Thesis (M.S.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains v, 37 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 33-37).

With the rapid increase in energy demand and the rise of CO2 levels due to traditional energy production from fossil fuels, it is critical to the transition to a sustainable and renewable energy sources. Recently, photovoltaic technology has been raised as a promising alternative to fossil fuel energy production. Solar cells, predominately crystalline silicon technology, are currently 3.6% of electricity production. To maintain this progress, coupling the perovskite and silicon in tandem devices has enormous potential to increase the efficiency of solar energy production, where perovskite solar cells emerged as a promising technology. Textured silicon solar cells are a well-established technology; keeping the advantage of this technology, it is crucial to employ the perovskite to be a compatible top cell for silicon-based tandems. Here, we optimize the silicon bottom cell by understanding the influence of temperature, time, and etchant concentration on the optical properties and performance of the device. Then, we investigate the impact of the textured silicon on the optoelectronic properties of perovskite. Using hyperspectral imaging, we demonstrate that different texturing substrates influence the PL of perovskite, which is associated with the thickness of the perovskite. Lastly, we explored the delamination of the devices due to the weak adhesion between C60/SnO2 after the deposition of IZO and MgF2, which was found to be caused by the deposition conditions. The high temperature and power density caused a weak adhesion between C60/SnO2. Overall, these findings will help to alter the design of Perovskite/Silicon tandem devices to accelerate the commercialization of tandem technology.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Semiconductor multijunction solar cell is a cutting-edge photovoltaic technology aimed at developing a frontier solution to the clean energy demand and environmental problem. Due to the efficient photoabsorption and energy conversion in the visible and near-infrared spectral ranges of the solar spectrum, the multijunction solar cell structures have shown an unprecedented application potential by demonstrating a solar conversion efficiency of over 44 %. Among various multijunction solar cell structural designs, the GaxIn1-xP/GaAs double-junction tandem structure is considered as the most fundamental building block for developing the industry-standard triple- and even more junction photovoltaic cells with super high efficiency. Therefore, obtaining a better and more in-depth understanding of physical properties of the GaxIn1-xP/GaAs double-junction tandem device structure, especially some fundamental optoelectronic processes in the individual structural layer, including photoexcitation, transport and the mid-way recombination of charge carriers, is crucial for further improving the energy conversion efficiency. In this thesis, the mid-way radiative recombination, diffusion transport, localization mechanism, and photocurrent spectra of charge carriers in the GaxIn1-xP/GaAs double-junction tandem solar cells grown on GaAs substrates with different misorientation angles were investigated in detail. Our main findings are summarized as below. Efficient radiative recombination of carriers in the GaxIn1-xP/GaAs double-junction tandem solar cell samples was demonstrated by using electroluminescence (EL) and photoluminescence (PL) techniques. The radiative recombination intensity was shown to be dependent on the intrinsic material-related parameters such as the doping concentration, growth thickness and the substrate misorientation angle both experimentally and theoretically. The radiative recombination was thus revealed to be an important loss channel of carriers in the GaxIn1-xP/GaAs double-junction tandem solar cells. Super strong transverse diffusion of minority carriers in the top GaxIn1-xP subcell was found by the micro-EL image surveying. Theoretical simulation on the experimental data shows that the minority carrier diffusion length is as long as ~93 μm at a forward bias of 2.75 V, which is ~30 times longer than that of unbiased GaxIn1-xP epilayer. Origin of this super transverse diffusion was argued, and its influence on device performance was also discussed. Significant correlations of carrier localization and luminescence behaviors with the substrate misorientation angle in the top GaxIn1-xP subcells were unveiled by excitation intensity- and temperature-dependent PL. The large difference in potential energy profile of GaxIn1-xP layers, caused by the different degrees of atomic ordering, was argued to interpret the observed PL distinctions. Vertical transport and photoresponse mechanisms of charge carriers in the GaxIn1-xP/GaAs double-junction tandem solar cells were studied by temperature- and reverse bias-dependent photocurrent (PC) spectroscopy. Both the temperature and reverse bias were shown to have significant impact on the device photoresponse, in particular on the photoresponse due to the absorption of photons with energy above the bandgap of GaAs and GaxIn1-xP, namely the supra-bandgap photoresponse. A model was proposed to simulate the observed temperature- and reverse-bias dependence of the supra-bandgap photoresponse. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Presented here is an analysis of some of the existing methods used for the determination of the series resistance of solar cells which is one of the key parameters in solar cell fabrication and technology together with the diode ideality factor and the reverse saturation current. These methods are based on the network analysis of the single-exponential lumped constant parameters model which has been accepted as being operationally sufficient to describe the current-voltage characteristics of the solar cell. The methods analyzed in this study are divided into two main groups. Methods using two I-V characteristics and methods using a single I-V output curve. For comparison purposes, all methods are applied first using data extracted from existing I-V curves and then using in-lab measurements of a commercial solar cell. It is demonstrated that the determination of the series resistance of solar cells using two I-V characteristics has several advantages over methods using a single I-V output curve. It becomes evident that methods which use a single I-V output curve are only accurate for cells operating under very high illumination conditions. At normal intensities, however, such methods result in erroneous R(s) values. This is due to the assumption of a constant diode ideality factor along the entire I-V output curve used in the derivation of these methods. It is shown that this assumption is inaccurate at normal intensity levels and can be appropriate only under very high illuminations. Three new methods are proposed in this study. One of the methods presents a new approach in determining the solar cell equation parameters. The new approach relies upon treating the diode ideality factor of the solar cell as a variable that is a function of both the terminal current and the light intensity level. The method uses two I-V output curves at different illumination levels in determining all solar cell parameters: The series resistance, the diode ideality factor and the reverse saturation current. Although somehow tedious, the new approach shows that, for accurate modeling of solar cells and prediction of illuminated characteristics at different light levels based on the single-exponential model, the diode ideality factor should be treated as a variable while the series resistance is held constant. Comparison between all methods is presented and a reasonable judgement and recommendations concerning the best method to be used are given.

