Grain Boundary Passivation in Multicrystalline Silicon Using Hydrogen Sulfide

Abstract

A new grain boundary passivation method for multicrystalline silicon using hydrogen sulfide has been developed in this work. It has the added benefit of both hydrogen and sulfur for grain boundary passivation. Minority carrier lifetime of the samples is measured to monitor the effect of passivation. It is found that sulfur passivation takes place at higher temperatures, ∼100 • C higher, than hydrogen passivation, and sulfur passivation results in much higher lifetime gains than hydrogen passivation. Post-annealing in ambient further improves the lifetime of the samples, which is attributed to improved surface passivation on the p-type silicon samples by aluminum oxide. The highest lifetime gain achieved after post-annealing is 6750% over the control sample for hydrogen sulfide annealed samples vs. ∼2400% for forming gas annealed samples. Post-annealing also improves the stability of the passivation. Multicrystalline silicon (multi-Si) solar cells are the most popular among various cell technologies today. They are lower cost than monocrystalline silicon (mono-Si) solar cells, although they come with a lower efficiency. The current record efficiency for laboratory multi-Si cells is 20.4%, whereas it is 25.6% for mono-Si cells. 1One of the fundamental bottlenecks for higher-efficiency multi-Si solar cells is the defects along the grain boundaries such as dangling bonds. These defects act as recombination centers for minority carriers, thus reducing the minority carrier lifetime.2,3 To improve the efficiency of multi-Si cells, passivation of defects along the grain boundaries is important, along with good surface passivation.2 A number of grain boundary or bulk passivation methods have been developed for multi-Si solar cells. [4][5][6][7][8][9][10][11][12][13][14][15] These methods include passivation by hydrogen plasma, 4,5 low-energy hydrogen implantation, [6][7][8] forming gas annealing (FGA) 9,10 and hydrogenated silicon nitride (SiN x :H).11-15 Among them, SiN x :H passivation is widely employed because SiN x :H serves multiple purposes in multi-Si solar cells: antireflection, surface passivation and bulk passivation. An important commonality in all the methods above is that hydrogen bonding to dangling bonds is the mechanism responsible for grain boundary passivation. In metal-oxide-Si field-effect transistors, deuterium passivation has been demonstrated which terminates dangling bonds at the oxide/Si interface. 16 Sulfur is a double-valency element which has been proven effective in terminating dangling bonds on the Si(100) surface. Sulfur passivation of Si(100) can be performed either in solution [17][18][19] or from gaseous hydrogen sulfide (H 2 S).20 Using H 2 S for grain boundary passivation could take advantage of both hydrogen and sulfur. This allows sulfur to terminate double dangling bond sites, while hydrogen terminates single dangling bond sites along the grain boundaries. In this paper, we report a new method for grain boundary passivation using H 2 S. The experiment was carried out on p-type mult...

Impact of carrier doping on electrical properties of laser-induced liquid-phase-crystallized silicon thin films for solar cell application

“…5(b). However, the highest lifetime of ∼400 ns obtained at a carrier density of 2 × 10 15 cm −3 (undoped LPC-Si) is two or three orders of magnitude lower than the typical lifetime of multicrystalline 36) and monocrystalline silicon. 35) Although the dominant origin of the carrier recombination in LPC-Si has not been identified, the passivation of the intragrain defects and grain boundaries (e.g., by hydrogen plasma treatment 19,37,38) ) is expected to be a key factor for obtaining higher lifetimes.…”

Valence-Mending Passivation of Si(100) Surface: Principle, Practice and Application

“…the relative lifetime over a control sample, of p-type multicrystalline-Si wafers as a function of annealing temperature in H 2 S. Two peaks in lifetime are noticed. The peak at 450˚C is identified as due to H passivation of grain boundaries and the second peak at 525˚C is attributed to S passivation of grain boundaries [26]. Significant lifetime improvement up to 28-fold is noticed by S passivation.…”

“…Valence-mending passivation is also applied to grain-boundary passivation in multicrystalline-Si for solar cells [26]. Single-dangling-bond sites along grain boundaries require hydrogen (H) or group VII atoms to passivate, while double-dangling-bond sites take group VI.…”

“…Significant lifetime improvement up to 28-fold is noticed by S passivation. When this new grain-boundary passivation method is combined with surface passivation and post annealing, a 68-fold increase in minoritycarrier lifetime is demonstrated [26]. Improvement in efficiency of multicrystalline-Si solar cells is expected with S passivation of the grain boundaries.…”