CIGS Thin-Film Solar Cells : Toward high efficiency & low cost

1. CIGS Thin-Film Solar Cells : Toward high efficiency & low cost

2. High efficiency • High stability • Inexpensive substrate (Soda lime glass) • Quaternary alloy • Rare elements (In,Ga) • Cd in buffer layer CIGS Solar cell

3. Typically, CIGS forms chalcopyrite structure • Bandgap can be adjusted by substitute elements • CuInSe2 (1.04eV) • CuInS2 (1.53eV) • CuGaSe2 (1.7eV) • CuGaS2 (2.5eV) • Cu content is most essential factor • In/Ga ratio also affects photovoltaic efficiency CIGS Crystal Structure

4. Overall composition is slightly Cu-deficient • More Cu-deficient surface layer enhances efficiency • Ordered Vacancy Compound (CuIn3Se5) • Formation of buried p/n junction CIGS Composition

5. Homojunction solar cell: • A significant part of photogeneration takes place close to the surface of the emitter • pn junction locates near surface • Heterojunction solar cell: • Photogeneration maximum is at the pn junction • Exhibit a high density of states at the interface • Interface recombination is dominant p n Bandgap Design

6. OVC layer have wider bandgap than CIGS absorber layer • Barrier for recombination is reduced by bandgap widening at interface • thicker inverted layer with material properties such as low carrier density, high resistivity, low carrier mobility, and high density of recombination centers could further worsen the device performance. Ordered Vacancy Compound

7. Shallow acceptor Vcu • Deep donor InCu • Vcu+ InCu pair is electrically inactive • InCu is responsible for n-type nature of OVC • OVC is weakly n-type • CuIn3Se5 OVC is not degenerate by defect compensation Ordered Vacancy Compound

8. The formation of the OVC layer occurs automatically on the top surfaces of slightly Cu-deficient CIGS thin films at high deposition temperatures when the In/(In+Cu) ratio in the • bulk of the film is higher than 0.52 • The more Cu-poor the CIS film, the thicker • the OVC layer • CuGaSe2 is always p-type, which prevents the formation of the inverted surface • CuInS2 is difficult to be prepared with a Cu-deficient composition: • attempts to prepare Cu-deficient CuInS2 lead often to the formation of n-type CuIn5S8 • CuInSe2 14.5% • CuGaSe2 9.5% • CuInS2 11.1% Ordered Vacancy Compound

9. Large distance between the Fermi-level and the CdS conduction band would cause a • poor fill factor due to series resistance • Recombination with the conduction band spike • High fill factors can be achieved in the experiment • Possibilities: • CdS is highly doped • the band offsets are smaller than calculation • Large density of charged interface states Ordered Vacancy Compound

10. Open circuit voltages of CuInSe2 solar cells increase linearly with the addition of Ga to the absorber until optimum ratio • Optimum Ga/(Ga+In) ratio is about 30% and a band gap of about 1.2 eV • The increase of the open circuit voltage is faster than that of the band gap, and is accompanied by a decreasing defect density CIGS Composition

11. Bulk diffusion length equals or exceeds the absorber thickness, recombination at the CIGS/back contact interface contributes to the electronic losses • Effective diffusion lengths Leff of up to 3 μm • Bandgap grading enhances the separation of the photogenerated charge carriers and reduces recombination at the back contact Bandgap Grading

12. Red illumination causes a metastable increase of net carrier concentration, which decreases the width of the space charge layer in the absorber • The open circuit voltage increases due to the reduced recombination in the narrower space charge layer • Narrower spacer charge layer width is confirmed from increased junction capacitance Metastable Behavior

13. Short-wavelength illumination affects mostly the regions at or close to the CdS/CIGS interface. Blue light is to a great extent absorbed into the buffer layer, and the photogenerated holes are injected into the near-surface region of the CIGS absorber • Illumination by blue light has been reported to improve the fill factor, which probably results from the ionization of deep donors in CdS. The positively charged fixed donors cause downward band bending in the CdS and reduce the barrier height to electrons • The photogenerated holes have also been suggested to neutralize the negative defect states that are present on the CIGS surface. The improvement of the FF upon illumination is therefore related to the CIGS/CdS interface Metastable Behavior

14. Defects that are caused by deviations from the stoichiometry are compensated by new defects that neutralize them • Formation energies of the compensating ionic defects are low. • Most of the defects or defect complexes • are inactive with respect to the carrier recombination • CIGS solar cells have shown exceptionally stable performances even under X-rays, electrons, or protons • Suitable to space applications • The self-healing mechanism is a result of the mobility of Cu and reactions involving Cu-related defects and defect complexes • Stability of the CIGS and related materials seems to be of dynamic Stability

15. smoothen the surface morphology • increase the grain size • enhanced crystallinity and (112) orientation • Increase in carrier concentration, leading to a higher p-type conductivity Na Effect on CIGS Absorber

16. Increase in carrier concentration, leading to a higher p-type conductivity • The increased p-type conductivity of Na-containing Cu chalcopyrite films is generally attributed to the suppression of donor-type defects such as InCu that act as majority carrier traps • On the other hand, the removal of a minority carrier trap has also been reported Na Effect on CIGS Absorber

17. sodium has been observed to suppress the diffusion of Ga which helps to achieve a graded Ga content • Sodium has also been suggested to aid the formation of the beneficial MoSe2 layer between Mo and CIGS Na Effect on CIGS Absorber

18. Se vacancies decrease the effective p-type doping of the film • Se vacancies act as recombination centers for the photogenerated electrons • Oxygen passivate positively charged Se vacancies (VSe) that are present at the surfaces and grain boundaries • Sodium has been suggested to promote the formation of chemisorbed O2− ions by weakening the O-O bond • Oxygen is needed for the diffusion of sodium from soda lime glass: suppression of Na • diffusion was observed in 1 × 10−8 Torr vacuum, whereas diffusion occurred in 1 × 10−5 Torr of either air, oxygen, or water vapor Na Effect on CIGS Absorber

19. Solution-based process • Low-cost • Compatible with buffer layer CBD process • Continuous process • High material utilization • Composition control is easy CIGS from Nanoparticle Ink

20. T. Arita et al. 1988 • A. Vervaet et al. 1991 • D. L. Schulz et al. 1997 • S. Ahn et al. 2006 • Y. Chiba et al. 2006 • C. Eberspacher et al. 2001 • V. K. Kapur et al. 2001 • M. Kaelin et al. 2004 • M. Kaelin et al. 2005 Micropowder CIGS nanopowder Oxide nanopowder Metal nanopowder Nitrate & Chloride nanopowder CIGS from Nanoparticle Ink

21. Selenization process is required for nanoparticle-derived process • Gases used for Selenization (H2Se, Se) are toxic and/or flammable • When metal oxide nanoparticle is used, additional reduction step is required • Additional processes need high temperature • Increase cost / Decrease producibility CIGS from Nanoparticle Ink

22. Selenization process is even used in case of Selenidenanoparticle CIGS from Nanoparticle Ink

23. Nanosolar – Industrial level development • Decreased particle size lower melting point and sintering temperature • Selenization process is inhibited • (Probably) Particle size less than 10nm is required CIGS from Nanoparticle Ink

24. Synthesis of CIGS nanoparticle by atmospheric pressure plasma • Low temperature sintering technique: • Microwave • Spark plasma Future Work