The Improvement of Bulk-Heterojunction Order in Polymer Photovoltaic Device

The blend morphology and vertical arrangement are critical to the performance of organic bulk-heterojunction photovoltaic devices.In the present paper,the authors proposed a new annealing method that controls the blend morphology and vertical arrangement of two materials by means of simultaneously applying external electrical field and violet irradiation on the active layer of poly(3-hexylthiophene) (P3HT) and -phenyl C61-butyric acid methyl ester(PCBM) during annealing process.By using this annealing method,the power conversion efficiency increased by 36%,which was caused by vertical phased-separated blend of crystalline P3HT and PCBM and better charge extraction of electrodes.X-ray photoelectron spectroscopy(XPS) was measured to prove more fullerene derivatives at the organic/cathode interfaces by using this annealing method.The X-ray diffraction(XRD) analysis and UV-Vis absorption spectrum analysis also revealed more ordered polymer crystallization.

Small molecular non-fullerene electron acceptors for P3HT-based bulk-heterojunction solar cells

Three small molecules with the same arms and different cores of perylene diimide(PDI)or indaceno[2,1-b:6,5-b']dithiophene(IDT)were designed and synthesized as the acceptor materials for P3HT-based bulk-heterojunction(BHJ)solar cells.The impacts of the different cores on the optical absorption,electrochemical properties,electron mobility,film morphology,photoluminescene characteristics,and solar cell performance were thoroughly studied.The three compounds possess a broad absorption covering the wavelength range of 400–700 nm and relatively low lowest unoccupied molecular orbital(LUMO)energy levels of?3.86,?3.81 and?3.99 eV.The highest power conversion efficiency of 0.82%was achieved for the BHJ solar cells based on SM3 as the acceptor material,the compound with a PDI core.

Two-dimensional like conjugated copolymers for high efficiency bulk-heterojunction solar cell application:Band gap and energy level engineering

Three two-dimensional like conjugated copolymers PFSDCN,PFSDTA and PFSDCNIO,which consist of alternating fluorene and triphenylamine main chain,and different pendant acceptor groups (malononitrile,1,3-diethtyl-2-thiobarbituric acid and 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile) with thiophene as π-bridge,have been designed,synthesized and characterized.The structure-property relationships of the two-dimensional like conjugated copolymers were systematically investigated.The absorption spectra,band gaps,and energy levels of the polymers were effectively tuned by simply attaching different acceptor groups.As the electron-withdrawing ability of the acceptors increased,the band gaps of the polymers were narrowed from 2.05 to 1.61 eV;meanwhile,the LUMO energy levels of the polymers decreased from -3.27 to -3.75 eV,whereas their relatively deep HOMO energy levels of ～-5.35 eV were preserved.BHJ solar cells were fabricated and characterized by using the three polymers as donor materials and the highest power conversion efficiency of 2.87% was achieved for the device based on PFSDTA:(6,6)-phenyl-C71-butyric acid methyl ester blend.

An in-situ room temperature route to CuBiI_4 based bulk-heterojunction perovskite-like solar cells

Both bismuth and copper are non-toxic and earth-abundant elements suitable for lead-free halide perovskite-like photovoltaic devices. Here, we report a highly facile route for in-situ producing copper-bismuth-iodide(CuBiI4) thin films directly on ITO substrate at room temperature, by utilizing a Bi-Cu alloy layer as precursor. X-ray diffraction and transmission electron microscopy(TEM) results verified the formation of well crystallized CuBiI4 thin films with [222] orientation. The transient photovoltage(TPV) analysis revealed that the CuBiI4 is an n-type semiconductor with a suitable band gap of ~1.81 eV, preferable to photoelectric conversion compared with CH3NH3PbI3. It is very interesting that the subsequent spin-coating process of the classical Spiro-MeOTAD organic solution with TBP and acetonitrile resulted in a dense and smooth CuBiI4:SpiroMeOTAD bulk-heterojunction film. The preliminarily fabricated simple sandwich structures of ITO/CuBiI4:Spiro-MeOTAD/Au hybrid solar cell devices displayed efficient photovoltaic performance with the PCE up to 1.119% of the best sample. The room temperature direct metal surface elemental reaction(DMSER) method may provide a new insight for all-inorganic lead free perovskite-like AaBbXx compounds and high performance photovoltaic devices.

