Kannan Ramanathan

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.

High efficiency graded bandgap thin-film polycrystalline Cu(In,Ga) Se2-based solar cells

Abstract Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m2). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 16.8-cm2 monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm2 single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here were made from compositional graded absorbers. The compositional Ga/(In + Ga) variations result in absorbers with graded bandgaps and graded carrier concentrations. Two types of bandgap gradings have been fabricated and characterized. We discuss their background for PV action enhancement along with the experimental concepts to grow such structures via coevaporation methods.

High efficiency Cu(In,Ga)Se/sub 2/-based solar cells: processing of novel absorber structures

Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m/sup 2/). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 16.8-cm/sup 2/ monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm/sup 2/ single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here are made from graded bandgap absorbers. Bandgap grading is achieved by compositional Ga/(In+Ga) profiling as a function of depth. The fabrication schemes to achieve the graded absorbers, the window materials and contacting are described.

Direct observation of Na and O impurities at grain surfaces of CuInSe2 thin films

The authors use field-emission Auger electron spectroscopy to investigate the spatial nature of trace Na and O impurities in thin films of photovoltaic-grade CuInSe2 thin films. They give the first direct proof that Na and O reside at grain surfaces and not in the grain interiors of CuInSe2 (CIS) thin films, and discuss the improvement in photovoltaic conversion efficiency of CIS with Na.

Cathodoluminescence of Cu(In,Ga)Se2 thin films used in high-efficiency solar cells

Cathodoluminescence spectroscopy and spectrum imaging are employed to investigate Cu(In,Ga)Se2 (CIGS) thin films used in high-efficiency solar cells. We have found a nonuniform spatial distribution for the photon energy. The shift by decade of the emission spectrum is also found to depend systematically on the location of excitation. In addition, the photon energy at grain boundaries is not affected by the external excitation. A model for radiative recombination to be applied to these chalcopyrite compounds should explain these results, and some suggestions are considered.

Electrical and optical properties of ion beam sputtered ZnO:Al films as a function of film thickness

Electrical and optical properties of as‐deposited, ion beam sputtered, Al‐doped ZnO films have been studied as a function of film thickness and carrier concentration. Hall effect measurements reveal that the bulk electrical resistivity of the film generally decreases with increasing film thickness. Additionally, it is observed that the rate of decreasing resistivity depends on the particular film thickness regime. For thinner films (100–200 nm), the resistivity decreases rapidly with increasing film thickness and is due to increases in both carrier concentration and Hall mobility. However, for thicker films, the resistivity decreases more slowly with increasing film thickness and approaches a nearly constant value at a thickness of 1100 nm. In this thickness regime, the slight decrease in resistivity with increasing film thickness is found to be due to an increase in carrier concentration alone. The above observations suggest the presence of at least two scattering mechanisms. It is speculated that grain ...

Prospects for in situ junction formation in CuInSe2 based solar cells

Abstract In this paper we describe our research efforts directed towards the understanding of the CdS/CuInGaSe 2 junctions and, specifically, the interaction of the chemical bath with the CuInGaSe 2 . Information gained from these studies has been used to develop a set of criteria for forming junctions without the need for chemical bath deposition or CdS. Our approach differs from many others previously used “alternative buffer layer” methods which appear to be somewhat problematic in implementation as well as in the quality of the results. This “buffer-free” technology has resulted in a 13.5% efficiency cell.

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.

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We present experimental results in three areas. Solar cells with an efficiency of 19% have been fabricated with an absorber bandgap in the range of 1.1-1.2 eV. Properties of solar cells fabricated with and without an undoped ZnO layer were compared. The data show that high efficiency cells can be fabricated without using the high-resistivity or undoped ZnO layer. Properties of CIGS solar cells were fabricated from thin absorbers (1 /spl mu/m) deposited by the three-stage process and simultaneous co-deposition of all the elements. In both cases, solar cells with efficiencies of 16%-17% are obtained.

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

The defect chalcopyrite material CuIn 3 Se 5 has been identified as playing an essential role in efficient photovoltaic action in CuInSe 2 -based devices; it has been reported to be of n-type conductivity, forming a p-n junction with its p-type counterpart CuInSe 2 . Because the most efficient cells consist of the Cu(In 1-x Ga x )Se 2 quaternary, knowledge of some physical properties of the Ga-containing defect chalcopyrite Cu(In 1-x Ga x ) 3 Se 5 may help us better understand the junction phenomena in such devices. Polycrystalline Cu(In l-x Ga x ) 3 Se 5 (with O 2 counterparts). Micrographs of the thin films show a substantial change in morphology as the Ga content is increased—for identical conditions of growth rate and substrate temperature. X-ray diffraction patterns agree with previously publish data for the ternary case (x=0), where these materials have been referred to as ordered vacancy compounds. Pole figures confirm a high degree of texturing in the films and a change in preferred orientation as Ga content is increased.