Brian E. McCandless

DEVICE AND MATERIAL CHARACTERIZATION OF CU(INGA)SE2 SOLAR CELLS WITH INCREASING BAND GAP

Thin‐film solar cells have been fabricated from Cu(InGa)Se2 films which were deposited by four‐source elemental evaporation with [Ga]/([In]+[Ga]) from 0.27 to 0.69 corresponding to a band gap from 1.16 to 1.45 eV. The films were intentionally deposited with no grading of the Ga and In to avoid gradients in their electrical and optical properties. X‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and Auger electron spectroscopy show that the films have uniform composition with no change in structure and morphology. Glass/Mo/Cu(InGa)Se2/CdS/ZnO devices have open‐circuit voltage increasing over the entire band gap range to 788 mV and 15% total area efficiency for band gap less than 1.3 eV, or [Ga]/([In]+[Ga]) less than 0.5. A decrease in device efficiency with higher Ga content is caused primarily by a lower fill factor. Analysis of current–voltage and quantum efficiency measurements show that this results from a voltage‐dependent current collection.

Recrystallization and sulfur diffusion in CdCl2‐treated CdTe/CdS thin films

The role of CdCl2 in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl2. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl2 treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl2 at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe1−xSx region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained.

Controlling Growth Chemistry and Morphology of Single-Bath Electrodeposited Cu ( In , Ga ) Se2 Thin Films for Photovoltaic Application

Single-bath electrodeposition of polycrystalline Cu(In,Ga)Se 2 thin films for photovoltaic applications is described. Cu(In,Ga)Se 2 was deposited onto Mo electrodes from low concentration aqueous baths containing CuCl 2 , InCl 3 , GaCl 3 , and H 2 SeO 3 . Buffering the solutions to pH ∼ 2.5 stabilized bath chemistry and improved Cu(In,Ga)Se 2 film composition. Bath concentrations were shown to affect composition of deposited films, with a bath [Se 4+ ]/[Cu 2+ ] ratio of 1.75 required to maintain suitable deposited Se and Cu levels, while [In 3+ ] could be adjusted to control deposited In and Ga. Deposited films initially exhibited significant cracking, which was prevented by lowering the [Se 4+ ] in the bath, and contained Cu 2-x Se as secondary phases, resembling cauliflower florets, embedded in the film surfaces. The formation of these secondary phases was overcome by pretreating the Mo electrodes with a short 1 min deposition from the Cu(In,Ga)Se 2 bath. This, coupled with a multipotential deposition regime, led to growth of smooth, compact, crack-free films of near stoichiometric values. Mechanisms of film growth and morphology control are discussed. All as-deposited films exhibit low crystallinity, and for device processing require recrystallization by annealing in an H 2 Se atmosphere. Promising preliminary results of electrodeposited Cu(In,Ga)Se 2 devices are presented.

Analysis of post deposition processing for CdTe/CdS thin film solar cells

A post-deposition process for optimizing the efficiency of thin film CdTe/CdS solar cells deposited by physical vapor deposition has been developed and the effects of the individual process steps on the materials and device properties have been analyzed. A 400 °C heat treatment with CdCl2 restructures the CdTe resulting in enhanced grain size and crystallographic reorientation. Structural and optical measurements indicate interdiffusion of sulfur and tellurium during the heat treatment resulting in formation of a CdSxTe1−x layer with a narrower band gap than CdTe. Bifacial current-voltage and quantum efficiency analysis of the CdTe devices at various stages of the optimization process shows the evolution of the device from a p-i-n structure to a heterojunction. A chemical treatment improves the open circuit voltage (Voc) and Cu/Au contact to the CdTe. The optimization process can be applied to cells using CdTe and CdS deposited by different methods.

Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S

The reaction pathways to form Cu(InGa)Se2 or Cu(InGa)S2 films at 450°C from metallic precursors were evaluated by reacting Cu–In–Ga films in H2Se or H2S for 10, 30, or 90min and characterizing the phase composition of the resulting films. A starting composition comprising Cu9(In0.64Ga0.36)4 and In phases was detected by x-ray diffraction in Cu–Ga–In precursors annealed at 450°C in an Ar atmosphere. When the precursors were reacted in H2Se, a graded Cu(InGa)Se2 film was formed with a Ga-rich composition and residual Cu–Ga intermetallics at the interface with the Mo back contact. The intermetallic compounds were observed to evolve from Cu9(In0.64Ga0.36)4 to Cu9Ga4 with increasing selenization time. Reaction in H2S formed inhomogeneous Cu(InGa)S2 with Cu–In intermetallics. The results are consistent with thermochemical predictions of the preferential reaction of In with Se, and Ga with S. These reaction preferences can explain the formation of a graded Cu(InGa)Se2 film during reaction in H2Se and provide a r...

