Studying polymer/fullerene intermixing and miscibility in laterally patterned films with X-ray spectromicroscopy

Films of the fullerene derivatives [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61) BM) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71) BM) are patterned on silicon nitride membranes using photolithography to study, with X-ray spectromicroscopy, the lateral, solid-state diffusion of fullerene derivatives into conjugated polymer films. After patterning of the fullerene film, a film of conjugated polymer is laminated on top and the structure is annealed in order to study lateral intermixing and facilitate measurement of fullerene miscibility. Lateral intermixing of polymer and fullerene readily occurs for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and regiorandom poly(3-hexylthiophene) (RRa-P3HT). A 42 wt.% miscibility of PC(61) BM in PBTTT is measured, while miscibilities of 20 and 41 wt.% are measured for PC(61) BM and PC(71) BM, respectively, in RRa-P3HT, thereby demonstrating a significant difference in the miscibilities of these two fullerene derivatives. For regioregular poly(3-hexylthiophene) (RR-P3HT), incomplete lateral intermixing of fullerene and RR-P3HT is observed with PCBM crystallite formation competing with the lateral diffusion of PCBM molecules into the polymer film.     

新型窄带隙聚方酸菁的合成与光电性质

为了寻找更好的聚合物太阳能电池材料,我们首先通过Leimgruber-Batho法合成了苯并双吡咯M1及其正丁基取代的衍生物M1a,并以它们为给电子性单体,分别与吸电子性单体方酸交替共聚得到聚合物P1和P1a。用TGA、GPC表征聚合物的热稳定性和分子量,并通过紫外-可见光吸收光谱、循环伏安法研究了它们的光电性能。结果表明,两种聚合物分解温度均高于300℃,其中P1a的紫外-可见光最大吸收峰出现在597nm,带隙为1.73eV,而且其LUMO能级为-3.85eV,与最常用的有机n型材料PCBM的HOMO能级相当(-3.91eV),有可能在聚合物太阳能电池中得到应用。     

Effect of nanoscale SubPc interfacial layer on the performance of inverted polymer solar cells based on P3HT/PC71BM

The effect of a nanoscale boron subphthalocyanine chloride (SubPc) interfacial layer on the performance of inverted polymer solar cells based on poly (3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) was studied. When a 1 nm SubPc layer was introduced between the active layer (P3HT:PC(71)BM) and MoO(x) in the device with ITO/ZnO/P3HT:PC(71)BM/SubPc/MoO(x)/Al configuration, the power conversion efficiency (PCE) was increased from 3.42 (without SubPc) to 3.59%. This improvement is mainly attributed to the enhanced open-circuit voltage from 0.62 to 0.64 V. When the Flory-Huggins interaction parameters were estimated from the solubility parameters through the contact angle measurement, it revealed that the interaction between SubPc and PC(71)BM is more attractive than that between SubPc and P3HT at the interface of P3HT:PC(71)BM/SubPc, through which charges are well transported from the active layer to the anode. This is supported by a decrease of the contact resistance from 5.49 (SubPc 0 nm) to 0.94 MΩ cm (SubPc 1 nm). The photoelectron spectra provide another evidence for the enhanced PCE, exhibiting that the 1 nm thick SubPc layer extracts more photoelectrons from the active layer than other thicknesses.     

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.     

Efficient inverted bulk heterojunction photovoltaic devices using a transparent polymeric interfacial buffer layer with C60 pendant and UV curable groups

We demonstrate the synthesis of a transparent, polymeric n-type material (M1) consisting of C60 pendant and UV curable groups in side chains. This material (M1) is employed as a polymeric n-type interfacial buffer layer for an efficient inverted bulk heterojunction (BHJ) photovoltaic device based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PC61BM) active layer. Under simulated solar illumination of AM 1.5G (100 mW/cm2), the highest efficient devices fabricated with a configuration of ITO/interfacial buffer layer (M1,10 nm)/P3HT:PC61BM (1:0.9 w:w) (120 nm)/PEDOT:PSS (30 nm)/Ag (100 nm) achieve an average power conversion efficiency PCE of 2.16%, with short-circuit current J(SC) = 6.70 mA/cm2, fill factor FF = 54.2%, and open-circuit voltage V(OC) = 0.60 V. This result is comparable to the inverted BHJ photovoltaic devices fabricated with Cs2CO3, one of widely used as a buffer layer. The synthesized M1 have thus proven to be promising polymeric interfacial buffer layer for high efficient BHJ photovoltaic devices.     

