Higher Jsc Research Articles

Effect of permodified β-cyclodextrin on the photophysical properties of poly[2,7-(9,9-dioctylfluorene)-alt-(5,5′-bithiophene)] main chain polyrotaxanes

The photophysical properties of two polyrotaxanes (PFBTh•PSβCD and PFBTh•PMeβCD) composed of fluorene and bithiophene encapsulated into permodified β-cyclodextrin cavities have been investigated and compared with those of the reference PFBTh. Rotaxane formation results in improvements of the thermal stability, solubility in common organic solvents, as well as better film forming ability combined with a high transparency. As expected PFBTh and its encapsulated forms absorb at wavelengths beyond 510 nm, and time-resolved photoluminescence (PL) in solution shows a well-define vibronic structures with a predominance of the 0-0 transitions and an energy difference of 0.16 eV. The fluorescence lifetimes follow a monoexponential decay with a value τ = 630 ± 30 ps. Atomic force microscopy, AFM, indicated a tendency of polyrotaxanes to organize into fibers. The advancing contact angles indicated higher surface hydrophobicity and lower surface free-energy values for polyrotaxanes compared with their unthreaded analogues. The device based on PFBTh•PSβCD:PCBM in a 1/1 w/w ratio under simulated AM 1.5G illumination at 100 mW cm−2 exhibited improved photovoltaic parameters of cells, resulted in high Voc (0.68 V), Jsc (1.65 mA cm−2), FF (31.6%), and PCE (0.35) values, compared with PFBTh or PFBTh•PMeβCD, respectively. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 460–471

Band-gap-graded Cu2ZnSn(S1-x,Sex)4 Solar Cells Fabricated by an Ethanol-based, Particulate Precursor Ink Route

Solution processing of earth-abundant Cu2ZnSn(S1-x,Sex)4 (CZTSSe) absorber materials is an attractive research area in the economical and large-scale deployment of photovoltaics. Here, a band-gap-graded CZTSSe thin-film solar cell with 7.1% efficiency was developed using non-toxic solvent-based ink without the involvement of complex particle synthesis, highly toxic solvents, or organic additives. Despite the high series resistance due to the presence of a thick Mo(S,Se)x layer and Zn(S,Se) aggregates, a high short-circuit current density (JSC) was generated. In addition, there was no significant difference in open circuit voltages (VOC) between CZTS (0.517 V) and CZTSSe (0.505–0.479 V) cells, despite a significant band gap change from 1.51 eV to 1.24 eV. The high JSC and less loss of VOC are attributed to the effect of band gap grading induced by Se grading in the CZTSSe absorber layer. Our environmentally benign ink approach will enable the realization of low-cost, large-area, high-efficiency thin-film solar cells.

Effect of Sintering Profiles on Titania Interparticle Connectivity, Electron Transport and Interfacial Resistance in Dye-Sensitized Solar Cells

TiO2 films, which are often sintered at 450°C for 30 or 60 minutes for application in dye-sensitized solar cells (DSSCs), show no appreciable connectivity between the TiO2 particles. The present work deals with connectivity between TiO2 particles and its effect on electron diffusion and short circuit current density (Jsc) of DSSCs made from TiO2 films sintered at lower temperature for longer time (450°C, 550°C, 650°C for 60 minutes) and higher temperature for shorter time (450°C for 60 min followed by 700°C and 800°C for 10 and 20 minutes). TiO2 films sintered at higher temperature (700°C) but for shorter time (10 min) exhibited better connectivity between the particles with slight reduction in surface area. This caused faster transport of electron through the films sintered at 700°C/10 min than 450°C/60 min and 550°C/60 min and hence, resulted in highest Jsc (~ 7 mA/cm2). Films sintered at 650°C/60 min and 700°C/20 min showed better interparticle connectivity but had significantly lower surface area, dye loading and therefore, despite faster diffusion of electron in these films Jsc was measured to be lower. Sintering at 700°C/10 min following 450°C/60 min could be considered the best in terms of dye loading, electron transport and efficiency.