EFFECTS OF PROCESSING ON CdTe/CdS MATERIALS AND DEVICES

By analyzing CdTe/CdS devices fabricated by vacuum evaporation, a self consistent picture of the effects of processing on the evolution of CdTe cells is developed which can be applied to other fabrication methods. In fabricating CdTe/CdS solar cells by evaporation, a 400°C CdCI2 heat treatment is used which recrystallizes the CdTe and interdiffuses the CdS and CdTe layers. The interdiffuson can change the bandgap of both the CdTe and CdS which modifies the spectral response of the solar cell. After this heat treatment a contacting/doping procedure is used which converts the CdTe conductivity to p-type by diffusion from Cu from the contact. Finally, the cell is treated with Br2CH3,OH which improves both Voc and FF. Analogous process steps are used in most fabrication processes for CdTe/CdS solar cells.

Interdiffusion of CdS/CdTe thin films: Modeling x-ray diffraction line profiles

A method for analyzing the diffusion process for CdS into CdTe thin films using x-ray diffraction is presented, allowing both bulk and grain boundary diffusion coefficients to be estimated. The equilibrium phase diagram for the CdTe1−xSx and CdS1−yTey alloy system was determined for temperatures from 625 °C to 415 °C. Measured diffraction line profiles for time-progressive diffusion of CdS into CdTe films resulting from thermal treatment at 440° were modeled using bulk and grain boundary diffusion coefficients of 1.25×10−13 cm2/s and 1.5×10−8 cm2/s, respectively. Modeling diffraction line profiles of samples treated at temperatures from 380 °C to 480 °C yielded Arrhenius activation energies for bulk and grain boundary diffusion processes of 2.8 eV and 2.0 eV, respectively. The bulk diffusion coefficients obtained from thin film structures were comparable to those obtained by Auger depth profiles for CdS/CdTe couples using CdTe single crystals.

Optimization of vapor post‐deposition processing for evaporated CdS/CdTe solar cells

The effects of thermal annealing in conjunction with CdCl2 vapor heat treatment on the properties of CdTe/CdS thin films and devices deposited by physical vapor deposition are reported. Results are compared for three treatment variations: high-temperature anneal only, high-temperature anneal followed by CdCl2 vapor heat treatment and CdCl2 vapor heat treatment only. X-ray diffraction, transmission electron microscopy and scanning electron microscopy show improved crystallographic properties of the CdTe film and reduced CdS/CdTe interdiffusion when a high-temperature anneal is used prior to CdCl2 treatment. The CdTe/CdS solar cells fabricated using an anneal at 550°C in argon prior to the CdCl2 vapor heat treatment exhibited improved electrical characteristics compared to cells fabricated with no anneal step, yielding an open-circuit voltage exceeding 850 mV. Copyright

CdTe/CdS solar cells with transparent contacts

Evaporated CdTe/CdS solar cells with a transparent Cu-indium tin oxide contact have been made with an efficiency greater than 8.5%. The deposition of single-phase CdTe films from a compound source required a cadmium-to-tellurium flux ratio of 1.7 incident on the substrate. To obtain the needed p-type conductivity of the CdTe films required a high temperature heat treatment in air which reduced the transmission through the CdTe film owing to the formation of a CdTeO3 surface layer. The heating and cooling rates used for the heat treatment affected the open-circuit voltage and contact resistance of the cells. The total subband gap absorption of the entire cell is 40% – 50%.

Characterization of 19.9%-efficient CIGS absorbers

We recently reported a new record total-area efficiency, 19.9%, for CuInGaSe2 (CIGS)-based thin-film solar cells [1]. Current-voltage analysis indicates that improved performance in the record device is due to reduced recombination. The reduced recombination was achieved by terminating the processing with a Ga-poor (In-rich) layer, which has led to a number of devices exceeding the prior (19.5%) efficiency record. This paper documents the properties of the high-efficiency CIGS by a variety of characterization techniques, with an emphasis on identifying near-surface properties associated with the modified processing.