Results concerning the properties of polycrystalline cadmium selenide films, as well as the metal insulator semiconductor (MIS) and pn-type solar cells produced from these films, show that remarkable photovoltaic parameters can be achieved.
Starting from the II-VI compound semiconductor cadmium selenide, thin film solar cells have been prepared and characterised in detail.
Not only the MIS system, but also a pn-hetereojunction has been investigated as the basis for a cadmium selenide solar cell.
A comprehensive set of scientific methods has been applied to characterise the diode structures prepared.
The photovoltaic parameters of the diode structures investigated were found to be significant.
On the whole, the barrier formation of diode structures has been found to be unsatisfactory. This is thought to be due to a high, continuous distribution of density states in the energy gap that results from selenium correlated defects in polycrystalline cadmium selenide films.
TWO DIFFERENT APPROACHES FOR A SOLID STATE SOLAR CELL ON THE BASIS OF THIN POLYCRYSTALLINE CDSE FILMS ARE INVESTIGATED: A MIS STRUCTURE AND A P-N HETEROJUNCTION.
THE INHERENTLY SIMPLY MIS SYSTEM CONSISTS OF THE FOLLOWING FILM SEQUENCE: CR COATED GLASS SUBSTRATE, POLYCRYSTALLINE N-CDSE, ZNSE (I-LAYER), AU (SCHOTTKY METAL), CURRENT GRID, AND ANTIREFLECTING COATING. STANDARD VACUUM EVAPORATION PROCESSES ARE APPLIED TO DEPOSIT THE DIFFERENT METAL AND SEMICONDUCTOR FILMS. DETAILED INVESTIGATIONS APPLYING SEM, ADMITTANCE SPECTROSCOPY, CURRENT VS. BIAS, AND SPECTRAL RESPONSE MEASUREMENTS REVEAL ORIGINS OF THE PHOTOVOLTAIC LIMITATIONS OF THE MIS SYSTEM. THE MAXIMUM ACHIEVABLE PHOTOVOLTAIC PARAMETERS ARE I SC = 20 MA/CM2, U OC = 700 MV, FF = 58% AND AN EFFICIENCY <= 7.2%.
THE ALTERNATIVE N-CDSE/P-ZNTE HETEROJUNCTION IS EXPECTED NOT TO SUFFER FROM THE PROBLEMS OF THE MIS SYSTEM BECAUSE OF THE VERY DIFFERENT INTRINSIC CONSTRUCTION. ON THE BASIS OF THE ANDERSON MODEL DIFFUSION VOLTAGES OF 1-1.5 V ARE EXPECTED.
THE MATCHING OF CDSE/ZNTE IS THE MOST CRUCIAL PART DURING THE DEPOSITION PROCESS. THE CHANGE FROM THE CDSE DEPOSITION TO THE ZNTE DEPOSITION HAS TO BE PERFROMED BY CONTINUOUSLY CHANGING THE IMPEDING FLUX FROM CDSE TO ZNTE. OPEN CIRCUIT VOLTAGES OF UP TO 850 MV HAVE BEEN ACHIEVED. THE LOW SHORT CIRCUIT CURRENTS AND FILL FACTORS ARE ASSUMED TO BE DUE TO THE NOT YET OPTIMISED DOPING OF THE CDSE DEPLETION REGION.