Tailoring organic bulk-heterojunction for charge extraction and spectral absorption in CsPbBr perovskite solar cells

All-inorganic CsPbBr_(3)perovskite solar cells(PSCs)are promising candidates to balance the stability and efficiency issues of organic-inorganic hybrid devices.However,the large energy barrier for charge transfer and narrow spectral response are still two challenging problems for performance improvement.We present here an organic bulkheterojunction{poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester(P3HT:PCBM)}photoactive layer to boost the charge extraction and to widen the spectral absorption,achieving an enhanced power conversion efficiency up to 8.94%by optimizing the thickness of P3HT:PCBM photoactive layer,which is much higher than 6.28%for the pristine CsPbBr_(3)device.The interaction between the carbonyl group in PCBM and unsaturated Pb atom in the perovskite surface can effectively passivate the defects and reduce charge recombination.Furthermore,the coupling effect between PCBM and P3HT widens the spectral response from 540 to 650 nm for an increased short-circuit current density.More importantly,the devices are relatively stable over 75 days upon persistent attack by 70%relative humidity in air condition.These advantages of high efficiency,excellent long-term stability,cost-effectiveness and scalability may promote the commercialization of inorganic PSCs.

Heavy metal complex containing organic/polymer materials for bulk-heterojunction photovoltaic devices

The application of heavy-metal complexes in bulk-heterojunction（BHJ） solar cells is a promising new research field which has attracted increasing attention,due to their strong spin-orbit coupling for efficient singlet to triplet intersystem crossing.This review article focuses on recent advances of heavy metal complex containing organic and polymer materials as photovoltaic donors in BHJ solar cells.Platinum-acetylide containing oligomersor and polymers have been firstly illustrated due to the good solubility,square planar structure,as well as the fairly strong Pt-Pt interaction.Then the cyclometalated Pt or Ir complex containing conjugated oligomers and polymers are presented in which the triplet organometallic compounds are embedded into the organic/polymer backbone either through cyclometalated main ligand or the auxiliary ligand.Pure triplet small molecular cyclometalated Ir complex are also briefly introduced.Besides the chemical modification,physical doping of cyclometalated heavy metal complexes as additives into the photovoltaic active layers is finally demonstrated.

Chlorophyll-based organic solar cells with improved power conversion efficiency

Chlorophylls(Chls), and associated chlorophyll derivatives, are one of the oldest, most versatile organic semiconductors found in nature. Herein, we present two easily semi-synthesized chlorophyll derivatives, namely, chlorin e6 trimethyl ester(Ce6Me3) and its copper complex(Cu–Ce6 Me3), as the p-type dopants for organic semiconductors and their impact in organic solar cells(OSCs). In our study, both Chls showed intense Soret and Q y bands in the UV-visible spectra, leading to an effect means for capturing solar light and energy. Chls also exhibited high carrier mobility owing to the partial formation of aggregates through the spin-coating process. Using Chls, we fabricated OSCs in both planar-heterojunction(PHJ) and bulkheterojunction(BHJ) solar cell configurations, together with C70/PC70 BM as electron acceptors. In PHJ solar cells, we received solar power conversion efficiencies(PCEs) of only 0.85% and 0.93% for Cu–Ce6 Me 3-and Ce6Me3-based devices, respectively, with the thickness of the donor layer at 5 nm. In BHJ cells, we achieved much higher PCEs of 1.53% and 2.05% for Cu–Ce6Me3 :PC 70 BM and Ce6Me3 :PC 70 BM respectively, where both blending ratios were set to 1:8. The improvement on PCE in BHJ cells may be attributed to the better charge separation increase at the donor–acceptor interface.

