Polymer-fullerene bulk heterojunction solar cell

When electrons of donor molecules are photoexcited, they jump from the HOMO to LUMO energy level. The electrons now in the LUMO energy level can travel to nearby acceptor molecules, which are more electronegative and thus lower in energy. The driving force for the electron transfer between donor and acceptor is the difference in LUMO energy levels.^[1]

Polymer-fullerene bulk heterojunction solar cells are a type of solar cell researched in academic laboratories. Polymer-fullerene solar cells are a subset of organic solar cells, also known as organic photovoltaic (OPV) cells, which use organic materials as their active component to convert solar radiation into electrical energy. The polymer, which functions as the donor material in these solar cells, and fullerene derivatives, which function as the acceptor material (such as PCBM, or phenyl-C61-butyric acid methyl ester), are essential components.^[2] Specifically, fullerene derivatives act as electron acceptors for donor materials like P3HT (poly-3-hexyl thiophene-2,5-diyl), creating a polymer-fullerene based photovoltaic cell.^[3] The Polymer-fullerene BHJ forms two channels for transferring electrons and holes to the corresponding electrodes, as opposed to the planar architecture when the Acceptor (A) and Donor (D) materials were sequentially stacked on top of each other and could selectively touch the cathode and anode electrodes. Hence, the D and A domains are expected to form a bi-continuous network with Nano-scale morphology for efficient charge transport and collection after exciton dissociation. Therefore, in the BHJ device architecture, a mixture of D and A molecules in the same or different solvents was used to form a bi-continual layer, which serves as the active layer of the device that absorbs light for exciton generation. The bi-continuous three-dimensional interpenetrating network of the BHJ design generates a greater D-A interface, which is necessary for effective exciton dissociation in the BHJ due to short exciton diffusion.^[4] When compared to the prior bilayer design, photo-generated excitons may dissociate into free holes and electrons more effectively, resulting in better charge separation for improved performance of the cell.

Photovoltaic cells featuring a polymeric blend of organics have shown promise in a field largely dominated by inorganic (e.g. silicon) solar cells. Some of the improvements that organic solar cells have over inorganic solar cells are that they are flexible and therefore can be applied to a larger range of surfaces.^[5] They can also be produced much more easily via inkjet printing or spray deposition, and therefore are vastly cheaper to manufacture.^[6] A downside is that, because they are not crystalline (like silicon), but instead are produced in a purposely disordered blend of electron-acceptor and -donor materials (hence the name bulk heterojunction), they have a limited efficiency of charge transport.^[7]

However, the efficiencies of these new types of photovoltaic cells have risen from 2.5% in 2001, to 5% in 2006, to greater than 10% in 2011.^[8] This is because improved methods for solution processing of acceptor and donor materials led to more efficient blending of the two materials. Further research can lead to polymer-fullerene based photovoltaic cells that approach the efficiency of current inorganic photovoltaic cells.

Structure

Three different schematic representations of blending electron donor and acceptor materials. (a) Bilayer representation, with efficient charge generation but poor charge transport.[10] (b) Solution processed representation, in which rapid drying leads to a randomized network of acceptor/donor blending, currently the most optimal way to blend. (c) Theoretical, ideal representation of acceptor/donor blending.^[9]

Materials used in polymer-based photovoltaic cells are characterized by their total electron affinities and absorption power. The electron-rich, donor materials tend to be conjugated polymers with relatively high absorption power, whereas the acceptor in this case is a highly symmetric fullerene molecule with a strong affinity for electrons, ensuring sufficient electron mobility between the two.^[5]

The typical structural layout of photovoltaic devices. Transparent, conductive ITO is applied onto glass, and a hole transport layer of PEDOT:PSS (poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)) on top of that. The photoactive layer is a blend of electron acceptor and donor atoms, and the cathode interlayer is a low work function metal used to lower the work function of the electrode on top to accept electrons.^[10][11]

The arrangement of materials essentially determines the overall efficiency of the heterojunction solar cell. There are three donor-acceptor bulk morphologies: (a) the bilayer, (b) the bulk heterojunction, and (c) the "comb" structure. Typically, a polymer-fullerene bulk heterojunction solar cell has a layered structure.