Investigation of structural, optical, and electrical properties of regioregular poly(3-hexylthiophene)/fullerene blend nanocomposites for organic solar cells

We investigated the influence of thermal annealing on the structural and optical properties of two variations of organic nanocomposite materials; 1) poly [3-hexylthiophene] and [6,6]-phenyl C₆₁ butyric acid methyl ester ( P3HT:PC₆₁BM), and 2) poly [3-hexylthiophene] and [6,6]-phenyl C₇₁ butyric acid methyl ester) (P3HT:PC₇₁BM). The evolution of surface roughness and morphology was investigated using atomic force microscopy while device electrical properties were investigated by measuring current-voltage (I-V) characteristics. Upon thermal treatment, results show that P3HT:PC₇₁BM nanocomposites produce a more homogeneous mixture of finer grain size than P3HT:PC₆₁BM. Furthermore, stronger optical absorption in the visible region is observed in P3HT:PC₇₁BM compared to P3HT:PC₆₁BM. Since optical absorption is closely related to film crystallinity, it is inferred that P3HT:PC₇₁BM composites undergo more extensive crystallization upon annealing. Photoluminescence spectra of both P3HT:PC₆₁BM and P3HT:PC₇₁BM nanocomposites (dissolved in chlorobenzene) show that each has excellent quenching effects. I-V characteristic curves show that P3HT:PC₇₁BM registers higher current density under AM 1.5 illumination than does P3HT:PC₆₁BM. For the devices described in this paper having active areas of approximately 12 mm², efficiency is approximately 33% better for C₇₁-based solar cells than that observed for devices made using C₆₁ fullerene.     

Sequential processing: control of nanomorphology in bulk heterojunction solar cells

Bulk heterojunction organic photovoltaic devices based on poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT)/[6,6]-phenyl C(70) butyric acid methyl ester (PC(70)BM) can be successfully fabricated by a sequential solution deposition process. When the top layer is deposited from an appropriate cosolvent, the PC(70)BM penetrates a predeposited bottom layer of PCDTBT during the spin-casting process, resulting in an interdiffused structure with a layer-evolved bulk heterojunction (LE-BHJ) nanomorphology. The PCDTBT:PC(70)BM LE-BHJ solar cells prepared with an optimized cosolvent ratio have comparable power conversion efficiency to the conventional BHJ solar cells. The nanomorphology of the optimized PCDTBT:PC(70)BM LE-BHJ mixture was found to have better vertical connectivity than the conventional BHJ material.     

Inverted polymer solar cells with an ultrathin lithium fluoride buffer layer

An ultrathin lithium fluoride (LiF) buffer layer was applied to inverted polymer solar cells with P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. By inserting the LiF layer between the transparent electrode and the P3HT:PCBM blend film, all parameters, including the short-circuit current, the open-circuit voltage and the fill factor, were enhanced compared to those of a reference cell without the LiF layer. The power conversion efficiency of the device with the LiF layer was thereby improved by more than 300% relative to the reference cell.     

Improvement of photovoltaic properties by addition of a perylene compound in P3HT:PCBM BHJ system

The synthesized n-type perylene derivative, N,N'-bis-(4-bromophenyl)-1,6,7,12-tetrakis(4-n-butoxy-phenoxy)-3,4,9,10-perylene tetracarboxdiimide (PIBr), was applied as an additive to polymer solar cells (PSCs) with P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. Without post thermal annealing, a considerable improvement of about 98% in power conversion efficiency was achieved by the addition of 1 wt% PIBr into a P3HT:PCBM layer, when compared with that of reference cell without the additive. The results, in combination with relevant data from UV-Vis. absorption, photoluminescence, X-ray measurements and carrier mobility studies, revealed that the addition of the perylene compound within active layer contributed to more effective charge transfer and enhanced electron mobility.     

Efficient and 1,8-diiodooctane-free ternary organic solar cells fabricated via nanoscale morphology tuning using small-molecule dye additive

The ternary strategy for incorporating multiple photon-sensitive components into a single junction has emerged as an effective method for optimizing the nanoscale morphology and improving the device performance of organic solar cells (OSCs).In this study,efficient and stable ternary OSCs were achieved by introducing the small-molecule dye (5E,5'E)-5,5'-(4',4″-(1,2-diphenylethene-1,2-diyl)bis(biphenyl-4',4-diyl))bis(methan-1-yl-1-ylidene)bis(3-ethyl-2-thioxothia zolidin-4-one) (BTPERn) into poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiopheneco-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films processed using a 1,8-diiodooctane (DIO)-free solvent.The incorporation of BTPE-Rn enhanced the short-circuit current density and fill factor of the ternary OSCs compared with those of binary OSCs.An investigation of the optical,electronic,and morphological properties of the ternary blends indicated that the third component of BTPE-Rn not only promoted the photon utilization of blends through the energy-transfer process but also improved the electron mobility of the blends owing to the fullerene-rich nanophase optimization.More importantly,this ternary strategy of utilizing a small-molecule dye to replace the photounstable DIO additive enhanced the operational stability of the OSCs.     

