Joseph D. Beach

Rapid thermal processing of ZnTe:Cu contacted CdTe solar cells

Cadmium telluride (CdTe) is a leading absorber for thin-film solar cells. However, state-of-the art open circuit voltages (Voc) of CdTe thin film solar cells fall ~350 mV below the value expected based on the band gap of CdTe. One of the reasons is due to barriers at the back contact. In this paper, we report on the development of a the back contact process for ZnTe:Cu contacted device that employs rapid thermal processing (RTP) to precisely control the activation and distribution of Cu. It is shown that 30 s annealing steps significantly improve fill factor and Voc without compromising the current density. Devices with >14% efficiency and >825 mV Voc are obtained under optimal conditions.

Atom probe tomography for nanoscale characterization of CdTe device absorber layers and interfaces

Cadmium telluride (CdTe) solar cells are a leading thin film technology with relatively high efficiencies. However, even the highest published efficiency CdTe cell is well below the theoretical achievable efficiency. Atomic scale characterization would provide important feedback on optimization of CdTe cells for further efficiency improvements. Atom probe tomography (APT), with both high spatial resolution and ppm composition sensitivity, is a technique well-suited for providing these details. It is demonstrated here that the compositions measured for CdTe and ZnTe by APT are sensitive to the analysis conditions, in particular the incident laser energy. Experiments demonstrating the relationships of the analysis parameters are presented. Using optimized values, APT analyses of the absorber layer and interfaces in CdTe devices were performed.

Non-uniformity mitigation in CdTe solar cells: the effects of high-resistance transparent conducting oxide buffer layers

High-resistance transparent conducting oxide buffer layers (HRTs) are commonly added between the doped transparent conducting oxide (TCO) and CdS in both CdTe and Cu(In,Ga)Se/sub 2/ (CIGS) based solar cells. The addition of the HRT is known to increase efficiency and has been employed in the production of record CdTe and CIGS devices. Here, we directly and unambiguously observe the non-uniformity mitigation effects of the HRT in CdTe cells through spatially resolved electroluminescence and light-beam induced current measurements. This decrease in non-uniformity has been measured in CdTe cells prepared both at the National Renewable Energy Laboratory and at the Colorado School of Mines both with and without HRT layers. Our measurements suggest that the nonuniformity is caused by thin CdS regions.

Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

Cadmium telluride (CdTe) is the leading commercialized thin-film photovoltaic technology. Copper is commonly used in back contacts to obtain high efficiency, but has also been implicated as a harmful factor for device stability. Thus it is critical to understand its composition and distribution within complete devices. In this work the composition and structure of the back contact region was examined in high efficiency devices (~16%) contacted using a ZnTe:Cu buffer layer followed by gold metallization. The microstructure was examined in the as-deposited state and after rapid thermal processing (RTP) using high resolution transmission electron microscopy and EDX chemical mapping. After RTP the ZnTe exhibits a bilayer structure with polycrystalline, twinned grains adjacent to Au and an amorphous region adjacent to CdTe characterized by extensive Cd-Zn interdiffusion. The copper that is co-deposited uniformly within ZnTe is found to segregate dramatically after RTP activation, either collecting near the ZnTe/Au interface or forming CuxTe clusters in CdTe at defects or grain boundaries near the interface with ZnTe. Chlorine, present throughout CdTe and concentrated at grain boundaries, does not penetrate significantly into the back contact region during RTP activation.

High-efficiency flexible CdTe superstrate devices

Flexible, superstrate CdTe devices combine the advantages of a commercially demonstrated, low-cost manufacturing process with a lightweight, flexible form factor. Here, we present data on cell efficiencies greater than 16%, and the critical processing changes that have enabled recent efficiency increases. The devices in this study were made on Corning® Willow® Glass, which is a highly transparent, flexible, hermetic, and dimensionally stable substrate that can withstand high processing temperatures. To date, we have produced devices with several different combinations of front and back contacts on this glass and have found that it is compatible with most of our standard processing steps. One of our best devices to date has a certified efficiency of 16.2%, with a short-circuit current density (Jsc) of 25.6 mA/cm2, an open-circuit voltage of 820 mV, and a fill factor (FF) of 77.3%. The increased Jsc in this cell is due to an improved sputtered CdS:O deposition process, and the high FF is due to a co-evaporated ZnTe:Cu back contact.