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銅銦鎵硒薄膜太陽能電池是一種清潔、環保的發電技術。 最近，銅銦鎵硒太陽能電池實現了20.9%的光電轉換效率，超出了多晶硅太陽能電池所保持的20.4%的薄膜太陽能電池的最高紀錄。 多種技術改進促成了這項薄膜太陽能電池的新紀錄。 其中一種重要改進是將1 到2 微米厚的硫化鎘硫化鋅混合窗口層替換成薄層硫化鎘和摻鋁氧化鋅透明導電層。 / 基於本實驗室在生長高質量銅銦鎵硒吸收層的先進技術，本工作重點研究了位於吸收層和透明窗口層之間的緩衝層和高阻窗口層。 這兩層的常規結構是由化學水浴法生長的硫化鎘層和本征氧化鋅層組成。 本論文的第一部分是關於這種常規結構的參數優化。 經過優化，本實驗室實現了在小型組件（總面積60 平方釐米）上15.6%的最高轉換效率。 / 本論文的第二部分關於用化學水浴法生長緩衝層。 我們發展了一種新型生長制備，用於避免氣泡和孔洞在吸收層表面的形成。 表面形貌測試結果顯示，使用此種設備生長的緩衝層能均勻的覆蓋銅銦鎵硒吸收層的表面。 其它硫化鎘的生長參數也根據新設備的特點進行了優化。 優化結果顯示，在空間電荷區的復合對電池轉換效率影響較大，而這種復合損失可以經過調整緩衝層與吸收能之間能帶結構得到減少。 我們研究了另外一種用化學水浴法生長的緩衝層：硫化鋅。 硫化鋅是一種無毒的寬禁帶材料，在短波部分有較少的光吸收。因此，它是一種很好的硫化鎘替代物。 我們研究了在不同生長溫度下的生長動力學機制。 最優的生長溫度是95 攝氏度。 經過生長結束後的退火過程，硫化鋅的禁帶寬度由3.61eV 下降到3.2eV。 再經過在氧氣環境中的退火，禁帶寬度可由3.2eV 繼續下降到2.9eV。 在單結電池中，硫化鋅的最優厚度在43 納米到62 納米之間。 在此厚度範圍中，具有硫化鋅緩衝層的電池實現了相對於具有硫化鎘緩衝層的電池更高的轉換效率。硫化鋅電池實現了與硫化鎘電池相近的開路電壓。 此項改進主要是由於在高溫條件下生長的硫化鋅與銅銦鎵硒層形成了更合適的能帶結構。 / 本論文的第三部分是關於用共濺射的方法生長鋅鎂氧化物緩衝層。 實驗結果顯示，鋅鎂氧化物的晶體結構和禁帶寬度與鎂含量相關。 當鎂含量小於0.4 時，鋅鎂氧化物具有(002)從優方位的纖鋅礦結構。 晶體質量隨鎂含量的增加而降低，同時，鋅鎂氧化物的禁帶寬度隨鎂含量的增加線性增加。 對於濺射方法生長的緩衝層，吸收層的表面鈍化對提高轉化效率非常重要。 / 本論文的最後一部分是關於高阻窗口層的研究。 相比於由本征氧化鋅構成的高阻窗口層，由鋅鎂氧化物構成的高阻窗口層能使電池有更優的穩定性。對於單結電池，本層的最優厚度是50 納米。對於小型組件，最優厚度在100 納米左右。 關於鎂的最優組分，結果仍爭議，但可以確定的是由較高濺射功率（大於2.2 瓦每平方釐米）產生的濺射損傷是應當盡量避免的。關於光照產生的亞穩定性的研究表明，亞穩定性強度與濺射環境中的氧氣含量正相關。 相對於無氧氣摻雜的電池，通過將1%的氧氣摻入氬氣濺射環境中，電池效率提高了0.5 個百分點。 / Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been regarded as a promising technology for cheap and environmentally friendly electricity generation. CIGS based solar cell has achieved 20.9% conversion effciency, while the offcial record for multicrystalline Silicon solar cell is 20.4%. A series of improvements have lead to this record for thin film based solar cell. An important improvement originated from the replacement of 1- to 2-um-thick doped (Cd,Zn)S layer by a thin, undoped CdS and a transparent conductive oxide(TCO). / Based on our techniques on growing high quality CIGS absorber layer, this work focuses on further optimization of buffer layer and high resistance window layer located between the CIGS absorber and the TCO window layer. The standard buffer structure includes a chemical-bath-deposited CdS layer and an intrinsic ZnO layer. The first part of this thesis is about optimization of this standard structure carried out in our laboratory. The best conversion effciency achieved on mini-module with total area of 60 cm² is 15.6 %. / The second part is about the fabrication of alternative buffer layers by chemical bath deposition. New deposition equipment has been invented to eliminate stationary bubbles and uncovered pinholes on absorber surface in the deposition of CBD CdS. Surface morphology studies shown that the buffer layer grown by this equipment has uniform coverage on the CIGS surface. Other deposition parameters in the chemical bath deposition of CdS buffer layer have been systematically studied employing this new equipment. Our results suggest that the detrimental effect of recombination in SCR region can be mitigated by proper band alignment