Bulk Heterojunction Solar Cell

25.7 MW Lauingen Energy Park, Germany

As part of solutions for the climate change, the demand for 'clean' energy has increased in the recent years. One of the piece of technologies that can fulfil this demand is solar cell which it can harness the solar energy. At this stage, the conventional solar cell, which is made of a thick layer of doped-silicon, is very expensive and the manufacturing process is energy consuming process. Another type of solar cell, dye sensitised solar cell (DSSC) or Grätzell cell, can be a solution for the expensive silicon solar cell. Instead of expensive silicon as semiconductor, cheaper TiO₂ is used and ruthenium-based complex compound as solar light harvester. However, this cell has a problem with solvent leakage, which can be  not environmentally friendly. The efficiency of this cell is also a problem; DSSC's efficiency is up to 11% while Si-based solar cell can be up to 25%. To address these problems, another type of solar cell has been developed in the recent years. This solar cell is called bulk heterojunction (BHJ) solar cell.

The idea of BHJ solar cells came up from the discovery a phenomenon called ultrafast electron transfer. This phenomenon was observed when the luminescence of the semiconducting polymer was heavily quenched by the addition of fullerenes which implied the electron transfer must happen significantly faster than the photoluminescence decay. From this idea, the whole field of BHJ solar cells was created and because the electron transfer is much faster than any competing processes within the system, the photoinduced efficiency must be nearly 100%.

In manufacturing bulk hetero-junction solar cells, the donor (hole conducting) component is provided by conjugated polymers while the acceptor (electron conducting) is provided by fullerene or its derivatives.

The structure of fullerene materials and conjugated polymers used in organic solar cells.

Both components are blended and mixed in a bulk volume to form a homogeneous bulk structure but heterogeneous structure in nanoscale. This solid state nanostructure is formed by spontaneous phase separation where both components self-assemble to form bicontinuous interpenetrating network. This self-assembly process happens because high polymers always tend to phase separate due to small entropy of mixing and crystallinity. It is noteworthy that the ultrafast electron transfer, the fundamental operation of BHJ cells, is sensitive to the morphology but controlling the morphology in this case is not a trivial process. Despite this withdraw, bulk heterojunction devices provide larger interface than the conventional bilayer devices.

As repeatedly mention earlier, the working principles of BHJ solar cells are based on ultrafast electron transfer and in term of electron movement within the cell is no different than the conventional solar cells.

The working principles (left) and diagram of inverted BHJ solar cells

When the cell is irradiated by the sunlight, the photons excite the electron conducting polymer into its LUMO (1). The preferred excitation in conduncting polymer is simply due to its higher molar absorption coefficient. This excitation creates a pair of hole in HOMO and the electron in LUMO. As the excited electrons want to relax, it drifts into lower energy level which is in acceptor's LUMO (2). This process is generally directed by the ultrafast electron transfer which is the fundamental principle within this cell. It has to be remembered, during this process the electron-hole pairs are still created as long as the cell is still irradiated. This electrons and holes drift creates a charge build up at both ends. Then, external circuit can be connected to allow the charge carriers flowing, i.e. allowing the electrical current to flow (3).

The BHJ solar cells operation can be affected by several factors, mainly related to the energy gap od donor and acceptor. Firstly, the most studied BHJ solar cells commonly use poly(3-hexylthiophene) as the donor polymer but this polymer does not absorb light beyond 670 nm.

If we see the graph above, there are a lot of solar energy that can be harnessed at longer wavelength. This means a necessary change in energy gap by lowering the band gap is required to allow longer wavelength to be used. One way to do this is by using polymers with alternating donor acceptor repeat units.

By making both donor and acceptor in the same unit, charge transfer happens intramolecularly.

Another factor that can affect the BHJ solar cells operation is the offset of energy levels of donors and acceptors used. The way to rationalise this factor is by surveying again their energy levels.

The energy level of donor of acceptor in BHJ solar cells

The first parameter from the diagram above is the open circuit potential (V_OC), which is the maximum voltage delivered by the solar cell. It is worthy to note that at this voltage, the current is zero. However, the offset between donor and acceptor (ΔE) dictates the charge (electron) transfer from donor to acceptor. This offset has to high enough to ensure there is an insentive for electrons to move to LUMO's acceptor but it has to be low enough to make the process is not energetically wasteful.

To conclude, we've seen a new generation of solar cell of bulk heterojunction solar cells where the p-n junction, the interface between donor and acceptor, is created by blending the donor and acceptor. This process creates a homogenous macrostructure but in its microstructure the morphology is hard to control. This solar cell offers a cheaper option for an alternative of the conventional silicon-based solar cells. However, this piece of technology is still in its infancy stage with the main drawback of much lower efficiency than Si-based and dye sensitised solar cells. In the future, a higher efficiency BHJ solar cells, hitting the 20% efficiency target, will be a major achievement with important consequences to energy technology.

Parking structure with plastic BHJ solar panels fabricated by Konarka Inc.

References
A. J. Heeger, Adv. Mater., 2014, 26, 10-28.