Ismael Cosme

Study of electrical conductivity of PEDOT:PSS at temperatures >300 K for hybrid photovoltaic applications

Temperature dependence of DC conductivity σDC(T) of spin-coated poly (3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) films has been studied. σDC(T) was measured in films deposited from mixtures of PEDOT/PSS: H₂O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1). Room temperature conductivity σ_RT of PEDOT/PSS films is enhanced from 4.46×10^-5 S/cm to 6.07×10^-4 S/cm after dilutions with H₂O and IPA (1:1), respectively. Experimental data is fitted with two different transport models: thermal activated conduction and variable range hopping (VRH) model. It is found that σ_DC(T) of PEDOT/PSS:IPA (1:1) sample can be described by the one-dimensional (1D) VRH model with an pre-exponential factor σ₀=28.5 S/cm and material-dependent parameter T₀=39,204 K.

Electronic properties of Ge y Si 1−y :H films deposited by LF PECVD at low temperatures

This paper reports the study of Ge<inf>y</inf>Si<inf>1−y</inf>:H films deposited at temperatures in the range of T<inf>d</inf>= 70 to 300 °C. The films were grown in capacitive low-frequency (f=110 KHz) discharge from Si:H<inf>4</inf> and Ge:H<inf>4</inf> feed gases diluted with H<inf>2</inf>. Other parameters were as follow: hydrogen flow Q<inf>H2</inf>= 3750 sccm, silane flow Q<inf>SiH4</inf>=50 sccm, germane flow Q<inf>GeH4</inf>= 500 sccm hydrogen dilution ratio R= Q<inf>H2</inf>/(Q<inf>SiH4</inf>+Q<inf>GeH4</inf>)=75, discharge power W= 300 Watt and pressure P= 0.76 Torr. The deposition rate of the films was varied not monotonically in the range of Td from 70 to 300 °C. Hydrogen bonding was characterized by Fourier transform infrared (FTIR) spectroscopy. The electrical parameters were extracted from the measurements of temperature dependence of conductivity σ(T). Activation energy and room temperature conductivity of the films were observed in the range of the values E<inf>a</inf>=0.27 to 0.37 eV and σ<inf>RT</inf>=5.7 × 10⁻⁵ to 9.6 × 10⁻⁴ Ω⁻¹ cm⁻¹, respectively. The electronic properties characterized by different electrical and optical measurements showed optimal properties within the deposition temperature range T<inf>d</inf>∼160 °C to 220 °C

AZO/PEDOT:PSS Polymer Frontal Interface Deposited on Flexible Substrates for a-Si:H Photovoltaic Applications

Thin-film hybrid organic-inorganic photovoltaic structures based on hydrogenated silicon (Si:H), poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) polymer Al-doped ZnO (AZO) films deposited on different types of flexible substrates have been fabricated and investigated. The compatibility of the polymer and inorganic materials regimes and deposition techniques used for device fabrication has been demonstrated on flexible substrates. Morphological characteristics of transparent Al-doped ZnO (AZO) films deposited on substrates have been measured by atomic force microscopy. Electronic characteristics of the fabricated photovoltaic structures have been measured and analyzed for different thicknesses of the transparent electrodes and different substrate types. Photovoltaic hybrid structure on polyethylene naphthalate (PEN) substrate showed the best characteristics: short circuit current density Jsc = 9.79 mA/cm2, open circuit voltage Uoc = 565 mV, and PCE η = 1.3%. To analyze the mechanisms governing the device performance, short circuit current density spectral dependence of the devices fabricated on different types of flexible substrates has been measured. As demonstrated by our analysis, the structures on PEN substrates, besides better substrate transmittance, also show better junction properties.

