Lioz Etgar

Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells.

We report for the first time on a hole conductor-free mesoscopic methylammonium lead iodide (CH(3)NH(3)PbI(3)) perovskite/TiO(2) heterojunction solar cell, produced by deposition of perovskite nanoparticles from a solution of CH(3)NH(3)I and PbI(2) in γ-butyrolactone on a 400 nm thick film of TiO(2) (anatase) nanosheets exposing (001) facets. A gold film was evaporated on top of the CH(3)NH(3)PbI(3) as a back contact. Importantly, the CH(3)NH(3)PbI(3) nanoparticles assume here simultaneously the roles of both light harvester and hole conductor, rendering superfluous the use of an additional hole transporting material. The simple mesoscopic CH(3)NH(3)PbI(3)/TiO(2) heterojunction solar cell shows impressive photovoltaic performance, with short-circuit photocurrent J(sc)= 16.1 mA/cm(2), open-circuit photovoltage V(oc) = 0.631 V, and a fill factor FF = 0.57, corresponding to a light to electric power conversion efficiency (PCE) of 5.5% under standard AM 1.5 solar light of 1000 W/m(2) intensity. At a lower light intensity of 100W/m(2), a PCE of 7.3% was measured. The advent of such simple solution-processed mesoscopic heterojunction solar cells paves the way to realize low-cost, high-efficiency solar cells.

Depleted hole conductor-free lead halide iodide heterojunction solar cells

Lead halide perovskite is an excellent candidate for use as a light harvester in solar cells. Our work presents a depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell using a thick CH3NH3PbI3 film. The CH3NH3PbI3 formed large crystals which function simultaneously as light harvesters and as hole transport materials. We performed capacitance voltage measurements, which show a depletion region which extends to both n and p sides. The built-in field of the depletion region assists in the charge separation and suppresses the back reaction of electrons from the TiO2 film to the CH3NH3PbI3 film. This depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell provides a power conversion efficiency of 8% with a current density of 18.8 mA cm−2, the highest efficiency achieved to date for perovskite based solar cells without a hole conductor.

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.

Depleted Bulk Heterojunction Colloidal Quantum Dot Photovoltaics

The first solution-processed depleted bulk heterojunction colloidal quantum dot solar cells are presented. The architecture allows high absorption with full depletion, thereby breaking the photon absorption/carrier extraction compromise inherent in planar devices. A record power conversion of 5.5% under simulated AM 1.5 illumination conditions is reported.

Light Energy Conversion by Mesoscopic PbS Quantum Dots/TiO2 Heterojunction Solar Cells

Solid state PbS quantum dots (QDs)/TiO(2) heterojunction solar cells were produced by depositing PbS QDs on a 500 nm thick mesoscopic TiO(2) films using layer-by-layer deposition. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO(2) device produces a short circuit photocurrent (J(sc)) of 13.04 mA/cm(2), an open circuit photovoltage (V(oc)) of 0.55 V and a fill factor (FF) of 0.49, corresponding to a light to electric power conversion efficiency (η) of 3.5% under AM1.5 illumination. The electronic processes occurring in this device were investigated by transient photocurrent and photovoltage measurements as well as impedance spectroscopy in the dark and under illumination. The investigations showed a high resistivity for the QD/metal back contact, which reduces drastically under illumination. EIS also indicated a shift of the depletion layer capacitance under illumination related to the change of the dipole at this interface.

High Efficiency Quantum Dot Heterojunction Solar Cell Using Anatase (001) TiO2 Nanosheets

This is the first report of using anatase TiO(2) nanosheets with exposed (001) facets in a high-efficiency PbS quantum dot/TiO(2) heterojunction solar cell. The TiO(2) nanosheets have higher conduction band, and surface energy compared to normal anatase (101) TiO(2) nanoparticles. This PbS QD/TiO(2) heterojunction solar cell produces power conversion efficiency of 4.7% which is one of the highest reported in literature.

Temperature dependence of hole conductor free formamidinium lead iodide perovskite based solar cells

Organometal halide perovskite is a promising material in photovoltaic (PV) cells. Within a short time, its performance has increased dramatically to become a real competitor to silicon solar cells. Here we report on the temperature dependence (annealing temperature and the dependence of the photovoltaic parameters on temperature) of formamidinium (FA) lead iodide (FAPbI3), methylammonium (MA) lead iodide (MAPbI3) and their mixture (MAPbI3 : FAPbI3) in hole conductor free perovskite solar cells. These three types of perovskites function both as light harvesters and as hole conductors. Surface photovoltage and optical characterization reveal the p-type behavior and the band gap of the different perovskites. We observed that the ratio between the MA and FA cations might change during the annealing process, affecting the band gap and the stability of the layers. The PV parameters at different temperatures show better stability for the pure MAPbI3 and FAPbI3 solar cells compared to their mixture. Using intensity modulated photovoltage/photocurrent spectroscopy, we found that the diffusion length is weakly dependent on the light intensity, while the charge collection efficiency drops with light intensity for the FAPbI3-based cells. However, for MAPbI3 and the mixture, the charge collection efficiency remains constant for a wide range of light intensities.

Parameters Influencing the Deposition of Methylammonium Lead Halide Iodide in Hole Conductor Free Perovskite-Based Solar Cells

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent years, the power conversion efficiency of perovskite solar cells has been dramatically increased, making them a competitive source of renewable energy. An important parameter when designing high efficiency perovskite-based solar cells is the perovskite deposition, which must be performed to create complete coverage and optimal film thickness. This paper describes an in-depth study on two-step deposition, separating the perovskite deposition into two precursors. The effects of spin velocity, annealing temperature, dipping time, and methylammonium iodide concentration on the photovoltaic performance are studied. Observations include that current density is affected by changing the spin velocity, while the fill factor changes mainly due to the dipping time and methylammonium iodide concentration. Interestingly, the open circuit voltage is almost unaffected by these parameters. Hole conductor free perovskite solar cells are used in this work, in order to minimize other possible effects. This study provides better understanding and control over the perovskite deposition through highly efficient, low-cost perovskite-based solar cells.

Enhancing the efficiency of a dye sensitized solar cell due to the energy transfer between CdSe quantum dots and a designed squaraine dye

The power conversion efficiency of a dye-sensitized solar cell with tailored squaraine dye enhanced by 47%, due to Forster resonance energy transfer from CdSe quantum dots to the squaraine dye. The incident photons to collection efficiency of electrons indicate panchromatic response from the visible to the near-infrared spectrum.

High voltage in hole conductor free organo metal halide perovskite solar cells

Organo metal halide perovskite has attracted considerable attention recently due to its distinctive properties that make it especially useful in photovoltaic solar cells. In this work we demonstrate high open circuit voltage of 1.35 V using Al2O3/CH3NH3PbBr3 perovskite solar cells without a hole conductor. The contact potential difference under light measured by surface photovoltage spectroscopy of CH3NH3PbBr3 was more than twice that of CH3NH3PbI3, which results in smaller surface potential for the Al2O3/CH3NH3PbBr3 cells. Incident modulated photovoltage spectroscopy shows a longer recombination lifetime for the Al2O3/CH3NH3PbBr3 cells than for the TiO2/CH3NH3PbI3 cells or for the TiO2/CH3NH3PbBr3 cells, further supporting the high open circuit voltage. The possibility to gain high open circuit voltage even without a hole transport material in perovskite solar cells shows that the perovskite/metal oxide interface has a major effect on the open circuit voltage in perovskite based solar cells.