Sunyong Ahn

A Mechanistic Understanding of a Binary Additive System to Synergistically Boost Efficiency in All-Polymer Solar Cells

All-polymer solar cells are herein presented utilizing the PBDTTT-CT donor and the P(NDI2OD-T2) acceptor with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) binary solvent additives. A systematic study of the polymer/polymer bulk heterojunction photovoltaic cells processed from the binary additives revealed that the microstructures and photophysics were quite different from those of a pristine system. The combination of DIO and CN with a DIO/CN ratio of 3:1 (3 vol% DIO, 1 vol% CN and 96 vol% o-DCB) led to suitable penetrating polymer networks, efficient charge generation and balanced charge transport, which were all beneficial to improving the efficiency. This improvement is attributed to increase in power conversion efficiency from 2.81% for a device without additives to 4.39% for a device with the binary processing additives. A detailed investigation indicates that the changes in the polymer:polymer interactions resulted in the formation of a percolating nasnoscale morphology upon processing with the binary additives. Depth profile measurements with a two-dimensional grazing incidence wide-angle X-ray scattering confirm this optimum phase feature. Furthermore impedance spectroscopy also finds evidence for synergistically boosting the device performance.

Morphology fixing agent for [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) in planar-type perovskite solar cells for enhanced stability

Here, we report that the delamination of [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) from the CH3NH3PbI3 (MAPbI3) layer is a critical reason for the degradation of inverted perovskite solar cells (PSCs). In this work, an ∼10 nm-thick titanium oxide (TiOx) layer can function as a morphology-fixing agent for preventing PC60BM delamination, which in turn induces greatly improved long-term stability of the PSCs. In addition, the devices with the TiOx layer exhibited increased open circuit voltages, current densities, and fill factors, which correspond to improved initial device efficiency. Surface morphology changes of the PC60BM layer on the MAPbI3 layer during long-term operation of PSCs were confirmed by SEM and AFM, and were also found to be correlated with various electrical properties of the devices such as the hole mobility and charge generation and extraction efficiencies from the space-charge-limited current (SCLC) measurements and Jph versus Vint characterizations.

Vacuum-process-based dry transfer of active layer with solvent additive for efficient organic photovoltaic devices

Stamping transfer process for fabricating organic electronics has recently been demonstrated to improve device performance because it allows the control of morphology, patterning, and transfer of various materials. However, it still has issues related to reproducibility because complex solvent conditions, that is, several active layers consisting of solvent additives or mixtures used for improving morphology, unfavorably affect the film transfer. To transfer uniform thin films, poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM)-based organic photovoltaic cells were fabricated in this study by utilizing the optimized surface energy of the hydrophilic 2-hydroxyethyl methacrylate–polyurethane acrylate stamp. Upon application of vacuum treatment based on dry transfer, a bulk-heterojunction (BHJ) layer with the solvent additive, 1,8-diiodooctane, was completely transferred onto a poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer without any change in its domain distribution and composition. Consequently, BHJ devices fabricated by stamping transfer showed power conversion efficiency comparable to those of devices fabricated by spin-coating. We also observed significant improvement in long-term stability of the PTB7:PC71BM-based photovoltaic cells fabricated by stamping transfer under dry conditions.

Dry-Stamping-Transferred PC71BM Charge Transport Layer via an Interface-Controlled Polyurethane Acrylate Mold Film for Efficient Planar-Type Perovskite Solar Cells

