The layer stack

Making a polymer solar cell is often done using polymers dissolved in organic solvents, which are transferred by printing or coating methods to a substrate. The materials are added in layers in a certain order to build a solar cell stack. The materials needed in the solar cell stack are; a central active (light absorbing) layer, which translate the impinging photons into separate electrons and holes, a selective charge transport layer on each side of the active layer, allowing only passage of either electrons (ETL) or holes (HTL), and finally two electrodes for extracting the charges from the solar cell, with at least one of the electrodes having a requirement of transparency such that the light can pass through and reach the active layer.

Figure 1: The layers of an organic solar cell.

Geometries

Polymer solar cells are often divided into two groups based on the solar cell stack geometry. A normal and an inverted geometry.
The definition of the two geometries lies within the direction of the charge flow. In a normal geometry solar cell the substrate and the transparent electrode on it is the positive electrode, with the light passing through the substrate and this electrode before being absorbed in the active layer. The top electrode is then the negative electrode. In the inverted geometry the two electrodes and the charge selective layers are switched around, such that the transparent electrode at the substrate is the negative electrode, with a ETL layer between it and the active layer, while the top electrode is the positive electrode with a HTL layer between it and the active layer.

Figure 2: Energy levels for normal and inverted geometry solar cells.

Active layer

The active layer consists of two components in the polymer solar cells. A donor which absorbs the light and an acceptor which extracts the electron from the excitonic bound electron hole, resulting in an electron travelling in the acceptor phase of the active layer and a hole travelling in the donor phase. For this to occur successfully the low lifetime of an exciton in the donor materials necessitates a donor-acceptor boundary at which the exciton can be broken within approximately 10 nm. Furthermore, since the holes and electrons have to travel out of the active layer towards the electrodes, the domains of donor and acceptor needs to be connected in an interconnected network allowing both efficient dissociation of the excitons and efficient transport of the charge carriers to the respective electrodes.

Transport layers

The transport layers are based on materials which have the capability of being able to primarily transfer either electrons or holes due to a suitable positioning of the energy levels. Examples of hole transport materials include the polymer based Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) and a metal oxide such as molybdeenum oxide (MoOx). Examples of often used electron transport layers are lithium floride (LiF), calcium (Ca) and metal oxides such as zinc oxide (ZnO) and titanium oxide (TiO).

Electrodes

The most commonly used electrode material has been indium tin oxide (ITO), due to a high optical transmission combined with a low resistance; on glass a transmission of >85% at <10 Ohm/sq. is often seen. The main issue with choosing the electrodes is to find electrodes with a suitable energy level and with one of the electrodes being transparent to allow sufficient light to enter the solar cell. PEDOT:PSS is a material that has started to become popular as use as an electrode, since it can be doped to allow conductivities of more than 500 Scm with a transmission of >80%. The doped PEDOT:PSS solutions tend to allow easier fabrication on flexible substrates, since the polymer based films have a better tolerance towards bending compared to an ITO electrode.

Substrates

When making polymer solar cells the substrates used for supporting the layered solar cell stack, can be divided into two distinct groups: glass and plastics. The two most commonly used types being floated glass substrates with ITO transparent electrodes used in lab scale production and flexible PET foil used in upscaling focused on manufacturing, where the transparent electrodes are either ITO as for the glass substrates or printed transparent electrodes.

Authors
    Henrik Friis Dam, Thue Trofod Larsen-Olsen