Efficient and stable, structurally inverted poly(3-hexylthiopen): [6,6]-phenyl-C61-butyric acid methyl ester heterojunction solar cells with fibrous like poly(3-hexylthiopen)

We investigated an inverted organic photovoltaic device structure in which a densely packed ~ 100 nm thin TiO₂ layer on fluorine doped conducting glass serves as anode and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Au layer on top of the active layer serves as cathode. The active layer is comprised of a blend of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The rectification behavior of such a device is improved significantly and injection losses are minimized compared to devices without any compact TiO₂ layer. Moreover, nanostructured P3HT active layer was achieved in-situ by spin coating concentrated pure P3HT and P3HT:PCBM blend and solar cell performances on thickness of the active layer were also investigated. For the inverted solar cells constructed with different concentrations of P3HT and PCBM keeping the P3HT:PCBM ratio 1:0.8 (wt.%), the highest short circuit current and efficiency was observed when the P3HT and PCBM concentration was equal to 1.5 (wt.%) and 1.2 (wt.%) respectively. This leads to highly stable and reproducible power conversion efficiency above 3.7% at 100 mW/cm² light intensity under AM 1.5 conditions.     

Direct writing of semiconducting polythiophene and fullerene derivatives composite from bulk heterojunction solar cell by inkjet printing

The direct writing approach of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) composite from bulk-heterojunction (BHJ) solar cell was efficiently addressed by inkjet printing technology using conventional chlorobenzene ink solution. The structure of inkjet-printed P3TH:PCBM BHJ film was fabricated by the repetitive direct writing of new line overlapped partially on former line. The best structure of P3HT:PCBM film for BHJ solar cell was observed from inkjet printing condition of around 50% of droplet overlaps with 2 wt.% BHJ ink at 25 °C of substrate temperature. The maximum power conversion efficiency reached 2.83% with an open circuit voltage of 0.62 V, a short circuit current density of 8.60 mA/cm², and a fill factor of 0.53 under air mass 1.5 G irradiation (100 mW/cm²).     

Fabrication of high-performance fluorine doped-tin oxide film using flame-assisted spray deposition

A high-performance fluorine-doped tin oxide (FTO) film was fabricated by flame-assisted spray deposition method. By varying the NH₄F doping concentration, the optimal concentration was established as 8 at.%. X-ray diffractograms confirmed that the as-grown FTO film was tetragonal SnO₂. In addition, the FTO film was comprised of nano-sized grains ranging from 40 to 50 nm. The heat-treated FTO film exhibited a sheet resistance of 21.8 Ω/? with an average transmittance of 81.9% in the visible region (λ = 400-800 nm). The figures of merit shows that the prepared FTO film can be used for highly efficient dye-sensitized solar cells electrodes.     

Self-assembly monolayer of anatase titanium oxide from solution process on indium tin oxide glass substrate for polymer photovoltaic cells

Titanium dioxide (TiO₂) thin films were deposited on indium tin oxide (ITO) coated glass substrates by a liquid phase deposition method using two different precursor concentrations of 0.01 M and 0.005 M [NH₄]₂TiF₆. Characterizations of the deposited TiO₂ films were performed by using different spectroscopic techniques including X-ray photoelectron spectroscopy, Raman spectroscopy and X-ray diffraction. From scanning electron microscopy observation, the surface of the TiO₂ films show a ricelike morphology. We have fabricated polymer-based photovoltaic cells (PCs) using the studied TiO₂ films and compared their performances to those using bare ITO-coated glass substrates. The structure of the PCs is glass/ITO/TiO₂/PCBM:P3HT/Au where PCBM:P3HT is poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. The power conversion efficiency of these devices is determined to be 0.85%, which is higher than that of similar devices without TiO₂ films (0.13%).     

Formation of phenyl-C<Subscript>61</Subscript>-butyric acid methyl ester nanoscale aggregates after supercritical carbon dioxide annealing

In this study, we successfully developed a novel method to create [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO₂) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO₂ annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO₂-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO₂ treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO₂-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO₂ is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.     

