Modulated Photocurrent Spectroscopy Research Articles

Enhanced conversion efficiency of flexible dye-sensitized solar cells by optimization of the nanoparticle size with an electrophoretic deposition technique

To optimize the conversion efficiency of plastic dye-sensitized solar cells fabricated by the electrophoretic deposition technique, anatase TiO2 nanoparticles of various sizes from 10 nm to 27 nm have been synthesized via a simple hydrothermal process. The obtained TiO2 nanoparticles have been characterized by X-ray diffraction and high resolution transmission electron microscopy, which confirmed that the synthesized nanoparticles are in the pure anatase phase. Rigid devices based on D149-sensitized TiO2 particles with a size of 19 nm showed the highest conversion efficiency of 7.0% among the four different devices, which was measured under illumination of AM 1.5G, 100 mWcm−2. The effect of the particle size on the photovoltaic performance of DSSCs has been systemically studied using photoelectrochemical characterizations, including intensity modulated photocurrent spectroscopy and intensity modulated photovoltage spectroscopy. The good photovoltaic performance for 19 nm TiO2 is ascribed to the good dye loading, an efficient electron transport and the high charge collection efficiency in the photoanode. Moreover, plastic DSSCs based on 19 nm TiO2 presented a conversion efficiency of 6.0% (AM 1.5G, 100 mWcm−2) under optimized conditions, showing about a 20% enhancement in the conversion efficiency as compared to that based on commercial Degussa P25 TiO2 (5.2%). These results demonstrate that optimization of the TiO2 nanoparticle size for devices fabricated using the EPD technique is an alternative method to achieve highly efficient plastic dye-sensitized solar cells.

Comparison of the photoelectrochemical oxidation of methanol on rutile TiO2 (001) and (100) single crystal faces studied by intensity modulated photocurrent spectroscopy

The photooxidation of methanol as a model substance for pollutants on rutile TiO(2) (001) and (100) surfaces was investigated using intensity modulated photocurrent spectroscopy (IMPS). The results are analyzed in view of the influence of the surface structure, the methanol concentration and the electrode potential on the rate constants of charge transfer and recombination. The obtained results have been explained with a model combining the theory of IMPS for a bulk semiconductor surface and the nature of the surface-bound intermediates (alternatively mobile or immobile OH˙ radicals). The results indicate that water photooxidation proceeds via mobile OH˙ radicals on both surfaces, while methanol addition gives rise to the involvement of immobile OH˙ radicals on the (100) surface. Detailed analysis in view of the surface structures suggests that the latter observation is due to efficient electron transfer from bridging OH˙ radicals on the (100) surface to methanol, while coupling of two of these radicals occurs in the absence of methanol, making them appear as mobile OH˙ radicals. In the case of the (001) surface, the coupling reaction dominates even in the presence of methanol due to the smaller distance between the bridging OH˙ radicals, leading to more efficient water oxidation, but less efficient methanol photooxidation on this surface.

Enhanced Open Circuit Voltage in Aluminum Confined Post-Annealing of poly(3-hexylthiophene)/fullerene Bulk Heterojunction Solar Cells under Electric Field

ABSTRACTThe effect of an external electric field during post-annealing on the device characteristics of poly(3-hexylthiophene) (P3HT) and phenyl-C61butyric acid methyl ester (PCBM) bulk heterojunction solar cells was studied. The application of external electric field in forward bias resulted in significant enhancement in Voc and fill factor whereas devices annealed under reverse bias had an enhanced Jsc. Both forward and reverse bias annealing increased the shunt resistance. The Al - blend interface topography and carrier dynamics were studied using conducting atomic force microscopy and frequency dependent intensity modulated photocurrent spectroscopy (IMPS). The results indicate that post-annealing under external electric field can be used to engineer the interface composition to enhance the charge transport in bulk heterojunction solar cells to improve the device performance.

Defect levels in the epitaxial and polycrystalline CuGaSe2 by photocurrent and capacitance methods

The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovoltaic performance of solar cells based on chalcopyrites.

Facile synthesis of ZnO nanocrystals via a solid state reaction for high performance plastic dye-sensitized solar cells

We report the facile synthesis of ZnO nanocrystals via a one-step solid state reaction at room temperature and their application as the photoanode in plastic dye-sensitized solar cells (DSCs). ZnO nanoparticles were prepared utilizing zinc acetate dihydrate and sodium hydroxide with a short grinding time and without a sintering process. The as-prepared samples with the polycrystalline hexagonal wurtzite structure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The obtained ZnO nanoparticles exhibited high crystallinity even without a high temperature sintering treatment during the preparation process. The effects of compression post-treatment on the photovoltaic performance of DSCs were also investigated using intensity modulated photocurrent spectroscopy (IMPS), incident photo-to-current conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS). The results indicate that the improvement of power conversion efficiency after compression post-treatment of ZnO photoelectrode can be attributed to its high photoelectron collection efficiency and effective electron transport. Under the optimized conditions, a full plastic D149-sensitized ZnO solar cell measured under illumination of 100 mW·cm−2 (AM 1.5G) presents an energy conversion efficiency of 3.76% with open-circuit voltage of 0.688 V, short-circuit current density of 8.55 mA·cm−2, and fill factor of 0.64. These results demonstrate that the one-step solid state reaction is a convenient and effective method for the synthesis of ZnO nanocrystals for use in plastic DSCs. Open image in new window

