ESTABLISH THE SCIENTIFIC AND TECHNICAL BASIS FOR A VARIABLE EUROPEAN AMORPHOUS SILICON SOLAR CELL INDUSTRY
The development of device grade amorphous silicon germanium hydrogen material is described. The development of the TCO/p+ interface and p/i interface is of great interest for good quality pin diodes, especially for high fill factors. Some properties of these barriers are discussed. Since the role of the internal electric field of the pin structure was demonstrated, some experiments influencing the electric field were carried out.
The effect of electric field modifications have been studied. The microcrystalline n+ layer can be used in pin cells as a good semiconductor contact to a metal or as a semiconductor contact inside a stacked pin structure, for which a low ohmic contact is of great importance.
The semiconductor contacts of a pin cell are very critical and the same seems to be valid for the TCO and metal contact. A detailed analysis of these contacts has been carried out. The integrated type of module, which can be manufactured by amorphous silicon solar cells, has a great potential for cost reduction. One method for this is laser scribing. Some results on this method are summarised. Last, but not least, the important technical problem of how to handle deposition gases has been studied.
Material deposition techniques have been developed to produce amorphous silicon modules with sizes up to 1 x 0.6 m and production problems such as high throughput and machine cleaning have been solved.
Studies on the physics of amorphous silicon devices have been conducted with a strong emphasis on stability.
Module integration on a monolithic glass substrate was achieved by a set of flexible techniques compatible with any module pattern. All losses related to the making of a module were introduced into a model allowing size optimization. All assumptions and predictions of the model were confirmed by a systematic scanning procedure of a series of 30 x 30 cm modules.
THE QUALITY OF BOTH, FRONT AND BACK CONTACT IN THIN FILM SOLAR CELLS WITH STRUCTURES OF GLASS/CTO/PIN-A-SI/METAL HAS AN IMPORT INFLUENCE ON PERFORMANCE AND THERMAL STABILITY OF THE DEVICE.
THE GLOW DISCHARGE PROCESS DURING THE DEPOSITION OF THE P-LAYER INDUCES A DEGRADATION OF THE CTO LAYER,ESPECIALLY SNO2 IN OUR CASE (FIG.1). IT WAS FOUND THAT SUBSTRATE TEMPERATURE IS A CRITICAL PARAMETER FOR CELL FABRICATION: TEMPERATURES HIGHER THAN APPROXIMATELY 200 DEGREES CELCIUS FOR P-LAYER DEPOSITION INITIATE A DETERIORATION OF CELL PERFORMANCE. AES DEPTH PROFILING HAS BEEN CARRIED OUT TO ANALYZE THE CHEMISTRY OF THE CTO-P INTERFACE. ELEMENTAL TIN CAN BE OBSERVED AT THE INTERFACE AND IN THE P-LAYER. THE BEHAVIOR OF THE CELL CHARACTERISTICS COULD BE EXPLAINED BY THE ELECTRICAL AND OPTICAL DATA OF THE P+-LAYER (SEE FIG.2).
TWO MAIN PROBLEMS OCCUR AT THE BACK SIDE OF THE CELL, NAMELY AT THE N/A1 INTERFACE: 1) HIGH CONTACT RESISTANCE WHICH EFFECTS INITIAL CELL EFFICIENCY AND 2) THERMAL DEGRADATION OF THE INTERFACE WHICH LIMITS THE INTERFACE WHICH LIMITS THE LIFETIME OF THE DEVICE. BOTH AMORPHOUS AND MICROCRYSTALLINE MATERIALS WERE USED AS N-LAYERS. THE ALUMINUM ELECTRODE WAS DEPOSITED BY E-BEAM EVAPORATION AND MAGNETRON SPUTTERING. IN THE CASE OF EVAPORATED A1, A SERIES RESISTANCE WHICH IS LIKELY TO BE DUE TO A NATIVE SILICONOXIDE FILM, DETERIORATES THE I-V CURVE OF THESE CELLS. AN APPROPRIATE ANNEALING PROCESS AFTER THE METALLIZATION STEP CAN RECOVER THE DEVICE PERFORMANCE WHICH IS COMPARABLE TO CELLS WITH SPUTTERED A1 ELECTRODE. THERMAL STRESS APPLIED TO A-SI CELLS AT ELEVATED TEMPERATURES BETWEEN 100 C AND 200 C LEADS TO A SHORT-CIRCUIT OF THE DEVICE (FIG.3.). AES PROFILES OF SUCH DEGRADED CELLS REVEAL INTERDIFFUSION OF SI AND A1 AT THE N/A1 CONTACT. ACTIVATION ENERGY OF THE DEGRADATION MECHANISM IS ESTIMATED TO BE AROUND 1,2 EV (FIG.4 AND 5).
Funding SchemeCSC - Cost-sharing contracts