Enhancement of VOC without Loss of JSC in Organic Solar Cells by Modification of Donor/Acceptor Interfaces

Organic solar cells (OSCs) are promising low‐cost devices for generating electricity. In addition to fill factor, the short circuit current density (JSC) and the open circuit voltage (VOC) are two key factors that have critical influence on the device performance. The energy levels of the donor and acceptor materials are crucial for achieving a high JSC and VOC. However, the interfacial structures between the organic materials substantially affect the JSC and VOC through the energy of the charge transfer (CT) states and the charge separation and recombination reaction kinetics. Here, it is reported that separating the donor and acceptor layer in bilayer OSCs with a thin insulating layer increases the energy of the CT state by weakening the Coulomb interaction at the interface and this also suppresses photoinduced CT and recombination. Although these effects usually increase VOC and decrease JSC, the trade‐off is avoided by doping the insulating layer with a dye to utilize the energy transfer process. The increase in VOC without the reduction in JSC enhances the conversion efficiency of the OSCs by 30%.

Temperature of InxGa1−xN/GaN solar cells with a multiple-quantum-well structure on SiCN/Si(111) substrates

The characterization of InxGa1−xN/GaN-based solar cells with a InxGa1−xN multiple-quantum-well (MQW) structure on SiCN/Si(111) substrates at various operation temperatures is reported. Solar cell operation with a low dark-current density (Jd), a high open-circuit voltage (Voc), and a high short-circuit current density (Jsc) is demonstrated. An increase in the operation temperature results in increases in Jd and Jsc, but decreases in Voc, the fill factor (FF), and the photovoltaic efficiency (η). Device configurations with various levels of indium content are investigated under an air mass 1.5 global solar spectrum. The proposed structure can be used for fabricating solar cells with a low Jd of 2.01–4.27μA/cm2, a high Voc of 2.34–2.94V, a high Jsc of 2.71–2.82mA/cm2, a high FF of 64.40–75.01%, and a high η of 4.25–5.99%.

A strategy to enhance both VOC and JSC of A–D–A type small molecules based on diketopyrrolopyrrole for high efficient organic solar cells

Four different diketopyrrolopyrrole (DPP)-based small molecules (SMs) with A–D–A type structure were synthesized, where electron-donating unit (D) was systematically varied with different electron-donating power (thiophene vs. phenylene; thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene; and biphenylene vs. naphthalene). The small molecules with weak donating unit (phenylene or naphthalene) have deeper HOMO energy levels than those with strong donating unit (thiophene or thienothiophene), and thus exhibit higher VOC. When the fused aromatic ring (thienothiophene or naphthalene) with planar molecular structure is introduced in SMs, the SMs exhibit high hole mobility and thus afford high JSC. As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both VOC and JSC, resulting in a promising power conversion efficiency of 4.4%. This result provides a valuable guideline for rational design of conjugated small molecules for high performance organic solar cells.

Enhancement of the solubility, thermal stability, and electronic properties of carbon nanotubes functionalized with MEH-PPV: a combined experimental and computational study

Multi-walled carbon nanotubes (MWCNTs) functionalized with poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MWCNT-f-MEH-PPV) nanocomposites were successfully prepared by employing a “grafting from” approach. The content of the functionalizing MEH-PPV in the composites was observed as 76 wt.%. Compared with pristine MWCNTs (p-MWCNT), the aqueous solubility and thermal stability of the former are significantly enhanced. The effect of covalently and non-covalently functionalized nanotubes on dye-sensitized solar cell performance was also studied. Solar cells were successfully fabricated from isolated MEH-PPV, p-MWCNT/MEH-PPV, and MWCNT-f-MEH-PPV/MEH-PPV counter electrodes. The devices based on a MWCNT-f-MEH-PPV/MEH-PPV counter electrode demonstrated the best photovoltaic performance as observed by higher J SC, V OC, and fill factor (FF) values. The experimental phenomena can be explained by quantum-chemical calculations: Charge transfer from MEH-PPV oligomers to nanotubes is greater when covalently functionalized compared with non-covalently functionalized. This suggests that the improvement in the photovoltaic parameters of the cells containing covalently functionalized nanotubes results not only from the higher concentration present in the nanotube films of the counter electrode, but also from the greater electron delocalization between the oligomers and nanotubes. .

