太阳能电池是基于光生伏特效应原理, 将太阳光直接转变为电能的光电器件。在各种不同类型的太阳能电池中, 钙钛矿太阳能电池(Perovskite Solar Cells, PSCs)的发展最为迅速, 2009年首次被报道, 目前实验室单结电池器件的最高认证PCE已经高达26%[1], 超过了碲化镉(CdTe)和硒化铜铟镓硒(CIGS)等传统半导体薄膜电池, 甚至接近了传统晶硅太阳能电池(Si)的最高纪录, 引领了新一代光伏技术的发展。
尽管PSCs展现了优异的光电性能, 其商业化进程仍存在较多问题和挑战, 如器件的迟滞效应、铅毒性、离子迁移现象、大面积薄膜的均匀性以及器件稳定性等。其中, 器件稳定性是衡量太阳能电池能否长期运行的一个重要指标, 也是目前产业化过程中最需要解决的问题。据报道[2-3], PSCs稳定性主要受两方面因素影响: (1)钙钛矿的本征稳定性。钙钛矿材料具有软晶格特性, 在光、水、氧等外界条件作用下, 容易发生分解, 进一步加剧离子迁移, 进而造成器件性能衰减。(2)钙钛矿器件的其他功能层材料(传输层材料和电极材料)。其中传输层材料与钙钛矿材料直接接触, 其本征稳定性以及光电性能也是影响PSCs稳定性的关键因素。因此, 有必要开发并应用高稳定性的传输材料来提升器件的稳定性, 尤其是探索无机空穴传输材料(Hole Transport Materials, HTMs)。本文讨论了无机HTMs在PSCs中的应用研究进展, 并重点介绍了主要无机HTMs对于PSCs性能和稳定性的影响。此外, 还总结了针对无机HTMs器件的性能优化策略, 包括元素掺杂、添加剂工程和界面工程等。最后,针对目前无机HTMs在PSCs中面临的挑战, 展望了未来的发展方向, 以期为研究者们深入理解无机HTMs在PSCs中的作用及实现方法提供参考和思路。
1 钙钛矿太阳能电池
1.1 器件结构
图1
图1
(a)正式(n-i-p)型结构; (b)反式(p-i-n)型结构
Fig. 1
(a) Formal (n-i-p) structure and (b) inverted (p-i-n) structure
正式器件中, HTL位于钙钛矿和电极之间, 需要具有合适的能级位置、可低温制备和良好的工艺兼容性, 一般采用有机材料作为HTMs, 如2,2′,7,7′-四[N, N-二(4-甲氧基苯基)氨基]-9,9′-螺二芴(Spiro-OMeTAD)等, 器件性能比较优异, 但稳定性仍然欠佳。而反式器件中, HTL的主要作用是传输空穴并阻挡电子, 位于钙钛矿和导电基底之间。因此, 除了合适的能级位置, 一般还需要具有较高的可见光透过率。目前大多采用的是无机HTMs, 如氧化镍(NiOx)和氧化铜(CuO)等。其稳定性方面具有一定优势，但是本征光电特性不足, 光电性能并不理想。
1.2 空穴传输材料
按照化学性质, HTMs大致可以分为有机材料和无机材料两种类型。常用的有机HTMs按照官能团, 又可以细分为共轭聚合物、共轭聚电解质和共轭自组装小分子等[7]。可低温制备、价带能级合适、成膜均匀和工艺简单等优点是研究者们选择有机HTMs的主要原因。目前高效PSCs大多都是基于有机HTMs, 例如Spiro-OMeTAD[8⇓-10]和聚[双(4-苯基)(2,4,6-三甲基苯基)胺](PTAA)等[11-12]。这些材料一般需要引入锂盐(Li-TFSI)、钴盐和4-叔丁基吡啶(TBP)等添加剂来提升导电性(>1×10−3 S·cm−1)[13], 进而提高PCE。然而温度超过90 ℃,掺杂剂会蒸发, 导致Spiro-OMeTAD不可逆地降解, 从而造成器件性能严重衰退。此外, 锂盐等添加剂具备一定吸湿性。Li离子虽然只带一个正电荷, 但是离子半径特别小, 具有很大的离子势, 很容易通过与水结合来降低自身的能量(水合能大)。这往往使基于Spiro-OMeTAD的器件在高湿度环境下稳定性欠佳。更重要的是, 有机HTMs在持续光照和较高温度条件下容易断裂或变形, 从而产生大量离子迁移通道, 使得钙钛矿材料出现分解、组分损失、晶格坍塌、相变等一系列问题, 最终导致PSCs的光电性能衰减[14]。
相对于有机HTMs, 无机HTMs主要是通过共价键结合, 因此面对水分、氧、热、光照射和电场的长期侵蚀, 仍可以保留较高的光电特性,避免钙钛矿材料进一步降解[6]。此外无机HTMs还具备相对良好的导电性(>1×10−3 S·cm−1)和空穴浓度(>1×1018 cm-3), 材料制备成本也较低。
2 无机空穴传输材料
表1 无机空穴传输材料的基本性质(Spiro-OMeTAD作为对比)
Table 1
| Material | Hole concentration, N/cm-3 | Hole mobility, μ/(cm2·V-1·s-1) | Conductivity, σ/(S·cm-1) |
|---|---|---|---|
| Sprio-OMeTAD with Li-TFSI, etc. | 7.13×1015[19] | 0.779[19] | 1.53×10-3[13] |
| NiO | 5.3×1018[20] | 0.12[20] | 1.66×10-4[21] |
| Cu:NiO | 7.3×1019[22] | ~0.2[22] | 1.25×10-3[23] |
| Ni0.8Li0.05Mg0.15O | 6.46×1018[24] | - | 2.23×10-3[24] |
| CuGaO2 | 3.098×1019[25] | - | 4.625×10-3[25] |
| Zn:CuGaO2 | 1.328×1020[25] | - | 1.39×10-2[25] |
| CuCrO2 | - | 0.1-1.0[26] | 2.9×10-2[27] |
| In:CuCrO2 | 7.1×1018[27] | 0.75[27] | 6.9×10-2[27] |
| CuScO2 | - | - | 2.11×10-3[28] |
| CuSCN | - | 1.2×10-3[21] | - |
| Co3O4 | - | 1.49×10-2[29] | - |
| Co3O4-SrCO3 | - | 6.33×10-2[29] | - |
图2
图2
代表性的无机空穴传输材料的最高占据分子轨道(HOMO)(或价带)和最低未占据分子轨道(LUMO)(或导带)能级(Spiro-OMeTAD作为对比)[18]
Fig. 2
Highest occupied molecular orbital (HOMO) (or valence-band) and lowest unoccupied molecular orbital (LUMO) (or conduction-band) energy levels relative to the vacuum of representative inorganic hole transport materials (HOMO and LUMO of Spiro-OMeTAD for comparison)[18]
目前常用的无机HTMs大致可以分为镍基氧化物、铜基氧化物、铜铁矿类氧化物和非氧化物等, 下文分别进行介绍。
2.1 镍基氧化物
