Abstract
Harnessing natural energy might be the best approach toward satisfying today’s growing world energy demands, solar energy the undisputed frontrunner among all such sources. Organic solar cells have seen enormous progress over the last two decades because of cheap, light weight, mechanically flexible and large scale production. Polymers or small molecules used in organic solar cell has large bandgap on the order of 2 eV, it limits the solar harnessing potential to <50 % of the total incident solar energy. Organic-inorganic hybrid (OIH) materials are having the combined properties of inorganic semiconductors and organic (polymer or small molecules) materials. OIH solar cells could adopt the merits of inorganic materials, such as stability, enhanced light absorption, high carrier mobility and compatible fabricating process, and utilize the advantages of organics, such as light weight, flexible, adjustable molecular structures for energy band alignment, facile solution processability. A buffer layer is an important constituent between the electrode and active layer; it plays the crucial role of extracting and transporting the photogenerated carriers (holes or electrons). The use of low-temperature-solution–processable bismuth iodide (BiI3) nanosheets as hole transport layers in organic photovoltaics was demonstrated. The performance of the resulting devices was comparable with that of corresponding conventionally used Polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). In order to push perovskite photovoltaic technology to real market, the large area and low material wastage method has to be realized. Study of the perovskite film formation by utilizing new technique is very important. This work demonstrated that the spray deposition is promising technique for fabricating large area perovskite solar cells. The champion device shows PCE of 10.2 % and large area devices with 60 and 342 mm2 achieved the PCE of 6.88 and 4.88%, respectively. To further improve the efficiency, droplet-assisted two-step process of spin coating the lead iodide (PbI2) followed by spray coating the methylammonium iodide (CH3NH3I) was used to prepare continuous lead iodide perovskite film. Power conversion efficiency (PCE) of 11.66% was achieved at 100 nm of PbI2 followed by 300 μl of CH3NH3I and annealed at 100°C for 120 min. The main issues in planar perovskite solar cells are the coverage and crystallinity of the perovskite film on the PEDOT:PSS layer. To enhance these features, we introduced alkali metal halides (salts) as additives in the perovskite precursor solution used in a two-step preparation method. These alkali metal halides chelate with Pb2+ ions and enhance the crystal growth of PbI2 films, resulting in nanostructured morphologies. The nanostructured PbI2 films promote homogeneous nucleation and larger crystallite sizes, thereby enhancing the morphology and crystallinity of the perovskite films. The alkali metal halides recrystallize the small grains and passivate the grain boundaries and interface states, allowing effective charge generation and dissociation in perovskite films. The power conversion efficiency of the device incorporating a small amount of a salt additive increased by approximately 33% from 11.4 to 15.08% and device was more stable. The major disadvantage of the perovskite (CH3NH3PbI3) is that it contains lead (Pb), which limits the commercialization of this perovskite material due to its toxicity. To overcome this issue, antimony (Sb) based perovskite materials were used for lead free planner perovskite solar cells. Preliminary results shows that, Sb based materials are potential candidate for less toxic, environmental friendly photovoltaic applications.