Improving the efficiency of ITO/nc-TiO2/CdS/P3HT:PCBM/PEDOT:PSS/Ag inverted solar cells by sensitizing TiO2 nanocrystalline film with chemical bath-deposited CdS quantum dots

An improvement in the power conversion efficiency (PCE) of the inverted organic solar cell (ITO/nc-TiO₂/P3HT:PCBM/PEDOT:PSS/Ag) is realized by depositing CdS quantum dots (QDs) on a nanocrystalline TiO₂ (nc-TiO₂) film as a light absorption material and an electron-selective material. The CdS QDs were deposited via a chemical bath deposition (CBD) method. Our results show that the best PCE of 3.37% for the ITO/nc-TiO₂/CdS/P3HT:PCBM/PEDOT:PSS/Ag cell is about 1.13 times that (2.98%) of the cell without CdS QDs (i.e., ITO/nc-TiO₂/P3HT:PCBM/PEDOT:PSS/Ag). The improved PCE can be mainly attributed to the increased light absorption and the reduced recombination of charge carriers from the TiO₂ to the P3HT:PCBM film due to the introduced CdS QDs.     

Semitransparent inverted polymer solar cells employing a sol-gel-derived TiO2 electron-selective layer on FTO and MoO3/Ag/MoO3 transparent electrode

We report a new semitransparent inverted polymer solar cell (PSC) with a structure of glass/FTO/nc-TiO₂/P3HT:PCBM/MoO₃/Ag/MoO₃. Because high-temperature annealing which decreased the conductivity of indium tin oxide (ITO) must be handled in the process of preparation of nanocrystalline titanium oxide (nc-TiO₂), we replace glass/ITO with a glass/fluorine-doped tin oxide (FTO) substrate to improve the device performance. The experimental results show that the replacing FTO substrate enhances light transmittance between 400 and 600 nm and does not change sheet resistance after annealing treatment. The dependence of device performances on resistivity, light transmittance, and thickness of the MoO₃/Ag/MoO₃ film was investigated. High power conversion efficiency (PCE) was achieved for FTO substrate inverted PSCs, which showed about 75% increase compared to our previously reported ITO substrate device at different thicknesses of the MoO₃/Ag/MoO₃ transparent electrode films illuminated from the FTO side (bottom side) and about 150% increase illuminated from the MoO₃/Ag/MoO₃ side (top side).     

Using water‐soluble nickel acetate as hole collection layer for stable polymer solar cells

We report polymer solar cells (PSCs) based on poly(3‐hexylthiophene (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) using water‐soluble nickel acetate (Ni(CH₃COO)₂, NiAc) instead of acidic poly(3,4‐ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) as hole collection layer (HCL) between the indium tin oxide (ITO) electrode and photoactive layer. The NiAc layer can effectively decrease R_s and increase R_p and shows effective hole collection property. Under the illumination of AM1.5G, 100 mW/cm², the short‐circuit current density (J_sc) of the NiAc based device (ITO/NiAc/P3HT : PCBM/Ca/Al) reach 11.36 mA/cm², which is increased by 11% in comparison with that (10.19 mA/cm²) of PEDOT : PSS based device (ITO/PEDOT : PSS/P3HT : PCBM/Ca/Al). The power conversion efficiency of the NiAc based devices reach 3.76%, which is comparable to that (3.77%) of the device with PEDOT : PSS HCL under the same experimental conditions. Moreover, NiAc based PSCs show superior long‐term stability than PEDOT : PSS based PSCs. Our work gives a new option for HCL selection in designing more stable PSCs. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Optimization of inverted bulk heterojunction polymer solar cells

We have successively fabricated inverted bulk heterojunction polymer solar cells employing ZnO and MoO₃ as electron and hole selective layers, respectively. The device structure is ITO/ZnO/P3HT: PCBM/MoO₃/Al. Differently from conventional polymer solar cells, ITO and Al work as electron and hole collecting electrodes in this inverted structure, respectively. We have found the optimal thickness of ZnO and MoO₃ to be 100 nm and 5 nm, respectively. The highest PCE was obtained to be 3.32% under AM 1.5 illumination at 1,000W/m², which is the highest PCE of inverted solar cells reported previously in the literature.     

