本論文利用低價製程製備氧化鋅奈米柱/氧化亞銅異質接面太陽能電池，並探討改善短路電流之方法。在過去研究提到平面太陽能電池具有較高的開路電壓和較低的短路電流，而奈米結構則是較低的開路電壓和較高的短路電流。本研究中，平面太陽能電池導入奈米結構，增加接面面積使之兼具兩者優點，達成低成本製程得出高短路電流密度。
奈米結構可以增加接面接觸面積、縮短載子收集之傳輸距離、減少復合機率，因此可以增加短路電流，但由於氧化鋅/氧化亞銅接面品質不如磊晶氧化鋅/銅片高溫成長氧化亞銅之平面式電池，因此損失了開路電壓及填充因子。平面結構電池使用濺鍍氧化鋅及電鍍氧化亞銅薄膜，常常因薄膜品質不佳，其太陽能電池有著低開路電壓、低短路電流及轉換效率不佳之缺點。本研究採用平面式太陽能電池結構及低成本製程，針對低短路電流密度之缺點做有效的改善，達到大幅提升轉換效率之目的。
電鍍主動層氧化亞銅於銦錫氧化物(ITO)玻璃基板上，接著以熱水預法成長氧化鋅奈米柱於晶種層上。最後濺鍍ITO透明導電膜及完成上電極。以兩次電鍍法改進氧化亞銅於接面處薄膜品質，調整電鍍電位得出最佳晶相。並在成長氧化鋅奈米柱中，改變熱水浴法加熱時的試片放置方向，有大幅減少雜質沉澱，有效降低缺陷造成的漏電流，再探討溶液預加熱對奈米柱成長影響。經由太陽光模擬器與I-V量測系統測量，研究不同製程參數製作之元件其特性差異，最後探討不同參數對太陽能電池之轉換效率的影響，並且提供最有效提升效率的方式。
氧化鋅晶種層上成長氧化鋅奈米柱，晶種橫向(氧化亞銅表面之方向)成長高品質氧化鋅，低阻抗有助於降低串聯電組，可改善異質接面之品質進而提升開路電壓，太陽能電池之填充因子也因此得以增加，光激發之載子藉由奈米柱傳輸不須通過晶界，可免除載子於晶界間被復合，因此短路電流密度可以大幅增加。
總合上述之太陽能電池研究成果，得出最佳結果：使用兩次電鍍氧化亞銅，-0.6V, 50s →-0.57V, 50s，並在第二次電鍍時過濾雜質；以晶種層朝下方式成長氧化鋅奈米柱。其太陽光模擬量測結果分別為：Jsc= 6.61 mA/cm2、Voc= 0.28 V、F.F. = 35.3 %以及轉換效率0.62 %。

SUMMARY
The thesis describes using low-cost processing methods to improve short circuit current density (Jsc) of the ZnO nanorods/Cu2O heterojunction solar cells. Low-cost processed planar solar cells usually used sputtered ZnO film and electrochemical deposition (ECD) of Cu2O. Because of poor quality of the ZnO film, the solar cell results in low open circuit voltage, low short circuit density (Jsc) and poor efficiency. This study used ZnO nanorods, so that the transport paths of the photo-generated carriers can avoid possible recombination at grain boundaries, therefore, it can improve Jsc. Using nanorods will increase the total surface area that contact with oxygen in the air, the ZnO nanorods become photoconductors, which allow the carrier easier to pass, once oxygen adsorbed on the ZnO surface and under UV exposure. In addition, the ZnO nanorod is a quasi-single crystalline material, its low impedance can lower cell’s series resistance and improve fill factor as well.
Key words: solar cells, cuprous oxide, ZnO nanorods, lithium hydroxide, two steps electrodeposition, high short circuit current density

INTRODUCTION
Cuprous oxide (Cu2O) is a direct band-gap (Eg ~2.2 eV) semiconductor with a high absorption coefficient and a high theoretical conversion efficiency of ~23 %. It has been considered as a potential alternative material for photovoltaic applications because of its non-toxicity, indium free, abundance, and possibly low-cost manufacturing process, such as electrodeposition (ED). However, Cu2O film contains ionic defects, single-charged copper vacancies, and doubly charged oxygen interstitials, and is a native p-type semiconductor. Producing n-type Cu2O film is not feasible due to the self-compensation mechanism, the low solubility of doping impurities, or both. Therefore, much effort has been focused on searching suitable n-type semiconductors for heterojunction Cu2O solar cells. ZnO/Cu2O heterojunction solar cells have recently attracted significant interest. Related studies are summarized in table below. One can notice that the short-circuit current density (Jsc) and the power conversion efficiency (η) of these devices vary widely, depending on how the Cu2O films were synthesized (formed via the thermal oxidation of Cu foil or by a low-cost non-vacuum ED process). It indicates that solar cells produced using TO Cu2O film achieved much better conversion efficiencies than those of solar cells produced using ED Cu2O film.

