Sudhakar Narra

Control of preferred orientation with slow crystallization for carbon-based mesoscopic perovskite solar cells attaining efficiency 15%

To produce mesoscopic carbon-electrode-based solar cells free of a hole-conduction layer, we developed a simple one-step drop-cast method (temperature 20 °C, relative humidity 50%) to grow uniform and compact perovskite nanocrystals via slow crystallization (SC). X-ray diffraction (XRD) results indicated that perovskite nanocrystals produced inside the mesoporous TiO2/Al2O3/C layers according to our SC approach with N-methyl-2-pyrrolidone (NMP) as a precursor solvent showed a preferred orientation at facet (004) with large crystalline grains, whereas those produced with other solvents or via thermal annealing (TA) methods showed greater intensity at facet (220) with small crystals. After the SC (NMP) procedure was completed, additional TA treatment would change the preferred orientation from (004) back to facet (220), which would break the connectivity and lose the uniformity of the crystals, causing the grains to become significantly smaller. Transient photoluminescence (PL) decay profiles of the SC devices made with four precursor solvents, NMP, γ-butyrolactone (GBL), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), were recorded to show the trend of charge separation kinetics of the perovskite crystals inside the mesoporous films, which is consistent with the trend of the corresponding device performance showing the same order. The SC (NMP) device attained the best efficiency of power conversion (PCE), 15.0%, with average value (13.9 ± 0.5%), which is much superior to those devices from either the traditional one-step TA (DMF) method (5.2 ± 1.0%) or the traditional sequential TA method (10.1 ± 0.7%). This work emphasizes the significance of the control of preferred orientation of perovskite nanocrystals using the SC approach, to obtain high-performance carbon-based mesoscopic solar cells with excellent reproducibility and stability.

Inverted planar solar cells based on perovskite/graphene oxide hybrid composites

Mixing graphene oxide (GO) with CH3NH3PbI3 (PSK) in varied proportions (0.025, 0.05 and 0.075 mg mL−1) and using GO nanosheets as a p-type electrode for devices with a layer-by-layer thin-film configuration ITO/GO/PSK:GO/PCBM/Ag produced hybrid composite perovskite solar cells. The efficiency of power conversion (PCE) of the device prepared with GO in PSK solution (0.05 mg mL−1) attained 15.2%, which is greater than that of a conventional planar heterojunction device (PCE 12.3%) fabricated using pristine PSK. In the homogeneous PSK:GO active layers, PSK acted as a hole donor and GO as a hole acceptor. The optimum GO concentration (0.05 mg mL−1) increased the charge mobility (investigated using Hall effect measurements), enhanced the charge separation (investigated using photoluminescence decays) and retarded the charge recombination (investigated using electrochemical impedance spectra), consistent with the trend of corresponding device performances. Our results thus indicate that the hybrid PSK:GO layer increased the interfacial contact between the donor and acceptor of holes to balance the charge mobility and improved the photovoltaic performance with excellent reproducibility and stability.

Simultaneous Observation of an Intraband Transition and Distinct Transient Species in the Infrared Region for Perovskite Solar Cells

Solar cells based on organometal-halide perovskites such as CH3NH3PbI3 have emerged as a promising next-generation photovoltaic system, but the underlying photophysics and photochemistry remain to be established because of the limited availability of methods to implement the simultaneous and direct measurement of various charge carriers and ions that play a crucial role in the operating device. We used nanosecond time-resolved infrared (IR) spectroscopy to investigate, with high molecular specificity, distinct transient species that are formed in perovskite solar cells after photoexcitation. In CH3NH3PbI3 planar-heterojuction solar cells, we simultaneously observed infrared spectral signatures that are associated with an intraband transition of conduction-band electrons, Fano resonance, and the spiro-OMeTAD cation having an exceptionally short lifetime of 1.0 μs (at ∼1485 cm(-1)). The present results show that the time-resolved IR method offers a unique capability to elucidate these important transients in perovskite solar cells and their dynamic interplay in a comprehensive manner.

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

Low-lying excited electronic states of an important class of molecules known as push-pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high-sensitivity nanosecond time-resolved dispersive IR spectroscopy and DFT calculations on p-nitroaniline (PNA), a prototypical push-pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO(2) group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge-transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited-state structure of highly polarizable molecular systems.

