Session We-C1

New Nano-materials: Photovoltaics I

Chair: Elvira Fortunato, Universidade Nova de Lisboa

We-C1.1 8:20–8:40

Local (Photo) Electronic Properties in Nanostructured Solar Cells

Antonín Fejfar (1), Matĕj Hývl (1), Martin Ledinský (1), Aliaksei Vetushka (1), Jan Kočka (1), Soumyadeep Misra (2), Martin Foldyna (2), Linwei Yu (2), and Pere Roca i Cabarrocas (2)

1. Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague 6, Czech Republic

2. Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), CNRS, Ecole Polytechnique, 91128 Palaiseau, France

Nanostructured solar cells based on thin film silicon are designed to improve the photovoltaic conversion efficiency by better light trapping, photogenerated charge collection, and stability. They can be based for instance on structured TCO electrodes (for example ZnO nanorods or hole arrays) or they may use Si nanowires [1] as a base for radial p-i-n junctions. The elements may be self-organized or prepared by lithography, but they always exhibit random variations which influence local photovoltaic conversion. Such cells thus operate as random arrays of microscopic photodiodes in which the weak diodes limit the performance of neighbors within the screening length determined by the resistance of the contact layers [2].

The structural elements of the cells have sizes comparable to the visible light wavelengths. Therefore, microscopic measurements of photoresponse are needed to assess the distribution of properties of the local photodiodes, their connections and the resulting limits to overall performance by weak diodes.

We demonstrate the use of the conductive atomic force microscopy (C-AFM) for the study of local electronic properties of silicon nanostructures: p-i-n radial junctions of amorphous Si grown on top of Si nanowires or mixed phase microcrystalline films. Mapping the dark local conductivity requires switching off the laser used by the AFM to detect the cantilever deflection during the scan [3] which is possible in several ways:

• constant height scan,

• dark lift-mode

• or using the laser with wavelength for which the Si layer is transparent (1260 nm).

We will compare these different approaches for the same sample of mixed phase silicon consisting of microcrystalline grains embedded in amorphous matrix. We have already observed variations of the conductivity of the radial junction solar cells based on Si nanowires. Possible reasons for the variations will be explored together with local I-V characteristics measured in the dark and with external illumination.

The goal of this study is to develop a characterization method correlating the local photoresponse with the local photovoltaic conversion efficiency, which can allow the microscopic study of photovoltaic energy conversion at nanoscale.

[1] L. Yu, B. O'Donnell, M. Foldyna, P. Roca i Cabarrocas, Nanotechnology 23 (2012) 194011

[2] V. G. Karpov, A. D. Compaan, D. Shvydka, Phys. Rev. B 69 (2004) 045325

[3] M. Ledinský, A. Fejfar, A. Vetushka, J. Stuchlík, B. Rezek, J. Kočka, Physica Status Solidi - Rapid Research Letters 5 (2011) 373

Keywords: silicon, thin films, nanowires, radial junctions, random diode arrays, atomic force microscopy, photoresponse

We-C1.2 8:40–9:00

Towards a Perfect System for Solar Hydrogen Production: An Example of Synergy on the Atomic Scale

Ramy Nashed (1,2,3), Faisal M. Alamgir (4), Seung Soon Jang (4), Yehea Ismail (3), Mostafa A. El-Sayed (1), and Nageh K. Allam (2)

1. Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA

2. Energy Materials Laboratory, Physics Department, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt

3. Center of Nanooelectronics and Devices (CND), American University in Cairo/ Zewail City of Science and Technology, Cairo, Egypt

4. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

The effect of metal doping on electronic band structure and charge carriers effective mass of Ta2O5 were studied using hybrid functionals in density functional theory. PBE0 hybrid functional proved to be very efficient in predicting the band structure with less than 5% error compared to experimental data. The bandgap decreases monotonically as the percentage of the dopant increases. Furthermore, the indirect bandgap behavior of Ta2O5 was found to initially increase on doping before it decreases again to its original value of pristine Ta2O5. We found that high percentage doping or even mixing with another metal is required in order to modify the band structure of Ta2O5. The effect of doping on the crystal structure was also studied. XRD measurements show that the crystal lattice tends to expand upon doping with metals with larger atomic radius than Ta and this effect is more pronounced as the dopant concentration increases.

Keywords: density functional theory, Ta2O5, band structure, water splitting, solar cells

We-C1.3 9:00–9:20

Intrinsic Doping and Band Gap Control Mechanisms of Crystalline Cu2ZnSnS4 Revealed By In-depth Study of Amorphous/Disordered Cu2SnS3-CZTS-ZnS Alloys

Peter T. Erslev (1), Matthew R. Young (1), Hui Du (1), Jian Li (1), Robert Lad (2), Sin Cheng Siah (3), Rupak Chakraborty (3), Rafael Jaramillo (3), Tonio Buonassisi (3), and Glenn Teeter (1)

