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(3,4ethylenedioxythiophene): 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

Determining the photoelectric parameters of an organic photoconductor by the photoelectromotive-force technique

We report on a theoretical model to describe the non-steady-state photoelectromotive-force (photo-EMF) effect in organic photoconductors. Unlike the conventional theory of the photo-EMF effect developed for crystalline materials, this model accounts for the field dependence of the charge generation quantum efficiency and charge carrier mobility. To verify our findings a detailed experimental study of the charge carrier generation and transport processes in a organic photorefractive composite was performed using the photo-EMF effect and ac photocurrent measurements. The investigated composite was based on a conjugated triphenyldiamine based polymer (TPD-PPV) sensitized with a highly soluble fullerene derivative (PCBM). Our results show that at zero and low dc field the dependence of the photo-EMF signal on frequency, grating period and external electric field is well described by the standard model originally developed for an inorganic monopolar photoconductor with finite charge carrier lifetime. In this regime the photo-EMF effect was used to determine zero- and low-field photoelectric material parameters including the low-field hole mobility (μ0,h=1.3×10−4cm2/Vs), the effective charge carrier lifetime (τ=45μs), the diffusion length (LD=114nm), and the primary charge carrier generation efficiency (Φ=5.3×10−3%). In addition, the validity of the Einstein relation (D∕μ=25meV) was verified for the low-field regime. At high electric fields the signal behavior deviates significantly from the trend predicted by the standard model for inorganic photoconductors. This behavior which is mainly attributed to the strong field dependence of the charge generation rate is well described by our model

Initial stage of the active mode-locking in semiconductor heterolasers

We make an attempt to develop a novel approach to describing the initial stage of the active mode-locking in semiconductor laser structures based on analyzing the properties of dispersion relations in terms of stability for small initial perturbations. Nonlinear process of shaping optical pulses is interpreted as manifesting instability of diffusion type. For the purposes of experimental investigations, the auto-manual opto-electronic measuring system detecting average time parameters inherent in ultra-short optical pulse trains has been designed. This system is able to register auto-correlation functions of the second order exploiting the interferometric technique as well as to identify a pulsed character of the incoming light radiation. Experimental confirmations of appearing the diffusive instability within the active mode-locking process in semiconductor laser structures operating in the near infrared range are presented.CONACyT, Mexico: project # 61237-FPeer reviewe

Study of Si and Ge Atoms Termination Using H-Dilution in SiGe:H Alloys Deposited by Radio Frequency (13.56 MHz) Plasma Discharge at Low Temperature

In this work, we present the study of the atomic composition in amorphous SiXGeY:HZ films deposited by radio frequency (RF—13.56 MHz) plasma discharge at low deposition temperature. A study and control of Si and Ge atoms termination using H-dilution in SiGe:H alloys deposited by RF plasma discharge was conducted and we made a comparison with low-frequency plasma discharge studies. Solid contents of the main elements and contaminants were determined by SIMS technique. It was found that for low dilution rates from RH = 9 to 30, the germanium content in the solid phase strongly depends on the hydrogen dilution and varies from Y = 0.49 to 0.68. On the other hand, with a higher presence of hydrogen in the mixture, the germanium content does not change and remains close to the value of Y = 0.69. The coefficient of Ge preferential incorporation depended on RH and varied from PGe = 0.8 to 4.3. Also, the termination of Si and Ge atoms with hydrogen was studied using FTIR spectroscopy. Preferential termination of Si atoms was observed in the films deposited with low RH < 20, while preferential termination of Ge atoms was found in the films deposited with high RH > 40. In the range of 20 < RH < 40, hydrogen created chemical bonds with both Si and Ge atoms without preference

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

Influence of the sensitizer reduction potential on the sensitivity of photorefractive polymer composites

We report on a series of near infrared (NIR)-sensitive photorefractive polymer composites (PPCs) based on the hole-conducting polymer PF6-TPD, which are sensitized by soluble fullerene-derivatives as electron-accepting agents. We demonstrate a direct correlation between the electron accepting capability of the sensitizer and the holographic response time. The holographic recording speed is found to improve by one order of magnitude when lowering the reduction potential of the sensitizer by approx. 400 mV, while all other physical parameters of the materials remain essentially identical. Furthermore, the lifetime of the mobile charge carriers is found to correlate linearly with the reduction potential, thus indicating a decrease in recombination rates for stronger accepting capability of the sensitizer. Finally, we found that pre-illumination enhanced the holographic sensitivity. The effect is found to be most pronounced for the strongest acceptor due to reduced recombination of the preformed carriers. Overall, the PPCs reported here feature the currently highest sensitivity in the NIR spectral region.

Nanostructural Modification of PEDOT:PSS for High Charge Carrier Collection in Hybrid Frontal Interface of Solar Cells

In this work, we propose poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) material to form a hybrid heterojunction with amorphous silicon-based materials for high charge carrier collection at the frontal interface of solar cells. The nanostructural characteristics of PEDOT:PSS layers were modified using post-treatment techniques via isopropyl alcohol (IPA). Atomic force microscopy (AFM), Fourier-transform infrared (FTIR), and Raman spectroscopy demonstrated conformational changes and nanostructural reorganization in the surface of the polymer in order to tailor hybrid interface to be used in the heterojunctions of inorganic solar cells. To prove this concept, hybrid polymer/amorphous silicon solar cells were fabricated. The hybrid PEDOT:PSS/buffer/a-Si:H heterojunction demonstrated high transmittance, reduction of electron diffusion, and enhancement of the internal electric field. Although the structure was a planar superstrate-type configuration and the PEDOT:PSS layer was exposed to glow discharge, the hybrid solar cell reached high efficiency compared to that in similar hybrid solar cells with substrate-type configuration and that in textured well-optimized amorphous silicon solar cells fabricated at low temperature. Thus, we demonstrate that PEDOT:PSS is fully tailored and compatible material with plasma processes and can be a substitute for inorganic p-type layers in inorganic solar cells and related devices with improvement of performance and simplification of fabrication process