Working Principle

Working principle of the Fullerene based BHJ OPVs device involves four fundamental steps namely (i) photons absorption and exciton creation, (ii) exciton diffusion and splitting at the D-A interface, (iii) charge transportation, and, (iv) charge collection.^[12] In a BHJ OPV device, the donor material is the one that absorbs the incoming light. Due to a substantial potential energy drop, the excitons must diffuse to the D-A interface, where they will be split into free charge carriers such as electrons and holes.^[13] There can be some limitations and losses during the device operation steps discussed above, which include absorption loss due to spectral mismatch, thermalization loss, the energy required for exciton splitting might be inefficient, charge recombination loss, etc.^[14]

Device Architecture

For Fullerene-based OPV, there are two device architectures in use today: traditional (conventional) and inverted. The BHJ conventional architecture has set a significant milestone in terms of improving efficiencies in OPVs in order to commercialize them. However, due to oxygen and moisture intrusion into the electrodes, as well as damage caused by air or oxidation of the electrodes, the environmental stability of these OPVs remains the most difficult challenge to overcome. To overcome this challenge researchers had established inverted device architecture for BHJ PSCs. In an inverted device, the bottom transparent electrode serves as the cathode while the top electrode is an anode. The inverted devices exhibited higher environmental stability,^[15] and higher efficiencies in most cases in comparison with the conventional architecture of OPVs, which is achieved by using high work function metal or metal oxides as a cathode and the low work function metal as an anode. In the normal architecture the low work function cathode would easily get oxidized in the air by oxygen and moisture, thus using a higher work function cathode minimizes this tendency and improves efficiency and stability.

Functions/Applications

The primary function of a solar cell is the conversion of light energy into electrical energy by means of the photovoltaic effect.^[16] In particular, polymer-fullerene bulk heterojunction solar cells are promising because of their potential in low processing costs and mechanical flexibility in comparison to conventional inorganic solar cells.^[17][18] Solution processing potentially allows reductions in manufacturing costs through screen printing, doctor blading, inkjet printing, and spray deposition at low temperatures.^[19][20] To overcome the narrow spectral overlap of organic polymer absorption bands, experiments have blended conjugated polymer donors with high electron affinity fullerene derivatives as acceptors to extend the spectral sensitivity. Ternary solar cells are a promising approach to increased efficiency and light harvesting properties of organic photovoltaic cells (OPV).^[21]

Challenges in Fullerene based BHJ OPV

The Fullerene based BHJ OPV Devices are expected to possess the following characteristics for successful commercialization: high performance, environmentally friendly, simple fabrication process, high stability, and low cost. However, efficiency and stability are the major challenges faced by PSC for its successful commercialization.^[22][23][24] Shorter diffusion lengths of excitons in conjugated polymers are limited to a few nanometers (less than 20 nm), which is shorter than the optical absorption path length (~ 100–200 nm),^[25] contributes to lower power conversion efficiencies in PSCs. Another factor that limits device efficiencies is lower charge carrier mobility in the conjugated polymers causing, their recombination before reaching their respective electrodes.^[26] Consequently, the solar cell experiences a significant loss in photo generated current, and hence poor device performance. Poor charge carrier collection at the electrodes due to bad mismatch is also another factor that limits device performance. If there is a mismatch between the anode and cathode with that of donor HOMO or acceptor LUMO, respectively, then no Ohmic contacts would be established, which ultimately, results in poor performance of the solar cell. In most Fullerene based BHJ OPVs, there is a mismatch between the anode and cathode with that of donor HOMO and acceptor LUMO respectively, which poses a great challenge in charge carrier collection in the respective electrodes, and overall device performance.

The stability of PSC device is the most important factor that should be given great attention to realize their commercialization, though there have been limited literature on the stability of PSCs compared to the literature on the efficiency of PSCs. A study on the stability of PSCs helps in understanding how a device degrades during its operation.^[27] Device instability occurs due to a range of complex phenomena that are in play simultaneously.^[23] These degradation factors include mechanical stress, irradiation (time & intensity), water, oxygen, heating, etc. (Fig 2.6), and may affect the active layer, the transport layers, the contacts, and the interface of every layer with the adjacent layers.

References

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