R. Noufi

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin‐film solar cells

This short communication reports on achieving 18·8% total-area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se2/Mo polycrystalline thin-film solar cell. We also report a 15%-efficient, Cd-free device fabricated via physical vapor deposition methods. The Cd-free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se2 thin-film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m2, 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin-film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

High‐efficiency CuInxGa1−xSe2 solar cells made from (Inx,Ga1−x)2Se3 precursor films

In, Ga, and Se were coevaporated to form precursor films of (Inx,Ga1−x)2Se3. The precursors were then converted to CuInxGa1−xSe2 by exposure to a flux of Cu and Se. The final films were smooth, with tightly packed grains, and had a graded Ga content as a function of film depth. Photovoltaic devices made from these films showed good tolerance in device efficiency to variations in film composition. A device made from these films resulted in the highest total‐area efficiency measured for any non‐single‐crystal, thin‐film solar cell, at 15.9%.

Enhanced stability of photoelectrodes with electrogenerated polyaniline films

Polyaniline was electrodeposited on platinum and semiconducting (Cd-chalcogenides, Si, GaAs, GaP) electrodes from aqueous solution (pH = 1) containing aniline. The electrochemical behavior of such films was examined in aqueous and nonaqueous solutions by cyclic voltammetry. Results show that the polyaniline film is conducting in both cathodic and anodic regions at pH < 3 as evidenced by the electrochemical redox reactions of various redox couples on polyanilinecoated electrodes. In general, all polyaniline-coated semiconductor electrodes examined in this work exhibited enhanced stability of the photocurrent when compared to that of the naked electrodes.

Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se2-based solar cells

Abstract We present an optimization of the CdS chemical bath deposition process as applied to high-efficiency Cu(In,Ga)Se2 photovoltaic thin-film absorber materials. Specifically, we investigated deposition time (thickness), bath temperature (65, 80 and 90°C) and a Cd2+ partial-electrolyte treatment of the chalcopyrite absorber prior to CdS deposition. We found that thinner CdS layers (grown on as-deposited absorbers) allowing more light to reach the junction are not necessarily conducive to higher short-circuit current density. Device performance was found to be dependent on the CdS layer thickness, but rather independent of the growth temperature. On the other hand, devices prepared from absorbers subjected to a Cd2+ partial electrolyte treatment show that the device performance dependence on CdS thickness is somewhat alleviated, and devices incorporating thinner CdS layers are possible without loss of parameters, such as open-circuit voltage and fill factor.

Electronic properties versus composition of thin films of CuInSe2

The electrical properties of thin‐film CuInSe2 (<4 μm thick) deposited by coevaporation of the elements have been measured by different techniques as a function of material composition. A correlation between the Cu/In and Se/metal ratios versus majority‐carrier concentration is established. A qualitative scheme is developed, based on experiments, which predicts the majority‐carrier type and concentration in relation to the stoichiometry of the material.

Band-gap engineering in Cu(In,Ga) Se2 thin films grown from (In,Ga)2Se3 precursors

Abstract A three-stage process starting with the deposition of (In,Ga) 2 Se 3 precursor films has been successful in the fabrication of graded band-gap Cu(In,Ga)Se 2 thin films. In this work we examine (1) the reaction of Cu + Se with (In,Ga) 2 Se 3 , which leads to a spontaneous grading in the Ga content as a function of depth through the film, and (2) modification of the Ga content in the surface region of the film through a final deposition of In + Ga + Se. We show how band-gap grading can be enhanced by the formation of non-uniform precursors, how counterdiffusion limits the degree of grading possible in the surface region, and how the Cu x Se secondary phase acts to homogenize the film composition.

Defect chemical explanation for the effect of air anneal on CdS/CuInSe2 solar cell performance

We formulate a consistent defect chemical model of the effect of air/O2 anneals on CdS/CuInSe2 devices. The model centers on O‐induced neutralization of (near) surface donor states in CuInSe2 grains. The simplest identification of these states is with ionized Se vacancies, due to coordinatively unsaturated In on grain surfaces and boundaries.

Na impurity chemistry in photovoltaic CIGS thin films: Investigation with x-ray photoelectron spectroscopy

Thermal processing of Cu(In1−xGax)Se2 thin-films grown as part of photovoltaic devices on soda-lime glass leads to the incorporation of Na impurity atoms in the Cu(In1−xGax)Se2. Na contamination increases the photovoltaic efficiency of Cu(In1−xGax)Se2-based devices. The purpose of this investigation is to develop a model for the chemistry of Na in Cu(In1−xGax)Se2 in an effort to understand how it improves performance. An analysis of x-ray photoelectron spectroscopy data shows that the Na concentration is ∼0.1 at. % in the bulk of Cu(In1−xGax)Se2 thin films and that the Na is bound to Se. The authors propose a model invoking the replacement of column III elements by Na during the growth of Cu(In1−xGax)Se2 thin films. Na on In and Ga sites would act as acceptor states to enhance photovoltaic device performance.

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.

CIGS‐based solar cells for the next millennium

Thin-film photovoltaic (PV) solar cells based on Cu(In,Ga)Se2 (CIGS) have two key distinctive features: highest performance of any true thin-film solar cell (18·8% efficient) and leading performance on the module level. There is no evidence of limits to further improvement of the efficiency. Device stability is not curtailed by intrinsic material properties. The obstacles to large-scale production and commercialization of Cu(In,Ga)Se2-based modules are the complexity of the material and the manufacturing processes. Published in 2000 by John Wiley & Sons, Ltd.