Advances in Solution-Processed Multijunction Organic Solar Cells.

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution-processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.

Near-Infrared Tandem Organic Photodiodes for Future Application in Artificial Retinal Implants.

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near-infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single-junction and tandem OPDs are compared. In the latter, two nominally identical single-junction cells are processed on top of each other, effectively doubling the open-circuit voltage (V OC ). Both single-junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high-resolution retinal prostheses based on flexible OPD arrays.

Quadruple Junction Polymer Solar Cells with Four Complementary Absorber Layers.

A monolithic two-terminal solution-processed quadruple junction polymer solar cell in an n-i-p (inverted) configuration with four complementary polymer:fullerene active bulk-heterojunction layers is presented. The subcells possess different optical bandgaps ranging from 1.90 to 1.13 eV. Optical modeling using the transfer matrix formalism enables prediction of the fraction of absorbed photons from sunlight in each subcell and determine the optimal combination of layer thicknesses. The quadruple junction cell features an open-circuit voltage of 2.45 V and has a power conversion efficiency of 7.6%, only slightly less than the modeled value of 8.2%. The external quantum efficiency spectrum, determined with appropriate light and voltage bias conditions, exhibits in general an excellent agreement with modeled spectrum. The device performance is presently limited by bimolecular recombination, which prevents using thick photoactive layers that could absorb light more efficiently.

New n-Type Solution Processable All Conjugated Polymer Network: Synthesis, Optoelectronic Characterization, and Application in Organic Solar Cells.

The efficient synthesis of a new solution-processable n-type conjugated polymer network (PNT1) is reported through palladium-catalyzed Stille cross-coupling reaction conditions following the A3 + B2 synthetic approach. A benzo[1,2-b:3,4-b':5,6-b″]trithiophene derivative is used as the A3 knot and an alkyl functionalized naphthalenediimide is utilized as the B2 linker. The thermal, optical, and electrochemical properties are examined in detail, showing high thermal stability, absorbance in the visible part of the solar spectrum, and reversible reduction characteristics similar to those of the fullerene derivative [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM). PNT1 is employed as the electron acceptor in solution-processed bulk heterojunction organic solar cells, demonstrating the potential of this new type of materials for optoelectronic applications.