(Nanowerk Spotlight) In the pursuit of sustainable energy solutions, organic solar cells (OSCs) have emerged as a promising technology for semitransparent applications, such as building-integrated photovoltaics and greenhouses.
The ability to fine-tune the optical properties of OSCs by manipulating the light-absorbing materials, known as donors and acceptors, has opened up exciting possibilities for creating devices that generate clean energy while allowing visible light to pass through. However, the challenge of balancing power conversion efficiency (PCE) with transparency has hindered the widespread adoption of semitransparent OSCs.
Now, a team of researchers from the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy and the University of Liverpool has made a significant breakthrough in addressing this challenge. In a recent study published in Advanced Functional Materials (“Guidelines for Material Design in Semitransparent Organic Solar Cells”), the authors present a novel simulation framework that builds upon the well-established Scharber model, which has been widely used to predict the performance of OSCs based on the energy levels of donor and acceptor materials.
a) Schematic of the energy levels used to calculate the Voc, b) Model absorbance spectra used for the donors PCE10 and PM6 and the acceptors Y6 and FOIC. The bandgap values are determined by the intersection point of the rising flank of the spectrum with the wavelength axis. In particular, the values were set as Eg = 1.82 eV or λg = 680 nm for PM6, Eg = 1.6 eV or λg = 775 nm for PCE10, Eg = 1.36 eV or λg = 910 nm for Y6 and Eg = 1.3 eV or λg = 950 nm for FOIC. (Adapted from doi: 10.1002/adfm.202314116, CC BY)
What sets this new model apart is its incorporation of realistic absorption spectra of representative donor and acceptor materials, allowing for a more accurate prediction of OSC performance. By systematically varying the bandgaps (the energy difference between the highest occupied and lowest unoccupied molecular orbitals) of these materials, the researchers were able to map out the landscape of achievable PCE and average visible transmittance (AVT) values, providing valuable insights into the optimal combinations of donor and acceptor materials for semitransparent OSCs.
The model identified three distinct regions of high light utilization efficiency (LUE), a figure of merit that balances PCE and AVT. The first optimum occurs when both donor and acceptor have relatively small bandgaps around 1.3 electron volts (eV), allowing for strong absorption in the near-infrared region while maintaining good transparency in the visible spectrum. The second optimum arises from a combination of a wide-bandgap donor (≈2.5 eV) and a narrow-bandgap acceptor (≈1.35 eV), which minimizes absorption overlap and enables a broad transmission window. The third optimum involves very wide bandgaps for both materials (≈2.9 eV for the donor and ≈2.3 eV for the acceptor), resulting in exceptional transparency at the cost of reduced PCE.
Interestingly, when comparing their theoretical predictions with experimental data from the literature, the researchers found that most high-performance semitransparent OSCs reported to date utilized donor and acceptor materials with bandgaps that do not correspond to any of the identified optima. This discrepancy suggests that the new guidelines could potentially shift research directions in the field, encouraging the development of OSCs with materials that are better suited for achieving high LUE values.
The implications of this research extend beyond the realm of academic curiosity. By providing a roadmap for designing semitransparent OSCs with optimal bandgap combinations, this work could accelerate the development of practical applications such as energy-generating windows and greenhouses. The integration of semitransparent OSCs into buildings could significantly reduce their energy consumption, while their use in agricultural settings could enhance crop yields by allowing for the selective transmission of photosynthetically active radiation (PAR).
To demonstrate the versatility of their approach, the researchers also adapted their model to optimize OSCs for transparency in the PAR region of the solar spectrum. By replacing the photopic response curve used for visible light with the plant action spectrum, they identified four distinct regions of high LUE, with the most promising involving a donor bandgap of ≈2.0 eV and an acceptor bandgap of ≈1.3 eV.
This finding highlights the potential for tailoring OSCs to specific applications, further expanding their utility.
While the results of this study are promising, there are still challenges to be overcome in translating these theoretical insights into practical devices. The researchers conducted a search of a database containing the energy levels of over 50,000 known organic semiconductors and found that all of the identified LUE maxima corresponded to chemically-accessible bandgap values. However, synthesizing novel organic semiconductors with the desired bandgaps and optimizing their performance in OSCs will require further experimental work.
Additionally, scaling up the production of semitransparent OSCs and ensuring their long-term stability and durability will be crucial for their widespread adoption. Addressing these challenges will require collaborative efforts from researchers across multiple disciplines, including materials science, chemistry, and engineering.
Despite these hurdles, the work of Forberich et al. represents a significant milestone in the development of semitransparent organic solar cells. By providing a powerful tool for predicting and optimizing the performance of these devices, their model opens up new avenues for research and innovation in the field.
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