High Dark Conductivity Research Articles

Development of high conducting phosphorous doped nanocrystalline thin silicon films for silicon heterojunction solar cells application

We have investigated the plasma-enhanced chemical vapor deposition growth of the phosphorus-doped hydrogenated nanocrystalline silicon (n-nc-Si:H) film as an electron-selective layer in silicon heterojunction (SHJ) solar cells. The effect of power densities on the precursor gas dissociation are investigated using optical emission spectra and the crystalline fraction in n-nc-Si:H films are correlated with the dark conductivity. With the P d of 122 mW cm−2 and ∼2% phosphorus doping, we observed Raman crystallinity of 53%, high dark conductivity of 43 S cm−1, and activation energy of ∼23 meV from the ∼30 nm n-nc-Si:H film. The n-nc-Si:H layer improves the textured c-Si surface passivation by two-fold to ∼2 ms compared to the phosphorus-doped hydrogenated amorphous silicon (n-a-Si:H) layers. An enhancement in the open-circuit voltage and external quantum efficiency (from >650 nm) due to the better passivation at the rear side of the cell after integrating the n-nc-Si:H layer compared to its n-a-Si:H counterpart. An improvement in the charge carrier transport is also observed with an increase in fill factor from ∼71% to ∼75%, mainly due to a reduction in electron-selective contact resistivity from ∼271 to ∼61 mΩ-cm2. Finally, with the relatively better c-Si surface passivation and carrier selectivity, a power conversion efficiency of ∼19.90% and pseudo-efficiency of ∼21.90% have been realized from the SHJ cells.

High conductivity characteristics of phosphorus-doped nanocrystalline silicon thin films by KrF pulsed excimer laser irradiation method.

The microstructure and high conductivity properties of phosphorus-doped nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique and the KrF pulsed excimer laser irradiation method. The results of Fourier transform infrared spectroscopy and Raman spectroscopy show that Si nanocrystallites with an average diameter of 2 nm to 3 nm are formed in the film. The degree of crystallinity increases with the increase of laser radiation intensity, while the content of hydrogen decreases gradually. More phosphorus atoms are substitutionally incorporated into the nc-Si dots under higher laser irradiation fluence, which is responsible for the high dark conductivity. By controlling the laser fluence at 1.0 J cm-2, the dark conductivity as high as 25.7 S cm-1 can be obtained. Based on the measurements of temperature-dependent conductivity, the carrier transport processes are discussed. The phosphorus doping and the increase of electron concentration are considered to be the reason for high dark conductivity and extremely low conductivity activation energy.

X-ray induced photoconductivity and its correlation with structural and chemical defects in heteroepitaxial diamond

Three sets of heteroepitaxial diamond crystals grown under nominally identical process conditions on Ir/YSZ/Si(001) substrates have been studied with respect to structural defects, chemical purity, and x-ray induced photoconductivity (PC). The dislocation density that varied systematically between 107 and 109 cm−2 had a minor influence on dark conductivity and photoconductive gain G. In contrast, the substitutional nitrogen (NN) and substitutional boron (NB) defects, which are both present at very low concentrations (≤1 ppb), turned out to be the crucial factors controlling the electrical behavior. Small differences between both resulted in variations of the photocurrents by up to 5 orders of magnitude. The maximum in G of 0.75×104 was measured in the sample with highest dark conductivity. It could be explained conclusively within our model calculations on gain formation by NN≤NB. For low gain samples, we found NN>NB. However, the measured G values were far from theoretical predictions. This indicates a dominating role of additional traps. After x-ray switch-off, persistent photoconductivity (PPC) was observed in high gain samples. It was attributed to an energetic barrier hampering the recharging of nitrogen atoms by hole capture. As a possible source, strain fields generated by dislocations are suggested.

