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Electrodeposited Thin Film Solar Cells
The p-type absorber layer of a hetero-junction solar cell can be electroplated either as an alloy2, 3 or as a series of thin metallic films4. Different processing approaches have been tried such as electrodeposition of Cu/In/Ga/Se thin films or alloy deposition of binary, ternary or even quarternary alloys such as CuInS2 or CuInGaSe2 (CIGS). All these precursor processing approaches require a subsequent high temperature 530 – 590o C rapid thermal annealing step in a selenium or sulfur atmosphere. A key challenge is to deposit very uniform nanometer scale thin films on meter large resistive substrates. Nucleation and growth phenomena and tailoring of the thin film microstucture is essential for controlling the composition of the final chalcopyrite layer and increasing the efficiency of the final device.4,5 Another challenge is to convert by annealing the uniform metal layers to good quality semiconductor material. For the fabrication of the CuInGa precursor material, surface roughness and thickness control are highly influenced by nucleation and growth of indium and gallium on copper. Electrodeposition of thin films of InGa and Se was demonstrated. The metal stack process was scaled up to a panel size of 30 cm x 60 cm and recently to a panel of 60 cm x 120 cm.
A promising candidate for low cost absorber layers is the quarternary compound of Cu2ZnSn(Se,S)4 which is the equivalent of CuInS2 when replacing In with Sn and Zn in a 50/50 ratio. The need to replace indium and gallium stems from the fact that these are two orders of magnitude more expensive than Sn, Zn and Cu. In terms of availability on the planet, indium and gallium are about as available as Cd and Se and one or more orders of magnitude less available than Sn, Cu, Sn and S.
With a similar methodology as in CuInS2, we have deposited Cu2ZnSn(Se,S)4 using an electroplated metal stack precursor. 6, 7 The metal precursor was converted to kesterite by annealing for 10-15 min in a sulfur containing atmosphere. The microstructure of the resulting absorber layer, the different phases detected by Raman spectroscopy will be discussed as well as cell efficiencies achieved to-date.
Rigid glass and flexible metallic substrates are used in the solar industry. The form factor of substrates for the solar industry varies between 225 cm2 to m.2 Methodologies similar to the one used for the scale-up of electrodeposited CIS can be used for the electrodeposited earth abundant materials. Flexible substrates can be processed continuously in a roll-to-roll process but these require a diffusion barrier to prevent diffusion of metallic impurities into the p-type absorber. A low cost barrier material that can be plated was evaluated for this application8.
Acknowledgement
The authors are thankful to many colleagues in IBM’s Microelectronics Research and Central Services Scientific Laboratory.
REFERENCES
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