摘要
铜铟镓硒(Cu(In,Ga)Se2, CIGS)太阳电池产业化受到全世界广泛关注。作为高转换效率薄膜电池,其效率可与晶硅电池相比,目前最高效率达到23.35%。对于小面积实验室电池而言,研究重点是精确控制吸收层的化学计量比和效率;对于工业化生产而言,除化学计量比和效率外,成本、重现性、产出和工艺兼容性在商业化生产中至关重要。重点介绍了不同制备工艺、吸收层组分梯度调控、碱金属后沉积处理、宽带隙无镉缓冲层、透明导电层和柔性衬底等研究进展。从CIGS电池的效率来看,将实验室创纪录的高效电池技术转移到平均工业生产水平带来显而易见的挑战。
由于硅无毒、稳定和原材料储量丰富,其是最早用于制造太阳电池的材料。在组件生产中,机械强度、厚和刚性单晶硅晶圆(180 μm)作为吸收层,约50%的电池生产成本由晶圆成本决
无机半导体薄膜太阳电
铜铟硒(CIS)是一种三元化合物吸光半导体材料,属于I–III–VI2族,晶体结构为四方黄铜矿结

图1 CIGS太阳电池实验室最高认证效率
Fig. 1 Best laboratory certified efficiencies for CIGS solar cells

图2 (a-b) CIGS太阳电池结构和组件组件结构示意图,(c) CIGS吸收层中典型剖面能带示意图。前部和后部分别对应于缓冲层/CIGS和CIGS/Mo界
Fig. 2 (a, b) Schematics of the device structure of the small-area cell and the monolithically-connected module structure, respectively. (c) Schematics of the typical band profile in the CIGS absorber layer. EC and EV represent the energetic positions of the conduction band minimum and the valence band maximum, respectively. Front and back correspond to the buffer/CIGS and the CIGS/Mo interfaces, respectivel
CIGS太阳电池的工作原理如下:n型CdS缓冲层(Eg~2.4 eV)透过小于2.4 eV的光子到吸收层,从而在吸收层中产生电子-空穴对。然而,高能光子(≥2.4 eV)被CdS薄膜吸收,对光电流没有贡献。这就是异质结的“窗口效应”。如果CdS和i-ZnO很薄,会有部分高能光子穿过这些薄膜进入到CIGS吸收层中,在CIGS太阳电池起到窗口作用。由于p-n结界面(在CIGS/CdS之间)贯穿内置电场中,扩散长度区域内的电子从p型吸收层漂移到n型缓冲层,并被n型电极收集。同样地,空穴从n型层漂移到p型层,并被p型电极收集。通过调节Ga梯度(靠近Mo背电极)在CIGS层中产生的背表电场是一种额外的机制,它将电子漂移向p-n结处,最终由n型电极收集。背表电场降低了电池器件背面的少数载流子复合。
对于高效率CIGS太阳电池而言,磁控溅射技术是用于制备Mo、i-ZnO和TCO等薄膜。真空和非真空(化学浴沉积,CBD)工艺方法分别用于沉积CIGS吸收层和CdS缓冲层。p型CIGS吸收层是由浅缺陷铜空位(VCu)决定
采用各种沉积技术,CIGS薄膜可以沉积在刚性或柔性基底上。因此CIGS太阳电池除了在陆地上应用外,还可用于空间应用由于其具有很高的抗辐射能力。目前CIGS太阳电池商业化应用还有一些限制。在连续化生产过程中,成分均匀性是一个限制,从而在线监测是至关重要。另一个限制是CIGS太阳电池的大规模制造,多源共蒸发和后硒化设备的标准化。实验室小面积电池(23.35%)和商用组件(19.2%)之间存在很大的效率差
CIGS薄膜在450-600 ºC之间生长,以获得高质量的吸收层。尽管沉积方法种类繁多,但在实验室小面积和大规模生产中占主导地位的方法很少。这些沉积方法可分为三大类,即(a)共蒸发,(b)磁控溅射金属预制层后硒化/硫化,(c)非真空沉积技术。
多元共蒸发法是沉积CIGS薄膜使用最广泛也是最成功的方法,用这种方法成功地制备了高效率CIGS太阳电
一步法就是在沉积过程中,保持Cu、In、Ga、Se四蒸发源的流量不变,沉积过程中衬底温度和蒸发源流量变化见