Phosphorus-doped nanocrystalline silicon-oxycarbide thin films

P-doped nc-SiOxCy:H films are grown at moderately low substrate temperature ~250°C and rf power ~200 W via controlled incorporation of C and O from CH4 and CO2 precursor gases by PECVD. The optoelectronic and structural properties of the films have been studied in detail. On gradual increase in CH4 flow rate (F) in the plasma, systematic formations of the Si-C and Si-O-C bonds in a-SiOxCy:H network are evident from the gradual shift of the Si-H wagging and Si-O-Si stretching vibrational modes towards higher and lower wavenumbers, respectively. XPS studies have confirmed on the presence of Si-O-C bonds and the P-atoms as dopants in the network. The Si-C bond density has increased from 1.56×1021 to 4.1×1021 cm−3 and the grain size of the nanocrystals has reduced from 6 to 3.2 nm with the increase in F from 0 to 0.3. An enhancement in the optical band gap is accomplished via the addition of stronger Si-C bonds in the a-SiOxCy:H matrix and also due to quantum confinement effect arising from the reduced size of the nanocrystals, together. The optimum n-type nc-SiOxCy:H film possessing ~50.5% crystallinity with nanocrystal grain size ~5 nm has demonstrated an optical band gap ~1.96 eV and high dark conductivity ~6.41 × 10−1 S cm−1, which altogether appear promising for the window layer of all-Si tandem solar cells. A switching of the electrical characteristics from the degenerate-like crystalline network to its non-degenerate amorphous-like configuration is identified at higher C incorporation in the P-doped SiOxCy:H complex network, corresponding to the CH4 flow rate above 0.2sccm.

Optoelectronic and structural properties of Ge-rich narrow band gap nc-SixGe1-x absorber layer for tandem structure nc-Si solar cells

Germane (GeH4) and silane (SiH4) have grossly different dissociation behaviors. In the conventional preparation process, the inclusion of GeH4 in the gas mixture under nc-Si compatible growth conditions results in the spontaneous dissociation of GeH4 in the plasma into various lower hydrides of Ge with lower mobility than Si-hydrides. Ge-radicals exhibit a higher viscosity co-efficient and smaller surface diffusion length when compared to Si-radicals, which prompt the arbitrary and excess residence of Ge atoms at the growth sites, eventually destroying the development of nanocrystallinity in the film network. In the present work, the controlled inclusion of Si atoms during the growth of the network at ~220 °C under plasma conditions compatible for growing high-quality nc-Ge film has been pursued to produce nc-SiGe alloy material through which the dissociation of SiH4 becomes admissible for sustained nanocrystallinity. Systematic changes in the optoelectronic and structural characteristics of the intrinsic nc-SixGe1–x films from H2-diluted (SiH4 + GeH4)-plasma have been studied and correlated. The fabricated nc-SixGe1–x (x > 0) films belong to the Ge-nanocrystal dominated mixed-phase materials bearing a-SiGe matrix. The nc-SixGe1–x film sustaining significant crystallinity with an adequately widened optical band gap to Eg ~1.23 eV due to the incorporation of Si and its corresponding high dark conductivity (σD~4.72×10−4 S cm−1) appears appropriate for the absorber layer in the bottom sub-cell of nc-Si tandem solar cells.

Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.

Structural and optoelectronic properties of p-type SiO:H films deposited in transition zone

