Window-integrated solar cells for electric vehicles

Summary:

Window-integrated solar cells are beneficial components to electrical vehicles, since the power generated by these components can be used for powering diverse car appliances, sparing the driving battery from running these appliances and increasing the driving range per charging. Examples of car appliances include air conditioning, radio, seat heating, etc.

However, the development of highly transparent solar cells to be used in window-integrated solar cells is not a trivial task, given that these devices are less efficient compared to their non-transparent counterparts, owing to the decline of light absorption in the solar cell active layer due to a high light transmission coefficient.

Background:

Electrically driven vehicles open up new challenges but also new opportunities regarding all aspects of their performance, implementation, acceptance etc. This working package focuses on tackling some major challenges:

  • A huge limiting factor for the implementation of electrically driven vehicles is their limited driving range. This is dictated by the capacity of available batteries as well as by the consumers of electricity on board the vehicle. Besides the motor, secondary consumers such as systems for air conditioning, lighting, safety and entertainment limit the possible range. By integrating transparent solar cells in the vehicle windows, these could generate energy to be used to run those secondary consumers.
  • Another limiting factor for electric driven vehicles is the missing acceptance among the users, which can be retraced to non-reliable components as well as a gap between vehicle price and offered driving standard. Window-integrated solar cells will help to overcome that gap by increasing reliability, decreasing prices, adding a green component and highlighting ‘specialty’ of the electric vehicle, i.e. immediately demonstrating that increased price also means driving a very special kind of vehicle.

Activities:

On this background applied research work is being performed that bases on the long-year expertise of NanoSYD at SDU-MCI in cleanroom laboratory related device technology. The research work will is performed as a postdoctoral work at NanoSYD in collaboration with existing PhD students and supervised by permanent staff at the centre. Through the NanoSYD representation in the board of Lean Energy Cluster the immediate applied research needs of small and medium sized companies developing solar cells and windows will be taken into account.

Specifically, in this working package the plan is to develop a fully transparent organic solar cell to be integrated in the vehicle´s windows. The organic solar cells will contain specially designed nanostructures which trap the infrared light of the solar spectrum, increasing the solar cell efficiency without compromising its transparency. The energy generated by the solar cells could be used for example for the air conditioning in the car so that the energy from the main battery can be saved for driving the vehicle itself.

Within this context the following main results are targeted:

1. Detailed research plan elaborated for a fully transparent solar cell. Company involvement will be established through Lean Energy Cluster involvement.

2. Optimization of organic bulk-heterojunction solar cell architecture.

3. Enhance light absorption into the cell by including nanostructures.

4. Identify suitable materials for transparent cells and enhance the light absorption by including nanostructures.

5. Solar cell fabrication on flexible foil.

6. Window-integration.

7. Elaboration of publications and dissemination.

Results and publications:

The first milestone is to design a solar cell with inverted geometry, with the metal contact as the bottom contact. There were many critical steps to be optimized for fabricating such a solar cell. As a result of the fabrication optimization, we have obtained reproducible results, where a clear photoelectric effect is observed, where a typical cell has an open circuit voltage of 600 mV and a current density of 8 mA/cm^2, resulting in a total efficiency of 2.3 %.

The second milestone is to fabricate the optimized solar cells on a flexible substrate. Polyimide was spin-coated on rigid Silicon substrates, followed by hard baking. The standard solar cells fabrication steps as optimized in milestone 1 are then applied to the substrate. After the solar cell fabrication, the polyimide substrate can be peeled-off the Silicon substrate, resulting in a flexible device. The flexible solar cell presents the same electrical performance as the cells fabricated in rigid substrates.

