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OPTIMIZATION OF HIGH EFFICIENCY MULTILAYER SOLAR CELLS BASED ON III/V COMPOUNDS.

Objective

THE AIM OF THIS PROJECT IS THE REALIZATION OF HIGH EFFICIENCY SOLAR CELLS, MADE BY MONOLYTHIC MULTILAYER GROWTH OF STRUCTURES BASED ON III-V COMPOUND SEMICONDUCTORS. THE GROWTH OF SUCH CELLS CAN BE EFFECTED BY MOCVD.

HIGH EFFICIENCY SOLAR CELLS CAN BE MADE BY STACKING DIFFERENT SOLAR CELLS IN SERIES. IN SUCH A STRUCTURE A MORE EFFICIENT ABSORPTION OF THE SOLAR SPECTRUM IS TO BE EXPECTED, LEADING TO AN INCREASE IN EFFICIENCY. IN PRINCIPLE TWO DIFFERENT WAYS ARE OPEN TO ACHIEVE THIS, VIZ. MECHANICALLY STACKING OF DIFFERENT CELLS OR, ALTERNATIVELY, MONOLYTHIC MULTILAYER GROWTH OF CELLS, E.G. BASED ON III-V COMPOUND SEMICONDUCTORS. THE FIRST APPROACH RECENTLY LEAD TO A 31% EFFICIENT CELL (AM 1.5, 300-500 SUNS) MADE BY MECHANICALLY STACKING OF A HETEROJUNCTION GAAS AND A CRYSTALLINE SI CELL. HOWEVER, THE SECOND APPROACH WHICH IS IN PRINCIPLE MORE PROMISING, UP TILL NOW HARDLY YIELDS MORE EFFICIENT PV-STRUCTURES THEN SIMPLE-JUNCTION CELLS. THIS PROBABLY IS A RESULT OF CRYSTALLOGRAPHIC IMPERFECTIONS IN LAYERS AND ON INTERFACES, WHICH ACT AS EFFECTIVE RECOMBINATION CENTERS, THEREBY REDUCING THE PHOTOCURRENT AND DIMINISHING THE MINORITY CARRIER DIFFUSION LENGTH. THESE DEFECTS APPEAR TO BE INTRODUCED DURING GROWTH AND PROCESSING OF THE LAYERED STRUCTURES.
IN THIS PROJECT MONOLYTHIC SOLAR CELLS ARE GROWN BY MOCVD AND MBE. A STUDY HAS BEEN PERFORMED OF SEVERAL ASPECTS OF THE MOCVD GROWTH OF GAAS AND ALGAAS. STARTING WITH THE SUBSTRATE, IT WAS SHOWN EARLIER THAT DEFECTS PROPAGATE AND EVEN MULTIPLICATE IN EPILAYERS GROWN ON TOP OF IT. HARDENING OF THE SUBSTRATE MATRIX BY IN ALLOYING LEADS TO SEVERE MISMATCH DISLOCATIONS IN THE EPILAYERS, WHICH ARE DELETERIOUS TO CELL PERFORMANCE. SUBSEQUENTLY, OPTIMUM CONDITIONS FOR THE MOCVD GROWTH OF GAAS AND A1XGA1-XAS AS A FUNCTION OF PROCESS VARIABLES WERE DEFINED EXPERIMENTALLY. IN ADDITION, WE ALSO PERFOMED A SYSTEMATIC STUDY IN CONNECTION WITH BOTH P-AND N-DOPING OF GAAS AND A1GAAS; THE INCORPORATION OF THE DOPE MATERIALS WAS RELATED TO VARIOUS MOCVD GROWTH PARAMETERS. USING THE RESULTS OF THE ABOVE-MENTIONED GROWTH- AND DOPINGSTUDIES, WE GREW GAAS SOLAR CELLS ON SI-DOPED SUBSTRATES AS A FIRST STEP TOWARDS MINOLYTHIC MULTILAYER CELLS. CELL EFFICIENCIES OBTAINED NOWADAYS REACH A REPRODUCIBLE LEVEL OF 18.5%.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

STICHTING KATHOLIEKE UNIVERSITEIT
Address

6500 GL Nijmegen
Netherlands