Research On Light Absorption Enhancement In Polymer Solar Cell

Polymer solar cells have received considerable attention as an attractive alternative to silicon-based photovoltaic technology due to their unique advantages of low cost, light weight, and potential application in flexible large-area devices. Optical absorption of the active layers is very important for the high efficiency achievement in polymer solar cells. As a result, how to enhance the optical absorption of the device is the topic in this dissertation. Based on the system of P3HT:PCBM heterojunction, light absorption enhancement of the devices are achieved by optimizing the thicknesses of active layer according to optical electromagnetic field and opto-electronic model, by inserting metal nano-particle layer or optical spacer layer, or by appling the stacked structure or grating structure in polymer solar cells. This dissertation is divided into serval parts, which including: the process of the device fabrication, the determination of optical constants and thickness for polymer films, and the electromagnetic field distrbution and optical or opto-electronic device performances for the special devices by appling the methods of FDTD and RCWA.The photocurrent of polymer solar cell is studied based on the electromagnetic theory and opto-electronic model. The electromagnetic field distribution, reflectivity and transimissivity of the polymer solar cell are derived from the optical transfer matrix. The process of photons conversion into photocurrent is qualitatively described by the model of Onsager-Braun, Drift-Diffusion equations and Space Charge Limited Current (SCLC). From these models, the dissociation probability of exciton, net charge production and the applied voltage dependence of photocurrent can be achieved.The optical constants and thicknesses of the polymer films are obtained by fitting the reflectance and transmitance curves based on Forouhi-Bloomer model. It can be found that the additional annealing process increases the surface roughness of P3HT:PCBM film, which leads to the rise of refraction index and extinction coefficient in visible spectrum.The thickness of active layer in polymer solar cell is optimized based on optical transfer matrix and the function of dissociation probability of exciton. The results from the theories and emperiments demonstrate that the optimal thickness of active layer is about 100 nm for the device structure of ITO(100nm)/PEDOT:PSS(40nm)/P3HT:PCBM /LiF(1nm)/Al(120nm), as both light absorption and charge dissociation are concerned. Inserting TiO2 into polymer solar cell as an optical spacer layer can decrease the optimal thickness of the active layer without reducing the light absorption of the device. To further widen light absorption spectrum for polymer solar cell, small organic molecules active layer AlClPc:C6o is paralelled with P3HT:PCBM layer in the device. Polymer solar cells with the structure:ITO(100nm)/AlClPc:C60(5nm)/NiO(15nm)/ITO(60nm) /PEDOT:PSS(40nm)/P3HT:PCBM(100nm)/LiF(1nm)/Al(120nm) are studied based on the calculation from FDTD, which demonstrates that light absorption of this structure has still about 20% in light spectrum of 780 nm-900 nm.The light absorption enhanced by inserting Ag clusters into polymer solar cell is also investigated in this paper. The calculation is performed for the electromagnetic field distribution in polymer solar cell with Ag clusters surrounded by different mediums based on FDTD. Ag clusters layer is fabricated through the method of thermal deposition and the annealing process. The IPCE curves are obtained for the devices with different surrounding mediums of Ag clusters. The experimental results demonstrate that IPCE of device with Ag clusters layer is higher than that of the reference device, and Ag clusters device with NiO medium has highest IPCE among the devices.In order to investigate the light absorption enhancement by introducing sub-micron scale gratings in polymer solar cell, micro-imprint is employed to fabricate the gratings device. With polydimethylsiloxane (PDMS) served as the stamp, channels with a minimal period of 1.6μm are micro-imprinted on the PEDOT:PSS layer as the alignment layer. The calculations are performed for electromagnetic field distribution inside polymer solar cell with gratings based on FDTD method, and the effect of gratings depth and fill factor on light absorption of the devices is analyzed based on RCWA method. It demonstrates that the gratings on the PEDOT:PSS layer can effectively improve the light absorption of P3HT:PCBM layer. While the grating period is 1 um and the depth is 10 nm, a smaller fill factor will lead to an apparent improvement of light absorption. The experimental results show that the gratings in polymer solar cell can improve the performance of the device.