Electrochemical Photovoltaic Cell with 6.9% Efficiency using Polycrystalline CdSe Grown by a Simple Liquid Metal-Vapour
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ELECTROCHEMICAL PHOTOVOLTAIC CELL WITH 6.9% EFFICIENCY USING POLYCRYSTALLINE CdSe GROWN BY A SIMPLE LIQUID METAL-VAPOUR REACTION MARCUS F. LAWRENCe, ZHITSING DENG AND LOUIS GASTONGUAY* *Concordia University, Dept. of Chemistry, 1455 de Maisonneuve Blvd. West, Montreal, Quebec, Canada H3G IMB. **I.N.R.S.-Energie, C.P. 1020, Varennes, Quebec, Canada JOL 2P0
INTRODUCTION Research on 1I-VI semiconducting compounds during the last two decades has been motivated by possible device applications such as thin film transistors, photodetectors and solar energy converters. In particular, the direct gap n-type semiconductor CdSe, has remained the subject of studies aimed at developing efficient photovoltaic and photoelectrochemical cells. For the low-cost production of polycrystalline CdSe layers, many noteworthy methods have been employed, such as: vacuum deposition, spray pyrolysis, chemical bath deposition and electrodeposition [1-5]. The best reported performances of photoelectrochemical cells using CdSe films obtained by these methods, in contact with an aqueous polysulfide electrolyte and under solar or simulated solar radiation, have varied between 5 and 7% [6,7]. More recently, studies based on the work of Iwanov and Nancy concerning the direct synthesis of epitaxial II-VI films on single crystal metal substrates [8], have led to the development of another promising technique for the low-cost production of polycrystalline CdSe layers. This method, referred to in the past as the "tarnishing reaction" [9] or as the "gas-solid process" [10], involves the reaction of the chalcogen vapour (Se) with the surface of the heated metal substrate (Cd), under a constant argon flow. The results presented here show that, under the proper experimental conditions, the liquid metal--vapour reaction enables the synthesis of polycrystalline CdSe semiconductor layers with a 6.9% conversion efficiency when in contact with a 1 M polysulfide electrolyte 2 and under 80 mW cm- of white light illumination. The highly textured surface of the samples thus obtained, would seem to be responsible for the high photovoltaic efficiency.
EXPERIMENTAL METHODS Liquid metal-vapour reaction: The liquid metal-vapour reaction system is illustrated in Fig. 1. 2 Cadmium substrates (2 x 2 cm ) were cut from a 1 mm thick plate of Cd
Pyrex Reaction chamber
Ar
inle t
_
_Se
_
_
/d
Ar
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C
I////////
outlet
Muffle Fig.
Furnace
1: Liquid metal-vapour reaction apparatus.
Mat. Res. Soc. Symp. Proc. Vol. 131. 61989 Materials Research Society
150
(Ventron, 99.999%). The substrates were etched for 5 sec in a 5:4:1 mixture of 110: HCI:HNOý, thoroughly rinsed with doubly deionized water, and then quickly dried with nitrogen before being introduced in the reaction chamber. To remove oxygen, the reaction chamber containing a Cd substrate is purged for 30 min (100 cm mirfI) with ultra-high purity argon (99.999%) which has passed through an Oxysorb oxygen removal unit (SPECTREX). The argon is introduced via a 1/4 inch glass tube which acts simultaneously as the
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