Black silicon significantly enhances phosphorus diffusion gettering

Abstract

Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm−3 to less than 1010 cm−3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.

Impact of black silicon on light‐ and elevated temperature‐induced degradation in industrial passivated emitter and rear cells

“…Chakraborty et al presented similar results and reported that phosphorus diffusion gettering reduced the concentration of light‐induced defects, which were suspected to be Cu, Ni, and Ti, by nearly one order of magnitude . Interestingly, our previous study demonstrated that heavy phosphorus‐doping in the b‐Si spikes after an industry‐typical POCl 3 diffusion process enhances iron segregation to the emitter, which substantially improves minority carrier lifetime in iron‐contaminated substrates . Consistent with earlier reports, sheet resistance is indeed substantially lower in the b‐Si than in the acidic‐textured cells (~50 Ω/□ vs ~80 Ω/□).…”

“…Furthermore, the large surface area of b‐Si is no longer a hindrance for high‐efficiency b‐Si solar cells due to the advancements in novel surface passivation methods . In fact, it has been recently shown that the large surface area can provide also an advantage, since it significantly enhances gettering of deleterious metal impurities during phosphorus diffusion …”

“…1,[3][4][5] In fact, it has been recently shown that the large surface area can provide also an advantage, since it significantly enhances gettering of deleterious metal impurities during phosphorus diffusion. 6 The evidence for enhanced gettering implies that b-Si could also influence light-and elevated temperature-induced degradation (LeTID), which is the main culprit for efficiency deterioration in today's multicrystalline silicon (mc-Si) solar cells. 7,8 LeTID may result in over 10% relative efficiency loss in cells with dielectrically-passivated rear side, 7,9,10 which is a significant issue for the PV industry that is shifting towards passivated emitter and rear cells (PERC).…”

Black silicon back‐contact module with wide light acceptance angle

“…Black silicon surfaces can be achieved using different techniques that can be easily incorporated in the PV industry such as metal‐assisted wet‐chemical etching (MACE), dry reactive ion etching (RIE), inductive couple plasma–reactive ion etching (ICP‐RIE), laser structuring, and electrochemical etching . Additionally, collateral benefits of b‐Si have arisen recently improving low‐cost substrates by removing saw surface damage in diamond‐wire‐sawn (DWS) wafers or increasing the gettering effect during doping diffusion on Czochralski (CZ) c‐Si solar cells . All these advantages make black silicon etching a very attractive candidate to replace conventional acidic and alkaline texturizing approaches in the PV fabrication chain.…”

“…4 Additionally, collateral benefits of b-Si have arisen recently improving low-cost substrates by removing saw surface damage in diamond-wire-sawn (DWS) wafers 5 or increasing the gettering effect during doping diffusion on Czochralski (CZ) c-Si solar cells. 6 All these advantages make black silicon etching a very attractive candidate to replace conventional acidic and alkaline texturizing approaches in the PV fabrication chain. For instance, Modanese et al 7 demonstrated the economic viability of replacing acidic texturing by ICP-RIE b-Si nanostructures using a convectional industrial process on the basis of mc-Si substrates.…”