A new alcohol-soluble electron-transporting molecule for efficient inverted polymer solar cells

A new electron-transporting material 4,7-diphenyl-1,10-phenanthroline-2,9-dicarboxylic acid (DPPA) was synthesized by modifying a n-type small molecule bathocuproine (BCP). The introduced carboxyl groups make DPPA soluble in polar solvent and compatible with large-scale solution-processing techniques. The anchoring of carboxyl on ZnO (or ITO) substrates helps to form a DPPA electron transporting layer, building an improved interfacial contact between the substrate and active layer. Furthermore, the highest occupied molecular orbital level of DPPA shifts to ?6.45 eV, which is 0.38 eV deeper than that of BCP, suggesting enhanced hole-blocking. Inverted polymer solar cells using P3HT:PCBM blend as the active layer and DPPA modified ZnO as the electron transporting layer were fabricated. A power conversion efficiency (PCE) of 3.55% was obtained, which is about 10% higher than that of the conventional ZnO buffered devices (3.25%). The DPPA was also used to replace ZnO as the sole electron-extracting layer, resulting in an improved PCE of 3.46%, which indicates that DPPA-ETL/ITO forms a better cathode than conventional ZnO/ITO.     

Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells

Here we report that poly(N-dodecyl-2-ethynylpyridiniumbromide) (PDEPB) interlayers between electron-collecting zinc oxide (ZnO) layers and bulk heterojunction (BHJ) layers act as a universal interfacial layer for improving the performances of inverted-type polymer:fullerene solar cells. Three different BHJ layers, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), poly[(4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(N-2-ethylhexylthieno[3,4-c]pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD):PC₆₁BM, and 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] (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), were employed so as to prove the role of the PDEPB interlayers. Results showed that the power conversion efficiency (PCE) of polymer:fullerene solar cells with the three different BHJ layers increased in the presence of the PDEPB interlayers prepared from 0.5 mg/ml solutions. The improved PCE was attributed to the conformal coating of the PDEPB layers on the ZnO layers (by atomic force microscopy measurement), lowered work functions of ZnO induced by the PDEPB layers (by Kelvin probe measurement), and reduced interface resistance (by impedance spectroscopy measurement), as supported by the noticeable change in the atom environments of both the ZnO and PDEPB layers (by X-ray photoelectron spectroscopy measurement).     

Acid-functionalized fullerenes used as interfacial layer materials in inverted polymer solar cells

Two types of carboxylic acid functionalized fullerence derivatives, 4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (p-EHO-PCBA) and bis-4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (bis-p-EHO-PCBA), were synthesized and investigated as an interfacial layer for inverted polymer solar cells (iPSCs). The –COOH groups on the PCBAs chemisorb to inorganic metal oxide (TiO_X), generating fullerene-based self-assembled monolayers (FSAMs). The devices with the mono- and bis-FSAMs exhibited substantially lower series resistance (R_S) values of 2.10 Ω cm² and 1.46 Ω cm², compared to that (4.15 Ω cm²) of the unmodified device. The TiO_X films modified with mono- and bis-FSAMs showed higher contact angles of 50° and 91°, respectively, than that of the pristine TiO_X film (33°). The increased contact angles were attributed to the enhanced hydrophobicity, improving the wetting properties with the organic photoactive layer. In addition, a comparison of device characteristics with electroactive FSAMs and non-electroactive benzoic acid SAMs clearly indicates that the FSAMs may suggest an additional pathway for photo-induced charge transfer and charge collection to ITO. After surface modification with FSAMs, the short-circuit current density (J_SC) and fill factor (FF) values increased substantially. The iPSCs based on poly(5,6-bis(octyloxy)-4-(thiophen-2-l)benzo[c][1,2,5]thiadiazole) (PTBT) and [6,6]phenyl-C₆₁-butyric acid methyl ester (PCBM) as an active layer showed remarkably improved power conversion efficiency up to 5.13% through incorporation of the FSAMs-based interfacial layer.     

Spraycoating of silver nanoparticle electrodes for inverted polymer solar cells

Inverted polymer solar cells using non-vacuum processed spraycoated silver nanoparticles (Ag-NPs) as the anode electrode were compared to vacuum deposited Ag electrodes. The number of spray coating layers can affect its final device performance showing with a higher number of coating layers, a better nanoparticle interconnectivity and morphology is achieved which reduces the sheet resistance and transparency of the Ag electrode leading to improved fill factor and device performances (～3.0%). These devices can be fabricated onto flexible ITO substrates devices showing a performance of ～1.40%. Using a non-vacuum technique to deposit the electrode is important for low-cost polymer solar cells.     