Spatially resolved derivative spectroscopy of vertical-cavity surface-emitting lasers using near-field scanning optical microscopy

A near-field scanning optical microscope was used in collection mode to generate spatially resolved maps of the first derivative with respect to operating current of power emission from vertical-cavity surface-emitting lasers (VCSELs). This technique is highly sensitive to the formation of new modes and can be used to identify mode cutoff points. An example of the usefulness of this technique is demonstrated as we estimate the index of refraction profile of the VCSEL under study at the single mode cutoff point. This profile is a primary feature of the waveguiding characteristics of such lasers.

Reliability and performance of organic down-shifting films for CdS/CdTe solar cells

An organic luminescent film is attached to CdS/CdTe solar cells to improve device performance. The luminescent film serves to absorb high energy photons that would normally be unharvested due to parasitic window layer absorption and re-emits photons at lower energies, better matched to the quantum efficiency peak of the solar cell. Efficiency improvements of up to 8.5% were obtained after optimizing dye concentration and film thickness in the luminescent layer. Long time stability tests show that the organic dye is stable for at least 5000 hours under 1 sun illumination provided that the dye is encapsulated in an oxygen and water free environment.

Characterization of VCSEL modal output using near-field scanning optical microscopy

The Near-Field Scanning Optical Microscope (NSOM) is a tool that combines the spatial resolution of scanning probe microscopy with optical characterization techniques. Using this technique, we have generated high-resolution spatial intensity maps of the output from vertical-cavity surface-emitting lasers (VCSELs) in the near-field region of the facet as a function of operating current. The VCSELs studied were proton implanted, gain guided devices designed to operate at ~850nm. Optical signals that have been spatially imaged include total intensity, the spectrally resolved intensity of individual transverse modes, and the derivative of intensity with respect of operating current. Deviations from expected mode patterns in the devices have been qualitatively linked to unacceptable levels of noise in operating lasers. These deviations can be observed at operating currents below the actual onset of unacceptable noise. We have also found that derivative spectroscopy can be used to sensitively detect the cutoff points of transverse modes. Using the spatial intensity profile at the cutoff point of an allowed mode, a first approximation to the index of refraction profile can be made that is in good agreement with prior work. A series of index profile estimates from the cutoff points of a VCSEL can provide information on the evolution of the index profile and the thermal lens as the power is ramped up.

Direct Patterning of Hydrogenated Amorphous Silicon by Near Field Scanning Optical Microscopy

Practical methods for directly patterning hydrogenated amorphous silicon (a-Si:H) films have been developed. Direct patterning involves selectively oxidizing the hydrogen passivated aSi:H surface, with the oxide then serving as an etch mask for subsequent hydrogen plasma removal of the unoxidized regions. Photo induced oxidation has been extensively studied using both far field projected patterns and near field scanning optical microscopy (NSOM) for direct write patterning. Examination of the threshold dose for pattern generation for excitation wavelengths from 248 to 633nm provides indirect evidence for involvement of electron-hole recombination in optically induced oxidation. Optical exposure of a-Si:H in vacuum demonstrated that oxygen must be present in the ambient atmosphere during exposure for successful pattern generation. This suggests that oxidation of the surface may not involve removal of hydrogen, but rather breaking of Si-Si backbonds and insertion of oxygen. An additional mechanism for oxide generation was observed whereby pattern generation resulted from simple proximity of an NSOM probe within ∼30nm from the sample surface. The probe dither amplitude was found to greatly affect the line width and height of patterns generated without light. Line widths of approximately 100nm, comparable to the probe diameter, were obtained.