Intensity Modulated Photocurrent Spectroscopy of Organic Solar Cells

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Mechanism of Recombination Losses in Bulk Heterojunction P3HT:PCBM Solar Cells Studied Using Intensity Modulated Photocurrent Spectroscopy

Intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies were used to study the mechanism of photoprocesses in P3HT:PCBM bulk heterojunction organic solar cells at various light intensities. The use of the frequency domain techniques allowed us to separate the bulk and interfacial processes and gain a valuable insight into the mechanism of losses in these devices. The results provide direct evidence that interfacial nongeminate recombination is one of the dominant loss and aging mechanisms in bulk heterojunction organic solar cells. The trapping of photoexcited holes in the P3HT phase was found to contribute to the increased recombination rate. The results suggest that promising ways of improving the efficiency of bulk heterojunction solar cells may be reducing the charge trapping both at and near the P3HT:PCBM interface, as well as improving the efficiency of charge extraction at contacts.

Influence of Ga content on defects in CuIn xGa 1 − x Se 2 based solar cell absorbers investigated by sub gap modulated photocurrent and admittance spectroscopy

In this work, we investigate the influence of gallium content on the defects properties of co-evaporated CuIn xGa 1 − x Se 2 by sub gap modulated photocurrent spectroscopy and admittance spectroscopy techniques. A series of CuIn xGa 1 − x Se 2 based solar cells with different gallium content in the range from 0% to 33%, and with the same CdS buffer layer have been investigated. On one hand, photocurrent spectroscopy results show 2 types of defects named D1 and D2, and on the other hand, admittance spectroscopy results exhibit only one type of defect. I–V curves show that one of the two defects probed by photocurrent is responsible of the dominant recombination mechanisms next to the heterointerface, between the absorber and the buffer layers. Moreover, I–V curves under AM 1.5 conditions show that the cell with no probed D2 defect presents the best photovoltaic performances.

Defect levels in CuGaSe 2 by modulated photocurrent spectroscopy

Results of high frequency modulated photocurrent spectroscopy (HF MPC) performed on epitaxial and polycrystalline CuGaSe 2 thin films are presented. Frequency and temperature scans of MPC in the high frequency regime are compared, and the advantages of the second mode of measurement over the first one are demonstrated. Electronic parameters of defect levels in Cu-rich and Ga-rich stoichiometry are obtained and discussed in comparison to the literature data on defect levels derived from capacitance junction techniques. An attempt to correlate levels observed by MPC technique with material stoichiometry has been made.

Dynamic Characterization of Hybrid Solar Cells Based on Polymer and Aligned ZnO Nanorods by Intensity Modulated Photocurrent Spectroscopy

For characterization of polymer-based solar cells with vertically aligned ZnO nanorod arrays (ZnO-NAs) by intensity modulated photocurrent spectroscopy (IMPS), a dynamic IMPS model is developed, where the structure-related charge generation and transport dynamics are considered. The model describes the IMPS responses affected by the phase shift ϕn(ω) due to the exciton diffusion property (ω0) and the structure-related device ideality factor N, the electron diffusion coefficient De, the exciton dissociation rate S, and the device structure (e.g., nanorod length d and interspacing l). The main expectations of the model are confirmed by the experimental data of the polymer/ZnO-NA cells with d = 180−650 nm, offering mechanistic information on the structure-related charge generation, charge transport, and device performance. The presence of the ϕn(ω) makes IMPS response not spiral into the origin and the phase angle in its Bode plot not tend to 90°; the d-dependent direct diffusion (DD) and diffusion-reflectio...