镍基氧化物NiOx作为HTMs, 起初广泛应用于有机太阳能电池、染料敏化太阳能电池等光电器件中, 具有空穴迁移率高、透光率高和热稳定性良好[24⇓⇓⇓⇓⇓-30]等优点。2014年NiOx基PSCs首次被报道[31], 但当时NiOx薄膜和钙钛矿光吸收层的工艺较差, 器件的PCE仅达到7.8%。随后, Yang课题组[32]采用溶胶-凝胶法在500 ℃下制备NiOx层, 具有多面和波纹表面的NiO薄膜能够形成连续且致密的良好结晶的MAPbI3层, 器件PCE进一步提升到9.11%。为了获得高质量的NiO膜, Han课题组[33]采用喷雾热解工艺, 制备了一层超薄NiOx致密层(10~20 nm), 并在其上设计一层介孔氧化铝(meso-Al2O3)支架,该复合结构具有电子阻挡效应和高的光学透明度, 可以降低光的寄生吸收和界面复合, 器件的最佳PCE提升至13.49%。为进一步提升NiOx薄膜的电导率, 2015年, Han课题组[21]又开发了p型重掺杂Ni0.8Li0.05Mg0.15O(NiLiMgO)。电导率从参比样品的1.66×10−4 S·cm−1提升到2.32×10−3 S·cm−1, 并且薄膜更平整、针孔更少(图3(a)), 基于NiLiMgO的器件具有更低的串联电阻(Rs)和更高的并联电阻(Rsh)。最终, 1 cm2孔径面积的PSCs的PCE达到15%, 器件在持续光照1000 h后, 仍保留初始PCE的90%, 这在当时是非常优秀的结果。进一步提升NiOx的器件性能, 需要更好地优化NiOx能级, He教授[34]采用化学沉淀法制备NiOx纳米晶, 并利用p型掺杂剂1,3,4,5,7,8-六氟四氰基萘并醌二甲烷(F6TCNNQ)提高了NiOx的功函数, 将价带顶从-4.63 eV提升到-5.07 eV, 并降低了NiOx和钙钛矿之间的能级偏移, 显著提高了空穴提取效率, 降低了NiOx/钙钛矿之间的界面接触电阻。最终, 基于Cs/FA/MA(FA: NH2CH2=NH2+, MA: CH3NH3+)三元阳离子钙钛矿和基于MAPbI3的器件的PCE分别达到20.86%和19.75%(图3(b))。除了NiOx的电导率以及能级匹配可以进一步改善之外, NiOx表面的化学状态较为复杂, 存在Ni2+、Ni3+以及化学反应性羟基NiOOH等, 想要获得超过20%的器件性能, 修饰和改性是必不可少的环节。2022年, 基于NiOx的反式PSCs, Liu课题组[35]提出了一种表面氧化还原工程(SRE)的方法, 具体包括Ar等离子体引发的NiOx薄膜氧化过程和Brønsted酸介导的还原过程。其中在氧化作用下, 高能氩等离子体使NiO和Ni(OH)2从低价态转变到高价态Ni≥3+。Ni3+和Ni4+的浓度随之增大, 而NiO和Ni(OH)2物种的百分比则有所下降(图3(c))。该方法不仅提高了NiOx薄膜的电导率和空穴迁移率(1.38→ 1.65 cm2·V-1·s-1), 还改善了NiOx薄膜的表面能, 有利于在其表面沉积钙钛矿薄膜。所组装的刚性(柔性)PSCs的PCE高达23.4%(21.3%) (图3(d)), 且器件在最大功率输出(MPP)跟踪下(AM 1.5G, 湿度<20%, 25 ℃), 超过1300 h后, 仍可以保持初始效率的90%, 器件稳定性优异。最近, Chen课题组[36]通过构建低维卤化物/钙钛矿异质结构, 有效消除了钙钛矿/C60接触处的非辐射复合路径。反式PSCs(NiOx基)的电压损失仅为370 mV, PCE达到24.09%(参比样品PCE=21.07%)。此外, 在AM 1.5G光照下最大功率点运行1008 h后, PSCs可以保持初始PCE的95%。除了NiOx之外, 三元镍基氧化物也可以作为PSCs的HTMs。Choy课题组[37]通过可控脱氨(图3(e))合成了三元氧化物NiCo2O4纳米颗粒(图3(f))。相对覆盖良好的NiCo2O4薄膜有助于形成大钙钛矿颗粒, 从而减少薄膜缺陷, 最终器件的PCE达到18.23%(图3(g))并且稳定性良好, 500 h后PCE保持~90%。但NiCo2O4材料的价带能级较浅(-4.98 eV), 且纳米晶分散性相对较差, 容易团聚, 无法获得均匀的薄膜, 限制了其在器件中的应用。
图3
图3
镍基氧化物的物理形貌、合成工艺和相关性能
Fig. 3
Physical morphology, synthesis process and related properties of nickel-based oxide materials
(a) Comparison of conductivity mapping results for NiO (left) and Li0.05Mg0.15Ni0.8O (right) films[21]; (b) J-V curve of NiOx-based PSCs with molecular doping of F6TCNNQ[34]; (c) Synthetic process of the SRE NiOx (top), Ni species changed with different synthetic processes (bottom-left) and spectrum changes in Ni species caused by SRE (bottom-right), and (d) champion J-V curves of PSCs[35]; (e) Schematic diagram of synthesis process and (f) high-resolution transmission electron microscopy (TEM) image of NiCo2O4 nanocrystals, as well as (g) J-V curves of the champion PSCs[37]. Colorful figures are available on website
NiOx虽然是目前高效、高稳定性反式PSCs的首选无机HTMs。但是其本征电导率仍然相对较低(表1), 能级也不能完美匹配, 需要通过掺杂和改性来调节其性能, 更为重要的是NiOx的表面化学状态较为复杂, NiOx薄膜中的高价Ni≥3+位点作为Lewis电子受体, 具有使钙钛矿中的阳离子胺去质子化和氧化碘化物等倾向, 这些因素制约了器件性能。未来应该进一步调节NiOx本征特性, 提高成膜质量, 同时通过表面修饰等方法改善界面缺陷, 这样才能更好地推进PSCs向低温、柔性、大规模制备方向发展。
2.2 铜基氧化物
铜基氧化物HTMs可以分为铜氧化物和铜铁矿类氧化物等。其中, 铜氧化物一般是指氧化铜(CuO)和氧化亚铜(Cu2O), 其空穴迁移率高达100 cm2·V-1·s-1, 并且具备合适的价带能级, 是一种很有前途的无机HTMs[38-39]。Ahmadi课题组[26]首次采用磁控溅射技术在钙钛矿层上制备了相对均匀、致密和无裂纹的Cu2O作为HTL(图4(a)), 改善了顶部金属接触的沉积和表面屏蔽防潮和机械损伤, 提升了器件的稳定性(图4(b))。但磁控溅射过程中产生的高能离子对钙钛矿材料有较大危害, 会产生较多的缺陷态, 导致器件的PCE仅为8.93%。而基于溶液法制备的CuxO薄膜表面光滑、相对均匀, 在可见光下, 具有良好的透明度。Ding课题组[40]在反式PSCs中引入CuO和Cu2O作为HTMs, 通过自旋包覆的CuI膜浸在氢氧化钠水溶液中, CuI与氢氧化钠发生反应, 得到Cu2O膜, 并进一步氧化生成CuO薄膜(图4(c))。基于Cu2O和CuO薄膜制备的器件的PCE分别为13.35%和12.16%(图4(d)), 优于有机空穴导体聚(3,4-乙烯二氧噻吩):聚苯乙烯磺酸(PEDOT:PSS)组装的PSCs (PCE=11.04%)。Huang课题组[41]进一步优化溶液法的制备工艺, 获得了性能更加优异的CuxO膜, 使得器件PCE达到17.1%。阻抗和光致发光(PL)表征结果表明,器件性能优异可以归因于优化后CuxO层具有的快速空穴提取能力和较低的界面接触电阻。然而, 由于铜氧化物能隙较窄, 对长波长区域的光透过率较低, 因此PSCs的性能相对较低, 极大限制了其在PSCs中的应用。
