低溫製程高效率鈣鈦礦型太陽能電池

隨著人類的科技發展及世界石化燃料的逐漸枯竭，能源議題在這個世紀以來受到廣泛的重視，各界極力投入發展替代能源，而太陽能具有取之不盡用之不竭的優勢，再加上對環境友善不排放溫室氣體，使太陽能電池的發展成為目前人類解決能源短缺的絕佳方案。在眾多的太陽能電池種類中，鈣鈦礦太陽能電池以非常快的速度串起，在短短5年間從效率3%提升到效率20%，使原本研究高分子及染料敏化太陽能電池的團隊紛紛投入，並將原本的技術應用在鈣鈦礦太陽能電池上，得到不錯的結果。鈣鈦礦太陽能電池除了高效率的優點，也有大面積元件製程的潛力，並能將其製作在可撓性基板上，使電池具有更多元的應用，本研究針對鈣鈦礦太陽能電池進行研究，發展低溫製程，達到節省生產能源及兼容可撓性基板應用的方法，並改良製程技術以增進元件的光電轉換效率。氧化鋅由於其具有良好的電子傳輸能力及光穿透率，在高分子太陽能電池中廣被使用，本論文首先以溶液製程成長氧化鋅薄膜，並使用簡單的二步驟製程步驟，先旋塗PbI2再滴上CH3NH3I溶液使兩者反應，在氧化鋅基板上成長鈣鈦礦CH3NH3PbI3層，利用調變二步驟製程的濃度參數、氧化鋅層之架構來提升元件效率，並探討鈣鈦礦層和氧化鋅層的介面形貌對元件效率的影響，最後加入鈣鈦礦層的後退火步驟，使鈣鈦礦層反應更完全，成功將元件轉換效率提升至13.38%。由於鈣鈦礦的生長過程中非常易受環境中氣體的影響，再加上鈣鈦礦層的形成如果太快會導致晶粒小且雜亂，使介面接觸不好影響載子傳輸，因此我們發展了一套新的低壓鄰近蒸鍍法的製程，先旋塗一層PbI2，再使用較慢速的蒸鍍方式來成長鈣鈦礦層，首先應用在氧化鋅基板上，但因為氧化鋅上的鈣鈦礦層其熱穩定性不佳，及無法避免的遲滯現象，使本製程應用在氧化鋅上稍有困難，但即便如此我們還是得到了8.87%的元件轉換效率，及非常不錯的開路電壓1.03V。接著本研究將低壓鄰近蒸鍍法的製程應用到順式結構的PEDOT:PSS基板上，利用此電洞傳輸層能使整個製程溫度控制在130℃以下，且鈣鈦礦層的熱穩定性較佳，我們發現旋塗PbI2後退火的時機會影響其表面形貌，旋塗完靜置10分鐘再退火之PbI2表面形貌出現許多孔洞影響了載子傳輸，元件的轉換效率只有3.80%，而旋塗完馬上退火的PbI2其薄膜較均勻緻密，元件的轉換效率可達7.55%，這是現今文獻中首次在PEDOT:PSS基板上成功使用低壓鄰近蒸鍍法製作鈣鈦礦太陽能電池。為了改善低壓鄰近蒸鍍法不易使底層PbI2反應完全，且容易使表層CH3NH3I蒸鍍過量的現象，我們提出一個溶液製程搭配壓鄰近蒸鍍法的應用，即製作CH3NH3I/PbI2/CH3NH3I的結構，第一層的CH3NH3I使底部PbI2能夠反應完全，並縮短了低壓鄰近蒸鍍法製成時間，使第二層CH3NH3I不會蒸鍍過量，經過最佳化製程時間及退火參數後，我們將元件效率提升至13.74%，且全程在大氣或真空下進行，不需使用低水氧環境，是鈣鈦礦電池發展及大量生產上的重大突破。