MATERIALS AND METHODS
We dip ITO-coated glass into 1:20 HCl Aqueous solution, fowllowed by ultrasonically cleaned in acetone, isopropanol, sodium hydroxide and de-ionized water for 20 min and N2 gas blow-drying. Conventional three-electrode electrochemical plating was used to deposit Cu2O film. Two solutions were made, first one consist of 0.4 M copper sulfate, 2.7 M lactic acid, and 4 M sodium hydroxide with a volume ratio of 1:1:1, then the pH value of the as-prepared solution adjust to 12.5 with 6 M sodium hydroxide. Second one is 0.4 M copper sulfate, 2.7 M lactic acid, and 5 M lithium hydroxide without pH value adjusting. The plating solution was maintained at 65oC using a water bath. The ITO glass serves as the working electrode, platinum as the counter electrode and a saturated silver/silver chloride (Ag/AgCl) electrode as the reference electrode. The first step ECD was performed at a constant potential of -0.6 V with NaOH electrolyte. The second step ECD is -0.63 V, -0.6 V, -0.57 V with filtered LiOH electrolyte.
A 15 nm ZnO seed layer was deposited on the ECD-Cu2O-coated ITO glass via magnetron sputtering. The sputtering was performed with 40-W radio-frequency power (power density of 1.97 W/cm2) at room temperature using a two-inch ZnO target. The purity of the ZnO target was above 99.99%. The base pressure of the sputtering chamber was evacuated to below 2 x 10-5 torr. The working pressure during sputtering was set at 1.2 x 10-2 torr with a 14 standard cubic centimeters per minute Argon flow.
CBD was used to grow ZnO NRs after seed layer deposition using a mixed aqueous solution containing 0.06 M zinc acetate and 0.06 M hexamethylenetetramine. The length of the ZnO NRs is about 1100 nm for all samples. During CBD deposition, the solution was maintained at 88oC using a water bath. We investigate seed layer face-up and face-down while CBD because of particles precipitating. After CBD ZnO NR growth and DI water cleaning, a 110 nm-thick transparent electrode ITO film, which also served as a current-collecting layer, was deposited on top of the ZnO NRs followed by top-contact Ag film (~300 nm) deposition using direct-current sputtering. All samples had a cell area of 0.13 cm2.

RESULTS AND DISCUSSION
This study used electrodepositing of Cu2O film on indium tin oxide (ITO) glass substrate. Conventional one step and two-step ECD Cu2O film with different electrolyte solutions and ECD potentials were investigated. It can improve absorption and crystal quality of the Cu2O film. Electrolyte filtering can prevent defects precipitate on surface, which could reduce electron-hole recombines at the interface. In addition, effects of sample face-up or face-down during plating, preheat solutions during CBD or not, and filtering the plating solution were studied. Face-down CBD can reduce particles on the ZnO layer and restrains current leakage caused by the defects therefore increases Jsc. Solar simulator and I-V measurements were used to measure the conversion efficiency and parameters of the devices. Effects of different processes on the Jsc of the solar cells were compared. ZnO NRs are quasi-crystalline without grain boundary, it can help to collect photo-generated carrier effectively. Using NRs can increase surface area to adsorb oxygen and turn into photoconductors under UV exposure, which allow the carrier easier to pass.

Figure 3 Use two step ECD(-0.6 V, 50 s →-0.57 V, 50 s) and filter the electrolyte after 2nd step, followed by face-down CBD ZnO nanorods. The best solar cell parameters are: Jsc 6.61 mA/cm2, Voc 0.28 V, fill factor 35.3 %, and power conversion efficiency of 0.62 %.

CONCLUSION
The techniques to fabricate solar cell with better Jsc using the low-cost processes are: two-step ECD (-0.6 V, 50 s →-0.57 V, 50 s), filtering the 2nd step ECD solution, and sample face-down during CBD deposition of the ZnO nanorods. The best solar cell parameters are: Jsc 6.61 mA/cm2, Voc 0.28 V, fill factor 35.3 %, and power conversion efficiency of 0.62 %.

*Author
** Advisor

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