1. Computational Materials Design for highly efficient In-free CuInSe 2 solar sells Yoshida Lab. Yoshimasa Tani

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

3. Computational Maerials Design Idea of new matrials CuIn 1-x Ga x Se 2 Calculate by computerGet properties ! INTRODUCTION

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

5. Companycountryproceedsprofitprofitabilitycell type Q-Cells : 2009 2nd priod (million euros) Germany １４２ - ６２ Single crystal First Solar : 2009 1st priod (million dollars) USA ４１８１６８４０．２％ compound Santec : 2009 1st priod (million dollars) China ３１６２１６．６％ Single crystal Sharp : 2008 4st priod (hundred million yen) Japan ２６５ - １４ ８ Single crystal Kyocera : 2008 4st priod (hundred million yen) Japan ２６５ -５-５ Single crystal Yingli Solar : 2009 1st priod (million dollars) China １４６３２．１％ Single crystal JA Solar : 2009 1st priod (million dollars) China ３４ - ２８ Single crystal The present of solar cells (2) The present of solar cells (2) INTRODUCTION ー Comparison of solar cells company ー

6. CuInSe 2 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 CuInSe 2 is rather easy due to the self- regeneration. In CuIn 1-x Ga x Se 2, the conversion efficiency of about 20 ％ can be realized. Crystal structure of CuInSe 2 Cu In Se CuInSe 2 (My theme) INTRODUCTION Low cost ! Thin film ! High efficiency !

7. 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 CuInSe 2. Co-doing : 2In Zn + Sn I calculate the electronic structure of CuIn 1-x Zn 0.5x Sn 0.5x Se 2 and compare with that of CuIn 1-x Ga x Se 2. Making In-free CIS (My theme) INTRODUCTION

8. Why using the Co-doping ? INTRODUCTION p-type doping In 3+ → Zn 2+ n-type doping In 3+ → Sn 4+ Co-doping 2In 3+ → Zn 2+ + Sn 4+

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

10. Density of states (DOS) INTRODUCTION Valence band Conduction band 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 Fermi level

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

12. RESULT AND DISCUSSION Electronic structure of CuInSe 2 Band diagram 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.

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

14. CBM E VBM p-type n-type Semiconductor (2) RESULT AND DISCUSSION 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. Band gapFelmi level Ex. In 3+ → Zn 2+ Ex. In 3+ → Sn 4+

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

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

17. Electronic structure of CuIn 1-x Zn 0.5x Sn 0.5x Se 2 (1) RESULT AND DISCUSSION X = 0X = 0.1X = 0.5

18. Electronic structure of CuIn 1-x Zn 0.5x Sn 0.5x Se 2 (2) RESULT AND DISCUSSION X = 0.9X = 1 (disordered alloy) X = 1 (ordered alloy)

19. Electronic structure of CuIn 1-x Zn 0.5x Sn 0.5x Se 2 (3) RESULT AND DISCUSSION Direct band gap Sn-5s and Se-4p X = 0.5

20. Comparison of CuIn 1-x Ga x Se 2 RESULT AND DISCUSSION CuIn 1-x Zn 0.5x Sn 0.5x Se 2 (x = 0.5)CuIn 1-x Ga x Se 2 (x = 0.5) Band gap 0.48 eVBand gap 0.65 eV Sn-5s and Se-4p Ga-4s and Se-4p

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

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

24. SUMMARY In all concentration of Indium, CuIn 1-x Zn 0.5x Sn 0.5x Se 2 have 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.