Synthesis and properties of polyfluorene copolymers bearing thiophene and porphyrin

To search for more wider absorption and higher charge carriers mobilities materials of polymer solar cell, a series of soluble alternating polyfluorene copolymers were synthesized by palladium-catalyzed Suzuki coupling reaction. Their structures were determined by 1H NMR, IR and UV-vis. And their UV-vis absorption spectra indicated that they had strong absorption over 600 nm spectral range and nearly cover 400-700 nm visible region. The band gaps of copolymers calculated according to cyclic voltammetry （CV） were between 1.96 and 2.03 eV. Polymer：TiO2 bulk-heterojunction films were made from mixtures of polymer and titanium isopropoxide, a precursor for TiO2, via hydrolysis in air overnight. The photoluminescence at 380-800 nm of the blend film of PT-TPP20 （5 mg/mL）：Ti（OC3H7）4 （80 μL/mL） （20% volume fraction） was significantly quenched in the 50% Ti（OC3H7）4 blend film, indicating that rapid and efficient separation of photoinduced electron-hole pairs.

Ternary organic solar cells based on polymer donor,polymer acceptor and PCBM components

In this work,ternary organic solar cells(OSCs)combining a fullerene derivative PC71BM with a nonfullerene acceptor N2200-F blended with a polymer donor PM6 were reported.Compared with the binary systems,the highest power conversion efficiency(PCE)of 8.11%was achieved in ternary solar cells with 30 wt%N2200-F content,mainly due to the improved short-circuit current density(Jsc)and fill factor(FF).Further studies showed that the improved Jsc could attribute to the complementary abso rption of the two acceptors and the enhanced FF was originated from the higher hole mobility and the fine-tuned morphology in the ternary system.These results demonstrate that the combination of fullere ne and nonfullerene acceptors in ternary organic solar cells is a promising approach to achieve high-performance OSCs.

High-efficiency quaternary polymer solar cells enabled with binary fullerene additives to reduce nonfullerene acceptor optical band gap and improve carriers transport

The polymer/small-molecule electron donor and nonfullerene organic electron acceptor are of structural similarity with both donor and acceptor molecules consisting of polycyclic fused-ring backbone and being decorated with alkyl-chains.In this study,we report that the introduction of binary fullerenes(C_(60)-/C_(70)-PCBM and C_(60)-/C_(70)-ICBA)into a nonfullerene binary system PBDB-T:ITIC reduces the polymer-nonfullerene acceptor intermixing,obtaining higher crystallinity with(100)crystal coherence length from 28 to 29–33 nm for the ITIC,and from 14 to 20–24 nm for the PBDB-T,and improved electron and hole mobilities both.Unprecedentedly,such a protocol reduces the ITIC optical band gap from 1.59 to 1.55 eV.As consequences,higher short-circuit current-density(17.8–18.4 vs.15.8 m A/cm^2),open-circuit voltage(0.92 vs.0.90 V)and fill-factor(0.72–0.73 vs.0.68)are simultaneously obtained,which ultimately afford higher efficient quaternary polymer solar cells with power conversion efficiencies(PCEs)up to 12.0%–12.8%comparing to the host binary device with 9.9%efficiency.For the polymer,ITIC,and ICBA/PCBM ternary blends,11%PCEs were recorded.The use of PCBM leads to larger red-shifting in thin film absorption and external quantum efficiency(EQE)response.Such effect is more pronounced when ICBA:PCBM mixture is used.These results indicate the size and shape of C_(60)and C_(70)as well as the substituent position of the second indene unit on C_(60)-/C_(70)-ICBA affect not only the blend morphology but also the electronic coupling in BHJ mixtures:the quaternary device performance increased in sequences of C_(70)-PCBM:C_(70)-ICBA→C_(70)-PCBM:C_(60)-ICBA→C_(60)-PCBM:C_(70)-ICBA→C_(60)-PCBM:C_(60)-ICBA.The resonant soft X-ray scattering(RSoXS)data indicated the most refined phase separation in the C_(60)-PCBM:C_(60)-ICBA based blend,corresponding to its best device function among the quaternary devices.These results indicate that the using of binary fullerenes as the acceptor additives allows for tuning nonfullerene blended film’s optical properties and filmmorphologies,shedding light on the designing high-performance multi-acceptor polymer solar cells.