Sputtered nickel oxide thin film for efficient hole transport layer in polymer-fullerene bulk-heterojunction organic solar cell

Bulk-heterojunction (BHJ) organic photovoltaics (OPV) are very promising thin film renewable energy conversion technologies due to low production cost by high-throughput roll-to-roll manufacturing, an expansive list of compatible materials, and flexible device fabrication. An important aspect of OPV device efficiency is good contact engineering. The use of oxide thin films for this application offers increased design flexibility and improved chemical stability. Here we present our investigation of radio frequency magnetron sputtered nickel oxide (NiO_x) deposited from oxide targets as an efficient, easily scalable hole transport layer (HTL) with variable work-function, ranging from 4.8 to 5.8 eV. Differences in HTL work-function were not found to result in statistically significant changes in open circuit voltage (V_oc) for poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) BHJ device. Ultraviolet photoemission spectroscopy (UPS) characterization of the NiO_x film and its interface with the polymer shows Fermi level alignment of the polymer with the NiO_x film. UPS of the blend also demonstrates Fermi level alignment of the organic active layer with the HTL, consistent with the lack of correlation between V_oc and HTL work-function. Instead, trends in j_sc, V_oc, and thus overall device performance are related to the surface treatment of the HTL prior to active layer deposition through changes in active layer thickness.     

Effect of cadmium sulphide quantum dot processing and post thermal annealing on P3HT/PCBM photovoltaic device

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT) on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nm blue shift in the UV-vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles. Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying paths of the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDs have been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively. Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (?) of the solar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves device performance due to enhancement in the device parameters like FF, V_OC and improvement in contact between active layer and Al.     

Light illumination effects in ambipolar FETs based on poly(3-hexylthiophene) and fullerene derivative composite films

The effects of light illumination on field effect transistors based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric methyl ester (PCBM) composite films have been studied. It is found that the light illumination on pure P3HT and PCBM generally resulted in decrease of the threshold voltages and increase of the mobilities by a little. In the composite film at the PCBM contents of x = [P3HT] / ([P3HT] + [PCBM]) = 0.67 ∼ 0.9, an ambipolar field transport appeared. The light illumination effect was observed remarkably in the shift of threshold voltage for the hole generation at x = 0.75. Variations of Hole and electron mobilities and threshold voltage of electron generation upon light illumination were basically similar to those of the pure materials. The results were discussed in terms of the light assisted carrier generation in field effects.     

Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors

The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed.     

The effect of growth conditions on the properties of ZnO nanorod dye-sensitized solar cells

In this article, we report ZnO nanorod samples grown on transparent conductive SnO₂:F (FTO) glass substrates by two different growth routes through hydrothermal method in a closed autoclave. One route is one-step continuous growth for 10 h. The other route is discrete multi-step growth for total 48 h. In this process, fresh solution was repeatedly introduced in every step. The structural, photoluminescence (PL) and photovoltaic properties of the as-prepared nanorod arrays were investigated. The nanorod arrays obtained through multi-step growth show longer rods, higher aspect ratio, larger spacing, better crystalline quality. The PL spectrum of nanorod arrays obtained through multi-step growth shows a strong and sharp near-band-gap emission (UV) peak and a weak green-yellow emission (GY) peak (I_UV/I_GY = 7.7), which also implies its good crystallinity and high optical quality. Dye-sensitized solar cells based on ZnO nanorod arrays were fabricated, and those grown with discrete multi-step procedure present better photovoltaic properties duo to its special morphology and better crystal quality.     

The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases - a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer film - the evolution of the P3HT-PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.     

Comparison of the dye-sensitized solar cells performances based on transparent conductive ITO and FTO

We investigated dye-sensitized solar cell (DSSC) performances with regard to transparent conducting oxide substrates: indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO). The DSSCs were in a standard configuration: a photoelectrode of TiO₂ nanoparticles (9 nm size, anatase phase) deposited on transparent and electrically conductive substrates, counter electrodes of Pt-coated glass, ruthenium 535 dye, and AN50 iodolyte electrolyte (Solaronix). The cells manufactured from ITO (FTO) had an open circuit voltage of 705 (763) mV and short-circuit current of 7.87 (34.3) mA/cm². A direct correlation was found between transparent conductive film resistivity and cell efficiency. Resistivities of 52 Ω/sq for ITO substrates and 8.5 Ω/sq for FTO led to major differences in internal global efficiency: from 2.24% for ITO to 9.6% for FTO.     

