藉由動態蒙地卡羅模擬方法探討高效率高分子太陽能電池其高轉換效率之關鍵因素

Abstract

本研究以厚膜型高效率PffBT4T-2OD:PCBM太陽能電池為研究對象，結合光學轉換矩陣、易辛模型及動態蒙地卡羅模擬三種方法，探討主動層厚度、分離區塊尺寸及電荷遷移率對於光電性質之影響，並且和P3HT:PCBM太陽能電池的模擬結果進行比較，以歸納出PffBT4T-2OD:PCBM系統高效率之關鍵因素。為了確認動態蒙地卡羅參數的正確性，我們固定PffBT4T-2OD:PCBM的混摻體積比為1:0.75，調整電子遷移率、電荷再結合速率常數及電荷提取速率常數，將模擬結果和實驗文獻進行比較，在最佳情況之下兩者之誤差值僅有5.52%，足以說明所採用之參數適合用於各項變因之模擬。 針對主動層厚度的影響，較厚的主動層會使FF數值下降，原因和再結合的增加有關，不過由於厚膜能夠大幅提升光子吸收效率，因此當主動層厚度為240 nm時有明顯較佳的JSC值，整體來看，FF的改變對於PCE的影響十分有限，PCE主要的變化趨勢還是受到JSC的影響所致。針對分離區塊尺寸的結果，我們觀察到孤島的介面積比例在系統中扮演舉足輕重的角色，當予體區塊尺寸為15 nm時，雖然予體/受體的比介面積較低，使得激子解離效率略低於9 nm的結果，不過因為較低的孤島介面積比例，使電荷不易於傳遞的過程中發生再結合，因此在厚膜及薄膜時皆擁有較佳的PCE表現。針對電荷遷移率的影響，結果指出較高的 使電洞更容易傳遞至電極，也會略微提高電子的移動能力，使IQE及JSC有效提升，我們也發現較佳的電荷傳遞能力能夠克服高膜厚時內部生成之電荷不易傳遞至電極的缺陷，有效減少電荷停留在層內的時間、降低再結合情況的發生，進一步使PCE和薄膜之間的差距拉大。最後從PffBT4T-2OD:PCBM和P3HT:PCBM兩系統的比較中，我們觀察到不論如何調整各項變因，前者皆擁有較佳的PCE值，原因是PffBT4T-2OD擁有優異的光子吸收效率及電荷傳遞能力，有效增加電荷提取數目、提高PCE值。經由一系列的模擬，我們歸納了各項變因對於PCE的影響程度，期許研究成果有助於優化太陽能電池的結構設計，促成高分子太陽能電池之發展。

In this study, the optical transfer matrix, Ising model and kinetic Monte Carlo method are combined to explore the influence of active layer thickness, domain size and carrier mobility on high-efficiency PffBT4T-2OD:PCBM polymer solar cells. The simulation results of PffBT4T-2OD:PCBM are compared with P3HT:PCBM for the purpose of searching the key factors in high efficient PffBT4T-2OD:PCBM system. Parameters applied in kinetic Monte Carlo, such as electron mobility, charge recombination rate constant, and charge extraction rate constant, are regulated to fit the experimental results. It shows that the error of J-V curve is merely 5.52% between our simulation and experiment, which indicates that these parameters are representative to be employed. First of all, from the perspective of thickness in active layer, it shows that although thicker active layer would result in decreasing FF for more recombination, it can greatly improve the photon absorption efficiency which contributes to higher JSC. As a result, there is better JSC when the thickness of active layer is 240 nm. As a whole, the trend of PCE is mainly affected by JSC instead of FF. Next, regard to the results of domain size, we observe that the isolated site interface ratio plays a significant role in the system. Although the exciton dissociation efficiency is slightly lower than the result of 9 nm when the donor has domain size of 15 nm, there is less recombination occurs because of lower isolated site interface ratio. For this reason, there is better performance for both thin and thick active layer when the donor domain size is 15 nm. After that, for the influence of charge mobility, the results indicate that the higher hole mobility makes it easier for the holes to transfer to electrodes. Besides, it slightly promotes the electron's ability to move, which effectively improves IQE and JSC. It also shows that better charge transfer ability can overcome the limitation for thicker film, which effectively reduces the time that charges stay in the layer and much easier transfer to the electrodes. Finally, from the comparison of PffBT4T-2OD:PCBM and P3HT:PCBM systems, we observe that the former has a better PCE value regardless of how to adjust the various factors because PffBT4T-2OD has excellent photon absorption efficiency and better carrier mobility, which effectively increases the number of charge extraction, and improves the PCE. Through a series of simulations, the effects of various parameters to PCE have been summarized. These simulation results are in the hope of optimizing the device structure and also assists the development of polymer solar cells.