J. Keane

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.

CuIn1−xGaxSe2-based photovoltaic cells from electrodeposited and chemical bath deposited precursors

We have fabricated 13.7%- and 7.3%-efficient CuIn1−xGaxSe2 (CIGS)-based devices from electrodeposited and chemical bath deposited precursors. As-deposited precursors are Cu-rich films and polycrystalline (grain size is very small) in nature. Only preliminary data is presented on chemical bath deposited precursors. Additional In, Ga, and Se were added to the precursor films by physical evaporation to adjust the final composition to CuIn1−xGaxSe2. Addition of In and Ga and also selenization at high temperature are very crucial to obtain high efficiency devices. Three devices with Ga/(In+Ga) ratios of 0.16, 0.26, and 0.39 were fabricated from electrodeposited precursors. The device fabricated from the chemical bath deposited precursor had a Ga/(In+Ga) ratio of 0.19. The films/devices have been characterized by inductive-coupled plasma spectrometry, Auger electron spectroscopy, X-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance–voltage, and spectral response. The compositional uniformity of the electrodeposited precursor films both in the vertical and horizontal directions were studied. The electrodeposited device parameters are compared with those of a 17.7% physical vapor deposited device.

Electrodeposition of In‐Se, Cu‐Se, and Cu‐In‐Se Thin Films

Indium-selenium, copper-selenium, and copper-indium-selenium thin films have been prepared by electrodeposition techniques on molybdenum substrates. Electrodeposited precursors are prepared at varying potentials, pH, and deposition times. The adhesion and uniformity of indium selenide on molybdenum substrates are improved by electrodepositing an initial copper layer (500 {angstrom}) on molybdenum. The films (In-Se, Cu-Se, and Cu-In-Se) are annealed at 250 and 450 C in Ar for 15 min and are slow-cooled (3 C/min). The films are characterized by electron microprobe analysis, inductive-coupled plasma spectrometry, X-ray diffraction analysis, Auger electron spectroscopy, and scanning electron microscopy. The as-deposited precursor films are loaded in a physical evaporation chamber and addition In or Cu and Se are added to the film to adjust the final composition to CuInSe{sub 2}. The device fabricated using electrodeposited Cu-In-Se precursor layers resulted in a solar cell efficiency of 9.4%.

12.3% Efficient CuIn1 − x Ga x Se2‐Based Device from Electrodeposited Precursor

Of the emerging materials for solar cell applications, CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) is a leading candidate and has received considerable attention in recent years. Copper-indium-gallium-selenium (Cu-In-Ga-Se) precursor thin films have been prepared by electrodeposition techniques on molybdenum substrates. The films have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, x-ray diffraction, electron probe microanalysis, current-voltage characteristics, spectral response, and electron-beam-induced current. Additional In or Cu, Ga, and Se have been added to the electrodeposited precursor film by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}, and allowed to crystallize at 550 C. A ZnO/CdS/CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} device fabricated using electrodeposited Cu-In-Ga-Se precursor layers resulted in an efficiency of 12.3%.

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.

Patterning of transparent conducting oxide thin films by wet etching for a-Si:H TFT-LCDs

The patterning characteristics of the indium tin oxide (ITO) thin films having different microstructures were investigated. Several etching solutions (HC1, HBr, and their mixtures with HNO3) were used in this study. We have found that ITO films containing a larger volume fraction of the amorphous phase show higher etch rates than those containing a larger volume fraction of the crystalline phase. Also, the crystalline ITO films have shown a very good uniformity in patterning, and following the etching no ITO residue (unetched ITO) formation has been observed. In contrast, ITO residues were found after the etching of the films containing both amorphous and crystalline phases. We have also developed a process for the fabrication of the ITO with a tapered edge profile. The taper angle can be controlled by varying the ratio of HNO3 to the HC1 in the etching solutions. Finally, ITO films have been found to be chemically unstable in a hydrogen containing plasma environment. On the contrary, aluminum doped zinc oxide (AZO) films, having an optical transmittance and electrical resistivity comparable to ITO films, are very stable in the same hydrogen containing plasma environment. In addition, a high etch rate, no etching residue formation, and a uniform etching have been found for the AZO films, which make them suitable for a-Si:H TFT-LCD applications.

CuIn(Ga)Se2-based devices via a novel absorber formation process

Abstract A novel pathway for the formation of copper–indium (gallium) diselenide has been developed. This two-stage process consists of (a) the formation of Cu–In–(Ga)–Se precursors, and (b) subsequent thermal treatment to form CuIn(Ga)Se2. The morphology, structure and growth mechanism for several different precursor structures prepared under various conditions were studied and correlated to the deposition parameters as well as the structure and morphology of the annealed films. Photovoltaic devices prepared from CuInSe2 and CuIn0.75Ga0.25Se2 resulted in efficiencies of 10% and 13%, respectively.

Properties of high-efficiency CIGS thin-film solar cells

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.

A new thin-film CuGaSe/sub 2//Cu(In,Ga)Se/sub 2/ bifacial, tandem solar cell with both junctions formed simultaneously

Thin films of CuGaSe/sub 2/ and Cu(In,Ga)Se/sub 2/ were evaporated by the 3-stage process onto opposite sides of a single piece of soda-lime glass, coated bifacially with an n/sup +/-TCO. Junctions were formed simultaneously with each of the p-type absorbers by depositing thin films of n-CdS via chemical bath deposition (CBD) at 60/spl deg/C. The resulting four-terminal device is a nonmechanically stacked, two-junction tandem. The unique growth sequence protects the temperature-sensitive p/n junctions. The initial device (/spl eta/ = 3.7%, V/sub oc/ = 1.1 V [AM1.5]) suffered from low quantum efficiencies. Initial results are also presented from experiments with variations in growth sequence and back reflectors.