Fabrication and characterization of vacuum deposited fluorescein thin films

Simple vacuum evaporation technique for deposition of dyes on various solid surfaces has been developed. The method is compatible with conventional solvent-free nanofabrication processing enabling fabrication of nanoscale optoelectronic devices. Thin films of fluorescein were deposited on glass, fluorine-tin-oxide (FTO) coated glass with and without atomically layer deposited (ALD) nanocrystalline 20 nm thick anatase TiO₂ coating. Surface topology, absorption and emission spectra of the films depend on their thickness and the material of supporting substrate. On a smooth glass surface the dye initially forms islands before merging into a uniform layer after 5 to 10 monolayers. On FTO covered glass the absorption spectra are similar to fluorescein solution in ethanol. Absorption spectra on ALD-TiO₂ is red shifted compared to the film deposited on bare FTO. The corresponding emission spectra at λ = 458 nm excitation show various thickness and substrate dependent features, while the emission of films deposited on TiO₂ is quenched due to the effective electron transfer to the semiconductor conduction band.     

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a V_OC ~350?mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC₆₀BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.     

Solid state dye-sensitized solar cell with TiO2/NiO heterojunction: Effect of particle size and layer thickness on photovoltaic performance

The present work demonstrates the usefulness of nickel oxide as a hole transporting material in solid state dye-sensitized solar cells (SSDSSCs). We report on the photovoltaic performances of sensitized TiO₂/NiO heterojunctions, and demonstrate that the TiO₂ film thickness and morphology, as well as NiO film thickness, have significant effects on the photovoltaic behaviour of TiO₂/NiO SSDSSC. Under 1 sun AM1.5G simulated illumination, the SSDSSCs demonstrated best photovoltaic performance with a short circuit photocurrent density, open circuit voltage, fill factor and efficiency of 0.91 mA cm⁻², 780 mV, 40% and 0.3%, respectively. This study draws attention to the feasibility of enhancing the photovoltaic performance in SSDSSC devices through development of appropriately designed sensitized TiO₂/NiO heterojunctions.     

Thermal annealing effect on the nanomechanical properties and structure of P3HT:PCBM thin films

Nanostructured polymer-fullerene thin films are among the most prominent materials for application in high efficient polymer solar cells. Specifically, poly(3-hexylthiophene) (P3HT) and fullerene derivatives (PCBM) blends are used as the donor/acceptor materials forming a bulk heterojunction. Although P3HT:PCBM properties have been extensively studied, less light has been set on its nanomechanical properties, which affect the device service life. In this work Atomic Force Acoustic Microscopy (AFAM), Atomic Force Spectroscopy and Nanoindentation were used to study the effect of the fullerene presence and the annealing on the P3HT:PCBM nanomechanical behavior. The P3HT:PCBM thin films were prepared by spin coating on glass substrates and then annealed at 100 °C and 145 °C for 30 min. Large phase separation was identified by optical and Atomic Force Microscopy (AFM) for the annealed samples. Needle-like PCBM crystals were formed and an increase of the polymer crystallinity degree with the increase of the annealing temperature was confirmed by X-ray diffraction. AFAM characterization revealed the presence of aggregates close to stiff PCBM crystals, possibly consisting of amorphous P3HT material. AFM force-distance curves showed a continuous change in stiffness in the vicinity of the PCBM crystals, due to the PCBM depletion near its crystals, and the AFM indentation provided qualitative results about the changes in P3HT nanomechanical response after annealing.     

