Samrana Kazim

Perovskite as Light Harvester: A Game Changer in Photovoltaics

It is not often that the scientific community is blessed with a material, which brings enormous hopes and receives special attention. When it does, it expands at a rapid pace and its every dimension creates curiosity. One such material is perovskite, which has triggered the development of new device architectures in energy conversion. Perovskites are of great interest in photovoltaic devices due to their panchromatic light absorption and ambipolar behavior. Power conversion efficiencies have been doubled in less than a year and over 15% is being now measured in labs. Every digit increment in efficiency is being celebrated widely in the scientific community and is being discussed in industry. Here we provide a summary on the use of perovskite for inexpensive solar cells fabrication. It will not be unrealistic to speculate that one day perovskite-based solar cells can match the capability and capacity of existing technologies.

Hole‐Transport Materials for Perovskite Solar Cells

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.

A dopant free linear acene derivative as a hole transport material for perovskite pigmented solar cells

A solution processable, molecular organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), was employed as hole transport material (HTM) in mesoscopic methylammonium lead iodide perovskite solar cells. TIPS-pentacene is potentially cost effective, exhibits a relatively high hole mobility and has a favourable HOMO level with respect to the valence band of perovskite. The photovoltaic performance of perovskite solar cells with TIPS-pentacene as HTM in its pristine form and with a dopant/additive was investigated and compared with classical spiro-OMeTAD based devices. Through solvoneering (solvent engineering) and concentration optimization TIPS-pentacene in its pristine form gave a very competitive power conversion efficiency (PCE) of 11.8% under 1 sun conditions. The open circuit voltage of 0.92 V and a short circuit current density of 20.86 mA cm−2 for the devices with pristine TIPS-pentacene were higher compared to doped spiro-OMeTAD based devices under similar conditions, thus paving the use of TIPS-pentacene as an alternative to an expensive spiro-OMeTAD for large area integration in perovskite based solar cells.

Rational design of triazatruxene-based hole conductors for perovskite solar cells

Triazatruxene core based hole transporting materials (HTMs), HMDI (5,10,15-trihexyl-3,8,13-trimethoxy-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole) and HPDI (5,10,15-tris(4-(hexyloxy)phenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole) were synthesized and exploited in perovskite based solar cells. The energy levels of star-shaped HMDI and HPDI were tuned by symmetrically introducing electron-rich alkoxy side groups. These soluble and easily synthesized materials exhibit optical transparency in the visible region, high thermal stability and have suitable HOMO values with respect to perovskite, making them an ideal HTM candidate for efficient perovskite solar cells. The HPDI molecule-based devices gave competitive power conversion efficiencies of ∼11% under AM 1.5G illumination. The facile synthetic approach using inexpensive precursor materials will facilitate triazatruxene-based molecules to be further exploited in thin film organic–inorganic perovskite solar cells and needs optimization to enhance power conversion efficiency.

Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film

We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 μm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.

Universal Features of Electron Dynamics in Solar Cells with TiO2 Contact: From Dye Solar Cells to Perovskite Solar Cells

The electron dynamics of solar cells with mesoporous TiO2 contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current-voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 μs, 1 order of magnitude longer, is required to reproduce the current-voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO2 contact with electrons.

Influence of the mixed organic cation ratio in lead iodide based perovskite on the performance of solar cells

Lead halide based perovskite solar cells are presently the flagship among the third generation solution-processed photovoltaic technologies. The organic cation part in the perovskite plays an important role in terms of crystal structure tuning from tetragonal to trigonal or pseudocubic or vice versa depending on the organic cations used, while it also displays different microstructure. In this paper, we demonstrate the influence of the organic cation part with respect to optical properties, hysteresis behavior, and stability. This study offers a clear understanding of the perovskite properties and how they can be modulated by compositional engineering. With a rational choice, light harvesting abilities and hysteresis behavior can be controlled in these systems. The substitution of formamidinium cation by methylammonium cation allows achieving low temperature annealing and inducing stability in perovskites together with enhanced photovoltaic properties. By the use of in-situ scanning force microscopy experiments the conversion of precursors to perovskite at a particular temperature can be visualized.

How photon pump fluence changes the charge carrier relaxation mechanism in an organic–inorganic hybrid lead triiodide perovskite

This study explores the excitation wavelength and fluence dependence of processes occurring in formamidinium lead triiodide (FAPbI3) film using time-resolved transient absorption and terahertz spectroscopies. The results indicate that second-order processes are responsible for charge carrier recombination at low fluences of the absorbed photons (below 8.4 × 1012 ph per cm2). An increase in fluence leads to the appearance and successive reduction of the time component assigned to the Auger recombination of free charge carriers (240-120 ps). Simultaneously, the bimolecular recombination time decreases from ∼1400 to ∼700 ps. Further increasing the pump fluence produces an exciton population that recombines in 6 ps. The comparison of two characteristic bleaching bands located at 480 and 775 nm provides evidence for the validity of the two valence bands model. Excitation with higher fluences results in a marked difference in the probed dynamics at these bands, reflecting the action of two excited states at the conduction band. Our results demonstrate that a single model cannot be applied in characterizing the perovskite absorber transitions at all pump fluences. These findings are relevant in understanding their operating mechanism under specific experimental conditions, which should differ for perovskite based solar cells, lasing media or photon detectors.

Judicious design of lithium iron phosphate electrodes using poly(3,4-ethylenedioxythiophene) for high performance batteries

LiFePO4 electrodes were built in different architectures using a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) mixed conductor as an additive. Conductivity enhancement of PEDOT:PSS was achieved by the addition of ethylene glycol and dimethyl sulfoxide solvents. The amounts of conducting polymer and solvent additives strongly influence the discharge capacity and potential of LiFePO4 electrodes at high rates. The initial impedance and the direct current resistance were correlated with the discharge performance at high rates. The optimized amount of PEDOT:PSS added within the bulk resulted in a lower value of impedance, lower load resistance and higher capacity as compared to the standard preparation. Furthermore 57Fe Mossbauer spectroscopy and X-ray photoelectron spectroscopy were employed to probe the bulk transformation of the LiFePO4 active material and the surface changes of the composite electrodes with the conducting polymer upon lithiation. The electrode with PEDOT:PSS coated on the aluminium current collector and doped with ethylene glycol showed highly competitive performance (132 mA h g−1 at 5 C and 145 mA h g−1 at 2 C for 50 cycles).

Towards Extending Solar Cell Lifetimes: Addition of a Fluorous Cation to Triple Cation-Based Perovskite Films

Organohalide perovskites have emerged as highly promising replacements for thin-film solar cells. However, their poor stability under ambient conditions remains problematic, hindering commercial exploitation. The addition of a fluorous-functionalized imidazolium cation during the preparation of a highly stable cesium-based mixed perovskite material Cs0.05 (MA0.15 FA0.85 )0.95 Pb(I0.85 Br0.15 )3 (MA=methylammonium; FA=formamidinium) has been shown to influence its stability. The resulting materials, which vary according to the amount of the fluorous-functionalized imidazolium cation present during fabrication, display a prolonged tolerance to atmospheric humidity (>100 days) along with power conversion efficiencies exceeding 16 %. This work provides a general route that can be implemented in a variety of perovskites and highlights a promising way to increase perovskite solar cell stability.