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.     

Simple 3,6-bis(diphenylaminyl)carbazole molecular glasses as hole transporting materials for hybrid perovskite solar cells

3,6-bis(diphenylaminyl)carbazole molecular glasses were initially designed as solid hole conductor for solid-state dye-sensitized solar cells. Herein we employed these simple and easy-to-synthesize carbazole derivatives in CH₃NH₃PbI₃ regular perovskite solar cells. Devices using these hole transporting materials (HTM) gave comparable efficiency to the conventional Spiro-OMeTAD based control device made under the same conditions, thus demonstrating the huge potential of carbazole-based molecular glasses as an emerging class of lower cost organic hole conductors with easier synthetic pathways for solid state hybrid solar cells.     

Versatile preparation method for mesoporous TiO2 electrodes suitable for solid-state dye sensitized photocells

Nano-structured TiO₂ electrodes, suitable for dye sensitized solid-state solar cells were prepared by a new simple spraying technique (SPT). Physical properties of these electrodes were compared with the electrodes prepared by the ‘doctor blade’ technique (typical sliding method, DB). Dye sensitized solid-state solar cells, comprising of CuI as the hole conductor, were fabricated with these electrodes and enhanced photo responses were obtained with SPT electrodes. The effects of additives, either to the spray solution or to the hole conductor on the photoresponses of the above devices were also studied. The cells fabricated with SPT electrodes containing Al(BuⁱO)₃ showed ∼ 2.4% efficiency and addition of 1-ethyl-3-methyl imidazolium thiocyanate into CuI layer further enhanced the efficiency up to 2.75% under the irradiance of 100 mW cm⁻² (AM 1.5).     

Toward interaction of sensitizer and functional moieties in hole-transporting materials for efficient semiconductor-sensitized solar cells

Sb(2)S(3)-sensitized mesoporous-TiO(2) solar cells using several conjugated polymers as hole-transporting materials (HTMs) are fabricated. We found that the cell performance was strongly correlated with the chemical interaction at the interface of Sb(2)S(3) as sensitizer and the HTMs through the thiophene moieties, which led to a higher fill factor (FF), open-circuit voltage (V(oc)), and short-circuit current density (J(sc)). With the application of PCPDTBT (poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)) as a HTM in a Sb(2)S(3)-sensitized solar cell, overall power conversion efficiencies of 6.18, 6.57, and 6.53% at 100, 50, and 10% solar irradiation, respectively, were achieved with a metal mask.     

Dithiapyrannylidenes as efficient hole collection interfacial layers in organic solar cells

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.     

High-efficiency inverted polymer solar cells with double interlayer

We have studied the performance of normal and inverted bulk-heterojunction solar cells with an active layer composed of a blend of poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PDTS-BTD) and {6,6}-phenyl-C71 butyric acid methyl ester (PC(71)BM). For inverted cells, a thin layer of ZnO nanoparticles and MoO(3) were used as interlayers for the bottom cathode and the top anode respectively. To enhance the device performance, a thin film of 4,4',4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA) was used along with MoO(3) as an anode interlayer to improve the hole extraction from the photoactive layer to the anode. The inverted polymer solar cells with double interlayer exhibit a higher power conversion efficiency of 6.45% compared to the conventional cell of 4.91% due to efficient charge extraction and favorable vertical morphology of active layer blend. Our ultraviolet photoemission spectroscopy results indicate that the formation of band bending due to interlayer leads to the enhancement in hole extraction.     

Two-dimensional photonic crystal arrays for polymer:fullerene solar cells

We report the application of two-dimensional (2D) photonic crystal (PC) array substrates for polymer:fullerene solar cells of which the active layer is made with blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The 2D PC array substrates were fabricated by employing a nanosphere lithography technique. Two different hole depths (200 and 300 nm) were introduced for the 2D PC arrays to examine the hole depth effect on the light harvesting (trapping). The optical effect by the 2D PC arrays was investigated by the measurement of optical transmittance either in the direction normal to the substrate (direct transmittance) or in all directions (integrated transmittance). The results showed that the integrated transmittance was higher for the 2D PC array substrates than the conventional planar substrate at the wavelengths of ca. 400 nm, even though the direct transmittance of 2D PC array substrates was much lower over the entire visible light range. The short circuit current density (J(SC)) was higher for the device with the 2D PC array (200 nm hole depth) than the reference device. However, the device with the 2D PC array (300 nm hole depth) showed a slightly lower J(SC) value at a high light intensity in spite of its light harvesting effect proven at a lower light intensity.     

Oxygen-Induced Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar Cells and Its Impact on Device Performance

Solid state dye-sensitized solar cells (sDSCs) employing the hole conductor 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) require the presence of oxygen during fabrication and storage. In this paper, we determine the concentrations of oxidized spiro-MeOTAD within devices under different operating and storage conditions by UV-vis spectroscopy. Relative concentrations of spiro-MeOTAD(+) were found to be greater than 10% after illumination for standard sDSCs, where no chemical dopant had been used in the solar cell fabrication but oxygen and lithium ions were present. We suggest that oxidized spiro-MeOTAD is created as a byproduct of oxygen reduction at the TiO(2) surface during cell illumination. Furthermore, we studied the effect of light soaking under different conditions and associated changes in spiro-MeOTAD(+) concentration on the solar cell measurements. Our findings give insights to photochemical reactions occurring within sDSCs and provide guidelines for which doping levels should be used in device fabrication in absence of oxygen.     

