Polyera Corporation

Introduction

Organic photovoltaic (OPV) cells are electronic devices that converts electromagnetic energy into electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, or photovoltaic arrays.

Architecture & Operation

The basic architecture of an OPV cell can be seen in the schematic below:

Standard OPV Architecture

Aside from the substrate (usually PET foil) and metal contacts (usually Al), an OPV cell comprises two main layers:

Interlayer. Usually made from the conducting polymer PDOT:PSS, this layer serves to smooth ITO surface and increase its work function.

The Active Layer. Comprised of both n- and p-type semiconductor materials, this is where photons are converted into electrical current.

The basic way an OPV cell works is as follows:

1. Photons travel through the material until they hit the active layer.
   
   
2. At this point, one of three things can happen:
   
   • The photon can pass straight through the active layer — this (generally) happens for lower energy photons.
     
     
   • The photon can reflect off the surface.
     
     
   • The photon can be absorbed by the active layer if the photon energy is higher than the active layer's band-gap value (which is determined by the particular materials used).
     
     
3. When the energy from the photon is absorbed, electrons and holes slightly decouple to form what's known as an exciton. These excitons then diffuse to the interface between the n- and p-type semiconductors, where the electron and hole fully separate from one another.
   
   
4. This current is then carried out of the cell through connections. An array of cells converts light energy into a usable amount of direct current (DC) electricity.

Performance

The overall performance of an OPV cell is primarily measured by its energy-conversion efficiency, often just referred to as efficiency, and is designated by the Greek letter η (eta). This is a measure of the amount of energy converted to electric current relative to the total energy incident upon the cell.

This efficiency is often thought of as being affected by three main parameters:

Absorption. This is the percentage of light that is absorbed by the active layer. This is primarily affected by the band-gap and thickness of this layer, but is also affected by absorption in other layers, as well as reflection and scattering.

Charge Separation. Once a photon has been absorbed and an exciton created, the electron and hole must be physically separated from one another so they do not recombine to be lost as heat or light. This is primarily influenced by the energy levels of the p- and n-type materials used. The morphology of the active layer also plays an active role: since the exciton can only diffuse a short distance, the morphology must be such that there is an n-/p- interface within that distance for charge separation to occur.

Charge Transport. Once the charge-carriers have been separated within the active layer, they need to be efficiently transported out of the active layer to the circuit contacts. The effectiveness of this process is largely determined by the mobility of these materials (which determine how effectively charge can be propagate through them), and by the ability of a charge to find a contiguous path from its current location to the appropriate contact (i.e. anode for electrons, cathode for holes).

The formula that typically used to calculate efficiency is as follows:

Another common indicator of OPV performance is knows as its external quantum efficiency (EQE). This is the ratio of the number of charge carriers generated in the OPV cell relative to the number of photons of a given energy incident upon it. It thus measures the response of a cell to a particular wavelength (i.e. energy) light. The ideal shape for the EQE would be a square, where nearly all the energy within a given wavelength range would be converted. In practice, however, a number of factors make this difficult to achieve in reality.

Morphology

As mentioned previously, once a photon is absorbed by the active layer and an exciton has been formed, it is important that they are separated from one another quickly before they have time to recombine. The way to do this is to have both n- and p-type materials close to the exciton-generation site, so that the n-type materials can transport the electrons away and the p-type can transport the holes.

Because this requires that the exciton-generation site is never far from a p-n interface, the morphology of the active layer becomes particular important (to ensure this is the case). Currently, the most common active-layer architecture is known as bulk heterojunction (BHJ). In a BHJ, the electron donor and acceptor materials are blended together and put in a mixture that then separates into distinct regions. Each region is separated by only several nanometers, a distance optimized for charge-carrier diffusion. Although devices based on BHJs are a significant improvement over planar designs (in which the n- and p-type materials were each in a separate layer), BHJs require sensitive control over materials morphology on a nanoscale. Moreover, many variables including choice of materials, solvents, and the donor-acceptor weight ratio can dramatically affect the BHJ structure that results. These factors can make optimizing BHJs difficult.

Applications

Although there are many applications possible (sensors of various wavelengths, etc.), by far the most common and most promising application of OPVs are solar cells. Because of the lower costs possible with organic, printable photovoltaics, there is the possibility of making such solar cells ubiquitous in almost any location, including power stations, in buildings, on roads, in stand-alone applications, in mass transit, and in 3rd world countries and other rural areas where little electrical infrastructure exists. Learn more.

Challenges

The biggest challenge that remains is increasing OPV cell efficiency. With current champion efficiencies in the 6% range, significant progress still has to be made before OPV-based solar cells reach the 10% mark that most experts agree would be necessary for large-scale commercialization.

References