Tandem solar cells have been the home of III-V compound semiconductor devices for many decades used to power space satellites and concentrating PV systems. These solar cells reach nearly 40 % efficiency without concentration and 47% under concentration, stacking up to 6 subcells on top of each other. More recently perovskites are entering this field with performances close to 30% for tandems with Silicon and this offers a path to lower cost but still high efficiency tandem cells for a wide range of terrestrial applications. The tutorial shares insights from the fundamentals of III-V tandem and perovskite based tandem solar cells as well as the related industrialisation challenges.

Speakers / Authors:
Dr. Ryan France/NREL
Prof. Christoph Ballif/EPFL

In the first part, this tutorial will give a general introduction to the field of multijunction solar cells for the use in terrestrial and space systems. It will start from basic theoretical considerations and explain the benefits of using several pn-junctions to convert the broad solar spectrum into electricity. Since III-Vs have historically dominated the field of multijunctions and high efficiency photovoltaics, we will review the III-V alloy system, multijunction cell components, and the various techniques that are available for growing and characterizing these highly crystalline semiconductors. Some of the latest developments in III-V research will be discussed as well as the specific requirements for the use of multijunctions in different environments will be introduced, such as high current capacity for concentrators, radiation hardness for space applications, and low cost for one-sun applications.

In the second part, new and on-going efforts toward hybrid tandems, combining cells of different material systems, will then be covered in detail. We’ll focus on silicon- and perovskite-based multijunction devices which offer the promise of similarly high conversion efficiencies as traditional tandem III-Vs, but at a fraction of the cost. We’ll review, in particular, some of the material and device properties of perovskites for use in tandem (Si/PK and PK/PK) devices, covering electronic and optical aspects. We will have a look at the advantages and disadvantages of different material and interconnection combinations (e.g. 2T, 3T, 4T). Finally we’ll discuss some of the remaining challenges towards mass commercialization, including device stability, process upscaling, and potential costs.