图3 不同共蒸发工艺制备CIGS薄膜的示意图 (a)一步法,(b)两步法,其中第一层在较低衬底温度下沉积,第二层在较高衬底温度下沉积,(c)三步共蒸发工艺,其中铟和镓在第一和第三阶段沉积,而铜在第二阶段沉积. 引自文献[
Fig. 3 Schematic illustration of different co-evaporation processes: (a) single stage process, (b) bilayer or Boeing process in which the first layer was deposited at lower substrate temperature, and the second layer was deposited at a higher substrate temperature, (c) three stage process in which In and Ga are deposited in the first and third stage, whereas Cu was deposited in the second stage (after Ref. [

图4 Solar Frontier CIGS太阳电池吸收层的标准制造工
Fig. 4 Fabrication process of Solar Frontier’s baseline CIGS solar cell
通过考察薄膜的结构和不同温度下的组分,可以揭示薄膜的生长途径。其中最重要的两点是,晶体生长过程中存在着大量的参数和自由度,所采用的退火炉的独特性对晶体生长有很大的影响。因此,必须研究晶体生长途径,并开发一个独特的退火工艺,以获得高质量CIGS薄膜。后硒化工艺的优点是易于精确控制薄膜中各元素的化学计量比、膜的厚度和成分的均匀分布,已经成为目前产业化的首选工艺。与蒸发工艺相比,后硒化工艺中,Ga的含量及分布不容易控制,很难形成双梯度结构。因此有时在后硒化工艺中加入一步硫化工艺,掺入部分S原子替代Se原子,在CIGS薄膜表面形成一层宽带隙的Cu(In,Ga)S2。这样可以降低电池器件的界面复合,提高电池器件的Voc。
在高效率CIGS太阳电池中,吸收层需要低Ga含量[Ga/(Ga+In)~0.26]和Cu/(In+Ga)比在0.88∼0.92之间变

图5 (a) CIGS吸收层成分和带隙分布示意图,以解释器件模拟的条件,(b)不同表面[S]/([S]+[Se])成分和带隙最小值位置的器件模拟结果(xEg,min). 引自文献[
Fig. 5 (a) Schematics of the profiles of composition and bandgap in the CIGS absorber layer to explain the condition of the device simulation. (b) Device simulation results with varying surface [S]/([S]+[Se]) composition and position of the bandgap minimum (xEg,min) (after Ref. [
二十多年前,人们发现以钠钙玻璃为衬底的CIGS太阳电池的性能远优于其他衬底。研究表明,这是由于玻璃衬底中的Na进入CIGS薄膜中起到优化的作

图7 将Na掺入CIGS薄膜后,观察到的变化 (a)载流子密度增强、晶界钝化,(b)Ga偏析,(c)晶体取向发生变化。 引自文献[
Fig. 7 After Na incorporation into the CIGS film, the following changes were observed: (a) enhanced carrier density and grain boundary passivation, (b) gallium segregation, and (c) changes in crystallographic orientation (after Ref. [

图8 表面化学分析 (a)三个被研究的吸收层的示意图,KF吸收剂上的紫色层表示表面改性,(b-e)分别从CIGS吸收剂表面获得的Cu 2p3/2、In 3d5/2、Ga 2p3/2和Se 3s的XPS峰,无碱蒸发(无PDT),仅添加NaF,仅添加KF,(f)进行了KF-PDT的CIGS吸收层的溅射显示出在大约20 nm处出现了Cu 2p3/2峰,其强度与无PDT的情况相似,(g)显然,在大约20 nm的深度处可测量K,(h)通过CdS层溅射后测量的两个吸收层的示意图,(i)在CdS/CIGS界面上,仅添加NaF和仅添加KF时,Cu 2p3/2、In 3d5/2、Ga 2p3/2和Se 3s的XPS峰,(m)不同溅射深度下K的XPS光谱. 引自文献[
Fig. 8 Surface chemical analysis (a) Schematic view of the three investigated absorbers. The purple layer on the KF absorber indicates the modified surface composition, (b-e) XPS peak of Cu 2p3/2, In 3d5/2, Ga 2p3/2 and Se 3s, respectively, obtained from the surface of CIGS absorbers with no alkali evaporation (no PDT), only NaF addition and only KF addition, (f) Sputtering of the CIGS absorber subjected to KF-PDT shows the appearance of the Cu 2p3/2 peak within the first approximately 20 nm with similar intensity as in the case of no PDT, (g) K is clearly measurable at the surface up to a depth of approximately 20 nm, (h) Schematic view of two absorbers measured after sputtering through the CdS layer, (i) XPS peak of Cu 2p3/2, In 3d5/2, Ga 2p3/2 and Se 3s, respectively, at the CdS/CIGS interface with only NaF addition and only KF addition, (m) XPS spectra of K at different sputtering depths (after Ref. [

