Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

New organic dyes containing fluorene appended dithienopyrrole as electron rich linker, different arylamine/heterocyclic units as conjugating donors and cyanoacrylic acid as acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The effect of different conjugated donors such as triarylamine, carbazole and phenothiazine on the photophysical, electrochemical and photovoltaic properties is investigated. The optical and electrochemical properties of the dyes are strongly influenced by conjugating donors. The dye containing phenothiazine donor exhibited longer wavelength absorption and low oxidation potential. The time dependent density functional calculations performed on the dye models reveal charge transfer character for the longer wavelength absorption. The dye-sensitized solar cells fabricated using a dye containing fluorenyldiphenylamine donor displayed highest power conversion efficiency (6.81%) in the series originating from the high short circuit current density (J_SC = 14.01 mA cm⁻²) and high open circuit voltage (V_OC = 738 mV).     

Improved performance of nanoporous TiO2 film in dye-sensitized solar cells via ZrCl4 and TiCl4 surface co-modifications

In this study, nanoporous TiO₂ films were modified by a dip-coating process using a mixture aqueous solution of ZrCl₄ and TiCl₄ followed by calcination to prepare a photoanode for dye-sensitized solar cells. Compared with the control film modified with 0.04 mol L⁻¹ TiCl₄, the power conversion efficiency of the TiO₂ film modified with a mixed solution of 0.05 mol L⁻¹ ZrCl₄ and 0.04 mol L⁻¹ TiCl₄, was 18.67% higher because of the improved short circuit current (J_sc) and open circuit voltage (V_oc). The improvement in J_sc was due to the suppression of charge recombination, which was demonstrated by a series of measurements, including electrochemical impedance spectroscopy, monochromatic incident photon-to-electron conversion efficiency spectroscopy, and the open-circuit voltage decay technique. The Mott-Schottky measurement results indicated that the negative shift of a flat band led to the increased V_oc.     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

Sb2(SxSe1−x)3 sensitized solar cells prepared by solution deposition methods

Solid state semiconductor sensitized solar cells are a very active research subject in emerging photovoltaic technologies. In this work, heterojunctions of antimony sulfide-selenide (Sb₂(S_xSe_1−x)₃) solid solution as the absorbing material and cadmium sulfide coated titanium dioxide (TiO₂/CdS) as the electron conductor have been developed with solution deposition methods such as spin-coating, successive ionic layer adsorption and reaction (SILAR), and chemical bath deposition. In particular, CdS has been deposited on mesoporous TiO₂ layers by SILAR deposition, followed by the chemical deposition of Sb₂(S_xSe_1−x)₃. It was found that by increasing the number of CdS SILAR deposition, both the open circuit voltage V_oc and the short circuit current density J_sc of the Sb₂(S_xSe_1−x)₃ sensitized solar cells had been increased from 153 to 434 mV and 0.77–9.73 mA/cm², respectively. This improvement was attributed to the fact that the presence of the CdS on TiO₂ surface reduces the formation of undesired Sb₂O₃ and promotes a better nucleation of the Sb₂(S_xSe_1−x)₃ during the chemical bath deposition. The best result was obtained for the solar cell with 30 cycles of CdS which produced a V_oc of 434 mV, a J_sc of 9.73 mA/cm², and a power conversion efficiency of 1.69% under AM1.5 G solar radiation.     

Large-area organic solar cells by accelerated blade coating

Large-area photovoltaic devices have been fabricated using the blade coating technique. In this study, the use of accelerated blade motion in this technique significantly improved the thickness uniformity of blade-coated layers of polymer solar cells on an A4 glass substrate. Two types of active layers, P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, were studied. For the P3HT:PC₆₁BM film, a thickness of 221 ± 14 nm was realised in a 12 × 15 cm² active region with a coating blade acceleration of 8 mm/s². For the POD2T-DTBT:PC₇₁BM film, a thickness of 98 ± 6 nm was realised with a coating blade acceleration of 10 mm/s². Ten cells, each measuring 0.9 cm × 12 cm and monolithically fabricated, were connected in series, yielding a total active area of 108 cm². The power conversion efficiency of the resulting 10-cell module was 2.66% and 3.64% for P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, respectively. The blade coating technique involving the accelerated blade motion is therefore useful for fabricating low-cost large-area organic solar cells, and it may be a promising alternative for the commercialisation of organic solar cells.     