1. National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401

2. Laboratory for Surface Science and Technology and Department of Physics, University of Maine, Orono, ME 04469, USA

3. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Kesterite Cu2ZnSnS4 (CZTS) is an attractive material for widespread photovoltaic energy generation due to the earth-abundance and non-toxic nature of its constituent elements. However, the complexity of the crystalline lattice in this quaternary compound presents challenges in controlling and understanding the native defect chemistry of the system. For example, high-efficiency CZTS devices seem to require significant variations off-stoichiometry, usually near the compositional ratios [Zn]/[Sn] = 1.2 and [Cu]/([Zn]+[Sn]) = 0.8. However, it is still somewhat undetermined exactly which compositional metric is the most important: a brief review of the current literature will yield several differing metrics through which the films are characterized, for example the [Cu]/([Zn]+[Sn]) vs. [Cu]/[Zn] vs. [Cu]/[Sn] ratios are all used. We have undertaken a detailed study of room-temperature deposited alloys of (Cu2SnS3)1–x(ZnS)x (d-CZTS) (CZTS @ x = 0.5) in order to 1) evaluate the viability of the resulting semiconducting materials for low-cost photovoltaics and 2) use the resulting amorphous or highly disordered (as characterized by XRD and SEM) materials to understand relationship between composition and electro-optical properties of crystalline CZTS. The disordered, alloyed films (along the Cu2SnS3 – ZnS tie line in the quasi-ternary phase diagram) have shown a high degree of control over the optical and electronic properties. As the ZnS fraction is increased, the optical band gap smoothly increases from 1.1 eV to 2.8 eV, with a high optical absorption coefficient, and the sheet resistance increases by over eight orders of magnitude. Independent of the band gap, control over the carrier concentration in the films of at least three orders of magnitude is obtained through relatively small variations in the Cu:Sn ratio; that is, composition variations perpendicular to the Cu2SnS3 – ZnS tie line. We propose to consider the increases in the ZnS fraction in the material is as quasi-isovalent alloying, where although the cation ratios vary, the mean cation oxidation state stays the same. Changes in the Cu:Sn ratio, however, alters the mean cation oxidation state in what we term intrinsic aliovalent alloying. Fundamental optical and electronic characterizations of these tetrahedrally-coordinated amorphous or disordered thin film chalcogenide alloys will be presented and discussed in the framework of quasi-isovalent versus aliovalent alloying. Maintaining the tetrahedral coordination of the crystalline semiconductor counterparts such as Cu2ZnSnS4 gives the network of these disordered films sufficient structural rigidity to unpin the Fermi level, in sharp contrast to the characteristics of most amorphous chalcogenides. We have also developed a kinetic growth model that provides qualitative insight into the transition from disordered or amorphous to crystalline growth conditions. Results and characterization from preliminary PV solar cells with several (Cu2SnS3)1−x(ZnS)x alloys as the absorber layer will be discussed. Strong correlations appear when the compositional and electronic properties of state-of-the-art CZTS solar cells are considered within the framework of our understanding of the disordered CZTS alloys.

Keywords: amorphous, chalcogenide, photovoltaic, electrical and optical properties

We-C1.4 9:20–9:40

Semiconductor-less Photovoltaic Device

Fatih B. Atar (1,2), Enes Battal (1,2), Levent E. Aygun (1,2), Bihter Daglar (2), Mehmet Bayindir (2,3), and Ali K. Okyay (1,2)

1. Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey

2. UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey

3. Department of Physics, Bilkent University, Ankara 06800, Turkey

In this study, we propose a novel semiconductor-less photovoltaic device. The proposed device is made of metals and dielectrics and the operation is based on hot carrier collection. We also present the use of surface plasmons to improve energy conversion efficiency. The field localization provided by plasmons confine the incident light in the metal layer, increasing the optical absorption and hot electron generation rate inside the metal layer. The device operation principle as well as the topology will be discussed in detail.

Thermal evaporation and atomic layer deposition (ALD) are used for the deposition of metal and insulator layers, respectively. Au-HfO2-Al layers form the bottom MIM junction, and gold nanoparticles (NPs) spin coated on an insulating spacer layer (Al2O3) complete the device structure. Scanning Electron Microscopy (SEM) is used to confirm the randomly distributed 50 nm diameter Au nanoparticles on the device surface.

Short circuit current measurements of the fabricated devices under monochromated light show clear evidence of the resonant characteristics of Au nanoparticles. The excitation of localized surface plasmon modes and propagating surface plasmon modes results in more than an order of magnitude enhancement in the short circuit current at the resonance wavelength (~600 nm) of the structure. Au nanoparticles were quite sparsely distributed; hence such an enhancement in the short circuit current is significant and clearly shows that the efficiency of MIM photovoltaic devices can be greatly enhanced by surface plasmon excitation. The simple planar MIM structure and use of chemically synthesized nanoparticles for plasmon excitation make this device promising for large area fabrication. We also show that plasmon excitation structures can be introduced without modifying the tunneling (rectifying) MIM junction. This gives additional degrees of freedom to separately optimize the electrical and optical properties of the MIM hot electron photovoltaic device. Further studies can reveal a path for very low cost and sufficiently efficient photovoltaic cells.

Keywords: hot electron, photovoltaics, surface plasmons, metal-insulator-metal (MIM)

We-C1.5 (invited) 9:40–10:10

Solar Cells on Paper to Power Paper Electronics

Rodrigo Martins, A. Vicente, Luís Pereira, D. Nunes, H. Águas, and Elvira Fortunato

CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP-UNINOVA, 2829-516 Caparica, Portugal

Imagine a world where all objects of our daily lives can produce energy, such as a book or even the clothes we wear, turning alive electronics you can embedded there. If so, the doors to a green electronics energy-sustained revolution would be opened! This is the approach we will present in this study where we will use paper, one of the most common and inexpensive materials available, fully recyclable, to embedded electronics circuits to be powered by solar cells grown on paper. Using the current technology of Plasma Enhanced Chemical Vapour Deposition, it was possible to manufacture solar cells on package paper with an efficiency of 4.3% (4.3 mW cm-2), for solar cells covering an area up to 16 cm2. This concept can be used in feeding devices (e.g. smart tags, or electro chromic auto sustainable displays) whose limit is our imagination. Thus, we believe that this opens the door to the development of devices of great added value to the disposable electronics industry in general and in particular for the nascent paper electronic industry.