The screen printed and sintered CdS/CdTe solar cell shows a conversion efficiency of over 12% under sunlight. However, the cell area in only 40 mm2 which is too small for terrestrial applications. Because the resistivity of CdS window layer in relatively high, the series resistance of the cell becomes large as the cell size increases. In order to reduce the series resistance, we adopted transparent conducting oxides (TCOs). such as indium tin oxide (ITO) and fluorine doped tin oxide ($SnO_2$:F), which have a very low resistivity. Adapting TCO films to screen printed CdS/CdTe solar cells as a front contact material increases the junction leakage current, resulting in the degradation of cell conversion efficiency. This thesis analyzed the origin of junction leakage current by C-V measurement, I-V measurement, AES and SEM. The origin of the leakage current is formed to be the penetration of carbon paste into the CdS/CdTe interface through porous CdTe layer. Using (Cd+Te) mixed slurry, instead of CdTe slurry, enabled us to make more dense CdS/CdTe interface morphology and to reduce the junction leakage current. Thus, the screen printed ITO/CdS/CdTe with high cell conversion efficiency was realized for the first time. The cell efficiency with 10 mm length and 9 mm width was improved 9.5% under 50 mW/㎠ of tungsten lamp.
The following paragraphs describe the preparation and characterization of $SnO_2$:F as a transparent conducting oxide by spray pyrolysis and chemical mist deposition method, the analysis of the origin of TCO/CdS/CdTe junction leakage, the improvement of solar cell efficiency with a larger area.
$SnO_2$ film that is one of the TCO films was prepared by spray pyrolysis and chemical mist deposition method (CMD) to use front conducting material in screen printed CdS/CdTe solar cell. CMD method did not show a good reproducibility. Thus, spray pyrolysis method was used to deposit $SnO_2$ films. Electrical and optical properties of $SnO_2$ films have been investigated by measurements of electrical resistivity, Hall effects, optical transmission and observation of microstructure. From the results of resistivity and transmittance of $SnO_2$ films that were deposited by solution with various volumetric ratios of D.I. water to ethanol, the content of oxygen vacancy in the films increase as increasing amount of ethanol in solvent. As the deposition temperature increased, the texture of $SnO_2$ films changed from (200) to (211) and (110) planes that are more stable planes. The variation of optical and electrical properties of $SnO_2$ is due to the change of orientation. $SnO_2$:F film which was deposited by the solution with a ratio of F/Sn=2.3 had a resistiviy of $4Ⅹ10^{-4}$ ohm-cm and transmittance of over 90%.
CdS/CdTe solar cell on $SnO_2$/glass and ITO/glass were prepared by CdTe compound slurry to reduce the series resistance. However, the screen printed CdS/CdTe solar cells with TCO front contacts showed a large leakage current. The leakage current in screen printed CdS/CdTe solar cells might be caused by two factors, the increase of interface state by diffusion from substrate and the formation of three dimensional imperfections at junction during the sintering process. The former effect has been investigated by C-V and I-V curve at various temperature, and the latter effect has been investigated by scanning electron microscope (SEM) and I-V plot. The results of C-V and I-V plots showed that the amounts of interface states of CdS/CdTe cells with TCO ($SnO_2$, ITO) electrodes were larger than those of interface state of cells without TCO. The C-V analysis of $SnO_2$/CdS/CdTe cells showed the pinning of depletion width as increasing applied reverse bias. It appears that the phenomena is caused by the increase of interface state. However, this diffusion factor can't explain the large leakage current in screen printed TCO/CdS/CdTe cells despite of its some degradation of cell properties. SEM microstructure revealed that carbon paste (ohmic contact of CdTe) can flow to junction area through porous screen printed CdTe layer. If the carbon paste is in contact with CdS, current can transport from carbon contact to CdS layer directly and can be origin of leakage current. I-V plots of a cell with two types of carbon contact materials, SPI (paste form) and GA-66D (coated only surface of CdTe), show that the porous CdTe of junction area is the origin of leakage current in the screen printed CdS/CdTe cells.
(Cd+Te) mixed slurry was adopted instead of CdTe slurry to fabricate a denser CdS/CdTe during CdTe compound formation from (Cd+Te) mixed powder. The densest junction was obtained when the increasing temperature of CdTe was increased at the rate of 14℃/min, up to 500℃.CdTe compound was synthesized fully from (Cd+Te) mixture at the rate of 14℃/min and then the denser CdTe junction was obtained during sintering process. When (Cd+Te) slurry was used to form CdTe layer in ITO/CdS/CdTe cell, the leakage current and the series resistance of the cell were significantly reduced. As the cell width increased from 3 to 9 mm, the efficiency of CdS/CdTe cells with ITO electrode degraded slightly from 10.5 to 9.4%, while that of cells without ITO decreased from 10.4 to 7.4% under the illumination of 50 mW㎠. The estimated module efficiency of the screen printed CdS/CdTe cells without ITO had a maximum value of 6.5% at only 4 mm width. However, that of the cells with ITO electrode increased to 7.6% at 9 mm width. Thus, adopting ITO electrode enabled us to improve the module efficiency of the screen printed CdS/CdTe solar cells with large active area