Hybrid Silicon-Organic Heterojunction Structures for Photovoltaic Applications

The concept for inorganic-organic device is an attractive technology to develop devices with better characteristics and functionality due to the complementary advantages of inorganic and organic materials. This chapter provides an overview of the principal requirements for organic and inorganic semiconductor properties and their fabrication processes and focus on the compatibility between low temperature plasma enhanced chemical vapor deposition (PECVD) and polymer organic materials deposition. The concept for inorganic-organic device was validated with the fabrication of three hybrid thin film photovoltaic structures, based on hydrogenated silicon (Si:H), organic poly(3-hexythiophene): methano-fullerenephenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM), and poly(3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) films. Optoelectronic characteristics, performance characteristics, and interfaces of the different configurations aspects are discussed. Hybrid ITO/PEDOT:PSS/(i)Si:H/(n)Si:H structure results in a remarkably high short circuit current density as large as 17.74 mA/cm2, which is higher than the values in organic or inorganic reference samples. Although some hybrid structures demonstrated substantial improvement of performance, other hybrid structures showed poor performance, further R&D efforts seem to be promising, and should be focused on deeper study of organic materials and related interface properties.

Effect of germane in gas phase on electronic properties of GeXSiY:H alloys deposited by RF plasma discharge

We have studied the effect of Ge-concentration in gas phase on electronic properties of GeXSiY:H alloys deposited by RF-PECVD. The relative gas phase Ge concentration was varied from [Ge]_gas= 50 % to 100 %. The electronic properties of the films were studied by the measurements of temperature dependence of dark conductivity σ_dark (T) in the range of T= 300 to 430 K. The temperature dependence curves were described by the thermal activated conduction model and activation energy and Fermi level parameters were calculated. A change in dark conductivity was found from 1.7×10^-6 Ω^-1cm^-1 at [Ge]_gas= 50 % to 1.1×10^-4 Ω^-1cm^-1 at [Ge]_gas =100 %. The correlation between Ge in gas phase and conductivity was reflected in activation energy and Fermi level values that varied from E_a= 0.37 to 0.23 eV and E_F=0.27 to 0.10 eV, respectively.

Study of optoelectronics properties of indium tin oxide films fabricated by sputtering in oxygen atmosphere

Indium tin oxide (ITO) thin films were deposited by magnetron sputtering with different oxygen flows rates (%O2 = 1 % to 0 %). The films were deposited by RF discharge in argon/oxygen atmosphere with a RF power W = 150 W at substrate temperature T = 175° C and pressure P = 6 mTorrs. Photoelectric parameters of fabricated ITO films were characterized by four-point probe method and transmittance measurements. It was found that reducing oxygen concentration the sheet resistance decreases. For samples fabricated in oxygen atmosphere the transmittance reached values above 90 %. However, the sample with post-deposition thermal treatment at T = 300 °C in oxygen atmosphere shows the best transmittance (95 %) and lowest value of sheet resistance (Rs= 27 ohms-square). Finally, the transparent conducting oxides were applied in hybrid organic-inorganic photovoltaic structures. The best power conversion efficiency in AM-1 sunlight η = 2.13 % was obtained for a structure with ITO fabricated at oxygen concentration %O2 = 0.25 % in comparison with an efficiency η = 1.87 % obtained for a structure fabricated with commercial ITO.

Effect of frontal interface configuration on electronic properties of organic-inorganic hybrid solar cells

Inorganic-organic hybrid solar cells with different frontal interface configurations have been studied. Spin-coated conducting polymer Poly (3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) or boron doped a-Si:H films were used as p-type layer. Intrinsic amorphous silicon (a-Si:H) films and P3HT:PCBM heterojunctions were investigated as absorbing layers. Solar cell performance was characterized by current-voltage measurements under AM1.5 illumination and spectral measurements in the range from hν = 1.5 to 3.5 eV. The stack structure with a-Si:H (absorbing layer)/PEDOT:PSS (p-layer) interface exhibited the highest short circuit current density (Jsc). The best open circuit voltage (Voc) as a large as 0.73 V was observed in the stack structure with a-Si:H/P3HT:PCBM heterojunction as absorbing layer. Finally, in the spectral response of hybrid configurations is possible to distinguish the Jsc contributions of both organic and inorganic layers.

