硒化鎘量子點改善磷化銦鎵/砷化鎵/鍺串聯式太陽能電池之電流匹配與效率提升之研究
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2014
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三五族多接面串聯式太陽能電池(III-V-base tandem solarcell)是由不同材料逐層堆疊所組成的化合物半導體結構，每一接面材料對應不同的能隙(energy band-gap)分佈，可分別吸收太陽光譜中不同波段以提高元件之光電功率轉換效率。在設計一高功率轉換效率之多接面串聯式太陽能電池時，有一項重要的物理因素值得探討，即在各個接面電池間，其光致電流(photon-generated current)是否有達到電流匹配(current matching)之議題。根據基本電學原理，一串聯電路中其導通電流須為一致。因此，對一多接面串聯式太陽能電池而言，其各個接面電池在照光後產生不同光致電流時，將以最小電流作為整個元件之輸出電流。換言之，整體太陽能電池最終輸出光電流會被產生最小電流的該層接面電池所限制。本論文主要研究使用硒化鎘(cadmium selenide, CdSe)量子點(quantum dots, QDs)來增強磷化銦鎵/砷化鎵/鍺 (InGaP/GaAs/Ge)三接面串聯式太陽能電池之光電功率轉換效率。量子點具有獨特的量子侷限效應(quantum confinement effect)，可以用來調變太陽光譜，以利於多接面串聯式太陽能電池整體轉換效率的提升。我們發現，其所增加之光電功率轉換效率與元件本身的結構，以及量子點的尺寸有很大的關連。我們進一步建立一套物理模型來設計與優化量子點尺寸，以達到最佳之元件功率轉換效率輸出。最後，本論文在磷化銦鎵/砷化鎵/鍺串聯太陽能電池上旋轉塗佈(spin coating)濃度7 mg/mL、直徑為4.2nm的硒化鎘量子點，相較於未塗佈量子點的裸片太陽能電池，光電功率轉換效率大幅且穩定地增加了10.39%。由於合成與旋轉塗布CdSe量子點是低成本且成熟的製程技術，並可直接融於現行太陽能元件的製造流程；因此我們相信，本論文所提出之新穎結構將可廣泛應用於下世代高效率之相關能源光電元件。
A III-V multi-junction tandem solar cell is the most efficient photovoltaic structure that offers an extremely high power conversion efficiency. Current mismatching between each subcell of the device, however, is a significant challenge that causes the experimental value of the power conversion efficiency to deviate from the theoretical value. In this work, we explore a promising strategy using CdSe quantum dots (QDs) to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism of the enhancement can be attributed to the QD’s unique capacity for photon conversion that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD’s dimensions. As a result, by appropriately selecting and spreading 7 mg/mL of CdSe QDs with diameters of 4.2 nm upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the cell’s counterpart without integrating CdSe QDs.
A III-V multi-junction tandem solar cell is the most efficient photovoltaic structure that offers an extremely high power conversion efficiency. Current mismatching between each subcell of the device, however, is a significant challenge that causes the experimental value of the power conversion efficiency to deviate from the theoretical value. In this work, we explore a promising strategy using CdSe quantum dots (QDs) to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism of the enhancement can be attributed to the QD’s unique capacity for photon conversion that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD’s dimensions. As a result, by appropriately selecting and spreading 7 mg/mL of CdSe QDs with diameters of 4.2 nm upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the cell’s counterpart without integrating CdSe QDs.
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Keywords
三五族串聯太陽能電池, 量子點, 電流匹配, III-V tandem solar cell, quantum dots, current matching