Conductive-probe atomic force microscopy as a characterization tool for nanowire-based solar cells

The photonic properties of nanowires advocate for their utilization in next generation solar cells. Compared to traditional devices, the electric scheme is transformed from a single into an ensemble of pn junctions connected in parallel. This new configuration requires new schemes for the characterization. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of this parallel-connected nanowire devices. With C-AFM it is possible to obtain both surface topography and local electrical characterization with nanoscale resolution. We demonstrate topography and current mapping of nanowire forests, combined with current-voltage measurements of the individual nanowire junctions from the ensemble. Our results provide discussion elements on some factors limiting the performance of a nanowire-based solar cell and thereby to provide a path for their improvement.

Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

Thomas Dufaux

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

EPFL2012

Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

Corsin Battaglia

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector. (C) 2012 Optical Society of America

Optical Society of America2012

Explaining Morphological and Electrical Features of Boron-doped Zinc Oxide to Tailor New Electrodes for Photovoltaics

Lorenzo Fanni

TCOs are a class of metal oxides that combine transparency to visible light with electrical conductivity. Each TCO is characterized by the trade-off of these two properties which can be tuned for a particular application. This thesis is dedicated to the investigation of one TCO material: boron doped zinc oxide (ZnO:B) deposited by low-pressure chemical vapor deposition (LP-MOCVD). Its main distinction is low absorptance which makes ZnO films deposited by LP-MOCVD ideal as transparent electrodes in thin-film solar cells. Although ZnO:B properties have previously been optimized for this application, the exact processes behind the film formation have not been fully described. This thesis substantially clarifies the processes of the film nucleation and growth evolution, and the mechanisms of incorporation of the B atoms into the ZnO. First, for non-intentionally doped ZnO we establish that the main deposition parameters influencing the film properties are the deposition temperature, the gas precursor ratio and the total gas flow. We experimented with these three parameters across a wide range of parameter values. The films deposited were characterized at different stages of their growth using atomic force microscopy, X-ray diffraction and automated crystallographic orientation mapping. These data reveal the dependence of the film preferential orientation on the deposition conditions. We propose a model based on the adsorbed atom mean free path to explain this dependence. Using deposition parameters learnt from this model, we control the preferential orientation during film growth to increase the grain sizes. Second, we analyze the conductivity to B-doped ZnO films. Boron atoms act as electron donors in ZnO, increasing the electrical conductivity and decreasing the transparency. This work quantifies how the B concentration and spatial distribution affect the film conductivity. Quantification of the B atoms incorporated in the film was performed using nuclear reaction analysis. We found that a high level of O precursor gas favors B incorporation in the film. Combining nano secondary ion mass spectroscopy and Kelvin probe force microscopy we demonstrate that the dopant atoms incorporate in only one of the two sides of each grain. This is a new observation because dopant atoms are commonly assumed to be uniformly distributed in the film. The transparency and electrical conductivity of ZnO depend also on the carrier mobility. The sources of carrier scattering are investigated for intrinsic (i.e. O vacancies and Zn interstitials) and extrinsic (B atoms) impurity concentrations. We observed that for the same B concentration the source of electron scattering depends on the concentration of intrinsic defects: for films with a lower concentration of intrinsic defects, the main source of scattering is the grain boundaries; for films with a higher concentration of intrinsic defects, the main sources are the mechanisms happening in the crystalline region. Finally, LP-MOCVD ZnO:B films were optimized using the previous results and successfully applied in four different types of solar cells: amorphous/microcrystalline silicon, amorphous silicon, copper indium gallium selenide and silicon heterojunction. The advantages of the developed films to these four cells include the simplification of the fabrication process, the reduction of reflectance and parasitic absorption, and a better understanding of carrier transport mechanisms through the device.