Enhanced Thermal Stability of Inverted Polymer Solar Cells with Pentacene

In this study, we focused on the thermal stability of organic solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM), fabricated by blends of P3HT : PCBM : pentacene. Enhanced thermal stability of organic solar cells was achieved by introducing pentacene (Pc) into blends of P3HT : PCBM in organic solar cells with the structure indium tin oxide/ZnO/P3HT : PCBM : Pc/poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate/Ag (ITO/ZnO/P3HT : PCBM : Pc/PEDOT : PSS/Ag). The donor-acceptor interfaces of devices with Pc were more stable than those without Pc in the active layer. During the thermal annealing process, the Pc in the P3HT : PCBM blends suppressed the crystallization of P3HT and PCBM, which was confirmed by optical microscopic images and UV-visible absorption spectra. The power conversion efficiency (PCE) of the device with Pc was reduced to no less than 70 % of its original efficiency after keeping it at 120 °C for 24 hours, while that of the non-Pc device was reduced to 13 % of its original efficiency after 24 hours at the same temperature. Based on these results, we propose a new Pc-blended organic solar cell that has advantages in the thermal annealing process.     

Towards optimization of functionalized single-walled carbon nanotubes adhering with poly(3-hexylthiophene) for highly efficient polymer solar cells

In this paper, we present the optimization of single-walled carbon nanotubes (SWCNTs) by acid-treatment, solution ultrasonication time and dispersion in photoactive layer for efficient organic solar cells. After non-covalently adhering with poly(3-hexylthiophene) (P3HT), pre-functionalized SWCNTs were blended into the composites of P3HT and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as photoactive layer, and a maximum power conversion efficiency (PCE) of 3.02% with a short-circuit current density of 11.46 mA/cm² was obtained from photovoltaic cell indium-tin oxide (ITO)/poly(ethylene-dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/P3HT:PCBM:SWCNTs/Al with an optimum 0.3 wt% SWCNTs in P3HT:PCBM:SWCNTs nanocomposite, the PCE can be enhanced by more than 10% as compared to the control device ITO/PEDOT:PSS/P3HT:PCBM/Al. The performance improvement by incorporating with functionalized SWCNTs is mainly attributed to the extension of excitons dissociation area and fastening charge carriers transfer across the active layer.     

In situ-prepared composite materials of PEDOT: PSS buffer layer-metal nanoparticles and their application to organic solar cells

We report an enhancement in the efficiency of organic solar cells via the incorporation of gold (Au) or silver (Ag) nanoparticles (NPs) in the hole-transporting buffer layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which was formed on an indium tin oxide (ITO) surface by the spin-coating of PEDOT:PSS-Au or Ag NPs composite solution. The composite solution was synthesized by a simple in situ preparation method which involved the reduction of chloroauric acid (HAuCl₄) or silver nitrate (AgNO₃) with sodium borohydride (NaBH₄) solution in the presence of aqueous PEDOT:PSS media. The NPs were well dispersed in the PEDOT:PSS media and showed a characteristic absorption peak due to the surface plasmon resonance effect. Organic solar cells with the structure of ITO/PEDOT:PSS-Au, Ag NPs/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/LiF/Al exhibited an 8% improvement in their power conversion efficiency mainly due to the enlarged surface roughness of the PEDOT:PSS, which lead to an improvement in the charge collection and ultimately improvements in the short-circuit current density and fill factor.     

Study of the processing pathway for cosolvent addition in active layer preparation of inverted organic solar cell

Investigation on processing pathway for co‐solvent addition in active layer preparation of an inverted organic solar cell indicated that the composite films cast out of simple marginal solvent (like cyclohexanone) addition in poly(3‐hexyl thiophene)‐[6,6]‐phenyl C₆₁‐butyric acid methyl ester (P3HT‐PCBM) solution were not effectively crystallized. A comparison of one step direct marginal solvent addition to the P3HT‐PCBM solution with the modified two step process consisting of primary P3HT crystallization in first step by marginal solvent addition to a solution of only P3HT in good solvent (like ortho‐dichloro benzene) followed by mixing of PCBM solution revealed improvement in P3HT crystallization in the latter method. Grazing incidence X‐ray diffraction measurements supported favorable vertical concentration gradient in the P3HT‐PCBM composite film. This modification of active layer morphology with the two step individually mixed cyclohexanone addition into the active layer spin coating solution results in an improved power conversion efficiency of 3.39%, an improvement of more than 10% compared to the conventional one step blended co‐solvent addition method. POLYM. ENG. SCI., 55:1758–1766, 2015. © 2014 Society of Plastics Engineers     

Hydrothermal synthesized AZO Nanorods layer as a high potential buffer layer for inverted polymer solar cell

