Polyera Corporation

Introduction

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

Architecture

The architecture of a bilayer OPV cell is schematically presented below:

Bilayer OPV Cell Architecture

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

• Active Layer. Comprised of both n- and p-type semiconductors. It is where photons are converted into electrical current.

• Interlayer. Usually made from conducting polymer PDOT:PSS. It serves to smooth ITO's surface and increase its work function.

Operation

The way OPV cells convert electromagnetic energy is described below:

The photons travel through the contact materials and hit the active layer.

This current is then carried out of the cell through connections. In the end, arrays of cells convert light energy into DC electricity.

Performance

The performance of an OPV cell is measured by its energy-conversion efficiency, or just efficiency, and is designated by the Greek letter η (eta). η measures the amount of energy converted to electric current relative to the total energy incident upon the cell. And it is often thought to be mainly affected by three 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. When excitons are created, electrons and holes must be separated from each other so that they do not recombine and their energy is lost as heat or light. This is influenced by the energy levels of the n-type and p-type materials. The morphology of the active layer also plays an active role. Since excitons can only diffuse a short distance, the morphology must be such that there is an n/p interface within that short distance for charge separation to occur.

• Charge Transport. When charge carriers are separated within the active layer, they need to be transported out of the active layer to the circuit contacts. The effectiveness of this process is determined by the mobility of these materials (that determines 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 and cathode for holes).

The formula for calculating efficiency is:

η = J_sc X V_oc X FF,

where J_sc is the short-circuit current (when there is maximum current flowing and no voltage difference across the circuit), V_oc is the open-circuit voltage (when there is no current flowing - a break in the circuit), and FF is the fill-factor (the actual power relative to the theoretical power produced by the cell).

To increase the efficiency of OPV cells, we need to improve these 3 factors. J_sc is primarily affected by band-gap, carrier mobility, and film formation properties of the active layer. V_oc is primarily affected by the material band-gap and the device structure. FF , is particularly difficult to predict and design, but seems related to the relative mobilities of the electrons and holes.

Another common indicator of OPV performance is external quantum efficiency (EQE). This is the ratio of the number of charge carriers generated in an OPV cell relative to the number of photons of a given energy incident upon it. It measures the response of a cell to a given wavelength (i.e. energy) of 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.

Morphology

As mentioned previously, when a photon is absorbed by the active layer and excitons are formed, it is important that they are separated from each other quickly before they have time to recombine. The way to do this is to have both the n-type and p-type materials close to exciton-generation sites, so that the n-type materials can transport the electrons away and the p-type can transport the holes away.

This requires that an exciton-generation site is usually close to a p-n interface. As a result, the morphology of the active layer becomes very important. 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 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 nano scale. Moreover, many variables, including materials, solvents, and the donor-acceptor weight ratio, can dramatically affect the BHJ structure. These factors can make it difficult to optimize BHJs.

Applications

Although there are other possible applications (sensors of various wavelengths, etc.), the most common and promising application of OPV cells are in organic solar cells. Because of the lower costs with printed photovoltaics, there is great potential of installing organic solar panels at any location, including stand-alone power stations and on buildings or roads, for developing countries and rural areas, where electrical infrastructure lacks.

Challenges

Our biggest challenge is how to improve the OPV cell efficiency. Currently, commercial inorganic photovoltaic cells have efficiencies between 12-20%, but their production cost is too high for ubiquitous use. Organic solar photovoltaics, in contrast, have efficiencies between 5-6%, but can be produced at a much more cost. However. the threshold efficiency for commercial use is generally agreed to be 10%. With current champion OPV efficiency at only around 6%, significant progress has to be made before OPV solar cells reach or exceed the 10% mark necessary for commercialization.

References