Shaibal K. Sarkar

Inorganic Hole Conducting Layers for Perovskite-Based Solar Cells

Hybrid organic-inorganic semiconducting perovskite photovoltaic cells are usually coupled with organic hole conductors. Here, we report planar, inverse CH3NH3PbI3-xClx-based cells with inorganic hole conductors. Using electrodeposited NiO as hole conductor, we have achieved a power conversion efficiency of 7.3%. The maximum VOC obtained was 935 mV with an average VOC value being 785 mV. Preliminary results for similar cells using electrodeposited CuSCN as hole conductor resulted in devices up to 3.8% in efficiency. The ability to obtain promising cells using NiO and CuSCN expands the presently rather limited range of available hole conductors for perovskite cells.

Atomic Layer Deposited Molybdenum Nitride Thin Film: A Promising Anode Material for Li Ion Batteries

Molybdenum nitride (MoNx) thin films are deposited by atomic layer deposition (ALD) using molybdenum hexacarbonyl [Mo(CO)6] and ammonia [NH3] at varied temperatures. A relatively narrow ALD temperature window is observed. In situ quartz crystal microbalance (QCM) measurements reveal the self-limiting growth nature of the deposition that is further verified with ex situ spectroscopic ellipsometry and X-ray reflectivity (XRR) measurements. A saturated growth rate of 2 Å/cycle at 170 °C is obtained. The deposition chemistry is studied by the in situ Fourier transform infrared spectroscopy (FTIR) that investigates the surface bound reactions during each half cycle. As deposited films are amorphous as observed from X-ray diffraction (XRD) and transmission electron microscopy electron diffraction (TEM ED) studies, which get converted to hexagonal-MoN upon annealing at 400 °C under NH3 atmosphere. As grown thin films are found to have notable potential as a carbon and binder free anode material in a Li ion battery. Under half-cell configuration, a stable discharge capacity of 700 mAh g(-1) was achieved after 100 charge-discharge cycles, at a current density of 100 μA cm(-2).

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

Pseudohalide (SCN(-))-Doped MAPbI3 Perovskites: A Few Surprises.

Pseudohalide thiocyanate anion (SCN(-)) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN(-) pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI3-x(SCN)x reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.

High thermoelectric performance of (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites having all-scale natural hierarchical architectures

Recent studies have shown that thermoelectric materials exhibit a high figure-of-merit if it consists of hierarchically organized microstructures that significantly lower the lattice thermal conductivity without any appreciable change in the power factor. Here, we report a new class of thermoelectric (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites synthesized via the vacuum hot pressing of a mixture of the constituents, which naturally consists of phonon scattering centers in a multiscale hierarchical fashion, i.e. atomic scale disorder, nanoscale amorphous structure, natural grain boundaries due to layered structure and mesoscale grain boundaries/interfaces. The presence of a natural hierarchical architecture of different length scales in the composite samples is confirmed by scanning electron and transmission electron microscopy. Detailed characterization reveals that in the composite samples there is a slight migration of Cu into the Ag site. Composite samples exhibit extremely low thermal conductivity ∼2 mW cm−1 K−1 at 773 K, which is nearly one third of the pure AgCrSe2 and CuCrSe2. The composite samples exhibit a high ZT ∼ 1.4 at 773 K, which is attributed to the scattering of heat carrying phonons of all wavelengths via the natural hierarchical architecture of the material. The ease of synthesis of such high performance (AgCrSe2)0.5(CuCrSe2)0.5 nanocomposites with a natural hierarchical architecture offers a promise for replacing conventional tellurides.

