Synthesis and photovoltaic properties of novel PPV-derivatives tethered with spiro-bifluorene unit for polymer solar cells

New thermally robust photoactive arylenevinylene-based conjugated polymers, poly[3,6-bis(3,7-dimethyloctyloxy)-9,9-spirobifluorenyl-2,7-vinylene] [(OC₁₀)₂-spiro-PFV] and poly[{3,6-bis(3,7-dimethyloctyloxy)-9,9-spirobifluorenyl-2,7-vinylene}-co-2-{methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene}] [(OC₁₀)₂-spiro-PFV-co-MEH-PPV], were synthesized and used to fabricate polymer solar cells. Bulk heterojunction solar cells fabricated by blending the new copolymers, spiro-PFV and (OC₁₀)₂-spiro-PFV-co-MEH-PPV, as an electron donor with the fullerene derivative, [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) as an electron acceptor. The effects of electron donor to acceptor ratio, thickness of photoactive layer, and the cathode structures on the power conversion efficiency (PCE) in polymer solar cells were studied. The copolymer feed ratio was found to have a considerable effect on the PCE. The maximum PCE of 1.30% was achieved with (OC₁₀)₂-spiro-PFV-co-MEH-PPV.     

Temperature dependence of open-circuit voltage and recombination processes in polymer-fullerene based solar cells

In this article, we have studied the temperature and illumination dependence of open-circuit voltage (V_OC) in polymer-fullerene based solar cells. It has been observed that V_OC at higher illumination intensities gets converged at 0 K which gives information about maximum achievable V_OC in a particular donor-acceptor blend. Besides this, recombination processes have been studied by transient open-circuit voltage decay (TOCVD) and the transition between recombination regimes has been observed for the first time. At low V_OC carrier lifetime exhibits a constant value around 500 μs, which is interpreted in terms of a monomolecular recombination regime. At higher V_OC carrier lifetime decreases as derived from a bimolecular relaxation law. The method allows estimating the recombination coefficient, which results in 2×10⁻¹³ cm³ s⁻¹. The results have been explained by considering Gaussian density-of-states (DOS) for highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO).     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Molecular engineering of Diketopyrrolopyrrole-based photosensitizer for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, DPP-bis[ter-3HT-TPA] and DPP-bis[ter-3HT] as novel derivatives of Diketopyrrolopyrrole-thiophene with/without triphenylamine (TPA) end group have been synthesized and characterized. And the effects of TPA moiety were investigated. Compared to DPP-bis[ter-3HT], DPP-bis[ter-3HT-TPA] shows red-shifted absorption and significantly higher molar absorption coefficient. And the HOMO level of DPP-bis[ter-3HT-TPA] is elevated than DPP-bis[ter-3HT]. Moreover, DPP-bis[ter-3HT-TPA] exhibited one order higher hole mobility than DPP-bis[ter-3HT], suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices with DPP-bis[ter-3HT-TPA] showed better efficiencies than DPP-bis[ter-3HT], showing the best power-conversion efficiency (PCE) of 1.5% (±0.12) under 100 mW/cm² with a short-circuit current (J_sc)=5.73 mA/cm², a fill factor (FF)=0.45, and an open-circuit voltage (V_oc)=0.59 mV.     

High-performance polymer photovoltaic devices with inverted structure prepared by thermal lamination

Use of a lamination process for the introduction of Au electrode, instead of conventional metal evaporation, improved the power conversion efficiency (PCE) of inverted-structure photovoltaic devices from 1.6% to 2.6% based on a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C₆₁ butyric acid methyl ester (PCBM). X-ray photoelectron spectroscopy indicated that a thin layer of P3HT is spontaneously formed at the air/polymer blend layer interface during the spin-coating process. It is suggested that the vacuum-evaporated gold could destroy the surface-segregated thin layer of P3HT, while the lamination process preserves the surface structure working as an electron-blocking layer. The insertion of a PEDOT:PSS layer, between the metal electrode and polymer layer, in the lamination process further improved the PCE to 3.3% with a short-circuit current density of 9.94 mA cm⁻², an open-circuit voltage of 0.60 V, and a fill factor of 55% under AM1.5 100 mW cm⁻² irradiation.     

