ZKC

A laser-based patterning technique—compatible with flexible, temperature-sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. In particular, rGO films with an initial transparency of ≈20% are patterned, resulting in rGOMMs films with a ≈59% transmittance and a sheet resistance of ≈565 Ω sq⁻¹, that is significantly lower than the resistance of ≈780 Ω sq⁻¹, exhibited by the pristine rGO films at the same transparency. As a proof-of-concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution-processed graphene-based electrodes. The controllable and highly reproducible laser-induced patterning of rGO hold enormous promise for both rigid and flexible large-scale organic electronic devices, eliminating the lag between graphene-based and indium–tin oxide electrodes, while providing conductivity and transparency tunability for next generation flexible electronics.

A direct laser writing technique is demonstrated for the fabrication of reduced graphene oxide micromesh electrodes with high conductivity and transparency. Their utilization as the transparent electrode in flexible organic photovoltaic (OPV) devices leads to a power conversion efficiency of 3.05%, which is the highest ever reported for flexible OPVs based on solution-processed graphene electrodes.

Coffee and milk creates rich flavor benefited from both ingredients. The same thing happens in the exciplex system which has great potential to realize highly efficient OLED by forming an exciplex between two different molecules. K.-T. Wong, J.-J. Kim, and co-workers demonstrate on page 361 a novel exciplex forming co-host system composed of N,N'-dicarbazolyl-3,5-benzene (mCP) and PO-T2T, resulting in an unprecedented high performance blue phosphorescent OLED.

Zero-dimensional ZnS/CdS nanocomposites (NCs) are designed based on the controlled growth of ZnS nanoparticles by X. S. Fang and team on page 445. The as-obtained NCs are functionally versatile and offer great optoelectronic properties. For example, the photo-degradation rate of ZnS/CdS NCs towards organic dyes under UV light is three times as much as that of pure ZnS nano-particles, due to the effective charge separation and increased specific surface area.

The field dependence of the photocurrent in a bilayer assembly is measured with the aim to clarify the role of excess photon energy in an organic solar cell comprising a polymeric donor and an acceptor. Upon optical excitation of the donor an electron is transferred to the acceptor forming a Coulomb-bound electron–hole pair. Since the subsequent escape is a field assisted process it follows that photogeneration saturates at higher electric fields, the saturation field being a measure of the separation of the electron–hole pair. Using the low bandgap polymers, PCDTBT and PCPDTBT, as donors and C₆₀ as acceptor in a bilayer assembly it is found that the saturation field decreases when the photon energy is roughly 0.5 eV above the S₁–S₀ 0–0 transition of the donor. This translates into an increase of the size of the electron-hole-pair up to about 13 nm which is close to the Coulomb capture radius. This increase correlates with the onset of higher electronic states that have a highly delocalized character, as confirmed by quantum-chemical calculations. This demonstrates that accessing higher electronic states does favor photogeneration yet excess vibrational energy plays no role. Experiments on intrinsic photogeneration in donor photodiodes without acceptors support this reasoning.

Exciton dissociation in bilayer solar cells is facilitated by exciting into higher-lying, more delocalized excited states of the donor polymer. This is shown by measuring the field dependence of the photocurrent in PCDTBT/C₆₀ cells and in PCPDTBT/C₆₀ cells for different excitation energies and comparing this to the delocalization of the associated excited states as determined by quantum-chemical calculations.

Highly thermal stable and durable organic bulk heterojunction photovoltaic cells are demonstrated by C.-P. Chen, C.-Y. Huang, and S.-C. Chuang on page 207 by incorporating ≈10–15 wt% crosslinkable open-cage fullerenes (COF) as additives in the active layer (weight ratio of P3HT:PC61BM = 1:0.9), via building up three-dimensional local borders upon thermal treatment at 150 °C.

Recently, there have been extensive research efforts on developing high performance organolead halide based perovskite solar cells. While most studies focused on optimizing the deposition processes of the perovskite films, the selection of the precursors has been rather limited to the lead halide/methylammonium (or formamidium) halide combination. In this work, we developed a new precursor, HPbI₃, to replace lead halide. The new precursor enables formation of highly uniform formamidium lead iodide (FAPbI₃) films through a one-step spin-coating process. Furthermore, the FAPbI₃ perovskite films exhibit a highly crystalline phase with strong (110) preferred orientation and excellent thermal stability. The planar heterojunction solar cells based on these perovskite films exhibit an average efficiency of 15.4% and champion efficiency of 17.5% under AM 1.5 G illumination. By comparing the morphology and formation process of the perovskite films fabricated from the formamidium iodide (FAI)/HPbI₃, FAI/PbI₂, and FAI/PbI₂ with HI additive precursor combinations, it is shown that the superior property of the HPbI₃ based perovskite films may originate from 1) a slow crystallization process involving exchange of H⁺ and FA⁺ ions in the PbI₆ octahedral framework and 2) elimination of water in the precursor solution state.

HPbI₃is introduced as a novel precursor to solve the non-uniformity problem of formamidium lead iodide (FAPbI₃) perovskite films from one-step solution-­processed method. Interestingly, the FAPbI₃ films exhibit high crystallinity with (110) plane orientation and the corresponding devices yield an average photo­voltaic efficiency of 15.4% under 1 sun illumination. Present results demonstrate that precursor engineering is an effective approach to produce perovskites with attractive properties.

