Computational Materials Design for highly efficient In-free CuInSe 2 solar sells

1. Computational Materials Design for highly efficient In-free CuInSe2 solar sells Yoshida Lab. Yoshimasa Tani

2. CONTENTS 1. INTRODUCTION 2. RESULT AND DISCUSSION 3. SUMMARY

3. INTRODUCTION Computational Maerials Design Idea of new matrials CuIn1-xGaxSe2 Get properties ! Calculate by computer

4. INTRODUCTION The present of solar cells (1) • Solar cells are mainly made by Si single crystals. However, the cost is very high. Si solar cells • Nowadays, CIGS solar cells attract attention. It can be low cost than the Si based solar cells. CIGS solar cells

5. INTRODUCTION The present of solar cells (2) ー Comparison of solar cells company ー

6. INTRODUCTION CuInSe2 (My theme) • CuInSe2 has the direct band gap suitable for absorption of sunlight and the large light absorption coefficient (100 times of Si). • The solar cells product processing of CuInSe2 is rather easy due to the self-regeneration. • In CuIn1-xGaxSe2, the conversion efficiency of about 20 ％ can be realized. Thin film ! Low cost ! Crystal structure of CuInSe2 Cu In Se High efficiency !

7. INTRODUCTION Making In-free CIS (My theme) The supply of Indium is limited around the world. Therefore, it is important to propose new photovoltaic materials without (or with low concentration of) In with high efficiency than CuInSe2. Co-doing : 2In Zn + Sn I calculate the electronic structure of CuIn1-xZn0.5xSn0.5xSe2 and compare with that of CuIn1-xGaxSe2 .

8. INTRODUCTION Why using the Co-doping ? p-type doping In3+ →　Zn2+ n-type doping In3+ →　Sn4+ Co-doping 2In3+ →　Zn2++ Sn4+

9. INTRODUCTION What is electronic structure ? • I mainly calculate electronic structure as density of states and band dispersion. Most of electronic property can be explained by these. Density of states (DOS) Band dispersion (Band diagram)

10. INTRODUCTION Density of states (DOS) • Density of states (DOS) means the number of states per interval of energy at each energy level that are available to be occupied. Occupied by electron Valence band Conduction band Fermi level

11. INTRODUCTION Band dispersion (Band diagram) • Band dispersion (Band diagram)isthe plotting of imaginary part of single particle Green’sfunction. It indicates electronic property of materials. Band gap Fermi level k : wave vector

12. RESULT AND DISCUSSION Electronic structure of CuInSe2 Density of state • Band gap is direct. • Calculated band gap is 0.71 eV (the experimental gap is 1.04 eV ). • The valence band is constructed of the hybridized orbitals of Cu-3d and Se-4p, the conduction band from hybridized Se 4p and In 5s. Band diagram

13. RESULT AND DISCUSSION Semiconductor (1) E excitation recombination • In the semiconductor, the electron excites and makes a hole when it absorbs sunlight whose energy is larger thanthe band gap. • Excited electron recombines with a hole and releases a light. CBM light Band gap VBM electron hole excited electron

14. RESULT AND DISCUSSION Semiconductor (2) • P-type semiconductor is obtained by carrying out a process of doping, that is adding a certain type of atoms in order to increase the number of free positive-charged carriers. • N-type semiconductor is adding the dopant atoms which are capable of providing extra conduction electrons to the host material. This creates an excess of negative-charged carriers. E p-type n-type CBM Band gap Felmi level VBM Ex. In3+ →　Zn2+ Ex. In3+ →　Sn4+

15. RESULT AND DISCUSSION Mechanism of solar cells (1) E p-type n-type p-n junction electron hole excited electron

16. RESULT AND DISCUSSION Mechanism of solar cells (2) E electron hole excited electron

17. RESULT AND DISCUSSION Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (1) X = 0 X = 0.1 X = 0.5

18. RESULT AND DISCUSSION Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (2) X = 0.9 X = 1 (disordered alloy) X = 1 (ordered alloy)

19. RESULT AND DISCUSSION Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (3) X = 0.5 Sn-5sand Se-4p Direct band gap

20. RESULT AND DISCUSSION Comparison of CuIn1-xGaxSe2 CuIn1-xZn0.5xSn0.5xSe2 (x = 0.5) CuIn1-xGaxSe2 (x = 0.5) Sn-5sand Se-4p Ga-4sand Se-4p Band gap 0.48 eV Band gap 0.65 eV

21. RESULT AND DISCUSSION Possibility of multi-exiciton (1) • In this structure, we can expect possibility of multi-exciton effect. Multi-exciton is the generation of multiple electron-hole pairs from the absorption of a single photon. X = 0.1 mechanism

22. RESULT AND DISCUSSION Possibility of multi-exiciton (2) Electrons excite to conduction band and impurity level based on Sn (process 1). The electron which absorbs more energy than impurity level loses excess energy by phonon process to the imputity level (process2). the electron which transition from impurity level to bottom of conduction band loses excess energy (process 3). Using this energy, another two electrons (due to the energy and momentum conservation, k = -k1, k1) are excited to conduction band (process 4).

23. RESULT AND DISCUSSION Possibility of multi-exiciton (3) electron hole excited electron

24. SUMMARY SUMMARY • In all concentration of Indium, CuIn1-xZn0.5xSn0.5xSe2have a direct band gap. • No impurity band is formed in the band gap. • Fano-antiresonce of Sn impurity state for the formation of multiexciton appears in the conduction band. • Based on these findings, it is expected that CuInSe2 in which all of In are replaced with Zn and Sn can be used as materials of photovoltaic solar cells with low production cost.