Mixed Halide Perovskite Solar Cells: Influence of Fabrication and Post Fabrication Conditions on the Device Performance
Organic-inorganic hybrid perovskite solar cells have attracted tremendous attention due to the dramatic improvement in efficiency from 3.8% to 22% in seven years. The technology, however, faces several challenges such as degradation due to the effect of humidity, hysteresis in the current-voltage characteristics, and lead toxicity. These problems adversely affect the commercial viability. In perovskite solar cells, 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene which is generally termed as spiro-MEOTAD, is commonly used as the hole transport layer. It is well known that spiro-OMeTAD shows high conductivity in oxidized form and it needs chemical dopants in order to promote the oxidative reaction between spiro-OMeTAD and oxygen. Some of the common dopants such as tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) bis(trifluoromethylsulphonyl) imide (FK dopant) are expensive. Low materials cost and fabrication cost is a pre-requisite for the commercialization of a solar cell technology. We developed a surface-treatment process for enhancing hole transport properties of spiro. The as-fabricated cells consisting of a spiro layer at the top was treated with an oxygen plasma. This post-fabrication treatment dramatically improved the performance of the cells without causing any degradation. The studies revealed that the enhancement in the device performance was due to increase in the concentration of oxidized spiro-OMeTAD. The presence of oxygen in the spiro-OMeTAD was revealed using XPS studies. Our method of doping could be used as a low-cost alternative for the conventional FK doping. This process is clean, fast, safe and environmentally friendly. In order to understand the factors influencing the cell performance, we made a comprehensive study on the effect of fabrication parameters as well as post-fabrication conditions. We developed a chamber for the solar cell characterization in a well- controlled atmosphere. Impedance spectroscopy studies were done to understand the carrier transport and recombination characteristics. The studies revealed that charge transport through the bulk was affected by plasma treatment depending on plasma conditions. We attributed it to the interaction of oxygen with perovskite layer. Nevetheless, the spiro-OMeTAD conductivity and charge transfer at interface with perovskite improved with treatment and thus giving a significant overall improvement in the device performance.
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