Oxygen vacancies in tungsten oxide and their influence on tungsten oxide/silicon heterojunction solar cells

Abstract

Tungsten oxide (WO x ) can be incorporated into amorphous/crystalline silicon heterojunction solar cells as hole contact and for interface modification between p-type amorphous silicon and indium tin oxide. This paper aims at understanding the influence of tungsten oxides properties on silicon heterojunction solar cells. Using in-system photoelectron spectroscopy on thermally evaporated WO x layers, it was verified that WO x with a stoichiometry close to WO 3 features a work function close to 6 eV and is therefore suitable as hole contact on silicon. Additionally the oxygen vacancy concentration in WO x was measured using photoelectron spectroscopy. High oxygen vacancy concentrations in WO x lead to a low band bending in the WO x /silicon-junction. Furthermore solar cells were fabricated using the same WO x , and the band bending in these cells is correlated with their fill factors (FF) and open circuit voltages (V OC ).Combining these results, the following picture arises: Positively charged oxygen vacancies raise the Fermi-level in WO x and reduce the band bending at the WO x /silicon-junction. This, in turn, leads to reduced V OC and FF. Thus, when incorporating WO x into silicon solar cells it is important to minimize the oxygen vacancy density in WO x . Therefore deposition methods, enabling adjustment of the WO x stoichiometry are preferable.

Toward Annealing‐Stable Molybdenum‐Oxide‐Based Hole‐Selective Contacts For Silicon Photovoltaics

“…Figure a shows the changes in the MoOx[/a‐Si:H]/c‐Si band bending of our solar cells throughout annealing (temperatures of 100–250 °C, each step 5 min), as extracted from SPV measurements. Note that although the absolute values obtained with the setup used in this study are typically lower compared to the ones measured in other places, we are only interested here in relative differences, which correlate well with device properties . The band bending of the MoO X ‐based solar cell with the a‐Si:H(i) buffer layer is significantly reduced by annealing, whereas the changes in the MoO X cell without the a‐Si:H(i) layer are much smaller (especially between 170 and 210 °C).…”

“…The use of passivating contacts—like the ones used in silicon heterojunction (SHJ) solar cells—suppresses these recombination routes by separating the metal from the surface of the Si wafer and omitting heavy doping of the Si wafer. To reduce further the optical losses in passivating contacts, the application of wide‐bandgap materials like molybdenum‐, tungsten‐, titanium‐, or nickel‐oxide (MoO X ,, WO X , TiO 2 , NiO ), as well as lithium‐ or magnesium‐fluoride (LiF, MgF 2 ) has received much attention in recent years. These materials feature a work function that is either higher than the ionization energy, or lower than the electron affinity of crystalline Si (c‐Si).…”

“…Note that although the absolute values obtained with the setup used in this study are typically lower compared to the ones measured in other places, [23] we are only interested here in relative differences, which correlate well with device properties. [8] The band bending of the MoO X -based solar cell with the a-Si:H(i) buffer layer is significantly reduced by annealing, whereas the changes in the MoO X cell without the a-Si:H(i) layer are much smaller (especially between 170 and 210 C). The similar onset temperatures for band-bending reduction and H effusion from a-Si:H/MoO X stacks suggests that these effects are linked.…”

Crystalline silicon solar cells with tetracene interlayers: the path to silicon-singlet fission heterojunction devices

“…The design of the electron contact was based on the highefficiency silicon heterojunction concept, and was fabricated as described in ref. 88, omitting the conventional indium-doped tin oxide layer to reduce the device complexity. The result is a structure consisting of the crystalline silicon wafer, a thin intrinsic amorphous Si layer (5 nm) and a thicker highly-doped n-type amorphous silicon layer (15 nm).…”

Parasitic Absorption Reduction in Metal Oxide-Based Transparent Electrodes: Application in Perovskite Solar Cells

“…21 It was also recently shown that WO x requires a nearly perfect stoichiometry to achieve similar band bending as obtained with p-type amorphous silicon. 48 A CO 2 plasma, or another similar technique, could therefore be interesting to control and adjust the stoichiometry of sub-stoichiometric TMO layers deposited by thermal evaporation. MoO x /WO x stacks could also be explored as they could provide the better electronic properties of MoO x and at the same time the resilience towards plasma damages of WO x .…”