From Groundwork to Efficient Solar Cells: On the Importance of the Substrate Material in Co‐Evaporated Perovskite Solar Cells

Abstract

Vacuum-based deposition of optoelectronic thin films has a long-standing history. However, in the field of perovskite-based photovoltaics, these techniques are still not as advanced as their solution-based counterparts. Although high-efficiency vacuum-based perovskite solar cells reaching power conversion efficiencies (PCEs) above 20% are reported, the number of studies on the underlying physical and chemical mechanism of the co-evaporation of lead iodide and methylammonium iodide is low. In this study, the impact of one of the most crucial process parameters in vacuum processes-the substrate material-is studied. It is shown that not only the morphology of the co-evaporated perovskite thin films is significantly influenced by the surface polarity of the substrate material, but also the incorporation of the organic compound into the perovskite framework. Based on these studies, a selection guide for suitable substrate materials for efficient co-evaporated perovskite thin films is derived. This selection guide points out that the organic vacuum-processable hole transport material 2,2″,7,7″-tet ra(N,N-di-p-tolyl)amino-9,9-spirobifluorene is an ideal candidate for the fabrication of efficient all-evaporated perovskite solar cells, demonstrating PCEs above 19%. Furthermore, building on the insights into the formation of the perovskite thin films on different substrate materials, a basic crystallization model for co-evaporated perovskite thin films is suggested.

Intrinsic Organic Semiconductors as Hole Transport Layers in p–i–n Perovskite Solar Cells

“…[24,42,43] Recently, Abzieher et al showed the advantage of using organic semiconductors as substrates for the vacuum processing of methylammonium lead iodide (MAPI) films. [44] The hydrophobic surface of organics favors the crystallization and orientation of the perovskite film, as opposed to polar metal oxide surfaces, confirming previous observations. [45][46][47] Interestingly, they found that a very thin (<3 nm) undoped film of 2,2',7,7'-tetra(N,N-di-p-tolyl)amino-9,9spirobifluorene (Spiro-TTB) can be employed to ensure ohmic contact between ITO and the MAPI absorber layer, leading to very efficient solar cells.…”

“…As previously observed by us and others, co-evaporated MAPI films crystallize in a cubic perovskite phase at room temperature (space group Pm-3m), with only a minor contribution from a residual PbI 2 phase. [44,49] Devices were prepared on ITO-coated glass substrates with the following device configuration: HTL (x nm)/MAPI (500 nm)/C60 (25 nm)/BCP (8 nm)/Ag (100 nm) where BCP is bathocuproine and the HTL is N4,N4, N4 0 0 ,N4 0 0 -tetra([1,1 0 -biphenyl]-4-yl)-[1,1 0 :4 0 ,1 0 0 -terphenyl]-4,4 0 0diamine (TaTm), with thickness varying from 2 to 10 nm. Details of the device fabrication are reported in the Experimental Section.…”

Understanding the PEDOT:PSS, PTAA and P3CT-X Hole-Transport-Layer-Based Inverted Perovskite Solar Cells

“…In the inverted perovskite solar cell, the p-type polymers and p-type metal oxides are widely fabricated on top of the ITO/glass substrates with a post-thermal annealing process at about 100 °C. The high-quality perovskite crystalline thin films can be fabricated by using the two-step spin coating method with an interdiffusion process [ 71 , 72 , 73 ], the one-step spin coating method with a washing-enhanced nucleation (WEN) process [ 74 , 75 , 76 ] and the vacuum thermal co-evaporation technique [ 77 , 78 , 79 ]. The organometal trihalide perovskite can be CH 3 NH 3 PbI 3 (MAPbI 3 ), CH(NH 2 ) 2 PbI 3 (FAPbI 3 ) and Cs x (MA y FA 1−y ) 1−x Pb(I z Br 1−z ) 3 , mainly due to the low absorption bandgap [ 80 , 81 , 82 ], large absorption coefficient [ 83 , 84 , 85 ], small exciton binding energy [ 86 , 87 , 88 ], long exciton (carrier) lifetime [ 89 , 90 , 91 ] and high carrier mobility [ 92 , 93 , 94 ].…”

Upscaling of perovskite solar modules: The synergy of fully evaporated layer fabrication and all‐laser‐scribed interconnections

“…The employed all-evaporated layer stack sequence ITO/spiro-TTB/CH 3 NH 3 PbI 3 /C 60 /BCP/Au was introduced in an earlier work as a highperformance, low-hysteresis architecture with good short-term stability and reproducibility (see Figures S8-S10). 117 Champion minimodules with an aperture area of 4.0 cm 2 (GFF of 96%) demonstrate PCEs as high as 18.0% with a fill factor (FF) of…”