Highly conductive Zinc-Tin-Oxide buffer layer for inverted polymer solar cells

Thin-films of Zinc Tin Oxide (ZTO) with an extremely high charge carrier mobility and superior optical transmittance are synthesized using a simple solution method. These ZTO films have been systematically studied for the application in inverted polymer solar cells (PSCs). The Hall effects measurements show that the charge mobility of the ZTO semiconductor is over 16.5 cm².V⁻¹.S⁻¹, which is the highest mobility value ever reported for oxide buffer made by using solution process. By applying the ZTO buffer layer in the inverted PSCs of P3HT:PC₆₁BM, the power conversion efficiency of the device is 30% higher than that of the devices made with other common buffer layers such as ZnO and TiO₂. Light intensity-dependent JV studies and PL measurements also indicate that ZTO buffer layer reduces surface recombination. This work demonstrates that the solution-synthesized ZTO is a promising new buffer layer with superior electron extraction capability for the solar cells.     

Improved performance of inverted polymer solar cells by utilizing alcohol-soluble oligofluorenes as efficient cathode interlayers

Two star-shaped oligofluorenes with hexakis(fluoren-2-yl)benzene as core are designed and synthesized, namely Tn0 and Tn1. Diethylamino groups are attached to the side chain of fluorene units of Tn0 and Tn1 and enable them alcohol solubility, additional hydrophobic nhexyl chains are grafted on the π-extended fluorene arms of Tn1. Power conversion efficiency (PCE) as high as 8.62% and 8.80% are achieved when utilizing Tn0 and Tn1 as cathode interlayers in inverted polymer solar cells, respectively. The work function of ITO effectively decreased by introducing interlayer, resulting in high Voc of the device, besides, the wetting properties of the interlayers can be tuned by modifying the oligofluorenes with π-extended structure, and the more hydrophobic interlayer will benefit the device performance with enhanced Jsc and FF.     

Solution processed bulk heterojunction polymer solar cells with low band gap DPP-CN small molecule sensitizer

The improvement of near infrared wavelength sensitivity in the bulk heterojunction organic polymer solar cell based on poly (3-hexylthiophene) (P3HT) and PC₇₀BM, by the addition of soluble DPP-CN small molecule is reported. By incorporating DPP-CN, the photosensitivity in the longer wavelength region was improved and the power conversion efficiency (PCE) has been reached to 4.37% as compared to 3.23% for the device based on P3HT:PC₇₀BM blend. The increase in the PCE is attributed to the increase in light harvesting property of the blend and efficient dissociation of excitons into free charge carriers due to the increased number of D–A sites. The PCE has been further enhanced to 4.70%, when mixed solvent cast P3HT:DPP-CN:PC₇₀BM blend is used as photoactive layer. The optical absorption spectra of the blend showed that the blend film cast from mixed solvent broadened the absorption wavelength range. This occurred as result of a large red shift of P3HT absorption peak and same time a widening and small red shift of DPP-CN absorption peak in the blend film. The improved light harvesting property of thermally annealed film is considered to the factor responsible for the improvement in the PCE.     

Nanoparticle-based,spray-coated silver top contacts for efficient polymer solar cells

We demonstrate a solution-processed top electrode for large area organic electronic devices. A Ag nanoparticle solution is spray-coated directly on top of an inverted bulk-heterojunction organic solar cell through a shadow mask. After sintering the Ag nanoparticle film at 150 °C, a temperature which is compatible with processes on flexible substrates, cells show performances comparable to those of reference devices with evaporated top-contacts.     

Relationship between HOMO energy level and open circuit voltage of polymer solar cells

A series of π-conjugated polymers (PDHF-BT and PDHF-TBT) with 4-(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (BT), 4,7-bis(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (TBT), and 9,9′-dihexylfluorene were synthesized by the Suzuki coupling reaction. The HOMO energy level of PDHF-BT was −5.47 eV, which was lower than that of PDHF-TBT (−5.22 eV), while the LUMO energy level of PDHF-BT (−3.45 eV) was very similar to that of PDHF-TBT (−3.42 eV). These energy levels of PDHF-BT and PDHF-TBT were also supported by a DFT calculation. The power conversion efficiency (PCE) of the polymer solar cell (PSC) with a structure of ITO/PEDOT:PSS/PDHF-BT:PCBM (1:1)/Al was determined as 0.34% and it was larger than that of the device based on PDHF-TBT (0.22%). Correspondingly, the V_oc of the PSC based on PDHF-BT (0.71 V) was much larger than that of the device based on PDHF-TBT (0.40 V). The results support that the V_oc of polymer based PSCs is strongly related to the HOMO energy level of the active polymers.     