图9 在低温共蒸发CIGS薄膜上应用NaF和NaF与KF后退火的示意
Fig. 9 Schematic drawing of the NaF PDT and the NaF&KF PDT applied on low-temperature coevaporated CIGS thin films
到目前为止,高效率CIGS太阳电池通常采用传统化学浴沉积CdS薄膜作为缓冲层来实现。CdS似乎满足缓冲层的大多数要求,具有与吸收层和本征杂氧化锌相匹配的适当导带,并且具有有益的界面缺陷化学特性。据报道,带正电荷的Cd可能在贫铜CIGS表面形成稳定的施主型缺陷,导致适当的电荷密度和良好的费米能级位置。然而,CdS是有害由于Cd是重金属有毒元素。因此,CIGS太阳电池正在寻找无镉缓冲层材料,如ZnS和In2S3。一般无Cd缓冲层的带隙材料比Cd高;大带隙无镉缓冲层改善了CIGS太阳电池的蓝色响应,从而改善了电池的Jsc,如

图10 不同缓冲材料的CIGS太阳电池的外部量子效率,所有电池都具有抗反射层。曲线下方的阴影区域表示相对于相应CdS参考的当前增益. 引自文献[
Fig. 10 External quantum efficiency for CIGS cells with different buffer materials. All cells with anti-reflection coatings. The shaded areas below the curves represent the current gain relative to the corresponding CdS reference when available (after Ref. [
其他几个无镉缓冲层,如Zn1-xMgxO和ZnxSn1-xOy等,作为CIGS太阳电池中的异质结进行了研究。当CIGS太阳电池器件采用无镉双缓冲层Zn(O,S,OH)x/Zn0.8Mg0.2O时,最高效率为23.35%(Voc = 734 mV,Jsc = 39.6 mA c
在缓冲层上沉积一层高透明、合适的带隙(3.3 eV)和n型氧化物(比如,TCOs)薄膜,作为前电极,也称为窗口层,因为它将光透射到产生载流子的CIGS吸收层。光生电流横向传输到TCO前电极而不造成明显光电流损失的必要条件。典型的双层ZnO薄膜由非常薄的本征ZnO层(100 nm)和n型Al或In元素掺杂ZnO层(300 nm)组成。本征ZnO层在CdS和掺铝ZnO(ZnO:Al)之间提供隔离层作用,以便本征ZnO层防止Al扩散到吸收层。通常采用高导电性ZnO:Al、ZnO:B、ZnO:In或ZnO:Ga作为TCO导电
TCOs的选择是基于其导电性、透光率、水分子稳定性以及与后续加工的兼容

图11 (a)对比了AZO和BZO导电薄膜的光学性能,非封装条件下不同TCO导电薄膜材料的湿热稳定性(85 ℃,相对湿度85%).引自文献[
Fig. 11 (a) Comparison of the optical properties of AZO and BZO with comparable sheet resistance. The high transmission in the near-infrared region for BZO stems from the reduced carrier density (AZO n=4.4*1
CIGS薄膜技术主要是在玻璃基板上发展起来的,长期以来,沉积在塑料或金属箔等柔性基板上的CIGS太阳电池无法达到类似的效率。由于杂质扩散或衬底的选择对生长温度的要求较低是造成这种效率差距的主要原因。柔性基板上太阳电池为构建一体化开辟了新的市场,如

