Carbon nanotube – silicon heterojunction solar cells with surface ‐ textured Si and solution ‐ processed carbon nanotube films

Carbon nanotube (CNT)–silicon (Si) heterojunction solar cells are fabricated with surface-textured Si substrates. Using a dilute alkaline solution, common etchant in the Si solar cell industry, we formed a pyramidal texture on the Si substrate surface. The texture effectively enhances the absorption of the incident light, improving the short-circuit current density by ∼1.3-fold, up to 33.1 mA cm−2. We fabricated CNT–Si solar cells with a power conversion efficiency (PCE) of 10.4% without any anti-reflective coatings or doping of the CNTs. Moreover, the CNT films were prepared from commercialized CNT agglomerates by a mild solution-based process, which is well suited for the fabrication of CNT–Si solar cells with large area. We also achieved a PCE of 9.57% for a flat cell with careful removal of surfactant from and doping by nitric acid of the CNT films. These findings suggest that with the combination of surface-textured Si and solution-processed CNT films, efficient and low-cost CNT–Si solar cells may be realized.

The reflectivity of the flat (without texture) and textured Si surfaces without and with CNT films was measured using a UV-Vis spectrophotometer (Jasco V-650, Tokyo, Japan).The same Si substrates utilized in the fabrication of CNT-Si solar cells were used for this measurement.
The thermal oxide layer was fully removed by hydrofluoric acid.Two substrates were then textured by the method described in the Experimental section.The CNT films (T = 90% at 550 nm) washed for 70 min with hot water before drying were attached to one flat Si substrate and one textured Si substrate.
Texturing effectively decreased the reflectivity of the Si surface with CNT films; from 39.5% without texture to 14.9% with texture at a wavelength of 550 nm.The CNT-Si solar cells were assembled by attaching as-filtrated CNT films (T = 90%) to the Si substrates with texture.The V oc , FF, and PCE values (0.450 V, 0.438, 6.32%) are significantly lower than those with the CNT films washed before drying (0.536 V, 0.575, 9.87%, shown in Fig. S2).The impurities in the CNT films, SDBS or particularly Na contained in the SDBS molecules, possibly trap carriers and promote the carrier recombination.
Fig. S3.J-V curves of the CNT-Si solar cells with texture made using as-filtrated CNT films.
The hydrophobicity of three differently treated CNT films were estimated by contact angle measurement of water droplets.Three differently treated CNT films (T = 90%) were prepared on HF-treated Si wafers and contact angles were measured for 10-12 water droplets of 1 μL for each film (blue open circles).The mean contact angles (black closed circles) were 86.9°, 100.9°, and 118.0° with standard deviations (shown with red error bars) of 5.9°, 6.5°, and 7.6° for the film as-filtrated, that dried on Si, washed, and that washed before drying, respectively.This result suggests that the SDBS and/or its residue are removed from the CNT film surface by washing with hot water and it becomes more difficult to remove them once the film is dried.Two CNT films (T = 90%) were analyzed using FT-IR (Thermo Fischer Nicolet6700, Kanagawa, Japan) spectrometry.One film was dried on a Si substrate without post-treatment and the other film was washed by hot water before drying and transferring onto the Si substrate.For reference, SDBS aqueous solution was coated on a Si substrate, dried and analyzed.Some absorption peaks indicated by red arrows appeared for the SDBS layer coated on the Si substrate (red line).Characteristic peaks were observed for the C-H stretching vibration at ~2950 cm -1 and for the sulfonate group at ~1200 and 1000-1050 cm -1 . 37None of such peaks were present on the spectra of the CNT films (blue and green lines).The amount of SDBS in both of the films was presumably too small for detection.Three CNT films (T = 80%) were analyzed using XPS (JEOL JPS-9010TR, Tokyo, Japan).Note that Fe and S are used as catalysts in CNT synthesis. 34The optical transmittance at 550 nm was measured using UV-Vis spectrophotometer (JASCO V-630, Tokyo, Japan).The data were fitted by the following equation, where T, a, ρ, and R sheet represent optical transmittance, optical absorption coefficient, electric resistivity and sheet resistance, respectively.T = exp(-aρ/R sheet ) R sheet was measured by the 4-probe method.

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Fig. S1 UV-Vis reflection spectra of flat and textured Si surfaces without and with CNT films.

Fig. S4 .
Fig. S4.Contact angles of water droplets with differently treated CNT films.

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Fig.S5FT-IR spectra of SDBS, as-filtrated CNT film and CNT film washed before drying.
Fig. S7 XPS spectra and elemental composition of as-filtrated and washed CNT films on Si substrates.

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Fig. S8 Relationship between the optical transmittance and sheet resistance of the CNT films prepared on slide glass substrates.