Robel Y. Bekele

Quaternary Sputtered Cu(In,Ga)Se 2 Absorbers for Photovoltaics: A Review

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H2Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed

Interdigitated Bulk Heterojunction Organic Photovoltaic Cells With Aligned Copper Phthalocyanine Nanorods

We show that vertically aligned nanorod arrays composed of copper phthalocyanine (CuPc) molecules can be grown on various substrates using the oblique angle deposition technique in high vacuum. High-density nanorod arrays with diameters of 20-70 nm and spacing of 10-100 nm have been achieved with either stationary or rotated substrates. Scanning electron and atomic force microscopies are combined to study the morphology of CuPc nanorods grown under different conditions. X-ray diffraction reveals that the crystallinity of the CuPc nanorods is similar to a flat CuPc film deposited under normal molecular incidence. Interdigitated bulk heterojunctions (BHJs) have been formed by successfully infiltrating [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to fill the spacing between the CuPc nanorods via spin coating. The resulted CuPc nanorod/PCBM photovoltaic cells possess a maximum power conversion efficiency approximately doubling that of bilayer CuPc/PCBM devices, demonstrating the effectiveness of the nanorod-based BHJ in enhancing both the donor-acceptor interfacial area and the exciton diffusion efficiency in the active layer.

Structural and electronic characteristics of Cu(In,Ga)Se 2 thin films sputtered from quaternary targets

Although the advantages of sputter deposition for large area, uniform deposition are well known, it has long been believed that sputtering Cu(In,Ga)Se2 (CIGS) from a quaternary sputtering target yields films with morphological and electronic properties that make them unsuitable for use in high-efficiency photovoltaic devices. Recent work, however, has demonstrated that this deposition method can produce dense, polycrystalline, highly oriented films with the desired stoichiometry. Devices built with these films exhibit efficiencies >;10%. While effective parameters for target composition and deposition conditions have been achieved, variation from these conditions can result in a wide array of morphologies, even while composition remains near that of stoichiometric CIGS. In this paper, we review the broad range of structural and electronic properties that result from various sets of target compositions and deposition conditions. Films deposited under some conditions are similar in important respects - their composition, a dense structure composed of ~1 μm sized grains, and the presence of a MoSe2 layer - to those of evaporated CIGS. We discuss how these results point towards the possibility of higher-efficiency sputtered CIGS.

Characterization of Cu(In, Ga)Se 2 thin films and devices sputtered from a single target without additional selenization

Typically, Cu(In, Ga)Se 2 (CIGS) thin films for photovoltaic devices are deposited by co-evaporation or, alternately, by deposition of the metals with, or followed by, treatment in a selenium environment. In this proceeding, we describe CIGS films that are instead deposited by RF magnetron sputtering from a single quaternary target without any additional selenization. Devices built with these films exhibit efficiencies as high as 9.9%. These results represent the first report of working CIGS devices fabricated by sputtering without additional selenization. We demonstrate that deposition power can be varied in order to change the film morphology and improve device performance.

Effects of sputtering technique on quaternary Sputtered Cu(In,Ga)Se2 Films

While Cu(In,Ga)Se2 (CIGS) has established itself as the thin film photovoltaic material of choice with current record efficiencies in excess of 20%, current high-efficiency laboratory-scale fabrication techniques, such as multi-stage evaporation, are ill suited to mass production. Quaternary-sputtering is a promising alternative technique for CIGS deposition, where a single sputtering target made from CIGS itself in the desired stoichiometry is used as the sole deposition source. Devices made using this technique do not require any additional post deposition selenium treatment and have demonstrated peak efficiency in our laboratory in excess of 10%, showing the potential of quaternary sputtering. In an effort to reduce deposition times, we have fabricated films using pulsed DC sputtering, which substantially reduces the substrate time-at-temperature during absorber formation. DC-sputtered films are observed to have reduced surface roughness and different internal morphology from RF-sputtered films, but show increased crystallographic alignment along the (112) plane. DC-sputtered CIGS is thickness-limited to less than 600 nm due to excessive target damage and exhibits power conversion efficiencies of 5-6%.

