以濺鍍技術製備氧化銦鈰透明電極並應用於四接點鈣鈦礦/矽晶串疊型太陽能電池

Abstract

有鑑於矽晶太陽能電池具有高效率及良好的穩定性，而在太陽光電市場居於主導地位，其光電轉換效率最高可達到26.7 %，已經非常接近理論效率極限29.4 %。為了突破此極限，與成本低、製程簡單且效率高的鈣鈦礦電池搭配開發鈣鈦礦/矽晶串疊型太陽能電池逐漸受到矚目。鈣鈦礦上電池負責吸收太陽光譜中短波長、高能量之可見光，而長波長、低能量之近紅外光則穿過上電池傳遞至矽晶下電池吸收。因此，開發具有高導電性、高可見光穿透度及近紅外光穿透度之透明電極對於鈣鈦礦上電池顯得極為重要。然而，市面上常見的氧化銦錫(ITO)及氟摻雜氧化錫(FTO)因具有相當高的自由載子密度，使其近紅外光穿透度相對較低，故不適用於鈣鈦礦上電池中作為透明電極。有鑑於此，本研究引入具有高載子遷移率的氧化銦鈰(Cerium doped indium oxide, ICO)作為透明電極材料，可以較低的氧化鈰摻雜量降低自由載子密度而增加近紅外光穿透度，同時維持高導電性。 本研究依據半透明鈣鈦礦太陽能電池的製程需求與限制，分別以低功率及鍍率快的直流濺鍍技術製備ICO透明上電極，並以高功率、成膜均勻且緻密度高的射頻濺鍍技術製備ICO透明下電極。透過熱退火處理、在濺鍍製程中添加適量氧氣以及調整薄膜厚度等方式來改善ICO薄膜的電性及光學性質。經過最佳化的ICO透明上電極及透明下電極，載子遷移率分別可達到32.51 cm2/Vs及104.7 cm2/Vs，用以實現低片電阻28.75 Ω/sq及9.99 Ω/sq，並具備優異的近紅外光穿透度80.00 %及84.42 %，相較於ITO上電極及FTO下電極之近紅外光穿透度為75.43 %及77.69%皆有顯著改善。進一步將ICO薄膜依序取代鈣鈦礦電池中常見的ITO透明上電極及FTO透明下電極，使鈣鈦礦上電池之近紅外光穿透度由53.73 %提升至76.74 %，並使矽晶下電池獲得2.9 mA/cm2之電流密度及1.92 %之效率提升。最後以機械式堆疊四接點鈣鈦礦/矽晶串疊型太陽能電池，其光電轉換效率最高可達到26.2 %，與同樣以甲基胺基碘化鉛(MAPbI3)作為鈣鈦礦材料之相關文獻相比為效率最高值。

For decades, crystalline silicon (c-Si) solar cells have dominated the solar industry because of its high power conversion efficiency (PCE) and good stability. However, the current efficiency record of c-Si solar cells is 26.7 %, approaching a theoretical efficiency limit of 29.4 %. To surpass this single junction efficiency limit, developing silicon-based tandem solar cells is one of the feasible ways to improve PCE. Among various solar cells, perovskite solar cell is considered as a promising candidate for being the top cell of silicon-based tandem solar cells, not only for its solution processibility and low cost but also for its high efficiency. In a perovskite/silicon tandem solar cell, perovskite top cells with wide bandgap (Eg) absorb high energy visible light in the solar spectrum, while silicon bottom cells with narrow Eg absorb the passed through low energy near-infrared light (NIR). For that reason, establishing transparent electrodes with high conductivity and high visible-NIR transmittance is extremely important for perovskite top cells. Nevertheless, mostly used commercial tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) are not suitable for the application of tandem solar cells because of their high free carrier density, which results in low NIR transmittance. Here, we introduce cerium-doped indium oxide (ICO) with excellent carrier mobility and low free carrier density as transparent electrodes. These two characteristics make ICO transparent electrodes maintain high conductivity and increase NIR transmittance, respectively. According to the requirements and limitations of fabricating semi-transparent perovskite solar cells, ICO top electrodes are grown by low power direct current (DC) sputtering system with a high deposition rate. On the other hand, ICO bottom electrodes are grown by high power radio frequency (RF) sputtering system with uniform thin film formation. Furthermore, we systematically investigate the processing parameters such as post-annealing treatment, oxygen fraction in the sputtering ambient, and film thickness, aiming to improve the electrical and optical properties of ICO thin films. Optimized ICO transparent top electrode and bottom electrode feature very high carrier mobility up to 32.51 cm2/Vs and 104.7 cm2/Vs, and achieve low sheet resistance of 28.75 Ω/sq and 9.99 Ω/sq, respectively. Moreover, the high NIR transmittance of 80.00 % and 84.42 % can be attained, which are significantly improved compared to 75.43 % of ITO top electrode and 77.69 % of FTO bottom electrode. Overall, a 23 % NIR transmittance improvement in perovskite top cells can be obtained by replacing FTO bottom electrode and ITO top electrode with ICO thin film, respectively. Thus, an absolute current gain of 2.9 mA/cm2 in silicon bottom cells is achieved, resulting in an improving PCE of 1.92 %. For four-terminal perovskite/silicon tandem solar cells, we reach a PCE improvement from 24.3 % to 26.2 %, which is the highest performance reported for tandem solar cells using methylammonium lead iodide (MAPbI3) as perovskite materials.