Low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole diffusion lengths (typically about 10 nanometers). Recent reports of highly efficient CH3NH3PbI3-based solar cells in a broad range of configurations raise a compelling case for understanding the fundamental photophysical mechanisms in these materials. By applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective-electron or selective-hole extraction materials, we have uncovered concrete evidence of balanced long-range electron-hole diffusion lengths of at least 100 nanometers in solution-processed CH3NH3PbI3. The high photoconversion efficiencies of these systems stem from the comparable optical absorption length and charge-carrier diffusion lengths, transcending the traditional constraints of solution-processed semiconductors.

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH 3 NH 3 PbI 3) and [6,6]-phenyl-C 61-butyric acid methyl ester (PC 61 BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm À2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH 3 NH 3 PbI 3 relative to its thickness. Furthermore, the low exciton binding energy of CH 3 NH 3 PbI 3 implies that exciton splitting at the CH 3 NH 3 PbI 3 /PC 61 BM interface is very efficient. With further increase in CH 3 NH 3 PbI 3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solutionprocessable bilayer solar cells to date. Broader context Low-temperature solution-processable bilayer solar cells with their simple architecture provide an inexpensive and straightforward platform for device fabrication without the necessity for extensive morphological optimization. For efficient solar cells, good light absorption accompanied by efficient conversion of photons to electrons is critical. This work shows that solar cells based on hybrid organic-inorganic lead halide as the donor and [6,6]-phenyl-C 61-butyric acid methyl ester (PC 61 BM) as the acceptor are able to resolve the conicting lm thickness requirements of high absorption together with efficient exciton diffusion. As a result, practically all the photons absorbed by the active layer can be converted to electrons.

This review provides an overview of factors affecting film morphology and how together with device architecture they impact perovskite cell performance.

The chemically reduced graphene oxide (rGO) was transferred onto polyethylene terephthalate (PET) substrates and then used as transparent and conductive electrodes for flexible organic photovoltaic (OPV) devices. The performance of the OPV devices mainly depends on the charge transport efficiency through rGO electrodes when the optical transmittance of rGO is above 65%. However, if the transmittance of rGO is less than 65%, the performance of the OPV device is dominated by the light transmission efficiency, that is, the transparency of rGO films. After the tensile strain (∼2.9%) was applied on the fabricated OPV device, it can sustain a thousand cycles of bending. Our work demonstrates the highly flexible property of rGO films, which provide the potential applications in flexible optoelectronics.

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors -chemical, electronic and structural -that govern strong multi-exciton correlations.Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA) 2 PbI 4 (PEA = phenylethylammonium). We determine the binding energy of biexcitons -correlated two-electron, two-hole quasiparticles -to be 44 ± 5 meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by ∼ 25% upon cooling to 5 K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder. * FT and SN are to be considered first co-authors of this manuscript. †

Broader contextA photo-supercapacitor (PSC) made up of energy harnessing (organic photovoltaic (OPV)) and energy storage (supercapacitor) components has been demonstrated using a layer-by-layer device fabrication approach, utilizing single-walled carbon nanotube (CNT) networks as a common integration platform. This hybrid device, in which photogenerated charge is stored at the electric double layer, was constructed by assembling a supercapacitor electrode and a polymer electrolyte on top of a multilayered OPV. This hybrid device architecture was thinner (< 0.6 mm), lighter (< 1 g) and lead to a 43% reduction in device internal resistance as compared to external wire connected OPVs and supercapacitors. The thickness of the CNT network is tunable to accommodate the charges generated from OPVs with different area coverages. Under an illumination of 100 mW cm À2 for 70 s, the hybrid device with an unoptimized OPV yielded a specific capacitance of 28 F g À1 when discharged in the dark. This specific capacitance could be further improved to 80 F g À1 by connecting two OPVs in series. Such integration also opens up the possibility of printability and flexibility for hybrid devices, with further possibilities of integration with printed electronics, which may comprise functional circuitry as well as display elements.

We study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH3NH3PbI3 thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. The carrier mobility values calculated from the low-temperature phonon mode frequencies, via two theoretical approaches, are found to agree reasonably with the experimental value (∼2000 cm(2) V(-1) s(-1)) from a previous time-resolved THz spectroscopy work. Thus, we have established a possible link between terahertz phonon modes and the transport properties of perovskite-based solar cells.

Block copolymers have unique associative properties that facilitate self-assembly into nanostructures that have been widely used in soft lithography, [1] templating, [2] drug delivery, [3] biomedical, [4,5] and chemical catalytic [6] applications. Of special interest is the in situ preparation of metallic or semiconducting nanoparticles in amphiphilic block copolymers. [7][8][9][10][11][12] The synthesis of nanoparticles in block copolymer micelles solves the problem of particle size control and stabilization compared to classical stabilization systems that employ surfactants [13][14][15] or microemulsions. [16,17] Nanocrystal-based organic memories [18][19][20][21] are attracting widespread interest owing to their simple structure and the prospect of creating 2D/3D stacks of these memory cells for increased bit densities. Recent reviews [20,22,28] summarize the literature for these nanoparticle-based organic memories comprehensively, and have identified the main operating mechanisms to be one of the following: (i) an electricfield-induced charge transfer between the nanoparticles and the surrounding conjugated compounds, [19,22] (ii) filamentary conduction, [23,24] (iii) charge trapping-detrapping, [25,26] and (iv) space-charge field inhibition of injection in the nanoparticles through a high-voltage pulse. [20,27] Besides the widely used two-terminal bistable organic memory devices, an alternative memory architecture that can be adopted is based on an organic thin-film transistor (OTFT) with a non-volatile floating gate memory [29] which allows a direct integration of the memory element with the transistor for integrated circuit applications. The ability to have a one-step fabrication process to generate arrays of metallic nanoparticles using solutionprocessing methods makes this approach amenable to potential implementations in the nanoparticle-based organic memory devices mentioned above. This is in contrast to past designs of organic memory devices, which involved the use of multistep approaches of presynthesizing nanoparticles followed by surface modification to prevent agglomeration prior to embedding them in multiple functional layers through solution processing or physical vapor deposition. Furthermore, this solution-processing approach is especially suitable for low-cost, large-area processing on flexible substrates, which may be considered to be the cornerstone of organic electronics applications. We demonstrate herein, for the first time, a polymeric memory that comprises an in situ synthesis strategy of gold nanoparticles in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). This system serves as a prototype for a generic memory device using nanoparticles as floating gate charge storage centers and, in particular, for integration into OTFT-based circuits. The block copolymer micelles turn out to be an excellent model system that is simple, forms a self-assembled ordered nanostructure, and provides optimum control over nanoparticle size formation and isolation. The response of the memory device is c...