Tuning the LUMO level of the acceptor to increase the open-circuit voltage of polymer-fullerene solar cells: A quantum chemical study

In the present study, semiempirical and density functional molecular orbital calculations are performed on fullerene derivatives with varying reduction potentials, successfully used as an electron acceptor in bulk heterojunction solar cells. The geometries of all the compounds were optimized with the semiempirical PM3 method. Density functional theory (DFT) single-point calculations, B3LYP/3-21G*, have been carried out with the aim to investigate the energy levels of the frontier orbitals. We have correlated the theoretical lowest unoccupied molecular orbital (LUMO) levels of different fullerenes with the open-circuit voltage of the photovoltaic device based on the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]–1,4-phenylenevinylene (MDMO–PPV) with the acceptor molecules. We have also investigated the influence of new substituents on the LUMO level of the parent fullerene showing the possibility to further increase the open-circuit voltage of the MDMO–PPV:fullerene device.     

Air-stable triarylamine-based amorphous polymer as donor material for bulk-heterojunction organic solar cells

A new air-stable triarylamine-based amorphous polymer, TSP-T11, which consists of thiophene and triarylamine units, can be successfully utilized to fabricate bulk-heterojunction organic photovoltaics (OPVs) using PC₆₀BM or PC₇₀BM as acceptor materials. The highest level of performance of OPVs optimized at TSP-T11:PC₇₀BM (weight ratios of 1:4) with thicknesses of 68 nm exhibited an open circuit voltage (V_oc) of 0.75 V, a short circuit current (J_sc) of 8.03 mA cm⁻², and a power-conversion efficiency (PCE) of 2.22% under simulated air mass 1.5 solar irradiation at 100 mW cm⁻². Although TSP-T11 has a lower hole mobility (1.5×10⁻⁴ cm² V⁻¹ s⁻¹) than P3HT, the use of amorphous film of TSP-T11 as a donor material for OPVs offers advantages over the use of polycrystalline film of P3HT in terms of its air-stability and pinhole-free homogeneous morphology.     

Higher fullerenes as electron acceptors for polymer solar cells: A quantum chemical study

In the present study, we have used quantum chemical methods to study the energy levels of the frontier orbitals of higher fullerene derivatives (from C₇₀ to C₈₄ and having the same addend as in [6,6]-phenyl C₆₁-butyric acid methyl ester) with the aim to understand if they can be used as electron acceptors in bulk heterojunction polymer–fullerene solar cells. Higher fullerenes have a stronger and broader absorption compared to C₆₀ and they can improve the current output of the corresponding devices. The geometries of all the compounds were optimized with the density functional theory at the B3LYP/3-21G* level of calculation. The lowest unoccupied molecular orbital (LUMO) levels of the investigated compounds correlate well with the reduction potentials (obtained by cyclic voltammetry) of the already prepared species. We found that the LUMO level depends not only on the fullerene size (number of carbons of the cage) and constitutional isomer, but also on the position and, in some cases, the addend orientation. This issue should be considered because for a proper device operation, a well-defined LUMO is required. The position of the LUMO level of some higher fullerene derivatives can be suitable for low-bandgap polymers.     

Patternable polymer bulk heterojunction photovoltaic cells on plastic by rotogravure printing

We study the fabrication of poly(3-hexylthiophene)—P3HT and [6,6]-phenyl-C61 butyric acid methyl ester—PCBM based polymer bulk heterojunction photovoltaic cells using rotogravure printing. By studying the dependencies of device performance on material and process parameters including contact angles, ink concentrations, ink viscosities, solvent characteristics, and gravure printing parameters, optimized hole transport layers [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)—PEDOT:PSS] and active layers (P3HT:PCBM) are printed, resulting in devices with power conversion efficiencies as high as 1.68% under AM 1.5 G and a spectrally matched intensity of 100 mW/cm².     

