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 <i>J</i>_SC 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 <i>J</i>_SC, 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

Structure–Property Correlation: A Comparison of Charge Carrier Kinetics and Recombination Dynamics in All-Polymer Solar Cells

We report a comparison of charge carrier kinetics and recombination dynamics correlated with the device performances of <b>PBDTTT-C</b> and <b>PBDTTT-CT</b> in nonfullerene <b>P­(NDI2OD-T2)</b> solar cells. The nanoscale morphological characteristics are found to be remarkably different for these two polymers in all-polymer bulk heterojunction (BHJ) blends. Important insights into the carrier dynamics and crystalline packing features of these different donor materials are provided in detail. The higher device performance of the <b>PBDTTT-CT:P­(NDI2OD-T2)</b> device [2.78% compared to 1.56% for the <b>PBDTTT-C:P­(NDI2OD-T2)</b> cell] is shown to result from efficient charge generation and faster carrier separation, reducing nongeminate recombination during charge collection, which is attributed to the perfect intermixing between the donor and acceptor polymer networks

Efficient Hole Extraction from Sb₂S₃ Heterojunction Solar Cells by the Solid Transfer of Preformed PEDOT:PSS Film

Here, we report significant improvements of <i>V</i>_oc and FF in Sb₂S₃ quantum dot (QD)-based, solid-state heterojunction solar cells prepared from the solid transfer of preformed PEDOT:PSS hole extraction layers. Despite the moderate optical properties of Sb₂S₃ QDs, the solid state QD solar cells suffer from poor power conversion efficiency (PCE) resulting from the disappointing <i>V</i>_oc and the high series resistance since there is inefficient charge extraction from QDs to the metal top electrode. In order to improve the hole extraction performance, a significantly uniform PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)/poly­(styrenesulfonate)) layer was transferred on the hole transport layer (P3HT, poly­(3-hexylthiophene-2,5-diyl)) by using a simple solid-transfer method. In contrast with conventional spin-cast methods, the hydrophilic PEDOT:PSS layer was uniformly coated on the hydrophobic P3HT layer without any significant detriment to P3HT film properties. Due to improved contact surface for the Au top electrode and hole conductance resulting in significantly improved charge extraction, the power conversion efficiency was dramatically enhanced. Furthermore, the thickness of the PEDOT:PSS film was precisely optimized by layer-by-layer solid transfer, and thereby the PCE of the PEDOT:PSS solid-transfer device (30 nm) was improved by 25.7% in comparison to the PEDOT:PSS spin-cast device and by 76% in comparison to the PEDOT:PSS free device

Stamping Transfer of a Quantum Dot Interlayer for Organic Photovoltaic Cells

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode

Conflicted Effects of a Solvent Additive on PTB7:PC₇₁BM Bulk Heterojunction Solar Cells

Recently, polymer–fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly­({4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene-2,6-diyl}­{3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]­thiophenediyl}) (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), have been widely studied due to exhibiting high power conversion efficiency (PCE) and well-defined nanomorphology. Because of the short exciton diffusion pathway (less than 10 nm) in organic thin films, the optimization of PTB7:PC₇₁BM BHJ with optimized morphology is very important between the donor and acceptor. In order to increase nanoscale phase separation, the chemical additives of 1,8-diiodooctane (DIO) have been used in PTB7:PC₇₁BM blend systems. However, the mechanism studies of DIO in BHJ solar cells and its effectiveness on device stability are unclear. In this study, we fabricated polymer solar cells (PSCs) based on PTB7:PC₇₁BM BHJ with various DIO concentrations to investigate not only correlation between device performances and different morphologies, but also the influence of additives on device stabilities. Positive effects of DIO, which were induced by efficient charge separation in BHJ at optimized blending ratio, are proved by the results of time-resolved photoluminescence (TRPL), and negative effects of DIO on a device stability have been investigated according to the ISOS-D-1 protocol

Intensity Dependence of Current–Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells

Solution-processed small-molecule p-DTS(FBTTh₂)₂:PC₇₁BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600–650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current–voltage (<i>J–V</i>) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the <i>J–V</i> curves are dominated by first-order recombination from the short-circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley–Read–Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor

Discrepancy of Optimum Ratio in Bulk Heterojunction Photovoltaic Devices: Initial Cell Efficiency vs Long-Term Stability

Organic photovoltaic devices are difficult to commercialize because of their vulnerability to chemical degradation related with oxygen and water and to physical degradation with aging at high temperatures. We investigated the photophysical degradation behaviors of a series of poly­(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PC₆₀BM) bulk heterojunctions (BHJs) as a model system according to the donor–acceptor ratio. We found that the optimum P3HT:PC₆₀BM ratio in terms of long-term stability differs from that in terms of initial cell efficiency. On the basis of cell performance decays and time-resolved photoluminescence measurements, we investigated the effects of oxygen and material self-aggregation on the stability of an organic photovoltaic device. We also observed the changes in morphological geometry and analyzed the surface elements to verify the mechanisms of degradation

Improved Light Harvesting and Improved Efficiency by Insertion of an Optical Spacer (ZnO) in Solution-Processed Small-Molecule Solar Cells

We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV–vis absorption spectroscopy and by measurements of the photoinduced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.9%, that is, comparable to that of polymer counterparts

Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for High-Efficiency Bulk Heterojunction Solar Cells

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDsgraphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDswere tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%)

Self-Position of Au NPs in Perovskite Solar Cells: Optical and Electrical Contribution

Metallic nanoparticles (NPs) exhibit a localized surface plasmon resonance (LSPR) and act as scattering centers and subwavelength antennas, so metallic NPs can be incorporated into perovskite solar cells (PSCs) to effectively improve the light absorption of light harvesting devices. Here, we have embedded Au nanoparticles (NPs) into the hole transport layer (HTL) of the PSCs to investigate the photovoltaic effect of the PSCs with Au NPs. Interestingly, it was found that Au NPs dispersed spiro-OMeTAD HTL solution could naturally end up located near the perovskite layer as the result of the spin-coating step. Solar cell performance observations indicate that the LSPR and electrical effects of Au NPs enhance the photovoltaic response of PSCs, in spite of a slight decrease in the open-circuit voltage (<i>V</i>_OC), by causing an incredible improvement in the photocurrent density as a dominant factor