John R. Tuttle

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%.

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.

Structure, chemistry, and growth mechanisms of photovoltaic quality thin‐film Cu(In,Ga)Se2 grown from a mixed‐phase precursor

The formation chemistry and growth dynamics of thin‐film CuInSe2 grown by physical vapor deposition have been considered along the reaction path leading from the CuxSe:CuInSe2 two‐phase region to single‐phase CuInSe2. The (Cu2Se)β(CuInSe2)1−β (0<β≤1) mixed‐phase precursor is created in a manner consistent with a liquid‐phase assisted growth process. At substrate temperatures above 500 °C and in the presence of excess Se, the film structure is columnar through the film thickness with column diameters in the range of 2.0–5.0 μm. Films deposited on glass are described as highly oriented with nearly exclusive (112) crystalline orientation. CuInSe2:CuxSe phase separation is identified and occurs primarily normal to the substrate plane at free surfaces. Single‐phase CuInSe2 is created by the conversion of the CuxSe into CuInSe2 upon exposure to In and Se activity. Noninterrupted columnar growth continues at substrate temperatures above 500 °C. The addition of In in excess of that required for conversion produce...

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.

Composition-structure relationships for multisource evaporated CuGaSe2 thin films

X‐ray diffraction (XRD) analysis of doublet formation and peak shifts were used to observe the compositional dependence of the order–disorder transition in polycrystalline CuGaSe2 thin films deposited by multisource evaporation. Cu‐poor material had a strong tendency to disorder as evidenced by the simultaneous presence of both the chalcopyrite and sphalerite phases. Stoichiometric and Cu‐rich material contained only the tetragonal phase as observed by XRD. Comparison of Cu‐poor XRD patterns with theoretical calculations reflecting probable defect chemistries (VCu, GaCu, VSe) suggests an interesting microstructure. The absence of higher index group (iii) reflections, notably the (103) and (211) peaks, in chalcopyrite material suggest that the tetragonal phase maintains a near‐stoichiometric composition. Overall Cu‐poor film compositions may therefore be obtained by adjustment within the cubic phase which implies compositional segregation between the phases.

Photoemission investigation of the electronic structure at polycrystalline CuInSe2 thin‐film interfaces

The surface versus bulk composition and electronic structure of polycrystalline CuInSe2 thin‐film interfaces were studied by synchrotron radiation soft‐x‐ray photoemission spectroscopy. An n‐type In2Se3/CuIn3Se5 surface layer forms on enhanced‐grain polycrystalline thin‐film p‐type CuInSe2 during fabrication. Enhanced‐grain CuInSe2 films were sputter etched (500 V Ar) and analyzed in situ to determine core‐level binding energies and Fermi‐level positions for the n‐type surface and the p‐type CuInSe2 bulk within ±0.1 eV. The transition between the n‐type surface and the p‐type bulk was experimentally observed by noting the change in the position of the valence‐band maximum relative to the Fermi level EF. From these measurements, the valence‐band offset ΔEv between the layers was determined to be 0.50 eV. Measurement of the work functions φ was also completed and reveals φ=4.75 eV for the In2Se3 (CuIn3Se5) surface layer and φ=4.04 eV for the bulk CuInSe2. Combining these results allows construction of a sur...

Sodium diffusion, selenization, and microstructural effects associated with various molybdenum back contact layers for CIS-based solar cells

By varying the argon pressure during deposition, the authors have prepared a set of sputtered molybdenum films on soda-lime glass substrates with a range of mechanical and electrical properties. These films were subsequently exposed to several of the processing steps used in the fabrication of copper-indium-diselenide (CIS) solar sells. Processing steps of interest include heating in a vacuum, exposure to selenium vapor at elevated temperatures, and deposition of CIS and CIGS layers over the Mo. Resulting Mo films and structures were subsequently characterized using XPS, SEM, Auger, and SIMS. Here, they describe the results of these experiments and their implications for CIS cell fabrication.

High efficiency thin-film Cu(In,Ga)Se/sub 2/-based photovoltaic devices: progress towards a universal approach to absorber fabrication

The formation chemistry of Cu(In,Ga)Se/sub 2/ by several reaction paths has been considered, and growth models of these processes have been developed. The results suggest a simple, reproducible approach to the formation of the multinary compound. The foundation of a universal process for the fabrication of Cu(In,Ga)Se/sub 2/-based solar cells is presented. Several embodiments of the process make it self-limiting with moderate compositional tolerances and simple endpoint detection. It is applicable to different device structures, and scalable to a variety of hybrid deposition technologies. A growth model is presented that correctly describes this process and related ones. Novel CuInSe/sub 2/ single-layer and Cu(In,Ga)Se/sub 2CuGaSe/sub 2/ multilayer absorber structures have been fabricated by physical vapor deposition using this process. Laboratory-scale photovoltaic devices demonstrate a total-area efficiency of 13.3%.<<ETX>>

Graded band‐gap Cu(In,Ga)Se2 thin‐film solar cell absorber with enhanced open‐circuit voltage

An important development in polycrystalline Cu(In,Ga)Se2 (CIGS) thin‐film photovoltaic solar cells is the attainment of a high voltage device simultaneous with state‐of‐the‐art conversion efficiency. This letter describes a CIGS‐based solar cell that demonstrates an open‐circuit voltage (Voc) approaching 700 mV and a total‐area conversion efficiency of 12.2%. The high value of Voc was achieved by grading In/Ga through the absorber by a computer‐controlled physical vapor deposition (PVD) process that utilizes variable metal fluxes.