Michael Salvador

Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability

Fullerene dimerization has been linked to short circuit current (Jsc) losses in organic solar cells comprised of certain polymer–fullerene systems. We investigate several polymer–fullerene systems, which present Jsc loss to varying degrees, in order to determine under which conditions dimerization occurs. By reintroducing dimers into fresh devices, we confirm that the photo-induced dimers are indeed the origin of the Jsc loss. We find that both film morphology and electrical bias affect the photodimerization process and thus the associated loss of Jsc. In plain fullerene films, a higher degree of crystallinity can inhibit the dimerization reaction, as observed by high performance liquid chromatography (HPLC) measurements. In blend films, the amount of dimerization depends on the degree of mixing between polymer and fullerene. For highly mixed systems with very amorphous polymers, no dimerization is observed. In solar cells with pure polymer and fullerene domains, we tune the fullerene morphology from amorphous to crystalline by thermal annealing. Similar to neat fullerene films, we observe improved light stability for devices with crystalline fullerene domains. Changing the operating conditions of the investigated solar cells from Voc to Jsc also significantly reduces the amount of dimerization-related Jsc loss; HPLC analysis of the active layer shows that more dimers are formed if the cell is held at Voc instead of Jsc. The effect of bias on dimerization, as well as a clear correlation between PL quenching and reduced dimerization upon addition of small amounts of an amorphous polymer into PC60BM films, suggests a reaction mechanism via excitons.

Flexible organic tandem solar modules with 6% efficiency: combining roll-to-roll compatible processing with high geometric fill factors

Organic solar cell technology bears the potential for high photovoltaic performance combined with truly low-cost, high-volume processing. Here we demonstrate organic tandem solar modules on flexible substrates fabricated by fully roll-to-roll compatible processing at temperatures <70 °C. By using ultrafast laser patterning we considerably reduced the “dead area” of the modules and achieved geometric fill factors beyond 90%. The modules revealed very low interconnection-resistance compared to the single tandem cells and exhibited a power conversion efficiency of up to 5.7%. Bending tests performed on the modules suggest high mechanical resilience for this type of device. Our findings inform concrete steps towards high efficiency photovoltaic applications on curved, foldable and moving surfaces.

Correction: Balancing electrical and optical losses for efficient 4-terminal Si-perovskite solar cells with solution processed percolation electrodes

Correction for ‘Balancing electrical and optical losses for efficient 4-terminal Si-perovskite solar cells with solution processed percolation electrodes’ by Cesar Omar Ramirez Quiroz et al., J. Mater. Chem. A, 2018, 6, 3583–3592.

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.

Coloring semitransparent room-temperature fabricated perovskite solar cells via dielectric mirrors(Conference Presentation)

While the development of perovskite-based semitransparent solar cells with competitive levels of transparency and efficiency offer a promising perspective towards building integrated photovoltaics, the color perception of perovskite films is of limited visual aesthetics, compromising their applicability to facades and windows. In the present work, we develop a technique to grow crystalline, ultrathin perovskite films through a solvent-solvent extraction process featuring full crystallization within few seconds at RT and under 45%RH environmental conditions. As a result we obtained the highest combination of efficiency and transparency to date for perovskite solar cells. We further improved the visual aesthetics of our devices by implementing dielectric mirrors. EQE and UV-Vis spectroscopic measurements are performed to fully characterize the device stacks featuring four different dielectric mirror configurations. By customizing the mirror to the near-IR absorption region of the perovskite, we could increase the Jsc by 18.7%, yielding a light blue appearance and showing 31.4% transparency at 3.5% electrical power efficiency. Both, the solar cells and the dielectric mirrors are fully-solution processed under ambient conditions and are easily transferable to roll-to-roll upscaling. Optical simulations support our experimental findings and provide a global perspective emulating full device stack appearance covering all the colors in the visible spectra. Transparency, photocurrent density contribution and chromaticity are finally simulated and analyzed. Based on the detailed analysis, we give an outlook on the performance – color – transparency roadmap for perovskite solar cells.

Flexible organic tandem solar modules: a story of up-scaling

The competition in the field of solar energy between Organic Photovoltaics (OPVs) and several Inorganic Photovoltaic technologies is continuously increasing to reach the ultimate purpose of energy supply from inexpensive and easily manufactured solar cell units. Solution-processed printing techniques on flexible substrates attach a tremendous opportunity to the OPVs for the accomplishment of low-cost and large area applications. Furthermore, tandem architectures came to boost up even more OPVs by increasing the photon-harvesting properties of the device. In this work, we demonstrate the road of realizing flexible organic tandem solar modules constructed by a fully roll-to-roll compatible processing. The modules exhibit an efficiency of 5.4% with geometrical fill factors beyond 80% and minimized interconnection-resistance losses. The processing involves low temperature (<70 °C), coating methods compatible with slot die coating and high speed and precision laser patterning.