Performance enhancement in organic solar cells and photodetectors enabled by donor phase optimization at the surface of hole transport layer

The domain purity,material crystallinity and distribution at the interface between the active layer and the transport layer have an important impact on the performance of organic solar cells(OSCs)and organic photodetectors(OPDs),while this focal issue has received less attention in previous studies.From this perspective,a new method to simultaneously enhance the performance of OSC and OPD is proposed,namely,using a sequential deposition method to first construct a compact stacking structure of dualdonor(D18-Cl:PTO2)eutectic in the donor layer,and then induce the ordered deposition of the acceptor(Y6).Compared with the conventional bulk heterojunction(BHJ),the active layer realized by this method not only improves the crystallinity and stacking order of the constituent material on the surface of the transport layer,but also regulates a good vertical distribution,which is conducive to improving the charge transport and extraction efficiency,reducing the leakage current,and enhancing the stability of the device.As a result,the OSC device based on the D18-Cl:PTO2/Y6 structure achieves a power conversion efficiency of up to 17.65%and good light-degradation stability,which is much better than that of BHJbased OSC(PCE of 16.37%).For the OPD,the dark current at reverse bias is reduced by more than an order of magnitude,and the maximum responsivity is improved to 0.52 A/W through the optimization of the donor phase at the interface.Moreover,the strategy does not require additional post-processing compared to the BHJ preparation,which reduces the device construction cost and process complexity,providing an effective way for developing high-performance organic optoelectronic devices.

The effect of end-capping groups in A-D-A type non-fullerene acceptors on device performance of organic solar cells

A series of novel wide bandgap small molecules(IFT-ECA, IFT-M, IFT-TH and IFT-IC) based on the A-D-A structure with indenofluorene core, thiophene bridge, and different electron-deficient end-capping groups, were synthesized and used as non-fullerene acceptors in organic solar cells. The influences of end-capping groups on the device performance were studied.The four materials exhibited different physical and chemical properties due to the variation of end-capping groups, which further affect the exciton dissociation, charge transport, morphology of the bulk-heterojunction films and device performance. Among them, IFT-IC-based device delivered the best power conversion efficiency of 7.16% due to proper nano-scale phase separation morphology and high electron mobility, while the devices based on the other acceptors achieved lower device performance(4.14% for IFT-TH, <1% for IFT-ECA and IFT-M). Our results indicate the importance of choosing suitable electron-withdrawing groups to construct high-performance non-fullerene acceptors based on A-D-A motif.

Synergistic effect of solvent and solid additives on morphology optimization for high-performance organic solar cells

Controlling the photoactive layer morphology towards nanoscale bi-continuous donor/acceptor interpenetrating networks is a key issue to build high-performance organic solar cells(OSCs).Due to the distinct properties between donor and acceptor materials,casting an active layer from a single solvent solution usually results in either insufficient or excessive phase separation that reduces the device performance.In comparison to the fullerene acceptors with closed-cage structures,the currently dominant non-fullerene acceptors possess the similar anisotropicπ-πinteractions with p-type organic semiconductor donors,giving rise to the complexity of the morphology regulation.Herein,we employ 4,4′-dimethoxyoctafluorobiphenyl(OFP)with strong crystallinity as a volatile solid additive to optimize the active layer morphology of OSCs.The synergistic effect of 1-chloronaphthalene(CN)and OFP as dual additives shows supreme capability on optimizing the morphology over the conventional additive of CN,which is in favor of improving charge transport and suppressing charge recombination for higher fill factors in various systems.In particular,the PTQ10:m-BTP-C6 Ph-based device processed by the additive showed a remarkable powerconversion efficiency(PCE)of 17.74%,whereas the control device processed by CN additive yielded a relatively lower PCE of16.45%.

Synthesis and properties of a novel squaraine dye modified by ferrocene

A novel squaraine dye（SQ） modified by ferrocene has been synthesized through（E）-dodecyl-2-ferrocenyl vinyl-1H-pyrrole and squaric acid.The molecular structure of SQ was characterized by ~1H NMR,^（13）C NMR,MS and elemental analysis.SQ is high soluble in common solvents.The maximum absorptions of SQ in different solvents are in the range of 708-734 nm,exhibiting positive solvatochromism with increasing solvent polarity.The optical and electrical properties of SQ indicate that it is a promising electron donor material for bulk-heterojunction organic solar cell.

