以銀奈米線作為並聯式串疊型高分子太陽能電池之中間層

Abstract

並聯式串疊型太陽能電池，因只需以最優化之膜厚，將前後電池之光電流疊加，進而充分利用太陽光，而被認為能有效提升光能量轉換效率。另外並聯式串疊型太陽能電池之前後子電池的高分子材料，其材料能隙選取之範圍較串聯式串疊型太陽能電池寬廣。不過並聯式串疊型太陽能電池最重要的要求，是要有一層高穿透度及低片電阻的中間傳導層，而銀奈米線除了能滿足此要求，更有可溶液製程、可撓曲、無銦材料之優點，為此我們選擇以銀奈米線為並聯式串疊型太陽能電池之中間傳導層。 於此研究中，我們的反式前子電池以氧化鋅(ZnO)奈米粒子為電子傳導層，鋪於氧化銦錫薄膜電極上，主動層之材料以高能隙高分子聚三己基噻吩(P3HT)及新型富勒烯衍生物-茚-碳六十之雙加成物(ICBA)組成。我們於主動層之上，採用特殊的聚二氧乙基噻吩：聚苯乙烯磺酸複合物(PEDOT:PSS CPP; Clevios F CPP-105D)為電洞傳導層，此PEDOT:PSS CPP可直接附著並旋塗於主動層，並可輔助電荷載子，在銀奈米線空隙之間的橫向傳遞。但是因PEDOT:PSS CPP為親水性之關係，若旋塗分散在異丙醇(IPA)中的銀奈米線於PEDOT:PSS CPP薄膜之上，會摧毀PEDOT:PSS CPP薄膜，所以我們採用氣溶膠噴塗法噴塗銀奈米線，使銀奈米線溶液於PEDOT:PSS CPP薄膜上迅速揮發。再加上封裝後，可避免因銀奈米線之空隙，滲漏氧氣及水氣，所造成元件表現降低之情形。藉由以上的改善方法，我們成功實現以銀奈米線作為前子電池的半透明上電極。 然而我們的三電極架構，並聯式串疊型太陽能電池，其後子電池以銀奈米線為下電極，必然需處理銀奈米線粗糙度高的問題。為此我們噴塗稀釋後的PEDOT:PSS PH1000 (Clevios PH1000)以降低銀奈米線的粗糙度，之後再旋塗添加界面活性劑Triton X-100的PEDOT:PSS 4083 (Clevios P VP AI 4083)，以保護前子電池避免後子電池溶劑的侵蝕。以這些塗佈PEDOT:PSS之方法，應用於我們的三電極架構並聯式串疊型太陽能電池，我們可以得到高填充因子、高旁路電阻的後子電池。 藉由以上的技術，我們可以在三電極架構並聯式串疊型太陽能電池中，不論前後子電池都是P3HT或前子電池是P3HT、後子電池是PTB7，都可以達到高填充因子。但是因為後子電池之開路電壓較低，使得整體元件表現低於預期。為此，必須要開發更進一步的技術，以達成高效率三電極架構並聯式串疊型高分子太陽能電池。

Parallel tandem polymer solar cells have been proposed as a novel structure to improve power conversion efficiency of polymer solar cells owing to the advantage of simply adding two sub-cell’s current without the problem of current matching in series tandem cell. Furthermore, the band gap range choice of polymer materials for parallel tandem cells is wider than the one of series tandem cells. However, the primary requirement of parallel tandem cells is a high transparency and low sheet resistance intermediate layer. For this requirement, we choose silver nanowires as the intermediate layer parallel tandem cells because silver nanowires have several merits: solution process, flexibility, and indium-free material. In this research, our inverted front sub-cell consisted of ZnO as electron transportation layer at the top of ITO (Indium tin oxide) electrodes, and the bulk-heterojunction blend of poly(3-hexylthiophene) (P3HT) as wide band gap polymer with indene-C60 bisadduct (ICBA) as fullerene derivative to increase Voc. On top of active layer, we used a conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS CPP; Clevios CPP-105D) as the hole transportation layer for the reason that PEDOT:PSS CPP can directly be deposited by spin coating on top of active layer and facilitate lateral charge transport in the void spaces of silver nanowires. Nevertheless, spin coating silver nanowire solution can spoil the PEDOT:PSS CPP layer because of the hydrophilic property of both materials. Therefore, we chose aerosol spray coating for the deposition of silver nanowires to minimize the drying times of silver nanowire on top of the PEDOT:PSS CPP layer. Furthermore, encapsulation for silver nanowire as top electrode can prevent the device degradation by water and oxygen. With this improvement, we successfully implement the silver nanowires as semi-transparent top electrodes for front sub-cell. However, our three-terminal structure parallel tandem cells, which the back cell’s bottom electrodes are silver nanowires, have to tackle the problem of high surface roughness of silver nanowires. To suppress the roughness, we sprayed a layer of diluted highly conductive PEDOT:PSS PH1000 (Clevios PH1000) to modify the surface of the silver nanowire film and deposited a layer of PEDOT:PSS 4083 (Clevios P VP AI 4083) with an additive Triton X-100 by spin coating. Using these several deposition steps of PEDOT:PSS, we got a high fill factor back cell with low shunt resistance in our three terminal structure parallel tandem cell. From above techniques, we could fabricate two sub-cells with high FF values in our three-terminal structure parallel tandem cells, which both front and back cell are P3HT or front cells are P3HT but back cells are PTB7, but the overall performance was not as good as we had expected due to low open circuit voltage of back cells. For this reason, further improvements have to be developed for achieving high efficiency three terminal parallel tandem cells.