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.     

Fabrication of inverted pyramid structure by Cesium Chloride self-assembly lithography for silicon solar cell

Inverted pyramids were fabricated through a method combining cesium chloride (CsCl) self-assembly technology and anisotropy corrosion of silicon solar cells. Ti film with nanoporous masks was formed by lift-off the CsCl nanoislands for the inverted pyramids. The pyramids were then formed by anisotropy corrosion of alkaline solution. The average diameter and morphology of the pyramids were controlled by varying the average diameter of CsCl nanoislands from 400 nm to 1.5 µm and by varying the etching time of alkaline solution from 2 to 8 min. The inverted-pyramid texture could suppress reflection to below 10% at wavelengths from 400 to 1000 nm, which was much lower than that of planar wafer. A solar cell fabricated from the pyramids had higher short-circuit current density (J_sc) and photovoltaic conversion efficiency (PCE) compared with those of planar solar cells for the good antireflection property. The solar cell showed a PCE of 15.25%, a J_sc of 38.35 mA/cm², and an open-circuit voltage of 555.7 mV.     

Improvement in the light conversion efficiency of silicon solar cells by pure hydrogen annealing

In this report, the effects of pure hydrogen gas annealing on series resistance (R_s), shunt resistance (R_sh), open circuit voltage (V_oc), short circuit current (I_sc), fill factor, and efficiency were investigated systematically using standard, commercially available polysilicon solar cells. Improvements on the electrical characteristics, fill factors, and efficiency of the solar cells were observed after annealing by pure hydrogen gas at 350 °C for 15 min. In the best case, the conversion efficiency was raised by nearly 1% point. Judging from our experimental evidences, the improvement on cell performance could be mostly attributed to the reduction of R_s and improvement in Ag grid/emitter contact resistance in the cells during the annealing process.     

Improved efficiency of multicrystalline silicon solar cells by TiO2 antireflection coatings derived by APCVD process

This paper reports about the adaptation of the chemical vapor deposition (CVD) thin films technology to the fabrication process of multicrystalline silicon solar cells as a simple, low cost and very effective technology for efficiency device improvement by reducing reflection and improving the light-generated current. In this contribution, the higher reflection of a mc-Si solar cell surface is strongly reduced by the deposition of TiO₂ antireflection coating (ARC) on the front using the atmospheric pressure chemical vapor deposition method (APCVD). The surface morphology and elemental composition of the TiO₂ antireflective layers were revealed using scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy . The reflectivity was then reduced from 35% to 8.6% leading to the increase of the short circuit current J_sc which was 33.86 mA/cm² with a benefit of 5.23 mA/cm² (surface area=25 cm²) compared to the reference cell (without ARC). This simple and low cost technology induces a 14.26% conversion efficiency which is a gain of +3% absolute in comparison to the reference cell. The LBIC measurements of a typical multicrystalline cell confirmed the uniformity of the photocurrent distribution throughout the device. These results are encouraging and prove the effectiveness of the APCVD method for efficiency enhancement in silicon solar cells.     

Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

This paper reports the size-dependent performance in polymer/CuInS₂ solar cells with tunable synthesis of chalcopyrite CuInS₂ quantum dots (QDs) by the solvothermal method. The CuInS₂ QDs of 3.2–5.4 nm in size are fine tuned by the reaction time in the solvothermal process with the slow supply of In³⁺ ions during the crystallization, and the band gaps increased with QDs sizes decreasing according to the results from the characterization of sizes, morphologies, component elements, valence states and band gaps of CuInS₂ QDs. We fabricated MEH-PPV/CuInS₂ solar cells, and the photoactive layer of device displayed size-dependent light-harvesting, charge separation and transport ability. Moreover, the solar cells exhibit size-dependent short circuit current (J_sc) and open circuit voltage (V_oc), with higher performance in both J_sc and V_oc for smaller CuInS₂ QDs, resulting in the maximum power conversion efficiency of ca. 0.12% under the monochromic illumination at 470 nm; CuInS₂ QDs actually serve as an effective electron acceptor material for the MEH-PPV/CuInS₂ solar cells with the wide spectral response extending from 300 to 900 nm.     

Novel pyrene-based donor–acceptor organic dyes for solar cell application

Pyrene is an alternant polycyclic aromatic hydrocarbon consisting of four fused benzene rings with a large, flat aromatic system, showing high thermal stability, extensive electron delocalization and electron accepted nature. In this work, pyrene was firstly employed as π-conjugated bridge to construct electron donor–π–electron acceptor (D–π–A) organic dyes, where diarylamine or indoline was used as donor, and cyanoacrylic acid as electron acceptor. The peryne-based dyes were employed as sensitizers in dye-sensitized solar cells, and give a short circuit photocurrent density (J_sc) of 12.1 mA/cm², an open circuit voltage (V_oc) of 0.71 V, and a fill factor (FF) of 0.71, corresponding to an overall conversion efficiency (η) of 6.1% under AM 1.5 conditions.     

A star-shaped oligothiophene with triphenylamine as core and octyl cyanoacetate as end groups for solution-processed organic solar cells

A new star-shaped D–π–A molecule with triphenylamine (TPA) as core and donor unit, octyl cyanoacetate (CA) as end group and acceptor unit, and 2,2′-bithiophene vinylene (bTV) as π bridge, S(TPA-bTV-CA) was designed and synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound is soluble in common organic solvents. The thermal, optical and electrochemical properties of the star molecule were studied. The OSC devices were fabricated by spin-coating the blend solution of the molecule as donor and PC₇₀BM as acceptor (1:3, w/w). The OSC device based on S(TPA-bTV-CA)/PC₇₀BM demonstrated a high open circuit voltage of 0.91 V, a short circuit current density of 4.64 mA/cm², a fill factor (FF) of 50%, corresponding to a power conversion efficiency of 2.1%, under the illumination of AM 1.5, 100 mW/cm².     

Evidences of photocurrent generation by hole–exciton interaction at organic semiconductor interfaces

The charge–exciton interaction at the donor/acceptor interface plays a significant role in the exciton dissociation processes, and thus influences the performance of organic solar cells. In this work, the evidences of photocurrent generation via hole–exciton interaction (HEI) at the organic semiconductor interface in organic solar cells, which is the counterpart of photocurrent generated by electron–exciton interaction, is demonstrated. A heterojunction, composed of copper phthalocyanine (CuPc) and fullerene (C₆₀), is used to provide free holes that interact with the excitons supplied by perfluorinated hexadecafluorophthalo-cyaninatozinc (F16ZnPc). The fact that photocurrent generation via HEI is well evidenced by: (1) a short circuit current of 0.38 mA cm⁻²; (2) the jump of an external quantum efficiency (EQE) around 800 nm after adding a bias light; (3) the EQE variations under bias light of different wavelengths and light intensities; and (4) the superlinear dependence of the photocurrent on the light intensity.     

Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η = 8.01% under standard 100 mW/cm² AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh₂)₂/PC₇₀BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V < 0.65 V) and the interface state assisted thermionic emission at high forward bias (V > 0.65 V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.     

