Khan Mamun Reza

Incorporation of plasmonic Au nanostars into photoanodes for high efficiency dye-sensitized solar cells

We have significantly improved the performance of dye-sensitized solar cells (DSSCs) by incorporating plasmonic Au nanostars into the TiO2 photoanode. Gold nanostars were synthesized and used as localized surface plasmons (LSPs) to enhance the light absorption of DSSCs. The Au nanostars exhibit near infrared (NIR) light absorption at ∼785 nm. The N719 and N749 dye based DSSC devices were fabricated with and without the incorporation of Au nanostars. The power conversion efficiency (PCE) of DSSCs using Au nanostars was increased by ∼20% from 7.1% to 8.4% for N719 cells and by ∼30% from 3.9% to 5.0% for N749 devices. The incident photon-to-current conversion efficiency (IPCE) was improved and the spectral response was broadened over the wavelength range of 380–1000 nm. Electrochemical impedance spectroscopy (EIS) showed that by incorporating Au nanostars, the charge recombination resistance Rct value decreased under both open-circuit and illumination conditions, which indicated high electron density generated in the device photoanode.

Room temperature, air crystallized perovskite film for high performance solar cells

For the first time, room temperature heating free growth and crystallization of perovskite films in ambient air without the use of thermal annealing is reported. Highly efficient perovskite nanorod-based solar cells were made using ITO/PEDOT:PSS/CH3NH3PbI3 nanorods/PC60BM/rhodamine/Ag. All the layers except PEDOT:PSS were processed at room temperature thereby eliminating the need for thermal treatment. Perovskite films were spin coated inside a N2 filled glovebox and immediately were taken outside in air having 40% relative humidity (RH). Exposure to humid air was observed to promote the crystallization process in perovskite films even at room temperature. Perovskite films kept for 5 hours in ambient air showed nanorod-like morphology having high crystallinity, with devices exhibiting the highest PCE of 16.83%, which is much higher than the PCE of 11.94% for traditional thermally annealed perovskite film based devices. It was concluded that moisture plays an important role in room temperature crystallization of pure perovskite nanorods, showing improved optical and charge transport properties, which resulted in high performance solar cells.

Higher efficiency perovskite solar cells using additives of LiI, LiTFSI and BMImI in the PbI2 precursor

The efficiencies of perovskite solar cells have been significantly increased to 18%, 17.01% and 15.6% for the cells containing the additives BMImI, LiI and LiTFSI in the PbI2 precursor solutions, respectively, from 11.3% for the devices without any additives. Incorporation of these additives led to the formation of perovskites with larger grain size and higher crystallinity with reduced PbI2 residue as indicated by X-ray diffraction (XRD) and atomic force microscopy (AFM) results. Kelvin Probe Force Microscopy (KPFM) and current sensing (CS)-AFM results were in good agreement with external quantum efficiency (EQE) measurements and proved the great enhancement in short circuit current density (Jsc) as a result of doping. Transient photovoltage measurement results exhibited longer charge carrier lifetimes for the additive incorporated perovskites than those without additives, thus improving the fill factor (FF) and open circuit voltage (Voc). In addition to the improved efficiency, the incorporation of these additives led to higher stability of the CH3NH3PbI3 perovskite solar cells. The new additive BMImI, LiI and LiTFSI incorporated CH3NH3PbI3 perovskite solar cells exhibited a reduced degradation with a 57%, 60%, and 91% decrease in performance respectively after exposure to air for 70 days compared to a 93% decrease for the pristine cell after only 24 days. The lithium salt additives can serve as desiccants to absorb moisture preventing perovskite degradation. Further, the BMImI additive can prevent the formation of free radicals in perovskites upon exposure to light and heat.

Environmentally Friendly Plasma Treated PEDOT:PSS as Electrodes for ITO-free Perovskite Solar Cells

Solution processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) transparent electrodes (TEs) offer great potential as a low cost alternative to expensive indium tin oxide (ITO). However, strong acids are typically used for enhancing the conductivity of PEDOT:PSS TEs, which produce processing complexity and environmental issues. This work presents an environmentally friendly acid free approach to enhance the conductivity of PEDOT:PSS using a light oxygen plasma treatment, in addition to solvent blend additives and post treatments. The plasma treatment was found to significantly reduce the sheet resistance of PEDOT:PSS TEs from 85 to as low as 15 Ω sq-1, which translates to the highest reported conductivity of 5012 S/cm for PEDOT:PSS TEs. The plasma treated PEDOT:PSS TE resulted in an ITO-free perovskite solar cell efficiency of 10.5%, which is the highest reported efficiency for ITO-free perovskite solar cells with a PEDOT:PSS electrode that excludes the use of acid treatments. This research presents the first demonstration of this technology. Moreover, the PEDOT:PSS TEs enabled better charge extraction from the perovskite solar cells and reduced hysteresis in the current density-voltage (J-V) curves.

