Prakash Koirala

Iron Pyrite Nanocrystal Film As A Copper-Free Back Contact For Polycrystalline CdTe Thin Film Solar Cells

We report the application of thin film nanocrystalline (NC) FeS2 as the copper-free back contact for CdTe solar cells. The FeS2-NC layer is prepared from solution directly on the CdTe surface using drop-casting coupled with a hydrazine treatment at ambient temperature and pressure, and requires no thermal treatment. Copper-free solar cells based on the CdS/CdTe/FeS2-NC/Au architecture exhibit device efficiencies 490% that of a standard Cu/Au back contact devices. The FeS2-NC back contact solar cells show good thermal stability under initial tests. Devices prepared with untreated FeS2-NC back contacts display a strong “S-kink” behavior which we correlate with a high hole-transport barrier arising from inter-NC organic surfactant molecules. & 2015 Elsevier B.V. All rights reserved.

Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.

Direct observation of electrical properties of grain boundaries in sputter-deposited CdTe using scan-probe microwave reflectivity based capacitance measurements

Polycrystalline CdTe in 12% efficient solar cells has been studied using scanning microwave impedance microscopy (sMIM). The CdS/CdTe junctions were grown on transparent-conducting-oxide-coated soda lime glass using rf sputter deposition. sMIM based capacitance measurements were performed on the exposed surface of CdCl2 treated CdTe adjacent to thermal-evaporation-deposited Cu/Au back contacts. The sMIM instrument was operated at ∼3 GHz, and capacitance measurements were performed as a function of ac and dc voltage biases applied to the tip, with and without sample illumination. Although dc capacitance measurements are affected by sample topography, the differential capacitance measurement was shown to be topography independent. It was found that the grain boundaries exhibit a depleted carrier concentration as compared to the grain bulk. This depletion effect is enhanced under photo-generated carrier separation or under sufficiently large probe tip biases opposite to the majority carrier charge.

Electron beam induced current in the high injection regime.

Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron-hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the total electron-hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in one- and two-dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the three-dimensional model.

Mapping spectroscopic ellipsometry of CdTe solar cells for property-performance correlations

Mapping spectroscopic ellipsometry (M-SE) has been applied for optimization of polycrystalline CdS/CdTe solar cell fabrication on transparent conducting oxide (TCO) coated glass superstrates. During fabrication of these solar cells, the structure undergoes key processing steps after the sputter-deposition of the CdS/CdTe. These steps include CdCl2 treatment of the CdTe layer and subsequent deposition of ultrathin Cu. Additional steps involve final metal back contact layer deposition and an anneal for Cu diffusion that completes the device. In this study, we have fabricated cells with variable absorber thickness, ranging from 0.5 to 2.5 μm, and variable CdCl2 treatment time, ranging from 5 to 30 min. Because both CdS window and Cu back contact layers are critical for determining device performance, an understanding of their deposition processes and process-property-performance relationships is important for device optimization. We have applied M-SE to map the effective thickness (volume/area) of the CdS and Cu films over 15 cm × 15 cm substrates prior to the fabrication of 16 × 16 arrays of dot cells. The model for M-SE analysis has been established using single-spot real time SE (RT-SE). We report correlations of cell performance parameters with the CdCl2 treatment time and with the effective thicknesses from M-SE analysis. We demonstrate that correlations between optical/structural parameters extracted from M-SE analysis and device performance parameters facilitate process optimization.

Spectroscopic Ellipsometry Applied in the Full p-i-n a-Si:H Solar Cell Device Configuration

Assessment of the performance of single-junction hydrogenated amorphous silicon (a-Si:H) p-i-n configuration solar cells has been developed with a combination of real-time spectroscopic ellipsometry (RTSE) and current-voltage (I-V) measurements. For each layer, RTSE measurements enabled the determination of thickness and optical properties in the form of the complex dielectric function (ε = ε1 + iε2) spectra. RTSE tracked changes in a as a function of depth and was used to extract profiles in the i-layer bandgap and crystallite fraction in the n-layer. Through mapping I-V characteristic measurements, spatial variations in device performance were determined. By comparing individual devices at the location of the RTSE beam spot, the influence of a and thickness for each layer on device performance was identified through simulations of quantum efficiency yielding the shortcircuit current. This study compares two devices prepared with different superstrate preheating processes and finds that the combination of RTSE and I-V measurements along with quantum efficiency simulations were able to identify plasma damage to the transparent conducting oxide as the likely cause for variation in device performance. This comparison serves as one example of how the optically obtained information, such as thickness and a for each layer, can be used to understand the final device performance.

