Nir Tessler recently published a research paper - Adding 0.2V to the open circuit voltage of organic solar cells by enhancing the built-in potential - that explores the idea of useing a new device architecture to achieve a higher efficiency for BHJ photovoltaics.
According to Nir Tessler the idea/motivation behind this research was that the well established bulk-heterojuction (polymer:fullerene) solar cells have effectively reached their best performance. With about 12% being reported and that further improvements will be small but require substantial research efforts.
In the scientific community the Shockley-Queisser paper is often used as an indication that the open circuit voltage of BHJ devices is at its maximum value (or close to it). What many researchers often forget is that every theory has some assumptions behind it. For example, in that famous paper it was assumed that the optical gap (where light is absorbed) is equivalent to the electronic gap (where charges move). They didn’t need to state this since this is obvious for any inorganic cell being considered at the time.
However, organics are "strange" in many ways and there is a difference between the two gaps. In fact, in BHJ devices the difference could easily be 0.7V. This means that there might be a way to engineer a device that will have much better performance.
The paper by Nir Tessler describes the work to identify a new device structure that could make a 12% BHJ into 18% one (while keeping the stability and reliability already achieved in BHJ devices).
The research paper concludes by saying that advanced materials developed for BHJ devices have a huge potential yet to be uncovered and that the performance of high efficiency devices (at least those of 10% or above) is most likely limited by the device structure and not by the materials' properties.
One such device property is the pinning of the electrode work function below the energy level of the bulk of the device which effectively limits the built-in potential of the device. By modifying the material composition at the vicinity of the electrode opens the possibility to enhance the energy gap between the electrodes and thus also the built-in potential.
The calculations presented indicate that the PCE of P3HT based systems could be improved by up to 28%. The calculations shown indicate that higher quality material compositions4 may show up to a 0.20V increase in their open circuit voltage which together with the improved fill factor would lead to a 48% increase in their power conversion efficiency. The researchers find the prospect of going from 10% to 15% efficiency through device structuring most promising.
J. Appl. Phys. 118, 215501 (2015) | http://dx.doi.org/10.1063/1.4936367
Received 14 September 2015 | Accepted 11 November 2015 | Published online 01 December 2015
We present a systematic device model that reproduces the important features of bulk heterojunction organic solar cells. While examining the model outputs we find that one of the limiting factors in organic solar cells is the reduced built-in potential due to effective pinning of the electrodes relative to the energy gap at the bulk of the device. Having identified this as a problem we suggest a device structure that can enhance the open circuit voltage. Our detailed modeling shows that such a structure can enhance the open circuit voltage as well as the short circuit current leading to above 40% improvement in power conversion efficiency of state of the art organic solar cells.
We acknowledge the support of the Israeli Nanotechnology Focal Technology Area on Nanophotonics for Detection, the Grand Technion Energy Program (GTEP), and comprises part of The Leona M. and Harry B. Helmsley Charitable Trust reports on Alternative Energy series of the Technion, Israel Institute of Technology, and the Weizmann Institute of Science.