Session Th-B3

Organic/Inorganic Heterojunction Solar Cells

Chair: Eric Schiff, Syracuse University

Th-B3.1 14:00–14:20

Self-assembled Silver Nanowires Mesh as Top Electrode for Organic-Inorganic Hybrid Solar Cell

Ishwor Khatri, Qiming Liu, Ryo Ishikawa, Keiji Ueno, and Hajime Shirai

Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan

Poly (3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS)/n-Si hybrid solar cell has attracted great attention due to its high power conversion efficiency and solution processing at low temperature. Generally, PEDOT:PSS solution is spin coated on n-Si substrate for the fabrication of this type of solar cell with metal-insulator-semiconductor (MIS) Schottky junction. The photocarriers are separated under the built-in potential at the interface of PEDOT:PSS/n-Si. Silicon generates photocarriers and transport electrons, while the organic layer is used for hole transporting. Charges are collected by metal electrodes deposited on the top and bottom of the device.

In this work, we use self-assembled polygonal structured silver nanowires (AgNWs) mesh as top electrode for efficient charge collection. At first, AgNWs were prepared by polyol process, where ethylene glycol acts as solvent and reductant. Poly (vinylpyrrolidone) (PVP) is added as capping and induced agent. The rate of PVP to silver ions was most crucial factors to the growth of nanowire. After repeated cleaning of AgNWs in ethanol and water solution, it was dispersed in deionized water that was bubbled with surfactant and thickening agent. When the bubbles containing AgNWs were sandwiched between two glass substrates with spacer, the bubble ridges including the AgNWs formed continuous polygonal structures. Mesh structures were formed after air-drying, which was followed by heat-treatment at 200°C for 20 minutes. This self-assembled AgNWs mesh electrode exhibited a low sheet resistance of 50 Ohm/square with a transparency of 87%, and transformed as top electrode in PEDOT:PSS/n-Si hybrid solar cell using thermal release tape. Device performance with randomly distributed AgNWs mesh electrode was also compared in similar device structure, where efficiency deteriorated due to unconnected AgNWs. Scanning electronic microscopic(SEM) image shows that self-assembled polygonal AgNWs mesh are well connected. The width and height of this AgNWs mesh varied from 5–25 ìm and 2–10 ìm, respectively. Current-voltage (I-V) characteristic of the device shows the improvement in power conversion efficiency from 10.32% to 10.62% with AgNWs mesh electrode. Specially, the fill factor (FF) of the device improves from 0.61 to 0.67 due to the decrease in series resistance. This finding suggests that self-assembled AgNWs mesh as top electrode is a promising method to improve device performance of PEDOT:PSS/n-Si solar cell.

Keywords: hybrid solar cell, silver nanowires, PEDOT:PSS, n-Si

Th-B3.2 14:20–14:40

Self-assemble Ferroelectric Nanoarray and its Application in c-Si/PEDOT:PSS Heterojunction Solar Cells

Qiming Liu, Naoto Miyauchi, Ryo Ishikawa, Ishwor Khatri, Keiji Ueno, and Hajime Shirai

Graduate School of Science and Engineering, Saitama University, Saitama, Japan

In this paper, we incorporated the ultrathin ferroelectric (FE) polymer layer into c-Si/ poly(3,4-ethlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction photovoltaic devices to increase efficiency with the induced polarization electric field. The organic-inorganic heterojunction solar cells combined with solution processed poly(3,4-ethlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and silicon offer the advantage of simple fabrication process and potential low cost. Recently, a power conversion efficiency (PCE) of ~11% was obtained with such devices. However, the open-circuit voltage (Voc) was still relatively low, compare with that of usual Si based solar cell, due to intensive carrier recombination at polymer/Si interface. Notably, an external bias voltage can be used to efficiently separate the electrons and holes and thus prevent their recombination. In this study, a internal electric field has been generated by incorporating a self-assemble vinylidene fluoride_trifluoroethylene P(VDF-TrFE) ferroelectric polymer nanoarray at the Si/PEDOT:PSS interface, which supplied a native polarized electric field owning to original partial poling of FE layer as a replacement of the external bias. The inserted FE layer showed a nanoarray distribution after the annealing process and the cross-section of each lattice exhibited a shape of typical triangle due to electrostatic effect among the polarized electric charge. Via the insertion of FE layer, efficiency of c-Si/PEDOT:PSS heterojunction solar cell increased from 10.1% with a Jsc of 27.2 mA/cm2, a Voc of 0.538 V and an FF of 0.686 to 12.1% with a Jsc of 29.6 mA/cm2, a Voc of 0.572 V and an FF of 0.715. Here, the improved photovoltaic parameter should mainly attributed to the insertion of FE nanoarray which suppressed the charge recombination by following effects, 1): improved charge separation at the Si/PEDOT:PSS interface due to enhanced internal filed with additional polar electric field, 2): accelerated charge transfer through the PEDOT:PSS thin film and/or N type Si bulk promoted by electrostatic force owning to homogeneous distribution of polarized electric charge like dipole array. The carrier transport property of c-Si/FE/PEDOT:PSS junction will be discussed.

