Optical and structural characterization of mixed halide perovskite thin films by thermal co-evaporation


Thesis Type: Postgraduate

Institution Of The Thesis: Middle East Technical University, Turkey

Approval Date: 2016

Thesis Language: English

Student: Onur Yılmaz

Co-Supervisor: SELÇUK YERCİ, RAŞİT TURAN

Abstract:

Cost-effective, easy-to-produce, optically and electrically configurable properties of perovskite allow researchers to perform a profound investigation to better understand its physical and chemical nature. Additionally, band gap tunability by introducing bromide (Br) to replace iodine (I) also makes perovskite a suitable material for tandem solar cells. However, to fully benefit from perovskite cells, hysteresis in current-voltage curve, poor stability and relatively low fill factor in large area devices need to be overcomed. In the literature, various deposition techniques (spin coating, dip coating and thermal evaporation combined with one of the first two) have been used for fabricating Br containing perovskite. In this thesis, mixed halide (CH3NH3PbI3-xBrx) perovskite thin films were fabricated entirely by thermal co-evaporation in the full range (x is ranging from 0 to 3) for the first time. Five sets of samples having different Br/I ratio including two extremes (only iodide and only bromide containing) were fabricated successfully after achieving high crystallinity and uniform morphology by optimizing the deposition conditions. Perovskite films with different Br/I ratio were characterized optically and structurally using photoluminescence, X-ray diffraction, reflection and transmission, and scanning electron microscopy measurements. The results clearly demonstrate band gap widening and lattice constant increase with Br concentration. CH3NH3PbI2Br sample with a band gap of 1.65 eV, suitable for tandem solar cells were fabricated with no Stokes shift in PL.