The study of interlayers is important to enhance the performance of inverted perovskite solar cells (PSCs) because interlayers in PSCs align energy levels and improve charge transport. However, previous research into applying interlayers for PSCs has focused only on wet-coated methods, such as spin coating, to form the interlayer. Here, we fabricated planar-type PSCs deposited with a 6,6-phenyl-C71 butyric acid methyl ester (PC71BM) layer onto a CH3NH3PbI3 (MAPbI3) layer by stamping transfer through a relatively dry process condition. We demonstrated the effects of a stamping-transferred PC71BM layer using polyurethane acrylate (PUA), the surface energy of which was modified by 2-hydroxyethyl methacrylate (HEMA) to increase the transfer reproducibility. In PSCs with a stamping-transferred PC71BM layer, we observed an enhanced JSC and a comparable power conversion efficiency (PCE), which were caused by an enhanced coverage of the electron transport layer onto the MAPbI3 layer with preserved crystallinity, which occurs owing to improved electron mobility and exciton dissociation. The optimized device PCE through the dry-transferred PC71BM exhibited a JSC, fill factor, and PCE of 21.65 mA/cm2, 76.0%, and 15.46%, respectively. Moreover, morphological analysis and electrical measurements confirmed the improved durability of dry-stamping-transferred PSCs.

Counterbalancing of morphology and conductivity of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate based flexible devices

The importance of conductive polymer electrodes with a balance between the morphology and electrical conductivity for flexible organic photovoltaic properties has been demonstrated. Highly transparent PEDOT:PSS anodes with controlled conductivity and surface properties were realized by insertion of dimethyl sulfoxide (DMSO) and a fluorosurfactant (Zonyl) as efficient additives and used for flexible organic photovoltaic cells (OPVs) which are based on a bulk-heterojunction of polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]phenyl-C71-butyric acid methyl ester (PC71BM). We investigated the correlation between the electrical properties of PEDOT:PSS electrodes and their influences on the surface morphology of the active materials (PTB7:PC71BM). When the device was prepared from the PEDOT:PSS layer functioning as an anode of OPV through an optimized ratio of 5 vol% of DMSO and 0.1 wt% of fluorosurfactant, the devices exhibited improved fill factor (FF) due to the enhanced coverage of PEDOT:PSS films. These results correlate with reduced photoluminescence and increased charge extraction as seen through Raman spectroscopy and electrical analysis, respectively. The conductive polymer electrode with the balance between the morphology and electrical conductivity can be a useful replacement for brittle electrodes such as those made of indium tin oxide (ITO) as they are more resistant to cracking and bending conditions, which will contribute to the long-term operation of flexible devices.

The effect of processing additives for charge generation, recombination, and extraction in bulk heterojunction layers of all-polymer photovoltaics

Bulk heterojunction all-polymer solar cells, fabricated with poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b′]thiophen-4,6-diyl]} (PBDTTT-CT) as a donor polymer, and a acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), have been demonstrated and have achieved a power conversion efficiency exceeding 3.7% by using 1,8-diiodooctane (DIO) as a processing additive. Based on the analysis of charge carrier dynamics (charge generation, separation, and extraction), we found that the appropriate ratio of processing solvent additive (5 vol. % DIO) leads to enhanced device performance and favorable morphological characteristics. This research, therefore, indicates that the incorporation of a DIO additive in all-polymer blends is an effective way to form a morphologically ideal heterojunction network and thereby improve charge carrier kinetics for effi...

Nanopatterned bulk-heterojunction photovoltaic cells using polyurethane acrylate (PUA) film replica of colloidal crystal arrays via stamping transfer process

Bulk heterojunction (BHJ) films with nanopattern structure have been fabricated using a stamping transfer process with polyurethane acrylate (PUA) replica film based on 3-dimensional colloidal crystals. The uniformly arrayed nanostructure of the PUA film was investigated by SEM analysis. Compared to the organic photovoltaic cells wtih plain BHJ of poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA), the device from the nanopatterned BHJ exhibit improved E-field distribution intensity, as assessed using a finite-difference time-domain (FDTD) method. The current density (JSC) and power conversion efficiency (PCE) of the device were respectively increased by up to 27% and 20% due to the enhanced interfacial contact area and plasmonic effect, which can brought about effective light harvesting between the BHJ active layer and cathode. Through a simple low-temperature transfer process using a PUA mold, we can fabricate reproducible nanopatterned BHJ devices that have improved light absorption within the active layer of the devices.