Effect of solution-processed niO thin film as a hole transport layer in poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction solar cells

In this study, solution-processed nickel oxide (NiO) thin film was investigated as a hole transport layer on anode to improve the performance of bulk heterojunction solar cell based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We fabricated NiO thin film without any vacuum-related process. Characterization of the NiO film under this study shows that it has maximum transmittance of 93.22% and bandgap of 3.84 eV which are proper for solar cell. Insertion of the NiO layer affords to realize enhanced power conversion efficiency of 1.97% and fill factor of 52.11% showing improvement over existing cells. In addition, NiO suggests one solution of minimizing conventional problems of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) such as interfacial power losses, corrosion of indium tin oxide layer, and degradation of the devices. The value of such hole transporting and electron blocking properties is clearly demonstrated and could be applicable to other organic photovoltaics.     

Fourier transform photocurrent spectroscopy applied to a broad variety of electronically active thin films (silicon, carbon, organics)

Steady state and low frequency photocurrent spectroscopies have proved as a valuable tool for investigation of many different semiconductors, used for example as an absorber in photovoltaic solar cells or in the large area sensors. Fourier transform photocurrent spectroscopy (FTPS), described here, exhibits advantages as a high sensitivity (we demonstrate dynamical range up to 9 orders of magnitude of the optical absorption coefficient, connected with the absorption process leading to free carriers; or sensitivity for dopant detection better than 1 part-per-billion), fast acquisition of data (it can be of the order of seconds) or high resolution (under more lengthy acquisition of data). Results on amorphous silicon, microcrystalline silicon, diamond layers, nanocrystalline diamond and very thin organic films, as poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))-p-phenylene-vinylene (MDMO-PPV), regioregular poly(3-hexylthiophene (P3HT) and their blends with (6,6)-phenyl-C61-butyric-acid (PCBM) are reported, together with the results measured on various thin film silicon or polymer solar cells.     

Synthesis and characterization of new dithienosilole-based copolymers for polymer solar cells

A series of dithienosilole-based copolymers, poly [(4,4'-bis(2-hexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-5,5'-diyl] (P1), poly[(4,4'-bis(2-hexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,2'-bithiazole)-5,5'-diyl] (P2), poly[(4,4'-bis(2-hexyl)dithieno[3,2-b:2',3'-d]silole)-2, 6-diyl-alt-(10 -methyl-phenothiazine)-3,7-diyl](P3), poly[(4,4'-bis(2-hexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-9,10-anthracene)-5,5'-diyl] (P4) were synthesized by the Pd-catalyzed Stille polymerization method. Electron-deficient benzothiadiazole and bithiazole units and electron-rich phenothiazine and anthracene moieties were incorporated into the polymer backbone to obtain the broad absorption spectrum and to improve the hole-transporting characteristics, respectively. The polymer solar cell (PSC) was fabricated with a layered structure of ITO/PEDOT:PSS/polymer:C71-PCBM (1:3)/LiF/Al. The best performance of PSC was obtained at P3:C71-PCBM which reaches a power conversion efficiency (PCE) of 1.18%, with a short circuit current density (J(sc)) of 4.75 mA/cm2, an open circuit voltage (V(oc)) of 0.71 V, and a fill factor (FF) of 0.35 under AM 1.5G irradiation (100 mW/cm2).     

Understanding polycarbazole-based polymer:CdSe hybrid solar cells

We report for the first time the fabrication and characterization of organic-inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (V(OC )) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application.     

Effect of hydrocarbon chain length of amphiphilic ruthenium dyes on solid-state dye-sensitized photovoltaics

We studied the influence of the hydrophobic hydrocarbon chain length of amphiphilic ruthenium dyes on the device performance in solid-state dye-sensitized solar cells. We found that the dyes with longer hydrocarbon chains gave higher efficiency values when used as a sensitizer in solid-state dye-sensitized solar cells. With increasing chain length, we observed higher currents and open-circuit voltages up to a limiting chain length. We attribute this improvement to the expected larger distance between TiO2 and the hole conductor, which seems to suppress recombination effectively.     

Photoinduced charge transfer and efficient solar energy conversion in a blend of a red polyfluorene copolymer with CdSe nanoparticles

We present measurements of charge transfer and the photovoltaic effect in a blend of the alternating polyfluorene copolymer poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) with branched CdSe nanoparticles. Quasi-steady-state photoinduced absorption measurements identified a long-lived charged species that formed after photoexcitation at room temperature. Photovoltaic devices based on this blend system showed a spectral response extending to 650 nm and gave a solar power conversion efficiency of 2.4% under Air Mass 1.5 Global (AM1.5G) conditions.     

