1 Tandem and thin-film solar cells LECTURE 22 Si sliver cells tandem junction solar cells CIGS as a promising solar absorber CIGS solar cells heterojunction basics surface phenomena

2 2 Efficiency comparison: materials and modules Efficiency comparison: materials and modules

3 3 Thin sc-Si solar cells Thin sc-Si solar cells K.J. Weber et al., IEEE Photovoltaic Specialists Conf., , X reduction in Si use claimed

4 4 Multijunction cells: concept and practice Sec

5 5 Matching the materials J. M. Román, “State-of-the- art of III-V Solar Cell Fabrication Technologies, Device Designs and Applications,” Advanced Photovoltaic Cell Design, pdf

6 6 The world-record holder

7 Sec Cell mismatch

8 CIGS properties Sec. 7.2

9 Photovoltaic Materials, Richard Bube Electron minority carrier diffusion lengths: 0.5 – 2.5 micron More CIGS properties

10 Even more properties of CuIn 1-x Ga x Se 2 Even more properties of CuIn 1-x Ga x Se 2 Chalcopyrite structure, tetragonal bonding Vacancy doping* Direct bandgap E g (x) eV High absorption coefficient Can be printed onto glass and metal Needs heteroface cell structure Google has invested $$$$$ in it. *

11 Preparation of CuIn 1-x Ga x Se 2 Preparation of CuIn 1-x Ga x Se 2

12 CIGS cells: a lower cost alternative (?) CIGS cells: a lower cost alternative (?) Noufi, Rommel; Ken Zweibel. HIGH-EFFICIENCY CDTE AND CIGS THIN-FILM SOLAR CELLS: HIGHLIGHTS AND CHALLENGES. National Renewable Energy Laboratory. Sec

13 ECEC EVEV Wide bandgap window, but what happens at the interfaces? And why is CdS needed? Heterojunction advantages and problems Heterojunction advantages and problems x E

14 A proposed band diagram A proposed band diagram

15 Real heterostructures Real heterostructures What are the effects on the electrical properties of some previously unconsidered real surface effects? surface reconstruction dipole formation interruption of the periodicity of the semiconductor surface states