图4
图4
铜基氧化物的物理形貌和相关性能
Fig. 4
Morphology and related properties of copper-based oxide materials
(a) Cross-sectional SEM image and (b) stability performance of the device with different HTL (Spiro-OMeTAD and Cu2O)[26]; (c) Preparation technology, device structure, energy level diagram and (d) J-V curves of Cu2O and CuO films[40]; (e) TEM image of CuGaO2 nanocrystals and (f) stability of the device[45]; (g) J-V curves, structure diagram (PC61BM: [6,6]-phenyl-C61-butyric acid methyl ester) and (h) stability of device based on mp-CuGaO2[39]; (i) Schematic diagram of nanocrystalline structure and (j) stability of devices based on CuCrO2[46]; (k) TEM image of CuScO2 nanocrystals and (l) J-V curves of PSCs[28]. Colorful figures are available on website
近年来, 铜铁矿类氧化物以其合适的能带结构、较高的电荷收集效率以及优异的稳定性, 在PSCs中的应用也逐渐增多。铜铁矿类氧化物种类繁多, 是一大类p型半导体材料, 基本结构式为ABO2, A为Cu+或者Ag+, B为Al3+、Ga3+、In3+、Sc3+、Fe3+、Cr3+和Co3+等[42-43]。不同的元素匹配方式对应不同的光电性能, 性质丰富可调, 是PSCs非常具有发展潜力的无机HTMs[44]。2017年, Chen课题组[45]首次将偏镓酸亚铜(CuGaO2)纳米晶(图4(e))作为HTMs应用在PSCs中, 构建的n-i-p型器件的PCE达到18.51%。并且优化后器件在湿度30%~55%、25 ℃环境下存放1 m后, PCE仍超过初始值的80%(图4(f))。2018年, Zhang课题组[29]设计了一种具有多孔HTL的p-i-n反式介观结构PSCs(图4(g)), 引入超薄NiOx(<10 nm)作为致密层, 然后在其上制备介孔CuGaO2层(mp-CuGaO2), 形成了双层梯度能级排列(c-NiOx、mp-CuGaO2和钙钛矿的价带最大值分别为-5.25、-5.32和-5.4 eV)的无机HTL。在NiOx、CuGaO2和钙钛矿之间, 梯度能级排列有利于转移和收集载流子, 抑制电荷复合; 另外, 介观结构增大了钙钛矿光吸收材料与HTMs之间的接触面积, 提高了电荷的提取效率。最终, 该结构PSCs的PCE超过20%, 并且在惰性环境中保存2个月后, 仍保持其初始PCE的90%以上。而且在85 ℃加速老化1000 h后, 器件性能损失小于20%(图4(h))。2019年, Chen课题组[46]又开发了另一种p型无机CuCrO2纳米晶作为HTMs(图4(i))。CuCrO2的带隙约为2.9 eV, 在紫外光区域具有较宽的光吸收范围, 同时在波长大于400 nm的可见光区域保持较高的透射率, 可以作为优异的紫外线阻挡层, 用来延缓钙钛矿材料分解, 而且几乎不会阻碍收集其余部分的太阳光谱。基于CuCrO2 HTMs构建的PSCs的最优PCE达到19.0%, 且具有优异的光稳定性(1000 h光照后, PCE仍超过初始值的80%, 图4(j))。Zhang课题组[28]开发了CuScO2纳米晶(图4(k)), 并用作反式PSCs的介孔HTL。考虑到无机HTL与钙钛矿光吸收层之间的界面缺陷, 本课题组设计了一种原位埋底离子补偿策略诱导生长高质量钙钛矿薄膜, 可以降低缺陷态密度, 有利于电荷分离、转移与输运。CuScO2为HTL的CsFA PSCs的PCE达到22.42%。在此基础上进一步优化, 三元阳离子(Cs/FA/MA)PSCs的PCE达到23.11%(图4(l)), 在惰性环境下存储5000 h后PCE仍保持初始值的87.1%, 并且光照和热稳定性都同样优异。
铜铁矿类氧化物的材料体系庞大, 但是目前已经开发应用的材料非常有限。并且大多采用复杂的高温水热合成工艺, 影响因素众多, 难以精准调控纳米晶的物理特性和形貌。因此, 想要获得更高效、更稳定的PSCs, 未来不仅要拓宽这类材料体系在PSCs中的应用(如CuAlO2、CuInO2等), 还需要着重探究其合成工艺, 以提高成膜质量, 减少界面缺陷。
2.3 其他类氧化物
过渡金属氧化物如氧化钴(Co3O4)、氧化钒(V2O5)、氧化钨(WOx)和氧化钼(MoOx)等, 具有可见光透光率高和化学性质稳定等优点, 可以作为高稳定性的HTMs应用于PSCs中。Mhaisalkar课题组[47]合成了尖晶石Co3O4纳米晶材料(图5(a)), 发现在二氧化锆层和碳层之间加入Co3O4作为HTL不仅使PCE从11.25%提升至13.27%(图5(b)), 而且稳定性也得到了一定提升。Wu课题组[48]将WOx和MoOx应用于PSCs, 分别采用热蒸发WOx和MoOx薄层作为反式结构PSCs的HTL, PCE分别达到了9.8%和13.1%。除了单独作为PSCs的HTL外, 过渡金属氧化物也可以与有机染料构建有机-无机复合结构的HTL, 兼具两者的优势。Sun课题组[49]将镍酞菁(NiPc)和V2O5复合制备HTL。钙钛矿、NiPc与五氧化二钒之间形成梯度能带排列(钙钛矿、NiPc和V2O5的价带最大值分别为-5.48、-5.06和-5.0 eV), 有利于提取空穴和抑制反向电子转移。NiPc/V2O5器件的PCE达到了18.3%。在此基础上, 多组分过渡金属氧化物可以进一步调控界面能带排列, 提高电荷传输和收集效率。Yang课题组[50]开发了由窄带隙氧化物(如Co3O4、NiO、CuO、Fe2O3和MnO2)和宽带隙SrCO3氧盐构成的自组织渗透结构, 作为PSCs的高效HTL, 优化了界面带排列, 显著改善了电荷传输和收集(Co3O4-SrCO3: 6.33×10−2 cm2·V−1·s−1; Co3O4: 1.49×10−2 cm2·V−1·s−1, 图5(c)), 把器件的PCE(图5(d))从8.08%(SrCO3)和15.47%(Co3O4)大幅度提高到21.84%(Co3O4-SrCO3)。
图5
图5
其他氧化物和非氧化物的物理形貌和相关性能
Fig. 5
Physical morphology and related properties of other oxides and non-oxides
