Layers in the Organic/Polymer Solar Cell

There are many types of organic/polymer solar cells, but basically, there are 2 main types: normal geometry devices, and inverted geometry devices.  For normal geometry devices, holes are extracted from the front electrode (anode) where light enters, while electrons are extracted from the back electrode (see diagram above).  This extraction is reversed for inverted geometry devices.  The naming convention is because the inverted geometry was invented after the normal geometry.  The inverted geometry allows the use of other electrode materials.

The diagram above names the layout of a polymer solar cell in sequence from top to bottom.  The main materials for substrates are glass (stiff material) and plastic (flexible material).  Glass is often used in research due to its suitability with techniques such as spin coating and evaporation.  It is very heat stable, and a can be a very good barrier to oxygen and moisture.  However, the advantage of polymer solar cells is that they can be mass produced cheaply by simple printing and coating technologies.  Hence, plastic foil, mainly based on PET (polyethylene terephthalate), is also used in research.  Its barrier properties also had much improvements, resulting in very stable solar cells.

Due to the requirement of the front electrode to be transparent, ITO (indium doped tin oxide) is often used with a good hole conductor PEDOT:PSS.  However, indium is a scarce material and ITO is very brittle, so alternatives such as FTO are being researched into.  An alternative technique to ITO deposition is to use a very thin metal grid.  This has been described in High-Efficiency Concepts of c-Si Wafer Based Solar Cells.

Intermediate layers - HTL (hole transport layer) and ETL (electron transport layer) - are required to facilitate charge carriers transport to the electrodes.  HTL such as PEDOT:PSS, MoOx (molybdenum oxide) and V2O5 (vanadium(V) oxide), and ETL such as TiO2, ZnO, LiF (lithium fluoride) and CeCO3 (caesium carbonate), have been used.

The active layer, a BHJ (Bulk Heterojunction), is created quite randomly by mixing 2 components together in a solvent.  The components have a tendency to microphase separate once the solvent evaporates.  Much research is done into the generation and maintenance of better BHJ structures, such as by using heat treatment or exposure to solvent vapours to help phase separation.

Finally, there is the difficulty of creating a tandem solar cell, where efficiency is possibly doubled due to the addition of voltages from both individual solar cells.  The difficulty arises from the stacking of many layers of materials (see diagram above), the selection of polymer materials with approximately the same current production and with different absorption spectra, and the development of an intermediate layer that can form an ohmic contact to both individual cells to transport charge carriers between them.


Reference:
Layers in the Solar Cell, https://www.coursera.org/learn/solar-cell/lecture/smOjn/layers-in-the-solar-cell