“…Therefore, it is unlikely that the huge decrease of the series resistance is solely caused by increasing the oxygen deficiency of the TiO layer. Indeed, recent studies 20 , 29 on samples with virtually identical selective contact processing conditions to ours showed that the contact resistance of SiO /TiO contacts is much higher compared to TiO ones and almost independent of TiO thickness in the as-deposited state, while for the annealed samples the contact resistances are almost identical and a clear dependence on TiO thickness is observed. This identifies the TiO layer as limiting the contact resistance in the annealed sample, but excludes it for the as-deposited state.…”
Section: Discussionsupporting
confidence: 70%
“…For data on the passivation quality of the contacts, the reader is referred to Ref. 29 , where passivation data is extensively studied. Figure 1 Characteristics of the investigated solar cells.…”
Carrier-selective and passivating SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
/Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO$$_{\rm x}$$
x
and TiO$$_{\rm y}$$
y
layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO$$_{\rm x}$$
x
. However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
/Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO$$_{\rm x}$$
x
layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.
“…Therefore, it is unlikely that the huge decrease of the series resistance is solely caused by increasing the oxygen deficiency of the TiO layer. Indeed, recent studies 20 , 29 on samples with virtually identical selective contact processing conditions to ours showed that the contact resistance of SiO /TiO contacts is much higher compared to TiO ones and almost independent of TiO thickness in the as-deposited state, while for the annealed samples the contact resistances are almost identical and a clear dependence on TiO thickness is observed. This identifies the TiO layer as limiting the contact resistance in the annealed sample, but excludes it for the as-deposited state.…”
Section: Discussionsupporting
confidence: 70%
“…For data on the passivation quality of the contacts, the reader is referred to Ref. 29 , where passivation data is extensively studied. Figure 1 Characteristics of the investigated solar cells.…”
Carrier-selective and passivating SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
/Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO$$_{\rm x}$$
x
and TiO$$_{\rm y}$$
y
layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO$$_{\rm x}$$
x
. However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO$$_{\rm x}$$
x
/TiO$$_{\rm y}$$
y
/Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO$$_{\rm x}$$
x
layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.
“…A similar phenomenon of lifetime recovery was also observed on SiO x /TiO x /Al heterocontacts by Yang et al [ 30 ] and Titova et al [ 24 ] They grew ≈1.2 nm thermal and ≈1.3 nm native SiO x prior to TiO x deposition, respectively. One possible explanation for the recovery of lifetime is its similarity to the well‐known “alneal” process, in which a thin Al layer is evaporated on the SiO 2 passivation layers before annealing.…”
Section: Resultssupporting
confidence: 79%
“…Recently, Titova and Schmidt used the dynamic infrared lifetime mapping technique to investigate the passivation of the fully metalized n ‐Si/SiO x /TiO x /Al heterocontact, demonstrating that the Al electrode resulted in passivation degradation. [ 24 ] To the best of our knowledge, a comprehensive investigation of the impact of metal electrode on the surface passivation of TiO x ‐based heterocontacts is still lacking. We reported some preliminary test results last year in a conference presentation, [ 25 ] based on which this article delivers further comprehensive results and discussion on: 1) a new heterocontact based on non‐doped TiO x , 2) accurate passivation evaluation with derived effective lifetime from PL images, and 3) understanding the passivation deterioration mechanism with XPS, STEM and energy‐dispersive X‐ray spectroscopy (EDX) characterization.…”
This article presents a comprehensive study regarding the impact of the Al electrode on the surface passivation of three TiOx‐based passivating selective contacts: TiOx:Al/LiFx/Al,TiOx/LiFx/Al, and a‐Si/TiOx:Al/LiFx/Al. A deterioration in passivation is recorded after the deposition of the Al electrode at close to room temperature, where the deterioration correlated to the Al thickness. A thin Al (10 nm) electrode resulted in the most severe passivation decline, while samples with a 100 nm Al electrode showed much less passivation deterioration. Furthermore, it is found that a low‐temperature annealing step led to a partial recovery of the passivation, particularly in the case of TiOx:Al/LiFx/Al and a‐Si/TiOx:Al/LiFx/Al contacts. The presented discovery in this article provides crucial insight into the importance of characterization and evaluation of passivating contacts, which is demonstrated here to be highly sensitive to the deposited metal thickness and the interfacial layers, as well as to the post‐deposition annealing.
“…[12][13][14] One of the most studied metal oxides in this context is titanium oxide (TiO x ). [15][16][17][18][19][20] It is successfully applied in organic and dopant-free asymmetric hetero-contact (DASH) silicon solar cells. Due to different stoichiometries, TiO x exhibits a wide range of work functions between 3.8 and 4.4 eV and doping levels between 1E16 and 3E19 cm À3 .…”
In solar cells with an optical cavity as a light trapping mechanism, parasitic absorption is among the main detrimental optical losses. One approach to reducing these losses is the implementation of charge carrier‐selective contacts with wide bandgaps using thin metal oxides like titanium oxide (TiOx). Herein, it is proved that TiOx is a suitable alternative for lossy n‐doped hydrogenated amorphous silicon (n a‐Si:H) as electron selective contact in ultrathin solar cells based on intrinsic hydrogenated amorphous germanium (a‐Ge:H). The integration of the TiOx layer leads to an improved photocurrent generation in the blue light wavelengths. Furthermore, a substantial extraction of charge carriers is demonstrated from the a‐Ge:H quantum well nanoabsorber embedded between intrinsic a‐Si barrier layers. However, the substitution of n a‐Si:H by TiOx results in limited photovoltaic performance due to a lower open‐circuit voltage. This effect is analyzed via electrical simulation considering a variation in the electron affinity and the doping of TiOx as well as the defect density in the silicon buffer. Moreover, the TiOx‐based solar cell exhibits improved light transmittance when the opaque back contact is omitted, which is beneficial for semi‐transparent applications.
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