Construction of Nanocrystalline Film on Nanowire Array via Swelling Electrospun Polyvinylpyrrolidone-Hosted Nanofibers for Use in Dye-Sensitized Solar Cells

A 74% enrichment of the efficiency of ZnO nanowire (NW) dye-sensitized solar cells (DSSCs) is achieved by the addition of a novel light-scattering nanocrystalline film (nanofilm). The 100 nm thick nanofilm is derived from the polyvinylpyrrolidone-hosted SnO(2)/ZnO nanofibers electrospun on the top of ZnO NW arrays via methanol vapor treatment followed by high-temperature calcination. Structural characterizations show that the film is composed of SnO(2) and ZnO nanocrystals with a diameter of ∼10 nm. Short-circuit current, open-circuit voltage, and fill factor of the nanofilm/ZnO NW DSSCs are all superior to those of the ZnO NW DSSCs. The mechanism of photocurrent enhancement in the nanofilm/ZnO NW DSSCs has been investigated using optical modulation spectroscopy. Intensity modulation photocurrent spectroscopy (IMPS) measurements reveal that the dye-sensitized nanofilm does not contribute significant photocurrent in the nanofilm/ZnO NW DSSCs. The significant enhancement of the efficiency of the ZnO NW DSSCs is achieved by reflecting unabsorbed photons back into the NW anode using the novel light-scattering layer of nanofilm.

Mapping out the distribution of electronic states in the mobility gap of amorphous zinc tin oxide

Amorphous zinc tin oxide (ZTO) is a wide-band-gap (transparent) semiconductor which exhibits high electron mobilities irrespective of its disordered nature. Transient photocapacitance (TPC), drive level capacitance profiling (DLCP), and modulated photocurrent spectroscopy (MPC) were used to determine the electronic state distribution within the mobility gap of ZTO. Conduction band-tail and valence band-tail Urbach energies near 10 and 110 meV were obtained by MPC and TPC, respectively. DLCP indicated free carrier densities in the mid-1015 cm−3 range plus a 0.2eV wide band of defects 0.4eV from the conduction band. The MPC spectra for ZTO also disclosed a defect band near the conduction band-tail.

Electric Field Effects on Charge Transport in Polymer/TiO2 Photovoltaic Cells Investigated by Intensity Modulated Photocurrent Spectroscopy

The electric field effects on the charge transport dynamics in bilayer polymer/TiO2 photovoltaic cells are studied by intensity modulated photocurrent spectroscopy (IMPS), for the first time, where a small bias voltage (Va) in the range from −0.1 to 0.2 V is applied to a cell during each IMPS measurement. The bilayer cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and nanostructured TiO2 are constructed by using a polymer layer thickness l of 350 nm but varied TiO2 layer thicknesses (d = 40 and 65 nm). All the experimental IMPS responses are satisfactorily fitted to the previously developed IMPS model for the bilayer cells at Va = 0 V, providing a detailed analysis of charge transport properties under the applied voltages. It is revealed that the external electric field almost exclusively changes the exciton dissociation rate (S) at the donor/acceptor (D/A) interface, but imposes only a very faint effect on the transport dynamics of photogenerated electrons [ϕn(ω), ...

Photoelectrochemical reduction of nitrate on p-Si coated with metallic Re thin films

In this study we examined the rhenium electrodeposition process onto p-Si(100) from acidic media. The study was carried out by means of cyclic voltammetry and the potential-steps method from which the corresponding nucleation and growth mechanism were determined. Both methods were performed under illumination using a solar simulator for electron photogeneration. A 3D progressive nucleation, diffusion-controlled growth of rhenium films was found. Likewise, a morphologic analysis was completed for the deposits obtained at different potential values by means of atomic force microscopy. An energetic characterization through capacitance measurements (Mott–Schottky plots and parallel capacitance) of the p-Si/NO 3 − and p-Si/Re/NO 3 − interfaces was done. The photoelectrochemical reduction of nitrate ions, PERN, on the different p-Si/Re electrode systems synthesized was studied. An overpotential decrease of 0.3 V and a photocurrent increase for the PERN on p-Si(100)/Re electrode systems compared with p-Si(100) and metallic Re was found. Finally, the kinetic parameters of the cathodic reactions in the p-Si and p-Si/Re acidic media were estimated using intensity modulated photocurrent spectroscopy. A brief analysis from this technique was done. According to these results, the p-Si/Re electrode system could be a potential photoelectrocatalyst for the PERN.

Rhenium electroless deposition on p-Si(1 0 0) from HF solutions under illumination: Hydrogen evolution reaction onto p-Si/Re systems

In this study we examined the rhenium photoelectroless deposition process onto p-Si(1 0 0) from HF solutions. In the first stage an energetic characterization through capacitance measurements (Mott–Schottky plots) of the p-Si/HF interface was done. Before the rhenium photoelectroless deposition process, a voltammetric and photovoltammetric analysis of p-Si in ReO 4 - / HF solutions was done. In this study, a decrease of the rhenium deposition charge was observed as a function of the increased HF concentration. This behavior was attributed to competition with the silicon dissolution process when an increased HF concentration was present in the electrolytic bath. A morphologic analysis was completed by means of atomic force microscopy (AFM) for the rhenium deposits obtained by photoelectroless process at different HF concentrations and different deposition times. X-ray photoelectron spectroscopy (XPS) technique was utilized to characterize the rhenium deposits, and it was concluded that the films formed by the photoelectroless process were metallic rhenium with only the uppermost atomic layer containing rhenium oxide species. The hydrogen evolution reaction (HER) on the different p-Si/Re electrode systems synthesized was studied. For the HER on p-Si(1 0 0)/Re electrode systems an overpotential decrease of 0.2 V and a photocurrent increase between one and two orders of magnitude, compared with p-Si(1 0 0) and metallic Re, was found. Additionally, the kinetic parameters of the cathodic reactions in the p-Si and p-Si/Re acidic media were estimated using intensity modulated photocurrent spectroscopy (IMPS). A brief analysis from this technique was done. According to these results, the p-Si/Re electrode system could be a potential photoelectrocatalyst for the HER.

