在有機太陽能電池的研究中，常使用氧化鋅作為陰極緩衝層材料，以增進電子的傳輸，且具有阻擋電洞的效果，進而提升元件效率。本研究將鋰元素添加至氧化鋅中，分別用奈米顆粒合成法及溶液法製備出鋰鋅氧化物奈米顆粒(LZO NPs)及鋰鋅氧化物薄膜(LZO film)，並討論不同鋅鋰莫耳比(Zn:Li = 10:0, 10:0.5, 10:2)所製成的奈米顆粒或薄膜，作為陰極緩衝層對P3HT:ICBA有機太陽能電池之特性影響。本實驗之太陽能電池採用反轉式結構，藉以避免銦錫氧化物ITO基板受陽極緩衝層PEDOT:PSS腐蝕影響，其結構由下至上分別為：ITO/ LZO NPs or LZO film/ P3HT:ICBA/ PEDOT:PSS/ Au/ Ag，元件的工作面積為0.16 cm2。
研究結果顯示，以溶液法製備鋰鋅氧化物薄膜作為陰極緩衝層，在添加少量鋰的情況，可使光電轉換效率由2.20%(ZnO film device)提升至2.54%(5%-LZO film device)，此時開路電壓為0.84 V，短路電流為6.83 mA/cm2，填充因子為0.44。但若增加鋰添加量，則光電轉換效率下降至1.84%(20%-LZO film device)，此時開路電壓為0.78V，短路電流為6.29 mA/cm2，填充因子為0.38。而採用奈米顆粒合成法製備鋰鋅氧化物奈米顆粒，少量添加鋰可使光電轉換效率更加提升，由2.36%(ZnO NPs device)提升至3.15%(5%-LZO NPs device)，此時開路電壓為0.86 V，短路電流為7.54 mA/cm2，填充因子為0.49。但若增加鋰添加量，則光電轉換效率下降至2.05%(20%-LZO NPs device)，此時開路電壓為0.82 V，短路電流為6.18 mA/cm2，填充因子為0.40。
在材料分析方面，二次離子質譜儀可偵測出鋰元素確已添入氧化鋅奈米顆粒中。穿透式電子顯微鏡分析得知添加鋰元素於奈米顆粒後沒有產生其他結晶相，仍由氧化鋅結晶相主導，以溶液法製備的鋰鋅氧化物薄膜（經200°C退火）則呈現非晶相。由紫外光-可見光光學分析的結果得知，添加鋰元素不會使能隙寬度明顯改變。電阻率的量測結果顯示，相較於溶液法製備的薄膜，奈米顆粒有較好的導電性，且導電性隨鋰元素添加量增加而提升。表面形貌以掃瞄式電子顯微鏡及原子力顯微鏡進行分析，發現添加少量鋰元素表面較平坦，添加多量是表面粗糙度變大，界面平整性會直接影響載子於界面傳輸的阻力。
X光電子能譜可分析陰極緩衝層之表層與內層氧空缺的含量，佐證鋰原子進入氧化鋅結構中的角色，進而了解鋰鋅氧化物對於太陽能池之電子傳輸及再結合的影響。紫外光電子能譜可得能帶相關資訊，費米能階抬升0.6eV，佐證氧空缺缺陷能階的存在。阻抗分析求得之載子再結合壽命可對應到元件特性，5%-LZO NPs device有最高的再結合壽命2176 μs，較長的再結合壽命代表載子有較高的機會被傳遞至元件外部，以達到較高的光電轉換效率。
在奈米顆粒與薄膜兩種系統中，添加少量鋰元素都具有提升元件性質的效果，故導入適量的氧空缺可以幫助載子傳輸，降低再結合的發生。然而當添加量增至20%時，內層及表層氧空缺的增加不匹配，再結合的可能性增加，載子再結合壽命變短，導致元件光電轉換效率不增反降。

In the research of organic solar cell, zinc oxide is commonly used as cathode buffer layer awing to the ability of electron-transport and hole-blocking. In this work, lithium incorporated zinc oxide nanoparticles (LZO NPs) by nanoparticles synthesizing route and lithium incorporated zinc oxide film (LZO film) by solution process with different zinc lithium molar ratio (Zn:Li = 10:0, 10:0.5, 10:2) are successfully synthesized to form a cathode buffer layer in the inverted solar cell structure of ITO/ LZO NPs or LZO film/ P3HT:ICBA/ PEDOT:PSS/ Au/ Ag with 0.16 cm2 working area.
As compared with the ZnO film buffer layer, a substantial increase in power conversion efficiency from 2.20% to 2.54% is achieved by applying 5%-LZO film buffer layer, with Voc of 0.84 V, Jsc of 6.83 mA/cm2, and FF of 0.44. However, further addition of lithium precursor to 20% will reduce power conversion efficiency to 1.84%, with Voc of 0.78 V, Jsc of 6.29 mA/cm2, and FF of 0.38. As compared with the ZnO NPs buffer layer, power conversion efficiency can further enhanced from 2.36% to 3.15% by applying 5%-LZO NPs buffer layer, with Voc of 0.86 V, Jsc of 7.54 mA/cm2, and FF of 0.49. More incorporation of 20% lithium in buffer layer leads to lower power conversion efficiency of 2.05%, with Voc of 0.82 V, Jsc of 6.18 mA/cm2, and FF of 0.40.
Several analysis techniques are applied to understand the lithium incorporating effect. At first, the existence of lithium in zinc oxide nanoparticles is proved by secondary ion mass spectroscopy (SIMS). Transmission electron microscopy (TEM) analysis confirms that no other crystal phase formed in nanoparticles after lithium incorporation, zinc oxide crystal phase is dominant. The structure of lithium incorporated zinc oxide film after 200°C annealing is still amorphous. From the result of UV/visible spectrophotometer (UV-vis), no obvious change of energy bandgap is introduced by lithium doping. The resistivity of nanoparticles can be measured by four-point probe station, the conductivity increase as doping concentration increase.
The surface morphology of cathode buffer layer is analyzed by scanning electron microscope (SEM) and atomic force microscope (AFM), incorporation of 5% lithium leads to smoother surface, but roughness is obviously raised with incorporation of 20% lithium. The uniformity at interface will directly influence carrier transport resistance. X-ray photoelectron spectroscopy (XPS) analysis shows the distribution of oxygen vacancy at the surface and in the bulk, which results from incorporation of lithium in zinc oxide structure. Ultraviolet photoelectron spectroscopy (UPS) informs the upward shift of Fermi level with 0.6 eV, which proves the existence of suitable energy level. Impedance measurement demonstrates that the recombination lifetime is substantially extended, which indicates a higher probability for carrier extraction and leads to its advanced performance. The 5%-LZO NPs device shows the highest recombination lifetime of 2176 μs.
In both nanoparticles and film system, lithium incorporation of 5% can improve device performance. The suitable energy level of oxygen vacancy facilitates electron transport, and the easier electron extraction to the cathode implies lower probability to recombine with holes. However, when lithium precursor concentration increases to 20%, the non-comparable increase of oxygen vacancy at the surface may lead to more carrier recombination and poor device performance.

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