Bertrand J. Tremolet de Villers

Charge-carrier dynamics in hybrid plasmonic organic solar cells with Ag nanoparticles

To understand the effects of Ag nanoparticles NPs on the performance of organic solar cells, we examined the properties of hybrid poly3-hexylthiophene:6,6-phenyl-C61-butyric-acid-methylester:Ag NP solar cells using photoinduced charge extraction with a linearly increasing voltage. We find that the addition of Ag NPs into the active layer significantly enhances carrier mobility but decreases the total extracted carrier. Atomic force microscopy shows that the Ag NPs tend to phase segregate from the organic material at high concentrations. This suggests that the enhanced mobility results from carriers traversing Ag NP subnetworks, and that the reduced carrier density results from increased recombination from carriers trapped on the Ag particles.

Hybrid conjugated polymer solar cells using patterned GaAs nanopillars

In this work, we study hybrid solar cells based on poly(3-hexylthiophene)-coated GaAs nanopillars grown on a patterned GaAs substrate using selective-area metal organic chemical vapor deposition. The hybrid solar cells show extremely low leakage currents (I≅45 nA @−1V) under dark conditions and an open circuit voltage, short circuit current density, and fill factor of 0.2 V, 8.7 mA/cm2, and 32%, respectively, giving a power conversion efficiency of η=0.6% under AM 1.5 G illumination. Surface passivation of the GaAs results in further improvement, yielding η=1.44% under AM 1.5 G illumination. External quantum efficiency measurements of these polymer/inorganic solar cells are also presented.

Stability of inverted organic solar cells with ZnO contact layers deposited from precursor solutions

We report on investigations of the stability of inverted organic solar cells with ZnO electron collecting interlayer that are solution-processed from zinc acetate (ZnAc) or diethylzinc (deZn) precursors. Characterization of the respective solar cells suggests that the two materials initially function similarly in devices, however, we find that devices with ZnO from the deZn precursor are more stable under long-term illumination and load than devices with ZnO from the ZnAc precursor. A dipolar phosphonic acid that reduces the ZnO work function also improved device performance and stability when compared with unmodified ZnAc-based ZnO, but was problematic for deZn-based ZnO. The long-term device degradation analyses shows that the improved devices had increased and significantly more stable open-circuit voltage and fill factor characteristics. Chemical analyses suggests that defects in the ZnO films, most likely interstitial zinc, may be responsible for the observed disparities in stability within organic solar cells.

Destruction of amplified spontaneous emission via chemical doping at low-work-function metal/conjugated polymer interfaces

The authors investigate how the use of different metal electrodes affects the ability of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) films to undergo amplified spontaneous emission (ASE). High-work-function metals such as Ag or Au have little effect on the ASE threshold, but low-work-function metals such as Ca or Al completely shut off ASE. ASE is restored when a thin spacer layer, such as a few nanometers of polystyrene or oxidized Ca, is introduced between the MEH-PPV film and the Ca or Al electrode. This suggests that low-work-function metals chemically dope the polymer, creating polarons that destroy ASE not only by lowering the gain through emission quenching but primarily by increasing the loss via optical absorption. Thus, the exponential sensitivity of ASE to optical losses provides a spectroscopic probe of conjugated polymer/metal interfaces.

Interfacial Characteristics of Efficient Bulk Heterojunction Solar Cells Fabricated on MoOx Anode Interlayers

The role of the interface between an MoOx anode interlayer and a polymer:fullerene bulk heterojunction is investigated. Processing differences in the MoOx induce large variations in the vertical stratification of the bulk heterojunction films. These variations are found to be inconsistent in predicting device performance, with a much better gauge being the quantity of polymer chemisorbed to the anode interlayer.