In this study, the performance of inverted polymer solar cell was improved using optically and electrically tuned Zn_1-xAl_xO (x?=?0, 0.005, 0.01 and 0.015) nanorods (AZO) as a high potential electron transporting layer. AZO nanorods with different compositions were synthesized using facile, low temperature, and low cost hydrothermal method that was confirmed by energy dispersive x- ray spectroscopy (EDS) analysis. As revealed, the optical transmittance and optical band gap increased by increasing the Al concentration in AZO nanorods. The fabricated device with ITO/Zn_1-xAl_xO nanorods / P₃HT: PCBM /WO₃/Ag structure showed ascending trend for its short-circuit current density (J_sc) by increasing the amount of Al doping. The inverted polymer solar cell with 1% Al doped ZnO nanorods showed a power conversion efficiency of 3.64% that is around 40% higher than that of the device with pure ZnO nanorods (2.58%). The performance enhancement was attributed to the combined effects of the improvement in charge collection and the higher optical transmittance of AZO in the visible range.     

Improving the efficiency of cadmium sulfide-sensitized titanium dioxide/indium tin oxide glass photoelectrodes using silver sulfide as an energy barrier layer and a light absorber

Cadmium sulfide (CdS) and silver sulfide (Ag₂S) nanocrystals are deposited on the titanium dioxide (TiO₂) nanocrystalline film on indium tin oxide (ITO) substrate to prepare CdS/Ag₂S/TiO₂/ITO photoelectrodes through a new method known as the molecular precursor decomposition method. The Ag₂S is interposed between the TiO₂ nanocrystal film and CdS nanocrystals as an energy barrier layer and a light absorber. As a consequence, the energy conversion efficiency of the CdS/Ag₂S/TiO₂/ITO electrodes is significantly improved. Under AM 1.5 G sunlight irradiation, the maximum efficiency achieved for the CdS(4)/Ag₂S/TiO₂/ITO electrode is 3.46%, corresponding to an increase of about 150% as compared to the CdS(4)/TiO₂/ITO electrode without the Ag₂S layer. Our experimental results show that the improved efficiency is mainly due to the formation of Ag₂S layer that may increase the light absorbance and reduce the recombination of photogenerated electrons with redox ions from the electrolyte.     

Application of a polymer heterojunction in dye-sensitized solar cells

Using a blend heterojunction consisting of a C₆₀ derivative, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), and poly(3-hexylthiophene) (P3HT) as a charge carrier transfer medium to replace the I₃⁻/I⁻ redox electrolyte, a novel TiO₂/dye/PCBM/P3HT dye-sensitized solar cell was fabricated and characterized. It was found that the P3HT/PCBM heterojunction widened the incident light harvest range from ultraviolet to visible light, and improved the photoelectrical response of the dye-sensitized solar cell. We investigated the influence of the PCBM/P3HT ratio and barrier layer on the photoelectric performance of the solar cell and proposed optimized preparation conditions. The optimized solar cell with a barrier layer and PCBM/P3HT ratio of 1:2 had a short circuit current density of 5.52 mA cm⁻², an open circuit voltage of 0.87 V, a fill factor of 0.640 and a light-to-electric energy conversion efficiency of 3.09% under a simulated solar light irradiation of 100 mW cm⁻².     

Solvent effects on gravure-printed organic layers of nanoscale thickness for organic solar cells

The effects of different solvents on the fabrication of organic photovoltaic cells by gravure printing are reported. Polymer bulk heterojunction solar cells were fabricated with ITO/PEDOT: PSS/P3HT: PCBM/Al layer structures using 4–9 wt% mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) in 1,2-dichlorobenzene to optimize solution viscosity for gravure printing. 7 wt% P3HT: PCBM showed optimal efficiency of 1.64% and resulted in an active layer 340 nm thick. Three solvents, 1,2-dichlorobenzene, chloroform, and chlorobenzene, were tested and a 1: 1 ratio mixture of 1,2-dichlorobenzene and chloroform resulted in the best efficiency of 2.21%. This study demonstrates the importance of solvent effects in the gravure printing of organic photovoltaic devices.     

A Novel Method for Preparing Vertically Aligned CdS Nanorod Arrays and Its Application in Photovoltaic Cells with P3HT Fibers

Vertically aligned cadmium sulfide (CdS) nanorod arrays were prepared through a novel thermal annealing route. By embedding the as-prepared CdS nanorod arrays into the poly(3-hexylthiophene) (P3HT) nanofiber (NF) matrix, the photovoltaic devices were fabricated with the structure of ITO/PEDOT:PSS/CdS arrays/P3HT NF/Au. The device performance was highly dependent on the P3HT NF layer thickness in this structure, and a power conversion efficiency (PCE) of 0.23 % was obtained for optimal P3HT NF layer thickness of 150 nm. In addition, much higher PCE of 0.84 % was achieved after post-annealing. The significantly improved photovoltaic performance may be caused by the increased interfacial areas between P3HT NFs and CdS nanorods for efficient charge separation, as well as the decreased inter-nanocrystal distance caused by insulating organic ligands after the annealing treatment. The results demonstrate a promising inorganic–organic hybrid photovoltaic structure with vertically aligned CdS nanorods arrays.     