(a) High-resolution TEM image of Co3O4 and (b) J-V curves of PSCs[47]; (c) Time-resolved photoluminescence (TRPL) spectra and (d) J-V curves of PSCs based on Co3O4-SrCO3[50]; (e) PL absorption spectra and (f) J-V curve of PSCs based on CuSCN HTL[16]; (g) Diagram of device structure, (h) J-V curves and (i) light stability of capped PSCs (under constant illumination and different temperature) based on CuSCN HTL and 2D Cs2PbI2Cl2 capping layers[55]
相比于镍基和铜基氧化物, 其他类氧化物的光电特性(<0.1 cm2·V−1·s−1)仍有所不足, 使得器件性能普遍较低。并且制备工艺大多较为复杂, 稀有过渡金属(钴、钒、钨和钼等)的价格较高, 这些都不利于降低成本以及PSCs的商业化发展。因而, 其他类氧化物应该主要以提升本征光电特性为主, 以开发工艺简单、成本低廉的HTL为辅, 为开发和应用新型无机HTMs提供一定实验和理论基础。
2.4 非氧化物
非氧化物无机HTMs一般为铜基化合物, 包括碘化亚铜(CuI)、硫化铜(CuS)以及硫氰酸亚铜(CuSCN)等, 相关研究较少。
CuI作为一种廉价而稳定的无机HTMs具有较大的带隙(3.1 eV)、高电导率、良好的空穴迁移率0.5~2 cm2·V−1·s−1以及可溶液法加工等优势[51]。Bian课题组[52]采用室温溶液处理得到了CuI薄膜, 随后通过控制反溶剂的滴入时间来精准调控钙钛矿薄膜质量。优化钙钛矿薄膜形貌后, 基于CuI的反式器件的PCE达到了16.8%。此外, 与基于PEDOT: PSS的器件相比, 基于CuI的器件在未封装条件下显示出更好的空气稳定性。与CuI类似, CuS也是一种廉价而稳定的无机HTMs, 具备合适的价带能级(-5.1 eV)。2016年Huang课题组[53]首次在反式平面异质结PSCs中使用CuS纳米颗粒作为HTL, 结果显示, CuS不仅具有良好的透光率, 还能将表面功函数从-4.9 eV调整到-5.1 eV, 提高了空穴提取效率。优化后器件的最高PCE为16.2%, 迟滞效应可忽略不计, 且具备优异的稳定性。但是, 目前CuI和CuS的成膜质量无法满足要求, 导致器件性能迟迟未能突破, 有待进一步改善。
相比于CuI和CuS, CuSCN的带隙更宽(3.9 eV), 具有良好的可见光透过率。此外, 其还具有空穴迁移率较高(0.01~0.1 cm2·V−1·s−1), 热稳定性好, 以及可溶液法加工等诸多优点, 因此CuSCN常被用作正式和反式钙钛矿器件的HTMs[54]。例如, Grätzel课题组[16]采用二乙基硫化物作为溶剂, 在钙钛矿层上动态旋涂CuSCN以去除溶剂, 制备了紧密、高度共形的CuSCN层, 并且二乙基硫化物的高挥发性, 还可以有效降低溶剂对钙钛矿层的损害。荧光光谱结果显示基于CuSCN器件的空穴迁移能力显著增强(图5(e))。使用CuSCN作为HTL的PSCs的PCE超过了20%(图5(f)), 且器件在85 ℃持续加热1000 h后, PCE仍超过初始值的85%。基于CuSCN显示出的高性能和良好稳定性, Loo课题组[55]也采用CuSCN替代Spiro-OMeTAD作为HTL, 为了最大限度地提高器件的热稳定性和光稳定性, 实验还在钙钛矿活性层和CuSCN HTL之间原位形成一层二维(2D) Cs2PbI2Cl2无机钙钛矿的封盖层(图5(g)), 得到3D/2D异质结构。一方面2D Cs2PbI2Cl2起到了钝化界面的作用, 显著抑制非辐射复合; 另一方面, 作为无机化合物钝化层, 其稳定性远远超过有机功能分子。因此, 基于CsPbI3器件的PCE从14.9%提高到了17.4%(图5(h))。特别是, 在持续光照和110 ℃环境下老化处理2100 h以上, PCE衰减不到20% (图5(i))。并且, 通过模拟阿伦尼乌斯温度依赖性的降解加速因子, 预测该器件可以在35 ℃下连续运行(51000±7000) h(> 5年)。这些器件稳定性研究方面的巨大进步, 为进一步推进工程化应用进程提供了理论基础。
3 钙钛矿太阳能电池无机HTL的性能调控
尽管许多HTMs表现出优异的器件稳定性, 但是相对于使用有机HTMs的PSCs, 基于无机HTMs的PSCs器件的PCE普遍偏低。提升电池性能, 不仅需要优化无机HTMs的本征性能, 同时考虑到无机HTMs是共价晶体结构, 而钙钛矿材料是离子晶体结构, 两种不同晶体结构之间的界面接触需要进一步改善。
因此, 为了最大限度地提高电池的性能, 需要采取多种方案优化调控无机HTMs。典型的优化方案包括元素掺杂工程、添加剂工程和界面工程等。
3.1 元素掺杂工程
元素掺杂, 即在主体材料的晶格引入具有特定功能的异类元素, 是实现调节目标材料的能级结构和电荷迁移率等光电特性的最基本和最有效的方法之一[22]。
虽然NiO薄膜的空穴迁移率相对较高(>0.1 cm2·V−1·s−1), 但空穴浓度较低, 致使本征导电性较低, 不能够满足高效PSCs的需要, 元素掺杂是解决这个问题的有效途径[56]。在筛选合适的掺杂元素时, 相邻元素是一个重要的入手点, 例如紧邻Ni元素(原子序数28)的Cu元素(原子序数29)[20]。2015年, Jen课题组[23]在FTO导电衬底上制备了高质量Cu+掺杂的NiOx(Cu:NiOx)薄膜作为PSCs的HTL, 器件的PCE达到17.74%(图6(a)), 显著优于当时原始器件(PCE=15.52%)。一方面, Cu+掺杂提高NiOx的空穴浓度, HTL的空穴导电性能从7.54×10−4 S·cm−1提升到1.25×10−3 S·cm−1, 并降低了载流子输运势垒; 另一方面, 基于Cu:NiOx的器件具有更高的功函数(-5.3 eV)。同时, 荧光光谱证明其具备更好的空穴提取能力(图6(b)), 降低了界面电荷复合。除了相邻元素外, 进行P型掺杂的元素化学价态应该低于Ni2+。比如, He课题组[27]采用Cs+掺杂NiOx作为HTMs应用在反式PSCs中, 所得器件的PCE为19.35%(图6(c)), 明显优于未掺杂NiOx的器件(PCE=16.04%), 其内在原因也是Cs+掺杂提高了HTL的空穴导电性能(>1×10−3 S·cm−1, 图6(d))。此外, 由于无机Cs:NiOx的化学稳定性和稳定的阴极界面层特性, 器件在惰性环境中老化70 d后PCE仍然能够保持初始值的98%, 稳定性优异。对于同价态元素, 引入不同元素可能改变目标材料的化学环境从而影响其光电性能。Choy课题组[57]采用In3+掺杂CuCrO2改善其本征电学性能, In3+掺杂提供了更p型的掺杂特性, 并提高了HTL的透光率, 电导率从2.9×10−2 S·cm−1提升到6.9×10−2 S·cm−1 。他们在In:CuCrO2的传统水热合成工艺中引入乙醇作为共沸溶剂(图6(e)), 在160 ℃和高压下合成的纳米晶可以在200 ℃保持稳定, 使PSCs的PCE达到20.54%(图6(f)), 并且提高了器件的光稳定性和实验的可重复性。Zhang课题组[25]利用Zn2+掺杂优化CuGaO2纳米晶的光学及电学特性, 构建了反式介观结构器件。引入Zn2+将HTL的电导率从4.625×10−3 S·cm−1提升至1.39×10−2 S·cm−1, 空穴浓度也得以提升(图6(g)), 与钙钛矿吸光层的界面传输势垒也得到改善, 器件的缺陷态密度显著降低。基于Zn:CuGaO2的反式介观结构PSCs的PCE达到20.67%(图6(h))。此外, 器件在氮气气氛、85 ℃条件下热老化处理1000 h后, PCE超过初始值的85%。
