Hakan Karaagac

Synthesis of ZnO nanowires and their photovoltaic application: ZnO nanowires/AgGaSe 2 thin film core-shell solar cell

In this investigation, hydrothermal technique was employed for the synthesis of well-aligned dense arrays of ZnO nanowires (NWs) on a wide range of substrates including silicon, soda-lime glass (SLG), indium tin oxide, and polyethylene terephthalate (PET). Results showed that ZnO NWs can be successfully grown on any substrate that can withstand the growth temperature (∼90°C) and precursor solution chemicals. Results also revealed that there was a strong impact of growth time and ZnO seed layer deposition route on the orientation, density, diameter, and uniformity of the synthesized nanowires. A core-shell n-ZnO NWs/p-AgGaSe 2 (AGS) thin film solar cell was fabricated as a device application of synthesized ZnO nanowires by decoration of nanowires with ∼700 nm thick sputtering deposited AGS thin film layer, which demonstrated an energy conversion efficiency of 1.74% under 100 mW/cm2 of simulated solar illumination.

Nanowire enabled photodetection

The physics and technology of nanowire (NW) photodetectors offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics and emerging negative index metamaterials based devices. The successful integration of NW photodetectors on CMOS compatible substrates and various low cost substrates would enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this chapter, we review the advantages of NW-based photodetectors, current device fabrication and integration schemes, practical strategies, recent device demonstrations and discuss the numerous technical design challenges. We present discussions on (i) low resistance contact and interfaces for NW integration, (ii) high speed design and impedance matching, and (iii) CMOS compatible mass-manufacturable device fabrication. We also offer a brief outlook into the future perspectives on material growth and the future communication, consumer and defense application opportunities for NW photodetectors.

One-Dimensional Nano-structured Solar Cells

The solar light harvesting has long been regarded as promising way to meet the increasing world’s annual energy consumption as well as the solution to prevent the detrimental long-term effect of carbon-monoxide emission released by fossil fuel sources. Due to the high cost of today’s conventional PV technology, however, it is not possible to compete with the energy supplied from fossil fuel sources. The use of one-dimensional nanostructures, including nanowires (NWs), nanorods (NRs), nanopillars (NPs) and nanotubes (NTs) in solar cells with different device architectures (e.g. axial, radial, and nanorod/nanowire array embedded in a thin film) provides peculiar and fascinating advantages over single-crystalline and thin film based solar cells in terms of power conversion efficiency and manufacturing cost due to their large surface/interface area, the ability to grow single-crystalline nanowires on inexpensive substrates without resorting to complex epitaxial routes, single-crystalline structure and light trapping function. In this chapter, we review the recent studies conducted on nanowire/nanorod arrays based solar cells with different device architectures for the realization of high-efficiency solar cells at an economically viable cost.

Theoretical limits of the multistacked 1D and 2D microstructured inorganic solar cells

Recent studies in monocrystalline semiconductor solar cells are focused on mechanically stacking multiple cells from different materials to increase the power conversion efficiency. Although, the results show promising increase in the device performance, the cost remains as the main drawback. In this study, we calculated the theoretical limits of multistacked 1D and 2D microstructered inorganic monocrstalline solar cells. This system is studied for Si and Ge material pair. The results show promising improvements in the surface reflection due to enhanced light trapping caused by photon-microstructures interactions. The theoretical results are also supported with surface reflection and angular dependent power conversion efficiency measurements of 2D axial microwall solar cells. We address the challenge of cost reduction by proposing to use our recently reported mass-manufacturable fracture-transfer- printing method which enables the use of a monocrystalline substrate wafer for repeated fabrication of devices by consuming only few microns of materials in each layer of devices. We calculated thickness dependent power conversion efficiencies of multistacked Si/Ge microstructured solar cells and found the power conversion efficiency to saturate at 26% with a combined device thickness of 30 μm. Besides having benefits of fabricating low-cost, light weight, flexible, semi-transparent, and highly efficient devices, the proposed fabrication method is applicable for other III-V materials and compounds to further increase the power conversion efficiency above 35% range.

Synthesis of one-dimentional nanostructures for gas sensing and photovoltaic applications

In this work, three-dimensional (3-D) p-n junctions were formed for the fabrication of field ionization gas sensors and solar cells. P-Si micro-pillars/ZnO NWs, n-TiO2-nanorod/p-CdTe and n-Si-NW/p-CuInSe2(CIS) material combinations were preferred for the construction of p-n hetero-junction solar cells. Vertically well-aligned Si NWs were synthesized over the surface of n-type silicon wafer by using electroless etching technique. The synthesized Si-NWs embedded into a sputter deposited mono-phase chalcopyrite thin film (CIS) for the realization of nanowire array embedded in thin film type inorganic solar cell, which exhibited a 1.51% power conversion efficiency. In addition to Si nanowires, high aspect ratio vertically well- oriented p- silicon micropillars (MPs) were also synthesized using deep reactive ion Etching (DRIE) process with the BOSCH recipe of cyclical passivation and etching. Three-dimensional (3D) p-Si-MPs/n-ZnO-NWs heterostructures were constructed from hydrothermally grown dense arrays of ZnO nanowires onto these p-type silicon micropillars. The device structures were tested for both the field ionization gas sensor and photovoltaic applications, which showed very promising results. As a final part of this study, TiO2 nanorods (NRs) were grown on FTO glass substrates by using hydrothermal technique, which is sequentially coated with CdTe thin film (sputtering) and subjected to CdCl2 chemical solution treatment to fabricate a core-shell model solar cell with a power conversion efficiency over 0.4% power conversion efficiency.

Enhanced field ionization current enabled by gold induced surface states to silicon nanowires

The employment of nanosized materials has gained much interest for the fabrication of field ionization gas sensors (FIGS) since they have many advantageous properties such as low cost, high sensitivity and high selectivity. In this work, we introduce a physical gas sensor using Si Nanowires (NW) configured as anode. These NWs are synthesized by using electroless etching (EE) technique, a cost effective and scaleable process for vertically aligned Si NWs. A thin layer of gold (Au) coating is subsequently applied to improve the field ionization current by introducing unoccupied local states. Characterization of pristine Si NWs and Au doped Si NWs in terms of current and voltage is done under NH3 and O2 gases. Our structures show more than five orders of magnitude enhanced field ionization current due to unoccupied local states formed by Au doping.

3D silicon micro-pillars/-walls decorated with aluminum-ZnO/ZnO nanowires for opto-electronic device applications

In this paper, high aspect ratio vertically oriented p-silicon (100) micropillars and microwalls were fabricated using the deep reactive ion etching (DRIE) process with the BOSCH recipe of cyclical passivation and etching. Two different patterns were etched; uniform pillar arrays of dimensions ~15µm (height) x 2µm (diameter) and wall arrays of dimensions ~1.5µm (width) x 25µm (height). Three-dimensional (3D) heterostructures of n-ZnO/p-Si heterostructures were fabricated from growing hydrothermally dense arrays of ZnO nanowires (290-400 nm in length and 48-80 nm in diameter) and depositing Aluminum-ZnO (AZO) thin film onto the high aspect ratio vertically oriented p-silicon micropillars and microwalls. The performances of the fabricated heterostructure optoelectronic devices were characterized for different applications including solar cells, photodetectors and field ionization gas sensors.