Novel symmetric squaraine chromophore containing triphenylamine for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, SQ-bis[HP-HT2-TPA] and SQ-bis[HP-HT2-BT] incorporating as novel derivative of squaraine-pyrrole framework and π-extended thiophene with triphenyamine (TPA) and benzothiophene (BT) end group, have been synthesized and characterized. The effects of TPA moiety were investigated. Compared to SQ-bis[HP-HT2-BT], SQ-bis[HP-HT2-TPA] exhibited three times improved transporting property of hole carrier and four times enhanced absorptivity by more efficient intermolecular π−π interaction for high-efficiency bulk heterojunction (BHJ) device, suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices fabricated with SQ-bis[HP-HT2-TPA]/C₇₁-PCBM BHJ film had an average power-conversion efficiency of 1.83%(±0.12) under 100 mW/cm², with a short-circuit current (J_sc) of 9.32 mA/cm², fill factor (FF) of 0.30, and open-circuit voltage (V_oc) of 0.65 V, which has ∼42% higher efficiency compared to SQ-bis[HP-HT2-BT]/C₇₁-PCBM BHJ films.     

Gold nanomesh induced surface plasmon for photocurrent enhancement in a polymer solar cell

In this study, a gold (Au) nanomesh layer was manufactured on an ITO-coated glass substrate at room temperature. The Au nanomesh was used to induce surface plasmon resonance (SPR) to enhance the photocurrent of a polymer solar cell. The Au nanomesh was manufactured by lift-off on closely packed PS nanospheres (diameter ∼50 nm; density ∼10¹⁰/cm²). The PS nanospheres were fabricated by modified block copolymer nano-patterning on ITO. A transmittance-reflection-absorbance spectrum was used to explore the induced surface plasmon. An extinction peak was observed at ∼580 nm indicate the possibility of Au nanomesh induced surface plasmon resonance. The short-circuit current density of the polymer solar cell was enhanced from 7.02 to 14.2 mA/cm² by the addition of Au nanomesh. Consequently, the power conversion efficiency enhanced from 1.9% to 3.2%. By the normalized input photon-to-current conversion efficiency (IPCE) measurement, enhanced photocurrent conversion efficiency at approximately 580 nm was observed that coincided with the extinction spectrum, indicating that the surface plasmon enhanced the photocurrent.     

Effect of solvents in liquid electrolyte on the photovoltaic performance of dye-sensitized solar cells

The effect of solvents in liquid electrolyte on the photovoltaic performance of dye-sensitized solar cells was investigated. The solvents with large donor number enhanced the open-circuit voltage but reduced the short-circuit current density. By mixing 30 vol.% NMP with 70 vol.% GBL, the open-circuit voltage increased from 0.55 to 0.632 V and the fill factor increased from 0.607 to 0.613 while the short-circuit current density decreased little. The further addition of 0.4 M pyridine into the above mixed solvent caused a huge increase of overall conversion efficiency from 5.73 to 6.70% under irradiation of 100 mW cm⁻².     

A new protocol for characterization of dislocations in photovoltaic polycrystalline silicon solar cells

Main photovoltaic properties of polycrystalline silicon solar cells are often affected by dislocation effects. Dislocations degrade functional photocurrent and considerably alter relevant parameters such as short-circuit current density, dark current intensity and open-circuit voltage. In this study, we have developed an enhanced photothermal technical protocol for diagnosing dislocation spatial distribution inside photovoltaic polycrystalline silicon solar cells. We tried to establish a qualitative and quantitative correlation between the local thermal properties alteration and dislocation spatial range. Experimental imaging profiles, yielded by this technique are compared to other diagnostic techniques results.     

