Abstract
With the dwindling fossil energy and increased environmental concern, solar cells are considered as viable renewable green energy sources. The conventional silicon solar cells are expensive primarily due to energy intensive and expensive manufacturing technology. Polymer solar cells (PSCs) are promising solar cell technologies to deliver cheap solar energy as they can be processed by high throughput and cheap roll-to-roll technology. However, currently PSCs use ITO (indium tin oxide) as the transparent electrode which is still expensive, brittle and with other technical drawbacks. Conductive polymer poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonate) (PSS) is quite promising for next-generation transparent electrode as it can be solution processed and is highly flexible. However, pristine PEDOT:PSS has very low conductivity so that it cannot be used as standalone electrode. This work aims at developing novel methods to significantly enhance the conductivity, investigate the mechanism of conductivity enhancement through various techniques and assess the PSC device performance. Three novel methods using cheap and nontoxic chemical are developed and conductivity is enhanced by four orders of magnitude and these films are used as standalone anodes for PSCs. Finally, it is demonstrated that the highly conductive PEDOT:PSS has high thermoelectric performance.
The first method was treatment of PEDOT:PSS using different concentration and molecular weight polyethylene glycol (PEG) where it also helped to investigate the effect of molecular weight of additives on the conductivity of PEDOT:PSS. The conductivity enhancement depends on both the molecular weight and concentration of PEG used. Conductivity of PEDOT:PSS was enhanced from 0.3 S cm-1 to 805 S cm-1 with 2% PEG but to only 640 S cm-1 with 6% ethylene glycol (EG). PEG and EG with high dielectric constants screen the charge between PEDOT and PSS followed by phase separation and reorientation of PEDOT chains leading to bigger and better connected particles. ITO-free PSC devices fabricated using PEDOT:PSS treated with PEG anodes showed better performance than those treated with EG and comparable performance to that of ITO counterparts.
The second method is simple yet robust PEDOT:PSS film treatment with methanol. Film treatment was done either by dropping small amount of methanol on the PEDOT:PSS, immersing the film in methanol or a combination of both. The conductivity of PEDOT:PSS films was enhanced to 1362 S cm-1 after film treatment. Other alcohols were also tried but showed inferior performance. Removal of insulator PSS from the film, morphology changes through phase segregation leading to larger domain and better connected conductor PEDOT and conformation change from coiled to linear/extended-coil structure are the main mechanisms for the high conductivity enhancement. Methanol treated films were smooth, uniform and also showed high transmittance desirable for standalone electrode for PSCs. ITO-free PSCs with standalone PEDOT:PSS anodes using P3HT:PCBM as the active layer showed power conversion efficiency of 3.71 % while the ITO counterpart showed 3.77 %.
The third method was facile film treatment with formic acid. Film treatment was carried out by dropping small amount of formic acid on the annealed PEDOT:PSS film with an optional rinsing with DI water. Conductivity increased with increasing concentration of formic acid; the highest conductivity being 2050 S cm-1 using 26M concentration (>98 %). Treated films were of high quality with film uniformity and reproducibility. Up to 62% of PSS was removed. The mechanism of conductivity enhancement is similar to methanol treatment. ITO-free PSCs with standalone PEDOT:PSS anodes treated with formic acid using P3HT:PCBM as the active layer showed power conversion efficiency of 4.10% while the ITO counterpart showed 4.11%.
In addition to transparent electrodes, highly conductive PEDOT:PSS has other important application areas. Highly conductive PEDOT:PSS treated by the above three methods were used for thermoelectrics: materials which change heat to electricity or vice versa for power generation or cooling/heating applications. Both thin film and free-standing flexible films were investigated. The Seebeck coefficient did not show big variation with the tremendous conductivity enhancement being 21.4 and 20.6 µV K-1 for EG and formic acid treated papers, respectively. A maximum power factor of 80.6 µW m-1K-2 was shown for formic acid treated samples. Coupled with intrinsically low thermal conductivity of PEDOT:PSS, a ZT~0.32 was calculated at room temperature using Harman method. The thermoelectric performance was greatly enhanced by enhancing the electrical conductivity of the papers.

