CIGS thin film solar cells using transparent conducting oxide back contacts for bifacial and tandem

3rd World Conference on Phorovolroic Energy Conversion May 11-18.2003 Os&, Jopon

S40-C12-04

Cu(Inl.,,Ga,)Sez THIN FILM SOLAR CELLS USING TRANSPARENT

CONDUCTING OXIDE BACK CONTACTS FOR BIFCIAL AND TANDEM SOLAR

CELLS

T. Nakada, Y. Hirahayashi, T. Tokado and D.Ohmori

Department of Electrical Engineering and Electronics, Aoyama Gakuin University,

5-10-1 Fuchinohe, Sagamihara, Kanagawa, 229-8558, Japan

TEL:++SI -42-754-6251, FAX: +SI-042-754-6521, E-mail: nakada@ee.aoyama.ac.jp

ABSTRACT

Cu(In,.,,Ga,)Se, (C1GS)-based thin film solar cells have been fabricated using transparent conducting oxide

(TCO) hack contacts allowing light to pass through the entire solar cell structure. The cell performance of CIGS

devices using tin oxide (SnO,) and indium tin oxide (ITO)

back contacts were almost the same as that of a

conventional CIGS solar cell fabricated using a MO hack

contact. In contrast, fairly low efficiency was achieved for

a device fabricated using a Zn0:Al hack contact. The hest

cell performance was obtained using a substrate

temperatures of 500-520°C. However, the cell performance

was observed to deteriorate above 550°C. This is attributed

to increased resistivity of the TCO's due to the

outdifhsion of fluorine dopant atoms from SnOz and the

formation of a GazO, interfacial layer on the IT0 layer. We

propose a low-cost bifacial CIGS thin film solar cell for

the first time as one of the key applications of this type of

ClGS device; preliminary results of cell performance are

presented. A four-terminal tandem ClGS solar cell is also

briefly discussed.

1. INTRODUCTION

For conventional Cu(In,Ga)Sez thin film solar cells, metallic MO hack electrodes are commonly used, making it

impossible for light to pass through the metal electrode layer. However, a semitransparent solar cell is required for the top cell of multijunction (tandem) devices and in

addition such a cell has the potential for use in applications

such as bifacial devices and solar windows. We thus have investigated an alternative structure for

CIGS-based thin film solar cells using transparent

conducting oxide (TCO) back contacts in which light can

pass through the entire solar cell structure [I]. NREL [Z] and HMI 131 groups have also reported similar cell

structures and demonstrated four-terminal tandem devices.

However, several problems still remain before high

efficiency CIGS-based tandem solar cells including a low

efficiency top cell fabricated using CuGaSez or other wide-gap chalcogenide solar cells can be achieved.

Another problem is the increased manufacturing costs of

producing a complicated cell structure as compared to a single junction device. We therefore propose a bifacial

CIGS thin film solar cell as a candidate for a low-cost

high efficiency CIGS solar cell. In this paper, the details

of the fabrication and performance of ClGS thin film

solar cells using TCO hack contacts are first described.

Preliminary results on bifacial CIGS devices are then

presented.

2. CELL STRUCTURE AND

FABRICATION

Figure l(a) shows the cell structure of a

CIGS-based thin film solar cell using an n' type

transparent conducting oxide (KO) back contact as an

alternative to a metallic MO thin film. This type

Incident liaht

Incident liaht

1-1

-

Transmitted red light Front illumination

I Sodalime glass I

Rear ilurnination Glass I

(a) (b) (d

Fig. 1.

metallic MO thin film, (h) bifacial ClGS solar cell, and (c) four-terminal tandem solar cell. (a) Cell structure of a CIGS-based thin film solar cell using an d-type TCO hack contact alternative to a

2880 Oral

3rd world Conference on Phorovoltoic Energv Conversion May 11-18.2003 Osaka, Japan

of cell can be considered to be a basic unit of both

bifacial and tandem devices as shown in Fig. I(b) and (c).

respectively. In this work, commercially available F-doped

tin oxide (Sn02:F) was mainly used. The sheet resistance

and optical transmission of the Sn0,:F were 7 ohm/sq and

more than 85% at 550 nm, respectively. Indium tin oxide

(IT01 thin films were also used for additional experiments.

The sheet resistance and optical transmission were 10

ohmisq and more than 85% at 550 nm, respectively. The

detailed performance of devices using IT0 back contacts

has been reported elsewhere [4]. CIGS thin films were

deposited by the three-stage process onto TCO-coated

glass substrates at growth temperatures of 400-550°C.

The film composition of ClGS thin film measured by

inductively coupled plasma spectroscopy (ICP) was typically Cu:ln:Ga:Se=23.20: 16.54:9.20:5 1.07,

Ga/(In+Ga)=0.36, Cu/(In+Ga)=0.90 and Se/metal=l.O4.

Wide-gap CuCaSe, thin films were deposited by a