Intercalation based tungsten disulfide (WS2) Li-ion battery anode grown by atomic layer deposition

Thin films of tungsten sulfide (WS2) are prepared by atomic layer deposition (ALD) and its intercalation properties as an anode material in Li-ion battery are studied. The self-saturation growth of the material and the temperature window for ALD growth is confirmed by in situ quartz crystal microbalance (QCM). In situ Fourier transform infrared spectroscopy (FTIR) and residual gas analyzer (RGA) studies help to predict the two half reactions and FTIR further validates the self-limiting feature of ALD growth. The as-grown WS2 is amorphous in nature and characterized in detail by XPS and Raman spectroscopic analysis. The as-grown films are tested as a suitable intercalation based material for Li-ion battery anode. CV measurements are carried out extensively to explore the dominant intercalation property of the WS2 anode. Stable cycling performance with high coulombic efficiency (>99%) up to 100 charge–discharge cycles is observed. To enhance the performance further, multi walled carbon nanotubes (MWCNTs) scaffold layer is introduced that helps to deposit more active material for the same number of ALD cycles.

Growth of a polarity controlled ZnO nanorod array on a glass/FTO substrate by chemical bath deposition

We present a polarity controlled ZnO nanorod thin film deposition on a glass substrate by Chemical Bath Deposition (CBD). The polarity of ZnO is controlled by the anions of the Zn-salt used in the deposition solution. In the presence of –SO42− the rods grow in the +c direction (Zn-polar), while −c directional (O-polar) growth is observed in the presence of –NO3−. The polarity of the nanorods is confirmed by the Convergent Beam Electron Diffraction (CBED) technique. This study depicts that rods with different polarity possess different optical, electrical and chemical properties.

Atomic layer deposition of NiS and its application as cathode material in dye sensitized solar cell

Nickel sulfide (NiS) is grown by atomic layer deposition (ALD) using sequential exposures of bis(2,2,6,6-tetramethylheptane-3,5-dionate)nickel(II) [Ni(thd)2] and hydrogen sulfide (H2S) at 175 °C. Complementary combinations of in situ and ex situ characterization techniques are used to understand the deposition chemistry and the nature of film growth. The saturated growth rate of ca. 0.21 A per ALD cycle is obtained, which is constant within the ALD temperature window (175–250 °C). As deposited films on glass substrates are found polycrystalline without any preferred orientation. Electrical transport measurement reveals degenerative/semimetallic characteristics with a carrier concentration of ca. 9 × 1022 cm−3 at room temperature. The ALD grown NiS thin film demonstrates high catalytic activity for the reduction of I−/I3− electrolyte that opens its usage as cost-effective counter electrode in dye sensitized solar cells, replacing Pt.

Molecular layer deposition of alucone films using trimethylaluminum and hydroquinone

A hybrid organic–inorganic polymer film grown by molecular layer deposition (MLD) is demonstrated here. Sequential exposures of trimethylaluminum [Al(CH3)3] and hydroquinone [C6H4(OH)2] are used to deposit the polymeric films, which is a representative of a class of aluminum oxide polymers known as “alucones.” In-situ quartz crystal microbalance (QCM) studies are employed to determine the growth characteristics. An average growth rate of 4.1 A per cycle at 150 °C is obtained by QCM and subsequently verified with x-ray reflectivity measurements. Surface chemistry during each MLD-half cycle is studied in depth by in-situ Fourier transform infrared (FTIR) vibration spectroscopy. Self limiting nature of the reaction is confirmed from both QCM and FTIR measurements. The conformal nature of the deposit, typical for atomic layer deposition and MLD, is verified with transmission electron microscopy imaging. Secondary ion mass spectroscopy measurements confirm the uniform elemental distribution along the depth of ...

Atomic layer deposition of Zn3N2 thin films: growth mechanism and application in thin film transistor

In this paper we present atomic layer deposition (ALD) of zinc nitride thin films using diethylzinc (DEZ) and ammonia (NH3). Density Functional Theory (DFT) is used to calculate the atomistic reaction pathway. The self-limiting growth characteristic is verified at 315 °C. Saturated growth rate is found to be 0.9 A per ALD cycle. The as deposited films are found to be polycrystalline with preferential orientation in the {321} direction. The performance of the material is further investigated as channel layer in thin film transistor (TFT) applications.