Universality in Intensity Modulated Photocurrent in Bulk-Heterojunction Polymer Solar Cells

We observe a universal feature in the frequency dependence of intensity modulated photocurrent Iph based on studies of a variety of efficient bulk-heterojunction polymer solar cells (BHJ-PSCs). This feature of Iph appears in the form of a local maximum in the 5 kHz < frequency < 10 kHz range and is observed to be largely independent of the external parameters such as modulated light intensity (Lac), wavelength, temperature (T), and external field (EF) over a wide range. Simplistic kinetic models involving carrier generation, recombination and extraction processes are used to interpret the overall essential features of Iph and correlate it to the device parameters

Dual-edged sword of ion migration in perovskite materials for simultaneous energy harvesting and storage application [Perspective]

Portable electronic devices and Internet of Things (IoT) require an uninterrupted power supply for their optimum performance and are key ingredients of the futuristic smart buildings - cities. The off-grid photovoltaic cells and photo-rechargeable energy storage devices meet the requirements for continuous data processing and transmission. In addition, these off-grid devices can solve the energy mismanagement problem famously called as “duck curve”. The conventional approach is the external integration of a photovoltaic cell and an energy storage device through a complex multi-layered structure. However, this approach causes ohmic transport losses and requires additional complex device packaging leading to increased weight and high cost. Toward this narrative, in this viewpoint, we shed light on application of disruptive organic-inorganic hybrid halide perovskite bifunctional materials employed as smart photo-rechargeable energy devices. We also present hybrid halide lead-free perovskite materials for off-grid energy storage systems for indoor lighting applications

Area dependent efficiency of organic solar cells

Efficiency estimations of organic solar cells are observed to be dependent on the dimensions of electrode defining the active area. We address this issue and explore the manner in which efficiency scales in polymer solar cells by studying these devices as a function of electrode area and incident beam size. The increase in efficiency for smaller active areas can be explained by the reduced electrical resistive loss, the enhanced optical effects, and the finite additional fraction of photogenerated carriers in the vicinity of the perimeter defined by the metal electrode

Raveling the Role of Dopants on Charge Carrier Kinetics of TiO2_2 Electrodes using Electrochemical Impedance Spectroscopy

We synthesized the pure and co-doped titanium dioxide (TiO$_2$) electrodes via spin coating. We examined the optical and electronic properties of as-prepared thin film electrodes with co-doping of transition metals and non-metals. The co-doping of Cu, Zn, and N increase the absorption of the radiation in the visible region. The doping leads to the formation of defect states in the electrodes. In this article, we have studied the carrier kinetics in pristine and co-doped TiO2 electrodes. To study the role of dopants in carrier transport of the synthesized TiO2 based electrodes, the electrochemical impedance spectroscopy measurement is performed in the frequency range of 10-1 Hz to 106 Hz. The study reveals the influence of dopants on electron-hole recombination in the defect sites present in bulk and the transport mechanism of the electrons and ions to the surface/interface of the electrodes

Dynamics of Bulk Polymer Heterostructure:Electrolyte Devices

The application of organic semiconductors and bulk heterojunction (BHJ) devices in photoelectrochemical cells, electrolyte-gated field effect transistors, and neuromorphic devices involves the interface of the polymer with an electrolyte. We report the observation of interesting features arising from bulk and interfacial properties of stable polymer/electrolyte devices from photovoltage and differential photocapacitance measurements. A crossover in polarity of the photovoltage signal as a function of BHJ layer thickness and a crossover in the sign of differential photocapacitance as a function of frequency are observed in certain classes of these device structures. The presence of the critical thickness and crossover frequency can be understood in the framework of an electrical transport model and the parameters defining the interfacial capacitance

Single-Pixel, Single-Layer Polymer Device as a Tri-color Sensor with Signals Mimicking Natural Photoreceptors

Color sensing procedures typically involve multiple active detectors or a photodetector coupled to a filter array. We demonstrate the possibility of using a single polymer layer based device structure for multicolor sensing. The device structure does not require any color filters or any subpixelation, and it distinguishes colors without any external bias. The color sensing relies on an appropriate thickness of the active polymer layer that results in a characteristic polarity and temporal profile of the photocurrent signal in response to various incident colors. The device characteristics reveal interesting similarities to the features observed in natural photosensitive systems including retinal cone cells

Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH3NH3PbI3 Perovskite Nanocrystals

Photoinduced degradation of individual methylammonium lead triiodide (MAPbI3) perovskite nanocrystals was studied using super-resolution luminescence microspectroscopy under intense light excitation. The photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts in the emission localization position up to a few hundred nanometers were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the three-dimensional MAPbI3 crystal structure smoothly collapses to the two-dimensional layered PbI2 structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is primarily due to migration of methylammonium ions (MA+), which distorts the lattice structure causing alterations to the Pb–I–Pb bond angle and in turn changes the effective band gap

Unraveling the antisolvent dripping delay effect on the Stranski-Krastanov growth of CH3NH3PbBr3 thin films: a facile route for preparing a textured morphology with improved optoelectronic properties.