P-type hydrogenated silicon oxide (p-SiOx:H) films are prepared by radio frequency plasma enhanced chemical deposition with various CO2 flow rates. We use gas mixtures of carbon dioxide (CO2), hydrogen (H2), silane (SiH4) and diborane (B2H6) as reaction source gases. For all experiments the substrate temperature, pressure and power density are fixed at 200 oC, 200 Pa and 200 mW/cm2, respectively. The films are deposited on Corning Eagle 2000 glass substrates for optoelectronic measurements and on crystalline Si wafers for Fourier transform infrared (FTIR) measurement. The structural, optical and electronic properties of the films are systematically studied as a function of CO2 flow rate. The CO2 flow rate is varied from 0 to 1.2 cm3 min-1, with all other parameters kept constant. It is shown that with the CO2 flow rate increasing from 0 to 1.2 cm3 min-1, the Raman peak shifts from 520 cm-1 to 480 cm-1 and corresponding crystalline volume fraction decreases from 70% to 0. In addition, the FTIR spectrum shows that the oxygen content increases from 0 to 17% and the hydrogen bond configuration gradually shifts from mono-hydrogen (Si-H) to di-hydrogen (Si-H2) and (Si-H2)n complexes in the film. What is more, with the incorporation of oxygen, the optical band gap of each of all p-type SiO:H films increases from 1.8 eV to 2.13 eV, while the dark conductivity decreases from 3 S/cm (nc-Si:H phase) to 8.310-6 S/cm (a-SiOx:H phase). Furthermore, the oxygen incorporation tends to disrupt the growth of silicon nanocrystals due to the created dangling bonds that arises from an increased structural disorder. This leads to microstructural evolution of SiO:H film from a single nanocrystalline phase into first a mixed amorphous-nanocrystalline and subsequently into an amorphous phase. At a certain threshold of CO2 flow rate, a transition from nanocrystalline to amorphous growth takes place. The transition from nanocrystalline to amorphous silicon is confirmed by Raman and FTIR spectra. In the transition region or crystalline volume fraction of about 45%, Raman spectrum also reveals that the a mixture of nanocrystalline silicon and amorphous silicon oxide (a-SiOx:H) phase exists in the film. This means that nanocrystalline silicon oxide (nc-SiO:H) is a two-phase structural material consisting of a dispersion of silicon nanocrystals (nc-Si) embedded in the amorphous SiOx network. As is well known, the oxygen-rich amorphous phase can help enhance the optical band gap, while the nc-Si phase contributes to high conductivity. Finally, it is the SiO:H film deposited at phase transition that can realize a relatively high dark conductivity (about S/cm) with a wide optical band gap of 2.01 eV in the film. By using the transition p-layer as the window layer in conjunction with a suitable buffer thickness, we obtain a thin film solar cell with an open-circuit voltage of 890 mV, a short-circuit current density of 12.77 mAcm-2, fill factor of 0.73, and efficiency of 8.27% without using any back reflector.

H2-Ar dilution for improved c-Si quantum dots in P-doped SiNx:H thin film matrix

Phosphorus-doped hydrogenated silicon nitride (SiNx:H) thin films containing crystalline silicon quantum dot (c-Si QD) was prepared by plasma enhanced chemical vapor deposition (PECVD) using hydrogen-argon mixed dilution. The effects of H2/Ar flow ratio on the structural, electrical and optical characteristics of as-grown P-doped SiNx:H thin films were systematically investigated. Experimental results show that crystallization is promoted by increasing the H2/Ar flow ratio in dilution, while the N/Si atomic ratio is higher for thin film deposited with argon-rich dilution. As the H2/Ar flow ratio varies from 100/100 to 200/0, the samples exhibit excellent conductivity owing to the large volume fraction of c-Si QDs and effective P-doping. By adjusting the H2/Ar ratio to 100/100, P-doped SiNx:H thin film containing tiny and densely distributed c-Si QDs can be obtained. It simultaneously possesses wide optical band gap and high dark conductivity. Finally, detailed discussion has been made to analyze the influence of H2-Ar mixed dilution on the properties of P-doped SiNx:H thin films.

Influence of RF power on structural optical and electrical properties of hydrogenated nano-crystalline silicon (nc-Si:H) thin films deposited by PE-CVD

We report synthesis of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by using conventional plasma enhanced chemical vapor deposition (PE-CVD) system from gas mixture of pure silane (SiH4) and hydrogen (H2). We investigated the effect of RF power on structural, optical and electrical properties using various characterization techniques including Raman spectroscopy, FTIR spectroscopy, UV–visible spectroscopy etc. Low angle XRD and Raman spectroscopy analysis revealed that the RF power in PE-CVD is a critical process parameter to induce nanocrystallization in Si:H films. The FTIR spectroscopy analysis results indicate that with increase in RF power the predominant hydrogen bonding in films shifts from Si–H to Si–H2 and (Si–H2)n bonded species bonded species. However, the bonded hydrogen content didn’t show particular trend with change in RF power. The UV–visible spectroscopy analysis shows that the band tail width (E04–ETauc) with increase in RF power. The defect density and Urbach energy also increases with increase in RF power. The highest dark conductivity (and lowest charge carrier activation energy) was obtained for the film deposited at RF power of 125 W indicating that 125 W is optimized RF power of our PE-CVD unit. At this optimized RF power nc-Si:H films with crystallite size ~3.7 nm having good degree of crystallinity (~86.7 %) and high band gap (ETauc ~ 2.01 eV and E04 ~ 2.58 eV) were obtained with a low hydrogen content (6.2 at.%) at moderately high deposition rate (0.24 nm/s).

