beneficial as the consumption of POCl3 gas can be consid-erably reduced.Fig.2 SIMS and SRP profile into a Boron doped sample for 2 different partial doping time. 12’/18’ corresponds to 12 min of deposition followed by 18 min of drive in4 POCl3/Oxygen RatioAs the diffusion profiles are determined by the concentra-tion of Phosphorous into the generated PSG formed onto the Silicon surface, it was also deemed necessary to study the im-pact of the ratio of the two precursors gases, POCl3 and O2 upon the extent of the dead layer. Various diffusions having different ratio of POCl3/O2 were performed at 860o C to aim at the same emitter resistivity for a fair comparison. Since the content of Phosphorous varies for each condition into the gas phase, it was obvious that the overall diffusion time had to be adapted to ob-tain a given emitter resistivity of about 50 Ohm/sq. 5 textured multicrystalline solar cells were fabricated for each condition and bifacial test structures on Cz substrates were processed to measure the emitter saturation current. The results are summa-rised in Tab.2.Although the total diffusion time is considerably reduced for the highest ratio of POCl3/O2, which is attractive for high in-dustrial throughput, the excess of Phosphorous into the gas phase however contributes to a large increase of Joe and thus probably to a wider dead layer near the Silicon surface, giving rise to a loss of Voc as well as Jsc on the cell level. Nevertheless, it should be pointed out from the Joe measurements that, one could have predicted a larger difference on the cell level be-tween the groups of gas ratio of 1 and 0.5. This might indicate that the saturation current from the base is still too high for such type of moderate quality of industrial substrates in comparison with the saturation current from the emitter in order to observe a larger difference. The same analysis based on higher quality ma-terials and/or on substrates having a higher emitter sheet resis-tance may reveal such a discrepancy.5ConclusionsVarious types of standard homogeneous emitters were in-vestigated, using an industrial POCl3 diffusion furnace. It was shown that the precipitated Phosphorous, which was described to be electrically inactive near the Silicon surface, can be sig-nificantly reduced without changing the measured values of in-dustrial emitter sheet resistance of about 50 Ohm/sq. A partial doping time of 40 % of a 30 minutes diffusion and low POCl3/O2 ratio were found to be optimum to minimise the im-pact of the dead layer, which led to an absolute efficiency gain of 0.3%.References[1] G. Schubert et al. Proc. 20th EUPVSEC, Barcelona: (2005) in print.[2] Mohamed M.Hilali et al. Proc. 19th EUPVSEC, Paris: (2004) 1300-1303[3] M. S. R. Heynes et al. Electrochemical Technology, 1967, Vol. 5, No. 9-10: 464-467Tab.1 IV parameters of industrial mc-Si solar cells of 1 Ohm.cm. Average values of 5 cells per groupPartial doping time / Drive in(minute) R□ (Ohm/sq) Jsc (mA/cm2) Voc(mV) FF(%) η(%) Joe(A/cm2)30’ / 0’ 46- 48 32.3 606 76.5 15 3.63E-1320’ / 10’ 46- 48 32.3 608 76.9 15.1 3.52E-1312’ / 18’ 47- 51 32.5 611 77 15.3 3.29E-13Tab.2 IV parameters of industrial mc-Si solar cells of 1 Ohm.cmPOCl3/O2 ratio R□ (Ohm/sq) Jsc(mA/cm2) Voc (mV) FF(%) η(%) Joe(A/cm2)3 47-4832.3860676.815.064.26E-131 47-4832.6660976.815.283.45E-130.5 47- 48 32.69 610 76.715.292.68E-13。