Joseph J. Berry

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoOx) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air yield continuous thin films of MoOx. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoOx and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoOx thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo5+ gap states in the MoOx HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoOx HCL.

Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening

Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3−xBrx (MAPbI3−xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3−xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I–Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.

Organic light emitting diodes using a Ga:ZnO anode

We report the application of gallium doped zinc oxide (GZO) films as anodes in organic light emitting diodes (OLEDs). Pulsed laser deposited GZO films of differing Ga composition are examined. Bilayer OLEDs using GZO and indium tin oxide (ITO) anodes are compared. Relative to ITO, the GZO anodes have a slightly better sheet resistance and transparency in the visible spectral region. Device data suggest that GZO effectively injects holes into an aromatic triamine hole transporting layer. Indium-free anodes such as GZO are expected to improve OLED stability while lowering the cost per unit area, which is crucial for OLED based lighting applications.

Carrier separation and transport in perovskite solar cells studied by nanometre-scale profiling of electrical potential

Organometal–halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells—which are the fundamental mechanisms of device operation and critical factors for power output—by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p–n junction structure at the TiO2/perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.

Structural and chemical evolution of methylammonium lead halide perovskites during thermal processing from solution

Following the prominent success of CH3NH3PbI3 in photovoltaics and other optoelectronic applications, focus has been placed on better understanding perovskite crystallization from precursor and intermediate phases in order to facilitate improved crystallinity often desirable for advancing optoelectronic properties. Understanding of stability and degradation is also of critical importance as these materials seek commercial applications. In this study, we investigate the evolution of perovskites formed from targeted precursor chemistries by correlating in situ temperature-dependent X-ray diffraction, thermogravimetric analysis, and mass spectral analysis of the evolved species. This suite of analyses reveals important precursor composition-induced variations in the processes underpinning perovskite formation and degradation. The addition of Cl− leads to widely different precursor evolution and perovskite formation kinetics, and results in significant changes to the degradation mechanism, including suppression of crystalline PbI2 formation and modification of the thermal stability of the perovskite phase. This work highlights the role of perovskite precursor chemistry in both its formation and degradation.

Efficient charge extraction and slow recombination in organic–inorganic perovskites capped with semiconducting single-walled carbon nanotubes

Metal-halide based perovskite solar cells have rapidly emerged as a promising alternative to traditional inorganic and thin-film photovoltaics. Although charge transport layers are used on either side of perovskite absorber layers to extract photogenerated electrons and holes, the time scales for charge extraction and recombination are poorly understood. Ideal charge transport layers should facilitate large discrepancies between charge extraction and recombination rates. Here, we demonstrate that highly enriched semiconducting single-walled carbon nanotube (SWCNT) films enable rapid (sub-picosecond) hole extraction from a prototypical perovskite absorber layer and extremely slow back-transfer and recombination (hundreds of microseconds). The energetically narrow and distinct spectroscopic signatures for charges within these SWCNT thin films enables the unambiguous temporal tracking of each charge carrier with time-resolved spectroscopies covering many decades of time. The efficient hole extraction by the SWCNT layer also improves electron extraction by the compact titanium dioxide electron transport layer, which should reduce charge accumulation at each critical interface. Finally, we demonstrate that the use of thin interface layers of semiconducting single-walled carbon nanotubes between the perovskite absorber layer and a prototypical hole transport layer improves device efficiency and stability, and reduces hysteresis.

Extrinsic ion migration in perovskite solar cells

The migration of intrinsic ions (e.g., MA+, Pb2+, I−) in organic–inorganic hybrid perovskites has received significant attention with respect to the critical roles of these ions in the hysteresis and degradation in perovskite solar cells (PSCs). Here, we demonstrate that extrinsic ions (e.g., Li+, H+, Na+), when used in the contact layers in PSCs, can migrate across the perovskite layer and strongly impact PSC operation. In a TiO2/perovskite/spiro-OMeTAD-based PSC, Li+-ion migration from spiro-OMeTAD to the perovskite and TiO2 layer is illustrated by time-of-flight secondary-ion mass spectrometry. The movement of Li+ ions in PSCs plays an important role in modulating the solar cell performance, tuning TiO2 carrier-extraction properties, and affecting hysteresis in PSCs. The influence of Li+-ion migration was investigated using time-resolved photoluminescence, Kelvin probe force microscopy, and external quantum efficiency spectra. Other extrinsic ions such as H+ and Na+ also show a clear impact on the performance and hysteresis in PSCs. Understanding the impacts of extrinsic ions in perovskite-based devices could lead to new material and device designs to further advance perovskite technology for various applications.

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.

Fabrication and Characterization of MIM Diodes Based on Nb/Nb2O5 Via a Rapid Screening Technique

Metal–insulator–metal (MIM) structures are gaining signifi cant attention due to their applications in varied electronic devices such as rectennas for energy harvesting, [ 1–4 ] high-frequency detectors/infrared photo-detection, [ 5–7 ] high-frequency mixers, [ 8–10 ] as well as applications in static memory and switching devices. [ 11 , 12 ] Ideally, for most of these applications, the MIM structure should exhibit current–voltage ( I–V ) characteristics with high asymmetry ( f ASYM > 1), strong nonlinearity ( f NL > 3), fast responsivity ( f RES > 7 V − 1 ), low hysteresis and low turn-on voltage (close to zero bias). [ 3 ] Despite the widespread utility and simple architecture of MIM devices, there is a signifi cant lack of understanding as to which materials properties produce the desired device performance. Although it is commonly stated that a high work-function difference ( Δ φ ) between the metal electrodes is responsible for high f ASYM and f NL , [ 3 , 5 , 7 ]