Doped microcrystalline silicon oxide alloys for silicon‐based photovoltaics: Optoelectronic properties, chemical composition, and structure studied by advanced characterization techniques

Doped microcrystalline silicon oxide (μc‐SiOx:H) alloys attract significant attention as a functional material in photovoltaic devices. By using various advanced characterization methods, we have studied the relationship between optoelectronic properties, chemical composition, and structure of p‐type µc‐SiOx:H deposited by plasma enhanced chemical vapor deposition (PECVD). For a wide range of compositions with varying oxygen content, we show that the dominant components are Si and a‐SiO2, while the fraction of suboxides is minor. The μc‐SiOx:H material with sufficient oxygen content (x = 0.35) exhibits an enlarged optical gap E04 > 2.2 eV and sufficiently high dark conductivity >10−6 S cm−1; the crystalline silicon fraction has a filament‐like shape (with a typical width of around 10 nm) forming a branch‐like structure elongated in the growth direction over several hundreds of nanometers.

New insight into the microstructure and doping of unintentionally n-type microcrystalline silicon carbide

Microcrystalline silicon carbide (μc-SiC:H) deposited by hot wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) provide advantageous opto-electronic properties, making it attractive as a window layer material in silicon thin-film and silicon heterojunction solar cells. However, it is still not clear which electrical transport mechanisms yield dark conductivities up to 10−3 S/cm without the active use of any doping gas and how the transport mechanisms are related to the morphology of μc-SiC:H. To investigate these open questions systematically, we investigated HWCVD and PECVD grown layers that provide a very extensive range of dark conductivity values from 10−12 S/cm to 10−3 S/cm. We found out by secondary ion mass spectrometry measurements that no direct correlation exists between oxygen or nitrogen concentrations and high dark conductivity σd, high charge carrier density n, and low activation energy Ea. Higher σd seems to rise from lower hydrogen concentrations or/and larger coherent domain sizes LSiC. On the one hand, the decrease of σd with increasing hydrogen concentration might be due to the inactivation of donors by hydrogen passivation that gives rise to decreased n. On the other hand, qualitatively consistent with the Seto model, the lower σd and lower n might be caused by smaller LSiC, since the fraction of depleted grain boundaries with higher Ea increases accordingly.

Boron Doped Nanocrystalline Film with Improved Work Function as a Buffer Layer in Thin Film Silicon Solar Cells

We investigated thin film silicon solar cells with boron doped hydrogenated nanocrystalline silicon/ hydrogenated amorphous silicon oxide [p-type nc-Si:H/a-SiOx:H] layer. First, we researched the bandgap engineering of diborane (B2H6) doped wide bandgap hydrogenated nanocryslline silicon (p-type nc-Si:H) films, which have excellent electrical properties of high dark conductivity, and low activation energy. The films prepared with lower doping ratio and higher hydrogen dilution ratio had higher optical gap (Eg), with higher dark conductivity (σ(d)), and lower activation energy (Ea). We controlled Eg from 2.10 eV to 1.75 eV, with σ(d) from 1.1 S/cm to 7.59 x 10(-3) S/cm, and Ea from 0.040 eV to 0.128 eV. Next, we focused on the fabrication of thin film silicon solar cells. By inserting p-type nc-Si:H film into the thin film silicon solar cells, we achieved a remarkable increase in the built-in potential from 0.803 eV to 0.901 eV. By forming p-type nc-Si:H film between SnO2:F/ZnO:Al (30 nm) and p-type a-SiOx:H layer, the solar cell properties of open circuit voltage (Voc), short circuit current density (Jsc), and efficiency (η) were improved by 3.7%, 9.2%, and 9.8%, respectively.

