In the past two decades, polymer solar cells have gradually attracted considerable attention as a promising alternative energy candidate. Polymer solar cells are made of different kinds of semiconducting polymers, which can be quite similar in terms of their molecular structures and compositions compared to conventional polymers found in daily applications.
With the assistance of small organic molecules in polymer solar cell devices, it can trap and accumulate light and turn it into electricity. The choice of polymer in solar cell materials usually makes them light and capable of many environmentally-friendly processes.
The power conversion efficiency, which measures the percentage of incident light converted to electrons, is the primary quantitative index to determine the performance of a solar cell device. Recently, the certified power conversion efficiency of a state-of-the-art polymer solar cell achieved 12%, which means that 12% of the incoming solar energy can be converted into electric power. However, inorganic crystalline Si solar cell can already reach power conversion efficiencies exceeding 26%, and some of the multi-junction cells can be even more than 30%. Based on the above facts, people may ask: what is the major advantage of polymer-based solar cells to compensate for its obvious lower efficiency? Is it really worth developing in a nearly saturated global solar cell market?
The answer is simple: transparency. The solar spectrum is composed of light with different wavelengths. For the human eye, only light with wavelengths from 400 nm to 700 nm can be differentiated and imaged in the brain. In other words, the colors we see originate from only a small region of light. If light is not absorbed through an obstacle, this obstacle is deemed visibly transparent.
Due to the nature of the molecular orbitals of polymer compounds, the absorption spectra of polymer semiconductor materials are not as continuous as the ones of its inorganic counterparts. This results in a major shortcoming for single-junction polymer solar cells in terms of their insufficient light absorption and small electron current. However, this “disadvantage” of polymer solar cells can actually open a door to a new opportunity to achieve visible region transparency while maintaining decent electronic power output. According to the energy distribution of the solar spectrum, nearly half of solar energy is distributed within the infrared (IR) region. As a result, the theoretical efficiency of polymer solar cells with only IR absorption is able to be as high as the device with only visible absorption.
Because of the rapid and intensive research in this field recently, people begin to realize the potential of transparent polymer solar cells. Once again, they may ask, “Is there a purpose for utilizing this type of solar cell rather than inorganic ones?”
Of course, there are some special but also important applications that transparent polymer solar cells can be specifically preferred, ranging from indoor sensors for the internet of things (IoT) to building-integrated modules. Indoor applications usually operate at low illuminance and consume very low power. In contrast, building-integrated modules are designed to incorporate into or replace traditional roofs and windows to absorb the outdoor sunlight and supply additional power to the building or the other system.
For indoor applications, transparent polymer solar cells have an advantage over inorganic solar cells under weak lighting (0.002 to 0.003 sun, 300 to 500 lux) conditions. Polymer materials have a much higher light extinction coefficient, which refers to a better light absorption ability per unit thickness and lower dependence of output voltage on illuminance, than inorganic solar cells. It means that polymer solar cells can still perform with reasonable efficiency and output power with a thinner layer under a dimmed surrounding without compromising much.
Among all potential outdoor applications, utilizing transparent polymer solar cells in a greenhouse for agricultural applications has more obvious advantages over the traditional module due to the ability of spectrum control.
Agricultural plants utilize specialized pigments to intercept and capture photon energy for growth. Within the broad solar light spectrum, the photosynthetically active radiation activates the chlorophyll a and b pigments, transforming light energy into chemical energy for production of carbon molecules (such as sugars) within some specific regions from 400 to 700 nm. The products are then used to construct organs like roots, leaves, stems, flowers, and fruits.
Consequently, solar cells can be designed to capture electromagnetic energy in regions of the spectrum that are not used or used less predominantly for photosynthesis and photomorphogenesis. Therefore, the spectral response of the cell can be tuned to use that section of light as well and maximize the total solar energy conversion efficiency.
Considering this remarkable potential, there is a strong uprising trend for developing polymer solar cells, especially the materials that can achieve both high efficiency and visible transparency. Research groups should also demonstrate methods of scalability to assist in technology transition from laboratory to industry, as well as to demonstrate transparent solar cells in other applications such as agriculture, construction, automobiles, and aviation.
These findings are described in the article entitled Transparent Polymer Photovoltaics for Solar Energy Harvesting and Beyond, recently published in Joule. This work was conducted by Sheng-Yung Chang, Pei Cheng and Yang Yang from the University of California, Los Angeles and Gang Li from the University of Hong Kong Polytechnic University.