以有序性介孔二氧化鈦電極與氧化石墨烯混合材料為基礎之n型和p型鈣鈦礦太陽能電池

Abstract

本論文目的為利用有序性介孔二氧化鈦與氧化石墨烯材料為基礎之鈣鈦礦太陽能電池設計。對n型鈣鈦礦太陽能電池來說，介孔二氧化鈦膜厚與孔隙率對於改善太陽能電池效能是很關鍵的條件。我們藉由價格低廉的兩性接枝共聚物(PVC-g-POEM)來當作高分子模版製備出有序性介孔二氧化鈦薄膜。這個簡單且有效率的合成方法可以製備出具有高度介孔性與有序性的二氧化鈦奈米結構，其特色為擁有互聯性以及可調控的孔洞。其平均孔洞大小將隨著在接枝共聚物中的疏水主鏈(PVC)的比例上升而變大。鈣鈦礦層則是利用連續兩步法沉積在二氧化鈦薄膜上:將已沉積碘化鉛的二氧化鈦薄膜以異丙醇潤濕後，浸泡至含有甲基碘的異丙醇溶液當中。此異丙醇潤濕的前處理不僅提升鈣鈦礦的轉化率，並增加了鈣鈦礦的晶體尺寸。隨著調控二氧化鈦膜厚以及孔隙率，鈣鈦礦太陽能電池在70奈米的孔洞以及300奈米的厚度下達到最佳的元件效率11.9 %，同時我們利用電荷萃取法與量測其瞬態光電壓衰減來理解電荷再結合之動力學與相關太陽能元件的關聯性。 在鈣鈦礦太陽能電池中，材料的本質特性是非常重要的一環，我們以元件架構ITO/GO/PSK:GO/PCBM/Ag為基礎，將不同比例的氧化石墨烯(0.025, 0.05 and 0.075 mgmL-1)混入鈣鈦礦層中，並以氧化石墨烯為p型電極材料製備出鈣鈦礦太陽能電池。在氧化石墨烯混摻比例為0.05 mgmL-1時，我們得到最佳化的元件效率為15.9 %，其優於傳統平板鈣鈦礦太陽能電池的12.3 %。在均勻的鈣鈦礦混合氧化石墨烯主動層中，鈣鈦礦扮演電洞的供體，而氧化石墨烯則扮演受體。在最佳化的混摻條件下，藉由量測霍爾效應可以得知改善了鈣鈦礦的電荷傳遞，以及利用螢光衰減光譜來證明增進了材料介面的電荷分離，另外從電化學阻抗圖譜可以了解其減緩了介面電荷再結合，與相關的太陽能元件表現趨勢相符。我們的研究結果表明鈣鈦礦混合氧化石墨烯可以增加電洞供體與受體的接觸，進而平衡電荷的遷徙率並提升鈣鈦礦太陽能元件效率。

The objective of this thesis is to create the novel design of perovskite solar cells by utilizing organized mesoporous TiO2 (om-TiO2) and graphene oxide. For the regular type of perovskite solar cells, the control of the thickness and porosity of a mesoporous TiO2 layer is important to improve the photovoltaic performance of perovskite solar cells. We produced organized mesoporous TiO2 (om-TiO2) layers using a low-cost amphiphilic graft copolymer, poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM), as a sacrificial template. This simple but effective synthetic approach generates highly mesoporous and well-organized TiO2 nanostructures with interconnected and size-tunable features. Specifically, the average pore size increased with the amount of hydrophobic PVC main chain in the graft copolymer, which acted as the pore forming agent. Perovskite layers were prepared on top of an om-TiO2 layer according to a two-step sequential deposition: after coating the PbI2 solution in dimethyl formamide (DMF) on an om-TiO2 substrate, the substrate was pre-wetted in isopropyl alcohol (IPA) solvent before immersing into a CH3NH3I/IPA solution. This pre-wetting treatment not only improves the yields of conversion from PbI2 to CH3NH3PbI3 but also increases the size of perovskite crystals with cuboid morphology. On varying the pore size and the film thickness of the om-TiO2 layer, the device performance attained 11.9 % of power conversion efficiency (PCE) at pore size 70 nm and film thickness 300 nm. We measured extracted charge densities and decays of transient photovoltage to understand the kinetics of charge recombination in relation to the corresponding device performance. The intrinsic property of perovskites is an important issue for perovskite solar cells, we attempt to introduce the heterojunction contact engineering by mixing graphene oxide (GO) with CH3NH3PbI3 (PSK) in varied proportions (0.025, 0.05 and 0.075 mg mL-1) and using GO nanosheets as a p-type electrode for devices with a layer-by-layer thin-film configuration ITO/GO/PSK:GO/PCBM/Ag. The efficiency of power conversion (PCE) of the device prepared with GO in PSK solution (0.05 mg mL-1) attained 15.9 %, which is greater than a conventional planar heterojunction device (PCE 12.3 %) fabricated using pristine PSK. In the homogeneous PSK:GO active layers, PSK acted as a hole donor and GO as a hole acceptor. The optimum GO concentration (0.05 mg mL-1) produced increased charge mobility (investigated with Hall effect measurements), enhanced charge separation (investigated with photoluminescence decays) and retarded charge recombination (investigated with electrochemical impedance spectra), consistent with the trend of corresponding device performances. Our results thus indicate that the hybrid PSK:GO layer increased the interfacial contacts between the donor and acceptor of holes to balance the charge mobility and improved the photovoltaic performance with excellent reproducibility and stability.