Date of Award
Doctor of Philosophy (PhD)
The improvement in device efficiency has brought organic photovoltaic (OPV) devices closer to commercial viability, highlighting the importance of studying the lifetime and stability of OPV devices. At present, the lifetime and stability of OPV devices is much shorter and poor mainly caused by oxygen, moisture, and light resulting in the oxidation on low work function electrodes and the degradation of the morphology of the photoactive layer. To improve the lifetime and stability of the OPV devices, we used newly developed low bandgap polymer, PCDTBT, as the electron acceptor material and a solution based titanium sub-oxide (TiOx) interfacial layer inserted between the active layer and the cathode.
In our experiment, we fabricated unencapsulated bulk heterojunctions OPV devices based on the high and low bandgap polymers of P3HT:PC61BM and PCDTBT:PC71BM, respectively. We synthesized a solution based TiOx by using a sol-gel chemistry method. We performed stability tests on the OPV devices: (1) with and without the TiOx layer (Case (I)) to test the effectiveness of the TiOx layer in protecting the photoactive layer from degradation, (2) with and without a protection cover (a high research grade opaque Al foil) to observe the device performance in a dark/light environment (Case (II)), and (3) in different storage media conditions: (a) air, (b) glove box, (3) ante-chamber of a glove box, and (4) (Case (III)). We spent significant time and effort in optimizing the fabrication processing steps including; the thickness of the active layer, pre-annealing and post-annealing treatments. We fabricated the OPV devices by using the optimal fabrication procedure.
We found that the best PCE value of 4.1% achieved for the P3HT:PC61BM OPV cell and 5.1% for the PCDTBT:PC71BM OPV cell. On the air stability test, we found that the OPV cell of P3HT:PC61BM materials showed good air stability performance resulting in the PCE only dropping 26% over a period of 70 days (stored in a glove box). The PCDTBT:PC71BM devices stored in the glove box over a period of 30 days showed relatively good air stability performances; (1) the device with a TiOx, layer and an opaque Al cover the PCE dropped only 16%, (2) the device with the TiOx layer and without an opaque Al cover PCE dropped 34%, and (3) the device without a TiO x, layer and with an Al cover PCE dropped 48%. While the PCDTBT:PC71BM devices stored in the air; (1-2) with a TiOx layer and with/without opaque Al covers the PCE values dropped 92% after 18 days, and (3) without the TiOx, layer and with an opaque Al cover, the PCE dropped 100% after 3 days. These results highlight the effectiveness of the TiOx layer in protecting the active layer from degradation. We concluded that the TiOx, layer effectively improved the stability the OPV devices.
"Improving the Stability of High and Low Bandgap Polymers Organic Photovoltaic Devices Using a Solution Based Titanium Sub-Oxide Interfacial Layer"
(2013). Doctor of Philosophy (PhD), dissertation, Electrical/Computer Engineering, Old Dominion University, DOI: 10.25777/bgbx-ey61