图12 柔性CIGS太阳电池组件与建筑一体化等光伏应用产
Fig.12 Photovoltaic application products such as flexible CIGS thin film solar cell template and building integratio
这些柔性基底可分为三种类型:(i)金属箔,如钛、钼、铝、铜和不锈
对于低成本生产而言,高效率和大规模生产且高产量是产业化关键因素。对于商业化,长期稳定性是重要因素之一。柔性太阳电池存在一些问题:(i)缺乏满足最低要求的柔性基板,例如具有合适的物理、化学和机械性能;(ii)金属和特殊聚合物被用作柔性基材。铜和铝等金属能经受高温加工。然而,它们具有密度高、热膨胀系数大和粗糙度高等优点。热膨胀系数因制造商而异;(iii)大多数金属(例如钢)含有杂质。由于杂质元素从柔性(金属)箔扩散到吸收层而造成的污染对电池性能是有

图13 (a)四端钙钛矿/CIGS叠层太阳电池结构的示意图,(b)带有和不带有滤波的叠层太阳电池的J-V曲线,反向和正向扫描曲线(扫描速度为50 mV
Fig.13 (a) Schematic of the 4-T perovskite/CIGS tandem solar cell. (b) J-V curves of the CIGS cell with and without filtering, and reverse and forward scanning curves (scanning rate of 50 mV
为了降低热损失,为了更好地利用太阳光谱,串联使用两个或多个不同带隙的太阳电池,以达到更高的转换效