Keywords: charge recombination, ferroelectric material, heterojunction solar cells, internal field

Th-B3.3 14:40–15:00

Improved Photovoltaic Response by Incorporating Green Tea Modified Multiwalled Carbon Nanotubes in Organic/Inorganic Hybrid Solar Cell

Ishwor Khatri, Qiming Liu, Ryo Ishikawa, Keiji Ueno, and Hajime Shirai

The Graduation School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakuraku, Saitama 858-3676, Japan

Carbon nanotubes (CNTs) have strong tendency to form bundles (or ropes) due to strong van der Walls interaction, which prevent interfacial charge separation ability, solubility and alter the properties of isolated tubes. Therefore, isolated CNTs are necessary for the optimization of active layer morphology as well as easy exciton splitting and suppression of charge recombination. In recent year, a few works have been reported to improve efficiency of hybrid solar cell by incorporating CNTs at interface of polymer and n-Si substrate. However, the efficiencies of these solar cells are very small. It has been suggested that the intrinsic work function of single walled carbon nanotubes (3.4–4.0 eV) and multiwalled carbon naotubes (MWCNTs) (4.5–5.1 eV) are closer to valance and conduction band of polymer materials, which signifies possible electron and hole transportation in the device.

In this work, poly (3,4-ethylenedioxythiophene): poly(stylenesulfonate) (PEDOT:PSS)/n-Si hybrid solar cells were studied by incorporating green-tea modified MWCNTs at the interface. Green tea disperse MWCNTs to individual. Catechins are expected to be a candidate for MWCNTs separation. Green tea catechins are a group of polyphenols compounds belonging to the flavonoid family. It mainly consists of epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate (EGCG) where EGCG makes up about 50–60% of the total polyphenols, which is effective to disperse MWCNTs. The isolated MWCNTs were well embedded by the PEDOT:PSS film, which reduces shunting effect in the device. As a result, the power conversion efficiency of the device improved from 10.49% to 10.93%. We believed that green-tea modified MWCNTs improve built in potential for better hole transport and suppression of charge recombination, thereby improving photovoltaic response.

Keywords: hybrid solar cell, carbon nanotubes, green route, PEDOT:PSS, n-Si

Th-B3.4 15:00–15:20

Efficient Tandem Junction Organic/Inorganic Hybrid Solar Cells

Vikram Dalal, Mehran Samiee, Siva Konduri , Pranav Joshi, and Rana Biswas

Iowa State University, Dept.of Electrical and Computer Engr., Ames, Iowa 50011

Organic solar cells are an important new technology for inexpensive solar energy conversion. Organic materials suffer from a fundamental problem, a relatively narrow range of wavelengths over which absorption can take place. This fact means that only a narrow range of solar photons can be absorbed. In this paper, we discuss an alternative design that uses amorphous Si based solar cells in combination with organic solar cells to produce a higher efficiency structure. This system offers a significant flexibility in design since the bandgap and absorption range of both the inorganic and organic cells can be varied widely to match currents so that an efficient tandem junction structure can be achieved. The design is better than using tandem of multi-junction combination of amorphous cells of different bandgaps because the organic cells offer a much higher absorption coefficient than amorphous or nanocrystalline materials near their Tauc gap, and therefore, a more complete absorption in a much thinner film (~100 nm). We will show how to obtain solar conversion efficiencies of >20% through an appropriate combination of bandgaps and photonic/plasmonic structures. We also report on the fabrication of two different tandem junction structures, one employing P3HT as the organic semiconductor, and another employing PTB7 as the organic semiconductor. We show that the selection of the tunnel contact layer between the two cells is critical in assuring good performance. If care is not exercised in the choice of the contacting layer, a double-diode type characteristic with poor fill factors results. We use a doped oxide layer (ITO) combined with an appropriate heavily doped n+ a-Si layer on the high gap cell as an efficient tunneling layer. With such layers, an exceptionally high fill factor of 77% is obtained. We show that for P3HT cells, the efficiency of the tandem cells increased by 30%, and for PTB7, by 25% compared to the efficiency of the organic cell by itself. Conversion efficiencies near 10% are now being routinely achieved in the PTB7 system. What is equally important, the tandem combination results in a more stable cell. Details on fabrication and design principles and stability testing will be described in the talk.

Keywords: tandem solar cells, organic solar cells, PTB7