Transparent electrodes, indespensible in displays and solar cells, are currently dominated by indium tin oxide (ITO) films although the high price of indium, brittleness of films, and high vacuum deposition are limiting their applications. Recently, solution-processed networks of nanostructures such as carbon nanotubes (CNTs), graphene, and silver nanowires have attracted great attention as replacements. A low junction resistance between nanostructures is important for decreasing the sheet resistance. However, the junction resistances between CNTs and boundry resistances between graphene nanostructures are too high. The aspect ratios of silver nanowires are limited to ∼100, and silver is relatively expensive. Here, we show high-performance transparent electrodes with copper nanofiber networks by a low-cost and scalable electrospinning process. Copper nanofibers have ultrahigh aspect ratios of up to 100000 and fused crossing points with ultralow junction resistances, which result in high transmitance at low sheet resistance, e.g., 90% at 50 Ω/sq. The copper nanofiber networks also show great flexibility and stretchabilty. Organic solar cells using copper nanowire networks as transparent electrodes have a power efficiency of 3.0%, comparable to devices made with ITO electrodes.

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Electrospun metal nanofiber webs as high-performance transparent electrode.

Here, means to enhance power conversion efficiency (PCE or η) in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells by optimizing the series resistance (Rs)—also known as the cell internal resistance—are studied. It is shown that current state-of-the-art BHJ OPVs are approaching the limit for which efficiency can be improved via Rs reduction alone. This evaluation addresses OPVs based on a poly(3-hexylthiophene):6,6-phenyl C61-butyric acid methyl ester (P3HT:PCBM) active layer, as well as future high-efficiency OPVs (η > 10%). A diode-based modeling approach is used to assess changes in Rs. Given that typical published P3HT:PCBM test cells have relatively small areas (∼0.1 cm2), the analysis is extended to consider efficiency losses for larger area cells and shows that the transparent anode conductivity is then the dominant materials parameter affecting Rs efficiency losses. A model is developed that uses cell sizes and anode conductivities to predict current–voltage response as a function of resistive losses. The results show that the losses due to Rs remain minimal until relatively large cell areas (>0.1 cm2) are employed. Finally, Rs effects on a projected high-efficiency OPV scenario are assessed, based on the goal of cell efficiencies >10%. Here, Rs optimization effects remain modest; however, there are now more pronounced losses due to cell size, and it is shown how these losses can be mitigated by using higher conductivity anodes.

Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series Resistance

Measurements of the series resistance R/sub S/ are important for the localisation of dominant loss mechanisms in photovoltaic devices. The new measurement technique presented in this work uses the measured "J/sub SC/-V/sub OC/ curve" of a solar cell as an approximation to the unknown R/sub S/-corrected I-V curve and determines the "lumped series resistance" in dark and illuminated operating conditions (R/sub s.dark/ and R/sub s.light/) from the voltage shift between the "J/sub SC/-V/sub O/C curve" and the dark and illuminated I-V curve, respectively. Owing to multidimensional effects in practical devices, the lumped series resistance depends on the operating condition of the cell (i.e., dark or illuminated I-V measurements) and on the current density flowing through the device. This work not only provides a new, powerful method for the determination of the lumped series resistance of photovoltaic devices, but also considerably improves the general understanding of ohmic power loss effects in silicon solar cells. >

A new method for accurate measurements of the lumped series resistance of solar cells

Series resistance is an important parameter in solar cell design and fabrication. Methods reported in literature for its determination are not suitable for routine use. We describe a simple method for obtaining the series resistance. Universal current-voltage characteristics of the solar cells are also given to illustrate the effects of series and shunt resistance. We also offer an explanation for the often observed drop in efficiency when small area cells are scaled upward. Cells less than 0.25 cm2 represent the intrinsic potential of any given structure or diode but do not reflect series resistance effects that must be eliminated with larger area cells and the practical problems of griding to allow for current collection.

A better approach to the evaluation of the series resistance of solar cells

Thin-film polycrystalline CdS/Cu 2 S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm2have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu 2 S cell.

The design and fabrication of thin-film CdS/Cu 2 S cells of 9.15-percent conversion efficiency

Most photovoltaic solar cells use some type of light-transmitting grid electrode on the upper surface to reduce series resistance losses as the current is collected. The geometry of this grid is chosen through consideration of optical transparency and series resistance. A method of calculating the series resistance as a function of grid geometry is presented here with the following assumptions: (1) current generation is uniform over the area of the cell; (2) the thickness of the upper semiconductor layer is very much smaller than the lateral dimensions of the cell; (3) the resistance of the grid electrode is much less than the sheet resistance of the upper semiconductor layer, and (4) the current flow in the layer is ohmic. These assumptions will be satisfied for most solar cells of interest. The method is then applied to several representative grid geometries and the results are compared with previous calculations in the literature.

N. Convers Wyeth

Papers

Sheet resistance component of series resistance in a solar cell as a function of grid geometry