WP3 - Complex description of transient phenomena to reduce aging and degradation processesTeam leader: doc. RNDr. Bohuslav Rezek, Ph.D. Team members:  POSTDOC1
POSTDOC2
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Team is focused on microscopic studies of short-term and long-term transient phenomena in PV materials (WP1, WP2), structures and devices (WP4). The research is also tightly linked to practical PV integration in buildings (WP5). Studying the transient phenomena on microscopic level will help us uncover and identify function, aging and degradation process and possibly suggest ways for their minimization. Important part plays study of surfaces and interfaces (including internal interfaces such as grain boundaries, phase boundaries or material junctions). With view to the transient and aging phenomena the WP3 will investigate also influence of structural and chemical modifications of materials and surfaces (in particular by using plasmatic treatments). |
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Our previous studies on degradation processes in PV materials and cells for instance showed that surface potential of P3HT-fullerene blend shift under illumination due to generation of electric charge carriers [1]. The characterization was performed by Kelvin probe force microscopy (KPFM). When the measurement was periodically repeated on air, i.e. illumination was switched on and off, the photovoltage response was decreasing. Also overall potential was changing in the long-term. Comparison of these effects on two types of blends using different fullerene derivatives ([60]ThCBM a [70]ThCBM) showed different degree of such degradation: [70]ThCBM blend was more stable than the other. Similarly we have used such methodology to characterize exciton dissociation on the junction of organic dye and diamond [2]. KPFM showed that the use of diamond provided order of magnitude higher photovoltage than a gold electrode.
Method for microscopic characterization of opto-electronic phenomena by KPFM and interpretation of the obtained can be improved by using light sources with controlled, high enough intensity and well-defined wavelength region. In a simplified form this approach has been already used to characterized UV photodiodes based on diamond Schottky junction [3]. Photovoltage changed by as much as 200 mV depending on the applied light source (halogen lamp, UV diode, deuterium lamp). Grounded Ohmic contact was used as reference. This enabled us to confirm charging of the floating Schottky junction and elaborate model of photocarrier generation and transport in such device. Further information about the function of PV cells can be obtained by microscopic measurements of electrical (photo) currents using conductive atomic force microscopy (C-AFM). Such experiments characterize electronic transport with high spatial resolution and with sensitivity below 1 pA. They are thereby complementary to photovoltage characterization. We have used them previously for studies of silicon as well as organic PV materials [4,5]. The electrical regimes of AFM provide great advantage for understanding opto-electronic properties that can be studied directly without need for additional electrical contacts that are necessary for macroscopic characterization but may influence the outcome of experiments (i.e. simply by shunts, degradation etc.). At the same time, AFM can study microscopic structures (photonic structures, nanowires, etc.) or material inhomogeneities (grain boundaries, phase separation) that can significantly impact PV performance yet remain unclear from macroscopic characteristics [5,6,7]. Although material inhomogeneities can be revealed by using nanomechanical property mapping (QNM) or phase lag in AFM [8], getting straightforward chemical information is not possible (with few specific exceptions). For this task, micro-Raman spectroscopy can be used. In spite of being optical method, it can resolve features down to 130 nm. It can also nicely resolve unwanted segregation of organic phases in a blend [5].
