Nicola Gasparini

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700u2009nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20u2009mAu2009cm−2 along with open-circuit voltages >1u2009V, which is remarkable for a 1.6u2009eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells.The nonfullerene-based small molecules start to attract more attention for solar cell research than the fullerene acceptors due to their wider tunability. Here Baran et al. demonstrate nonfullerene-based solar cells with high power conversion efficiency of 12% and quantum efficiencies approaching 100%.

Fluorination Triggered New Small Molecule Donor Materials for Efficient As-Cast Organic Solar Cells

Solution-processable small molecules (SMs) have attracted intense attention due to their definite molecular structures, less batch-to-batch variation, and easier structure control. Herein, two new SM donors based on substituted isatin unit (DI3T, DI3T-2F) are synthesized and applied as electron donors with the mixture of PC71 BM to construct organic photovoltaics. As a result, 5,6-difluoro isatin derivative (DI3T-2F) obtains a power conversion efficiency of 7.80% by a simple solution spin-coating fabrication process without any additives, solvent, or thermal annealing process. More intuitively, due to stronger intermolecular interaction and higher hole mobility after the incorporation of fluorine atoms in end units, the devices present good tolerance to active layer thickness. The results indicate that DI3T-2F shows promising potential for large-scale printing processes and flexible application of efficient small molecule organic solar cells.

Strategies for high current densities in non-fullerene acceptors based organic solar cells

Here, we report the strategies to increase the photon harvesting in single junction organic photovoltaics by band gap engineering. Low band-gap non-fulllerene small molecule acceptors yield remarkable short-circuit current (26.6 mA/cm2) which comparable to existing high efficiency photovoltaic technologies.

Cs0.15FA0.85PbI3 perovskite solar cells for concentrator photovoltaic applications

Recently developed, highly stable perovskite materials show promise for use in concentrator photovoltaics where the illumination intensity far exceeds standard test conditions. Here, we demonstrate solar cell devices employing different perovskite absorber layers featuring balanced charge generation and extraction characteristics at high light intensities greater than 10 suns. Using a mixed cesium-formamidinium perovskite, we are able to achieve over 18% PCE at 1 sun and 16% PCE at 13 suns with negligable performance loss after several hours of high intensity light soaking.

P3HT: non-fullerene acceptor based large area, semi-transparent PV modules with power conversion efficiencies of 5%, processed by industrially scalable methods

The transfer from poly-3hexylthiophene (P3HT) based fullerene free organic photovoltaic (OPV) lab cells with IDTBR (rhodanine-benzothiadiazole-coupled indacenodithiophene) as acceptor material to fully solution processed roll-to-roll (R2R) compatible modules is reported. The developed R2R process is fully compatible with industrial requirements as it uses exclusively non-hazardous solvents. The combination of optimized ink formulation, module layout, and processing affords efficiencies of 5% on 60 cm2 total module area.

Overcoming the Ambient Manufacturability‐Scalability‐Performance Bottleneck in Colloidal Quantum Dot Photovoltaics

Colloidal quantum dot (CQD) solar cells have risen rapidly in performance; however, their low-cost fabrication under realistic ambient conditions remains elusive. This study uncovers that humid environments curtail the power conversion efficiency (PCE) of solar cells by preventing the needed oxygen doping of the hole transporter during ambient fabrication. A simple oxygen-doping step enabling ambient manufacturing irrespective of seasonal humidity variations is devised. Solar cells with PCE > 10% are printed under high humidity at industrially viable speeds. The devices use a tiny fraction of the ink typically needed and are air stable over a year. The humidity-resilient fabrication of efficient CQD solar cells breaks a long-standing compromise, which should accelerate commercialization.

Efficiency enhancement of semitransparent organic solar cells by using printed dielectric mirrors (Presentation Recording)

Building integrated thin film solar cells are a strategy for future eco-friendly power generation. Such solar cells have to be semi-transparent, long-term stable and show the potential to be fabricated by a low-cost production process. Organic photovoltaics are a potential candidate because an absorber material with its main absorption in the infrared spectral region where the human eye is not sensitive can be chosen. We can increase the number of absorbed photons, at the same time, keep the transparency almost constant by using a dielectric, wavelength-selective mirror. The mirror reflects only in the absorption regime of the active layer material and shows high transparencies in the spectral region around 550 nm where the human eye is most sensitive. We doctor bladed a fully solution processed dielectric mirror at low temperatures below 80 °C. Both inks, which are printed alternatingly are based on nanoparticles and have a refractive index of 1.29 or 1.98, respectively, at 500 nm. The position and the intensity of the main reflection peak can be easily shifted and thus adjusted to the solar cell absorption spectrum. Eventually, the dielectric mirror was combined with different organic solar cells. For instance, the current increases by 20.6 % while the transparency decreases by 23.7 % for the low band gap absorber DPP and silver nanowires as top electrode. Moreover we proved via experiment and optical simulations, that a variation of the active layer thickness and the position of the main reflection peak affect the transparency and the increase in current.