低溫溶液製程高效率n-i-p型鈣鈦礦太陽能電池

Abstract

近年，鈣鈦礦太陽能電池因為具有低成本、低溫溶液製程和高效率等特點，受到各方熱切的關注和研究。鈣鈦礦太陽能電池被評估具有極大的潛力成為新一代的再生能源。然而，鈣鈦礦層形貌的優劣對電池的效率有明顯的影響，所以近年來科學家們一直嘗試改良鈣鈦礦電池的製程方法，希望形成緻密的鈣鈦礦層以達到高效率。 我們以n-i-p結構的平面異質接面鈣鈦礦太陽能電池為基礎，結構為FTO/TiO2-NPs/CH3NH3PbI3-xClx/spiro-OMeTAD/Au，希望透過分別加入兩種不同的高子添加劑，polyvinylpyrrolidone(PVP)和 polyethylene glycol (PEG)，有效地控制鈣鈦礦層的形貌。經過實驗發現，加入PVP會使鈣鈦礦層在電子傳導層上的覆蓋率下降，導致元件效率下降。然而加入PEG則有效增加鈣鈦礦層的覆蓋率，避免電子傳導層和電洞傳導層的接觸，有效提升元件的開路電壓(Voc)。在此，我們提出一個機制解釋PEG添加劑如何有效改善鈣鈦礦層的形貌：由於PEG的融點低於製作鈣鈦礦層過程中的熱退火溫度，所以融化的高分子會在鈣鈦礦層中流動並包覆住鈣鈦礦晶粒。由於高分子的包覆會阻礙鈣鈦礦晶粒在熱退火時的晶粒成長。在熱退火結束後，小顆的晶粒會形成一個緻密的鈣鈦礦層。 為了進一步了解影響PEG添加劑控制鈣鈦礦層形貌的因素，我們分別對PEG添加劑的分子量和添加量進行研究。首先，我們把不同分子量的PEG添加劑(6k, 12k和20k) 加入鈣鈦礦層，發現高分子量的PEG添加劑會因為熔點提高和分子鏈長增加，在鈣鈦礦層中的流動性下降，而無法有效包覆住每一顆晶粒。在熱退火結束後，鈣鈦礦層的晶粒會較為大顆，無法形成一個緻密層並覆蓋底部的電子傳導層。接著，我們在鈣鈦礦層中加入不同比例的PEG添加劑(0.5wt%, 1wt%, 3wt% 和5wt%)，發現提高添加劑的數量可以有效提升鈣鈦礦層的覆蓋率並提升元件的開路電壓，但是加入過多的添加劑則會因為PEG的絕緣性質而使元件的短路電流(Jsc)因此下降。在經過調控之後，我們發現加入1 wt%的6k PEG添加劑可以最有效地改善鈣鈦礦層的形貌並把元件效率由11.56±0.40%提升至13.98±0.75%。

Recently, the perovskite solar cell draws lots of attention due to the properties of low cost, low temperature solution processable and high efficiency. It has great potential to substitute the silicon based solar cell and become a new pollution free and renewable energy source of next generation. However, the morphology control of perovskite layer significantly affect the device performance. The scientists endeavor to produce a dense perovskite layer for high efficiency by many strategies, including solvent engineering, solvent annealing and additive, etc. We fabricated the planar heterojunction perovskite solar with a n-i-p structure: FTO/TiO2-NPs/CH3NH3PbI3-xClx/spiro-OMeTAD/Au and incorporated two different polymer additives, polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), into perovskite layer to control the morphology. With the PVP additive, the perovskite layer had poor coverage on electron transport layer (ETL). On the other hand, with the PEG additive, the coverage of perovskite layer was enhanced and the direct contact of ETL and hole transport layer (HTL) was avoided which effectively improve the Voc of the device. Because the melting point of PEG additive is lower than the thermal annealing temperature during the fabrication of perovskite layer, the melted PEG additive may surround the perovskite grains and retard the grain growth. Finally, the small perovskite grains constitute a dense perovskite layer with improved coverage. We also investigated the molecular weight effect and optimal amount of PEG additive in the perovskite layer for high efficient perovskite solar cell. First, we added different molecular weight PEG additive (6k, 12k and 20k) into the perovskite layer to study the molecular weight effect of PEG on the morphology of perovskite. The results indicated that the higher molecular weight of PEG additive has higher melting point and longer polymer chain length that may reduce the flow ability of polymer in the perovskite layer. The grain growth of perovskite grains wouldn’t be retarded by the polymer additive. Due to the large grain size, the perovskite layer would have a poor coverage. On the other hand, we added the PEG additive into the perovskite layer with different amount of PEG (0.5wt%, 1wt%, 3wt% and 5wt%). The coverage of perovskite layer and the Voc were enhanced with an increased amount of PEG additive. However, the Jsc of device was decreased when adding too much insulating PEG. With compromising between the Voc increases and Jsc decreases, 1 wt% of 6k PEG additive is the most appropriated amount and molecular weight for the PEG additive in the perovskite layer. As a result, the Voc was improved from 0.85V to 0.96V and the power conversion efficiency PCE was increased from 11.56±0.40% to 13.98±0.75%.