Study of doping of Ge 0.96 Si 0.04 :H films with B, and P during low frequency plasma deposition at low temperature

The films studied were grown at the temperature T<inf>d</inf>= 160 °C and doped by boron (B) and phosphorus (P) by incorporation of diborane (B<inf>2</inf>H<inf>6</inf>) and phosphine (PH<inf>3</inf>), respectively, in gas mixture used for the deposition. For B-doped films changing boron concentration in gas phase C<inf>B</inf>^gas from 0.04% to 0.06%, resulted in changing electrical characteristics: conductivity activation energy E<inf>a</inf> and room-temperature conductivity σ<inf>RT</inf> from E<inf>a</inf>=0.32 to 0.52 eV and σ<inf>RT</inf>=10^𢄤4 to 10⁻⁷ Ω⁻¹ cm⁻¹, respectively, suggesting compensation of electronic conductivity of the films. Further increasing C<inf>B</inf>^gas from 0.06% to 0.14% caused a reduction of E<inf>a</inf> from 0.52 to 0.31 eV and increasing of σ<inf>RT</inf> from 10⁻⁷ to 10⁻⁴ Ω⁻¹ cm⁻¹. Similar behavior of electrical properties with boron incorporation has been reported in the B-doped Ge:H films deposited at T<inf>d</inf>=300°C in [1]. The P-doped films show continuous change in activation energy ranging from E<inf>a</inf>=0.32 to 0.18 eV and room temperature conductivity ranging from σ<inf>RT</inf>=2.3×10⁻⁴ to 1.8×10⁻² Ω⁻¹ cm⁻¹ with P incorporation. Thus effective B- and P-doping of Ge-Si films deposited at T<inf>d</inf>=160 °C has been demonstrated.

Noise in Different Micro-bolometer Configurations with Silicon-Germanium Thermo-sensing Layer

A recent interest in thermal detectors is due to plasma deposited materials with large temperature coefficient of resistance (TCR) and micro-machining technology for thermal isolation. Both are compatible with dominating Si CMOS technology that makes it possible developing device-on-chip configurations. In our previous works we have reported on the study of fabrication and characterization of single cell micro-bolometers based on silicon-germanium thermo-sensing films deposited by low frequency plasma [1-3]. Noise measurements are very important part of device characterization, which have been poorly reported in literature. Models proposed by present time for noise description in non-crystalline samples (either in films or in device structures) are still debated and no one can be considered as the only accepted. The goal of this work is to study experimentally noise spectral density in several configurations of micro-bolometers with silicon-germanium (a-SixGey:H) as thermo-sensing films. We studied four configurations: planar structure with a) a-SixGey, y=0.5, b) a-SixGeyBz:H, x=0.5, y=0.45 and z=0.05 ; c) x=0.5, y=0.5, z=0 and d)sandwich structure with x=0.5, y=0.5. These samples were characterized by SIMS (composition), FTIR (H-bonding and H content), conductivity measurements (σ(T), activation energy, TCR), current-voltage characteristics in dark and under illumination (responsivity). Noise spectral density (NSD) versus frequency S(f) was studied in the range of frequency f=1 to f=103 Hz under IR illumination modulated with this frequency and constant bias. The measurements were performed in vacuum chamber with P=10 mTorr. Generally S(f) dependence demonstrated three regions separated by two corner frequencies: fc1 and fc2: 1) f ≤ fc1 S1 α f -γ γ =0.15 to 0.5, 2) fc1 ≤ f ≤ fc2, S2 α f-γ γ = 0.7 to 1.34 and 3) f ≥ fc2 S3 const (f). Different samples studied showed different values of fc1, fc2 γ and S3. These noise characteristics observed experimentally are analyzed in comparison with data reported in literature and possible mechanisms for noise in the frequency regions studied are discussed.