Afshin Hadipour

Solution-processed organic tandem solar cells with embedded optical spacers

We demonstrate a solution-processed polymer tandem solar cell in which the two photoactive single cells are separated by an optical spacer. The use of an optical spacer allows for an independent optimization of both the electronic and optical properties of the tandem cell. The optical transmission window of the bottom cell is optimized to match the optical absorption of the top cell by varying the layer thickness of the optical spacer. The two bulk heterojunction subcells have complementary absorption maxima at λmax∼850nm for the top cell and λmax∼550nm for the bottom cell. The subcells are electronically coupled in series or in parallel using four electrical contacts. The series configuration leads to an open-circuit voltage of >1V, which is equal to the sum of both subcells. The parallel configuration leads to a high short-circuit current of 92A∕m2, which is equal to the sum of both subcells. The parallel configuration results in a much higher efficiency compared to the series configuration.

Nafion-modified MoOx as effective room-temperature hole injection layer for stable, high-performance inverted organic solar cells.

We present a hole injection layer processed from solution at room temperature for inverted organic solar cells. Bis(2,4-pentanedionato) molybdenum(VI) dioxide (MoO2(acac)2) is used as the precursor for MoOx. Small amounts of Nafion in the precursor solution allow it to form continuous films with good wetting onto the active layers. The hydrolysis of MoO2(acac)2 and the effects of adding Nafion to the precursor solution are studied by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The devices with solution-processed MoOx including Nafion exhibited comparable performance to the reference devices based on the commonly used hole injection layers such as poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) or evaporated MoO3. Inverted poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester devices with Nafion-modified MoOx maintain 80% of their initial power conversion efficiency upon exposure to ambient air for ∼5000 h, outperforming devices with PEDOT:PSS or with evaporated MoO3.

Organic multi-junction solar cells processed from solution with sensitivity from ultraviolet to the near infrared

One of the limitations of present organic solar cells is the relatively poor spectral overlap of their absorption bands with the solar spectrum. Semiconducting polymers as poly(3-hexyl thiophene) have a bandgap higher than 2.0 eV (600 nm), thereby limiting the maximum possible absorption of the solar spectrum to about 30%. A way to overcome this limitation is a tandem solar cell where two bulk heterojunction single cells are stacked in series, each with a different bandgap. The combined absorption then covers a broader region of the solar spectrum. So far, solution-processed tandem solar cells have not been realized due to incompatibility of the solvents. We demonstrate a solution-processed polymer tandem cells by stacking two single cells in series. The tandem cell consist of two bulk heterojunction subcells separated by a thin semitransparent electrode of gold. This middle electrode serves in three different ways; as a charge recombination centre, as a protecting layer for first cell during spin coating of the second cell, and as a semitransparent layer that creates optical cavities, which allows tuning of the optical transmission through the first (bottom) cell to optimize the optical absorption of the second (top) cell. To cover a broader region of the solar spectrum we combined a small bandgap polymer (λmax ~ 850 nm) with a large bandgap polymer (λmax ~ 550 nm). These sub cells are electronically coupled in series, which leads to an open-circuit voltage that equals the sum of each sub cell. A high open-circuit voltage of 1.4 Volt is achieved. The current density of the tandem cell follows the current of the top cell, which has a lower, limiting current. The tandem architecture and proper materials give us the possibility to cover a very broad spectral range of the solar spectrum to make highly efficient organic solar cells in the near future.