Projects

University of Delaware

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Project Objective:  Explore the feasibility and develop approaches for obtaining a transparent, high-efficiency wide band gap top cell for use in a multijunction polycrystalline solar cell.

Approach/Background:  The minimum requirements for the wide band gap cell in a high efficiency tandem cell are a band gap in the range 1.6 E_g 1.9 eV with efficiency = 15% and high optical transparency to sub-band gap illumination.  While single junction thin film solar cells based on CuIn_1-xGa_xSe₂ and CdTe are being actively developed, devices based on alloys of these films with the necessary bandgap range have not met the high bandgap cell requirements.  The approach taken in this proposal will be to develop thin film solar cells based on CuInSe₂-based films alloyed with Ga, Al, and/or S and on Cd_1-xZn_xTe alloys.  The project will be performed under two tasks focussing on the wide band gap I-III-VI₂ and II-VI materials respectively.

Under the first task, methods to deposit CuIn_1-xGa_x(Se_1-yS_y)₂ (CIGSS) films with uniform composition and E_g from 1.6 to 1.9 eV will be developed.  The structural, compositional and optical properties will be characterized as a function of the relative Ga/In and S/Se compositions.  Devices will be fabricated with CIGSS films, which have E_g from 1.6 to 1.9 eV. J-V and QE measurements of selected device will be analyzed to determine controlling mechanisms.

Under the second task, Cd_1-xZn_xTe alloy films will be deposited by thermal evaporation with x from 0 to 0.6, corresponding to E_g from 1.5 to 1.9 eV. The thermochemical reactivity of the Cd_1-xZn_xTe alloys with different ambient chemical species, including oxide and halide species, will be quantified.  Morphological, structural, and optical properties of Cd_1-xZn_xTe alloy films after thermal treatment in different ambients will be characterized. Devices will be fabricated and characterized as a function of composition and post-deposition treatment.

Status/Accomplishments:  A new system has been constructed to deposit CIGSS films by five-source elemental thermal evaporation. Sequential processes, in which metal or binary layers are deposited at low temperature and then reacted in mixed hydride gases or elemental vapors, were ruled out due to difficulties in attaining uniform incorporation of Ga and In. The deposition system uses boron nitride crucibles as sources for Cu, In, and Ga. These crucibles are heated in a boron nitride furnace, using tantalum wire resistive heaters, and multiple layers of thermal shielding. While these metal sources typically operate at temperatures between 1100°C and 1400°C, the evaporation temperatures for Se and S will range from 100 to 300°C. The chalcogen sources are mounted below the metal sources to increase the distance, minimizing thermal cross talk, and a new source design was developed to facilitate precise thermal control. The crucible for the S and Se sources consists of a stainless steel bottle connected to a tube that enables the S or Se vapor to pass between two of the metal sources. A water-cooled jacket surrounds each source to decrease the thermal response time and enhance control at low temperatures. The nozzles from all five sources are at the same height, 25 cm from the substrate. All components of the system are designed to withstand the corrosivity of the S vapor and the chamber includes a chilled Meissner trap to getter excess sulfur.

The deposition of thin Cd_1-xZn_xTe films with controlled compositions is facilitated by the congruent sublimation of CdTe and ZnTe compounds so that single-phase alloy films can be deposited by evaporation from binary compound CdTe and ZnTe sources. The alloy composition of the film is determined by the ratio of elemental impingement rates and their relative sticking coefficients. The impingement rates are determined by the effusion rates from the CdTe and ZnTe sources and are controlled by source temperature. To deposit alloy films at a reasonable growth rate, ~0.3 µm/min, a CdTe source temperature of 900°C was selected and alloy compositions were varied by adjusting the ZnTe source temperature. Cd_1-xZn_xTe films 3-4 µm thick with x from 0 to 1 were deposited onto ITO/glass and CdS/ITO/glass substrates at 325°C. The film compositions agree well with the effusion rate composition showing that, for the chosen substrate temperature of 325°C, the Cd and Zn sticking coefficients are comparable and, therefore, likely very high. The optical band gap was determined from optical transmission and reflection data, and varied from 1.5 to 2.25 eV. X-ray diffraction patterns indicated the presence of a single crystalline phase in all films. All films exhibited sharp reflections and strong (111) texture, indicative of homogeneous composition. The lattice parameter, determined using the Nelson-Riley-Sinclair-Taylor analysis, was linear over the entire composition range. Atomic force and scanning electron microscopy showed the presence of faceted grains with decreasing lateral dimension as relative ZnTe content increases. Thus, using thermal evaporation, control of the Cd_1-xZn_xTe alloy film composition, band gap, and lattice constant have been demonstrated.

Planned FY 2002 Activities: Processes will be developed to deposit CIGSS films with uniform, controlled composition. This will require calibration of the five elemental sources and verification of control of the two chalcogen sources. Films will be deposited with a wide range of composition and band gap and the structural, compositional and optical properties will be characterized. Complete solar cell devices will be fabricated and characterized. In addition, a process will be explored in which CuIn_1-xGa_xSe₂ and CuIn_1-xGa_xS₂ layers are deposited sequentially and then, if necessary, annealed in-situ to determine if layers with uniform composition can be obtained.  This would relax the need for precise control of the S and Se sources since each layer could be deposited with excess chalcogen flux.

Using Cd_1-xZn_xTe alloy films with x from 0 to 1, the control of film properties during post-deposition treatments will be characterized.  This will focus on halide, air and thermal anneal treatments of the films, which are typically used to optimize CdTe cell performance. Morphological, structural, and optical properties of the Cd_1-xZn_xTe films after thermal treatment in different ambients will be characterized. Devices will be fabricated and characterized as a function of composition and post-deposition treatment.

University of Delaware High-Performance PV Publications:
"Wide Band Gap CuInSe2 and CdTe-Based Thin Films for Tandem Solar Cells," National Center for Photovoltaics Program Review Meeting, Oct. 14-17, 2001 (Lakewood, CO). (PDF 723 KB)

W. Shafarman, M. Gossla and B. McCandless