图6
图6
元素掺杂对于器件性能的影响
Fig. 6
Effect of element doping on device performance
(a) J-V curves and (b) PL spectra of PSCs based on Cu:NiOx HTL[23]; (c) J-V curves of PSCs and (d) electrical conductivity of Cs:NiOx film[27]; (e) SEM image of In doped CuCrO2 film and (f) J-V curves of PSCs[57]; (g) Mott-Schottky curves and (h) J-V curves of PSCs based on the Zn doped CuGaO2[25]
3.2 添加剂工程
添加剂工程是一种类似于掺杂的策略, 但添加剂通常以一种分散的方式存在于宿主材料中, 而不是进入材料的晶格中。添加剂可以影响主体材料结晶, 诱导材料结晶生长, 抑制缺陷形成, 增强本体/表面钝化以及表面能量等[58], 是PSCs研究中提升器件性能的重要手段。
pH是无机材料晶体生长过程中影响晶体质量的重要因素之一。Park课题组[30]通过添加中性的单乙醇胺盐来调控溶胶-凝胶NiO前驱液的pH, 制备了形貌相对均一和结晶度较高的纳米晶材料 (图7(a)), 并改善界面性质、能级匹配(-5.37 eV)和空穴迁移率等。添加单乙醇胺盐可以改善NiOx薄膜与钙钛矿吸光层之间的界面特性, 提高钙钛矿薄膜的生长质量。最佳器件的PCE达到19.91%(图7(b)), 并且在封装条件下存放800 h后, PCE仍保持初始值的97%以上。Yang课题组[59]报道了一种硼酸辅助制备NiOx的策略(BA-NiOx), 添加硼酸可以将NiOx的空穴迁移率从2.41×10−2 cm2·V−1·s−1提升到5.35×10−2 cm2·V−1·s−1, 载流子浓度从4.02×1018 cm−3提升到2.17×1019 cm−3, 并加深价带边缘, 价带最大值从-5.21 eV (NiOx)提高到-5.39 eV (BA-NiOx), 从而更有效地提取和输运空穴。基于这种策略的MAPbI3 PSCs(图7(c))的开路电压高达1.131 V, 最优器件的PCE达到21.40%(图7(d)), 在持续光照(图7(e))或者室温下都展现出优异的稳定性。室温下保存近1400 h后, 仍然保持初始PCE的92.8%。此外, 添加离子液体也可以有效改良NiOx纳米晶。Li课题组[60]提出通过离子液体辅助合成获得高质量的NiOx(NiOx-IL)纳米颗粒。离子液体的阳离子可以通过强氢键和低吸附能抑制杂质离子吸附在氢氧化镍上, 从而获得具有高电导率(从9.2×10−3 S·cm−1提升到2.45×10−2 S·cm−1)和强空穴提取能力的NiOx-IL。重要的是, 去除杂质离子可以有效抑制NiOx薄膜与钙钛矿薄膜之间的氧化还原反应, 减缓器件性能衰减, 改进后反式器件的PCE超过22.62%(图7(f))。另外, 基于NiOx-IL未封装的PSCs在存储6000 h后仍保持初始效率的90%左右(图7(g)), 且在AM 1.5G光照下最大功率点运行1000 h后, PCE也能够保持初始值的92%(图7(h)), 显示出优越的稳定性。
图7
图7
添加剂工程对于器件性能的影响
Fig. 7
Effect of additive engineering on device performance
(a) SEM images of NiO film with different ammonium stabilizers and different concentrations and (b) J-V curves of PSCs[30]; (c) Schematic structure, (d) J-V curves and (e) I-t curves of PSCs based on NiO film with boric acid[59]; (f) J-V curves, (g) long-term stability and (h) maximum power output stability of PSCs based on ionic liquid-assisted synthesis of NiO NPs[60]; BCP: Bathocuproin; VMPP: Output voltage at maximum power. Colorful figures are available on website
3.3 界面工程
在钙钛矿与功能层之间引入特定界面材料可以有效钝化表面缺陷, 调整价带/导带位置, 降低电荷转移损失, 改善整体接触性能(如表面性质、黏附、电流收集), 进而获得优良的PSCs性能和稳定性[61-62]。为了改善NiOx薄膜和钙钛矿吸光层之间的接触问题, 常晶晶课题组[63]采用CsBr作为NiOx层和钙钛矿层之间的缓冲层, 可以缓解界面应力, 提升钙钛矿薄膜结晶质量, 提高电荷提取能力, 减少电荷复合。最终器件的PCE达到19.7%(图8(a)), 并且稳定性良好。二维结构也常被用来钝化界面接触, Sargent课题组[64]使用大体积的有机阳离子碘化胍(GUAI)作为界面修饰剂, 在埋底界面处(NiOx/钙钛矿)自发形成二维钝化层(GUA2PbI4)。GUA2PbI4抑制了界面上的非辐射复合, 使得开路电压(VOC)提高了65 mV。基于1.70 eV带隙的PSCs的PCE为20.1%(图8(b)), VOC更是达到了1.23 V, 并且表现出非常优异的光热稳定性(图8(c))。核壳包覆工艺可以有效降低界面电荷传输损失, 提升器件性能。Yao课题组[65]将核壳Au@NiOx纳米晶(图8(d))混入介孔NiOx (mp-NiOx)层。由于Au@NiOx异质结构的欧姆接触性质, 通过等离子激元辅助的金属到半导体的电荷转移(PACT)机制, 在黑暗和光照条件下空穴均可以从Au纳米晶核注入NiOx纳米壳(图8(e))。Au@NiOx异质结构将载流子浓度从3.9×1017 cm-3提升到4.6×1018 cm-3, 并实现了高效的电荷转移和输运。这实质上加深了价带边缘, 价带最大值从-5.00 eV (NiOx)提高到-5.08 eV (Au@NiOx), 降低了界面传输能垒, 促进了PSCs器件中的空穴提取, 相应器件的PCE达到了20.6%(图8(f))。未封装的PSCs在85 ℃、湿度50%~70%条件下保存500 h后, PCE仍保持了初始值的89%。考虑到NiOx-钙钛矿异质结的光致降解是限制其长期寿命的主要因素, Qi课题组[66]在NiOx/钙钛矿界面上构建了一个非质子三甲基磺化铵(TMSBr)缓冲层。它具有良好的光热稳定性, 与钙钛矿晶体匹配的晶格参数, 以及优良的缺陷钝化能力, 构筑的反式器件的PCE达到了22.1%。且在AM 1.5G光照条件下连续运行2000 h后, 仍保持了初始值的82.8%(图8(g))。近期, Yan课题组[67]采用少量1, 3-双(二苯基膦基)丙烷(DPPP)处理钙钛矿埋底界面(NiOx/钙钛矿), 结果显示这些分子在界面和晶界处提供额外的结合和桥接, 可以显著改善器件性能和耐久性。器件的PCE达到了24.5%, 并且封装后器件在最大功率点和85 ℃条件下保持1500 h后, PCE几乎没有任何衰减。
