Hole‐Selective Ultrathin Al‐Doped SiO_x Passivation Layer Formed by Immersing in Aluminum Nitrate Aqueous Solution

Abstract

Herein, ultrathin Al‐doped SiOx layer formed by immersing in Al(NO3)3 aqueous solution is introduced. This layer enables high‐level surface passivation with a maximum effective surface recombination velocity (Seff,max) < 16 cm s−1 at an excess carrier density (Δn) of 1 × 1015 cm−3 for 2.5 Ω cm n‐type c‐Si without any other processes such as film deposition in a vacuum chamber, high‐temperature annealing, and hydrogenation. This passivation effect presumably comes from negative fixed charges localized in Al‐doped SiOx layers, forming inversion layers due to upward Si band bending at the SiOx/Si interface. In addition, the Al‐doped SiOx layers facilitate accumulated holes at the interface to tunnel through the layers even though the applied bias is extremely low. The high tunnel current density of holes accumulated in the inversion layers implies that these ultrathin Al‐doped SiOx layers are expected to be favorably implemented for high‐efficiency passivating‐contact c‐Si solar cells.

Ultrathin Al-doped SiO x passivating hole-selective contacts formed by a simple wet process

“…31) On the other hand, we demonstrated iV oc of 670 mV even without such a vacuum process and hydrogenation: just immersing Si wafers into Al(NO 3 ) 3 aqueous solution to form an inversion layer induced by Al-doped SiO x . 19) In this case, since the negative fixed charges exist in the fourcoordinated Al with O (AlO 4 − ) structure which stabilized adjacent to the SiO 2 structure, the AlO 4 − structure formation with increasing SiO 2 structure might enhance hole contact. In our previous investigation, hole tunneling current densities were gradually increased with increasing Al(NO 3 ) 3 treatment time throughout 30 min process time.…”

“…In our previous investigation, hole tunneling current densities were gradually increased with increasing Al(NO 3 ) 3 treatment time throughout 30 min process time. 19) However, when the AlO 4…”

“…In addition, we expect to improve the V oc to at least the same level as an iV oc of 670 mV which we reported in the previous work. 19) To obtain further V oc , Ag/Al-doped SiO x /n-Si interface should be modified not to be affected by fermi-level pinning to the metal-induced gap states. 22) The promising approaches are optimizing Al(NO 3 ) 3 treatment conditions and applying a conductive buffer layer between the electrode and the Aldoped SiO x layer, such as ITO, ZnO, MoO 3 , WO 3 , and so on.…”

“…In this research, we used FZ-grown n-and p-type Si(100) substrates with a resistivity of 2.5-3.0 Ω cm and a thickness of 280 μm, respectively. After the cleavage of Si substrates into 20 × 20 mm square pieces, we performed RCA cleaning 20) and the removal of chemical oxide by immersing into 1% diluted hydrofluoric acid (HF) aqueous solution, followed by Al(NO 3 ) 3 oxidation treatment under one of the best conditions we found in the previous study: 19) Al(NO 3 ) 3 concentration of 0.25 mol l −1 , treatment time of 12.5 min, and temperature of ∼95 °C. After the Al(NO 3 ) 3 oxidation treatment and following the deionized water rinse, N 2 blow was carried out until the sample surface was completely dried.…”

“…Therefore, our previous work developed a simple process: just dipping Si wafers into an aluminum nitrate [Al(NO 3 ) 3 ] aqueous solution to form ultrathin Aldoped SiO x , which contains ∼1 at% Al-O on the SiO x structures. 19) Good surface passivation and hole tunneling properties have been realized simultaneously in the highly transparent ultrathin layer. However, the origins of the good properties still need to be clarified.…”