Subramani Thiyagu

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.

Significance of the ZnO nanorod array morphology for low-bandgap polymer solar cells in inverted structures

This paper reports an inverted solar cell with ZnO nanorods for electron collection. Because ZnO materials could function as the electron transport layer in inverted organic solar cells, the ZnO nanorods further provided a large number of carrier extraction channels deep inside the organic active layer to improve carrier collection efficiency. The relationship between the ZnO nanorod array morphology and the device performance was systematically studied. Different ZnO nanorod morphologies could be controlled by the ratio of zinc nitrate and hexamethylenetetramine in a low-cost hydrothermal method, including the average spacing between individual nanorods and the surface area. For a maximum organic–inorganic contact area and good polymer infiltration, the morphology of the ZnO nanorod array has been optimized. When the ratio of zinc nitrate and hexamethylenetetramine was controlled at 1.0 : 0.6 in the growth-promoting solution, the surface area of the nanorod array was 7.55 times more than a planar ZnO structure. The ZnO nanorods significantly enhanced the performance of inverted PBDTTT-C-T:PC71BM solar cells and the power conversion efficiency of the device increased from 5.22% to 7.26%.

Efficient Planar Heterojunction Perovskite Solar Cells via Low-Pressure Proximity Evaporation Technique

The sequential deposition process is widely used to fabricate planar structure perovskite solar cells because of better uniformity and perfect surface coverage of perovskite films. However, it is difficult to control the crystallization of perovskite finely. Here, we improve the sequential deposition process by using a low-pressure proximity evaporation technique. The slow grain growth rate of perovskite (CH₃NH₃PbI₃) would make large crystallization and high quality of perovskite film. Moreover, we manipulate an innovative double-side interdiffusion process, that is, sequentially deposit CH₃NH₃I/PbI₂/CH₃NH₃I to fabricate the perovskite layer. This way, PbI₂ can be effectively converted into perovskite from a double side and achieve good control of CH₃NH₃I deposition on the surface as well. The devices reach an efficiency over 14%, which is the first time for TiO₂ free cells fabricated by a low-pressure approach.

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%.

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.

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.

Fabrication of Silicon thin film by metal-assisted chemical etching

We fabricate a Silicon thin film by metal-assisted chemical etching (MacEch). Generally, it is hard to make a large size silicon thin film. Here we demonstrate a low cost and simple method to lift off the silicon thin film from silicon wafer. Besides, the substrate is reusable, so we can fabricate many thin films from the same wafer. Thus, this method is competitive for commercial applications.

Fabrication of large-scaled synergetic silicon nanowire arrays using metal-assisted chemical etching for solar cell applications

We fabricate Silicon nanowire (SiNW) arrays for solar-cell applications on 6-inch wafers employing metal-assisted chemical etching (MacEtch). It can reduce cost and energy consumption. However it is difficult to make uniform SiNW arrays on large size wafer. Here we demonstrate a simple method to achieve a uniform SiNW on 6 inch wafers. Moreover, optical properties and surface morphologies of 6 inch N-type pyramid/SiNW arrays and 6 inch P-type as-cut/SiNW arrays are investigated. Reflectance indicates the extensive light-trapping effect by the SiNW arrays. Thus, the improved MacEtch method is promising for future commercial mass production on large size wafers.