This work is a reinvestigation of the impact of blend morphology and thermal annealing on the electrical performance of regioregular-P3HT:PC₆₀BM bulk heterojunction organic solar cells. The morphological, structural, and electrical properties of the blend are experimentally investigated with atomic force microscopy, X-ray diffraction, and time-of-flight measurements. Current–voltage characteristics of photodiode devices are measured in the dark and under illumination. Finally, the existence of exponential electronic band tails due to gap states is experimentally confirmed by measuring the device spectral response in the subband gap regime. This method reveals the existence of a large density of gap states, which is partially and systematically reduced by thermal annealing. When the band tails are properly accounted for in the drift and diffusion simulations, experimentally measured charge transport characteristics, under both dark and illuminated conditions and as a function of annealing time, can be satisfactorily reproduced. This work further confirms the critical impact of tails states on the performance of solar cells.

The predominance of the band tail states reorganization, characterized by spectral response measurement, in the improvement of the solar cell performances during the annealing step of the active layer is discussed. This energetic reorganization leads to a lowering of the recombinations assisted by trap state (efficiency improvement) and a decrease of the thermal generation from trap states (reverse dark current).

It has been generally believed and assumed that organometal halide perovskites would form type II P–N junctions with fullerene derivatives (C₆₀ or PCBM), and the P–N junctions would provide driving force for exciton dissociation in perovskite-based solar cell. To the best of our knowledge, there is so far no experiment proof on this assumption. On the other hand, whether photogenerated excitons can intrinsically dissociate into free carrier in the perovskite without any assistance from a P–N junction is still controversial. To address these, the interfacial electronic structures of a vacuum-deposited perovskite/C₆₀ and a solution-processed perovskite/PCBM junctions is directly measured by ultraviolet photoelectron spectroscopy. Contrary to the common believes, both junctions are found to be type I N–N junctions with band gap of the perovskites embedded by that of the fullerenes. Meanwhile, device with such a charge inert junction can still effectively functions as a solar cell. These results give direct experimental evidence that excitons are dissociated to free carriers in the perovskite film even without any assistance from a P–N junction.

While perovskites/fullerene is commonly assumed to form a type II P–N junction with its internal E-field facilitating exciton dissociation, it is found that perovskite/C₆₀ (PCBM) is a charge inert type I N–N junction. Devices with such a junction show photovoltaic effects effectively, thus photogenerated excitons can indeed dissociate to free carriers in the perovskite film.

PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.

A ternary blend system with two donors and one acceptor provides an effective route to improve the performance of organic solar cells. A synergistic effect of polymer and small molecules is observed in ternary solar cells, and the power conversion effi ciency (PCE) of the ternary system (8.40%) is higher than those of binary systems based on small molecules (7.48%) or polymers (6.85%).

Thermal stability has been the important issue in organic solar cell, especially for the large scale fabrication and application in the future. In this work, a new strategy involving the introduction of porphyrin compound (BL) is proposed to prevent the [6,6]-phenyl C61 butyric acid methyl ester (PC₆₁BM) aggregation. The supramolecular interactions between PC₆₁BM and BL are first demonstrated in PC₆₁BM:BL binary blend, and then the effect of BL on P3HT:PC₆₁BM blend is qualitatively and quantitatively studied by differential scanning calorimetry, UV–vis absorption spectroscopy, atomic force microscopy, optical microscopy, and fluorescence techniques. It is found that the BL addition not only stabilizes the morphology of P3HT:PC₆₁BM blend films, but also shows a good ability to maintain the electron mobility by depressing the PC₆₁BM crystallization. And the thermal stability of the devices based on P3HT:PC₆₁BM:BL ternary blend films is therefore greatly improved. For example, 8 wt% BL doping drops the power conversion efficiency by 10.5% relative to its peak value after 48 h of annealing at 130 °C, while 71.5% of decrease is obtained for the device without BL after only 3 h of annealing. This strategy is preliminarily proved to be universal and will show great potentials in future commercialization of polymer solar cells.

An effective strategy involving the introduction of porphyrin and the formation of the supramolecular interactions between porphyrin and PC₆₁BM is proposed to prevent PC₆₁BM from thermal-driven aggregation. The device based on the P3HT:PC₆₁BM:BL blend film shows enhanced morphological stability, good ability to maintain the electron mobility, and most importantly, excellent thermal stability (10.5% of power conversion efficiency decreases at 130 °C for 48 h).

In this work, a high-performance ITO-free flexible polymer solar cell (PSC) is successfully described by integrating the plasmonic effect into the ITO-free microcavity architecture. By carefully controlling the sizes of embedded Ag nanoprisms and their doping positons in the stratified device, a significant enhancement in power conversion efficiency (PCE) is shown from 8.5% (reference microcavity architecture) to 9.4% on flexible substrates. The well-manipulated plasmonic resonances introduced by the embedded Ag nanoprisms with different LSPR peaks allow the complementary light-harvesting with microcavity resonance in the regions of 400–500 nm and 600–700 nm, resulting in the substantially increased photocurrent. This result not only signifies that the spectral matching between the LSPR peaks of Ag nanoprisms and the relatively low absorption response of photoactive layer in the microcavity architecture is an effective strategy to enhance light-harvesting across its absorption region, but also demonstrates the promise of tailoring two different resonance bands in a synergistic manner at desired wavelength region to enhance the efficiency of PSCs.

Highly efficient ITO-free, flexible polymer solar cells are successfully demon­strated by integrating the plasmonic effect into microcavity-based devices. By carefully controlling the embedded Ag nanoprisms sizes, the power conversion efficiency of the devices can be significantly enhanced to as high as 9.4% on both glass and flexible (PET) substrates.