Efficiency enhancement in DIBSQ:PC<Subscript>71</Subscript>BM organic photovoltaic cells by using Liq-doped Bphen as a cathode buffer layer

We have improved the photovoltaic performance of 2,4-bis[4-(N,Ndiisobutylamino)- 2,6-dihydroxyphenyl] squaraine:[6,6]-phenyl C71-butyric acid methyl ester (DIBSQ:PC₇₁BM) organic photovoltaic (OPV) cells via incorporating Liq-doped Bphen (Bphen-Liq) as a cathode buffer layer (CBL). Based on the Bphen-Liq CBL, a DIBSQ:PC₇₁BM OPV cell possessed an optimal power conversion efficiency of 4.90%, which was 13% and 60% higher than those of the devices with neat Bphen as CBL and without CBL, respectively. The enhancement of the device performance could be attributed to the enhanced electron mobility and improved electrode/active layer contact and thus the improved photocurrent extraction by incorporating the Bphen-Liq CBL. Light-intensity dependent device performance analysis indicates that the incorporating of the Bphen-Liq CBL can remarkably improve the charge transport of the DIBSQ:PC₇₁BM OPV cell and thus decrease the recombination losses of the device, resulting in enhanced device performance. Our finding indicates that the doped Bphen-Liq CBL has great potential for high-performance solution-processed small-molecule OPVs.     

Exploiting optical properties of P3HT:PCBM films for organic solar cells with semitransparent anode

In this study, we demonstrate optical properties of multilayer system in an organic solar cell based on poly (3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) with semitransparent anode through thermal annealing effect. The optical absorption is enhanced via optimizing annealing treatment which further elevates near-field electric field amplitude. The electric field amplitude at the interface (active layer/semitransparent anode) is enhanced after thermal annealing corresponding to effective absorption near to semitransparent anode. Moreover, the thickness of the active layer is optimized via optical thin-film model for enhancing the organic solar cell efficiency.     

Synthesis and characterization of novel D–A porphyrin-containing copolymers for polymer solar cells

A novel asymmetrical D–A zinc porphyrin derivative with dimehtyl triphenylamine (donor unit) and methyl benzoate (acceptor unit) as para-arms was first synthesized. Then, two new copolymers (P1 and P2) containing D–A zinc porphyrin derivatives were synthesized by the Stille coupling method and applied in PSCs. Their structures, photophysical and electrochemical properties were characterized by ¹H NMR, ¹³C NMR, gel permeation chromatography, thermogravimetric analysis, UV–vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The two copolymers exhibited good thermal stability and film-forming ability. The results showed that P1 containing D–A zinc porphyrin exhibits a strong absorption in the range of 400–500 nm. By the introduction of thiophene derivative with 4,7-di(4-hexylthiophen-2-yl)benzothiadiazole (T-DTBT) conjugated side-chain unit, P2 showed broader absorption in the region of 300–650 nm than P1. The photoluminescence spectra made clear that charge transfer between the whole main chain and side chain can be effective. Cyclic voltammograms revealed that the LUMO energy levels of P2 was reduced in comparison with P1 due to the introduction of electron-deficient T-DTBT conjugated side-chain unit, indicating that electron-injection and transporting properties have been improved. Polymer solar cells were fabricated based on the blend of the copolymers and methanofullerene[6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM). The PSC based on P2:PC₆₁BM (1:2, w/w) exhibited a power conversion efficiency of 1.26% under AM 1.5, 100 mW cm⁻².     

Enhancements in device efficiency of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based solar cells with incorporation of bathocuproine

The overall enhancement of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based organic solar cell with a thin layer of bathocuproine (BCP) inserted between active layers and aluminum (Al) cathodes was investigated. X-ray and ultraviolet photoemission spectroscopy show that no reaction occurs at the active layer/BCP interface and 2 nm of BCP could effectively suppress the chemical reactions between Al and active layers. Atomic force microscope images also indicate that BCP layers can provide smoother contact surfaces with Al cathodes and suppress the generation of shunt leakage, resulting in larger V_oc and better power conversion efficiency of devices.     

Preparation and characterization of high molecular weight low bandgap polymers based on poly(2,7-carbazole)s for organic solar cells

We report the PCDTBT {Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]}, an alternating copolymer of 2,7-carbazole and dithienyl-2,1,3-benzothiazole, has high molecular weight and narrow molecular weight distribution. Our PCDTBT can be successfully prepared as good yield by using tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] catalyst instead of Pd2dba3/P(o-Tol)3 catalyst. From the UV/Vis absorption spectroscopy, we can observe that absorption bands of PCDTBT are bathochromically shifted by increasing the molecular weight, that is to say, our high molecular weight PCDTBT can absorb much longer wavelength light compare to low molecular weight PCDTBT. The best performance can be obtained from device based on the mixture of PCDTBT (polymer-30) and PC70BM {[6,6]-phenyl C71-butyric acid methyl ester} (1:4) as an active layer, which shows 4.50% of PCE with 10.1 mA/cm2 of short-circuit current density (J(SC)), 0.85 V of open-circuit voltage (V(OC)), and 52.3% of fill factor which is very similar with Leclerc's published result.     

