Computational Materials Design for highly efficient In-free CuInSe2 solar sells

Yoshida Lab. Yoshimasa Tani

CONTENTS

1. INTRODUCTION

2. RESULT AND DISCUSSION

3. SUMMARY

Computational Maerials Design

Idea of new matrials

CuIn1-xGaxSe2

Calculate by computerGet properties !

INTRODUCTION

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.

Company countryproceed

sprofit

profitability

cell type

Q-Cells : 2009 2nd priod(million euros)

Germany １４２ - ６２ Single crystal

First Solar : 2009 1st priod

(million dollars)USA ４１８ １６８ ４０．２％ compoun

d

Santec : 2009 1st priod(million dollars)

China ３１６ ２１ ６．６％ Single crystal

Sharp : 2008 4st priod(hundred million yen)

Japan ２６５ - １４８

Single crystal

Kyocera : 2008 4st priod(hundred million yen)

Japan ２６５ - ５ Single crystal

Yingli Solar : 2009 1st priod

(million dollars)China １４６ ３ ２．１％ 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

• 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.

Crystal structure of CuInSe2

　　 Cu In Se

CuInSe2 (My theme) INTRODUCTION

Low cost !

Thin film !

High efficiency !

• 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 .

Making In-free CIS (My theme) INTRODUCTION

Why using the Co-doping ? INTRODUCTION

p-type doping In3+ → Zn2+

n-type doping In3+ → Sn4+

Co-doping2In3+ → Zn2+ + Sn4+

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.

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

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

RESULT AND DISCUSSION

Electronic structure of CuInSe2

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.

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

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 gap Felmi level

Ex. In3+ → Zn2+ Ex. In3+ → Sn4+

Ep-type

Mechanism of solar cells (1) RESULT AND DISCUSSION

n-type

electron hole excited electron

p-n junction

E

Mechanism of solar cells (2) RESULT AND DISCUSSION

electron hole excited electron

Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (1)

RESULT AND DISCUSSION

X = 0 X = 0.1 X = 0.5

Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (2)

RESULT AND DISCUSSION

X = 0.9 X = 1 (disordered alloy) X = 1 (ordered alloy)

Electronic structure of CuIn1-xZn0.5xSn0.5xSe2 (3)

RESULT AND DISCUSSION

Direct band gap

Sn-5s and Se-4pX = 0.5

Comparison of CuIn1-xGaxSe2

RESULT AND DISCUSSION

CuIn1-xZn0.5xSn0.5xSe2 (x = 0.5) CuIn1-xGaxSe2 (x = 0.5)

Band gap 0.48 eV Band gap 0.65 eV

Sn-5s and Se-4p Ga-4s and Se-4p

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

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).

Possibility of multi-exiciton (3) RESULT AND DISCUSSION

electron hole excited electron

SUMMARY SUMMARY

• In all concentration of Indium, CuIn1-xZn0.5xSn0.5xSe2 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.