Organic-Inorganic Heterojunctions for Application in Perovskite Based Photovoltaics
Organic-inorganic lead halide perovskite solar cells (PSC’s) research has seen most notable progress in the field of photovoltaics (PV). The very first PSC was reported in the year 2009 with efficiency of 3.8%, which rapidly increased to the record 25.2% in year 2020. These numbers are quickly approaching the record values achieved for single-crystal silicon based solar cells. Defect tolerant nature of perovskite, high carrier lifetime, ability to tune band-gap and low-cost solution-based processing in addition to many rare properties makes it an ideal candidate for future solar cell technology. These are the properties which also allows this material to find applications beyond PSC’s, like photodetectors, memory, thin-film transistors (TFT’s), etc. However, given the many valuable properties of this class of material, they also come with some dominating properties which hampers commercialization of this PV technology. Problems like, Ion-migration, degradation in ambient condition still have not fully solved and understood. This research field still have many unanswered questions like, finding suitable compositional engineered lattice to make system stable, role of interface on charge transport and device stability. In this work, we have developed a novel process to grow micron size grains of CH3NH3PbI3 (MAPI) using a custom-made glass reactor. Pristine films are spin-coated on substrate in a glove-box and transferred to methyl-amine gas filled reactor followed by annealing of the whole setup in controlled environment. The resulted films are conformal with average grain size of > 1 microns. The resulted films are used to demonstrate the increase in minority carrier lifetime upon annealing in methyl-amine gas environment. Fabricated devices also showed improvement in device performance upon inclusion of large-grained MAPI film as compared to pristine MAPI film. At later stage, optimization of compact layer (c-TiO2) and mesoporous layer (m-TiO2) is performed followed by improved device fabrication methodology. This yield, device efficiency up to 17.5% with high reproducibility. Major contribution in improvement of device efficiency is from fill-factor (FF) of the device. We found that most of the resistive drop is coming from transparent conductive oxide (TCO) and the current path is changed within the TCO to substantially lower the series resistance of the device and improve FF. FF can also be affected by interface defect density and to see the effect of interface defect density on device performance, simulations are performed by taking experimentally found parameters as input to the simulator. High efficiency device fabrication involves deposition of c-TiO2 at 250oC in ALD chamber and thermal annealing of meso-TiO2 at 500oC for 1 hour, increasing the thermal budget of the device. To address this problem, we replace the conventional FTO/TiO2(c)/TiO2(m) stack with Aluminum doped zinc oxide (AZO), considerably simplifying the fabrication process and reducing thermal budget. Photoelectron spectroscopy suggests that AZO is an effective ETL for perovskite (MAPI) thin films, with a large valence band-offset and a small conduction band offset, but with a possible path for carrier recombination at the interface. We show that treating the surface of AZO with ozone gas (AZO:O3) improves the charge carrier extraction at the interface and open-circuit voltage (Voc) and efficiency (η) of 1.03 V and 10.5% respectively are achieved. Given the stability issue of the MAPI, couple of inorganic materials are explored as potential candidate as solar absorber. First material is barium bismuth oxide (BaBiO3, BBO), thin-films of which are deposited by pulsed laser deposition (PLD). Complete electronic band-diagram of BBO is constructed and TiO2 has been used to make single-sided type-2 heterojunction to test BBO’s opto-electronic properties. Second material is cesium titanium bromide (Cs2TiBr6, CTB), thin-films of which are deposited by thermal annealing of cesium bromide (CsBr) thin-films in titanium bromide (TiBr4) vapors in a glove-box. Proof-of-concept CH3NH3PbI3 device with efficiency of 3.2% has been shown on flexible stainless steel (SS). Also, photodetectors have been made based on gold and carbon-based electrode and comparison has been made. This work provides fabrication and characterization of high-efficiency perovskite solar cell and understanding on charge transport across interface in planar device.