The extremely high electron mobility of graphene - under ideal conditions electrons move through it with roughly 100 times the mobility they have in silicon - combined with its superior strength and the fact that it is nearly transparent (2.3 % of light is absorbed; 97.7 % transmitted), make it an ideal candidate for photovoltaic applications. Recent research suggests, though, that doping is a necessity to harvest the full potential of graphene. The challenge then for researchers is to find suitable fabrication techniques for high-quality graphene flakes that exhibit high charge mobilities. Researchers now present a chemical approach towards non-covalently functionalized graphene, which is generated from vastly available and low-priced natural graphite.
In order to find replacement materials for ITO, scientists have been working with carbon nanotubes, graphene, and other nanoscale materials such as nanowires. While many of these approaches work fine in the lab, upscaleability usually has been an issue. Researchers at Empa, the Swiss Federal Laboratories for Material Science and Technology, have now demonstrated another solution: they presented a transparent and flexible electrode based on a precision fabric with metal and polymer fibers woven into a mesh. The team demonstrated organic solar cells fabricated on their flexible precision fabrics as well as on conventional glass/ITO substrates and found very similar performance characteristics.
The power conversion efficiency of solar cells made of conjugated polymer/nanorod nanocomposites can be maximized when the nanocomposites are aligned perpendicularly between two electrodes for effective exciton dissociation and transport. To realize this, external fields can be applied to induce the self-assembly/alignment. The challenge is how to assemble them over a large scale - current self-assembly studies of cadmium selenide nanorods in literature are limited to only a micrometer scale. New design approaches are therefore needed to solve this problem. Due to their intrinsic structural anisotropy, nanorods possess many unique properties that make them potentially better nanocrystals than quantum dots for photovoltaics and biomedical applications.
In a standard dye sensitized solar cell, an organic molecule adsorbed on the surface of a porous electrode absorbs light and then initiates the charge separation process eventually leading to generation of photocurrent. While the dye appears to have "sensitized" the large bandgap material, it never actually does, because only the dye molecules absorb the light and generate the carriers, the large bandgap material primarily serves the function of a conducting channel to take the electrons out. While wide bandgap materials alone can not absorb the sun light efficiently, it has been predicted that if two large bandgap materials with type-II band alignment form coaxial nanowires, the effective indirect bandgap could be substantially smaller than either of the individual materials. After a few years effort, one research team has now demonstrated a real functional device that exhibits the key feature of the idea: the use of two large bandgap materials to make a solar cell behaving like a small bandgap material.
Nanowires - particularly those of silicon - promise great potentials for high-efficiency, low-cost solar energy conversion. This promise has not yet been met by experimental evidence, raising fundamental questions whether silicon nanowires are intrinsically disadvantaged and whether the photovoltaic research community should continue working on this material. Despite intense efforts, the performance of silicon nanowire-based solar cells remains significantly lower than what has been achieved for bulk silicon or micrometer-scale wires. The gap between the predicted performance and the inability to deliver raises an important question with regard to the origin of this problem. New research shows that the poor performance is not a result of the nanowire morphology, but is intrinsic to the growth chemistry.
There is an increasing interest in flexible solar cells and researchers have been investigating weavable fiber solar cells based on metal wires, glass fibers, or polymer fibers. Unfortunately, the low efficiencies of these fiber-based solar cells greatly limit their promising applications. In order to improve these efficiencies, scientists are exploring various nanomaterials to improve charge separation and transport in these fiber-based photovoltaic devices. One recent promising result has been demonstrated by a research team in China who have developed a novel solar cell from flexible, light-weight, ultrastrong, and semiconductive carbon nanotube fiber. The high alignment of building nanotubes in the fiber allows charges to separate and transport along the fibers efficiently, which provides a fiber solar cell with high performance.
A key hurdle in realizing high-efficiency, cost-effective solar energy technology is the low efficiency of current power cells. In order to achieve maximum efficiency when converting solar power into electricity, ideally you need a solar panel that can absorb nearly every single photon of light - across the entire spectrum of sunlight and regardless of the sun's position in the sky. One way to achieve suppression of sunlight's reflection over a broad spectral range is by using nanotextured surfaces that form a graded transition of the refractive index from air to the substrate. Researchers in Finland have now demonstrated a scalable, high-throughput fabrication method for such non-reflecting nanostructured surfaces.
Spin coating has been the dominant fabrication method for polymer electronics. However, it is not a high-throughput process and numerous research groups are trying to find a scalable fabrication method for polymer solar cells. One such method, spray coating, is capable of delivering large-area, uniform polymer thin films through a relatively simple process, while offering ample processing possibilities of engineering the film structure. Spray-coating is a high-rate, large-area deposition technique that ensures an ideal coating on a variety of surfaces with different morphologies and topographies. It is frequently used for industrial coating and in-line deposition processes. In spray-coating systems, the ink is atomized at the nozzle by pressure or ultrasound and then directed toward the substrate by a gas. An added advantage of spray-coating is that it is efficient: compared to other techniques only a small amount of the solutions are wasted.