图14 钙钛矿/CIGS叠层太阳电池的性能。(a)钙钛矿/CIGS叠层太阳电池的原理图和截面SEM图,(b)J-V曲线和最高转换效率,(c)钙钛矿/CIGS叠层太阳电池的EQE光谱,(d)钙钛矿/CIGS叠层太阳电池稳定性试验. 在持续的1个太阳光照射和在30°C的周围环境下跟踪最大功率点的情况下,经过500小时的老化后,未封装的器件保持其初始PCE的88%。插图显示,该器件在12 h的静止时间后无需负载和照明,即可恢复其初始性能的93%. 引自文献 [
Fig. 14 Performance of the perovskite/CIGS tandem cells. (a) Schematic and cross-sectional SEM image of the monolithic perovskite/CIGS tandem solar cell. (b)J-V curve and efficiency at the maximum power point (inset) of the champion tandem device. (c) EQE spectra for the subcells of the monolithic perovskite/CIGS tandem solar cell. (d) Stability test of the monolithic perovskite/CIGS tandem solar cell. The unencapsulation device maintained 88% of their initial PCE after 500 hours of aging under continuous 1-sun illumination and maximum power point tracking at 30 °C ambient environment. The inset shows that the device can recover 93% of its initial performance after a 12-hour resting period without load and illumination (after Ref. [
美国加州大学洛杉矶分校工学院杨阳教授团队与日本Solar Frontier公司的ARC研究中心合作,运用双层叠层技术,研发出转换效率高达22.43%的钙钛矿/CIGS叠层太阳电
影响CIGS太阳电池效率的因素包括化学计量比、晶粒尺寸、表面形貌、吸收层中的缺陷及其界面。CIGS薄膜的粗糙度小于50 nm降低CIGS/CdS界面的表面积,从而降低了界面复合和反向饱和电流。然而,光滑薄膜的一个缺点是增加了薄膜的反射。通过带隙工程可以提高载流子收集效
为了改善Jsc,需要宽带隙缓冲层来增强光透射到吸收层,以产生更多的光生载流子。因此,由于Zn(O,S)的带隙比CdS大,所以用Zn(O,S)缓冲层代替CdS。同时,空间电荷区Ga浓度的增加也改善了Voc。在硒化过程中,由于Ga向CIGS薄膜背面偏析,导致Voc较低。为了提高Voc,采用了后硫化以加宽表面CIGS的带隙。除两步硒化/硫化工艺外,还进行了氟化钾沉积后处理,以达到改善Vo
对于带隙为1.14 eV的CIGS吸收层,理论效率极限为33.5
对于1000 MW/年的产能(组件效率15%),CIGS组件生产成本预计为0.34美元/每
(a) CIGS是一种四元化合物;大面积衬底上均匀性(如厚度和成分)是一个问题。均匀性是高效太阳电池电学和光学性能的主要要求。大面积的再现性和均匀性直接影响产量。对于大面积衬底而言,磁控溅射沉积是合适的。
(b) 在大规模生产中,电池效率较低。需要一种无损在线技术来评估CIGS薄膜的厚度和组成。
(c) 商业化的主要挑战是将实验室技术扩大到工业生产,同时保持高效率。实验室效率(23.35%)与商业组件效率(19.2%)存在较大差
(d) 如果非真空和真空工艺(例如CBD沉积CdS,真空技术沉积CIGS)都涉及到CIGS太阳电池的制造,那么不同工艺之间的基板处理成为一个问题。此外,流程之间的输出差异是另一个问题。这可以通过对所有薄膜沉积采用真空工艺来避免,即磁控溅射技术。
(e) 由于镉是有毒的,生产镉废液会对环境造成不利影响,并对人体产生毒害作用。需要开发无镉缓冲层CIGS太阳电池。
(f) 铟主要是作为锌的副产品获得的,其产量受到锌生产的限制。铟需求可能会随着CIGS大规模生产而增加,价格可能会因供需缺口而上涨。为了解决这个问题,器件中应该使用小于1 μm厚CIGS薄膜,而不损害Jsc。
(g) 减少CIGS和TCO层的生长时间。这可以通过使用更薄的CIGS层和TCO来实现高速生长。
(h) 含EVA/玻璃的CIGS组件性能表现出退化,因为组件性能对水蒸气敏感;主要原因是增加了氧化锌的电阻和钼的腐蚀。为了避免湿气进入,需要合适的边缘密封,如粘合性、耐久性、低湿气透过率和热稳定
(i) 激光划片后出现清洗问题,用于单片集成的串联互连。这个问题可以采用短脉冲宽度激光来解决。
(j) 梯度带隙CIGS改善了光学和电学性能。一个高产能可以降低生产成本。
(k) 对于高效实验室小面积电池,使用静态沉积工艺(固定了基板位置)。另一方面,在商业化制造中,动态沉积工艺(在线工艺中衬底在运动)用于生产大面积太阳电池。静态和动态过程的生长动力学不同。因此,小面积高效率不能在大面积上再现。在这两种情况下,沉积过程应该相同。
进一步展望CIGS太阳电池的未来,需要对材料和界面性能的优化进行研究,以提高效率。对于大面积、商业化生产,需要标准化多源蒸发设备和两阶段硒化/硫化工艺。目前,高效CIGS太阳电池中使用的吸收层厚度约为3 μm,沉积时间约为60 min。工业生产中,需要10 min左右的沉积时间,才能保证高产出,同时又不降低器件性能。此外,对于两阶硒化/硫化技术,需要更快的硒化过程。为了降低生产成本,CIGS薄膜厚度应该降低到1 μm左右,同时不降低器件性能,特别是长波区域的Jsc损耗。高Jsc要求更高带隙无镉缓冲层(比CdS高)。为使CIGS吸收层获得理想的带隙(1.4 eV),应该在不影响器件性能的前提下增加CIGS中Ga的量。
总结了CIGS太阳电池的吸收层材料性能、最新进展以及各种生长技术。为了获得高效率CIGS太阳电池,Ga/(Ga+In)和Cu/(Ga+In)的比值应该分别在0.26和0.88∼0.92之间变化。当Ga/(Ga+In)比值超过0.3,薄膜中的缺陷数量将会增加。为了获得高FF,S/(S+Se)比值应小于0.61。高效率CIGS太阳电池由贫Cu吸收层制成由于CIGS/CdS界面复合减少。为了提高效率,通过加入Ga,吸收层的能带可以从1.04 eV增加到1.14 eV。然而,如果能带大于1.14 eV,则电池器件的中间隙缺陷/复合增加,导带的不连续从一个小尖峰增加到一个悬崖。吸收层中的带隙梯度改善了电池性能。正面梯度(靠近CdS)提高Voc,背面梯度(靠近背面触)提高载流子收集效率和Jsc。此外,钠从玻璃衬底扩散到吸收层提高了实验室电池的效率。在工业生产中,采用三步共蒸发工艺。实验室电池效率(23.35%)与商业组件效率(19.2%)存在较大差异。由于实验室电池器件采用静态沉积工艺(固定基片的位置),而在工业生产中采用动态沉积工艺,因此实验室规模的高效率不能大面积复制;效率的差异是由于生长动力学的差异造成的。在实验室电池中使用大约3 μm厚度CIGS吸收层;生长需要大约60 min。对于高产出,1 μm厚度CIGS薄膜大约需要10 min的沉积时间,而不会降低器件性能。
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