Effect of thermal annealing on the optical and electrical properties of boron doped a-SiOx:H for thin-film silicon solar cell applications

The p-type layer in a p-i-n thin-film solar cell plays a crucial role in determining efficiency. The requirements for p-type layer films are high optical band gap (Eg), narrow valence band tail to minimize optical absorption, high dark conductivity, and low activation energy to reduce the parasitic series resistance of the solar cell. We investigated the effects of temperature during film growth and post-deposition thermal annealing on the optical and electronic properties of p-type amorphous silicon oxide films (p-a-SiOx:H) for thin-film silicon solar cell applications. The activation energy of thermally annealed p-a-SiOx:H film prepared at low substrate temperature decreased from 0.72eV to 0.56eV with similar Eg. Our improvements are explained in the changed ratio of conjugation with the three- and four-fold coordinated boron atoms by the shift of the B (1s) X-ray photoelectron spectrum. Taking into account the reversible electrical change by thermal annealing while maintaining high optical properties, we propose necessary process-procedure conditions for obtaining high photovoltaic performance in thin-film-Si solar cells with high-quality p-a-SiOx:H. We carried out device modeling of p-i-n junction amorphous silicon solar cells by employing a thermal annealing effect on p-type a-SiOx:H layer, using an advanced semiconductor analysis simulator. Due to reduced Ea with high Eg of p-type a-SiOx:H layer after thermal annealing, the solar cell performance of the open circuit voltage, fill factor, and conversion efficiency improved by 11.1%, 60.42%, and 53.75%, respectively.

Boron doped nanocrystalline silicon/amorphous silicon hybrid emitter layers used to improve the performance of silicon heterojunction solar cells

Boron doped nanocrystalline silicon/amorphous silicon hybrid thin films were deposited by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) to improve the performance of silicon heterojunction (SHJ) solar cells. Electrical and optical properties as well as structural and passivation characteristics of these thin films were systematically researched as a function of TMB gas mixture ratio. A high dark conductivity of 6.5×10−4S/cm and minority carrier lifetime (τs) of 1740μs on Czochralski (Cz) Si wafers were obtained with the hybrid p-type Si films. We applied this optimized film as an emitter layer on SHJ solar cells based on Cz silicon wafers; a significant improvement in the solar cell wavelength response at 400nm and output performance have been achieved.

Reduction of Tail State on Boron Doped Hydrogenated Amorphous Silicon Oxide Films Prepared at High Hydrogen Dilution

In this report, we have investigated on the defect state of diborane (B2H6) doped wide bandgap hydrogenated amorphous silicon oxide (p-type a-SiO:H) films prepared using silane (SiH4), hydrogen (H2) and nitrous oxide (N2O) in a radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) system with different hydrogen dilutions. The films prepared with higher hydrogen dilution show lower Urbach energy (Eu), lower microstructure (R*), lower short and medium range disorder (omegaTO, Gamma(TO), I(TA)/I(TO), I(LA)/I(TO)), higher dark conductivity (sigma d) and higher refractive index (n) with high optical gap (Eg). Eu decreases from 248 meV to 153 meV, and R* decreases from 0.46 to 0.26, Raman peak omegaTO-TO mode position shifts from 480.24 to 483.28, GammaTO-full width half maximum of omegaTO decreases from 78.16 to 63.87, I(TA)/I(TO)-the ratio of integrated area of TA and TO mode decreases from 0.624 to 0.474, I(LA)/I(TO)-the ratio of integrated area of LA and TO mode deceases from 0.272 to 0.151, sigma d increases from 4.6 x 10(-7) S/cm to 1.1 x 10(-6) S/cm, n increases from 3.70 to 3.86. Reduced Nd, Eu and R* at wide Eg indicates that the films are more useful for solar cell window layer. Applying this layer to a single junction solar cell shows open circuit voltage (Voc) = 0.80 V, short circuit current density (Jsc) = 16.3 mA/cm2, fill factor (FF) = 72%, efficiency (eta) = 9.4%.

Evolution of microstructure and opto-electrical properties in boron doped nc-Si:H films deposited by HW-CVD method