图8
图8
界面工程对于器件性能的影响
Fig. 8
Effect of interface engineering on device performance
(a) J-V curves of devices based on surface modification with different concentrations of CsBr, and stability of PSCs[63]; (b) J-V curves and (c) MPP stability of PSCs based on GUAI surface modification at different concentrations (in molar) with inset showing environmental stability[64]; (d) High-resolution TEM images of Au@NiOx NPs, (e) corresponding structure diagram and (f) J-V curves of PSCs[65]; (g) MPP stability of devices with TMSBr surface modification[66]; CsBr-2.5: 2.5 mg/mL CsBr; RS: reverse scan; FS: forward scan; T80: the time maintaining 80% initial PCE. Colorful figures are available on website
4 结论与展望
相比于有机HTMs, 无机HTMs具有得天独厚的稳定性, 使其在湿度、光、热等外场环境下, 仍然可以保持优异的光电特性。此外, 无机HTMs的制备方法简单多样, 成本低廉。因此基于无机HTMs的高稳定PSCs具有显著的优势与巨大的商业化潜力, 但仍然在多个方向上需要进一步深入研究。
1)从改善本征材料的能级、电导率、透光性、结晶度、均一性和稳定性等多个角度出发, 通过元素掺杂、添加剂工程和界面工程等协同作用, 调控优化无机HTMs的载流子传输性能和整体器件的稳定性。
2)从理论和实验双重维度深层次认知无机HTM与钙钛矿吸光层界面处的载流子动力学、能级偏差、非辐射复合缺陷和工艺兼容性等诸多问题。
3)面向未来的产业化发展, 将无机HTMs应用于大面积器件, 开发相应的涂层工艺以及优化串联电池模组结构, 也是一个非常具有发展潜力的研究方向。
参考文献
Revealing the strain- associated physical mechanisms impacting the performance and stability of perovskite solar cells
Rear electrode materials for perovskite solar cells
Stabilization of perovskite solar cells: recent developments and future perspectives
Perovskite solar cells for building integrated photovoltaics-glazing applications
Efficient and stable perovskite solar cells based on inorganic hole transport materials
Although power conversion efficiencies of organic-inorganic lead halide perovskite solar cells (PSCs) are approaching those of single-crystal silicon solar cells, the working device stability due to internal and external factors, such as light, temperature, and moisture, is still a key issue to address. The current world-record efficiency of PSCs is based on organic hole transport materials, which are usually susceptible to degradation from heat and diffusion of dopants. A simple solution would be to replace the generally used organic hole transport layers (HTLs) with a more stable inorganic material. This review article summarizes recent contributions of inorganic hole transport materials to PSC development, focusing on aspects of device performance and long-term stability. Future research directions of inorganic HTLs in the progress of PSC research and challenges still remaining will also be discussed.
Recent progress in hole-transporting layers of conventional organic solar cells with p-i-n structure
High performance hybrid solar cells sensitized by organolead halide perovskites
Ion-modulated radical doping of Spiro-OMeTAD for more efficient and stable perovskite solar cells
Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2',7,7'-tetrakis(,-di--methoxyphenyl-amine)9,9'-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4--butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.
