Semiconductors for Photovoltaic Devices: Electrochemical Approaches using Ionic Liquids

Can electrodeposition be used to create high quality p-type inorganic compound semiconductors for photovoltaic applications?

Thin film photovoltaic devices offer similar power conversion efficiencies to polycrystalline silicon devices and have the inherent advantages of consisting of less material and requiring less energy expenditure during processing. Thin film devices consist of a semiconductor pn heterojunction with front and back contacts to extract the excited charge carriers. The materials properties of the p-type layer are the most stringent, and determine the overall performance of the device. Common p-type semiconductors are CdTe, Cu(In,Ga)Se2, and Cu2ZnSn(S,Se)4. Typically the p-type semiconductor must form a continuous dense single phase layer two micron thick over metre squared areas. Most commercial producers of thin film photovoltaic modules choose evaporation or sputtering methods to deposit this layer. Of importance is the speed, cost, and quality of deposition.

Electrodeposition offers the ability to deposit thin films over large areas with high materials usage, potentially at high speed. Can electrodeposition be used to create high quality p-type inorganic compound semiconductors?

This talk will show that it is possible to directly deposit a working p-type semiconductor, but that a two step approach of depositing metals and then annealing them in a reactive atmosphere is a simpler, easier, and more robust approach. Both approaches can lead to semiconductors which provide working photovoltaic devices. However, improvements to the electrodepostion process are still required and the main challenges are outlined below.

Challenges in directly electrodepositing a p-type semiconductor are (i) the inherent lack of electrons necessary for a reductive deposition process and (ii) the low thermal energy available at normal deposition temperatures to create micron sized well ordered crystals.
Challenges for directly electrodepositing the metal alloys CuInGa or CuSnZn from aqueous solution are (iii) competition with hydrogen reduction leading to inefficient deposition, embrittlement, and dendritic growth (iv) control of the alloy composition over the micrometer and centimeter length scales due to the different reduction potentials, nucleation densities, and diffusion coefficients.

In this talk it will be shown how these challenges can be met by using ionic liquids to replace aqueous solvents. Ionic liquids offer larger electrochemical windows, higher processing temperatures, and the choice of new forms of starting reagent. Furthermore, task specific ionic liquids or liquid metal salts, may even be employed to allow extremely high speed deposition.