Optimization of a compact layer of TiO2via atomic-layer deposition for high-performance perovskite solar cells

We report the effect of thickness of a film consisting of a compact layer of TiO2 produced via atomic-layer deposition (ALD) for mesoporous perovskite solar cells (PSCs) with a n-i-p configuration. Uniform and pinhole-free TiO2 films of thickness from 10 to 400 nm were deposited on fluorine-doped tin-oxide substrates using ALD. The device performance of the PSC showed a trend systematic with the thickness of the ALD-TiO2 compact layer and attained the best efficiency, 15.0%, of power conversion at thickness 200 nm. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite (PSK) deposited on varied ALD-TiO2 films were recorded; the effective PL quenching is due to electron transfer from PSK into the ALD-TiO2 compact layer. The most efficient interfacial electron transfer occurred at film thickness 200 nm, for which the ALD-TiO2 film has the greatest surface roughness and conductivity. We found a systematic correlation between the device performance in relation to the conductivity and the rate of interfacial electron transfer as a function of thickness of the ALD-TiO2 film; the best performance occurred at thickness 200 nm. The devices showed great stability and reproducibility, providing an alternative for high-performance PSCs with a well-controlled TiO2 compact layer.

Interfacial Investigation on Printable Carbon-Based Mesoscopic Perovskite Solar Cells with NiOx/C Back Electrode

Solar cells with high efficiency, low cost, and high stability are the target for the new generation of solar cells. A fully printable perovskite (CH3NH3PbI3) solar cell (PSC) with device architecture FTO/TiO2/Al2O3/NiOx/C is fabricated in the current research as a low-cost and relatively stable structure and is investigated to determine how different fabrication factors such as the thickness of the insulating spacer layer (Al2O3) or treatments such as heat and UV-O3 treatments can affect the interfacial properties of this multilayer mesoporous structure. X-ray photoelectron spectra (XPS) show that UV-O3 treatment increases the Ni3+(Ni2O3) phase on the surface of the black nickel oxide layer leading to better charge extraction and increasing open-circuit voltage (VOC) up to 0.945 V. We observe improved CH3NH3PbI3 formation inside the mesoporous layers by the PbI2 penetration at a higher temperature. Impedance spectral together with current-voltage measurements show the effect of thickness for the insulator layer in the internal and interfacial resistances and photovoltaic characteristics of the cell. The best performance of the carbon-based PSC attains power conversion efficiency of 12.1% with the thickness of the Al2O3 layer at 450 nm.

Mechanism of back electron transfer in an intermolecular photoinduced electron transfer reaction: solvent as a charge mediator.

Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile (ACN) by using time-resolved near- and mid-IR spectroscopy. The Py dimer radical cation (Py2(·+)) and DCB radical anion (DCB(·-)) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCB(·-) to the solvent and charge recombination between the resulting ACN dimer anion and Py2(·+). The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye-sensitized solar cells.

Direct observation of the solvent effects on the low-lying nπ* and ππ* excited triplet states of acetophenone derivatives in thermal equilibrium.

Low-lying excited triplet states of aromatic carbonyl compounds exhibit diverse photophysical and photochemical properties of fundamental importance. Despite tremendous effort in studying those triplet states, the effects of substituents and solvents on the energetics of the triplet manifold and on photoreactivity remain to be fully understood. We have recently studied the ordering of the low-lying nπ* and ππ* excited triplet states and its substituent dependence in acetophenone derivatives using nanosecond time-resolved near-IR (NIR) spectroscopy. Here we address the other important issue, the solvent effects, by directly observing the electronic bands in the NIR that originate from the lowest nπ* and ππ* states of acetophenone derivatives in four solvents of different polarity (n-heptane, benzene, acetonitrile, and methanol). The two transient NIR bands decay synchronously in all the solvents, indicating that the lowest nπ* and ππ* states are in thermal equilibrium irrespective of the solvent polarity studied here. We found that the ππ* band increases in intensity relative to the nπ* band as solvent polarity increases. These results are compared with the photoreduction rate constant for the acetophenone derivatives in the solvents to which 2-propanol was added as a hydrogen-atom donor. Based on the present findings, we present a comprehensive, solvent- and substituent-dependent energy level diagram of the low-lying nπ* and ππ* excited triplet states.