A Theoretical Model for Organic Bulk-Heterojunction Solar Cells

在本研究中我們建構有機混摻異質接面型太陽能電池放電特性的數學模型，以描述電池內部光電轉換以及電荷傳遞的機制，包含了光子在活化層的吸收、光激子的移動、電荷轉移態的解離現象、自由電荷的流動及活化層和電極邊界上的電流熱注入效應。透過數值方法我們可以計算出電池內部的電位及自由電荷分佈，並計算電池的亮電流放電曲線，此外透過研究各種操作點情況下的電位及電荷分佈也幫助我們了解影響電流輸出的因素。 本研究中我們首先模擬活化層為OC1C10-PPV: PCBM的放電曲線並得到一組模擬的參數，以該組參數為基準的情況下，此模型拿來預測電池的外部量子效率得到了不錯的結果。另外在實驗光照強度及操作溫度變化的條件下，模型對於短路電流的預測也相當準確，雖然開環電壓在缺乏暗電流模型的情況下預測不甚完美，但在大部分的中高溫區段還是可模擬出隨光照強度及操作溫度變化的趨勢。另外我們也模擬以P3HT:PCBM為活化層的放電曲線並得到另一組參數，以實驗提供的各種熱退火溫度下的吸收係數及載子遷移率，也成功預測出經過熱退火程序程序後的電池放電表現，證明了我們模型除了光照及溫度之外，對於掌握遷移率影響電池的效應也同樣優異，相信善用此模型將可以有效節省優化電池效率的費用及時間成本。In this work, we established a mathematical model for the description of bulk-heterojunction solar cells. This model considers detailed mechanisms of the photovoltaic and the charge transport phenomenon, including the photon absorption, exciton diffusion, dissociation of charge transfer states, free charge transport and thermionic emission at the interface between the active layer and electrode. By solving the equation numerically, we obtain the profile of potential and free charge concentration, thus the photocurrent under the specific applied voltage. In additions, we also studied the details of the cells under 3 important conditions─short circuit(zero applied voltage), maximum power point and open circuit. In this study, firstly we simulate the I-V curve for the OC1C10-PPV: PCBM and obtain a set of parameters. By using those parameters, we can then predict the external quantum efficiency with the experiment in a good agreement. Besides, our result shows that the model can also predict the change in short circuit current due to the changes in illumination intensity and operation temperature. Although the prediction for open circuit voltage is not perfect, it can still simulates the tendency of Voc at high temperature region without the use of dark current model. Secondly, we simulate the I-V curve and obtain another set of parameters for P3HT:PCBM. By using the same set of parameters and only changing the absorption coefficient and mobility from the experiment, the performance of cells after thermal annealing can be well predicted. These results prove that this model can simulate the illumination intensity、 temperature、optical properties and mobility effects on the device. We believe that this model can really help tone in the design of high efficient bulk- heterojunction solar cells in a low cost and time saving manner