Revisiting CdS-PbS Solar Cell Structure
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1012-Y12-31
Revisiting CdS-PbS Solar Cell Structure Harumi Moreno Garcia, O. Gómez-Daza, J. Campos, M. T. S. Nair, and P. K. Nair Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Av. Xochicalco S/N, Temixco, Morelos, 62580, Mexico ABSTRACT Starting in 1969 and into 1970’s CdS-PbS cells were reported with open circuit voltage (Voc) up to 450 mV and short circuit current density (Jsc) < 1 mA/cm2. However, further reports are scarce. In this work we revisit this type of photovoltaic junction. Photovoltaic behavior of cell structures prepared by chemical deposition is presented for: glass/CdS/PbS/Ag, SnO2:F/CdS/PbS/Ag, and SnO2:F/CdS/(Bi2S3 or/and CdSe/PbS)/Ag. SnO2:F/CdS/PbS/Ag shows Voc of ≈ 500 mV and Jsc of ≈ 2 mA/cm2 under an illumination of 3 kW/m2, while SnO2:F/CdS/(Bi2S3 or/and CdSe)/PbS/Ag show Voc of ≈ 200 to 300 mV and Jsc of ≈ 2 – 6 mA/cm2 under tungsten-halogen illumination of 1 kW/m2. This work opens up possibilities for developing simple solar cell structures by sequential chemical deposition of semiconductors.
INTRODUCTION CdS (optical band gap Eg, ≈ 2.5 eV) is a photoconductor, which is famous as nwindow/buffer layer in thin film solar cells. PbS (Eg ≈ 0.4 eV) is also a photoconductor, which has risen to prominence during the past few years due to high efficiency carrier multiplication expected for photon energy, hν > 2-3 E g in the nanocrystal regime [see for example, 1]. These two materials are among the most investigated by chemical deposition technique [2]. Material parameters for CdS and PbS, for example, work function of ≈ 4.4 eV for (n)CdS and ≈ 3.4 eV for (p)PbS could give built-in voltages of nearly 1000 mV for the CdS/PbS junction [3]. Diffusion voltage of 400 mV and Voc of 400 mV (300 W/m2-tungsten) were reported for the CdS/PbS junction [4]. Heterojunctions of ZnCdS/PbS and CdS/PbS were prepared by spray pyrolysis which showed Voc 400-500 mV and Jsc 0.03-0.05 mA/cm2 under 1 kW/m2 (tungsten) radiation [5]. Efficiency analysis [3] shows that the practical cell efficiency would be < 10% for PbS absorber in single electron-hole pair/photon generation process with hν > Eg. This probably is a reason why CdS/PbS cells were not investigated further. However, we considered that CdS/PbS would be the easiest and simplest cell structure, which could be developed in any laboratory in an all-chemical deposition approach for photovoltaic cell production. In this work we discuss some characteristics of the materials X = CdS, PbS, Bi2S3 and CdSe and present the photovoltaic structures (n)CdS/X/(p)PbS developed by sequential chemical deposition of the different semiconductor films. EXPERIMENTAL DETAILS Thin film deposition For the measurement of thin film characteristics, the semiconductors were deposited on Corning microscope glass slides (75 mm x 25 mm, 1 mm thickness). The general reference for the solution composition and the conditions of depositions of the different semiconductor thin films mentioned below is [6].
CdS: TEA method – 5 ml 0.1 M cadmium nitrate, 5 ml 50% (3.
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