Characterization of Polymer/TiO2 Photovoltaic Cells by Intensity Modulated Photocurrent Spectroscopy

A dynamic model for the intensity modulated photocurrent spectroscopy (IMPS) of bilayer polymer/TiO2 photovoltaic cells is developed in consideration of the exciton generation in the polymer layer, exciton diffusion to the polymer/TiO2 interface, electron injection into the TiO2 layer, and electron diffusion through the TiO2 layer; particularly, the phase shift ϕn(ω) due to the time delay between exciton generation and dissociation is included in the continuity equations for electron transport. Bilayer polymer/TiO2 cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and nanostructured TiO2 were prepared for experimental purposes, with a TiO2 layer thickness d of 120 and 65 nm. Experimental data obtained confirm all the main expectations of the model, providing important information on incident photon-to-current conversion efficiency (IPCE), exciton dissociation, and electron transport. The frequency-dependent ϕn(ω) affects the location (Phigh) where IMPS response cross...

Trapping effects on the frequency response of dithiolene-based planar photodetectors

The paper analyses the interrelation between the signal frequency response in an organic planar photodetector and the distribution of traps of the active material playing a fundamental role in the transport of charge carriers. In particular, the response of a dithiolene-based photodetector spectrally matched to the near infrared has been acquired under optical pulses ranging from 10 Hz to 1 MHz in frequency. By means of modulated photocurrent spectroscopy, the trap density distribution of the dithiolene in the solid state has been extracted. The capture and release of photogenerated carriers exerted by a Gaussian density of trapping states, located 0.42 eV below the transport energy, is shown to be the responsible for the observed device frequency response.

Relationship between Phase Shift, Square-Wave Response and Density of States in Modulated Photocurrent Spectroscopy

AbstractLow- and high-frequency modulated photoconductivity measurements (LF and HF MPC) have been made on amorphous silicon films prepared by the expanding thermal plasma (ETP) and RF PECVD techniques. Time constants have been measured by decay of square wave excitation and behavior of complex frequency response. The influence of quasi-Fermi level position has been examined. Band tail slopes of 32 meV (ETP) and 37 meV (PECVD), and defect densities of order 1018 and 1017 cm-3 eV-1 respectively are found. Tail state capture coefficients of order 3×10-8 cm3 s-1 are calculated from overlapping LF and HF regimes. Defect state values for ETP (< 10-8 cm3 s-1) are smaller than for PECVD silicon films (> 10-7 cm-3 s-1).

Measurement of built-in voltage of organic semiconductor devices

The built-in voltage in organic semiconductors is usually measured by electroabsorption measurements. In this paper, we show that for tris-(8-hydroxy quinolato)aluminium (Alq) devices, modulated photocurrent spectroscopy provides a simple viable alternative to measuring the built-in voltage. This is possible because the quantum efficiency for photoconductivity in Alq is independent of the electric field. We also examine the limitations of modulated photocurrent spectroscopy in obtaining the built-in electric field.

Photocurrent spectroscopy of solution-grown CdS films annealed in CdCl2 vapour

Thin cadmium sulfide (CdS) films (100–130 nm) are deposited on glass from an alkaline solution. The films are annealed in air, in the vicinity of a CdCl2 source, at different temperatures (280–400°C) and for different time periods (below 75 min). The change in some of the optoelectronic parameters (resistivity, direct and indirect transition energies) as a result of annealing is studied using modulated photocurrent spectroscopy. The experimental results reveal that annealing in air, in the absence of a CdCl2 source, reduces the bandgap energy of CdS by 0.12 eV due to the formation of valence-band states. Annealing in CdCl2 vapour removes the tail states and establishes the bulk bandgap (2.42 eV). It also increases the film's photoconductivity by a factor of 10 under white light illumination of intensity 100 mW cm−2, yielding a dark-to-light resistivity ratio of 3 × 104. The film's optoelectronic properties are controlled by two defect energy levels that are located at 0.14 and 1.30 eV below the conduction band edge. The increase in photoconductivity by annealing is due to the improvement of excess carrier lifetime as a result of changes in the 1.30 eV energy level.