Chenyi Yi

Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency

Simultaneous modification of the dye and redox shuttle boosts the efficiency of a dye-sensitized solar cell. The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.

A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells

Metal halide perovskite solar cells (PSCs) currently attract enormous research interest because of their high solar-to-electric power conversion efficiency (PCE) and low fabrication costs, but their practical development is hampered by difficulties in achieving high performance with large-size devices. We devised a simple vacuum flash–assisted solution processing method to obtain shiny, smooth, crystalline perovskite films of high electronic quality over large areas. This enabled us to fabricate solar cells with an aperture area exceeding 1 square centimeter, a maximum efficiency of 20.5%, and a certified PCE of 19.6%. By contrast, the best certified PCE to date is 15.6% for PSCs of similar size. We demonstrate that the reproducibility of the method is excellent and that the cells show virtually no hysteresis. Our approach enables the realization of highly efficient large-area PSCs for practical deployment.

Tris(2-(1H-pyrazol-1-yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells

Chemical doping is an important strategy to alter the charge-transport properties of both molecular and polymeric organic semiconductors that find widespread application in organic electronic devices. We report on the use of a new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs). We show that the proposed compounds fulfill the requirements for this application and that the discussed strategy is promising for tuning the conductivity of spiro-MeOTAD in ssDSCs, without having to rely on the commonly employed photo-doping. By using a recently developed high molar extinction coefficient organic D-π-A sensitizer and p-doped spiro-MeOTAD as hole conductor, we achieved a record power conversion efficiency of 7.2%, measured under standard solar conditions (AM1.5G, 100 mW cm(-2)). We expect these promising new dopants to find widespread applications in organic electronics in general and photovoltaics in particular.

Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides

In the past few years, organic-inorganic halide perovskites have rapidly emerged as promising materials for photovoltaic applications, but simultaneously achieving high performance and long-term stability has proved challenging. Here, we show a one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient perovskite solar cells with enhanced stability. We modify the surface of methylammonium lead triiodide (CH3NH3PbI3) perovskite by spin-coating its precursor solution in the presence of butylphosphonic acid 4-ammonium chloride. Morphological, structural and elemental analyses show that the phosphonic acid ammonium additive acts as a crosslink between neighbouring grains in the perovskite structure, through strong hydrogen bonding of the -PO(OH)2 and -NH3(+) terminal groups to the perovskite surface. The additives facilitate the incorporation of the perovskite within a mesoporous TiO2 scaffold, as well as the growth of a uniform perovskite layer at the surface, enhancing the materials photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture.

A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials

Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I3−/I− redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)2]3+/2+(PF6)3/2 as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO2, yielded a power conversion efficiency of over 10% at 100 mW cm−2. This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I3−/I− redox shuttle.

Entropic stabilization of mixed A-cation ABX3 metal halide perovskites for high performance perovskite solar cells

ABX3-type organic lead halide perovskites currently attract broad attention as light harvesters for solar cells due to their high power conversion efficiency (PCE). Mixtures of formamidinium (FA) with methylammonium (MA) as A-cations show currently the best performance. Apart from offering better solar light harvesting in the near IR the addition of methylammonium stabilizes the perovskite phase of FAPbI3 which in pure form at room temperature converts to the yellow photovoltaically inactive δ-phase. We observe a similar phenomenon upon adding Cs+ cations to FAPbI3. CsPbI3 and FAPbI3 both form the undesirable yellow phase under ambient condition while the mixture forms the desired black pervoskite. Solar cells employing the composition Cs0.2FA0.8PbI2.84Br0.16 yield high average PCEs of over 17% exhibiting negligible hysteresis and excellent long term stability in ambient air. We elucidate here this remarkable behavior using first principle computations. These show that the remarkable stabilization of the perovskite phase by mixing the A-cations stems from entropic gains and the small internal energy input required for the formation of their solid solution. By contrast, the energy of formation of the delta-phase containing mixed cations is too large to be compensated by this configurational entropy increase. Our calculations reveal for the first time the optoelectronic properties of such mixed A-cation perovskites and the underlying reasons for their excellent performance and high stability.

Influence of the Donor Size in D−π–A Organic Dyes for Dye-Sensitized Solar Cells

We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

Cyclopentadithiophene Bridged Donor-Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the tris-Cobalt Bipyridine Redox Couple

Keywords: cobalt ; dye-sensitized solar cells ; electrochemistry ; photovoltaics ; sensitizers ; Photovoltaic Cells ; Transport ; Recombination ; Electrolyte ; Performance ; Mediators Reference EPFL-ARTICLE-166673doi:10.1002/cssc.201100120View record in Web of Science Record created on 2011-06-08, modified on 2017-05-12

Influence of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells employing cobalt redox shuttles

We highlight the effect of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells based on two cobalt redox shuttles, namely cobalt(III/II) tris(2,2′-bipyridine) and cobalt(III/II) tris(1,10-phenanthroline). Highly porous counter electrodes based on poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by electro-oxidative polymerization are compared to the typically employed platinized FTO glass, with the former showing much lower charge transfer resistances for both cobalt complexes, leading to improved fill factors and to linear response of the short circuit photo-current density to light intensity up to one sun. Based on these findings, an excellent power conversion efficiency of 10.3% was achieved with a recently reported organic sensitizer and PEDOT as counter electrode.

Subnanometer Ga2O3 Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells

Herein, we present the first use of a gallium oxide tunnelling layer to significantly reduce electron recombination in dye-sensitized solar cells (DSC). The subnanometer coating is achieved using atomic layer deposition (ALD) and leading to a new DSC record open-circuit potential of 1.1 V with state-of-the-art organic D-π-A sensitizer and cobalt redox mediator. After ALD of only a few angstroms of Ga(2)O(3), the electron back reaction is reduced by more than an order of magnitude, while charge collection efficiency and fill factor are increased by 30% and 15%, respectively. The photogenerated exciton separation processes of electron injection into the TiO(2) conduction band and the hole injection into the electrolyte are characterized in detail.