The influence of the organic/inorganic interface on the organic-inorganic hybrid solar cells

Organic-inorganic hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) hybridized with ZnO nanorods were fabricated by growing vertical ZnO nanorods on indium tin oxide (ITO) substrates and filling with bulk heterojunction polymers (P3HT:PCBM). The interface between the organic and inorganic nanostructures influences the performance of the organic-inorganic hybrid solar cells. In this paper, the influence of the state of the P3HT:PCBM/ZnO interface on the performance of organic-inorganic hybrid solar cells is examined. The solar cell performance was high when the P3HT:PCBM/ZnO junction area was large. The charge separation is effective when the active layer/electron transport layer junction area is large, resulting in increasing photocurrent and a high conversion efficiency. The bulk-heterojunction polymer concentration was kept low to infiltrate into the ZnO nanorods, resulting in a large active layer/electron transport layer junction area.     

Superiority of branched side chains in spontaneous nanowire formation: exemplified by poly(3-2-methylbutylthiophene) for high-performance solar cells

One‐dimensional nanostructures containing heterojunctions by conjugated polymers, such as nanowires, are expected to greatly facilitate efficient charge transfer in bulk‐heterojunction (BHJ) solar cells. Thus, a combined theoretical and experimental approach is pursued to explore spontaneous nanowire formation. A dissipative particle dynamics simulation is first performed to study the morphologies formed by rodlike polymers with various side‐chain structures. The results surprisingly predict that conjugated polymers with branched side chains are well suited to form thermodynamically stable nanowires. Proof of this concept is provided via the design and synthesis of a branched polymer of regioregular poly(3‐2‐methylbutylthiophene) (P3MBT), which successfully demonstrates highly dense nanowire formation free from any stringent conditions and stratagies. In BHJ solar cells fabricated using a blend of P3MBT and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM), P3MBT polymers are self‐organized into highly crystalline nanowires with a d₁₀₀ spacing of 13.30 Å. The hole mobility of the P3MBT:PC₇₁BM (1:0.5 by weight) blend film reaches 3.83 × 10^?4 cm² V^?1 s^?1, and the maximum incident photon‐to‐current efficiency reaches 68%. The results unambiguously prove the spontaneous formation of nanowires using solution‐processable conjugated polymers with branched alkyl side chains in BHJ solar cells.     

Polymer solar cells with a TiO2:Ag layer

Photovoltaic (PV) polymer solar cells with Ag and titanium dioxide were fabricated to improve the PV performance by increasing the amount of Ag in TiO₂ (by 3, 5, 7, and 10%). Sol–gel method was used to obtain amorphous or crystalline form of titanium dioxide layers. The solar cells with poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester active layer in two various positions of titanium dioxide in device were tested. Higher PV performance was received by introducing TiO₂ with 5% of Ag between ITO and PEDOT:PSS in device and by heating the layer at 130 °C. The viscosity of applied PEDOT:PSS strongly influences the values of power conversion efficiency of the constructed polymer devices with titanium dioxide.     

Probing the nanoscale phase separation and photophysics properties of low-bandgap polymer:fullerene blend film by near-field spectroscopic mapping

The effect of the additive 1,8-octanedithiol (ODT) on the nanometer-scale morphology and local photophysical properties of low-bandgap polymer blends of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b'] dithiophene)- alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl C(61) -butyric acid methyl ester (PCBM) is investigated. Phase separations of the PCPDTBT:PCBM blend film induced by ODT are visualized by the morphological changes from fibril-shaped features to spherical bumps, by the dramatically increased photoluminescence emission from PCPDTBT that was originally largely quenched, and by the fluctuations of spectral features at different locations of the sample surface. The correlations between the morphology and the local photophysical properties of the blend film with/without ODT at both the micrometer and nanometer scales are revealed by confocal and high-resolution near-field spectroscopic mapping techniques.     

An Efficient, “Burn in” Free Organic Solar Cell Employing a Nonfullerene Electron Acceptor

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PffBT4T‐2OD) as a donor polymer blended with either the nonfullerene acceptor EH‐IDTBR or the fullerene derivative, [6,6]‐phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) as electron acceptors is reported. Inverted PffBT4T‐2OD:EH‐IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH‐IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T‐2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T‐2OD:PC₇₁BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap‐assisted recombination through increased photoinduced trap states, PffBT4T‐2OD:EH‐IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T‐2OD:EH‐IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T‐2OD:PC₇₁BM devices.