T. M. Razykov*, K. M. Kouchkarov

Physical-Technical Institute, Scientific Association “Physics-Sun", G. Mavlyanov Str.2B, Tashkent 700084, Uzbekistan

Phone: + 998-71-135-4103, Fax: +998-71-135-4291, E-mail: razykov@uzsci. net

Abstract

Analysis of the temperature dependence of the current-voltage characteristics of polycrystalline thin film glass-ITO-n-CdS-p-CdTe-graphite solar cells is reported in this paper. CdTe films were fabricated by a low cost, novel, chemical molecular beam deposition (CMBD) method in an atmospheric pressure Ar flow, from separate Cd and Te sources.

Value of the series resistance determined from the direct current-voltage characteristics is RS= 1.2 x 105 Ohm. It is rather high for thin film solar cells. Changing of the forward current with temperature is caused by the temperature dependence of the diffusion potential V>. The slope of the forward current — voltage characteristics does not depend on temperature. It is shown that the predominant mechanism of the forward current is the multistep tunneling and recombination. The reverse current mechanism is the thermal excitement of carriers in the space charge region at kT/e < V < 1.0 V and the tunneling of carriers at 1 < V< 10 V.

1. Introduction

Polycrystalline CdTe films based solar cells remain one of candidates for large scale terrestrial application. The optimal band gap of 1.46 eV and sufficiently high absorption coefficient of 104 — 105 cm-1 make it attractive for a number of research groups. At present the worldwide record efficiency for CdS-CdTe thin film solar cells is 16.5 % [1]. This is highest value for thin film solar cells after Cu(In, Ga)Se2 (18-19 %) based solar cells.

More than 10 % efficient CdS-CdTe thin film solar cells were fabricated by different technologies: chemical vapor deposition, electrochemical deposition, physical vapor deposition etc. Novel chemical molecular beam deposition (CMBD) of polycrystalline II-VI binary and multinary films in atmospheric pressure gas flow [2] has many advantages of chemical vapor deposition and molecular beam epitaxy techniques. We have fabricated glass — ITO — CdS-CdTe-graphite solar cells by CMBD for the first time. The current mechanisms in this structure are discussed in this paper.

2. Experimental

n-CdS film was deposited on ITO-glass by vacuum evaporation. p-CdTe film was fabricated by CMBD in the atmospheric pressure Ar flow from separate Cd and Te sources. Details of CMBD process were described by Razykov [2]. The thickness of CdS and CdTe films were 0.1 pm and 3.0 pm respectively. The temperature dependence of the current-voltage characteristics was carried out in the wide range of 193-300 K.