Joel Pankow

Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films

We report on a direct measurement of two-dimensional potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on grain boundaries, and the grain boundary is positively charged. The local built-in potential on the grain boundary is expected to increase the minority-carrier collection area from one to three dimensional. In addition, a work function decrease induced by Na on the film surface was observed.

System voltage potential-induced degradation mechanisms in PV modules and methods for test

Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent. Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potential-induced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

Phase Identification and Control of Thin Films Deposited by Co-Evaporation of Elemental Cu, Zn, Sn, and Se

Kesterite thin films [(i.e., Cu2ZnSn(S,Se)4 and related alloys] have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (>104 cm−1), their similarity to successful chalcopyrites (like CuInxGa1−xSe2) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.

Effect of Cu deficiency on the optical bowing of chalcopyrite CuIn1−xGaxSe2

Optical bowing coefficients are used to describe the band gap variation of a composite semiconductor alloy. It is known to be related to the electronic structure and the lattice deformation in the semiconductor alloys. Spectroscopic ellipsometry study shows that the optical bowing coefficient of slightly Cu-poor polycrystalline Cu0.9In1−xGaxSe2 is larger than that of stoichiometric polycrystalline CuIn1−xGaxSe2 and band gaps are larger when Cu becomes poor. This can be explained by an increase in valence band offset due to reduced p-d coupling and an increase of perturbation potential ΔV due to lattice deformation.

Enhanced Performance in Cu(In,Ga)Se

Cu(In,Ga)Se2 (CIGS) solar cells fabricated with twostep selenization processes commonly suffer from low open-circuit voltage (Voc). We found that the Voc of solar cells made from selenized Cu/Ga/In stacked metal precursors can be increased by employing a potassium fluoride (KF) postdeposition treatment (PDT). This study presents a comparison of films and resulting devices with and without the KF PDT. By including the KF PDT, an 18.6%-efficient CIGS device with a Voc of 0.709 V was fabricated using a two-step selenization process.

_{\bf 2}

Zn(O,S) thin films were deposited by chemical bath deposition (CBD), atomic layer deposition, and sputtering. Composition of the films and band gap were measured and found to follow the trends described in the literature. CBD Zn(O,S) parameters were optimized and resulted in an 18.5% efficiency cell that did not require post annealing, light soaking, or an undoped ZnO layer. Promising results were obtained with sputtering. A 13% efficiency cell was obtained for a Zn(O,S) emitter layer deposited with 0.5%O2. With further optimization of process parameters and an analysis of the loss mechanisms, it should be possible to increase the efficiency.

Solar Cells Fabricated by the Two-Step Selenization Process With a Potassium Fluoride Postdeposition Treatment

The efficiency of Cu(In, Ga)Se2 (CIGS) solar cells is enhanced when Na is incorporated in the CIGS absorber layer. This work examines Na incorporation in CIGS utilizing Na-doped Mo sputtered from targets made with sodium molybdate-doped (MONA) powder. Mo:Na films with varying thicknesses were sputtered onto Mo-coated borosilicate glass (BSG) or stainless steel substrates for CIGS solar cells. By use of this technique, the Na content of CIGS can be varied from near-zero to higher than that obtained from a soda-lime glass (SLG) substrate. Targets and deposition conditions are described. The doped Mo films are analyzed, and the resulting devices are compared to devices fabricated on Mo-coated SLG as well as Mo-coated BSG with NaF. Completed devices utilizing MONA exceeded 15.7% efficiency without anti-reflective coating, which was consistently higher than devices prepared with the NaF precursor. Strategies for minimizing adhesion difficulties are presented.

A comparative study of Zn(O,S) buffer layers and CIGS solar cells fabricated by CBD, ALD, and sputtering

Crystalline W03 nanoparticles were made with hot-wire chemical vapor deposition. The powder was heat-treated at 300, 400, 500, and 600°C in air. The structural properties were measured with X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. Thin-film electrodes of the powder were made by ultrasonic spray deposition onto conducting substrates. The photocurrent was measured at wavelengths greater than 400 nm as the voltage was scanned from 0.2 to 1.8 V in an aqueous environment. A correlation was seen between the photocurrent and dye decomposition and the results were most favorable for the monoclinic (I) γ-phase of WO 3 .

Sodium-doped molybdenum targets for controllable sodium incorporation in CIGS solar cells

Recently, CdTe photovoltaic (PV) devices fabricated in the nonstandard substrate configuration have attracted increasing interest because of their potential compatibility with flexible substrates such as metal foils and polymer films. This compatibility could lead to the suitability of CdTe for roll-to-roll processing and building-integrated PV. Currently, however, the efficiencies of substrate CdTe devices reported in the literature are significantly lower (~6%-8%) than those of high-performance superstrate devices (~17%) because of significantly lower open-circuit voltage (Voc) and fill factor (FF). In our recent device development efforts, we have found that processing parameters required to fabricate high-efficiency substrate CdTe PV devices differ from those necessary for traditional superstrate CdTe devices. Here, we investigate how oxygen incorporation in the CdTe deposition, CdCl2 heat treatment, CdS deposition, and post-deposition heat treatment affect device characteristics through their effects on the junction. By adjusting whether oxygen is incorporated during these processing steps, we have achieved Voc values greater than 860 mV and efficiencies greater than 10%.

Photocatalytic Activity and Photoelectrochemical Property of Nano-WO3 Powders Made by Hot-Wire Chemical Vapor Deposition

We used time-resolved photoluminescence (TRPL) spectroscopy to analyze time-domain and spectral-domain charge-carrier dynamics in CuIn1−xGaxSe2 (CIGS) photovoltaic (PV) devices. This new approach allowed detailed characterization for the CIGS/CdS buffer interface and for the space-charge region. We find that dynamics at the interface is dominated by diffusion, where the diffusion rate is several times greater than the thermionic emission or interface recombination rate. In the space-charge region, the electric field of the pn junction has the largest effect on the carrier dynamics. Based on the minority-carrier (electron) drift-rate dependence on the electric field strength, we estimated drift mobility in compensated CuIn1−xGaxSe2 (with x ≈ 0.3) as 22 ± 2 cm2(Vs)−1. Analysis developed in this study could be applied to evaluate interface and junction properties of PV and other electronic devices. For CIGS PV devices, TRPL spectroscopy could contribute to understanding effects due to absorber compositional ...