Synthesis and photovoltaic properties of low band gap copolymers containing (bithiophenevinyl)-(2-pyran-4-ylidenemalononitrile) (TVM) moieties

Three novel conjugated copolymers containing coplanar bithiophenevinyl-(2-pyran-4-ylidenemalononitrile) unit linked with different electron-donating moieties by a double bond were synthesized by Heck polycondensation reactions. By introducing a double bond between the donor and acceptor unit within the backbone of polymers instead of a single bond, the conjugated length was enhanced, and consequently broader absorption spectral range and reduced band gap. Hence, the polymer can absorb more light to get higher Jsc and power-conversion efficiency.

Dye-sensitized solar cell characteristics of nanocomposite zinc ferrite working electrode: Effect of composite precursors and titania as a blocking layer on photovoltaic performance

This research investigates the performance of a zinc ferrite (ZF) as working electrodes in a dye-sensitized solar cell (DSSC). This ZF working electrode was prepared by sol–gel and thermal decomposition of four different precursors including: zinc acetate dihydrate (Zn(CH3COO)2⋅2H2O), ferric nitrate nonahydrate (Fe(NO3)3⋅9H2O), iron(III) acetate; Fe(C2H3O2)3, and zinc nitrate hexahydrate, Zn(NO3)2⋅6H2O. The effects of annealing temperature and precursors on the structural, morphological, and optical properties were investigated. The field emission scanning electron microscope images (FESEM) and scanning electron microscopy (SEM) show that ZFe films are polycrystalline in nature and homogeneous with densely packed grains. Nanoporous zinc ferrite coatings were prepared by doctor blade technique on the fluorine-doped tin oxide (FTO) and used as working electrodes in DSSC. In all DSSCs, platinized FTO and [Co(bpy)3]2+/3+ in 3-methoxy proponitrile were used as counter electrode and redox mediator system respectively. Comparing the fill factors of four different zinc ferrite nanocomposites, the highest fill factor was for ZnFe2O4–TBL sample. Cell fabricated with ZnFeA working electrode shows relatively higher Jsc.

Enhanced photovoltaic performance of cross‐linked ruthenium dye with functional cross‐linkers for dye‐sensitized solar cell

ABSTRACTPhotovoltaic performance of cross‐linkable Ru(2,2′‐bipyridine‐4,4′‐bicarboxylic acid)(4,4′‐bis((4‐vinyl benzyloxy)methyl)‐2,2′‐bipyridine)(NCS)2 (denoted as RuS dye) adsorbing on TiO2 mesoporous film was enhanced by polymerizing with either ionic liquid monomer, 1‐(2‐acryloyloxy‐ethyl)‐3‐methyl‐imidazol‐1‐ium iodide (AMImI), to form RuS‐cross‐AMImI or di‐functional acrylic monomer with ether linkage, triethyleneglycodimethacrylate (TGDMA), to form RuS‐cross‐TGDMA. Their cross‐linking properties were investigated by UV–vis spectroscopy by rinsing with 0.1 N NaOH aqueous solution. The power conversion efficiencies (PCEs) of dye‐sensitized solar cells (DSSCs) with RuS‐cross‐AMImI and RuS‐cross‐TGDMA both reached over 8% under standard global air mass 1.5 full sunlight. The increased PCE for DSSCs with RuS‐cross‐AMImI comparing with cross‐linked RuS was attributed to the I− counterion of AMImI increasing the charge regeneration rate of RuS dye, whereas that with RuS‐cross‐TGDMA was attributed to the Li+ coordination property of TGDMA. The photovoltaic performance of RuS‐cross‐TGDMA was also slightly better than that of RuS‐cross‐AMImI because of higher open‐circuit photovoltage (Voc) and short‐circuit photocurrent (Jsc). Its higher Voc was supported by the Bode plot of impedance under illumination and Nyquist plots at dark, whereas higher Jsc was supported by the incident monochromatic photon‐to‐current conversion efficiency spectra and charge extraction experiments. Copyright © 2013 John Wiley & Sons, Ltd.

