Origin of passivation in hole-selective transition metal oxides for crystalline silicon heterojunction solar cells

Abstract

Transition metal oxides (TMOs) have recently attracted interest as an alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type c-Si (n-Si) and three thermally evaporated TMOs (MoO 3 , WO 3 and V 2 O 5 ) was investigated by transmission electron microscopy and secondary ion-mass/x-ray photoelectron spectroscopy. For the oxides studied, chemical passivation of n-Si was attributed to an ultra-thin (1.9 -2.8 nm) SiO x~1.5 interlayer formed by chemical reaction, leaving oxygen-deficient species (MoO, WO 2 and VO 2 ) as byproducts. Field-effect passivation was also inferred from the inversion (hole-rich) layer induced on the n-Si surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function delta Δφ ≥1 eV). Therefore, the holeselective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower cost and simplify solar cell processing.

Simulation of an efficient silicon heterostructure solar cell concept featuring molybdenum oxide carrier-selective contact

“…From these analytical results, it can be inferred that the contact properties at the interface are dominated by the MoO x work function and should be chosen as high as possible to attain high efficiencies from CSC solar cells based on MoO x hole selective layer, which is in accordance with the reports by other groups. 29,31,44 According to this work, ψ lager than 5.5 eV is essential to create an excellent blocking potential against electrons as well as reducing the energy barrier for holes.…”

TiO2 Films from the Low‐Temperature Oxidation of Ti as Passivating‐Contact Layers for Si Heterojunction Solar Cells

“…Note that if an ionization process occurred during the measurement, then a reduced Ti 2+ oxide (TiO − ) signal is also detected for this 5 nm Ti film. At the TiO 2 /Si interface, a signal peak with an atomic mass unit of 59.87 is observed, corresponding to SiO 2 − ions . Note that each Ti ion (TiO − , TiO 2 − , TiO 3 − ) – contained spectrum shows significantly strong peaks at the Si/TiO 2 interlayer, indicating that Ti x (SiO y ) silicates may be present at this interlayer.…”

“…At the TiO 2 /Si interface, a signal peak with an atomic mass unit of 59.87 is observed, corresponding to SiO 2 À ions. [33] Note that each Ti ion (TiO À , TiO 2 À , TiO 3 À )contained spectrum shows significantly strong peaks at the Si/TiO 2 interlayer, indicating that Ti x (SiO y ) silicates may be present at this interlayer. This Ticontained silicate may responsible for the passivation property of this thermally oxidized TiO 2 film.…”

Dopant‐Free and Carrier‐Selective Heterocontacts for Silicon Solar Cells: Recent Advances and Perspectives

“…Bullock et al demonstrated that the ρ c and J 0c of MoO 3 on n‐Si (2.1 Ω cm) are quite high, though still applicable to high efficient c‐Si SC designs, with optimum values of 30 mΩ cm 2 and 300 fA cm −2 , respectively . As shown in Figure , Gerling et al extracted ρ c values of 110 mΩ cm 2 (V 2 O 5 ), 370 mΩ cm 2 (MoO 3 ), and 670 mΩ cm 2 (WO 3 ), all of which are close to the target series resistance values for most c‐Si SCs (0.1–0.5 Ω cm) . The lifetime values at 1 sun illumination are 240, 142, and 4.5 µs for V 2 O 5 , MoO 3 , and WO 3 , respectively, which translate into implied‐ V oc values of 653 (V 2 O 5 ), 637 (MoO 3 ), and 543 mV (WO 3 ), indicating a certain degree of surface passivation.…”