Fluoro-substituted low band gap polymers based on isoindigo for air-stable polymer solar cells with high open circuit voltages

Two low band gap polymers (PIDT2FBT and PIDT4FBZ), which are composed of fluorinated elctron donor units (dithienyldifluorobenzothiadiazole (DT2FBT) and dithienyltetrafluorobenzene (DT4FBZ)) and a isoindigo electron accepting unit were implemented in a photoactive layer to invesitigate thier photovoltaic performance. Both low band gap polymers possess exhibit low-lying energy levels and extended light absorption range, which are essential to generate high open circuit voltage and short circuit current density of polymer solar cell. In addition, the blend with PC₇₁BM formed affirmative bulk heterojunction mrophology, resulting in promising power conversion efficiency up to 4.1% with high openc circuit voltage exceeding 1.1 V. Furthremore, the low-lying energy levels of the polymers leaded to good ambient stability of the devices up to 250 h, enabling PIDT2FBT and PIDT4FBZ to be promising candidates for photoactive layer of ambient stable polymer solar cells.     

Rubrene-based interfacial engineering toward enhanced performance in inverted polymer solar cells

Rubrene, an organic semiconductor having stable fused-ring molecular structure was used as a double interfacial layer in inverted organic solar cells. When a thin, 3 nm-thick layer of rubrene was introduced between a MoO₃-based hole-collecting layer and a bulk-heterojunction (BHJ) photo-active layer consisting of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM), the power conversion efficiency was improved over 12% (from 7.2% to 8.1%). It was demonstrated that the insertion of thin rubrene layer showed suppressed exciton quenching and improved exciton dissociation, resulting in more efficient charge carrier collection and weaker charge recombination, thus improving the device performance.     

Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed polymer solar cells.     

Zwitterionic ammonium and neutral amino molecules as cathode interlayer for inverted polymer solar cells

Cathode interlayer is essential to inverted bulk heterojunction polymer solar cells (PSCs). A series of zwitterionic ammonium and neutral amino organic molecules are introduced into inverted PSCs as cathode interlayer and power conversion efficiency (PCE) as high as 8.07% is demonstrated. Compared to the devices without interlayer, all the devices exhibit significant improvements of the device parameters by reducing the work function of indium tin oxide (ITO) cathode. It is striking that the devices with neutral amino molecules as interlayer exhibit remarkably higher PCEs than the devices with zwitterionic ammonium molecules as interlayer. We attribute the improved performance to the better photoactive morphology induced by the hydrophobic properties of the neutral amino derivatives through research of ultraviolet photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. Interestingly, the PCEs of the inverted PSCs with cathode interlayer are positively correlated with the hydrophobic properties of the interlayer materials, since devices with neutral amino molecules or molecules with a more hydrophobic alkyl pendant (piperidine) as interlayer exhibit higher PCEs. These results pave the way to the design of effective cathode interlayer materials.     

The use of combinatorial materials development for polymer solar cells

We use a combinatorial approach to develop molecular plastic solar cells based on soluble fullerene derivatives blended with conjugated polymers. A combinatorial way of sample preparation is well suited to deal with the multitude of possible combinations of the components of such blends. We use high mobility poly(thiophene) and poly(phenylenevinylene) derivatives to be combined with acceptors. Gradients of methanofullerene/polymer concentration were formed by diffusion of the low molecular weight component in the spin‐cast polymer matrix. Likewise the gradients of zinc phthalocyanine/C₆₀ were prepared by co‐evaporation of the two materials from two sources to make a linear array of photodiode devices. Photo‐ and electrophysical properties, such as absorption, luminescence, short circuit photocurrent and open circuit photovoltage, were measured using a specially designed installation with a resolution of 70–100 µ. Clear evidence was obtained that the photoconversion efficiency increased with the amount of methanofullerene up to very high levels, in the case of methanofullerene/polymer blends, verifying the important role of the acceptor in the photoconversion. By choosing the optimal ratio between C₆₀ and ZnPc in the evaporated layer it is possible to obtain high photocurrent in the 600–700 nm range, due to the added contribution from photoinduced electron transfer between the two molecules. Copyright © 2000 John Wiley & Sons, Ltd.     

Rational design of perylenediimide-based polymer acceptor for efficient all-polymer solar cells

Perylenediimide (PDI)-based small molecules have significantly contributed to the development of non-fullerene acceptors, whereas the development of PDI-based polymer acceptors is relatively lagging behind. In this study, we designed and synthesized two PDI-based n-type polymers named as PF-PDI and PBDT-PDI, in which PDI was used as electron-deficient unit and fluorene (F) or benzodithiophene (BDT) were used as electronrich components. The density functional theory (DFT) calculations and grazing incidence wide-angle X-ray scattering (GIWAXS) results indicate that the PF-PDI shows larger steric hindrance and relatively weaker lamellar packing than that of PBDT-PDI. Comparing with PBDT-PDI, PF-PDI shows red-shift absorption and lower-lying HOMO level, which agrees well with the DFT results. A well-known wide bandgap polymer donor, PDBT-T1 was employed to fabricate polymer solar cells (PSCs) with the two acceptors. The all polymer solar cells (all-PSCs) based on PDBT-T1:PF-PDI showed a high power conversion efficiency (PCE) of 4.47%, which is approximately 2-fold larger than that of devices with PDBT-T1:PBDT-PDI (PCE = 2.70%).     

