Nitride layer screening as carrier-selective contacts for silicon heterojunction solar cells

Abstract

A three-architecture method for screening new materials' viability as carrier-selective contacts for silicon solar cells is presented. Test-structure solar cells were fabricated with a standard silicon heterojunction contact for the front side, and one of three treatments on the back: bare silicon, intrinsic amorphous silicon (i-aSi:H), or n-type amorphous silicon on an i-aSi:H passivation layer. Then, the candidate contact material of interest was deposited on the back of each test structure and the cells were finished with evaporated Al. By analysing the current-voltage characteristics of each type of architecture, the carrier selectivity and surface passivation quality of novel contact materials or material stacks can be rapidly screened. To demonstrate the utility of this method, we present a preliminary investigation of nitride compounds as electron-selective contacts in silicon solar cells, in particular zinc tin nitride (ZnSnN2), which is naturally n-type and has favourable band alignments with c-Si. ZnSnN2 was deposited by reactive sputtering as an electron contact on each test structure. No passivation was observed, but decent electron-selectivity was observed when in direct contact with the silicon wafer. When combined with intrinsic amorphous silicon, poor performances were obtained with poor selectivity and the occurrence of an S-shape, likely due to insufficient selectivity of the ZTN. Similar results were obtained for 2 nm and 20 nm thick layers, indicating selectivity did not result from the Al over-layer, but was in fact due to the ZTN itself. Overall, this work shows the three-architecture screening method is a useful tool for assessing novel contact materials for silicon heterojunction solar cells.

Dopant‐free passivating contacts for crystalline silicon solar cells: Progress and prospects

“…Reliable doping strategies or chemical potential tuning of relevant elements (e.g., oxygen) could be promising solutions and deserve efforts for further development 150,169 . Besides, other than the common metal oxides, materials such as III‐V compounds, 109 ternary semiconductors, 108 or hybrid semiconductors 225–227 could be further explored as potential CTLs due to their proper band matching with c‐Si and the high electrical conductivities 228,229 The instability of CTLs and its incompatibility with adjacent layers due to interfacial reactions or processing constrains need to be properly addressed.…”

“…In addition to oxides, other types of metal compounds such as nitrides, 59,108–111 sulfides, 112–114 halides, 115,116 fluorides, 63,117 phosphides, 118 and alkali/alkaline‐earth metal carbonates have also been exploited as dopant‐free passivating contact for c ‐Si solar cells 119,120 . Great attention has been paid to developing ETLs such as TiN x, 59 TaN x, 111 MgF x, 63 LiF x, 121 and so forth 122 .…”

“…Contact structure τ (μs) J 0c (fA/cm 5C) and a 4-nm MoO x layer contributes to the best cell performance showing a certified efficiency of 23.5% (Figure 5D). 48 In addition to oxides, other types of metal compounds such as nitrides, 59,[108][109][110][111] sulfides, [112][113][114] halides, 115,116 fluorides, 63,117 phosphides, 118 and alkali/alkaline-earth metal carbonates have also been exploited as dopant-free passivating contact for c-Si solar cells. 119,120 Great attention has been paid to developing ETLs such as TiN x,…”

Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

“…Unique materials include CuI, which is the only p-type material of this section and enabled over 20%-efficient devices when combined with Ag as partial rear contact. 346 Another one is ZnSnN 2 , which showed underwhelming performance, 347 but belongs to the very vast and underexplored family of ternary metal-nitrides. This family includes a very large number of thermodynamically stable compositions with relevant material properties and might deserve further exploration.…”

Gallium Nitride as Transparent Electron-Selective Contact in Silicon Heterojunction Solar Cells

“…Solar cells were prepared on textured, 4", n-type float zone wafers with resistivity of 2-3 Ωcm, and thickness of 180-220 µm. Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit front thin-film silicon layers to fabricate the 1x1 cm 2 standard all-silicon reference cell as described elsewhere [7]. For back GaN layers, PECVD conditions were as follows: tri-methyl gallium (TMG), N2, and H2 were used as precursor gases in a 110 MHz plasma held at 30 W power for all depositions.…”