本論文主要是針對反置型有機太陽能電池中的電洞傳輸層進行奈米結構翻印，可藉由奈米結構來提升對光捕捉的能力並增加電洞傳輸層與銀電極之間的接觸面積，進而提升短路電流密度(Jsc)和填充因子(FF)，使整體的光電轉換效率(PCE)提升。而本論文採用兩種不同奈米結構之PDMS做翻印，一為光柵結構、另一為洞狀結構，翻印後為光柵與柱狀結構。將兩者分析比較可得在翻印後開路電壓仍保持不變於0.56V，而短路電流密度與填充因子皆有提升，光柵結構其短路電流密度從8.45mA/cm2上升到8.58mA/cm2，填充因子從0.551增加到0.608，光電轉換效率由2.607%提升到2.921%。柱狀結構則使短路電流密度上升至8.63mA/cm2，填充因子增加到0.629，因此效率提升至3.039%。雖然兩者的短路電流密度增加的幅度沒有太大的變化，但在填充因子的方面有明顯的改善，而達到提升光電轉換效率的目的。

SUMMARY

The purpose of this study is to reprint nanostructures for inverted organic solar cells on the hole transport layer. It can improve the ability of light trapping, and also increase the contact area between the hole transport layer and the silver electrodes. And then enhance the short-circuit current density (Jsc) and fill factor (FF), so that the overall photoelectric conversion efficiency (PCE) is increased. This thesis uses two kinds of PDMS with nanostructures to reprint. One is gratings structure, the other is nanorods structure. The result of experiment shows that both of structures of Voc maintain 0.56V, the short-circuit current density and fill factor increase. In gratings structure, the short-circuit current density enhances from 8.45mA/cm2 up to 8.58mA/cm2, the fill factor enhances from 0.551 up to 0.608, the value of photoelectric conversion efficiency rises from 2.607% to 2.921%. In nanorods structure, the short-circuit current density up to 8.63mA/cm2, the fill factor up to 0.629, and then the value of photoelectric conversion efficiency rises to 3.039%. Although both the short-circuit current density of increase has not changed much, but there are significant improvements in fill factor to enhance the photoelectric conversion efficiency purposes.

Keywords : Organic Solar Cells ; Soft Lithography ; Nanoimprint ; Grating ; Nanorod

INTRODUCTION

Organic Solar Cells (OSCs) have received a significant attention, because they are potentials of renewable energy sources. OSCs can be fabricated onto large area and light weight substrates by solution process, their cost is relatively low and fabrication is easy to do. In addition, the inverted type of OSCs have also attracted significant attention, because their devices are air stable and the back electrodes use high work function metal electrodes. The inverted type devices insert a buffer layer between the active layer and the back electrodes to reduce oxygen and moisture diffusion into the polymer. Recently, a lot of researches have reported various method how to improve power conversion efficiency (PCE) of OSCs, one is to enhance light absorption in the 300-800 nm wavelength range, where typical photoactive materials have a weak absorption region, by developing organic materials with low bandgap energy or using nanostructures to improve light trapping. Another is to control the surface morphology and interaction between donor and acceptor in diffident layer. This study uses soft lithograpy to reprint nanostructures for inverted organic solar cells on the hole transport layer.

MATERIALS AND METHODS

The layer of OSCs with nanostructures was in the geometry of ITO/ZnO/P3HT:PCBM/ PEDOT:PSS/Ag. The ITO substrate was treated with the ultraviolet ozone for 15 minutes in the beginning. The sol-gel of the ZnO was spin coated on at 3000 rpm and baked at 350°C for 10 min. The blend solution of P3HT:PCBM was spin coated on the ZnO layer at 500 rpm in the nitrogen-filled glove box, remaining in the N2 environment for 1 hour for crystallization, and baked at 110°C for 10 min. The PEDOT:PSS was spin coated on the blend layer at 3000rpm and baked at 180°C for 10 min. The gratings are made by first peeling off the upper plate of a DVD disc. The size of grating has a depth of 140nm and 30nm, line width of 400nm and gap of 350 nm. Then the DVD disc is cleaned in alcohol to remove recorded signal, and washed in isopropanol and DI-water with sonication for 20 min. The nanorods are made from the silicon substrate with nanoholes, which made by E-beam process, by transfer onto the PUA mold. The nanorods have six sizes of diameters, spreads of 300,400, 500, 600, 700 and 800nm. The PDMS, which has very low surface enegy, can avoid serious problem of adhesion during reprint, is used as the soft mold of nanostructures with gratings or nanorods. The PDMS is prepared with primary agent versus hardener at a ratio of 10:1 and stirred in a plastic cup for a few minutes. After removing bubbles generated in the PDMS with a vacuum pump, it is poured onto the grating or the nanorods to form a PDMS with the grating or the nanoholes. Before the PDMS imprint the PEDOT:PSS, the PEDOOT:PSS must pre-back 60˚C for 20s in order to have better adhesion on the blend. Then the nanostructures on the PDMS mold is stamped onto the PEDOT:PSS and heat the device 110˚C for 5mins to be solidified at the same time. Peeling the PDMS can get the nanostructure of gratings or arrays of nanorods on the PEDOT:PSS, as shown in the SEM images of Fig. 1, and baking at 180°C for 10 min. The Ag electrode were thermally deposited on the substrate through corresponding masks to the thickness of 100nm at 5x10-5 Pa. Finally the substrate is post-anneal at 150°C for 30 min.

RESULTS AND DISCUSSION

The efficiency of PEDOT:PSS with the period of grating structures or the array of nanorods device is better than flat structures, because both of the short-circuit current density (Jsc) and fill factor (FF) are also improved. In part of the deep grating, the short-circuit current density enhances from 8.45mA/cm2 to 8.58 mA/cm2, and its fill factor rises from 0.551 to 0.608. So the efficiency can up to 2.921%, as shown in Fig. 2. Among the array of nanorods, the best efficiency is 500nm in diameter, which gets to 3.039%. Because the short-circuit current density and fill factor can up 8.63 mA/cm2 and 0.629, as shown in Fig. 3. Although the Jsc is increased for both of nanostructures, its performance is not well. Probably when the light enters the active layer being absorbed by first time, and then the remaining part of the light is reflected on the nanostructures. It is relatively weak when the light is scattered. The active layer absorbing this light is limited, so the short-circuit current density has increased slightly. The main contribution to the efficiency is the fill factor significant improvements. The nanostructures change morphology to enhance contact area between PEDOT:PSS and back electrode so that they can improve carries transportation.

CONCLUSION

We have successfully demonstrated that inverted OSCs with nanostructures on the hole transport layer. This study reprint two kinds of nanostructures on the PEDOT:PSS by capillary force. One is the period of grating structures, the other is the array of nanorods. We have developed a method for the fabrication of a nanoreprinted on PEDOT:PSS by using a soft mold which is made of PDMS material that can avoid serious problem of adhesion during reprint. The nanostructures can improve both light trapping and charge collection. Experimental results indicate that the deep grating and 500nm of diameter nanorods perform the best efficiency, because their short-circuit current density and fill factor are improved.

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