鈣鈦礦太陽能電池之光浸潤效應探討

Investigation of Light Soaking Effect in Perovskite Solar Cells

羅志紘(Lo, Chih Hung)

指導教授 ： 楊耀文

國立清華大學/理學院/化學系/碩士(2016年)

摘要

本論文分為兩部分，第一部分為p-i-n形式鈣鈦礦太陽能電池轉換效率最佳化的研究，p-i-n形式的元件較n-i-p形式元件有製程簡單、成本低、應用範圍廣的優勢，因此我們選擇p-i-n形式的元件進行研究，兩者之比較會在本論文的1-6-3章節有更詳細的介紹。第二部分則是光浸潤效應(light soaking effect)的探討，由於元件在照光後，我們發現Voc及FF會有很明顯的增加，進而使得轉換效率隨照光時間的增加而增加。根據Li等人所發表的文獻提到1，鈣鈦礦吸收層中的碘離子會因外加電場的作用而移動，因此造成在鈣鈦礦的介面處會有能帶彎曲現象的產生，此能帶彎曲就是造成Voc變大的原因。而鈣鈦礦吸收層因照光產生的電子及電洞在電池元件為開路的情況下，會累積在兩端電極而產生內建電場，此內建電場效果應與前面提到的外加電場相似。為了探討鈣鈦礦吸收層與電洞傳輸層及電子傳輸層兩者間的介面在接受光浸潤時發生的變化，因此我們以UPS及XPS來分析在光浸潤的過程中，鈣鈦礦吸收層之能階位置及元素組成的改變。
第一部分p-i-n形式一般型元件架構為ITO/PEDOT:PSS/perovskite/C60/Ag，首先藉由改變PEDOT:PSS旋轉塗佈時的轉速條件，希望能改變PEDOT:PSS的膜厚，探討膜厚對元件效率的影響。接著是改變不同的鈣鈦礦溶劑及反溶劑的搭配，並以SEM、AFM、XRD、UV-Vis及UPS等儀器分析，探討鈣鈦礦薄膜的表面形貌、晶相性、能階位置對整體元件的轉換效率的影響。最後會再改變C60的蒸鍍膜厚，探討膜厚對效率的影響。我們發現當PEDOT:PSS以3000 rpm維持60秒的條件下可得到最適的膜厚，鈣鈦礦吸收層是以DMF與甲苯當作溶劑與反溶劑可以得到較平整且晶相性較高之鈣鈦礦薄膜，最後蒸鍍20 nm的C60作為電子傳輸層，如此一來可以得到9.0 % 之轉換效率，相較於其他條件下是最好的。
第二部分為光浸潤效應的探討，首先從UPS的分析我們發現當光浸潤開始後，在鈣鈦礦吸收層中接近其與上、下兩層介面處的HOMO能量位置會有很明顯的改變，靠近與電子傳輸層介面之鈣鈦礦HOMO位置在光浸潤開始後會往低束縛能位移；靠近與電洞傳輸層介面之鈣鈦礦HOMO位置則是往高束縛能位移，如此能帶彎曲現象是由於內建電場造成部分的電子及電洞分別累積在電子及電洞傳輸層，使得此兩層之費米能階分別上升及下降，造成鈣鈦礦吸收層靠近兩介面處之能帶彎曲，此即是造成整體元件Voc上升的原因。XPS的部分我們設計了兩種實驗架構，首先在元件無內建電場作用下並無觀察到鈣鈦礦中碘的未知物訊號產生，而第二種實驗設計是可以在元件照光後產生內建電場，此部分可明顯觀察到碘未知物訊號的增加，因此我們推測元件照光後產生的內建電場會影響此未知物的產生。根據Kobayashi等人所發表的文獻提到2，碘離子會吸附至C60上，其吸附能為49.0 kJ mol-1，因此我們推測此未知物即為碘離子與C60或CuPc所形成之錯合物。帶負電的碘離子會除了往電洞累積處移動，同時也會有部分吸附在C60及CuPc上，此兩效應造成能帶彎曲的產生也會影響光浸泡效應的可逆性表現。

