Thermal stability improvement of metal oxide-based contacts for silicon heterojunction solar cells

Abstract

Metal oxides are interesting materials for use as carrier-selective contacts for the fabrication of doping-free silicon solar cells. In particular, MoOx and TiOx have been successfully used as hole and electron selective contacts in silicon solar cells, respectively. However, it is of paramount importance that good thermal stability is achieved in such contacts. In our work, we combined i-a-Si:H/MoOx based hole contacts with electron contacts featuring i-a-Si:H/TiOx/low work function metal (ATOM) to fabricate doping-free cells, termed MolyATOM cells. We found that the thermal stability of the ATOM contact was improved when the i-a-Si:H was annealed (300 °C for 20 min in N2) before depositing TiOx (i.e. pre-TiOx annealing), which reduces the hydrogen content in i-a-Si:H by about 27 %rel, and thereby the H-related degradation of the ATOM contact characteristics. Moreover, it was found that reducing the thickness of the low-work function metal on top of the TiOx enhanced the thermal stability of the ATOM contact. With these adaptations, the MolyATOM cell efficiency was improved by 3.5 %abs, with the highest efficiency of 17.6 %. Moreover, the cells show improved thermal stability after the above-mentioned pre-TiOx annealing, which is confirmed by annealing tests at cell level as well as damp-heat tests at module level. The insights of this study could be used to tailor other metal-oxide based electron or hole contacts.

Low Work Function Ytterbium Silicide Contact for Doping-Free Silicon Solar Cells

“…The cell fabrication process flows are summarized in Figure a, with the corresponding schematic structures of the 3 cell types depicted in Figure b–d. All 3 cell structures investigated had the same hole-selective contact featuring the i-a-Si:H/MoO x /ITO/Ag stack on the front side. , On the rear side, different electron-selective contacts were applied: i-a-Si:H/Yb/Ag and i-a-Si:H/YbSi x /Ag as doping-free electron-selective contact candidates and i/n-a-Si:H/ITO/Ag as the classical SHJ reference. These contact structures will simply be referred to as Yb-contacted, MolYSili (i-a-Si:H/MoO x hole contact + i-a-Si:H/YbSi x electron contact), and classical SHJ electron contact cells, respectively.…”

“…As a consequence, many works in the field of doping-free solar cells have focused on attaining low J 0,metal without compromising on ρ c , with promising results. For the hole contact, high work function metal oxides, especially MoO x , have been investigated intensively. − For the electron contact, low work function metal oxides, − metal nitrides, , rare-earth metal fluorides, , and low work function metals ,− have been studied.…”

Temperature and environmentally stable titanium carbide as an electron-selective heterocontact for crystalline silicon solar cells

“…Recently, transition metal oxides (TMOs) and alkali metal fluorides have been widely investigated as electron-selective contact (ESC) materials, forming excellent Ohmic contacts to the lightly doped n-type c-Si with mΩ cm 2 scale contact resistivities. 8–38 The studies of ESC materials have further extended to low-work-function alkali or alkaline earth metals, 39,40 transition metals, 22 and their compounds such as halides, 36,41 oxides, 14,42,43 nitrides, 28,29 sulfides, 30,44 silicides, 45 carbonates 46 as well as organic compounds. 47,48 As a result, c-Si solar cells with dopant-free ESC have exhibited a high conversion efficiency beyond 23%.…”

Dual Functional Dopant‐Free Contacts with Titanium Protecting Layer: Boosting Stability while Balancing Electron Transport and Recombination Losses

“…The transparent conducting oxide (TCO) layers in HIT SCs are a successful example that ensures sufficiently lateral conductivity and separation of the metal electrodes from the doped a‐Si:H. However, for future‐oriented PV techniques, TCO layers are no longer a good choice due to their higher parasitic absorption as well as economic considerations. [ 32 ] On the other hand, the atomic‐layer‐deposition (ALD)‐grown films (TiO X , [ 14 , 33 , 34 , 35 ] TiN X , [ 36 ] TiO X N Y , [ 37 ] TaN X , [ 38 ] ZnO X , [ 39 , 40 ] etc.) have a high density, allowing them to be employed as protecting layers in innovative electron‐selective contact (ESC) designs.…”