摘要
有限的石化能源與環境的議題考量下，太陽能電池被視為是可行的綠色再生能源。目前被廣泛使用的矽太陽能電池，由於製造時消耗大量的能源與製造設備昂貴，所以其發電成本居高不下。高分子太陽能電池可以使用高產出輪軸方式生產，將能提共便宜的太陽能再生能源。然而，高分子太陽能電池使用的銦錫氧化物作為透明電極，它仍然是昂貴的、脆性的、與其他技術上的缺點。因導電聚合物聚PEDOT:PSS可以溶液製作，並且具有極佳的可撓性，所以被視為下一世代透明電極。然而，原始的PEDOT：PSS導電度非常低，使得它不能單獨作為電極。這項工作的目的是開發新穎方法，來大幅改善其導電特性，並且探討不同技術來增強導電性的機制和評估高分子太陽能電池的表現。此論文中之三種新方法使用便宜的和無毒的化學藥劑可使PEDOT:PSS薄膜導電度增加四個數量級，且這高導電之PEDOT:PSS可單獨用作高分子太陽能電池的陽極。最後，高導電PEDOT：PSS也被證明有高的熱電性能。
第一種方法是使用不同的濃度和分子量的聚乙二醇來處理PEDOT:PSS，此項研究有助於觀察添加劑分子量對於PEDOT:PSS導電性的影響。導電性增強跟PEG分子量和添加濃度兩者有密切的關係。PEDOT：PSS的導電度從0.3 S/cm增加至805 S/cm，當填加2％的聚乙二醇，但當填加6%乙二醇時導電度僅能增640 S cm-1。 PEG和乙二醇具有高介電常數能將PEDOT和PSS之間的電荷吸引力隔絕，以致PEDOT鏈和PSS可重新定位形成像分離，導致較大較好的PEDOT顆粒。使用PEG添加後的PEDOT:PSS薄膜作為導電陽極的高分子太陽能電池表現出比乙二醇添加的PEDOT:PSS薄膜作為導電陽極的高分子太陽能有較好的轉換效率，且能有等同使用銦銻氧化物作為導電陽極的高分子太陽能有相同的轉換效率。
第二種增加PEDOT：PSS薄膜導電度的方法法是簡單而穩定的甲醇處理。 此方法是將PEDOT:PSS薄膜浸泡在甲醇或滴上甲醇或兩者的組合處理。 PEDOT:PSS薄膜經此方式處理後的導電性提高至1362 S/cm。也試著使用其它醇類，但表現出的導電度都沒有比甲醇來的好。處理後將PSS從薄膜去除、通過相分離產生較大的PEDOT粒子與PEDOT之間較好的連接、從糾纏捲曲改變成成較分散捲曲的形貌是使PEDOT:PSS導電度增加的原因。經甲醇處理過之PEDOT:PSS薄膜有平整與均勻的表面，並且有非常高的穿透度是非常適合單獨用於有機太陽能的電極。經實驗證明單獨用PEDOT:PSS作為高分子太陽能電池陽極的元件效率為3.71%，與使用銦銻氧化物透明電極的太陽能電池效率3.77%不相上下。
第三種方法是使用甲酸來處理PEDOT:PSS薄膜。處理方式是滴加少量的甲酸於PEDOT：PSS薄膜表面，並同時加熱薄膜，最後用去離子水清洗。電導度隨甲酸濃度的增加而增加，當甲酸濃度達26M (>98％），PEDOT:PSS薄膜導電度可達2050 S/cm¬。此方法製作出的導電薄膜品質優良且導電度的再現性與穩定度非常好。PEDOT:PSS薄膜中之PSS經處理後有62％被移除，所以其增加導電度機制類似於甲醇處理。只使用PEDOT:PSS作為有機太陽能電池的陽極元件效率可達4.10％與使用銦銻氧化物透明電極的太陽能電池效率4.11%幾乎沒有任何差別。
除了將PEDOT:PSS薄膜作為透明電極，導電性高的PEDOT：PSS也具有其它重要的應用領域。經上述三種方法處理後PEDOT：PSS薄膜也可用在熱電應用上。將熱轉電或電轉熱，可用於發電或製冷/加熱應用的材料就可稱為熱電材料。我們對薄膜和厚膜進行了熱電的研究。 經乙二醇和甲酸處理後的PEDOT:PSS薄膜表的Seebeck係數分別為21.4和20.6μV/K，Seebeck係數並沒有隨著導電度大幅增加而有很大的變化。經甲酸處理過的樣品可表現出最佳的能量因子為80.6μWm-1K-2。伴隨著PEDOT:PSS的低導熱係數，用哈曼法在室溫下計算可得到ZT~0.32。當PEDOT:PSS導電度增強可大大提高了其熱導性。

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