in the buffer/absorber interface. / Another buffer layer grown by CBD method is ZnS. Because the wider bandgap and less light absorption in short wavelength range, ZnS is a good candidate to replace the toxic CdS buffer layer. The growth kinetics under different deposition temperature have been studied. The optimal temperature profile has been achieved by setting temperature at 95°C. The results of post annealing after deposition indicate that the bandgap energy of CBD ZnS decreases from 3.61 eV to 3.2 eV by annealing in vacuum. A further decrease from 3.2 eV to 2.9 eV could be caused by annealing with oxygen gas. The optimum thickness of ZnS used in single solar cells is between 43nm and 62nm. In this range, devices with CBD-ZnS buffer layer have achieved higher conversion effciency than CBD-CdS buffer layer solar cell. The open circuit voltage for ZnS-buffer devices has approached the value with CdS-buffer. The improvement is mainly due to proper band alignment of ZnS/CIGS interface achieved under high deposition temperature of CBD process. / The third part of this thesis is to study how to deposit (Zn,Mg)O buffer layers by co-sputtering method. It was found that the crystalline structure and optical bandgap of sputtered (Zn,Mg)O varies with Mg concentration. (Zn,Mg)O thin films with Mg concentration less than 0.4 have preferential orientation with a wurtzite phase (002). The crystal quality decreases with increasing Mg concentration and the band gap of the (Zn,Mg)O films has a linear relationship with the Mg concentration in this range. An interesting finding to emerge from this study is that oxygen passivation of absorber surface is critical to improve device performance with (Zn,Mg)O buffer layer deposited by sputtering method. / The last chapter assesses the effect of replacing high resistance window layer with (Zn,Mg)O in devices with CBD-ZnS buffer layer. Compared to devices with i-ZnO (high-resistance window) HRW layer, better device stability has been confirmed on solar cells with (Zn,Mg)O HRW layer. For single cells, the optimum thickness of HRW layer is about 50 nm, and the optimum thickness for mini-modules is around 100nm. Although no conclusion can be drawn with the optimum Mg concentration, the sputtering damage caused by sputtering power density higher than 2.2 W/cm² should be avoided. It was also shown that the metastability effect activated by illumination has positive correlation with the number of energetic oxygen ions in sputtering process. Compared to devices without oxygen doping, a higher effciency (increase of 0.5 % unit) has been achieved by the oxygen/argon doping ratio of 1 %. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Zhu, Jiakuan = 基於銅銦鎵硒薄膜太陽能電池的緩衝層結構研究 / 朱家寬. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 121-134). / Abstracts also in Chinese. / Zhu, Jiakuan = Ji yu tong yin jia xi bo mo tai yang neng dian chi de huan chong ceng jie gou yan jiu / Zhu Jiakuan.

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