Solution-Processed Titanium Oxide for Rear Contact Improvement in Heterojunction Solar Cells

Abstract

In this work, we demonstrated a heterojunction Si solar cell utilizing chemically grown titanium oxide (TiOx) as an electron-selective contact layer at its rear surface. With TiOx, the rear surface was passivated to reduce carrier recombination. The reverse saturation current, which is an indicator of carrier recombination, exhibited a 4.4-fold reduction after placing a TiOx layer on the rear surface. With reduced recombination, the open-circuit voltage increased from 433 mV to 600 mV and consequently, the power conversion efficiency (PCE) increased from 9.57 to 14.70%. By x-ray photoemission spectroscopy, the surface passivation was attributed to a silicon oxide interfacial layer formed during the chemical growth process. This passivation results in a 625 cm/s surface recombination velocity for the TiOx-passivated Si surface, which is 2.4 times lower than the sample without TiOx, ensuring the carriers pass through the rear contact without extensive recombination. According to these results, the band alignment for the heterojunction solar cell with and without a TiOx rear contact layer was plotted, the reduced interfacial recombination and the electron and hole blocking structure are the main reasons for the observed efficiency enhancement.

Optimization of a Solution-Processed TiOx/(n)c-Si Electron-Selective Interface by Pre- and Postdeposition Treatments

“…Lee et al presented a high surface recombination velocity (SRV) of 625 cm/s with an 8 nm TiO x layer reflected in a V oc of 600 mV at the cell level with a front side passivated by a a-Si:H(i)/a-Si:H(p) stack. 39 In fact, the current state-of-the-art ALD-deposited TiO x substantially outperforms the reported solution-processed TiO x -based CSCs; thus, further progress has yet to be made to achieve a competitive level of performance. In this context, this work studies the effects of predeposition termination of silicon surface dangling bonds as well as postdeposition annealing atmospheres to discover viable approaches for optimizing the electrical properties of the Si/TiO x interface.…”

“…Although TiO x was manufactured by the spin-coating method, the observed efficiency could not be attained without a (i)­a-Si:H interfacial passivation layer, which necessitates the use of the vacuum-based PECVD technique. Lee et al presented a high surface recombination velocity (SRV) of 625 cm/s with an 8 nm TiO x layer reflected in a V oc of 600 mV at the cell level with a front side passivated by a a-Si:H­(i)/a-Si:H­(p) stack . In fact, the current state-of-the-art ALD-deposited TiO x substantially outperforms the reported solution-processed TiO x -based CSCs; thus, further progress has yet to be made to achieve a competitive level of performance.…”

Sputtered Ultrathin TiO2 as Electron Transport Layer in Silicon Heterojunction Solar Cell Technology

“…The main advantages of this concept stem from avoiding (i) dopants, (ii) high-temperature processing, and (iii) hazardous gas precursors. Recently, ultrathin TiO 2 layers were successfully incorporated into SHJ solar cells as a hole-blocking interface acting as ETL [ 18 , 19 , 20 , 21 ]. Relative to n-type c-Si, TiO 2 has a small conduction band offset (Δ E c ≈ 0.05 eV), which allows electrons to pass through it, and a large valence-band offset (Δ E v ≈ 2.0 eV), which results in holes being blocked [ 22 ].…”