The effect of methanol treatment on the performance of polymer solar cells

Interface Engineering for Highly Efficient Organic Solar Cells

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity

A small amount of PFN, introduced as an impurity, can have a dramatic impact on the performance of small molecule-based BHJ solar cells.

A Nonconjugated Zwitterionic Polymer: Cathode Interfacial Layer Comparable with PFN for Narrow-Bandgap Polymer Solar Cells

A nonconjugated, alcohol-soluble zwitterionic polymer, poly(sulfobetaine methacrylate) (denoted by PSBMA), is employed as cathode interfacial layer (CIL) in polymer solar cells (PSCs) based on PTB7-Th:PC₇₁ BM. Compared with the control device without CIL, PSCs with PSBMA CILs show significant enhancement on the resulting performance, and the highest power conversion efficiency (PCE) of 8.27% is achieved. Under parallel conditions, PSCs with PSBMA as CIL show comparable performance than those with widely used poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)] as CIL. The polar groups of PSBMA not only provide a solvent orthogonal solubility in the process of preparation of the devices but also lead to interfacial dipole to the electrode, which promises a better energy level alignment. In addition, PSBMA-based devices show better abilities of hole blocking. These results indicate that the zwitterionic polymer PSBMA should be a promising CIL in PSC-based narrow-bandgap polymers.

Diblock Copolymer PF-b-PDMAEMA as Effective Cathode Interfacial Material in Polymer Solar Cells

An alcohol-soluble diblock copolymer poly[2,7-(9,9-dihexylfluorene)]₁₅-block-poly[2-(dimethylamino)ethyl methacrylate]₇₅ (denoted as PF₁₅-b-PDMAEMA₇₅) was employed as the cathode interfacial layer (CIL) in p-i-n polymer solar cells (PSCs). PF₁₅-b-PDMAEMA₇₅ contains a conjugated rigid block and a nonconjugated flexible block grafted with polar amino groups, and it can effectively lower the work function of the Al cathode and decrease the series resistance of the devices. When applied as the CIL in PSCs based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl C71 butyric acid methyl ester, the champion power conversion efficiency of 8.80% was achieved, which is slightly higher than that of the PSCs using the well-known poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] as CIL under our experimental conditions, and much better than that of PSCs using Ca as CIL. The improvement of the performance is mainly attributed to the enhanced open-circuit voltage and fill factor. To the best of our knowledge, this is the first time a diblock copolymer has been used as a CIL in PSCs, and this study may provide a novel avenue for the design and synthesis of interfacial materials for high-performance PSCs.

High-Performance Polymer Solar Cells Employing Rhodamines as Cathode Interfacial Layers

The development of simple and water-/alcohol-soluble interfacial materials is crucial for the cost-effective fabrication process of polymer solar cells (PSCs). Herein, highly efficient PSCs are reported employing water-/alcohol-soluble and low-cost rhodamines as cathode interfacial layers (CILs). The results reveal that rhodamine-based CILs can reduce the work function of the Al cathode and simultaneously increase the open-circuit voltage, current density, fill factor, and power conversion efficiency (PCE) of PSCs. The solution-processed rhodamine-based PSCs demonstrated a remarkable PCE of 10.39%, which is one of the best efficiencies reported for thieno[3,4-b]thiophene/benzodithiophene:[6,6]-phenyl C₇₁-butyric acid methyl ester-based PSCs so far. The efficiency is also 42.3% higher than that of the vacuum-deposited Ca-based device (PCE of 7.30%) and 21.5% higher than that of the complicated solution-processable polymeric electrolyte poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-based device (PCE of 8.55%). Notably, rhodamines are very economical and have been extensively used as dyes in industries. Our work indicates that rhodamines have shown a strong potential as CILs compared to their counterparts in the large-area fabrication process of PSCs.

One-Pot Large-Scale Synthesis of Carbon Quantum Dots: Efficient Cathode Interlayers for Polymer Solar Cells

Cathode interlayers (CILs) with low-cost, low-toxicity, and excellent cathode modification ability are necessary for the large-scale industrialization of polymer solar cells (PSCs). In this contribution, we demonstrated one-pot synthesized carbon quantum dots (C-dots) with high production to serve as efficient CIL for inverted PSCs. The C-dots were synthesized by a facile, economical microwave pyrolysis in a household microwave oven within 7 min. Ultraviolet photoelectron spectroscopy (UPS) studies showed that the C-dots possessed the ability to form a dipole at the interface, resulting in the decrease of the work function (WF) of cathode. External quantum efficiency (EQE) measurements and 2D excitation-emission topographical maps revealed that the C-dots down-shifted the high energy near-ultraviolet light to low energy visible light to generate more photocurrent. Remarkably improvement of power conversion efficiency (PCE) was attained by incorporation of C-dots as CIL. The PCE was boosted up from 4.14% to 8.13% with C-dots as CIL, which is one of the best efficiency for i-PSCs used carbon based materials as interlayers. These results demonstrated that C-dots can be a potential candidate for future low cost and large area PSCs producing.