Implications of the negative capacitance observed at forward bias in nanocomposite and polycrystalline solar cells

Four different types of solar cells prepared in different laboratories have been characterized by impedance spectroscopy (IS): thin-film CdS/CdTe devices, an extremely thin absorber (eta) solar cell made with microporous TiO2/In(OH)xSy/PbS/PEDOT, an eta-solar cell of nanowire ZnO/CdSe/CuSCN, and a solid-state dye-sensitized solar cell (DSSC) with Spiro-OMeTAD as the transparent hole conductor. A negative capacitance behavior has been observed in all of them at high forward bias, independent of material type (organic and inorganic), configuration, and geometry of the cells studied. The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices. An equivalent circuit model is suggested to explain the impedance and capacitance spectra, with an inductive recombination pathway that is activated at forward bias. The deleterious effect of negative capacitance on the device performance is discussed, by comparison of the results obtained for a conventional monocrystalline Si solar cell showing the positive chemical capacitance expected in the ideal IS model of a solar cell.     

Polymer Sensitized Quasi Solid-State Photovoltaic Cells Using Derivatives of Polythiophene

Substituted thiophene sensitized, nanocrystalline TiO2-based quasi solid-state solar cells were fabricated by using either poly （3-thiophene acetic acid） （P3TAA） or a copolymer with poly （3-thiophene acetic acid）-poly （hexyl thiophene） （P3TAA-PHT） polymers and copper iodide （Cul） as a hole conducting material together with an ionic liquid 1-ethyl-3-methylimidazolium bis （trifluoromethylsulfonyl） amide and lithium bis （triflu- oromethanesulfone） imide as additives for charge transport promotion. Dramatic enhancements in the cell performances were observed with the additives in Cul. While the cell sensitized with P3TAA generated a short-circuit photocurrent of -1.45 mA.cm^-2, an open-circuit photovoltage of -345 mV with a total power conversion efficiency of -0.3% under simulated full sunlight of 100 mW-cm^-2 （air mass： 1.5）, the cell sensitized with copolymer P3TAA-PHT delivered -0.25% efficiency under the same conditions with -1.23 mA-cm^-2 as photocurrent and -371 mV as photovoltage.     

Diketopyrrolopyrrole-containing oligothiophene-fullerene triads and their use in organic solar cells

We report the characterization of a series of oligothiophene-diketopyrrolopyrrole-fullerene triads and their use as active materials for solution processed organic solar cells (OSCs). By incorporating the diketopyrrolopyrrole (DPP) core with electron rich oligothiophene units and electron withdrawing fullerene units, multifunctional electronic molecules have been prepared; these molecules show high solubility in common organic solvents, excellent photophysical properties with high extinction coefficients (1 × 10(4) to 1 × 10(5) M(-1) cm(-1)) and broad absorption spectra coverage (250-800 nm), as well as suitable molecular orbital energy levels (HOMO of approximately -5.1 eV, LUMO of approximately -3.7 eV). Solution-processed thin-film organic field effect transistors (OFETs) from these triads revealed good n-type characteristics with electron mobilities up to 1.5 × 10(-3) cm(2) V(-1) s(-1). With these multifunctional triads, single-component OSCs have been fabricated, exhibiting power conversion efficiencies (PCEs) of up to 0.5 % under AM 1.5 G simulated 1 sun solar illumination. Blending these molecules with poly(3-hexylthiophene) (P3HT) afforded bulk heterojunction OSCs with PCEs reaching as high as 2.41%.     

Investigation of hole transport layer in relation to the properties of organic solar cells

Organic solar cells based on a blend of copper phthalocyanine and bulk fullerene are fabricated with a double hole transport layer system. The double hole transport layer was composed of poly3,4-ethylenedioxythiophene:polystyrenesulfonate, and copper phthalocyanine and inserted between the anode and active layer. The double hole transport layer system utilizes advantages of both layer. The poly3,4-ethylenedioxythiophene:polystyrenesulfonate layer modifies the surface morphology of the ITO anode and the copper phthalocyanine layer enhances hole transport. In order to enhance the conductivity of the modification layer, the optimal amount of glycerol is doped into poly3,4-ethylenedioxythiophene:polystyrenesulfonate. Furthermore, the photovoltaic characteristics are further improved. Insertion of the double hole transport layer with a 4 nm-thick copper phthalocyanine layer resulted in open circuit voltage, short current, and power conversion efficiency as high as 0.46 V, 8.8 mA/cm2 and 1.37%, respectively.     

n-Type transition metal oxide as a hole extraction layer in PbS quantum dot solar cells

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer.     

Selective electron- or hole-transport enhancement in bulk-heterojunction organic solar cells with N- or B-doped carbon nanotubes

Doping improves performance. N- or B-doped carbon nanotubes (CNTs) uniformly dispersed in the active layer of P3HT/PCMB (poly (3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester) bulk-heterojunction solar cells selectively enhance electron or hole transport and eventually help carrier collection. Specifically, the incorporation of 1.0 wt% B-doped CNTs results in balanced electron and hole transport and accomplishes a power conversion efficiency improvement from 3.0% (without CNTs) to 4.1%.