Photovoltaics
The energy from the sun that reaches the earth’s surface each day exceeds global annual energy consumption. Solar energy is a vast resource – but it needs to be harvested efficiently and cost-effectively. Manufacturing solar cells using conventional semiconductors such as single-crystal silicon is expensive and energy-consuming. This is why solar cells have yet to meet a significant fraction of the world’s energy needs: per watt of power delivered, their cost is too high to compete with legacy energy technologies.
Our focus is on solar energy
conversion technologies that are simultaneously efficient and inexpensive. We employ low-cost fabrication methods such as spin-casting and spray-coating of semiconductor nanoparticles onto an arbitrary surface. We work with colloidal quantum dots (CQDs), semiconductor particles a few nanometres in diameter processed from the solution phase. CQDs’ energy gap, which determines the portion of solar spectrum the material can absorb, is programmed both by the choice of semiconductor material use
d, and by the size of the particles. We have the ability to tune the size of our CQD such that they absorb across the entire sun's spectrum. Our approach is naturally compatible with multilayer architecture, whose constituent layers efficiently harvest specific bands within the sun's spectrum: layers of different-bandgap CQDs are stacked atop one another, combining the power harvested within each layer. Additionally, solution processibility permits painting CQD on flexible conductive substrates and then coating the solar cell with final top electrode.
We reported in January 2005 [1] the first paint-on solar cell to harvest the sun’s abundant infrared rays. However this device exhibited low power conversion efficiency (PCE). Over the ensuing five years we have increased the PCE of our devices several thousand fold.
In 2007 we used a porous electrode-based solar cell architecture and a newly discovered thiol-based cross-linking technique to demonstrate the first solution-processed solar cell having an infrared power conversion efficiency greater than 1% [2]. In 2007 we reported the first Schottky diode based on colloidal quantum dots [3]. This led to the first CQD Schottky photovoltaic device and new performance records: a monochromatic infrared PCE of 4.2% and an AM1.5 PCE of 1.3% [4]. This was the first infrared CQD device to exceed 1% solar power conversion efficiency. We followed this report with an in-depth analysis on the inner workings of the CQD Schottky photovoltaics [5], focusing on the joint roles of charge carrier drift and diffusion in determining cell performance.
In parallel with increasing the performance of CQD PV devices, we have been improving the stability of our devices. By optimizing the choice of linker molecules, the methods of device fabrication [6], and the choice of electrical contact choice, we have been able to increase the life of our air-exposed encapsulated devices to weeks while simultaneously achieving 2% AM1.5 PCE [7], and soon after a new record AM1.5 PCE of 3.6% [8].
We recently reviewed international progress in this fast-moving field [9].
In 2010 [10] we reported the first CQD photovoltaic device to reach above 5% solar power conversion efficiency. Here we moved away from the Schottky diode architecture, employing instead a new device strategy that we termed the depleted heterojunction device.
Our focus today is on further enhancing device performance, stability, and understanding.
[5] K. W. Johnston, A. G. Pattantyus-Abraham, J. P. Clifford, S. H. Myrskog, S. Hoogland, H. Shukla, E. J. D. Klem, L. Levina, E. H. Sargent, “Efficient Schottky-Quantum-Dot Photovoltaics: The Roles of Depletion, Drift, and Diffusion,” Applied Physics Letters, vol. 92, pp. 122111, 2008.