Device Fabrication and Solar Cell Testing
The focus of much of the research activity in the Sêr SAM group is the use of solution-processable materials. This includes using organic and perovskite materials to make solar cells, photodetectors, light-emitting diodes, transistors, and more.
Part of the research interest in these materials is relatively straightforward fabrication processes compared to their traditional semiconductor counterparts. Solution-processable materials can be applied to substrates using techniques such as spin, spray, blade coating and roll-to-roll printing. This makes them candidate materials for fabrication on large scales and low cost.
One challenge for processing these types of materials is their sensitivity to the air. Oxygen and moisture will degrade the properties of many of these materials. One solution to this is to process these materials in an inert environment, such as nitrogen. This is achieved using a nitrogen glovebox which has monitored controlled oxygen and humidity levels.
Our Fabrication Equipment
In Sêr SAM's lab (Swansea University), we use a 6-port glove box system with an integrated spin coater and Lesker Nano 36 evaporator system. Our evaporator system has two high-temperature evaporation sources used for metal depositions, and two low-temperature evaporation (LTE) sources for other materials, such as organics.
Outside the glove box, we have a UV/Ozone cleaning system, and other spin coaters to process air-insensitive materials. We also have a spectroscopic ellipsometer (J.A Woolham), which enables us to accurately obtain the thickness and optical constants of these solution-processed films.
Figure 1: Six-port glove box system
Figure 2: Glovebox in use
Solar Cell Testing
The most important property of a solar cell is the efficiency (also called power conversion efficiency, PCE). This refers to the portion of energy from the sunlight that can be converted into electricity. The solar cell's efficiency can be obtained from the J-V characteristic curves, which is basically a graphical representation of a solar cell summarizing the relationship between the current and voltage. In addition, from the J-V characteristic curves, more devices metrics, such as the maximum voltage that the solar cell provides at open-circuit condition (Voc), maximum current that the solar cell provides at short-circuit condition (Jsc), and the maximum power point (MPP) relates to the point where the power supplied by the solar cell connected to the load also can be investigated. Testing will typically quickly follow the fabrication of solar cells, and looking at these metrics helps us make decisions on how to improve the devices.
Our Testing Equipment
Our test equipment is located close to the fabrication facility to ensure that devices can be rapidly measured following fabrication. To reliably simulate the light arriving from the Sun, a Newport-certified silicon reference cell was used to calibrate a solar simulator (LCS-100, Newport). whilst a source meter unit (2450, Keithley) and a J−V scanning software (Ossila Ltd., UK) were performed to obtain the J-V characteristics of our solar cells under AM 1.5G (100 mW cm−2) to. Furthermore, a home-built sensitive external quantum efficiency system also allows us to accurately characterize the quantum efficiency (EQE and IQE ) of our solar cells.