此次研究主要分為兩大部份，第一部份為選用製備矽晶太陽能電池外部抗反射材料作為光學膜使用，針對三種不同的高分子材料來測試其光學性質與聚光抗反射能力，並且討論其材料的適用性與光學特性，其中最重要的是此高分子材料必須要有高透光性及材料的折射率匹配問題，並且同時須具備有耐熱、耐低溫、抗濕及耐候性，對太陽能電池元件也必須要有良好的接著性。最後我們選用polydimethylsiloxane(PDMS)、ethylene-vinyl acetate(EVA)、以及UV膠(UV glue)高分子封裝材料來製備光學膜。
第二部份是在太陽能電池表面的表面上製作微米結構去探討其結構特性，利用常見的LCD/LED面板或手機螢幕背光模組中的增亮膜(Brightness enhanced film ,BEF)，藉由軟微影技術來取得增亮膜上的規則稜鏡(prism lens)微米結構，將17um、23um和48um的增亮膜稜鏡結構，取得具有高透光性並由PDMS翻製不同尺寸與結構的透明PDMS母膜，利用PDMS母模為基礎與不同的轉印技術，進而轉印出各種具有不同光學膜材料的稜鏡結構。當光進入含有微米結構的薄膜，使光反射下降進而提升電流密度(Jsc)，經由三種不同材料而翻製成透明薄膜後當作聚光層，轉印於太陽能電池元件上。
另外在之後為了探討光學膜的實用性，我們使用市售太陽能電池元件來當作第二測試元件，並且預期市售元件添加我們自製的光學薄膜可以有效的提升電流密度(Jsc)，進而使電池元件效率提升。
最終由量測分析結果得知開路電壓(Voc)維持在0.53 V，填充係數(FF) 維持在0.70~0.71，主要短路電流密度(Jsc)的提升最多可由22.29 mA/cm2提升到24.8 mA/cm2，因此光電轉換效率亦從8.3%增加至8.8~9.3%；而市售元件最終量測分析結果得知開路電壓(Voc)維持在0.59 V，填充係數(FF) 維持在0.71~0.73，主要短路電流密度(Jsc)的提升最多可由36.4 mA/cm2增加到38.8mA/cm2，因此轉換效率亦從15.5%增加至16~16.6%。

SUMMARY

The study has two primary parts. The first part is using anti-reflection film as an optical material for three different polymer materials to test their anti-reflective optical properties and concentrating ability, and discuss the applicability of the optical characteristics of the material, the most important is that this polymer material must have a high light transmittance and a refractive index matching material, and the optical film is also required to weather resistance of solar cell device must also have good adhesion. Finally we chose PDMS, EVA, and UV glue polymer packaging materials to fabricate optical film.

The second part is to fabricate micron structures on the solar cell surface with using array prism micron structure, 17um, 23um, 48um prism structure of BEF which is by soft lithography techniques, and achieved a high light transmittance with PDMS of different sizes and replicating transparent PDMS mold structure, and using different imprint technology on the device, and then replicate prismatic structures with different optical film materials. When light enters the film containing the micro structure, thereby enhancing the current density (Jsc) as well as light reflection decrease, and by the transfer structure of three different materials as a transparent film is fabricated the light-collecting layer transferred on the solar cell device.

Keywords：silicon solar cell；Brightness enhanced film, BEF；optical film；soft lithography；light trapping

INTRODUCTION

The performance of silicon solar cell which has been improved significantly due to large advancement of semiconductor processing technology has encountered bottle-neck currently. It is difficult to improve cell efficiency by further advancement in semiconductor processing. One of the possible to raise the cell performance is to increase light trapping. The traditional methods of light trapping for silicon solar cells can be found in good text books or review articles. More advanced methods of light trapping includes the use of submicrometer, periodic features of relief formed by soft imprint lithography as light trapping structures, the use of ordered arrays of silicon nanowires and the use of a double layer photonic crystal. This paper uses a very cost-effective optical film imprinted with prism lens microstructure, which is placed on top of a crystalline silicon solar cell, to enhance light trapping leading to increase in the current density and efficiency performance. The optical film is made of low cost polymer material and is imprinted by a soft mold which is formed by array prism lens microstructure on brightness enhanced film, (BEF).