Demand for an efficient window layer for a-Si:H based solar cells in terms of electrical conductivity and optical band gap is ever increasing since the inception of single junction and tandem solar cells. In this paper, we report synthesis of highly conducting boron doped p-type nc-Si:H films by HW-CVD using the mixture of silane (SiH4) and diborane (B2H6) without hydrogen (H2) dilution. Variation in film characteristics with B2H6 gas-phase ratio was studied, and revealed that the boron doping induces amorphization in nc-Si:H film structure. The AFM analysis show increase in rms surface roughness and micro-void density while FTIR spectroscopy analysis show shift of hydrogen bonding from Si–H2 and (Si–H2)n complexes to Si–H configuration on boron doping. The hydrogen content was found <2.66 at.% while the band gap remain as high as 1.8 eV or more. At optimized B2H6 gas-phase ratio, we have obtained p-type nc-Si:H films having high band gap (∼2.0 eV), high dark conductivity (∼1.2 S/cm) with low hydrogen content (∼1.78 at.%) at reasonably high deposition rate (∼9.2 A/s). The obtained films can be used as a window layer in a-Si:H based p–i–n and tandem solar cells and color light detectors.

One-Step Surface Doping of Organic Nanofibers to Achieve High Dark Conductivity and Chemiresistor Sensing of Amines

High dark electrical conductivity was obtained for a p-type organic nanofibril material simply through a one-step surface doping. The nanofibril composite thus fabricated has been proven robust under ambient conditions. The high conductivity, combined with the intrinsic large surface area of the nanofibers, enables development of chemiresistor sensors for trace vapor detection of amines, with detection limit down to sub-parts per billion range.

Effects of doping concentration on the microstructural and optoelectrical properties of boron doped amorphous and nanocrystalline silicon films

Boron doped hydrogenated amorphous silicon thin films were prepared by plasma-enhanced chemical vapor deposition technique at various flow rate of diborane (FB). As-deposited samples were thermally annealed at the temperature of 800 °C to obtain the doped nanocrystalline silicon (nc-Si) films. The effect of boron concentration on the microstructural, optical and electrical properties of the films was investigated. X-ray photoelectron spectroscopy (XPS) measurements demonstrated the presence of the substitutional boron in the doped films. It was found that thermal annealing can efficiently activate the dopants in films accompanying with formation of nc-Si grains. Based on the temperature-dependent conductivity measurements, it was shown that the dark conductivity of doped amorphous samples increases monotonously with the increase of doping content. While the dark conductivity of doped nc-Si films is not only determined by the concentration of dopant but also the crystallinity of the films. As increasing the flow rate of diborane, the crystallinity of doped nc-Si films decreases, which causes the decrease of dark conductivity. Finally, the high dark conductivity of 178.68 S cm−1 of the B-doped nc-Si thin films can be obtained.

Improvement of the electrical properties of nanocrystalline silicon films by the KrF pulsed excimer laser irradiation method

The effects of laser fluence on the microstructure and electrical properties of nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique. It was found that the content of hydrogen in the films could be reduced gradually by increasing the laser fluence, accompanied by the formation of nc-Si. The evolution of hydrogen is responsible for a change in the optical band gap and dark conductivity. By controlling the laser fluence at 1.0 J cm−2, a dark conductivity as high as 5.9 × 10−3 S cm−1 could be obtained. Based on measurements of the temperature-dependent conductivity, the carrier transport processes are discussed. The effusion of hydrogen and the increase of crystallinity are thought to be contributed to the high dark conductivity and low conductivity activation energy.

SYNTHESIS AND PHOTOSENSITIVE PERFORMANCE OF NANOSTRUCTURED ZnO/DYE HYBRID FILM FOR ENERGY CONVERSION

The ZnO film prepared by doctor blade method are highly porous in presence of structure directing agent PEG and then can be easily sensitized by various molecules. In the present work, the nanostructured ZnO /dye hybrid film prepared by doctor blade method has been investigated in conductivity and photoconductivity measurements in view of applications in dye-sensitized solar cells (DSSC) and in optoelectronics, since electron transport in these devices plays a decisive role for the electron collection efficiency and therefore for the overall efficiency. The absorption of ZnO film sensitized by Rose Bengal dye (RB) has been studied by UV spectroscopy which indicates that after the dye is adsorbed on the ZnO electrode, its absorption spectra showed red-shift in the peak position compared to the absorbance spectra of dye in ethanol. Highly porous ZnO films are found to have high dark conductivity, probably because of a higher n-doping, which is due to higher concentration of Zn atoms in the film. On the other hand, ZnO /RB hybrid film is found to show a much higher sensitivity to illumination with visible light in photoconductivity measurements due to a higher absolute photoconductivity and lower conductivity in the dark.