Poly(thiophenes) derivatized with oligo(oxyethylene) chains as donor materials for organic solar cells

Regioregular poly[3-(3,6-dioxaheptyl)thiophene] (PDHT) has been synthesized by Grignard metathesis (GRIM). The UV-vis absorption spectrum of spin-cast films of PDHT presents a well-resolved vibrational fine structure typical of well-ordered polymers. The characterization of bulk heterojunction solar cells fabricated using PCBM as acceptor shows that PDHT leads to power conversion efficiencies in ca 4-5 times lower than that obtained with regioregular poly(3-hexylthiophene) (P3HT). In contrast, the characterization of bilayer solar cells realized by thermal evaporation of C₆₀ on spin-cast polymer films shows that PDHT leads to performances similar or even better than P3HT both before and after thermal treatment.     

Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells

A new class of triphenylamine substituted methanofullerene derivatives, bis(4'-(diphenylamino)biphenyl-4-yl)methanofullerene (1) and the bisadduct (2), were synthesized. The incident photon to current efficiency (IPCE) studies revealed that the diphenylamino components have contribution to the photocurrent that expands the light harvesting window around 400 nm. When being blended with poly (3-hexylthiophene) (P3HT) to fabricate the solar cell, the device of P3HT:1 (1:0.7) shows high open circuit voltage (V_oc) of 0.69 V under the illumination of AM 1.5, 100 mW/cm² with high power conversion efficiency (PCE) of 3.16%, which is about 0.1 V higher than that of the corresponding [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) devices. This indicates that the arylamine substituents on 1 have played some special roles on the high V_oc performance. Similar effects are also observed for 2. The device of P3HT:2 (1:1) shows even higher V_oc of 0.87 V with the PCE of 1.83%. These results indicate that 1 and 2 are alternative high performance acceptors.     

p-OXA-X: A new Oligo photosensitizer for organic solar cells

A two-dimensional conjugated oligo p-phenylenevinylene-based photoactive electron donor (p-OXA-X) has been utilized for harvesting incident light and transferring electrons to generate charge carriers in PCBM-based solar cells demonstrating AM1.5 efficiencies of 0.6% and IPCE values of 27% at 400 nm. Nanoscale morphology variations induced in the photoactive layer by varied processing conditions were studied by a combination of SEM, AFM, and profilometry showing that thinner films with minimal surface roughness provided the best photovoltaic performance.     

Influence of alkylpyridine additives in electrolyte solution on the performance of dye-sensitized solar cell

The influence of alkylpyridines additive to an I⁻/I₃⁻ redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (J_sc), but most of the alkylpyridines increased the open-circuit photovoltage (V_oc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the J_sc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the V_oc of the cell. Under AM 1.5 (100 mW/cm²), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine.     

Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH₂ and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH₂ and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO₃ hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH₂ and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO₃ structure, and a high V_oc of 1.27 V has been achieved.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

Anode engineering for photocurrent enhancement in a polymer solar cell and applied on plastic substrate

In this work, a multilayer structure, PEDOT:PSS/insulator/PEDOT:PSS (CIC), was designed and used as the anode in a polymer solar cell (PSC) to enhance the efficiency at low annealing temperature. The efficiency for PSC with CIC multilayers could increase around 22% as compared to the reference cell. The internal electrical field enhancement due to the effective work function increase by CIC multilayer was assumed and responded to efficiency enhancement. The work function of the multilayer anode structure was explored by an electrostatic force microscopy (EFM) analysis. The EFM result shows that the surface potential of PEDPT:PSS in CIC structure is around 0.6 V higher than PEDOT:PSS in reference structure, indicating a higher work function for PEDOT:PSS in multilayer structure. By the input photon-to-current conversion efficiency (IPCE) study, the major enhancement in photocurrent occurred at solar spectrum range of 400-650 nm. Further applied to plastic substrate, the PSC exhibits 9.2% enhancement in efficiency.