Guy Olivier Ngongang Ndjawa

Characterization of the polymer energy landscape in polymer: fullerene bulk heterojunctions with pure and mixed phases

Theoretical and experimental studies suggest that energetic offsets between the charge transport energy levels in different morphological phases of polymer:fullerene bulk heterojunctions may improve charge separation and reduce recombination in polymer solar cells (PSCs). In this work, we use cyclic voltammetry, UV-vis absorption, and ultraviolet photoelectron spectroscopy to characterize hole energy levels in the polymer phases of polymer:fullerene bulk heterojunctions. We observe an energetic offset of up to 150 meV between amorphous and crystalline polymer due to bandgap widening associated primarily with changes in polymer conjugation length. We also observe an energetic offset of up to 350 meV associated with polymer:fullerene intermolecular interactions. The first effect has been widely observed, but the second effect is not always considered despite being larger in magnitude for some systems. These energy level shifts may play a major role in PSC performance and must be thoroughly characterized for a complete understanding of PSC function.

Hole‐Transporting Transistors and Circuits Based on the Transparent Inorganic Semiconductor Copper(I) Thiocyanate (CuSCN) Processed from Solution at Room Temperature

The wide bandgap and highly transparent inorganic compound copper(I) thiocyanate (CuSCN) is used for the first time to fabricate p-type thin-film transistors processed from solution at room temperature. By combining CuSCN with the high-k relaxor ferroelectric polymeric dielectric P(VDF-TrFE-CFE), we demonstrate low-voltage transistors with hole mobilities on the order of 0.1 cm(2) V(-1) s(-1) . By integrating two CuSCN transistors, unipolar logic NOT gates are also demonstrated.

Re‐evaluating the Role of Sterics and Electronic Coupling in Determining the Open‐Circuit Voltage of Organic Solar Cells

The effects of sterics and molecular orientation on the open-circuit voltage and absorbance properties of charge-transfer states are explored in model bilayer organic photovoltaics. It is shown that the open-circuit voltage correlates linearly with the charge-transfer state energy and is not significantly influenced by electronic coupling.

Efficient inverted bulk-heterojunction solar cells from low-temperature processing of amorphous ZnO buffer layers

In this report, we demonstrate that solution-processed amorphous zinc oxide (a-ZnO) interlayers prepared at low temperatures (∼100 °C) can yield inverted bulk-heterojunction (BHJ) solar cells that are as efficient as nanoparticle-based ZnO requiring comparably more complex synthesis or polycrystalline ZnO films prepared at substantially higher temperatures (150–400 °C). Low-temperature, facile solution-processing approaches are required in the fabrication of BHJ solar cells on flexible plastic substrates, such as PET. Here, we achieve efficient inverted solar cells with a-ZnO buffer layers by carefully examining the correlations between the thin film morphology and the figures of merit of optimized BHJ devices with various polymer donors and PCBM as the fullerene acceptor. We find that the most effective a-ZnO morphology consists of a compact, thin layer with continuous substrate coverage. In parallel, we emphasize the detrimental effect of forming rippled surface morphologies of a-ZnO, an observation which contrasts with results obtained in polycrystalline ZnO thin films, where rippled morphologies have been reported to improve efficiency. After optimizing the a-ZnO morphology at low processing temperature for inverted P3HT:PCBM devices, achieving a power conversion efficiency (PCE) of ca. 4.1%, we demonstrate inverted solar cells with low bandgap polymer donors on glass/flexible PET substrates: PTB7:PC71BM (PCE: 6.5% (glass)/5.6% (PET)) and PBDTTPD:PC71BM (PCE: 6.7% (glass)/5.9% (PET)). Finally, we show that a-ZnO based inverted P3HT:PCBM BHJ solar cells maintain ca. 90–95% of their initial PCE even after a full year without encapsulation in a nitrogen dry box, thus demonstrating excellent shelf stability. The insight we have gained into the importance of surface morphology in amorphous zinc oxide buffer layers should help in the development of other low-temperature solution-processed metal oxide interlayers for efficient flexible solar cells.