Engineering of ambient processing conditions to control solvent induced intermediate phase in mixed halide organic-inorganic perovskite (CH 3 NH 3 PbI 3−x Cl x ) film for efficient planar perovskite solar cells

One of the challenges with organic-inorganic hybrid perovskite films, is its degradation from moisture present in ambient atmosphere, which severely restricts its crystallization process using thermal annealing in ambient air having high humidity levels. A widely used method for perovskite film crystallization is use of thermal annealing in inert atmosphere to rapidly crystallize the film in perovskite phase, immediately after its deposition. Herein, we have explored several methods of crystallization of mixed-halide perovskite (CH3NH3PbI3-xClx) film crystallization in ambient air with an aim to obtain high crystallinity and complete conversion to perovskite phase in ambient air assisted with and without thermal annealing. We optimized the crystallization process of perovskite film by exposing the film in ambient air, partially exposure to air followed by thermal annealing, exposing it to air-flow at room temperature. Perovskite films were spin coated inside the N2 filled glove box and immediately were taken outside in air with 40% relative humidity (RH). Crystallized mixed-halide perovskite films obtained from different approaches were then used to fabricate planar perovskite solar cells with device structure as ITO/PEDOT:PSS/CH3NH3PbI3-xClx/PC60BM/Rhodamine/Ag. It was concluded that humidity plays an important role in crystallization of perovskite films in ambient air, but additional treatment could lead to improved perovskite crystallinity.

Higher efficiency perovskite solar cells using Au@SiO2 core–shell nanoparticles

In this work, we improved photovoltaic performance by about 27% in planar p-i-n perovskite solar cells (PSCs) using plasmonic Au@SiO2 core–shell nanoparticles (NPs). The devices have an architecture of ITO glass/PEDOT:PSS/perovskite(CH3NH3PbI3)/PCBM/Rhodamine/Ag. Four batches of devices were fabricated with different concentrations of Au@SiO2 NPs ranging from 0.4 to 1.6 wt% with an interval of 0.4 wt%. The Au@SiO2 NPs were integrated at the interface between the PEDOT:PSS layer and the active perovskite layer. At an optimized concentration of 1.2 wt% Au@SiO2 NPs, the PSCs achieved 25.1% of enhancement in photocurrent from 17.45 to 22.35 mA cm−2 and an improvement of 27.3% in power conversion efficiency (PCE) from 11.44 to 14.57%. This significant improvement in device performance is attributed to the localized surface plasmon resonance (LSPR) of Au@SiO2 NPs, which enhanced the light absorption in the active perovskite layer. The transient photocurrent and photovoltage measurements revealed that PSCs with Au@SiO2 NPs have a faster charge transport time and longer recombination lifetime than those without Au@SiO2 NPs. These results demonstrate that plasmonic metal nanoparticles substantially improved the efficiency of PSCs.

Urea treated WO 3 and SnO 2 as cost effective and efficient counter electrodes of dye sensitized solar cells

A novel method was developed to convert the electro-catalytically inactive commercial WO₃ and SnO₂ into high efficient WO_3-x and SnO_2-x counter electrodes for dye sensitized solar cells (DSSs) to replace Pt. Both WO₃ and SnO₂ was treated with Urea with different Urea mass ratio concentrations and annealed under N₂ environment at 500°C. The energy conversion efficiency (PEC) was significantly improved by urea treatment with 8.57% compared to 2.91 % for non-treated CE in case of WO₃ and increases from 2.76 % for non-treated CE to 8.7 % after urea treatment in case of SnO₂. All other characterizations performed for urea treated WO₃ and SnO₂ support the hypothesis of creating oxygen vacancies inside the metal oxides by the treatment. These oxygen vacancies facilitate the redox process in iodide/trioxide electrolyte. The density of these oxygen vacancies could be controlled by controlling the urea concentration during the treatment.

Crystallization of perovskite film using ambient moisture and water as co-solvent for efficient planar perovskite solar cell(Conference Presentation)

Smooth, compact and defect free morphology of perovskite is highly desired for enhanced device performance. Several routes such as thermal annealing, use of solvent mixtures, growth under controlled humidity has been adopted to obtain crystalline, smooth and defect free perovskite film. Herein we showed direct use of water (H2O) as co-solvent in precursor solution and have optimized the water content required to obtain smooth and dense film. Varying concentration of water was used in precursor solution of CH3NH3I and PbI2 mixed in γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO). Perovskite films were crystallized using toluene assisted solvent engineering method using GBL:DMSO:H2O as solvent mixture. The amount of water was varied from 1% to 25%, which resulted in change in film morphology and perovskite crystallinity. It was concluded that an appropriate amount of water is required to assist the crystallization process to obtain smooth pin-hole free morphology. The change in morphology led to improved fill factor in the device, with highest efficiency ~14%, which was significantly higher than devices made from perovskite film without adding water. We also showed that addition of up to 25% by volume of water does not significantly change the device performance.