Teddy Salim

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) 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 CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Organic Photovoltaic Devices Using Highly Flexible Reduced Graphene Oxide Films as Transparent Electrodes

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.

Perovskite-based solar cells: impact of morphology and device architecture on device performance

Organic–inorganic metal halide perovskites have recently shown great potential for application in solar cells with excitingly high performances with an up-to-date NREL-certified record efficiency of 20.1%. This family of materials has demonstrated considerable prospects in achieving efficiencies comparable to or even better than those of thin film solar cells. The remarkable performances thus far seem not to be limited to any specific device architecture. Both mesoscopic and planar cells showed good device performance and this eventually leads to the inevitable comparison between both architectures. Regardless of device architecture, device performance is highly dependent on the film morphology. The factors influencing the film morphology such as the deposition method, material composition, additives and film treatment will be discussed extensively in this review. The key to obtaining good-quality film morphology and hence performance is to essentially lower the energy barrier for nucleation and to promote uniform growth of the perovskite crystals. A comparison of the material selection for various layers as well as their corresponding impact on the perovskite film and device behavior in both device architectures will be presented.

Solvent additives and their effects on blend morphologies of bulk heterojunctions

Controlling the blend morphology is one of the ways to achieve high power conversion efficiency in organic bulk heterojunction (BHJ) photovoltaic devices. One simple yet effective method is “solvent additive” approach, which involves the addition of a small fraction of high boiling point solvent into the blend of donor/acceptor dissolved in another host solvent. Even though this method has been successfully applied in a number of polymer/fullerene BHJ devices, the selection rule of the choice of additive and the host solvent has yet to be fully established. In this work, we performed a systematic study of the effect of alkyl lengths of alkanedithiol additives on the nanoscale phase separation of P3HT:PC61BM blends and consequently, the power conversion efficiency (PCE) of the devices. The extent of the additive-induced phase separation is related to the additive boiling point and the degree of interaction between the additive and fullerene, as evident from grazing incidence X-ray diffractometry (GIXRD) and scanning transmission X-ray microscopy (STXM) data. We found that both the boiling point and the degree of interaction are correlated and should be considered simultaneously in the selection of the appropriate solvent additives. Lastly, PCE as high as 3.1% can be achieved in an optimally phase-separated blend due to an improvement in the charge dissociation and a decrease in bimolecular recombination.

Printable photo-supercapacitor using single-walled carbon nanotubes

A printable, all solid-state photo-supercapacitor (PSC) incorporating both organic photovoltaic (OPV) and supercapacitor (SC) functions has been demonstrated utilizing a single-walled carbon nanotube network, enabling a thinner (< 0.6 mm) and lighter (< 1 g) device architecture, which leads to a 43% reduction in device internal resistance as compared to external wire connected OPVs and SCs.

A new insight into controlling poly(3-hexylthiophene) nanofiber growth through a mixed-solvent approach for organic photovoltaics applications

One dimensional (1-D) nanostructures of conjugated polymers, such as nanofibers, offer the possibility of directed charge transport and improved absorption due to better chains ordering. Poly(3-hexylthiophene) (P3HT) nanofibers can be synthesized by utilizing its interaction with marginal solvents. This work explores the effect of different poor solvents in driving P3HT chain self-assembly into nanofibers and also the effect of a small amount of good solvent in such a poor solvent system in controlling the nanofiber morphology. The organic photovoltaic (OPV) devices based on the blend of P3HT nanofibers and PCBM showed an improved short circuit current when anisole was used compared to p-xylene. Surprisingly, the presence of a small amount of good solvent such as chlorobenzene (CB) in anisole resulted in a higher degree of crystallinity and thinner nanofibers compared to purely anisole system. These are evident from the absorption, scattering and morphology data. The presence of CB delayed crystallization, which is evident from the synchrotron small angle X-ray scattering (SAXS) measurements. This modification of fiber morphology with CB addition into P3HT/anisole results in an improved power conversion efficiency (PCE) of 2.3%; an improvement of more than 50% compared to the pure anisole system. Our investigation provides a new insight into self-assembly of polymers in a mixed solvent system, paving the way to new approaches of controlled self-assembly of organic nanofibers.

Elucidating the role of disorder and free-carrier recombination kinetics in CH3NH3PbI3 perovskite films.

Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in a perovskite film by the use of time-resolved terahertz spectroscopy. Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant.

Solution-Processed Nanocrystalline TiO2 Buffer Layer Used for Improving the Performance of Organic Photovoltaics

In this study, we use solution-processable crystalline TiO(2) nanoparticles as a buffer layer between the active layer and aluminum cathode to fabricate the P3HT:PCBM-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. The employment of the presynthesized TiO(2) nanoparticles simplifies the fabrication of OPV devices because of the elimination of an additional hydrolysis step of precursors in air. The fabricated OPV devices with the thermally stable TiO(2) buffer layer are subjected to the further postdeposition thermal annealing, resulting in a power conversion efficiency (PCE) as high as 3.94%. The improved device performance could be attributed to the electron transporting and hole blocking capabilities due to the introduced TiO(2) buffer layer.

Phonon Mode Transformation Across the Orthohombic–Tetragonal Phase Transition in a Lead Iodide Perovskite CH3NH3PbI3: A Terahertz Time-Domain Spectroscopy Approach

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.

From benzobisthiadiazole, thiadiazoloquinoxaline to pyrazinoquinoxaline based polymers: effects of aromatic substituents on the performance of organic photovoltaics

Here we report the syntheses of low bandgap polymers based on benzobisthiadiazole (BBT), thiadiazoloquinoxaline (TQ) and pyrazinoquinoxaline (PQ) core structures with different aromatic substituents. The effects of changing core structures from BBT to PQ and also substituents from biphenyl and bithienyl on the photophysical, electrochemical and morphology of the polymers were studied. These polymers were incorporated into solar cell devices as donors, with PC[71]BM as acceptors, and their device performances were correlated with their properties. It was found that the effect of these structural changes has significant consequences on the overall device performances.