Functionalized polythiophene for corrosion inhibition and photovoltaic application

Conjugated polymers are encouraging substitute for creating clean and renewable energy for photoinduced charge generation and transport media in polymer solar cells (PSCs). Successful synthesis of a new solution processable n-type polythiophene based π-conjugated polymer (P3HT-CN) is done where polythiophene units are substituted by cyano groups through post functionalization approach of synthesized P3HT which is cheap, stable, easily prepared, and inert against ambient conditions and is expected to be a competitive candidate for the acceptor material in non fullerene acceptors (NFAs) PSCs against fullerene derivative and used as the highly efficient active layer in the bulk heterojunction (BHJ) which increase the donor-acceptor interfacial area through controlling the phase separation indicating device structure ITO/PEDOT:PSS/P3HT:P3HT-CN/Al. Photovoltaic measurement of this PSCs device based on P3HT:P3HT-CN demonstrate the PCE of 0.008% with an increased short circuit current density (J_sc) of 0.11 mA cm⁻². The thermal, optical, and electrochemical properties are examined in detail showing high thermal stability, absorbance in the visible part of the solar spectrum, higher charge carrier mobility, and also mixed type corrosion inhibitive behavior with 90 and 78% inhibitor efficiency for P3HT and P3HT-CN, respectively, built this class of material as smart which can be used for many other applications.     

Synthesis and characterization of novel fulleropyrrolidine in P3HT blended bulk heterojunction solar cells

Synthesis of novel fullerene derived electron acceptors and characterization of their organic photovoltaic (OPV) properties is important for advancing fundamental knowledge towards developing next generation organic solar cells. We report the synthesis of a novel fulleropyrrolidine derivative C60-fused N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl)fulleropyrrolidine (NCPF) by 1,3-dipolar cycloaddition reaction and characterization of NCPF by ¹H NMR, ¹³C NMR, MALDI-TOFMS, FT-IR, UV–Vis and CV. The synthesized NCPF fullerene derivative showed good solubility in common organic solvents such as chlorobenzene and 1,2 dichlorobenzene important for film formation, with optical absorbance and electronic properties comparable to PCBM. Optical micrographs of P3HT:PCBM thin films reveal formation of sparse, phase segregated needle shape PCBM micro-crystalline aggregates after 1 h of annealing at 150 °C whose length follows nucleation and growth kinetics over 24 h. In contrast, the P3HT:NCPF thin films exhibit homogeneity over 24 h, possibly due to weaker interparticle vanderWaals forces and/or stronger interactions with P3HT. This long term morphological stability of P3HT:NCPF is important for extended use in OPV applications. At an order of magnitude smaller scale, AFM of as cast and 10 min annealed at 150 °C P3HT:PCBM and P3HT:NCPF films reveal mostly smooth surfaces, with some NCPF cluster formation. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements of P3HT:NCPF films indicate an increase of P3HT crystallinity with thermal annealing, leading to improvement in device performance. Photovoltaic devices fabricated with the active layer of P3HT:NCPF and P3HT:PCBM sandwiched between ITO/PEDOT:PSS and Al layer showed comparable performance upon short term annealing.     

Effect of co‐solvents on the photovoltaic performance of an inverted organic solar cell

We investigated the effect of varying polymer crystallinity, morphology, and optical property, produced by adding four different co‐solvents in to the poly(3‐hexylthiophene) (P3HT): [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) active layer blend solution, on the functioning of an inverted polymeric solar device. Photovoltaic devices primed with cyclohexanone co‐solvent showed the best performance with power conversion efficiency (PCE) reaching a value of 3.01 ± 0.04%. Improvement in efficiency is related to an increase in photocurrent which is due to a combined result of ordered P3HT crystallite growth, as well as of the precise size and phase separation of domains. POLYM. ENG. SCI., 55:1382–1388, 2015. © 2015 Society of Plastics Engineers     

Degradation in bulk heterojunction organic solar cells: changes in electrode interface and reduction in the occupation probability of the interface states

