Blog #2 – Hope For Solar Power Remains Unextinguished

In a study recently conducted by Vaynzof et al. (2010), it was found that the performance of a polymeric solar cell was greatly enhanced when combined with modified ZnO substrates.

Solar power has often been considered the environmentally friendly power source of the future, but it has equally often been criticized for its inefficiency. Photovoltaic (PV) devices remain too uneconomical as a result of their inability to effectively dissociate electrical charges. One potential solution favoured by environmental scientists is to combine polymeric solar cells with metal oxides, thus increasing the charge separation that is necessary to harness photo-induced electrical charges. ZnO in particular has been the subject of much attention due to certain properties it exhibits when applied as a film to polymeric solar cells. Vaynzof et al. (2010) build upon this potential solution, bringing the scientific community closer to an efficient hybrid PV device, and to a more eco-friendly future.

To begin the experiment, a film of ZnO was applied to the polymer poly[3-hexylthiopene] (P3HT), after having been altered by a self-assembled monolayer (SAM) of phenyl-C₆₁-butyric acid (PCBA). The PCBA monolayer was chosen to modify the ZnO substrate due to its high electron affinity and its ability to readily accept electrons from P3HT, increasing photo-induced electron transfer. This increased efficiency is of course beneficial to the cell’s function. In order to test and examine the benefits of the modified ZnO PV device, it was compared to a regular hybrid ZnO/P3HT PV device, which lacked the modified cell’s PCBA monolayer. Efficiency of the cells was then assessed by taking ultraviolet photoemission spectroscopy (UPS) measurements and external quantum efficiency (EQE) measurements. The UPS value for bare ZnO substrate was measured to be 3.6eV as compared to the value of ZnO modified with SAM of PCBA, which was 4.1eV. The total work function of the cell therefore increased by 0.5eV when modified ZnO was used. The EQE measurements similarly increased from 3% with the regular P3HT/ZnO device to 9% with the modified device. These values were shown to correspond to the cells’ internal quantum efficiency (IQE), meaning that the modified P3HT/ZnO was running at approximately 100%; a drastic increase from the 30% of the unmodified P3HT/ZnO. This increase to 100%, which was attributed to the P3HT/ZnO being modified by a SAM of PCBA, was interpreted as the solution to the unmodified device’s inability to efficiently separate photo-induced charge.

The miniscule interactions that take place at the interface between the polymer P3HT and the film of PCBA/ZnO explain this sudden leap in efficiency. Charge separation, assisted by the PCBA monolayer, occurs as excitons rapidly dissociate into holes in the P3HT, and electrons are transferred to the PCBA monolayer of the modified ZnO substrate. Meanwhile, ground state electrons are transferred from ZnO to the PCBA monolayer in order to equalize their chemical potentials. This second transfer creates a depletion zone in the surface of the ZnO film that separates the ZnO from the PCBA monolayer, and the electric field that exists at the ZnO/PCBA interface causes a swift transfer of electrons back to ZnO. This final transfer prevents ZnO from recombining with the PCBA monolayer. Thus, the increased efficiency of the modified ZnO device is explained empirically by increased charge separation (exciton dissociation) and by reduction of recombination.

The problem with solar power is its inefficiency, but this study has shown that charge separation can continue to be improved as better modifications are made to the materials in PV devices. The SAM of PCBA, as shown in the study, has unique properties that allow exciton dissociation and recombination reduction to increase dramatically. In this case, the monolayer is a strong electron acceptor and is able to create dipoles at the interface between P3HT and PCBA/ZnO, allowing for easy electron transfer. Not only does the study’s results reveal drastically improved charge separation, but it also shows potential for further development specifically with modified ZnO cells. Next steps could include the production of nano-structured ZnO substrates, which would allow for a larger interface and a greater area across which to carry photo-induced charge.

Solar power remains a strong contender for the future of energy production and, although they are far from perfected, PV devices are being continually improved through studies like this one. Efficient green energy is not too far out of reach, as long as the scientific community continues to strive towards it.

Vaynzof et al. Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices. Applied Physics Letters, 2010; 97 (3): 033309

Link to website containing PDF of primary source: http://apl.aip.org/resource/1/applab/v97/i3/p033309_s1