Improved aluminum-doped ZnO/metal back reflector for p-i-n amorphous silicon germanium thin film solar cells

Different aluminum-doped ZnO (AZO)/metal composite films, including AZO/Al, AZO/Ag/Al and AZO/Ag/NiCr/Al, were utilized as the back reflectors for p-i-n hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells. It was found that AZO/Al had a better performance than single Al electrode and the thickness of AZO film could strongly affect the short-circuit current density (Jsc) of the solar cells. However, AZO/Ag/Al, which was expected to improve the back reflectance further, gave out a bad effect, due to the inter diffusion of Ag and Al. While a nickel–chromium (NiCr) film was inserted between Ag and Al as a diffusion barrier, the highest Jsc was achieved. So AZO/Ag/NiCr/Al could be utilized as an improved AZO/metal back reflector for p-i-n a-SiGe:H solar cells.

Dye-sensitized solar cells based on organic dyes with naphtho[2,1-b:3,4-b′]dithiophene as the conjugated linker

Naphtho[2,1-b:3,4-b′]dithiophene is used for the first time as the π-spacer for metal-free organic dyes in dye-sensitized solar cells (DSSCs). Four new donor–(π-spacer)–acceptor organic dyes using triarylamine or carbazole as the electron-donating group, cyanoacrylic acid as the electron-withdrawing anchoring moiety, and naphtho[2,1-b:3,4-b′]dithiophene as the linker were synthesized. The absorption, electrochemical, and photovoltaic properties of the dyes were investigated. DSSCs based on these dyes as sensitizers had efficiencies ranging from 3.98% to 4.60%. The highest efficiency was 4.60 ± 0.16% with a Jsc of 10.8 ± 0.4 mA cm−2, a Voc of 657 ± 5 mV, and a fill factor of 64.0 ± 0.2% under 100 mW AM 1.5 irradiation. The incident photon-to-current efficiency (IPCE) measurement showed that these dyes had high quantum efficiency over the visible range, and the long π-spacer of the molecule resulted in broader IPCE at long wavelength and high Jsc. Photovoltage transient was used to understand the electron recombination and the trends of the photovoltaic parameters (e.g. Voc and efficiency) of the DSSCs based on these organic dyes.

A pentacyclic aromatic lactam building block for efficient polymer solar cells

A D–A conjugated polymer (PThTPTI) was developed by using a brand new pentacyclic aromatic lactam acceptor unit (TPTI). PThTPTI possesses good light absorption, thermal stability, and a deep HOMO level. PThTPTI/PC71BM cells afford an outstanding PCE up to 7.80%, with high Voc (0.87 V), Jsc (13.69 mA cm−2) and FF (65.6%), and over 70% EQE in the range of 435–640 nm.

Mixed photoelectrode based on spherical TiO2 nanorod aggregates for dye-sensitized solar cells with high short-circuit photocurrent density

Mixed photoelectrode was successfully prepared by dispersing spherical TiO2 nanorod aggregates with high surface area into nanocrystallites for dye-sensitized solar cells. This type of mixed photoelectrode demonstrated a considerably higher short-circuit photocurrent density value due to the multifunctions of the spherical TiO2 nanorod aggregates, including high surface area for large dye loading, sub-micrometre size for superior light-scattering ability and one-dimensional building units for fast electron transport. After maximizing the combined dye loading, light-scattering efficiency, and electron transport property in the mixed photoelectrode by adjusting the concentrations of the spherical TiO2 nanorod aggregates, a highest Jsc of 20.36 mA cm−2 and a highest energy conversion efficiency of 6.96% were obtained when the concentration of the spherical TiO2 nanorod aggregates was 40 wt%.