Heterocyclic amino acid molecule as a multifunctional interfacial bridge for improving the efficiency and stability of quadruple cation perovskite solar cells
Highly efficient flexible perovskite solar cells through pentylammonium acetate modification with certified efficiency of 23.35%
Organometallic-functionalized interfaces for highly efficient inverted perovskite solar cells
Further enhancing the performance and stability of inverted perovskite solar cells (PSCs) is crucial for their commercialization. We report that the functionalization of multication and halide perovskite interfaces with an organometallic compound, ferrocenyl-bis-thiophene-2-carboxylate (FcTc), simultaneously enhanced the efficiency and stability of inverted PSCs. The resultant devices achieved a power conversion efficiency of 25.0% and maintained >98% of their initial efficiency after continuously operating at the maximum power point for 1500 hours under simulated AM1.5 illumination. Moreover, the FcTc-functionalized devices passed the international standards for mature photovoltaics (IEC61215:2016) and have exhibited high stability under the damp heat test (85°C and 85% relative humidity).
Enhancing the hole-conductivity of Spiro-OMeTAD without oxygen or lithium salts by using Spiro(TFSI)2 in perovskite and dye-sensitized solar cells
Ion migration in halide perovskite solar cells: mechanism, characterization, impact and suppression
Metal halide perovskites are emerging as the most promising candidate for the next-generation Photovoltaics (PV) materials, due to their superior optoelectronic properties and low cost. However, the resulting Perovskite solar cells (PSCs) suffer from poor stability. In particular, the temperature and light activated ionic defects within the perovskite lattice, as well as electric-field-induced migration of ionic defects, make the PSCs unstable at operating condition, even with device encapsulation. There is no doubt that the investigation of ion migration is crucial for the development of PSCs with high intrinsic stability. In this review, we first briefly introduce the origin and pathways of ion migration, and also the essential characterization methods to identify ion migration. Next, we discuss the impact of ion migration on the perovskite films and cells with respect to photoelectric properties and stability. Then, several rep- resentative strategies to suppress ion migration are systematically summarized in the context of compo- sition engineering, additive engineering and interface engineering, with an in-depth understanding on the underlying mechanisms which may provide more clues for further fabrication of PSCs with improved stability. Finally, a perspective with some suggestion on future research directions and chemical approaches are provided to alleviate ion migration in perovskite materials and the entire devices.
Recent progress in perovskite solar cells: from device to commercialization
Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%
Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized only with expensive organic hole-transporting materials. We demonstrate PSCs that achieve stabilized efficiencies exceeding 20% with copper(I) thiocyanate (CuSCN) as the hole extraction layer. A fast solvent removal method enabled the creation of compact, highly conformal CuSCN layers that facilitate rapid carrier extraction and collection. The PSCs showed high thermal stability under long-term heating, although their operational stability was poor. This instability originated from potential-induced degradation of the CuSCN/Au contact. The addition of a conductive reduced graphene oxide spacer layer between CuSCN and gold allowed PSCs to retain >95% of their initial efficiency after aging at a maximum power point for 1000 hours under full solar intensity at 60°C. Under both continuous full-sun illumination and thermal stress, CuSCN-based devices surpassed the stability of spiro-OMeTAD-based PSCs.Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.
A solution-processed spinel CuCo2O4 as an effective hole transport layer for efficient perovskite solar cells with negligible hysteresis
Recent progress of inorganic hole transport materials for efficient and stable perovskite solar cells
Identification and mitigation of a critical interfacial instability in perovskite solar cells employing copper thiocyanate hole-transporter
Modulation of Ni valence in p-type NiO films via substitution of Ni by Cu
10 and 18 at. % copper incorporated NiO films were deposited by reactively sputtering from Ni targets covered with Cu strips of different sizes. A change in the valence of Ni from Ni2+ to Ni3+ is found by x-ray absorption near edge structure analysis after incorporating Cu into the films. All NiO films exhibit p-type conductivity, either without or with Cu additives. However, as compared with the pure NiO films, the carrier concentration of the films incorporating Cu increases, while the resistivity and mobility decrease. This study not only discusses the connections among the physical properties of the NiO films with different Cu contents but also clarifies the role of Cu additives in the NiO films.
Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers
The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking. Copyright © 2015, American Association for the Advancement of Science.
Electrical doping in halide perovskites
A low-temperature, solution- processable, Cu-doped nickel oxide hole-transporting layer via the combustion method for high-performance thin-film perovskite solar cells
Overcoming redox reactions at perovskite-nickel oxide interfaces to boost voltages in perovskite solar cells
Thermally stable methylammonium- free inverted perovskite solar cells with Zn2+ doped CuGaO2 as efficient mesoporous hole-transporting layer
Cuprous oxide as a potential low-cost hole-transport material for stable perovskite solar cells
Inorganic hole-transport materials are commercially desired to decrease the fabrication cost of perovskite solar cells. Here, Cu2O is introduced as a potential hole-transport material for stable, low-cost devices. Considering that Cu2O formation is highly sensitive to the underlying mixture of perovskite precursors and their solvents, we proposed and engineered a technique for reactive magnetron sputtering. The rotational angular deposition of Cu2O yields high surface coverage of the perovskite layer for high rate of charge extraction. Deposition of this Cu2O layer on the pinhole-free perovskite layer produces devices with power conversion efficiency values of up to 8.93%. The engineered Cu2O layers showed uniform, compact, and crack-free surfaces on the perovskite layer without affecting the perovskite structure, which is desired for deposition of the top metal contact and for surface shielding against moisture and mechanical damages.© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Cesium doped NiOx as an efficient hole extraction layer for inverted planar perovskite solar cells
Ion compensation of buried interface enables highly efficient and stable inverted MA-free perovskite solar cells
Design of an inorganic mesoporous hole-transporting layer for highly efficient and stable inverted perovskite solar cells
Improved stability and efficiency of inverted perovskite solar cell by employing nickel oxide hole transporting material containing ammonium salt stabilizer
Nickel oxide electrode interlayer in CH3NH3PbI3 perovskite/PCBM planar-heterojunction hybrid solar cells
High-performance hole-extraction layer of Sol-Gel-processed NiO nanocrystals for inverted planar perovskite solar cells
Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH3 NH3 PbI3 in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH3 NH3 PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Hybrid interfacial layer leads to solid performance improvement of inverted perovskite solar cells
Molecule-doped nickel oxide: verified charge transfer and planar inverted mixed cation perovskite solar cell
Surface redox engineering of vacuum-deposited NiOx for top-performance perovskite solar cells and modules
Efficient inverted perovskite solar cells with a low-dimensional halide/perovskite heterostructure
Strategic synthesis of ultrasmall NiCo2O4 NPs as hole transport layer for highly efficient perovskite solar cells
Review of inorganic hole transport materials for perovskite solar cells
Novel approaches and scalability prospects of copper based hole transporting materials for planar perovskite solar cells
Solution-processed Cu2O and CuO as hole transport materials for efficient perovskite solar cells
Solution-processed Cu2 O and CuO are used as hole transport materials in perovskite solar cells. The cells show significantly enhanced open circuit voltage Voc, short-circuit current Jsc, and power conversion efficiency (PCE) compared with PEDOT cells. A PCE of 13.35% and good stability are achieved for Cu2O cells, making Cu2O a promising material for further application in perovskite solar cells.© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
High-performance inverted planar heterojunction perovskite solar cells based on a solution-processed CuOx hole transport layer
Polymorph engineering of CuMO2 (M = Al, Ga, Sc, Y) semiconductors for solar energy applications: from delafossite to wurtzite
The cuprous oxide based ternary delafossite semiconductors have been well studied in the context of p-type transparent conducting oxides. CuAlO2, CuGaO2 and CuInO2 represent a homologous series where the electronic properties can be tuned over a large range. The optical transparency of these materials has been associated with dipole forbidden transitions, which are related to the linear O—Cu—O coordination motif. The recent demonstration that these materials can be synthesized in tetrahedral structures (wurtzite analogues of the chalcopyrite lattice) opens up a new vista of applications. We investigate the underlying structure–property relationships (for Group 3 and 13 metals), from the perspective of first-principles materials modelling, towards developing earth-abundant photoactive metal oxides. All materials studied possess indirect fundamental band gaps ranging from 1 to 2 eV, which are smaller than their delafossite counterparts, although in all cases the difference between direct and indirect band gaps is less than 0.03 eV.