Optoelectronic and photovoltaic properties of the air-stable organohalide semiconductor (CH3NH3)3Bi2I9

Lead halide perovskite materials have shown excellent optoelectronic as well as photovoltaic properties. However, the presence of lead and the chemical instability relegate lead halide perovskites to research applications only. Here, we investigate an emerging lead-free and air stable compound (CH3NH3)3Bi2I9 as a non-toxic potential alternative to lead halide perovskites. We have synthesized thin films, powders and millimeter-scale single crystals of (CH3NH3)3Bi2I9 and investigated their structural and optoelectronic properties. We demonstrate that the degree of crystallinity strongly affects the optoelectronic properties of the material, resulting in significantly different band gaps in polycrystalline thin films and single crystals. Surface photovoltage spectroscopy reveals outstanding photocharge generation in the visible (<700 nm) region, while transient absorption spectroscopy and space charge limited current measurements point to a long exciton lifetime and a high carrier mobility, respectively, similar to lead halide perovskites pointing to the remarkable potential of this semiconductor. Photovoltaic devices fabricated using this material yield a low power conversion efficiency (PCE) to date, but the PCE is expected to increase with improvements in thin film processing and device engineering.

Programmable and coherent crystallization of semiconductors

Programmable crystallization of thin films produces patterns and bespoke microstructures for semiconductor applications. The functional properties and technological utility of polycrystalline materials are largely determined by the structure, geometry, and spatial distribution of their multitude of crystals. However, crystallization is seeded through stochastic and incoherent nucleation events, limiting the ability to control or pattern the microstructure, texture, and functional properties of polycrystalline materials. We present a universal approach that can program the microstructure of materials through the coherent seeding of otherwise stochastic homogeneous nucleation events. The method relies on creating topographic variations to seed nucleation and growth at designated locations while delaying nucleation elsewhere. Each seed can thus produce a coherent growth front of crystallization with a geometry designated by the shape and arrangement of seeds. Periodic and aperiodic crystalline arrays of functional materials, such as semiconductors, can thus be created on demand and with unprecedented sophistication and ease by patterning the location and shape of the seeds. This approach is used to demonstrate printed arrays of organic thin-film transistors with remarkable performance and reproducibility owing to their demonstrated spatial control over the microstructure of organic and inorganic polycrystalline semiconductors.

The roles of bulk and interfacial molecular orientations in determining the performance of organic bilayer solar cells (presentation video)

Molecular orientation plays a significant role in determining the performance of small molecule solar cells. Key photovoltaic processes in these cells are strongly dependent on how the molecules are oriented in the active layer. We isolate contributions arising from the bulk molecular orientations vs. those from interfacial orientations in ZnPc/C60 bilayer systems and we probe these contributions by comparing device pairs in which only the bulk or the interface differ. By controlling the orientation in the bulk the current can be strongly modulated, whereas controlling the interfacial molecular orientation and degree of intermixing mediate the voltage.

Charge-transfer state energy determines open-circuit voltage in organic photovoltaics

Organic photovoltaics (OPVs) are a promising alternative to inorganic photovoltaics because they are made out of cheaper and non-toxic materials combined with low-energy and low-cost processing techniques. However, large energy losses between the optical gaps of the absorbing materials and the open-circuit voltage (VOC) limit how efficiently these OPVs convert solar energy into electricity. Materials used in OPVs commonly have optical gaps of between 1.7 and 2.1eV. However, the VOC seldom exceeds 1.0eV. This difference between the optical gap and qVOC, the potential energy at open-circuit voltage, represents a loss of almost half the photon’s original energy. In contrast, inorganic photovoltaic cells, such as polycrystalline silicon, gallium arsenide, and copper indium gallium selenide, show a difference of only 0.3–0.45eV between the material bandgap and qVOC. For OPVs to reach higher power conversion efficiencies (PCEs), these energy losses must be decreased and the VOC increased. This requires a better understanding of the origin of VOC and energyloss mechanisms. The VOC is the point at which the recombination current is equal to the photocurrent. It can be given by the equation Jrec D qkrecLnp, where krec is the recombination rate constant, L is the thickness of the recombination layer, and np is the product of electrons and holes (charges) in the device. The np product depends exponentially on the energy gap and applied or photogenerated voltage. Increases in the energy gap decrease np and Figure 1. Chemical structures of the donor materials used in the organic photovoltaic devices reported in this article, tetracene and rubrene.