We have investigated the degradation of P3HT:PCBM (poly(3-hexylthiophene):6,6-phenylC61 butyric acid methyl ester) solar cell beyond 150?h of fabrication in continuation to our earlier reported work up to 150?h of fabrication. The current- voltage characteristics of degraded Indium tin oxide/poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate)/poly(3-hexylthiophene):6,6-phenylC61 butyric acid methyl ester/Aluminum (ITO/PEDOT:PSS/P3HT:PCBM/Al) solar cell can be explained by considering the tunneling current through electrode interfaces, increase in both the interface states density and the thickness of interface with time for150-200?h. Beyond 200?h of fabrication, a significant reduction in the occupation probability at the electrode interfaces explains the experimental results up to 300?h fairly well. Calculations based on realistic parameters and activity at both the electrode interfaces (ITO/PEDOT:PSS and P3HT:PCBM/Al) confirm that degradation at P3HT:PCBM/Al interface is more prominent than that at ITO/PEDOT:PSS interface.     

Effect of the regioregularity of poly(3‐hexylthiophene) on the performances of organic photovoltaic devices

BACKGROUND: The highest efficiencies of bulk‐heterojunction solar cells from poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) reported so far are close to 6%. Phenomena occurring during the photovoltaic process, such as the creation, diffusion and separation of excitons, as well as charge carrier transport, are governed by the active layer morphology. The latter phenomenon, which depends on the self‐organization of P3HT, can be influenced by its degree of regioregularity. The aim of this work is to clarify the relationship between the regioregularity of P3HT, the composition of P3HT/PCBM blends and the performances of photovoltaic devices. RESULTS: Two types of P3HTs with different degrees of regioregularity have been synthesized and used as active layers with PCBM in photovoltaic cells. The higher performances in photovoltaic devices are obtained for high‐regioregular P3HT and can be explained considering the self‐organizing properties of high‐regioregular P3HT, leading to higher sunlight absorption and higher hole mobilities. In addition, this report demonstrates the importance of the ratio of P3HT versus PCBM in correlation with the regioregularity of P3HT on the optical properties, charge transport and characteristics of photovoltaic cells. CONCLUSION: We have investigated the dependence of the photovoltaic properties of P3HT/PCBM blend‐based photovoltaic devices on the degree of regioregularity of P3HT. We find that the best performance is exhibited by devices based on highly regioregular P3HT. Also, the best performances are not obtained for the same P3HT:PCBM weight ratios for high‐regioregular P3HT (1:0.8) and low‐regioregular P3HT (1:3). Copyright © 2007 Society of Chemical Industry     

Influence of NH3·H2O additive on the photovoltaic performance of dye-sensitized solar cells with chemical sintered scattering layers

A bi-functional nanocrystalline TiO₂ (nc-TiO₂) layer able to offer both light-scattering and electron generating properties was prepared with a simple method through adding the basic NH₃·H₂O agent into an acid nc-TiO₂ paste to form some big rod-like nc-TiO₂ aggregates by the chemical sintering process. The influence of additional amount of NH₃·H₂O on the photovoltaic performance of the dye-sensitized solar cell with this bi-functional nc-TiO₂ layer in the photoelectrode was studied. It was found that through controlling the additional amount of NH₃·H₂O and the thickness of the bi-functional nc-TiO₂ layer, the highest energy conversion efficiency about 8.11% could be obtained, which was much higher than that of the dye-sensitized solar cell containing a single nc-TiO₂ layer prepared with the original acid nc-TiO₂ paste (4.34%).     

Enhancing the light absorbance of polymer solar cells by introducing pulsed laser-deposited CuIn0.8Ga0.2Se2 nanoparticles

Evenly separated crystalline CuIn_0.8Ga_0.2Se₂ (CIGS) nanoparticles are deposited on ITO-glass substrate by pulsed laser deposition. Such CIGS layers are introduced between conjugated polymer layers and ITO-glass substrates for enhancing light absorbance of polymer solar cells. The P3HT:PCBM absorbance between 300 and 650 nm is enhanced obviously due to the introduction of CIGS nanoparticles. The current density-voltage curves of a P3HT:PCBM/CIGS solar cell demonstrate that the short-circuit current density is improved from 0.77 to 1.20 mA/cm². The photoluminescence spectra show that the excitons in the polymer are obviously quenched, suggesting that the charge transfer between the P3HT:PCBM and CIGS occurred. The results reveal that the CIGS nanoparticles may exhibit the localized surface plasmon resonance effect just as metallic nanostructures.

PACS

61.46. + w; 61.41.e; 81.15.Fg; 81.07.b