Eugene A. Katz

Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight

We report on accelerated degradation testing of MAPbX3 films (X = I or Br) by exposure to concentrated sunlight of 100 suns and show that the evolution of light absorption and the corresponding structural modifications are dependent on the type of halide ion and the exposure temperature. One hour of such exposure provides a photon dose equivalent to that of one sun exposure for 100 hours. The degradation in absorption of MAPbI3 films after exposure to 100 suns for 60 min at elevated sample temperature (∼45-55 °C), due to decomposition of the hybrid perovskite material, is documented. No degradation was observed after exposure to the same sunlight concentration but at a lower sample temperature (∼25 °C). No photobleaching or decomposition of MAPbBr3 films was observed after exposure to similar stress conditions (light intensity, dose, and temperatures). Our results indicate that the degradation is highly dependent on the hybrid perovskite composition and can be light- and thermally enhanced.

Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions

We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions ~25‐ 60 °C!. The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase ~coupled to a corresponding increase for the fill factor! was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47‐ 60 °C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility.

Toward ultrahigh-flux photovoltaic concentration

We report experimental results with a miniature fiber-optic photovoltaic concentrator with (a) deliverable power density up to 104 suns (10 W/mm2), (b) solar cell efficiencies above 30%, (c) completely passive cooling, (d) uniform and individualized cell illumination, and (e) assembly from readily available components. Measurements include the sensitivity of the conversion efficiency of tandem III–V cells to (1) power input, (2) flux distribution, and (3) the modified spectrum from the fiber-optic concentrators. Our results augur favorably for the feasibility of such designs at concentration levels as high as thousands of suns.

Effects of concentrated sunlight on organic photovoltaics

We report the effects of concentrated sunlight on key photovoltaic parameters and stability of organic photovoltaics (OPV). Sunlight collected and concentrated outdoors was focused into an optical fiber and delivered onto a 1 cm2 bulk-heterojunction cell. Sunlight concentration C was varied gradually from 0.2 to 27 suns. Power conversion efficiency exhibited slow increase with C that was followed by saturation around 2% at C=0.5–2.5 suns and subsequent strong reduction. Possible OPV applications in stationary solar concentrators (C≤2 suns) are discussed. Finally, experiments at C=55–58 suns demonstrated potential of our approach for accelerated studies of light induced mechanisms in the OPV degradation.

Photovoltaic characterization of concentrator solar cells by localized irradiation

The ability to determine the macroscopic parameters that characterize photovoltaic performance, including their spatial dependence, especially at high flux, is demonstrated with extensive solar measurements on high-efficiency concentrator solar cells. Two case studies explore (a) the impact of inhomogeneous flux distribution on photovoltaic behavior, (b) establishing how solar cell parameters vary across the cell surface (of particular interest for deployment in high-concentration optical systems), and (c), the sensitivity of photovoltaic parameters to the spatial variation of series resistance that stems from nonuniform cell metallization. In the process, we elucidate current-voltage trends unique to strongly inhomogeneous illumination and to series resistance losses at high flux.

Origin of size effect on efficiency of organic photovoltaics

It is widely accepted that efficiency of organic solar cells could be limited by their size. However, the published data on this effect are very limited and none of them includes analysis of light intensity dependence of the key cell parameters. We report such analysis for bulk heterojunction solar cells of various sizes and suggest that the origin of both the size and the light intensity effects should include underlying physical mechanisms other than conventional series resistance dissipation. In particular, we conclude that the distributed nature of the ITO resistance and its influence on the voltage dependence of photocurrent and dark current is the key to understanding size limitation of the organic photovoltaics (OPV) efficiency. Practical methods to overcome this limitation as well as the possibility of producing concentrator OPV cells operating under sunlight concentrations higher than 10 suns are discussed.

MoS2 Hybrid Nanostructures: From Octahedral to Quasi‐Spherical Shells within Individual Nanoparticles

MoS2, a layered compound with tribological and catalytic applications, is known to form a range of hollow closed nanostructures and nanoparticles, including graphene-like structures. These have been demonstrated experimentally through high-temperature synthesis and pulsed laser ablation (PLA), and theoretically with quantum chemical calculations. The smallest allowed structures are nanooctahedra of 3 to 8 nm size. Nicknamed the “true inorganic fullerene” in analogy to carbon fullerenes, they differ from larger multiwalled MoS2 fullerene-like nanoparticles both in their morphology and predicted electronic properties. The larger fullerene-like particles are quasi-spherical (polyhedral) or nanotubular, typically with diameters of 20 to 150 nm. Above a few hundred nm in size, these nanoparticles transform into 2H-MoS2 platelets. Fullerene-like particles have been recognized as superior solid lubricants with numerous commercial applications, and MoS2 nanooctahedra may have catalytic applications. Understanding the fundamental commonality of these two morphologies might prove essential in the development of new materials. The research on hollow MoS2 nanostructures of minimal size (< 10 nm in diameter) was initiated in 1993 upon the first independent proposal of the formation of nanooctahedra of MoS2 [3,5] (and BN) with six rhombi in their corners. In 1999, it was demonstrated that twoto four-walled MoS2 nanooctahedra, 3–5 nm in size and up to ca. 10 atoms, could be obtained by PLA. Similar results were subsequently reported in Ref. [1, 2] as illustrated in Figure 1a. Recent studies of high energy density methods such as laser ablation and arc– discharge resulted in small structures with only a limited number of atoms: Mo–S clusters or double-walled nanooctahedra.

Identifying Fundamental Limitations in Halide Perovskite Solar Cells.

The temperature dependence of the principal photovoltaic parameters of perovskite photovoltaics is studied. The recombination activation energy is in good agreement with the perovskites bandgap energy, thereby placing an upper bound on the open-circuit voltage. The photocurrent increases moderately with temperature and remains high at low temperature, reinforcing that the cells are not hindered by insufficient thermally activated mobility or carrier trapping by deep defects.

Hybrid photovoltaic-thermoelectric system for concentrated solar energy conversion: Experimental realization and modeling

An experimental demonstration of the combined photovoltaic (PV) and thermoelectric conversion of concentrated sunlight (with concentration factor, X, up to ∼300) into electricity is presented. The hybrid system is based on a multi-junction PV cell and a thermoelectric generator (TEG). The latter increases the electric power of the system and dissipates some of the excessive heat. For X ≤ 200, the systems maximal efficiency, ∼32%, was mostly due to the contribution from the PV cell. With increasing X and system temperature, the PV cells efficiency decreased while that of the TEG increased. Accordingly, the direct electrical contribution of the TEG started to dominate in the total system power, reaching ∼20% at X ≈ 290. Using a simple steady state finite element modeling, the cooling effect of the TEG on the hybrid systems efficiency was proved to be even more significant than its direct electrical contribution for high solar concentrations. As a result, the total efficiency contribution of the TEG reach...

High-flux characterization of ultrasmall multijunction concentrator solar cells

A characterization study of ultrasmall ultraefficient multijunction concentrator solar cells is presented, with emphasis on performance sensitivity to light intensity and distribution. Information of this type is essential in the design and optimization of the latest generations of high-flux photovoltaic systems. Cell miniaturization allows increasing the concentration at which efficiency peaks, facilitates passive heat rejection, and permits the use of all-glass optics. However, few device measurements have been published on ultrasmall cells. Extensive measurements, up to ∼5000 suns, on the 1.0mm2 active region within the busbars are reported and analyzed.