Microcavity-enhanced light-trapping for highly efficient organic parallel tandem solar cells.

A high-performance parallel tandem solar cell employing ultra-thin Ag as the intermediate anode is demonstrated, which comprises a semitransparent front sub-cell and a microcavity assisted back sub-cell. In addition to the extended optical field as a result of the tandem architecture, the prominent microcavity resonance formed in the back sub-cell enables such a parallel tandem configuration to possess high light utilization efficiency (the peak EQE value is over 80%) and a high photovoltaic performance of 9.2%. This study establishes an effective architecture that can be generally applicable to all organic materials for improving their performance.

Strong photocurrent enhancements in highly efficient flexible organic solar cells by adopting a microcavity configuration.

Organic solar cells often show inefficient light harvesting due to a short absorption path length limited by the low charge mobility of organic semiconductors. We demonstrate a flexible organic solar cell in a microcavity configuration using a TeO2/Ag semitransparent electrode to confine the optical field within the device with significant performance improvements and reaching a power conversion efficiency of 8.56%.

Polymer triplet energy levels need not limit photocurrent collection in organic solar cells.

We study charge recombination via triplet excited states in donor/acceptor organic solar cells and find that, contrary to intuition, high internal quantum efficiency (IQE) can be obtained in polymer/fullerene blend devices even when the polymer triplet state is significantly lower in energy than the intermolecular charge transfer (CT) state. Our model donor system comprises the copolymer PIDT-PhanQ: poly(indacenodithiophene-co-phenanthro[9,10-b]quinoxaline), which when blended with phenyl-C(71)-butyric acid methyl ester (PC(71)BM) is capable of achieving power conversion efficiencies of 6.0% and IQE ≈ 90%, despite the fact that the polymer triplet state lies 300 meV below the interfacial CT state. However, as we push the open circuit voltage (V(OC)) higher by tailoring the fullerene reduction potential, we observe signatures of a new recombination loss process near V(OC) = 1.0 V that we do not observe for PCBM-based devices. Using photoinduced absorption and photoluminescence spectroscopy, we show that a new recombination path opens via the fullerene triplet manifold as the energy of the lowest CT state approaches the energy of the fullerene triplet. This pathway appears active even in cases where direct recombination via the polymer triplet remains thermodynamically accessible. These results suggest that kinetics, as opposed to thermodynamics, can dominate recombination via triplet excitons in these blends and that optimization of charge separation and kinetic suppression of charge recombination may be fruitful paths for the next generation of panchromatic organic solar cell materials with high V(OC) and J(SC).

Increased open circuit voltage in fluorinated benzothiadiazole-based alternating conjugated polymers.

Small band-gap conjugated polymers based on monofluoro- and difluoro-substituted benzothiadiazole were developed. Highly efficient polymer solar cells (PCE as high as 5.40%) could be achieved for devices made from these polymers.

Development of new conjugated polymers with donor-pi-bridge-acceptor side chains for high performance solar cells.

Two new conjugated polymers have been designed and synthesized for polymer solar cells. Both of them exhibit excellent photovoltaic properties with a power conversion efficiency as high as 4.74%. Different from the traditional linear donor-acceptor (D-A) type conjugated polymers, these newly designed polymers have a two-dimensional conjugated structure with their tunable acceptors located at the end of D-A side chains and connected with the donors on the main chain through an efficient pi-bridge. This approach provides great flexibility in fine-tuning the absorption spectra and energy levels of the resultant polymers for achieving high device performance.

Design and synthesis of iridium(III) azacrown complex: application as a highly sensitive metal cation phosphorescence sensor.

A new metal cation probe bearing a central Ir(III) element and 1-aza-15-crown-5-ether substituted pyridyl pyrazolate as the chelate was synthesized. The octahedral molecular structure of was confirmed using single crystal X-ray diffraction analyses. Subsequent photophysical study showed yellow-green emission at approximately 560 nm in both fluid solution and solid state at room temperature. Remarkable differentiation in spectral properties upon metal cation (e.g. Ca2+) complexation makes complex a highly sensitive phosphorescence probe.