Novel D-A-D type small-molecular hole transport materials for stable inverted perovskite solar cells

Hole transport materials (HTMs), as a critical role in the hole extraction and transportation processes, highly influence the efficiency and stability of perovskite solar cells (PSCs). Despite that several efficient dopant-free HTMs have been reported, there is still no clear structure-property relationship that could give instructions for the rational molecular design of efficient HTMs. Thus, in this work, a series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been carefully designed and synthesized. By varing the electron acceptor unit from benzene to pyridine, pyrazine and diazine, their packing structure in single crystals, optical and electronic properties have shown a great difference. While as dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13%. Moreover, the unencapsulated device based on TM-2 retains about 80% of its initial efficiency after 500 h storage in ambient environment, showing the superior stability.     

Correlation between polymer molecular weight and optimal fullerene content in efficient polymer solar cells

In this contribution, a donor-acceptor (D-A) copolymer PTP8, consisting of alternating benzodithiophene and thienopyrroledione with conjugated side-chains on both donor and acceptor units, was sucessfully prepared. We further investigated the effect of polymer molecular weight on polymer physicochemical properties, solar cell device performance, polymer-PCBM blend morphology, and, most importantly, polymer/PCBM blend ratio. We found that increasing the molecular weight of the donor polymer can both effectively improve the device performance and simultaneously stabilize solar cell efficiency over a wide range of polymer/PCBM blend ratios (from 1:0.5 to 1:1.0), which may lead to more thermally stable and cost-effective devices. Through intensive morphological investigation, we propose a sound morphological evolution for PTP8/PCBM blends with different molecular weights at low fullerene content.     

Utilizing intermixing of conjugated polymer and fullerene from sequential solution processing for efficient polymer solar cells

Efficient polymer solar cells based on poly[2, 6-(4, 4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-fluorobenzothiadiazole)] (PCPDTFBT) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are successfully fabricated by a sequential processing (SqP). With appropriate orthogonal solvent and thermal treatment, the SqP film can form an inter-diffused layer, and the SqP devices show efficient photovoltaic performance in both conventional and inverted layouts. The SqP inverted device was firstly demonstrated and the highest power conversion efficiency (PCE) of 5.84% with the enhanced J_sc of 16.4 mA cm⁻² was able to be achieved with the high internal quantum efficiency (IQE). Photoluminescence quenching shows the SqP films can provide efficient exciton quenching. X-ray photoemission spectroscopy (XPS) and ellipsometry analysis shows a polymer-rich surface in SqP films after thermal annealing. The charge mobilities in the SqP films were significantly enhanced as measured by space-charge-limited-current (SCLC) method. All these contribute to the improved photovoltaic performance in the inverted SqP device. We believe that these results inspire a new way of forming the active layer with controllable morphology, efficient charge separation and collection in polymer solar cells.     

Alcohol soluble titanium(IV) oxide bis(2,4-pentanedionate) as electron collection layer for efficient inverted polymer solar cells

We demonstrate efficient inverted polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) by using solution-processed titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) as electron collection layer (ECL) between the indium tin oxide (ITO) electrode and photoactive layer. The TOPD buffer layer was prepared by spin-coating isopropanol solution of TOPD on ITO and then baked at 140 °C for 5 min. The power conversion efficiency (PCE) of the inverted PSC with TOPD buffer layer reaches 4% under the illumination of AM1.5G, 100 mW/cm², which is increased by 76% in comparison with that (2.27%) of the inverted device without TOPD ECL. The results indicate that TOPD is a promising electron collection layer for inverted PSCs.     

可调带隙硫硒化锑薄膜及太阳电池的研究进展

硫硒化锑(Sb₂(S,Se)₃)薄膜太阳电池因其制备方法简单、原材料丰富且低毒、性能稳定等本征优势成为研究热点。目前Sb₂(S,Se)₃太阳电池最高效率已超过10%,显示出产业化潜力。Sb₂(S,Se)₃太阳电池的研究重点是提高吸光层质量和优化器件结构。首先,系统介绍了Sb₂(S,Se)₃薄膜的主流生长工艺;其次,对Sb₂(S,Se)₃太阳电池各功能层选择和渐变带隙结构设计进行分析;最后,对Sb₂(S,Se)₃太阳电池的大面积制备和其在锑基多结叠层太阳电池中的应用潜力做了进一步展望,为其产业化发展提供可行性参考。