Hiram E. Martinez

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.

Effect of hydrogen dilution on morphology and electronic properties of silicon-germanium films deposited by RF plasma discharge

In this work we study the influence of hydrogen dilution on morphological properties and electronic of SiGe:H films. Surface morphology measured by Atomic Force Microscopy (AFM) technique and temperature dependence of conductivity, together with conductivity measurements in dark and under AM1.5 illumination at room temperature were used for characterization. Dark conductivity and Fermi energy varied from 10^-8 Ω^-1cm^-1 to 10^-4 Ω^-1cm^-1 and from 0.45 to 0.08 eV, respectively, with changing hydrogen dilution from R_H =9 to.80.

SIMS analysis of atomic composition of silicon-germanium films deposited by RF plasma discharge

In this work we study the atomic composition in SiGe:H films as a function of hydrogen dilution. Silane and germane fluxes are kept constant while hydrogen flow is varying to achieve dilutions up to 80 times. Solid content of principle elements as well as contaminants are determined by SIMS. For low dilutions (from 9 to 30), germanium solid content strongly depends on hydrogen dilution since it varies from 0.485 to 0.675. Higher presence of hydrogen in the mixture does not change germanium content which remains close to the value of 0.69.

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 solar cell based on a-Si/polymer flat heterojunction on flexible substrates

In this work, we present the results of investigation of thin film hybrid organic-inorganic photovoltaic structures based on flat heterojunction hydrogenated silicon (a-Si:H) and poly(3,4 ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) fabricated on polyethylene naphthalate (PEN). Different thicknesses of transparent AL doped Zn:O (AZO) electrodes have been tested on PEN substrate and studied by atomic force microscopy (AFM). The AZO films on PEN substrate were statistically processed to obtain surface morphological characteristics, such as root mean square roughness RQ, skewness SK and kurtosis KU. Performance characteristics of fabricated photovoltaic structures have been measured and analyzed for different thicknesses of the transparent electrodes under standard illumination (AM 1.5 I0= 100mW/cm2). Structures on flexible substrates show reproducible performance characteristic as their glass substrate counterpart with values of JSC= 6 mA/cm2, VOC= 0.535 V, FF= 43 % and PCE= 1.41%.

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.

High contrast distributed Bragg reflectors based on Si:H/SiC:H PECVD multilayer structure

In this work we study the high contrast distributed Bragg reflectors (DBRs) based on plasma deposited Si:H/SiC:H films. Optical constants of these materials were measured by means of spectral ellipsometry and used in theoretical calculations of spectral transmittance and reflectance. The stacks consisted of 13 layers of the films were designed to provide stop band center at λ= 835 (DBR1) and 565 nm (DBR2). High reflectance (>90%) for DBR1 and lower value of 70% for DBR2 was measured with bandwidth Δλ=260 nm. The properties of DBR1 are of promise as back reflector in PECVD PV solar cells because of his stop band characteristics and complete compatibility with solar cell fabrication. While DBR2 structure shows worse filter properties suggesting significant effect of light absorption in Si:H layers.

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.