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Solar Cells

To make the photovoltaic technology competitive with traditional fossil fuel combustion, cost photovoltaic systems needs to be reduced by at least by another 60%. The way to achieve this is to improve the power conversion efficiency and/or reduce fabrication costs of the state-of-the-art. Historically the crystalline silicon solar has been the technology of choice because it had the most success in making PV more affordable. However, going forward there seems to be little room for improvement in conventional Si PV. The efficiency of conventional Si solar cells has stagnated around 25% and the fabrication costs are limited by the high costs of ultrapure crystalline silicon. The goal of the solar research at CeNSE is to look beyond the current silicon technology and develop next generation solar PV that can boast of substantially higher efficiencies at substantially reduced costs.

Perovskite-like Solar Cells

The thin-film perovskite cell structure

The thin-film perovskite cell structure

Methylammonium lead trihalide or MAPbI3 perovskite solar cells are a new class of thin-film photovoltaic devices, discovered just 4 years ago, that has already shown exceptional performance (up to 20% efficiency). However, the MAPbI3 technology still faces two significant challenges – device instability and high-toxicity. At CeNSE we are trying to solve these problems by pursuing two strategies. First, we are trying to integrate a novel graphene/polymer encapsulating layer (WVTR ~ 10-6 gm/m2day) with the perovskite cells to protect them from water/oxygen and improve their lifetime. Second, we are searching for lead-free alternatives to MAPbI3. Using careful DFT calculations backed by experiments we are scanning the materials space for promising solar materials, with the ultimate goal of demonstrating a reliable efficient thin-film solar cell technology.

Silicon/Perovskite Tandem Cells

Efficiency of single-material solar cells is limited by thermodynamics in what is called the Shockley-Queisser limit (SQL). The SQL of perovskite (bandgap = 1.7 eV) and silicon (bandgap = 1.1 eV) cells is only 25% and 30%, respectively. Practical devices usually yield even lower efficiencies –perovskite and silicon solar cells have efficiency of 15-18% and 21-22%, respectively. One way to exceed the SQL is to integrate different absorber materials onto the same device, forming a tandem cell. Given the bandgap of perovskite and silicon, a perovskite/Si tandem cell can theoretically have efficiency of >40%. Even the realization of a more practical 30% efficient solar device would have tremendous impact on the cost and viability of renewable photovoltaic technology.

(Left) Maximum theoretical efficiency of a tandem cell with perovskite (bandgap~1.7 eV) as top cell and silicon (bandgap =1.12eV) as the bottom cell is more than 40%. (Right) The perovskite/Silicon tandem cell device structure.

(Left) Maximum theoretical efficiency of a tandem cell with perovskite (bandgap~1.7 eV) as top cell and silicon (bandgap =1.12eV) as the bottom cell is more than 40%. (Right) The perovskite/Silicon tandem cell device structure.

Thin-Film Electronics

While most of the traditional electronics focuses on making things smaller and smaller (following Moore’s Law), there is a significant need for electronics devices that are very large, e.g. transistors used in displays, large-area sensors, etc. Due to the very large sizes involved (displays can be more than 100 inches across), the conventional wafer-based technology becomes prohibitively expensive. So instead of wafers, industry uses thin-films of amorphous and polycrystalline semiconductors deposited on cheap substrates like glass or plastic. In the transition from crystalline wafers to thin-films, one loses performance (mobility is lesser than 100cm2 / Vs) but gains the ability to manufacture really large & flexible devices. At Centre we fabricate thin-film devices on a range of materials.

P-type Metal-Oxides Transistor

Structure of a proposed p-channel thin-film metal-oxide transistor. Other devices like above can also be fabricated

Structure of a proposed p-channel thin-film metal-oxide transistor. Other devices like above can also be fabricated

ZnO and IGZO metal-oxide thin films have been very successfully used to demonstrate n-type thin-film devices using a variety of thin-films depositions techniques. However, the complementary p-type metal-oxide has proved elusive. Due to the crystal structure, the hole mobility of most metal-oxides is up to a 1000 time lower than their electron mobility. The few metal-oxides which do have reasonable hole-mobility, like Cu2O and SnO, tend to form very defective thin-films which lead to poor device performance. At the Centre, we are trying to design new materials and use novel deposition techniques to demonstrate p-type metal-oxide transistors. Such devices will be useful low-power CMOS circuits for displays, sensors, etc.

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