Milestone 3 focuses on the fabrication of nanostructures in the solar cell for light trapping and efficiency enhancement. The chosen type of nanostructure was a grating fabricated on the bottom electrode. The gratings pitch distance was varied to three different values (250, 500 and 750 nm), diffracting the incident light along the active layer and increasing absorption. The gratings with a pitch distance of 500 nm shows to be the most effective design for light trapping for the P3HT:PCBM active layer. An enhancement on the cell absorption was observed for the structured solar cell, which also showed an enhancement on the conversion efficiency of about 36%. These results were presented at the “SPIE Photonics Europe 2012” and published in the conference proceedings: Proceedings of SPIE 8438-38 (2012), and in “Nanotechnology 24 (2013) 145301”.

For the fourth milestone, we have identified suitable materials for transparent organic solar cells including transparent electrodes and a molecular blend to be used in the active layer (PCPDTBT:PCBM) which absorbs light in the red/infrared wavelength range, resulting in a fairly transparent layer. The bottom electrode is made of ITO, the traditional conductive transparent electrode used in optoelectronics and a thin layer of titanium dioxide, which acts as a hole-blocking layer. The top electrode is the same used on the non-transparent cell, namely, the conductive polymer PEDOT:PSS in special formulations. In the past months, the device configuration has been optimized and the efficiencies have been increased to about 0.5 % for the PCPDTBT:PCBM material and 1% for the P3HT:PCBM material, which are pretty good for transparent devices, due to the low light absorption by the transparent active layer, and the absence of back-reflection from the bottom electrode.

The fifth milestone focuses in increasing the layer infrared light absorption.  A theoretical analysis based on the material properties (PCPDTBT:PCBM) and using the grating equation was performed in order to determine the ideal grating configuration, namely gratings with pitch distances of 500 and 750 nm. Experiments were performed with this molecular blend and these pitch distance and the experiments show an enhancement of about 14% on the efficiency of these solar cells. These results were published in the paper “Flexible PCPDTBT:PCBM solar cells with integrated grating structures” Proceedings of SPIE 8830, 883021-1, 2013.

The next step was to fabricate these gratings on transparent substrates. The ITO deposition (material for bottom-electrode) by sputtering has been optimized and the enhancement on the ITO conductivity and transparency has been obtained by post-annealing. Only by increasing the ITO conductivity we are able to fabricate gratings by e-beam lithography without charging up the sample. Preliminary gratings fabrication was performed, the results indicate an enhancement on the solar cell short-circuit current density and consequently on the power conversion efficiency for the 450 nm pitch gratings.

The sixth milestone consists in designing and fabricating window integrated solar cells and research on this side has been already performed. Devices fabricated in series have been designed and first fabrication trials have been performed in order to integrate the solar cells into LEGO windows.

The seventh milestone consists in dissemination of results, and has been fulfilled through the publication of scientific papers, participation in international conferences, and through the organization of a workshop on flexible and semi-transparent solar cells in June.

Conclusions:

The current status is that researchers at the University of Southern Denmark in Sønderborg have successfully demonstrated a method for increasing the power conversion efficiency of solar cells by including nanostructured optical features which enhances light absorption in the infrared range, leading to a remaining high transmission of visible light and visible transparent devices. These results were published in a number of international conferences and scientific journals. The figure below shows a window-integrated transparent solar cell. The marked area corresponds to the solar cell area, and one can observe the features on the background due to the device high transparency.

Semi-transparent window-integrated solar cell

Perspectives and applications:

Being able to fabricate semi-transparent solar cells opens the door for a large variety of applications, especially for window applications. A number of research groups have been researching this topic in the past years, with significant improvement in this directions, opening positive perspectives for the field.

Business opportunities:

Companies like Heliatek, in Germany are interested in semi-transparent solar cells, and claim to produce solar cells with 7% efficiency, but only 29% transparency. More research in this area will certainly be of their interest and also from other companies.

Responsible partner:

Syddansk Universitet
Mads-Clausen Institute (SDU-MCI)
Alsion 2, 6400 Sønderborg

Prof. Horst-Günter Rubahn



rubahn@mci.sdu.dk
+45 6550 1657