Chien-Ting Liu

Low-Pressure-Assisted Coating Method To Improve Interface between PEDOT:PSS and Silicon Nanotips for High-Efficiency Organic/Inorganic Hybrid Solar Cells via Solution Process

UNLABELLED Nanostructured silicon hybrid solar cells are promising candidates for a new generation photovoltaics because of their light-trapping abilities and solution processes. However, the performance of hybrid organic/Si nanostructure solar cells is hindered because of carrier recombination at surface and poor coverage of organic material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS) on nanostructures. Here we demonstrate low-pressure-assisted coating method of PEDOT PSS on surface-modified silicon nanotips with broadband light-trapping characteristics to improve interface property and to achieve high-efficiency hybrid solar cells through a solution process. The approach enhances the effective minority-carrier lifetime and the coverage of PEDOT PSS on the surface of nanostructures. Hybrid solar cells fabricated with surface-modified nanotips exhibit a high fill factor of 70.94%, short-circuit current density of 35.36 mA/cm(2), open-circuit voltage of 0.528 V, and power conversion efficiency of 13.36%. The high efficiency and the high fill factor are achieved because of conformal coating of PEDOT PSS via a low-pressure-assisted coating process, excellent light harvesting without sacrificing the minority-carrier lifetime, and efficient charge separation/collection of photogenerated carriers.

Optical trapping enhancement from high density silicon nanohole and nanowire arrays for efficient hybrid organic–inorganic solar cells

In this paper, we employ a series of metal-assisted chemical etching processes to fabricate low-cost silicon nanohole (SiNH) and silicon nanowire (SiNW) arrays for hybrid solar cells. The SiNH arrays and SiNW arrays are obtained by a two-step etching and one-step etching technique, respectively. Length and depth of SiNWs and SiNHs can be controlled by etching time. The SiNH arrays demonstrate higher optical trapping effect than SiNW arrays, resulting in leading performance power conversion efficiency of 11.25% in the hybrid organic–inorganic solar cells. SiNH arrays have a high surface area, compared to SiNW arrays, so they can give rise to more junction area in the organic–inorganic heterojunction structures. In addition, these SiNH arrays possess additional advantages of robust structures and higher density with low air-filling fraction as compared to SiNW arrays. Furthermore, the SiNH arrays show superior efficiency to SiNW arrays experimentally. In particular, the fabricated SiNH arrays with high density can suppress the optical reflection well below 5% over a broad wavelength range from 300 to 1100 nm in a short nanohole depth. The very low reflectance and excellent light trapping property are attributed to the sub-wavelength dimension of the SiNH structure. These SiNH arrays not only facilitate the optical trapping, but also provide efficient broadband and omnidirectional photon harvests for cost-effective future nanostructured photovoltaics.

Interface modification for efficiency enhancement in silicon nanohole hybrid solar cells

In this paper, the interface between Si nanoholes (SiNHs) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated and improved to achieve high-efficiency SiNH/PEDOT:PSS hybrid solar cells. The high-density SiNHs are fabricated using short-time Ag deposition before metal-assisted chemical etching (MacEtch) method. Also, a polymer coverage method is explored to overcome the difficulty of PEDOT:PSS infiltration into SiNHs. PEDOT:PSS is mixed with co-solvent dimethylsulfoxide (DMSO) to have better polymer infiltrate into SiNHs via two-step coating process. This technique significantly improves the interface between SiNHs and PEDOT:PSS; the greatly reduced contact angle from 90° to 16° at the interface of Si and PEDOT:PSS has established this fact. In addition, the minority carrier lifetime is dramatically increased from 31.52 to 317.20 μs. The property improvement enables the SiNH/PEDOT:PSS hybrid solar cell with high Jsc of 36.80 mA cm−2, Voc of 0.524 V, FF of 66.50%, and thus PCE of 12.82%. Also, the SiNH structures have an excellent light-trapping effect, which contributes to very low average total reflectance of 3%, due to internal multiple reflections caused by subwavelength features. At an angle of incidence up to 60°, the specular reflectance maintains at as low as 1%; even at a large angle of 70°, the reflectance is still below 10%. This work provides a feasible solution process to fabricate SiNH structure and to improve organic/Si hybrid solar cells in energy and cost-effective manner.

