Ultra‐thin electron collectors based on nc‐Si:H for high‐efficiency silicon heterojunction solar cells

Abstract Low parasitic absorption and high conductivity enable ( n )‐type hydrogenated nanocrystalline silicon [( n )nc‐Si:H], eventually alloyed with oxygen [( n )nc‐SiO x :H], to be deployed as window layer in high‐efficiency silicon heterojunction (SHJ) solar cells. Besides the appropriate opto‐electrical properties of these nanocrystalline films, reduction of their thickness is sought for minimizing parasitic absorption losses. Many strategies proposed so far reveal practical limits of the minimum ( n )‐layer thickness that we address and overcome in this manuscript. We demonstrated the successful application of an ultra‐thin layer of only 3‐nm‐thick based on ( n )nc‐Si:H PECVD plasma growth conditions without the use of additional contact or buffer layers. For simplicity, we still name ( n )nc‐Si:H this ultra‐thin layer and the solar cell endowed with it delivers a certified efficiency η of 22.20%. This cell shows a 0.61 mA/cm 2 overall J SC gain over the ( n )a‐Si:H counterpart mainly owing to the higher transparency of ( n )nc‐Si:H, while maintaining comparable V OC > 714 mV and FF > 80%. Our optimized ( n )nc‐Si:H layer yields low absorption losses that are commonly measured for ( n )nc‐SiO x :H films. In this way, we are able to avoid the detrimental effect that oxygen incorporation has on the electrical parameters of these functional layers. Further, by applying a MgF 2 /ITO double‐layer anti‐reflection coating, a cell with 3‐nm‐thick ( n )nc‐Si:H exhibits a J SC,EQE up to 40.0 mA/cm 2 . By means of EDX elemental mapping, we additionally identified the ( n )nc‐Si:H/ITO interface as critical for electron transport due to unexpected oxidation. To avoid this interfacial oxidation, insertion of a 2‐nm‐thick ( n )a‐Si:H on the 3‐nm‐thick ( n )nc‐Si:H contributes to FF gains of 1.4% abs. ( FF increased from 78.6% to 80.0%), and showing further room for improvements.