奈米結構異質接面高分子太陽能電池之材料開發及元件設計(I)

Abstract

本計畫主要的目的在於發展具高光電轉換效率之奈米結構異質接面高分子太陽能電池之材料開發 及新穎元件設計製作與特性研究，工作範圍包含前瞻性包含三方面，首先在材料方面為：(a)合成具多 重吸收及低能隙之高分子：合成主鏈(donor)及側鏈(acceptor)具不同吸收波長之高分子，藉由引進不同 能隙之側鏈基團進而調控共軛高分子之能隙，增加吸光範圍、電荷分離及傳輸效率，使元件之光電流 大幅提升，而提升光轉換成電之能量效率，或合成主鏈含共軛(donor-acceptor)之低能隙高分子以增加吸 光範圍。在元件內部結構方面為：(b)產生垂直奈米結構P-N 異質接面：由於有機材料結晶度較低，於 吸光後所產生激子之擴散距離僅為20 奈米而且電子與電洞在有機材料之傳導速度為在矽半導體之百分 之一，必須根據奈米技術設計元件結構以增加異質接合介面面積和增加電荷傳送之效率。其中以獨立 傳送電子與電洞之材料P-Njunction 交錯結構以避免電子與電洞重新結合之機會為最理想之結構，因而 形成具方向性並具能隙設計之奈米複合材料結構，並且(c)合成表面電漿效應之金屬奈米顆粒，表面電 漿子模式會侷限在金屬表面附近，形成高度增強的近場，表面增強的特性可利用在光學元件上增強吸 收，對於光電元件的應用是很理想的材料，以及(d)發展多層元件製程及最佳化：多層元件(tandem cell) 在增加元件效率之方法即經由串聯兩個或數個分別具有不同吸光光譜之電池，將其堆疊起來，使光通 過吸光層的路徑加長而且吸收波長不同，即可增加整體元件之吸光範圍及開路電壓。另外於元件結構 奈米化後進行元件最佳化以提昇元件之效率。在元件性能中，共軛高分子之能隙及奈米結構形態控制 為最重要的因素。期望藉由這些材料開發及元件設計之發展，三年後將能量轉換效率(Power conversion efficiency)提高至8%。

Solution-processed conjugated organic materials combine the electronic properties of semiconductors with the high processability of polymeric materials, allowing them to be cast using wet-processing techniques such as spin casting, dip coating, ink-jet printing, screen printing, and micro-molding. These techniques are enormously attractive for producing low-cost, flexible, large-area photovoltaic cells that are suitable for commercial applications in optoelectronics technologies. Our main objective for this project is to construct high-efficiency bio-inspired conjugated polymer heterojunction photovoltaic cells incorporating (i) donor–acceptor polymers presenting side chain-tethered dye or electron withdrawing moieties, (ii) ordered and bandgap-engineered nanorods (NRs), and (iii) metal nanoparticles (NPs) exhibiting surface plasmonic effects. The principal idea behind the design of these photovoltaic cells is that the heterojunctions between the ordered NRs and conjugated polymers will be formed in such a way that the inter-rod distance will be on the same order of magnitude as the diffusion length of the excitons. First, a high degree of absorption of solar energy will be accomplished using polymers consisting of a conjugated main chain, absorbing wavelengths in middle of the visible light spectrum, and side-chain-tethered dye or electron-withdrawing moieties, absorbing in the near-infrared or ultraviolet region. We aim to synthesize polythiophenes or polyfluorenes tethered to transition metal-based dyes, such as ruthenium(II) bipyridine complexes and platinum(II) diimine dithiolate complexes, that will absorb more than 30% of the solar energy, much higher than that (20%) adsorbed by such conjugated polymers alone. Second, monolayered polymer thin films incorporating aligned nanorods will be obtained by two steps. The first step involves in mixing pyridine-modified NRs and a polymer in pyridine; the NRs become distributed selectively on the side chains of the polymer as a result of their preference for experiencing polar interactions. Then, the nanorods in these composite films became aligned after they experienced external fields such as electric or magnetic field at elevated temperature. Alternatively, anodic aluminum oxide (AAO) templates can be used to fabricate vertical composite nanorods consisting of polymer and nanoparticles. Third, the effect that incorporating metal nanoparticles has on the plasmonic enhancement of polymer/NPs or NRs bulk heterojunction systems will examined. Because the dipole-allowed photogeneration of excitons in the active layer should scale with the square of the electric field, we hypothesize that enhancing the local electromagnetic field through the inclusion of surface plasmon-active materials such as metal nanoparticles will enhance the degree of photogeneration of excitons in the conjugated polymers and, to a lesser degree, in nanorods. Moreover, in the tandem cell structure, we will adopt an inverted structure using a multiple-wavelength absorbing, low bandgap polymer/fullerene composite. With our proposed cell architecture, the probability of diffusion of excitons to the heterojunction interfaces will be relatively high and their separation relatively more complete. Moreover, the presence of the NRs will provide direct pathways for the electrons to move through the NRs toward the cathode, while the conducting polymers will serve as channels for holes to move to the anode once the excitons have dissociated at the interfaces. Having the electrons and holes travel along their independent paths to their respective electrodes will decrease their chance of recombination and, thereby, increase the cell’s power conversion efficiency. By combining enhanced optical absorption of donor-acceptor conjugated polymer, aligned semiconductor nanorods device architecture, surface plasmonic effect of nanoparticles and tandem cell structure, we believe that we can achieve power conversion efficiency of at least 8% for the proposed heterojunction poymer/nanorod solar cells in three years.