關鍵字

鈣鈦礦太陽能電池；氧化鋅；溶液製程；二步驟製程；形貌控制；低壓鄰近蒸鍍法； PEDOT:PSS ；載子傳輸；大氣製程

並列摘要

Due to the development of technology and gradual depletion of fossil fuels, the energy issue has received wide attention in this century. The world is giving every effort to develop alternative energy. Solar energy is with inexhaustible source and also environment-friendly that it will not produce greenhouse gases. For these reason, the development of solar cells has become one of the best plan to solve the oncoming shortage of energy. Among many types of solar cells, perovskite solar cells suddenly appear on the horizon and the conversion efficiency has been improved from 3% to 20% in just five years. This makes many teams who originally study in polymer and dye-sensitized solar cells start doing research about perovskite. They apply their former technology of polymer and dye-sensitized to perovskite solar cells and also get good results. In addition to high efficiency, perovskite solar cells have other advantages such as large area solar cells potential, possible to fabricate on flexible substrates, light weight and with diverse applications. So in this study, we are going to develop low temperature process to ruduce the consumption during production. We will also optimize the methods to improve the conversion efficiency of the devices. Zinc oxide is widely used in polymer solar cells because of its good electron conductivity and high light transmittance. As a result, we first applied solution processed zinc oxide film and a simple two-step process steps to fabricate perovskite solar cells. We spin-coated PbI2 on ZnO and then dropped CH3NH3I solution on PbI2 film to form perovskite. Next, we modified the concentration of CH3NH3I solution and used different structure of ZnO to improve the efficiency. We analyzed the effect of interface morphology between perovskite and ZnO. After that, we used a postannealing step to let perovskite layer form more completely and finally gained a conversion efficiency of 13.38%. Because two-step process is very susceptible to the atmosphere and the formation of perovskite is too quick that causing perovskite grains become small and messy. We developed a new method called proximity evaporation technique (PET) to fabricate perovskite layer. At first we spin-coated PbI2 on ZnO and then slowly evaporated CH3NH3I under low pressure to form perovskite layer. However, due to poor thermal stability and unavoidable hysteresis effect of the perovskite layer, we found that PET process is difficult to be exercised on ZnO substrate. But even so we still got a conversion efficiency of 8.87% and a very good open-circuit voltage of 1.03V. After using ZnO, we utilized PET method on PEDOT: PSS substrate to make traditional structure solar cells. By using PEDOT:PSS, we could control the temperature of whole process under 130℃ and also got good thermal stability of perovskite layer. We found that the moment to anneal PbI2 after spin-coating would greatly affect its surface morphology. If we kept PbI2 film 10 minutes at room temperature then annealed it, the PbI2 morphology would be very rough and hindered the carrier transport. The conversion efficiency of the device was only 3.80%. But If we annealed PbI2 immediately after spin-coating, the PbI2 film would be uniform and giving a conversion efficiency of 7.55%. This is the first time in the literature to use a low pressure evaporation process to form perovskite on PEDOT: PSS substrate. In order to achieve complete formation of the bottom PbI2 and avoid the excess deposition of CH3NH3I on surface, we bring up an idea of using solution process along with PET to form a structure of CH3NH3I/PbI2/CH3NH3I. The first CH3NH3I could let the bottom PbI2 reacted completely and also shortened the PET process time. Consequently, the second CH3NH3I would not be deposited excessively. After optimized the process parameters, we achieved an efficiency of 13.74%. For a low temperature process under whole atmosphere or under vacuum condition, this is a breakthrough of the perovskite solar cells and mass production.

並列關鍵字

perovskite solar cells ； zinc oxide ； solution process ； two-step process ； morphology control ； low pressure proximity evaporation technique ； PEDOT:PSS ； carrier transport ； atmosphere process

參考文獻

[1] International Energy Agency, “2014 Key World Energy Statistics”.

[2] International Energy Agency, “ENERGY SUPPLY SECURITY 2014”.

[4] D. Chapin, C. Fuller and G. Pearson, 'A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power', J. Appl. Phys., vol. 25, no. 5, p. 676, 1954.

[5] D. Carlson and C. Wronski, 'Amorphous silicon solar cell', Applied Physics Letters, vol. 28, no. 11, p. 671, 1976.

[6] “Best Research-Cell Efficiencies”, National Renewable Energy Laboratory (NREL), Rev. 06-09-2015.

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低溫製程高效率鈣鈦礦型太陽能電池

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