Thermal oxide, Al 2 O 3 and amorphous-Si passivation layers on silicon

The effective passivation needs (1) higher bandgap than Si with type 1 alignment, (2) low interface density at the interface between passivation layer and Si, and (3) ionized charges for field effect passivation. The thermal oxide (SiO2) with low interface defect density seems most effective but requires high growth temperature (900 °C). Al2O3 with trapped negative fixed charges can serve as the field effect passivation. Moreover, doped amorphous Si can also have the field effect passivation with the controlled ionized charge density. The effective life time is measured by quasi-steady-state photoconductance (QSSPC). Photoluminescence (PL) measurement is consistent with QSSPC, and can probe a local area with mapping ability on large samples. The dependence of PL intensity on surface recombination velocity is theoretically studied. The passivation of a-Si becomes less effective after crystallization at high temperature annealing, indicating the larger bandgap is necessary.

Flexible silicon thin film organic/inorganic hybrid solar cells

Compare with different thickness silicon thin film, 30μm, 50μm and 100μm, we demonstrate a promising way to form 30μm silicon thin film hybrid solar cells with good bending property, by using the silicon nanostructure (SiNS) to achieve low reflection and increase heterojunction areas, the device performance is enhanced to 10.15%.

Thorough organic/Si nanostructure heterojunction provided by surfactant assisted PEDOT:PSS

In this work, water, isopropyl alcohol (IPA), and Zonyl fluorosurfactant are applied on PEDOT:PSS to try to improve PEDOT:PSS attachment on Si nanowires (SiNWs) thoroughly. After treatment, only Zonyl fluorosurfactant assisted PEDOT:PSS has thorough coating profiles on SiNWs. However, water and IPA diluted PEDOT:PSS only touch the top portion of SiNWs. Therefore, Zonyl-PEDOT:PSS/SiNW solar cells generate highest PCE of 10.87%, Jsc of 28.86 mA/cm2, and FF of 71.75%. In comparison, the water-PEDOT:PSS/SiNW and IPA-PEDOT:PSS/SiNW devices have lower PCE of 9.22% and 8.66%, respectively.

Modified Silicon nanotips with improved carrier lifetime by using solution process for efficient solar cells applications

In this work, a simple solution process (metal-assisted wet chemical etching [MacEtch] method) is used to fabricate high-density silicon nanohole (SiNH) arrays on n-type wafer. SiNH arrays generally produce a large surface-area-to-volume ratio, so that aid for strong light trapping effect between the nanostructures causes high absorption and charge collection via the formation of a core-sheath p-n junction. However, the photogenerated excited carriers are easily trapped by high-density surface defects due to higher surface area prolonging to depth of nanohole (NH). To reduce the surface defects and metal contamination of SiNHs formed by metal-catalyst etching, it is important to further proceed to feasible simple solution treatment. Applying the chemical polishing etching (CPE) treatment to SiNH surface leads to smooth and contamination-free surface. In addition, all the processes mentioned here are energy and cost-efficient.

Novel fabrication of Si thin film for solar cell applications

We demonstrate a novel method to fabricate crystalline silicon thin film for solar cell applications. It can reduce material cost and energy consumption. Here, we adopt a multi-step metal-assisted chemical etching procedure by using silver as catalyst to form silicon thin film. Through engineering the etching direction, this method provides possibility to fabricate transferable silicon thin film. Moreover, the substrate can be reused for further thin film fabrication.

Silicon nanowire/organic hybrid solar cells with zonyl fluorosurfactanct treated PEDOT:PSS

In this work, we added Zonyl fluorosurfactant into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to improve the affinity of PEDOT:PSS and silicon nanowire (SiNW) arrays. The concentration of Zonyl fluorosurfactant is 0%, 0.1%, 0.5%, 1%, and 10%. The 0.5%-Zonyl treated PEDOT:PSS/SiNW solar cell has the highest open-circuit voltage of 0.541 V, but the best efficiency is the device with 0.1%-Zonyl treated PEDOT:PSS. The efficiency is 9.18%.

High efficiency hybrid organic/silicon-nanohole heterojunction solar cells

In this work, a simple method of solution process to fabricate high density Silicon nanohole (SiNH) arrays on n-type wafer is experimented. SiNHs exhibit very low reflectance from range of wavelength 300 to 1100 nm irrespective of the angle of incidence, better than Si nanowires. The SiNH arrays have a strong light trapping effect between the nanostructures causes high absorption. We experimentally demonstrate high-efficiency organic-inorganic hybrid solar cells, Si/PEDOT:PSS with silicon nanoholes. Such Si/PEDOT:PSS hybrid solar cells exhibit high Jsc of 36.80 mA/cm2, Voc of 0.52V, FF of 66.50%, and thus power conversion efficiency (PCE) of 12.72%. SiNH arrays produce a large surface-area-to-volume ratio, hence allowing efficient light harvesting and charge collection via the formation of a core-sheath p-n junction.