The Optimization And Stability Study Of Inverted Organic Solar Cells

Due to their low cost, light weight, simple processibility, and mechanical flexibility, organic solar cells(OSCs) have exhibited great potential in their commercialization and attracted worldwide attention. In order to commercialize the photovoltaic devices in the future, the improved power conversion efficiency(PCE), fabrication cost and operational stability must be taken into account when considering the applications of OSCs. The aim of this work is to obtain low cost OSCs with higher PCE, simpler processibility and better stability. For example, the commonly used photoactive material of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester(P3HT:PCBM) bulk heterojunction blend is chosen to achieve simple processibility. Inverted structure is used to improve the device stability, and interfacial modification layers are used to improve device performance. In this thesis, the main targets include the comparison of OSCs and inverted OSCs(IOSCs) from optical and electrical performance, the effect of interfacial modification layer on inverted OSCs, and the long-term stability of IOSCs.The first chapter introduces the research background and emphasizes the importance of inverted structure and interfacial modification layers in OSCs. Then the second chapter provides an introduction on the working mechanism, device testing and various characterization methods. In addition, theoretical simulations, device structure and fabrication process for OSCs are also discussed. The detail studies pave the way for the following research work in this thesis.In chapter 3, based on the transfer-matrix model, the differences between OSCs and IOSCs are studied from optical and electrical aspects. From the optical aspect, with increasing the active layer thickness, the number(N) of photons absorbed in the active layer and the external quantum efficiency(EQE) tend to increase due to the obvious interference behavior for both OSCs and IOSCs. However, compared to OSCs, IOSCs show better performance except the thickness around which the interference maxima of OSCs are obtained. When considered the electrical aspect as well, an effective area in the active layer will be induced by the charge drift length(L), and only the photons absorbed in this effective area have contribution to the photocurrent. Then IOSCs obtain better performance for thin active layer, while OSCs achieve larger N and EQE for the relatively thick active layer. Meanwhile, it is found that the optical spacer layer can notably enhance the performance of OSCs with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers. Therefore, optical absorption and charge transport play important roles in device fabrication and optimization. And combining the better optical absorption and suitable interfacial modification, IOSCs are promising to achieve higher efficiency, which has been confirmed by the performance of referenced OSCs and IOSCs.In chapter 4, ITO-free IOSCs with aluminum doped zinc oxide(AZO) cathode and ultrathin calcium(Ca) interfacial modification layer are first realized in this work, and the fabricated IOSCs have a structure of Glass/AZO/Ca/P3HT:PCBM/Mo O3/Ag. Without the modification layer, AZO only IOSCs show a poor PCE of 1.34% and light soaking issue, which indicates that an interfacial layer is expected to modify the AZO electron selectivity. Then, a 5 nm Ca is used to lower down the AZO work-function and align the energy levels between AZO and P3HT:PCBM, which acts as an electron transport and hole blocking layer. The resulted AZO/Ca IOSC shows an increased PCE from 1.74% to 2.69% after 15 min illumination. It is thought that the increased photoconductivity of AZO/Ca(5 nm) film upon illumination and the enhanced electron transport across the AZO/Ca interface may be responsible for the light soaking issue. When an ultrathin Ca modifying layer of 1 nm is employed, a further improved PCE of 3.17% is obtained, and remarkably, no light soaking issue is observed in this case. And this could be understood by the highly efficient electron transport across the AZO/Ca/P3HT:PCBM interfaces. Furthermore, the AZO/Ca(1 nm) IOSC obtains superior performance among AZO based devices, which also shows a comparable PCE to the referenced ITO/Ca IOSC and presents a better air-stability, as well as lower cost.In chapter 5, ITO-free IOSCs with AZO cathode and low-temperature Zn O interfacial layer processed from aqueous solution are fabricated with a structure of Glass/AZO/Zn O/P3HT:PCBM/Mo O3/Ag. Here, AZO/Zn O IOSC with Zn O annealed at 150 °C shows the superior PCE of 3.01%, if decreasing the Zn O annealing temperature to 100 °C, the obtained IOSC remains a PCE of 2.76%, and no light soaking issue is observed. The results show that the low cost Zn O could efficiently modify the AZO cathode and align the energy levels at AZO/P3HT:PCBM interface. What’s more, this Zn O film also acts as a transmittance enhance layer which slightly improves the optical transmittance of AZO substrates. Further, AZO/Zn O IOSCs show comparable PCE and air stability to devices based on AZO/Ca cathode, and overcome the light soaking issue in AZO/Ca IOSCs.Furthermore, the long-term stability of IOSCs with ITO/Zn O cathode is also investigated during 4320 h storage in air-N2-air atmosphere. The statistical results show that all devices with Zn O annealing temperature ranging from 50 to 150 °C obtain an average PCE value over 3% and relatively good stability. All devices could remain approximately 80% of their stable PCE values. In detail, the air storage(480 h) is firstly used to study the stability of un-encapsulated devices. For IOSCs with Zn O annealed at 80, 100, and 150 °C, the device PCE could hold 90% of their initial values, and shows the similar tendency to that of short circuit current density with relatively steady open circuit voltage and fill factor. The main mechanism is the degradation of active layer and electrodes caused by the penetration of oxygen/moisture from air into devices. For IOSCs with Zn O annealed at 50 and 70 °C, their PCE climbs up and then declines in this period. The following N2-air storage is chosen to simulate the degradation process of encapsulated devices in air for a very long time. All devices show stable PCE in N2 storage and similar performance degradation to that in first air storage. Interestingly, at the beginning of second air storage, an improvement of photovoltaic parameters for most devices is observed, which may be related to the change of oxygen concentration in devices and oxygen desorption in light soaking. In short, IOSCs with low temperature Zn O interfacial modifying layer processed from aqueous solution obtain superior photovoltaic performance and good long-term stability, thus the simple Zn O deposition method is a promising technique in fabricating IOSCs and flexible devices with high PCE and long lifetime.