Inorganic-organic hybrid perovskite materials have been a topic of interest for the last few years due to their superior optoelectronic properties. However, the optical properties of perovskite materials are strongly dependent on the film morphology. A textured film morphology is expected to have higher light absorption as well as light out-coupling efficiency compared to a smooth film. There have been numerous methods for controlling and optimizing the film morphology to achieve high efficiency in solar cells and light emitting diodes. Here we have demonstrated that controlled anti-solvent treatment at low temperature can lead to Stranski-Krastanov growth in CH3NH3PbBr3 thin films with superior optical and electronic properties for light emitting diode applications. We have studied their photoluminescence properties at the micro- to nano-scale via fluorescence microscopy, hyper-spectral imaging and scanning near-field optical microscopy. We have demonstrated that the nanostructured micro-islands are highly emissive because of large quasi-Fermi level splitting (QFLS) due to the localization of free charges in the smaller crystals. We have shown that the photoluminescence as well as electroluminescence can be improved by at least seven-fold due to the presence of micro-islands on a smooth background film enhancing light out-coupling. Photo-induced photoluminescence enhancement is also observed in smooth films while micro-islands show photo-degradation

Influence of Halide Substitution and External Stimuli on Ion Transport in Inverted MAPb(I1-xBrx)3 Perovskite Solar Cells

The coupled electronic-ionic response in various MAPb(I1-xBrx)3-based inverted perovskite solar cells (PSCs) is studied in-operando by impedance spectroscopy (IS) under varied AM1.5G light intensities and electrical biases. We show that the concentration of Br- in the composition significantly alters the capacitance and resistive response of the PSC under external stimuli. For example, we observed that the low frequency capacitance does not increase proportionally with light intensity, instead it is highly dependent on the amount of Br- in the composition. We found that the recombination resistance (Rrec) has a linear inverse relationship with light intensity in MAPbI3 and MAPbBr3 whereas, the mixed compositions show deviation. Interestingly, the deviation of Rrec from linearity also scales with the increase in Br- concentration. Upon applying an electrical bias, a large deviation of Rrec from linearity was observed all mixed halide compositions exhibited a non-linear inverse trend. We further report the diffusion coefficient (D) for each MAPb(I1-xBrx)3 composition under different light intensity. Notably, the D values decreased on changing the composition from MAPbI3 (10-7 cm2 s-1) to MAPb(I0.8Br0.2)3 and MAPbBr3 (10-8 cm2 s-1). On the other hand, mixed compositions containing more than 20% Br- concentration show faster diffusion kinetics. Overall, our results emphasize on the complex and intertwined nature of electronic and ionic response in PSC that is tunable by changing the halide composition

Fabrication Conditions for Efficient Organic Photovoltaic Cells from Aqueous Dispersions of Nanoparticles

For environmentally friendly and cost-effective manufacturing of organic photovoltaic (OPV) cells, it is highly desirable to replace haloarenes with water as the active layer fabrication solvent. Replacing an organic solvent with water requires retooling the device fabrication steps. The optimization studies were conducted using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as active layer materials. These materials were dispersed in water as blend and separate nanoparticles using the miniemulsion method. Topologies of the active layers were investigated using atomic force microscopy and electron microscopy techniques. We have identified two essential steps to fabricate efficient OPVs from aqueous dispersions: (1) treatment of the hole-transport layer with UV-O3 to make the surface hydrophilic and (2) the use of an electron-transporting buffer layer for efficient charge extraction. We have also identified relative humidity and substrate temperature as key fabrication parameters for obtaining uniform active layer films. The OPV devices were fabricated using PEDOT:PSS as the hole-transport layer and PCBM as electron-transport layer with Ca/Al as the counter electrode. Efficiencies of 2.15% with a fill factor over 66% were obtained; the efficiency and the fill-factor is the highest among all aqueous processing of P3HT–PCBM nanoparticle solar cells