Organic photovoltaic cells fabricated on a SnOx/Ag/SnOx multilayer transparent conducting electrode

Transparent conducting multilayer structured electrode of a few nm Ag layer embedded in tin oxide thin film SnO_x/Ag/SnO_x was fabricated on a glass by RF magnetron sputtering at room temperature. The multilayer of the SnO_x(40 nm)/Ag(11 nm)/SnO_x(40 nm) electrode shows the maximum optical transmittance of 87.3% at 550 nm and a quite low electrical resistivity of 6.5 × 10^− 5 Ω cm, and the corresponding figure of merit (T¹⁰/R_S) is equivalent to 3.6 × 10^− 2 Ω^− 1. A normal organic photovoltaic (OPV) structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/polythiophene:phenyl-C60-butyric acid methyl ester/Al was fabricated on glass/SnO_x/Ag/SnO_x to examine the compatibility of OPV as a transparent conducting electrode. Measured characteristic values of open circuit voltage of 0.62 V, saturation current of 8.11 mA/cm² and fill factor of 0.54 are analogous to 0.63 V, 8.37 mA/cm² and 0.58 of OPV on commercial glass/indium tin oxide (ITO) respectively. A resultant power conversion efficiency of 2.7% is also very comparable with the 3.09% of the same OPV structure on the commercial ITO glass as a reference, and which reveals that SnO_x/Ag/SnO_x can be appropriate to OPV solar cells as a sound transparent conducting electrode.     

Surface treatment of TiO2 films for dye-sensitized solar cells using atmospheric-pressure non-equilibrium DC pulse discharge plasma jet

In this paper, we report the utilization of the DC pulse discharge plasma jet technique as a means for the preparation of titanium oxide (TiO₂) films on fluorine dope tin oxide (FTO) coated glass substrates used for dye-sensitized solar cells (DSCs). The TiO₂ film made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼25 nm. The DSCs made by the TiO₂ film exhibited an energy conversion efficiency of 5.7% at 100 mW/cm² light intensity. Consequently, we believe that the optimization between the specific surface area and photocurrent density of TiO₂ film was achieved by the plasma surface treatment which also contributed to the improvement of energy conversion efficiency of DSCs.     

Large-area SnO2: F thin films by offline APCVD

In this paper, we reported the successful preparation of fluorine-doped tin oxide (FTO) thin films on large-area glass substrates (1245 mm × 635 mm × 3 mm) by self-designed offline atmospheric pressure chemical vapor deposition (APCVD) process. The FTO thin films were achieved through a combinatorial chemistry approach using tin tetrachloride, water and oxygen as precursors and Freon (F-152, C2H4F2) as dopant. The deposited films were characterized for crystallinity, morphology (roughness) and sheet resistance to aid optimization of materials suitable for solar cells. We got the FTO thin films with sheet resistance 8-11 Ω/□ and direct transmittance more than 83%. X-ray diffraction (XRD) characterization suggested that the as-prepared FTO films were composed of multicrystal, with the average crystal size 200-300 nm and good crystallinity. Further more, the field emission scanning electron microscope (FESEM) images showed that the films were produced with good surface morphology (haze). Selected samples were used for manufacturing tandem amorphous silicon (a-Si:H) thin film solar cells and modules by plasma enhanced chemical vapor deposition (PECVD). Compared with commercially available FTO thin films coated by online chemical vapor deposition, our FTO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si:H solar cells and ideal open voltage and short circuit current for a-Si:H solar modules.     

Composition and annealing effects in polythiophene/fullerene solar cells

We have fabricated organic solar cells with blends of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM) as electron donor and electron acceptor, respectively. Blend composition and device annealing effects were investigated with optical absorption and photoluminescence spectroscopy, atomic force microscopy, photocurrent spectroscopy, and current-voltage characteristic measurements on devices under monochromatic or air mass (AM) 1.5 simulated solar light illumination. The highest efficiency was achieved for the 1:1 (P3HT:PCBM) weight ratio composition. The good performance is attributed to an optimized morphology that enables close intermolecular packing of P3HT chains. Inferior performance for the 1:2 composition is attributed to poorer intermolecular packing with increased PCBM content, while phase segregation on a sub-micron scale was observed for the 1:4 composition. The power conversion efficiency (AM 1.5) was doubled by the thermal annealing of devices at 140^∘C to reach a value of 1.4%.