MATERIALS AND METHODS

The PDMS is used the mold of prism lens mirostructure and optical material. The PDMS is prepared with primary agent versus hardener at a ratio of 10:1 and stirred in a plastic cup for 10 mins. After removing bubbles generated in the PDMS with a vacuum pump, the PDMS is poured onto the BEF with prism lens microstructure. The PDMS mold imprinted with prism lens microstructures of BEF can be readily peeled off from the BEF, so the prism lens PDMS mold is formed.

For the case to imprint microstructures of prism lens shape on the optical film of PDMS material. The PDMS mold with microstructures of prism lens shape is cleaned in a UV ozone cleaning system for 15 mins to make the surface of the PDMS mold hydrophilic so this optical film of PDMS can be separated from a PDMS mold during the imprint. Therefore, liquid-like PDMS material is spin coated on the solar cell and vacuumed to remove bubbles and ensure that PDMS can seep into microstructures with prism lens. After heated and hardened at 90oC for 1 hour, the optical film with microstructures of prism lens shape can be peeled off. For the case to imprint microstructures of prism lens shape on the optical film of EVA material. The EVA film placed on the solar cell device, then the PDMS mold placed on the EVA and three stacked in sequence, before laminating. And then start to the vacuum, heated to 150 oC and pressurized to about 80kpa, and the process time is about 10 minutes. After the device was cooled to ambient temperature, then PDMS mold can be peeled off. Finally, we can get EVA optical film of the prism lens on the solar cell. For the case to imprint microstructures of prism lens shape on the optical film of UV glue material. Spin coating the optical film material of UV glue on the solar cell with 1000rpm 30s, and using the PDMS mold to imprint the microstructure of prism lens, and vacuumed to remove bubbles and ensure that UV glue can seep into microstructures of prism lens. After exposing UV light with 4 minutes, the optical film with microstructures of prism lens shape can be peeled off. Finally, we can get UV glue optical film of the prism lens on the solar cell.

RESULTS AND DISCUSSION

To fabricate a soft mold for imprinting the PDMS layer with microstructure, three different pitch sizes of prism lens. In addition, light absorption increases with increasing the pitch sizes of the prism lens microstructure from 17 to 48 μm on the optical film. With the microstructures of 48 μm in size on the optical film, light trapping can increase by 10% over the entire spectrum from 250 to 1100 nm. This can increase the EQE uniformly over the entire spectrum from 400 to 1100 nm. Both the EQE and the short circuit current increase with increasing the pitch size of the prism lens microstructures on the optical film which leads to increase in the cell performance.

Finally, the results of the lab device show the measurement of the mainly short-circuit current density (Jsc) increased from 22.29 to 24.8 mA/cm2, therefore the photoelectric conversion efficiency was increased from 8.3 to 9.3%. The commercial device measurement and analysis was measured the mainly Jsc increased from 36.4 to 38.8 mA/cm2, therefore the photoelectric conversion efficiency is increased from 15.5 to 16.6 %.

CONCLUSION

A thin layer, low cost polymer material of optical film can be used as a good light trapping material on top of a solar cell because of its highly transparency. The techniques of imprint microstructures on the optical film are developed using PDMS soft mold, and the PDMS soft mold is made by BEF to form a array prism lens microstructures. Experimental results indicate that light trapping increases with increasing the pitch sizes of the microstructures imprinted on the optical film. All of quantum efficiency, current density and cell performance of the solar cells increase with increasing the pitch sizes of the microstructures imprinted on the optical film. It appears that larger pitch sizes of the microstructures on the optical film are more suitable for light trapping. The optical film developed is flexible and can be widely applied on any kinds of solar cells for improvements of light trapping and solar cell performance.

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