Through-the-glass optical metrology for mapping 60 cm × 120 cm CdTe photovoltaic panels in off-line and on-line configurations

Through-the-glass spectroscopic ellipsometry (SE) has been applied in mapping analyses of 60 cm × 120 cm panels consisting of soda-lime glass coated with a four-layer transparent conducting oxide (TCO) stack and a two-layer CdS/CdTe photovoltaic (PV) heterojunction. Both off-line and on-line measurement configurations have been evaluated, using a rotating compensator for modulation of the incident polarization state and a multichannel detection system for high speed spectroscopy. Because of the longer time available for signal averaging during off-line measurements, the off-line capability assists in model development and qualification of the on-line capability, which operates at a higher speed and a limited data point density over the panel area. For on-line analysis, the panel is lying film side up on a linear conveyer and is moved past an SE mapping station. In this case, the ellipsometer heads traverse from side to side beneath the panel for through-the-glass measurements. Comparisons of off-line and on-line analyses indicate good agreement in the variations of the CdS layer effective thickness over the panel. The deduced CdS effective thickness, which includes components of the bulk and interface layers, is found to be a robust parameter in both analyses, and is also critical for predicting light collection in CdTe PV modules.

Enhancing the efficiency of CdTe solar cells using a nanocrystalline iron pyrite film as an interface layer

We use thin film nanocrystalline (NC) iron pyrite as an interface layer at the back contact of CdS/CdTe solar cells. In both spattered and CSS deposited CdTe devices, improvements in Voc and FF were obtained after the inclusion of the NC FeS2 layer. Repeated tests show that Voc increases by >30 mV and FF increases by ~3.5% for a standard CdTe device having efficiency of ~13%. The devices tested at STC show a relative increase in the power conversion efficiency in the range of 5% to 9%.

Real time and post-deposition optical analysis of interfaces in CdTe solar cells

Real time spectroscopic ellipsometry (RT-SE) in the near-infrared to ultraviolet range, as well as pre-deposition and post-deposition mid-infrared spectroscopic ellipsometry (IR-SE) have been applied as probes of the formation of optical interfaces in sputter-deposited CdS/CdTe solar cell structures. Both optical probes are configured for reflection from the film side of the solar cell structure. One focus of this work is to assist in the development of optical models to be used for both on-line analysis and quantum efficiency modeling. Toward this goal, RT-SE during CdS deposition has provided information on (i) [transparent conducting oxide (TCO)]/CdS interface formation - the extent to which the TCO surface roughness is conformally covered by the depositing CdS film; (ii) CdS bulk layer growth, and (iii) CdS surface roughness evolution and the final roughness thickness, which influences interface formation with the overlying CdTe. Pre-deposition and post-deposition IR-SE has also proven valuable for exploring the TCO free electron characteristics and the CdS optical properties that determine their near-infrared absorption spectra. The TCO characteristics have been observed to change with the over-deposition of the semiconductor films.

Multichannel spectroscopic ellipsometry for CdTe Photovoltaics: From real-time monitoring to large-scale mapping

Real time spectroscopy ellipsometry (RTSE) has been applied to study the evolution of thin film optical structure during sputter deposition of polycrystalline CdS/CdTe solar cell stacks on transparent conducting oxide (TCO) coated glass substrates optimized for high efficiency. RTSE provides information on (i) interface formation to the underlying high resistivity transparent (HRT) layer during initial CdS growth, (ii) bulk layer CdS growth and its surface roughness evolution, (iii) CdS/CdTe interface formation when the overlying CdTe layer is deposited on the CdS, and (iv) CdTe bulk layer growth and its roughness evolution. Structural/optical models developed in the analysis of RTSE data acquired at a single point are also applied in the analysis of ex situ mapping SE data obtained over the area of the completed solar cell stack. As a result, maps of the structural parameters can be extracted, which then can be correlated with maps of the small area device performance. When uncorrelated non-uniformities exist over the area, optimization by combinatorial methods is possible.