Charge Carrier Dynamics in PDPP-F/PCBM Heterojunction Solar Cells

The morphology dependent of migration and recombination kinetics of charge carriers in the polymersolar cell based on poly {2,7′-9,9-dioctyfiuorene-alt-5-diethylhexyl-3,6-bis（5-bromothiophen-2-yl）pyrrolo[3,4-c]-pyrrole-1,4-dione}（PDPP-F） was investigated with photo-induced charge carrier extraction by linearly increasingvoltage technique. The recombination coefficient of charge carriers decreased and the mobility of charge carriersincreased, after the mass ratio of [6,6]-phenyl-C61-butyric acid methyl ester（PCBM） increased from 3：2 to 2：1.Meanwhile, both of them were sensitive to the applied electric field and could be together responsible for theimprovement of voltage performance of polymer solar cell.

The effect of interfacial diffusion on device performance of polymer solar cells: a quantitative view by active-layer doping

The diffusion of constituent materials at interfaces is one of the key factors for device performance and stability. In this work, the effect of interfacial diffusion of a classic interfacial material PFN on device performance of polymer solar cells was studied quantitatively by doping PFN into active layer based on P3HT：PC61BM blend. The PCEs of devices with 550 ppm PFN de- crease to half compared to those of the control devices without PFN, which are mainly attributed to the decrease of short-circuit current （Jsc） and fill factor （FF）. Advanced analyses of equivalent circuit, absorption spectra, and atomic force mi- croscopy indicates that the presence of PFN in the active layer increases the leakage current, decreases the aggregation of P3HT, and reduces the phase separation. This research reveals the physical mechanism of interfacial diffusion in device per- forrnance and provides a basis for further improving the performance and stability of PSCs.

Dipyrrin-based Complexes for Solution-processed Organic Solar Cells

Three dipyrrin-containing metal complexes and a boron dipyrromethene（BODIPY）-containing complex were designed and synthesized. The photophysical properties, electrochemical behaviours and photovoltaic performance were extensively investigated. Density functional theory calculations were also performed on those complexes These complexes, together with electron-acceptor [6,6]-phenyl-C71-butyric acid methyl ester, were utilized for the fabrication of solution-processed bulk heterojunction solar cells as the electron-donor materials. The more efficient electron acceptor BODIPY segment renders a lower energy gap and a relatively better photovoltaic conversion efficiency of 0.58%. These results prove that BODIPY segment has a great potential for constructing efficient organic solar cell materials.

Effect of Molecular Weight and Processing Additive on the Performance of Low Bandgap Polymer Solar Cells

In this work, we synthesized a low bandgap polymer polysilole（-2,6-diyl-alt-5-octylthieno[3,4-c]pyrrole-4,6- dione） （PDTSTPD） with different molecular weights （Mn）. The devices based on PDTSTPD/PC71BM composite are prepared and the dependence of power conversion efficiency （PCE） of the devices on the M,1 of conjugated poly- mers is addressed. We found the hole mobility of PDTSTPD is dependent on the Mn of the polymer, which should be the main reason contributing to the drastic difference of device performance, i.e. the PCE of the device using 10 kDa polymer is only 0.52%, in contrast to 2.3% for 24 kDa polymer device. This PCE data is then further improved to 5.0% via using 1,8-diiodoctane as processing additive to achieve an optimized morphology for the photoactive layer with an appropriate length-scale of phase separation for both exciton dissociation and charge transportation.

Mesoporous organic solar cells

The device configuration with mesoporous titanium dioxide(m-TiO2)has garnered considerable attention as a promising solution for high-stable perovskite and dye-sensitized solar cells,although its application in organic solar cells remains unexplored.In this communication,we have incorporated this structure into both bulk-heterojunction(BHJ)and single-component organic solar cells(SCOSCs).Surprisingly,mesoporous OSCs(M-OSCs)demonstrate a deteriorative efficiency in BHJ-type cells,whereas this configuration succeeds in SCOSCs,exhibiting competitive performance with planar OSCs(P-OSCs).This pioneering study has resulted in a competitive power conversion efficiency of 9.67%for m-TiO_(2)-based cells,marking a significant milestone in the advancement of OSCs.Importantly,profiting from the better ultraviolet resistance of m-TiO_(2)than zinc oxide,this M-OSC exhibits superior photostability than that of P-OSCs when subjected to continuous one-sun(AM1.5G)illumination.In its entirety,this research not only introduces the concept of M-OSCs for the first time but also unveils a novel device architecture poised to address the long-term stability concerns within the realm of OSCs.