關鍵字

鈣鈦礦太陽能電池 ； 光浸潤效應 ； 光電子能譜 ； p-i-n架構 ； n-i-p架構

並列摘要

This thesis is composed of two parts: the first part deals with the performance optimization of perovskite (PVK) solar cells exhibiting p-i-n architecture, and second part reports an electron spectroscopy investigation of the so-called light soaking effect in solar cells. The p-i-n device was chosen because it possesses some advantages that include easier fabrication, lower cost and wider application, as compared with n-i-p device. Light soaking effect is related to an interesting observation that the solar cell parameters like Voc and FF increase with a prolonged exposure to AM 1.5 sunlight, leading to an overall enhancement of power conversion efficiency. In the case of PVK solar cell, the present understanding tends to believe that the migration of halide ions under the solar illumination, a common occurrence, is primarily responsible for the light soaking effect. A previous study by Li et al,1 has presented a clear evidence showing iodide ions and positively-charged iodine vacancies, assisted by the applied electric field, will migrate toward the respective electrode and accumulate in the region of PVK that borders either hole transport layer (HTL) or electron transport layer (ETL). As a result, the band bendings at two interfaces of PVK/ETL and PVK/HTM take place, resulting in a change of Voc. The change in chemical makeup and the band bending at the interface can be conveniently studied by XPS and UPS, respectively, which forms the basis of the second part of this thesis.
The architecture of our p-i-n device is ITO/PEDOT:PSS/PVK/C60/Ag. The thicknesses of both PEDOT:PSS (HTL) and C60 (ETL) layers are to be optimized first. Frist, the thickness of PEDOT:PSS film is changed by varying spin-coating condition. The optimal PEDOT:PSS film that supports the fabrication of “best” solar cell was formed at a spin-coating speed of 3000 rpm for 60 s. Second, the properties of peroskites based on a different combination of solvent/antisolvent pairs were evaluated by using SEM, AFM, XRD, UV-Vis, and UPS with the derived information covering surface morphology, crystallinity, and electronic property. A recipe of using DMF as a solvent and toluene as an antisolvent results in smoother and more crystalline PVK films. Third, the optimal thickness of vacuum-deposited C60 film (an electron transport material) was concluded to be 20 nm by examining the thickness dependence of device performance. Putting all things together, we produced p-i-n structural devices exhibiting a power conversion efficiency of 9.0 %.
To investigate the light soaking effect with XPS and UPS while the device is in operation, a special solar cell exhibiting two different cross section profiles but electrically connected was fabricated. The large thickness region was fabricated with the thickness of the relevant layers conformed to the real solar cell requirement. The small thickness region is topped with a very thin Ag film (< 1nm) as an electrode layer and, underneath this layer, either a C60 ETL or a CuPc HTL of the same small thickness (< 1 nm) is formed on top of 300 nm thick PVK. The reason for making such a small thickness region is that the thickness of transport layer for real devices is much larger than the probing depth of XPS and UPS that are essentially surface sensitive. Without thinning down both electrode and ETL (or HTL), the PVK/ETL and PVK/HTL interfaces cannot be directly probed.
UPS data show that as the light soaking is in progress, valence band maximum (VBM) of PVK/ C60 interface shifts to lower binding energy (b. e.) while the VBM of PVK/CuPc shifts to higher b. e.. The direction of b. e. shift is in accord with the prediction based on the accumulation of iodide and iodine vacancy at PVK/CuPc and PVK/C60 interfaces, respectively. The accumulation of negatively-charged iodide ions generates a downward band bending at interface. Whereas an upward band bending is observed for PVK/C60 interface. Elemental composition analysis revealed by XPS data shows the formation of a new I 3d component at both interfaces. This new component is believed to be a complex formed between C60 (or CuPC) and iodide. This assignment is supported by the result published by Kobayashi et al2, in which iodide ions adsorb onto C60 with a modest adsorption energy of 49.0 kJ mol-1. Iodide anions not only migrate to the PVK/HTL interface under the influence of build-in potential, but are also capable of adsorbing onto C60 and CuPc.

並列關鍵字

perovskite solar cell ； light soaking effect ； photoemission spectroscopy ； p-i-n structure ； n-i-p structure

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