Influence of compact TiO2 layer on the photovoltaic characteristics of the organometal halide perovskite-based solar cells

A series of perovskite-based solar cells were fabricated wherein a compact layer (CL) of TiO₂ of varying thickness (0–390 nm) was introduced by spray pyrolysis deposition between fluorine-doped tin oxide (FTO) electrode and TiO₂ nanoparticle layer in perovskite-based solar cells. Investigations of the CL thickness-dependent current density–voltage (J–V) characteristics, dark current, and open circuit voltage (V_oc) decays showed a similar trend for thickness dependence. A CL thickness of 90 nm afforded the perovskite-based solar cell with the maximum power conversion efficiency (η, 3.17%). Furthermore, two additional devices, perovskite-based solar cell omitting hole transporting materials layer and cell without the TiO₂ nanoparticles, were designed and fabricated to study the influence of the CL thickness on different electron transport paths in perovskite-based solar cells. Solar cells devoid of TiO₂ nanoparticles, but with perovskite and organic hole-transport materials (HTMs), exhibited sustained improvement in photovoltaic performances with increase in the thickness of CL, which is in contrast to the behavior of classical perovskite-based solar cell and common solid state solar cell which showed optimal photovoltaic performances when the thickness of CL is 90 nm. These observations suggested that TiO₂ nanoparticles play a significant role in electron transport in perovskite-based solar cells.     

Indoline-based donor molecule for efficient co-evaporated organic photovoltaics

We demonstrated the use of an asymmetrical donor–acceptor-type indoline dye—D131, developed for dye-sensitized solar cells, as an electron donor and fullerene C₇₀ as an electron acceptor for thermal co-evaporated bulk-heterojunction organic solar cells (OSCs). In spite of the presence of intermolecular hydrogen bonds among D131 molecules, they can be thermally evaporated in high vacuum at a relatively low temperature of 220 °C. The blend ratio and thickness of the active layer of D131/C₇₀ blend films in OSCs were optimized to achieve a maximum power-conversion efficiency of 4.5% with a short-circuit current of 9.1 mA cm^?2, an open-circuit voltage of 0.89 V, and a fill factor of 0.56 under AM 1.5G solar illumination (100 mW cm^?2), which is the best value reported so far for OSCs based on indoline-based donor materials.     

Photovoltaic response of Cu2O/In2S3 hetero-structure grown on Cu substrate

A thin layer of p-type Cu₂O was grown over flexible 30 μm thick copper substrates. Using Injection Chemical Vapor Deposition technique, n-type In₂S₃ thin films were grown over the Cu₂O layer. A p–n junction was thus realized. The Cu₂O/In₂S₃ hetero-structure showed photovoltaic behavior. A solar cell with the structure Cu/Cu₂O/In₂S₃/Ag could be fabricated. An acidic texturization sequence was developed which increased the photo-sensitivity of the In₂S₃ window layer. The Cu/Cu₂O/In₂S₃/Ag hetero-structure with the textured window layer had an open circuit voltage of 377 mV, short circuit current density of 0.118 mA/cm² and fill factor of 33.34%. It was found that the efficiency of the solar cell depended upon the photo-sensitivity of the In₂S₃ window layer. The work demonstrates the use of copper substrate for thin film solar cell fabrication.     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Influence of mixed solvent on the morphology of the P3HT:Indene-C60 bisadduct (ICBA) blend film and the performance of inverted polymer solar cells

A new acceptor material, Indene-C60 bisadduct (ICBA), has reportedly improved the open circuit voltage. The published literature has reported that almost all groups dissolve the ICBA and P3HT in DCB. Nevertheless, the solubility of ICBA in DCB is poor, leading to high leakage current and lower open circuit voltage. To enhance the solubility of ICBA in photoactive ink, we use CB and DCB as a mixed solvent to fabricate the inverted polymer solar cells. We then add conductive polymer polyvinylcarbazole (PVK) to reduce the horizontal phase separation and control the drying time of the active layer by adjusting the ratios of DCB and CB. As a result, the open circuit voltage of inverted polymer solar cells is enhanced from 0.66 V to 0.82 V and the power conversion efficiency (PCE) improve from 2.6% to 4.27%.