Effect of the nature of the anode buffer layer – MoO3, CuI or MoO3/CuI – on the performances of organic solar cells based on oligothiophene thin films deposited by sublimation

An oligothiophene having a donor-acceptor-donor chromophore with hydrogen bonding groups is used as electron donor in planar heterojunction organic photovoltaic cells. We focus on the contact between the anode and the oligothiophene. Different anode buffer layers (ABLs) have been used, MoO3 and CuI, alone or coupled with MoO3. The thicknesses were 4 nm and 3 nm for MoO3 and CuI respectively. It is shown that the ABL improves the cells performances. The best results are achieved with the couple MoO3/CuI through an increase of the open circuit voltage and short circuit current. The optical absorption, the surface roughness and the organic film conductivity depend on the ABL. The conductivity of the oligothiophene film is one order of magnitude higher when the ABL is a CuI film. The influence of the ABL can be explained partly by the fact that it raises the anode work function. Nevertheless, the study of the structures ITO/ABL/oligothiophene shows that each ABL exhibits specific advantages and disadvantages. Therefore the couple MoO3/CuI allows summing up the advantages of both ABLs, MoO3 allows a very good band matching and avoids too high leakage current, while CuI allows achieving high Jsc thanks to its effect on the TTB conductivity.

Synthesis, characterization, and photovoltaic properties of diketopyrrolopyrrole-oligothiophene/fullerene dyads

Low-band-gap dyad molecules containing diketopyrrolopyrole (DPP) chromophore and hexathiophene were designed and synthesized in combination with C60 and C70 derivatives as the electron acceptors. The effect of alkyl side chains in the donor parts on film morphology, thermal properties, and optical properties were investigated by X-ray diffraction analysis, differential scanning calorimetry, UV–vis absorption spectroscopy, and fluorescence spectroscopy. The donor part with a shorter alkyl side chain showed better packing of the π-plane and an enhancement of absorption in the longer-wavelength region, which led to higher JSC of organic solar cells. The use of C70 as the electron acceptor led to stronger absorption in the visible region, resulting in improved external quantum efficiency and JSC compared with those of the C60 derivatives. The single-component solar cell of the dyad with the methyl side chains and C70 derivative showed the power conversion efficiency of 0.84% with a relatively high JSC of 4.4mAcm−2 under AM 1.5 illumination (100mWcm−2).

A Selenophene Analogue of PCDTBT: Selective Fine-Tuning of LUMO to Lower of the Bandgap for Efficient Polymer Solar Cells

In an attempt to further improve the performance of the PCDTBT-based polymer solar cells (PSCs), we have synthesized a selenophene analogue of PCDTBT, namely, PCDSeBT, in which diselenienylbenzothiadiazole (DSeBT) monomer alternately flanks with a 2,7-carbazole unit. The intrinsic properties of PCDSeBT are not only characterized by UV–vis absorption, cyclic voltammetry (CV), and organic field-effect transistors (OFETs) but also the surface morphology, mobilities of space charge-limited current (SCLC) model, and polymer solar cells (PSCs) in its bulk-heterojunction (BHJ) active layer with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) are evaluated in detail. It is found that PCDSeBT simultaneously has a low-lying highest occupied molecular orbital (HOMO) energy level at −5.4 eV and a low bandgap of 1.70 eV as required by the ideal polymers for BHJ PSCs. The high current of 11.7 mA/cm2 is obtained for PCDSeBT-based PSCs, to our knowledge, which is among the highest short-circuit current density (JSC) values obtained from a BHJ device consisting of PCDTBT derivatives and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The high JSC value, along with a moderate fill factor (FF) of 45% and a high open-circuit voltage (VOC) of 0.79 V, yields a power conversion efficiency (PCE) of 4.12%, which is about 37% increase in PCE from a PCDTBT-based reference device. On the basis of our results, one can be concluded that the DSeBT placement for construction of donor (D)-acceptor (A) polymers is an easy and effective way to realize both the higher JSC and VOC values in PSCs, as a consequence of the selective lower-lying lowest unoccupied molecular orbital (LUMO) with the HOMO being almost unchanged, together with the effective broadening on the absorption band.