Hydrothermal synthesis of ultrasmall CuCrO2 nanocrystal alternatives to NiO nanoparticles in efficient p-type dye-sensitized solar cells
Electronic structure of p-type conducting transparent oxides
CuGaO2: a promising inorganic hole-transporting material for highly efficient and stable perovskite solar cells
Low-temperature solution- processed CuCrO2 hole-transporting layer for efficient and photostable perovskite solar cells
Spinel Co3O4 nanomaterials for efficient and stable large area carbon-based printed perovskite solar cells
Efficient inverted-type perovskite solar cells using UV-ozone treated MoOx and WOx as hole transporting layers
Efficient perovskite solar cells based on a solution processable nickel(II) phthalocyanine and vanadium oxide integrated hole transport layer
Self-organized Co3O4-SrCO3 percolative composites enabling nanosized hole transport pathways for perovskite solar cells
An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide
Organo-lead halide perovskite solar cells have emerged as one of the most promising candidates for the next generation of solar cells. To date, these perovskite thin film solar cells have exclusively employed organic hole conducting polymers which are often expensive and have low hole mobility. In a quest to explore new inorganic hole conducting materials for these perovskite-based thin film photovoltaics, we have identified copper iodide as a possible alternative. Using copper iodide, we have succeeded in achieving a promising power conversion efficiency of 6.0% with excellent photocurrent stability. The open-circuit voltage, compared to the best spiro-OMeTAD devices, remains low and is attributed to higher recombination in CuI devices as determined by impedance spectroscopy. However, impedance spectroscopy revealed that CuI exhibits 2 orders of magnitude higher electrical conductivity than spiro-OMeTAD which allows for significantly higher fill factors. Reducing the recombination in these devices could render CuI as a cost-effective competitor to spiro-OMeTAD in perovskite solar cells.
Room-temperature and solution- processed copper iodide as the hole transport layer for inverted planar perovskite solar cells
Solution-processed CuS NPs as an inorganic hole-selective contact material for inverted planar perovskite solar cells
Copper(I) thiocyanate (CuSCN) as a hole-transport material for large-area opto/electronics
Accelerated aging of all-inorganic, interface-stabilized perovskite solar cells
To understand degradation routes and improve the stability of perovskite solar cells (PSCs), accelerated aging tests are needed. Here, we use elevated temperatures (up to 110°C) to quantify the accelerated degradation of encapsulated CsPbI PSCs under constant illumination. Incorporating a two-dimensional (2D) CsPbICl capping layer between the perovskite active layer and hole-transport layer stabilizes the interface while increasing power conversion efficiency of the all-inorganic PSCs from 14.9 to 17.4%. Devices with this 2D capping layer did not degrade at 35°C and required >2100 hours at 110°C under constant illumination to degrade by 20% of their initial efficiency. Degradation acceleration factors based on the observed Arrhenius temperature dependence predict intrinsic lifetimes of 51,000 ± 7000 hours (>5 years) operating continuously at 35°C.
Understanding the doping effect on NiO: toward high-performance inverted perovskite solar cells
Multifunctional synthesis approach of In:CuCrO2 nanoparticles for hole transport layer in high-performance perovskite solar cells
Transient quantum beatings of trions in hybrid organic tri-iodine perovskite single crystal
Utilizing the spin degree of freedom of photoexcitations in hybrid organic inorganic perovskites for quantum information science applications has been recently proposed and explored. However, it is still unclear whether the stable photoexcitations in these compounds correspond to excitons, free/trapped electron-hole pairs, or charged exciton complexes such as trions. Here we investigate quantum beating oscillations in the picosecond time-resolved circularly polarized photoinduced reflection of single crystal methyl-ammonium tri-iodine perovskite (MAPbI) measured at cryogenic temperatures. We observe two quantum beating oscillations (fast and slow) whose frequencies increase linearly with B with slopes that depend on the crystal orientation with respect to the applied magnetic field. We assign the quantum beatings to positive and negative trions whose Landé g-factors are determined by those of the electron and hole, respectively, or by the carriers left behind after trion recombination. These are [Formula: see text] = 2.52 and [Formula: see text]= 2.63 for electrons, whereas [Formula: see text]= 0.28 and [Formula: see text]= 0.57 for holes. The obtained g-values are in excellent agreement with an 8-band K.P calculation for orthorhombic MAPbI. Using the technique of resonant spin amplification of the quantum beatings we measure a relatively long spin coherence time of ~ 11 (6) nanoseconds for electrons (holes) at 4 K.© 2022. The Author(s).
Boric acid mediated formation and doping of NiOx layers for perovskite solar cells with efficiency over 21%
Critical role of removing impurities in nickel oxide on high-efficiency and long-term stability of inverted perovskite solar cells
Materials and methods for interface engineering toward stable and efficient perovskite solar cells
Buried interface modification in perovskite solar cells: a materials perspective
NiO/perovskite heterojunction contact engineering for highly efficient and stable perovskite solar cells
Passivation of the buried interface via preferential crystallization of 2D perovskite on metal oxide transport layers
Tailoring carrier dynamics in inverted mesoporous perovskite solar cells with interface-engineered plasmonics
Elimination of light- induced degradation at the nickel oxide-perovskite heterojunction by aprotic sulfonium layers towards long-term operationally stable inverted perovskite solar cells
This work introduces an aprotic sulfonium buffer layer at the nickel oxide–perovskite heterojunction to eliminate the multi-step photochemical reactions, which leads to inverted perovskite solar cells with long-term operational stability.
Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells
Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.
