Case and cd(_1-x)zn(_x)se single crystal photovoltaic devices

Abstract

With bandgap ranging from 1.74 to 2.67eV depending on composition, the ternary alloy (ZnCd)Se is an interesting system for optoelectronic applications. The main purpose of the work reported in this thesis was to characterise some of the electrical properties of crystals of Zn(_x) Cd(_1-x) Se and to assess the " potential in CdSe/Cu(_2)Se and zn(_x)Cd(_1-x)Se/Cu(_2)Se photovoltaic cells. Single crystals of this ternary compound have been grown from the vapour phase using two different methods. With each technique boules of graded composition were produced with the Cd/Zn ratio decreasing towards the end that was last to grow. The variation in composition was determined using atomic absorption spectroscopy and energy dispersive X-ray analysis. Lattice Parameters were determined using X-ray diffractometry and were found to vary linearly with composition over a wide range. This study showed that for x < 0.5 the crystal adopts the hexagonal Wurtzite structure, changing to the cubic sphalerite for higher values of x. The variation is bandgap energy with composition was determined for single crystals of Zn(_x)Cd(1-x)Se at 300K and 9OK and shows that the bandgap changes quadratically in x for x < 0.6. The barrier heights of Aūzn cd(_1-x)sc (x < 0.45) Schottky diodes were calculated from forward I-V characteristics, C-V and photoelectric measurements were also carried out. A good linear relationship with composition was obtained for barrier heights measured by the Photoelectric method. Deep levels were also investigated in these diodes using Photocapacitance, which revealed the presence of two dominant levels having activation energies of 0.4 - 0.5 eV and 0.9 - 1.0 eV (referred to the valence bandedge) that were independent of the composition The second part of the thesis described an investigation into CdSe/CU(_2)Se and Zn(_x)Cd(_1-x)Se/CU(_2)Se (x < 0,4) devices that had been prepared on orientated single crystal substrates by a chemiplating technique. Reflection high energy diffraction (RHEED) showed that the structure of the CU(_2)Se layer took the cubic modification. Cells formed on as-grown low resistivity substrates exhibited no rectification. However good Photovoltaic properties were produced by heating the devices in air or Argon at 200 C, However, for cells formed on higher resistivity CdSe, the resultant devices showed a Photovoltaic effect without any heat treatment. The Photovoltaic output characteristics were measured under simulated AMI illumination. The properties of the Photovoltaic cells prepared on Zn(_x)Cd(1-x)Se single crystals are closely related to those of devices fabricated on CdSe substrates. Cells formed on CdSe were found to have higher short circuit current densities (J(_sc)), but lower open circuit voltage (V(oc)) than those produced on the mixed Zn(_x)Cd(_1-x)Se crystal substrates. Thus the open circuit voltage was increased with zinc content to 420 mv with a Zng(_0.4)Cd(_0.6)Se based cell. However, there was a considerable decrease in the short circuit current. The characterisation of these cells has revealed the main threshold in all the devices Indicated a dominant level with an activation energy of between 1.0 and 1.1 eV with respect to the conduction band in both CdSe and Zn(_x)Cd(_1-x)Se.