Imagine solar panels so lightweight and adaptable they can be integrated into everyday objects like windows or clothing. This vision is closer to reality than ever with organic bulk heterojunction (BHJ) solar cells. Dr Neha Chaturvedi, head collar researcher at solar cell developer Nextgen Nano, explores how recent advancements in materials and fabrication techniques are set to enhance the efficiency of BHJ solar cells, making sustainable energy more accessible than ever.
Organic BHJ solar cells offer a significant departure from traditional silicon-based panels, primarily due to their flexible and lightweight design. Unlike rigid panels, BHJ cells are constructed with a mixed active layer that integrates donor and acceptor materials. This blending creates a network of donor-acceptor interfaces within the active layer, which improves the light absorption and the power conversion efficiency.
When sunlight strikes the cell, the donor material absorbs photons, generating excitons – electron-hole pairs. The unique mixed-layer structure allows these excitons multiple pathways to reach the donor-acceptor interface, facilitating their dissociation into free charge carriers. This efficient separation of charge-carriers leads to improved power conversion efficiency (PCE). BHJs’ flexibility and efficiency makes them suitable for a variety of applications, including wearable technology, portable chargers and building-integrated photovoltaics.
Materials in BHJ Cells
Material selection plays a vital role in the performance of BHJ solar cells. Traditional materials like indium tin oxide (ITO) have been widely used as electrodes due to its excellent conductivity and transparency. However, ITO is brittle and costly, costing as much as £28 per cubic centimetre. Nextgen Nano has adopted fluorine-doped tin oxide (FTO) as an alternative. FTO offers similar optical transparency and electrical conductivity but with improved chemical and thermal stability, increased flexibility and a low price tag at £4 per cubic centimetre, making it more suitable for the demands of flexible and large area applications.
In addition to transparent electrode materials, the choice of donor and acceptor materials is critical. Polymers like PM6, PTA10 and D18 are commonly used as donor materials, helping achieve record PCEs of up to 19.3%. This is much greater than traditional fullerene based organic photovoltaic cells that typically achieve PCEs of 10 to 12%. These combinations enhance photon absorption and improve charge transport, significantly influencing the overall performance and efficiency of BHJ solar cells.
Optimising for Efficiency
Enhancing the efficiency of BHJ solar cells involves several key strategies. At the lab scale, selecting high-performance donor and acceptor materials with complementary absorption spectra and well-matched energy levels is critical.
Advanced materials, like non-fullerene acceptors (NFAs) and Indacenodithiphene-acceptors (ITICs), improve light absorption and energy alignment with donor materials by optimising the photogenerated charge carrier separation and transport. This results in a 2-4% increase in PCE compared to traditional fullerene-based systems.
Optimising electron and hole transport layers are crucial for enhancing the efficiency of BHJ solar cells. Electron transport layer (ETLs) materials must improve electron mobility and reduce recombination losses by providing a high conductivity path for electrons to travel to the electrode.
Using a material like zinc oxide (ZnO) can result in a PCE gain of around 1-2%. ZnO’s performance can be further enhanced by optimising the solvent choices during deposition, as certain solvents can influence the uniformity and quality of the ZnO film, improving electron transport.
The solvent used during active layer deposition, like chlorobenzene and o-xylene, affects the morphology of the active layer, which can improve the efficiency by 2-3%. Solvents impact how well the donor and acceptor materials mix and phase-separate, affecting light absorption and charge transport.
The hole transport layers (HTLs) such as poly (3,4-ethylenedioxythiophene) (PEDOT) facilitate the movement of holes towards the anode. Using a material like PEDOT can boost PCE by three to five percent compared to devices without HTLs. Adding dimethyl sulfoxide (DMSO) can further enhance the stability of PEDOT and maintain its conductivity over time by reducing degradation from moisture, contributing an additional 0.2-0.5% in PCE.
Transitional metal oxides like molybdenum trioxide (MoO3) offer better chemical stability and alignment with donor materials compared to PEDOT, improving hole extraction and overall device stability. However, MoO3’s impact on PCE is dependent on process conditions, including layer thickness, which influence its effectiveness.
Additives like 1,8-diiodooctane (DIO), 1-Chloronaphthalene (CN), improve phase separation within the active layer, enhancing charge transport and efficiency by up to 1%. Post deposition treatments like thermal and solvent vapour annealing, where the film is exposed to solvent vapours, allowing controlled swelling and relaxation of the material, improving its morphology and crystallinity, potentially increasing efficiency by 1-2%. It does this by improving alignment and separation of the donor and acceptor phases.
These optimisations in materials and processing techniques have collectively pushed PCEs for BHJ cells, with an average of 18.5% and a record 19.3%.
Fabricating BHJ Solar Cells
Fabricating large-area BHJ solar cells presents significant challenges, such as achieving uniform film thickness and avoiding defects like pinholes and striations.
Blade coating is one effective method that addresses these challenges through its scalability and cost-effectiveness. This technique involves preparing a solution of active materials, cleaning and pre-treating the substrate, then spreading the solution with a blade. After coating, the film is dried to remove the solvent, resulting in a uniform active layer.
Alternative methods, such as roll-to-roll processing, also offer scalability but face different challenges, like managing continuous film quality and ensuring consistency over large areas. Blade coating integrates well with the overall focus on improving BHJ solar cells by enabling large-scale production while addressing key fabrication issues through precise control of parameters like blade speed, substrate temperature and solution viscosity.
With their lightweight and adaptable design, organic BHJ solar cells are poised to significantly impact the sustainable energy sector. Continued advancements in materials, such as FTO and fabrication techniques like blade coating are driving these cells towards widespread usage.
