Sreejith Karthikeyan

Revealing the Origins of 3D Anisotropic Thermal Conductivities of Black Phosphorus

Black phosphorus (BP) has emerged as a direct-bandgap semiconducting material with great application potentials in electronics, photonics, and energy conversion. Experimental characterization of the anisotropic thermal properties of BP, however, is extremely challenging due to the lack of reliable and accurate measurement techniques to characterize anisotropic samples that are micrometers in size. Here, we report measurement results of the anisotropic thermal conductivity of bulk BP along three primary crystalline orientations, using the novel time-resolved magneto-optical Kerr effect (TR-MOKE) with enhanced measurement sensitivities. Two-dimensional beam-offset TR-MOKE signals from BP flakes yield the thermal conductivity along the zigzag crystalline direction to be 84 ~ 101 W/(m*K), nearly three times as large as that along the armchair direction (26 ~ 36 W/(m*K)). The through-plane thermal conductivity of BP ranges from 4.3 to 5.5 W/(m*K). The first-principles calculation was performed for the first time to predict the phonon transport in BP both along the in-plane zigzag and armchair directions and along the through-plane direction. This work successfully unveiled the fundamental mechanisms of anisotropic thermal transport along the three crystalline directions in bulk BP, as demonstrated by the excellent agreement between our first-principles-based theoretical predictions and experimental characterizations on the anisotropic thermal conductivities of bulk BP.

In-situ stress and thermal stability studies of molybdenum bilayer back contacts for photovoltaic applications

Molybdenum bilayers have been used as back contacts for CIGS and CZTS based solar cell for years. Mo bilayer films undergo a heat treatment during the CIGS/CZTS deposition process which in turn changes the stress and morphological properties and affects the adhesion and morphological features of CIGS/CZTS layers. This work reports in-situ studies of Mo stress variation during anneals ranging from 20°C to 500°C and compares its morphological and electrical properties before and after heating to develop a thermally stable Mo bilayer. We have also used a low tensile stress bilayer developed in this way to demonstrate CZTS and CIAGS films with good adhesion properties.

Challenges in deposition of wide band gap copper indium aluminum gallium selenide (CIAGS) thin films for tandem solar cells

Copper indium gallium diselenide (CIGS) based solar cells have shown efficiencies > 20% on the lab scale and are already in commercial production. Even though the optimal band gap of 1.6eV to 1.7eV can be achieved by increasing the Ga content, these solar cells show a maximum efficiency at ~1.3eV and any further increase in the Ga concentration and band gap results in lower efficiencies due to bulk and interfacial traps. This also prevents the use of wide band gap CIGS layer as a top cell for harvesting the solar cell spectrum in a tandem cell configuration. This paper reports the manufacturing challenges on the production of wide band gap aluminum doped CIGS layers (CIAGS) and devices fabricated using this material. We have fabricated 11.3% efficient solar cells using the CIAGS absorber layers.

Synthesis and structural characterizations of Cu 2 ZnSnS 4 thin films

We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) thin films using thermal evaporation from copper, zinc and tin pellets and post-annealing in sulfur atmosphere at 600 °C. X-ray diffraction and Raman scattering results show that the films produced from a Cu/Sn/Cu/Zn precursor at 600°C for 4 hours are kesterite phase CZTS with traces of ZnS. Grain sizes of 1 um or above are obtained from SEM images. We explored the effect of the sulfurization temperature, sulfurization time and metal precursor evaporation sequence on the CZTS structure and film morphology. The beneficial effect of impurity diffusion in grain growth has also been observed in this work.

Single Phase Formation of CuInS2 Nanoparticles: Structural, Morphological, Thermal Studies with Annealing Effect

Single phase of CuInS2 nanoparticles was prepared by solid state melt growth process. The structural, morphological and elemental analyses were studied by using X-ray diffraction, Field Emission Scanning Electron Microscope and Energy Dispersive Analysis of X-rays. The differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) confirms the phase transformation and stability of CuInS2 with its annealing effect at 200oC. Micro-Raman studies evidencing a strong Raman A1 mode at 285 cm -1 and 302 cm -1 ,

Bulk and interface characterization and modeling of copper indium aluminum gallium selenide (CIAGS) solar cells

The saturation of VOc at larger band gaps in Cu(In, Ga)Se2 devices presents a major challenge in developing high efficiency solar devices for tandem solar applications. Although recent studies have shown that recombination at the buffer/absorber interface dominates in high Ga samples with wide band gaps, the interface parameters are not well understood to accurately model the device behavior. In this work we have applied temperature dependent CV and DLCP methods to estimate the interface state density along the bandgap in CIGS and CIAGS based solar devices. We have also used DLTS to study the nature of deep levels in CIGS and CIAGS devices. Based on our analysis and device simulation results, we attribute the VOC saturation in wide gap CIAGS devices to increased recombination rate at the interface.

Cu2ZnSnS4 thin film growth optimization and post rapid thermal annealing of solar cells and its influence on device performance

We investigated the synthesis of kesterite Cu2ZnSnS4 (CZTS) thin films using thermal evaporation from copper (Cu), zinc (Zn) and tin (Sn) pellets and post-annealing in a sulfur atmosphere. Two different metal precursor stacking orders (Cu/Sn/Zn/Cu and Cu/Sn/Cu/Zn) have been tested and experimental results prove that a layer of Cu on the top provides higher compositional uniformity and fewer secondary phases. CZTS thin films produced by a Cu/Sn/Zn/Cu precursor stacking order was used to study post rapid thermal annealing (RTA) influence on solar cell performance. The best performing solar cell using the CZTS thin film mentioned above with composition ratio Cu/(Zn+Sn) = 0.82, and Zn/Sn = 1.35 exhibited an open-circuit voltage of 433 mV, a short-circuit current of 7.78 mA/cm2, a fill factor of 46.23% and a conversion efficiency of 1.56%. Post RTA was conducted and an increase of device performance was observed. The solar cell which shows 1.56% at room temperature shows 2.72% efficiency after RTA at 350°C for 30 seconds. The cell performance improvement mechanism was studied.