Extension of the New Model to CIGS Thin-Film Solar Cells - Advances in Thin-Film Solar Cells - page 104

This is in fact the extension of the new model proposed recently by
Dharmadasa et al. [37, 38, 39] for CdS/CdTe thin-film solar cells.

5.6 Recent Work on Metal/p-CIGS Interfaces

The metal/semiconductor (MS) interface plays an important role
in electronic transport across device structures, as described in
chapter 1. However, work on metal/CIGS interfaces is very scarce in
theliterature.Onereasonforthismayhavebeenthenon-availability
of high electronic quality CIGS reference materials for this work.
One of the papers published in 2005 [39], however, is noteworthy
inthisworkonp-typeCIGSmaterialsgrownbyShowaShellinJapan
[40]. Since this material produced over 13% conversion e ciency
forlarge-area(3,600cm 2 )solarpanelsintheyear2003,theauthor's
group selected this high-performance p-CIGS material for electrical
contact studies. This work investigated metal/p-CIGS interfaces in
detail using Ag ( φ m =

4.26 eV), Cu (

φ m =

4.70 eV), and Au

(

φ m =

5.10 eV) covering a wide range of metal work functions

ϕ m ). The final results are summarised in Table 5.2 and in Fig. 5.4,
clearly showing four main discrete Fermi-level pinning positions.
This situation is identical to the one observed for n-CdTe/metal
interfaces [37-39, 41], as described in chapter 4.
A comprehensive literature search shows that these Fermi-
level pinning positions overlap with the major defect levels
independently observed for CIGS using current-voltage (I-V),
capacitance-voltage(C-V),photoluminescence(PL),deep-leveltran-
sient spectroscopy (DLTS), photo acoustic spectroscopy (PAS), and

(

φ b (eV) measured for 2 mm
diameter metal contacts fabricated on chemically etched Cu(InGa)(SeS) 2
layers

Table 5.2 The ideality factor ( n )andthe

Ag Cu/Au Au

n φ b (eV) n φ b (eV) n φ b (eV)

— — 1.40-2.20 0.75-0.79 — —

1.57-3.29 0.82-0.87 1.74-2.90 0.82-0.85 1.60-2.60 0.83

1.24-1.80 0.93-0.95 — — — —

1.24-1.53 1.01-1.04 — — — —