In the project here, we will focus on microscopic studies of short-term and long-term transient phenomena in PV materials (WP1, WP2), structures and devices (WP4). The studies are also tightly linked to practical PV integration in buildings (WP5). Mutual feedback of WP3 with WP1, WP2, WP4, WP5 will be established. Studying the transient phenomena on microscopic level will help us uncover and identify function, aging and degradation process and possibly suggest ways for their minimization. Important part plays study of surfaces and interfaces (including internal interfaces such as grain boundaries, phase boundaries or material junctions). The studies will be accomplished by systematic and correlated characterization of material microscopic structure (AFM, SEM), chemical composition (AFM, Raman, QNM), opto-electronic response (KPFM, CAFM) characterized as a function of time and as a function of (accelerated) aging/degradation processes due to illumination, humidity, temperature, strain. The data will be compared with measurements in the oxygen and humidity free atmosphere. With view to the transient and aging phenomena the WP3 will investigate also influence of structural and chemical modifications of materials and surfaces (in particular by using plasmatic treatments). The experimental results will be used for developing models of opto-electronic and aging mechanisms. The models will be corroborated by theoretical calculations of the structure and electronic properties at the surfaces and interfaces. The research in this field will highly benefit from close links of the team to other research groups in the Academy of Sciences CR (Institute of Physics and Institute of Macromolecular Chemistry). We expect that the newly obtained results, technological and characterization procedures, and knowledge will contribute to elucidation of function and aging of PV systems and to their innovative applications in buildings. The work envisioned within this WP can be divided into following tasks contributing to the two main objectives: Development of a specialized microscopic setup for characterization of opto-electronic transient phenomena. The experimental arrangement will be developed based on the already available AFM/CAFM/KPFM system, it will include coupled light source with adjustable light wavelength in visible spectral range and controlled illumination intensity. The setup will enable transient measurements of opto-electronic characteristics (electrical potential, photovoltage, electrical current, photocurrent) with high spatial resolution (10 nm or better) and appropriate time resolution (1 ms or better). It will be used both on novel and common PV materials studied within the project. Characterization of microscopic transient response and aging of standard PV materials (mainly silicon) in novel structured forms (application/building-based structuring, photonic microstructures, nanowires). We will study the material microscopic structure (AFM, SEM), chemical composition (AFM, Raman), opto-electronic response (KPFM, CAFM) characterized as a function of time and as a function of (accelerated) aging/degradation processes due to illumination, humidity, temperature, strain. The data will be correlated with measurements in the oxygen and humidity free (both < 1 ppm) glovebox that is presently being purchased. Correlation of microscopic structure, composition, and optoelectronic response of novel PV materials, junctions, and device concepts (in particular perovskite-silicon interfaces, diamond- organic heterostructures, diamond-graphene interfaces, graphene-organic interfaces, organic blends, transition metal dichalcogenides such as MoS2, use of nanocrystals and nanoparticles such as diamond and silicon, photonic structures, stabilization via chemical bonds and antioxidizing effects, etc). We will characterize the material microscopic structure (AFM, SEM), chemical composition (AFM, Raman), opto-electronic response (KPFM, CAFM). Characterization of microscopic transient response and aging of novel PV materials (described above). We will study the material microscopic structure (AFM, SEM), chemical composition (AFM, Raman), opto-electronic response (KPFM, CAFM) characterized as a function of time and as a function of (accelerated) aging/degradation processes due to illumination, humidity, temperature, strain. The data will be correlated with measurements in the oxygen and humidity free (both < 1 ppm) glovebox that is presently being purchased. Investigation of influence of material/device modifications (plasma treatments, coating, structuring) on the PV performance with view to the targeted practical applications (buildings) including aging/degradation characteristics. Characteristics after specific modification will be compared with prior characteristics obtained in Objective 3.3 and 3.4. Theoretical computations of molecular/material/surface interactions related with the above materials and devices. We will use DFT, MM, MD calculations. At first, we will prepare structures in GaussView 5 program and the structure is going to be optimized in Gaussian 09 program. The computational resources are going to be provided mainly by virtual organization MetaCentrum. In the later stage of the project we plan to use Materials Studio or similar software package for more diverse calculations including in particular forcefields and molecular dynamics. Calculation of structure and vibrational analysis of molecular states will be made, either in gas phase as well as in water solution. Physisorption and chemisorption of molecules will be investigated. Binding energies will be evaluated and the band gap will be calculated. Charge transfer and polarization between molecules and material surface functionalized with various surface groups will be calculated. The improvement of the theoretical models (if necessary) will be made to comply with the experimental results. Theoretical and experimental results will be combined to propose models of structural, electronic, chemical interactions in the specific PV systems. References
[1] J. Čermák, B. Rezek, V. Cimrová, A. Fejfar, A. Purkrt, M. Vaněček, J. Kočka. Thin Solid Films 519 (2010) 836-840
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