Non-mechanical beam steering in the mid-wave infrared

The mid-wave infrared (MWIR) portion of the electromagnetic spectrum is critically important for a variety of applications such as LIDAR and chemical sensing. Concerning the latter, the MWIR is often referred to as the “molecular fingerprint” region owing to the fact that many molecules display distinctive vibrational absorptions in this region, making it useful for gas detection. To date, steering MWIR radiation typically required the use of mechanical devices such as gimbals, which are bulky, slow, power-hungry, and subject to mechanical failure. We present the first non-mechanical beam steerer capable of continuous angular tuning in the MWIR. These devices, based on refractive, electro-optic waveguides, provide angular steering in two dimensions without relying on moving parts. Previous work has demonstrated non-mechanical beam steering (NMBS) in the short-wave infrared (SWIR) and near infrared (NIR) using a waveguide in which a portion of the propagating light is evanescently coupled to a liquid crystal (LC) layer in which the refractive index is voltage-tuned. We have extended this NMBS technology into the MWIR by employing chalcogenide glass waveguides and LC materials that exhibit high MWIR transparency. As a result, we have observed continuous, 2D MWIR steering for the first time with a magnitude of 2.74° in-plane and 0.3° out-of-plane.

Imaging EQE in CIGS solar cells with high spatial resolution

We use sub-micron scanning photocurrent microscopy to spatially resolve variations in the collection properties of CIGS thin-film solar cells made from a quaternary target. Spectrally dependent photocurrent measurements show remarkable variations in the current collection properties of the grain boundaries and grain cores. We combine EQE maps with Raman and EBSD scans to infer a direct correlation between the electrical and structural properties of the material comprising these polycrystalline devices.

Recent Progress in Sputtered Cu(In,Ga)Se2 Absorbers for Photovoltaics

Sputtering techniques have increasingly been applied for the fabrication of Cu(In,Ga)Se2 (CIGS) thin films for photovoltaics. For some of these methods, the metals are deposited via sputtering, and selenium is supplied from a secondary source either during or after deposition. In other CIGS sputtering techniques, selenium is included directly in the sputtering targets, eliminating the need for post-deposition selenization and the reliance on toxic H2Se gas. In general, sputtering offers several advantages over other deposition methods including excellent uniformity over large areas and high material usage. Interestingly, while record efficiencies for laboratory CIGS cells have been established using co-evaporation, the recent records for CIGS modules are based on CIGS films that have been deposited via multicomponent sputtering. We discuss recent progress on sputtered CIGS, reviewing various sputter deposition methods and discussing the advantages and limitations of each. Finally, we highlight areas of research that may result in improved performance.

Optimization of electrical performance of Cu(In,Ga)Se 2 thin film solar cells sputtered from quaternary targets

Utilizing a quaternary target to sputter Cu(In,Ga)Se2 (CIGS) yields films which are dense, polycrystalline and highly oriented. Devices fabricated from these films exhibited efficiencies >10%. In this paper, we study the electrical characteristics of these devices, including current-voltage (I-V), external quantum efficiency (EQE) and sheet and contact resistances measured by the transfer length method, to improve their overall performance. The effect of edge termination by different techniques is reviewed to investigate perimeter effects and edge defects. It was found in these particular devices that mechanical scribing of device areas contributed to both edge and bulk shunt effects. We discuss the extent of these effects in the context of increasing device efficiency through the optimization of edge termination.

Microstructured ZnO coatings for improved performance in Cu(In,Ga)Se 2 photovoltaic devices

The performance of thin film Cu(In,Ga)Se2 (CIGS) photovoltaics is typically degraded by light lost due to the high reflectivity of the transparent top contact and by recombination resulting from carrier generation far from the junction. Traditional antireflective (AR) coatings are insufficient to address the former issue, particularly at non-normal incidence. We present a novel microstructured ZnO coating that serves two functions; it acts an AR layer with superior non-normal performance in comparison to thin film AR coatings, and it scatters a significant fraction of the incoming radiation at a large angle, resulting in absorption that is on average closer to the junction. This coating, formed via a wet etch process, results in performance comparable to that of uncoated films at normal incidence and an increase of up to 25% in the short circuit current and 18% in device efficiency at non-normal incidence.