Enhancing the power conversion efficiency of polymer solar cells via selection of quinoxaline substituents

Improvement of the power conversion efficiency of polymer solar cells was successfully achieved by selecting appropriate quinoxaline substituents on the polymer backbone.

n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells

With the demonstration of small-area, single-junction polymer solar cells (PSCs) with power conversion efficiencies (PCEs) over the 10% performance milestone, the manufacturing of high-performance large-area PSC modules is becoming the most critical issue for commercial applications. However, materials and processes that are optimized for fabricating small-area devices may not be applicable for the production of high-performance large-area PSC modules. One of the challenges is to develop new conductive interfacial materials that can be easily processed with a wide range of thicknesses without significantly affecting the performance of the PSCs. Toward this goal, we report two novel naphthalene diimide-based, self-doped, n-type water/alcohol-soluble conjugated polymers (WSCPs) that can be processed with a broad thickness range of 5 to 100 nm as efficient electron transporting layers (ETLs) for high-performance PSCs. Space charge limited current and electron spin resonance spectroscopy studies confirm that the presence of amine or ammonium bromide groups on the side chains of the WSCP can n-dope PC71BM at the bulk heterojunction (BHJ)/ETL interface, which improves the electron extraction properties at the cathode. In addition, both amino functional groups can induce self-doping to the WSCPs, although by different doping mechanisms, which leads to highly conductive ETLs with reduced ohmic loss for electron transport and extraction. Ultimately, PSCs based on the self-doped WSCP ETLs exhibit significantly improved device performance, yielding PCEs as high as 9.7% and 10.11% for PTB7-Th/PC71BM and PffBT4T-2OD/PC71BM systems, respectively. More importantly, with PffBT4T-2OD/PC71BM BHJ as an active layer, a prominent PCE of over 8% was achieved even when a thick ETL of 100 nm was used. To the best of our knowledge, this is the highest efficiency demonstrated for PSCs with a thick interlayer and light-harvesting layer, which are important criteria for eventually making organic photovoltaic modules based on roll-to-roll coating processes.

Microscopic Investigations into the Effect of Surface Treatment of Cathode and Electron Transport Layer on the Performance of Inverted Organic Solar Cells

Surface treatments of various layers in organic solar cells play a vital role in determining device characteristics. In this manuscript, we report on the influence of surface treatment of indium tin oxide (ITO) electrode and electron transport layer (ETL), ZnO, on the photovoltaic performance of inverted organic solar cells (IOSC) and their correlation with the surface chemistry and surface potential as studied using X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM), using the device structure glass/ITO/ZnO/P3HT: PCBM/MoO3/(Au or Ag) (P3HT, poly(3-hexylthiophene-2,5-diyl), and PCBM, phenyl-C61-butyric acid methyl ester). Our results show that although ozonization of ITO leads to an improvement in the device power conversion efficiency, the ozonization of a subsequent ZnO layer results in a decreased performance mainly because of a decrease in the fill factor (FF). However, subsequent methanol (CH3OH) treatment of ZnO layer on an ozonized ITO electrode shows substantial improvement with device efficiencies exceeding ∼4% along with superior reproducibility of the devices. Furthermore, a detailed analysis of the surface wettability, chemistry, and surface potential using contact angle measurements, XPS, and KPFM attribute the improvements to the elimination of surface defects and the changes in the surface potential. Finally, impedance analysis suggests that methanol treatment of the ZnO layers leads to the development of a favorable nanophase structure with higher conductivity, which is otherwise indiscernible using microscopic methods.

Effect of alcohol treatment on the performance of PTB7:PC71BM bulk heterojunction solar cells

The effect of an environmentally friendly alcohol treatment on bulk heterojunction (BHJ) polymer solar cells using the low-bandgap copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene units and [6,6]-phenyl-C71-butyric acid methyl ester is systematically investigated. Different alcohols are tested, and besides the most commonly used methanol treatment, other alcohols such as ethanol, 2-propanol, and 1-butanol also improve the device performance to certain extents as compared to the untreated solar cells. Changes of the surface structure caused by the alcohol treatment are probed with atomic force microscopy, and the modification of inner film morphology is probed by time-of-flight-grazing incidence small-angle neutron scattering (TOF-GISANS). UV/vis measurements show that the thickness of all BHJ films remains unchanged by the different solvent treatments. Thus, the enhanced device performance induced by the alcohol treatments is correlated to the reconstruction of the inner film structures probed with TOF-GISANS and the modified energy levels at the interfaces between the BHJ layer and the aluminum electrodes, evident by the enhanced short-circuit current and open-circuit voltage of the I-V curves.

Water/alcohol soluble conjugated polymers for the interface engineering of highly efficient polymer light-emitting diodes and polymer solar cells

This review provides a summary of the recent developments and applications of water/alcohol soluble conjugated polymers in highly efficient polymer light-emitting diodes and polymer solar cells.

Organic photovoltaic cells with stable top metal electrodes modified with polyethylenimine

Efficient organic photovoltaic cells (OPV) often contain highly reactive low-work-function calcium electron-collecting electrodes. In this work, efficient OPV are demonstrated in which calcium electrodes were avoided by depositing a thin layer of the amine-containing nonconjugated polymer, polyethylenimine (PEIE), between the photoactive organic semiconductor layer and stable metal electrodes such as aluminum, silver, or gold. Devices with structure ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):indene-C60-bis-adduct (P3HT:ICBA)/PEIE/Al demonstrated overall photovoltaic device performance comparable to devices containing calcium electron-collecting electrodes, ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, with open-circuit voltage of 775±6 mV, short-circuit current density of 9.1±0.5 mA cm(-2), fill factor of 0.65±0.01, and power conversion efficiency of 4.6±0.3%, averaged over 5 devices at 1 sun.

Conventional polymer solar cells with power conversion efficiencies increased to >9% by a combination of methanol treatment and an anionic conjugated polyelectrolyte interface layer

The power conversion efficiency of a PTB7:PC₇₁BM polymer solar cell was improved up to 9.1% by a combination of methanol treatment followed by conjugation of a water- or alcohol-soluble polyelectrolyte thin layer.