A study of ZnO:B films for thin film silicon solar cells

Boron doped zinc oxide (ZnO:B) films with different thicknesses were prepared with low pressure chemical vapor deposition (LPCVD) technique and implemented in thin film silicon solar cells as front and back electrodes. It is found that thick back ZnO:B film electrode in thin film silicon solar cells leads to a high fill factors (FF), which is attributed to an improvement of the electrical properties of the thick ZnO:B films, and in the meanwhile a slightly low short circuit currents (Jsc) due to a high light absorption in the thick back ZnO:B films. Differently, the thicker front ZnO:B film electrodes result in a high Jsc but a low FF of solar cells compared to the thinner ones. The low FF of the solar cells may be caused by the local shunt originated from the pinholes or by the cracks (zones of non-dense material) formed in particular in microcrystalline silicon materials deposited on rough front ZnO:B films. As to the high Jsc, it is expected to be due to a good light trapping effect inside solar cells grown on rough front ZnO:B films. Moreover, the application of high reflective polyvinyl butyral (PVB) foils effectively enhances the utilization of incident light in solar cells. By optimizing deposition process of the ZnO:B films, high efficiencies of 8.8% and 10% for single junction thin film amorphous silicon solar cells (a-Si:H, intrinsic layer thickness<200nm) and amorphous/microcrystalline silicon tandem solar cells (a-Si:H/μc-Si:H, intrinsic amorphous silicon layer thickness<220nm), respectively, are achieved.

Comparison of AZO, GZO, and AGZO Thin Films TCOs Applied for a-Si Solar Cells

This study prepared aluminum and gallium co-doped zinc oxide (AGZO) thin films using pulsed direct current magnetron sputtering, and studied the electrical properties, transmittance, effects of texturing, and applicability as the front contact for a-Si:H solar cells. Textured ZnO:Al (AZO) and ZnO:Ga (GZO) thin films were also compared with AGZO thin film to evaluate their performance as front contacts. Experimental results show that AGZO films with the highest figure of merit ϕTC value (24.64 × 10−3 Ω−1) were obtained at Al and Ga doping concentrations of 0.54 wt% and 1.165 wt%, respectively. The textured AGZO films used as the front contact in a-Si:H solar cells achieved the following results: VOC = 0.77 V, JSC = 16.5 mA/cm2, FF = 59%, and conversion efficiency of 7.53%. AZO and GZO films have better electrical properties than AGZO film; however, AGZO film has superior transmittance in the near-infrared (NIR) region and higher JSC, conversion efficiency and external quantum efficiency (EQE) than AZO and GZO films under a similar haze value.

Highly efficient and stable dye-sensitized solar cells based on SnO2nanocrystals prepared by microwave-assisted synthesis

Highly efficient dye-sensitized solar cells (DSSCs) with excellent long-term stability were fabricated based on tin(IV) oxide (SnO2) nanocrystals with tunable morphologies and band energy levels. The nanocrystals were prepared by a facile, fast, and energy-saving microwave-assisted solvothermal reaction. Through variation of the precursor base used during nanocrystal synthesis control over morphology was achieved—precursor metal cations are known to have a strong influence on the growth process of SnO2 nanostructures. A simple and economic way to prepare semiconducting pastes for photoanodes was devised. The photovoltaic performance of dye-sensitized solar cells based on SnO2 photoanodes was investigated. A very high power conversion efficiency of up to 3.2%, based on very high Voc and comparable Jsc and FF [under 1 Sun condition (AM 1.5, 100 mW cm−2, with shading masks)] was achieved, reporting the highest efficiency value for the cells based on unmodified SnO2 nanocrystals so far. In order to elucidate the efficient cell behavior, electrochemical properties such as the charge transport in the photoanodes as well as SnO2/electrolyte interfacial properties were investigated. Uncharacteristically for DSSCs, all devices tested in the present study show an unusual long-term stability under ambient conditions over several weeks.