Organic Photovoltaics vs 3rd-Generation Solar Cell Technologies
|Written by Emma Spooner, a PhD student in Fullerene-Free Photovoltaic Devices at the University of Sheffield in collaboration with Ossila Ltd.|
Whilst the majority of commercial solar cells are currently made using crystalline silicon (c-Si), thin-film alternatives have the potential to be cheaper, flexible, and more straightforward to produce. These technologies can be classified as second-generation cells (e.g. cadmium telluride (CdTe)), or third-generation (e.g. perovskite solar cells). An overview of the working mechanism for many of these technologies can be found in this guide - Solar Cells: A Guide to Theory and Measurement. One of the longest-standing alternatives to silicon are organic photovoltaics (OPVs), which can be directly compared to both second-generation (found in the companion article to this one) and third-generation solar technologies.
Emerging Solar Technologies
Third-generation solar technologies include OPVs, copper zinc tin sulphide (CZTS), perovskite solar cells, dye-sensitised solar cells (DSSCs), and quantum dot solar cells. Whilst most second-generation solar technologies have been in research since the late 1970s, third-generation technologies are generally (with the exception of DSSCs) more modern offerings. As such, they are typically described as ‘emerging’ technologies. Record efficiencies achieved (over time) for different third-generation technologies can be seen in Fig. 1, with most being considerably lower than c-Si or second-generation technologies. The main advantages of these materials are their tunability and abundant components. As research progresses, record efficiencies of several technologies are likely to continue growing.
Comparison of solar cell technologies should consider typical efficiency, cost, weight, stability to a range of environmental factors, scalability, and the environmental impact of a technology. The efficiencies achieved for a technology - in comparison to the maximum possible theoretical efficiency - are also important, (as can be seen in Fig. 2). This is especially so when considering the number of research years spent achieving this efficiency.
The environmental impact of second-generation solar technology installations can be assessed with factors such as greenhouse gas emissions/global warming potential (GHG/GWP) or energy payback time (EPBT) - discussed more thoroughly in the companion article to this one. However, measuring these values is more difficult with technologies that are yet to be manufactured on a large scale (such as CZTS). Modelled factors are shown in Fig. 3, although these obviously are very dependent on multiple assumptions and factors and have a higher level of uncertainty than real life values. When considering large-scale potential and environmental impact, there are several other important factors that are not discussed here - such as freshwater use, eco-toxicity, and ease of recycling.
Copper Zinc Tin Sulphide (CZTS)
CZTS cells use an active layer with the general form Cu2ZnSnS4 and a kesterite crystal structure11 (shown in Fig. 4). These cells have many similarities with second-generation copper indium gallium selenide (CIGS) cells. However, they use significantly more abundant and non-toxic materials, and avoid using toxic cadmium or rare indium. Their use of readily-available materials is an advantage shared with OPVs, alongside a bandgap that can be easily tuned (in this case from 1.0 to 1.6eV11) by varying component stoichiometry. The record efficiency for a pure CZTS cell is 11.0%.12 However, this has been exceeded thanks to the inclusion of selenium to form a CZTSSe cell with an efficiency of 12.6%.13
Both CZTS and CZTSSe are complex materials that are difficult to optimise. Furthermore, material quality can limit the efficiencies of cells.14 In particular, this is due to a tendency for the material to form point defects through ‘cation disorder’.15 Inclusion of selenium also introduces issues in terms of material abundance and toxicity, and there have been few significant improvements in efficiencies in recent years of both variants. As with OPVs, CZTS remains attractive due to low GHG values and energy payback time, but is limited by poor efficiency.
Perovskite Solar Cells
Perovskite solar cells are covered in detail in the Ossila guide Perovskite Solar Cells: An Introduction. In short, their active material is based on a generic ABX3 structure, where A is an organic cation such as methylammonium (CH3NH3+), B is an inorganic cation, typically lead (Pb2+) and X is a halogen anion, such as chloride (Cl-) or iodide (I-). Perovskites are especially notable for their rapid rise in published efficiencies and having achieved performances that are comparable to second-generation inorganic cells, clearly a significant advantage over other third-generation solar technologies.
As with CZTS and OPVs, perovskites are advantageous compared to CdTe or CIGS because they use cheaper, abundant materials, and have a tunable band gap. However, they still have notable stability issues, particularly sensitivity to moisture11 and concerns in regards to toxicity, especially due to the water-solubility of the toxic compounds - potentially leading to environmental exposure. Several lifecycle analyses of perovskites have produced modelled GHG and EPBT values which are significantly larger than those of OPVs4,6,16 (which is an obvious area of improvement in terms of manufacture optimisation), but most conclude that the lead component has negligible eco-toxicity effects.
Perovskites have strong potential for use in tandem cells with silicon, and their high growth rate suggests that efficiencies may continue to rise significantly.14 Thus, obstacles to commercial adoption are mainly improvement of stability and environmental impact.
Dye-Sensitised Solar Cells (DSSCs)
DSSCs are the longest-standing third-generation solar technology. They are based on organic dye-absorbers in a liquid electrolyte. Many of their features and advantages overlap with those of OPVs - namely, abundant, solution-processable materials, and the potential for flexible and transparent devices. The two systems have also achieved similar certified efficiencies approaching 12%1 and small scale research efficiencies exceeding 13%.17,18 Despite this, advances on the record-certified efficiency and research-scale efficiency for DSSCs have not been exceeded since 2012 and 2014 respectively, which suggests a low potential for further improvements.
DSSCs can also suffer from instability issues. Mainly, these issues are temperature instability and solvent permeation19 of the liquid electrolyte. Theoretically, multiple dyes can be used to capture a wider range of the solar spectrum, but this results in complicated redox chemistry that can be difficult to optimise. As with OPVs, DSSCs are best suited to low-cost, niche applications, (especially those requiring a variety of colours),14 but lower efficiencies may restrict their development in the future.
Quantum Dot Solar Cells
Quantum dot solar cells encompass a variety of technologies based on semiconductor nanocrystals (most often metal chalcogenide nanocrystals such as PbS or PbSe), with the record efficiency achieved by a cell based on a CsPbI3 system.13 These share advantages with OPVs in terms of low-temperature solution-processable manufacture and easily tunable band gap,11 dependant on component choice, but often use toxic or rare materials, such as selenium. Tunable band gaps, dependent on size, could allow multi-junction cells using a single material.20 Difficulties in application include size distribution of nanoparticles leading to a distribution of band gaps, resulting in large voltage loss. Regardless, efficiencies have risen rapidly since initial publication, and certified efficiencies now exceed those of OPVs. With better understanding, efficiencies are likely to continue rising. If stability and ease of manufacture improve with alongside, quantum dots may become a promising technology for commercialisation.
Whilst OPVs have clear disadvantages (efficiency) compared to second generation solar technologies, and clear advantages in terms of environmental impact and material abundance, the comparisons are not so clear-cut for other third-generation cells. Many technologies, such as CZTS and DSSCs are similarly performing and have similar advantages, but arguably OPVs are the main low-efficiency technology that has continued to rise in reported efficiencies, especially with the advent of non-fullerene acceptors. Perovskite solar cells share the low-cost, abundant material advantages of OPVs, but have efficiencies comparable to that of second-generation technologies - and hence have received significantly more research attention in recent years. Despite this, OPVs still maintain an advantage in terms of environmental impact and material toxicity. Ultimately, it remains to be seen if this can compete with the higher efficiencies of perovskite solar cells, or if the two can co-exist in differently-suited applications.
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