It is an exciting time in solar energy for materials scientists. Dye sensitized solar cells are narrowing the efficiency gap, carbon solar cells have become a reality and transparent solar cells are becoming practical. Despite these developments there is still significant room for third generation solar cell technology to improve. In order to understand where solar research is heading, we need to explore where it started and how it works. This article will present recent breakthroughs in transparent solar cells in their historical context and explain the basics of this promising technology.
News and Background
Since Michael Gratzel’s landmark paper on dye sensitized solar cells in 1991, transparent cells have been a promising area of photovoltaic research. This concept promises cheap, easy to install, electricity generating windows in our near future. Up until recently, the properties of such devices had remained too weak for them to be practical. Most importantly the efficiency of transparent cells could not compete with crystalline technology, which remains the king of solar. Crystalline panels have 18% average efficiency, nearly double that of thin film, its nearest competitor. This is far beyond the 1% efficiency of most transparent cells. Though transparent cells, and other third generation technologies, are not very efficient or durable, they can be flexible, easy to install and have unique physical properties. Part of the reason these cells lag behind is that they are a relatively new field of study. Fortunately, the past few years have shed light on some novel techniques that have brought transparent photovoltaics closer to the marketplace.
In 2011, a research paper from MIT by Bulovic, professor of electrical engineering, and Lunt, post doctoral researcher, presented a cell with impressive properties. The team’s organic solar cell was 1.7% efficiency and greater than 65% transparency. While this was an important result that made global news, the cell’s efficiency was still rather low and compound stability concerns still existed. These are chronic problems in organic photovoltaic cells, whose materials are less efficient than silicon and degrade quickly with use. However, the new solar cell developed at UCLA has taken significant steps to improve performance.
UCLA researchers’ latest PV publication describes a new plastic photovoltaic cell, which has an efficiency of 4% and nearly 70% transparency. These are both milestones. 70% is a record for photovoltaic transparency and, while 4% may not seem like much, keep in mind that thin film solar cells have an average of 10% efficiency. Considering the easy implementation, high efficiency and transparency of these cells, we may have photoelectric displays and windows sooner than we thought. Yang Yang, the professor who led the study, said “These results open the potential for visibly transparent polymer solar cells as add-on components of portable electronics, smart windows and building-integrated photovoltaics and in other applications.”
Transparent solar cells operate using the same mechanism as more common solar panels to produce electricity: the photovoltaic effect. Incident light on a semiconductor is converted into electricity by promoting electrons to the conduction band. These electrons then create a current as they travel between the cell’s front contact and back contact. Transparent cells differ from crystalline and thin film panels in that they absorb only non-visible light, such as infra red and ultraviolet. Most visible wavelengths, which lie between 400nm and 700nm, pass through the material unimpeded. This allows us to see through the cell. In addition, the cells are often flexible since they are not made from silicon wafers and have efficient manufacturing methods, such as rolling, that produce huge economies of scale.
Many different designs and materials are used to make transparent photovoltaic cells. Naturally, a key part of the technology is ensuring that no visible light is absorbed. Plastics, composite and a whole host of exotic materials have been used to do this. In the case of the UCLA study, a near-infrared photoactive polymer was used. The cell therefore produces most of its electricity from infrared light. This means that it is relatively reliable, producing some electricity even in dark conditions. Unfortunately the material remains somewhat unstable. The researchers were not very concerned about this, saying that some additional research could extend the cell life considerably.
Often third generation technologies, such as transparent solar cells, use titania (TiO2) nanoparticles to improve their efficiency. This material is abundant and very effective in solar cells. The UCLA cell is no exception. The cell’s conductor, which is entirely transparent, consists of a silver nanowire film doped with titania nanoparticles. Not only does the material have excellent optical properties, it is also inexpensive to produce via solution processing. Many have pointed to this conductor as a key element to the device’s success. Previous designs included opaque conductors which, quite naturally, did not make for very good transparent photovoltaic cells.
Glass is everywhere. You can find it on building’s windows, greenhouses and practically any electronic device. Transparent photovoltaic material can be used in all of these applications to produce clean, reliable electricity. A major benefit of all these potential uses is that there are no extra mounting or installation costs. For example, when new windows are needed in a building, transparent solar cells can be replace glass panels for very little extra cost. Solar glass is an easy choice for businesses as the cells, which are likely to be very cheap, will lower their electricity costs. Unlike crystalline and thin film panels, experts do not even need to be called in for installation.
The application attracting the most intense interest is portable electronic displays. Cell phone batteries are notoriously short lived. Most of our devices run out of power in under a week. By partially recharging our devices using transparent solar displays, battery life can be prolonged and electricity consumption (from the grid) reduced. Designs have already been made where the cell would reside directly on the screen, below the capacitance level, allowing for users to keep using their phones as touch screens. A company called Wisips has recently been marketing this idea. Many also believe that the cells could be successfully installed in skyscrapers and other large businesses which have a lot of windows and sunshine. 3M has their own transparent solar cell that they are planning to pitch to government and corporate buildings.
It may be a while before this technology makes it into the average households, but this will probably happen eventually. Many homes have windows that face the equator (south in the northern hemisphere, north in the southern hemisphere) and would welcome lower electricity bills. Buying a grid tie inverter for solar windows may impose an extra cost on the homeowner, but it would not be very significant. Note that if the individual already has a distributed electricity source, adding solar windows becomes even more simple.
One area that few people have discussed is using this technology in crystalline solar panel glass. Low cost and durable transparent cells could complement current solar technology by increasing the range of wavelengths of light that are absorbed. Crystalline silicon has a band gap of around 1 eV, meaning it converts light from near infrared with the greatest efficiency, but also absorbs radiation with frequencies up to that of blue visible light. The UCLA cell, which runs mainly on infrared light, may not be complementary to this technology, but a cell with a different band gap might be. By combining the two technologies, the efficiency of panels would go up, lowering the cost of solar energy.