本論文主要探討以氣相方式成長甲基胺碘化鉛(CH3NH3PbI3)之鈣鈦礦薄膜，藉由不同於以往溶液塗佈的製程方式，我們可以透過控制碘化鉛的鍍率以及CH3NH3I的擴散反應時間、溫度及壓力等多項參數，進而得到多樣化的薄膜型態，我們期望可以更精確的控制鈣鈦礦薄膜的生長。搭配以電子、電洞傳輸層組合成P-N結構，製作出性能更穩定的鈣鈦礦薄膜太陽能電池。此研究的鈣鈦礦薄膜，是利用熱蒸鍍法(thermal evaporation or PVD)或旋轉塗佈法(spin coating)成長碘化鉛(PbI2)薄膜，再以真空輔助方式對甲基碘(CH3NH3I)粉末及PbI2基板進行化學氣相沉積反應(Low-Pressure Chemical Vapor Deposition)，利用二部法的製程成長出甲基胺碘化鉛(CH3NH3PI3)的鈣鈦礦薄膜。在適當的參數條件下可以得到晶界(grain size)大且無孔洞的薄膜，降低缺陷與漏電的產生，我們可以量測薄膜得到和文獻相近的吸收(UV-vis)與受激螢光放光(Photoluminescence)光學性質，結果說明此反應方式形成的鈣鈦礦薄膜具有相當的品質。用溶液塗佈搭配低壓化學氣相沉積的平面CH3NH3PbI3鈣鈦礦太陽能電池元件最高效率可達7.7%；而由熱蒸鍍得的PbI2薄膜緻密、無孔洞，與CH3NH3I的擴散較為不易，元件電流較低，使用熱蒸鍍搭配低壓化學氣相沉積的平面CH3NH3PbI3鈣鈦礦太陽能電池元件最高效率可達5.14%。

關鍵字:熱蒸鍍，低壓化學氣相沉積，平面鈣鈦礦太陽能電池

SUMMARY

In this study, we demonstrate a two-step method to fabricate perovskite thin-film by thermal evaporation (PVD) and low-pressure chemical vapor deposition (LPCVD) process. Compare with solution and thermal evaporation process, we can have much smooth and dense PbI2 film with roughness lower than 50 nm by latter method. But this kind of thin-film is much harder to transfer to CH3NH3PbI3 by reaction with CH3NH3I, it is full coverage and less pin-hole existence. The power conversion efficiency of CH3NH3PbI3 perovskite solar cell reached 5.14% by 30 minute anneal with perovskite layer.

Key words: Thermal evaporation, Low pressure chemical vapor deposition, Planar perovskite solar cell

INTRODUCTION

Organometal halide perovskite (CH3NH3PbI3-xClx and CH3NH3PbI3) have recently attracted tremendous attention in this few years because of its low cost, high carrier life-time, high power conversion efficiency over 20%. At present, mesosuperstructure-type and planar-type cells are popular for research. The planar-type device architecture is particularly interesting due to the simple cell configuration and possible low-temperature fabrication on flexible substrates. In this work, we applied thermal evaporation and low pressure chemical vapor deposition technique to fine control perovskite formation. Regulate the deposition rate of PbI2 with 30 Å/s by control the voltage of thermal heater in first-step, apply single heating zone to provide uniform reaction environment for hybrid CH3NH3PbI3 perovskite film fabrication in the quartz tube in second-step. After the film formation completely, anneal for 30 minute in low pressure and N2 filled tube. Then we can have perovskite solar cells with 5.14% PCE by this PVD and LPCVD two-step method.

MATERIAL AND METHODS

The devices were prepared on cleaned ITO substrates by spin coating a PEDOT:PSS thin film on it. Then, the substrates were loaded into a high vacuum chamber
(base pressure<5×10−6 Torr) to evaporate PbI2 (150 nm) with rate of about 30 Å/s. CH3NH3I (Methylammonium iodide, MAI) powder was spread around PbI2 substrate in the graphite boat and quartz cover ,then transferred into quartz tube to control pressure of 1 Torr. The temperature of quartz tube increases to 120˚C with a slope of 2 ˚C/min followed by maintain the working temperature of 120˚C for 1.5 hour to fabricate the CH3NH3PbI3 light absorber (300~400 nm). Afterward, C60 (20 nm) and BCP (10 nm) was evaporation layer-by-layer with low evaporation rate of 0.1~0.4 Å/s to ensure the well thin film quality and good device performance. Finally, Al electrode was thermal evaporated with a thickness of 100 nm. The active area of the as-fabricated solar cell was 0.06 cm2 defined by a mask. For annealing process, the perovskite was set into the same system as LPCVD method without CH3NH3I powder in it, working temperature had changed to 90˚C and keeping heating for 30 minute.

RESULT AND DISCUSSION

Figure showed the SEM view of PbI2 thin-film with different thermal evaporation rate of 0.1 Å/s and 30 Å/s. Non-uniform PbI2 film are formed by slow evaporation rate and grain size is about 100~300 nm, film surface is compact but rough than fast evaporation film. For increase evaporation rate, the film exhibit lower surface roughness and much uniform grain size with 100 nm, a little pin-hole show up at film surface is visible. We prefer to choose pin-hole filled film to react with CH3NH3I vapor in order to easily diffuse, similar size between grain to grain is also helpful for the perovskite formation much uniform. Spin coating PbI2 film is the most roughness surface in our study, large pin-hole filled the entire perovskite layer that is a great phenomenon for second step reaction.
CH3NH3PbI3 perovskite can be obtained after we put PbI2 film into quartz tube to be in progress of LPCVD. Both reacted film of different rate can show the same pattern with main peak position at 14.1˚, 28.5˚ and 31.9˚ that can be assigned to (110), (220) and (310) diffraction peaks, respectively. We use the ratio of this three peak intensity to compare to the ITO peak intensity from substrate (at 30.2˚), solid line is higher than dashed line all over the three diffraction peaks, photoluminescence intensity had the same trend for solid line higher than dashed line, both results show that the fast-rate film is more suitable to formation a high quality perovskite film. Fig showed the J-V curves of perovskite solar cells formed by the two-step method, indicating that the fast rate film had the better performance for higher short-circuit current density and open-circuit voltage.
Perovskite active layer with thermal annealing could improve short-circuit current density for the solar cell devices, we consider that residual CH3NH3I would react with perovskite surface again. Modified surface had more uniform grain size, flat appearance and grain boundary became much clear, less residual material at junction cause carrier transformation more easily, so the current density had increase. An annealing cell had PCE of 4.7 % (best PCE of 5.14%) with a VOC of 0.86 V, a JSC of 8.35 mA/cm2 and FF of 0.66 is better than au unannealing cell had a JSC of 7.07 mA/cm2 lead to 3.84 % efficiency. Cell fabricated by slow rate PbI2 film could had only 2.98% even dealing with annealing process.

CONCLUSION

PbI2 fabricated by thermal evaporation process is more compact and uniform than the conventional solution method, collocated with low pressure chemical vapor deposition process can fabricate excellent morphology and large grain of perovskite. It can be better control by this slow reaction technique and the film can be tune accurately. We don’t need to use organic solvent when make a solar cell, environment damage can be reduced.

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