Facile Fabrication of Self‐Assembly Functionalized Polythiophene Hole Transporting Layer for High Performance Perovskite Solar Cells

Abstract Crystallinity and crystal orientation have a predominant impact on a materials’ semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra‐uniform hole‐transporting material (HTM) by self‐assembling COOH‐functionalized P3HT (P3HT‐COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self‐assembly approach facilitates the P3HT‐COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down‐shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self‐assembled HTM show high open‐circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim‐light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.


The comparison of PCE for different HTM.
In this study, we mainly focus on development of self-assembly technique to provide costeffective approach for large scale planar cell fabrication, as well as the interconnecting layer needed for tandem solar cell. We agree that the power conversion efficiencies (PCE) of the P3HT-COOH based PSCs are inferior to the state-of-the-art devices based on the PTAA HTM, but PTAA cannot deposit on top of the ITO surface by self-assembled methods as shown in Figure S8 in the Supporting Information. We believe that the self-assembly approach is a simple, materials-saving, and cost-effective method to fabricate a large-area, uniform and highly reproducible HTM for photovoltaics and a promising interconnecting layer on the textured silicon cells to realize the high performance tandem solar cell. Nevertheless, we agreed that PEDOT:PSS might not a good reference HTM for p-i-n type PSCs due to the inferior performance, however, the sulfonate (-SO3H) group on PSS can anchor on ITO substrate through self-assembled technology. That's the mainly reason we choose it as a reference HTM to compare with our P3HT-COOH HTM. The result clearly showed the solar device based on self-assembled PEDOT:PSS exhibited an extremely poor performance and would not be further discussed in the manuscript. The J-V curve of the PSCs using PEDOT:PSS (SA) method is shown in the Figure S9 in the Supporting Information.
In addition, we fabricated PSCs with PTAA as HTM. However, we want to note that according to a previous literature published by Deng et al, [1] the perovskite solution spread on the ITO/PTAA substrate, but shrunk quickly during drying, leaving most of the area uncovered.
To improve the wettability of perovskite precursor solution on the PTAA layer, the PTAAcoated ITO substrates were pre-treated by spin-coating 80 μl of DMF at a speed of 4,000 rpm for 15 s prior to perovskite films deposition. [2,3] Briefly, the performance of our PTAA devices are summarized in Figure S10 in the Supporting Information. The wetting issues still remaining, even the PTAA film was pre-wetted by DMF. Compare to our studied devices, we introduce a facile method to self-assemble a new functionalized polythiophene (P3HT-COOH) monolayer on the conducting oxide electrodes and serve as hole-transporting layer in PSCs with high efficiency, reproducible and stable device performance. We believe that our demonstration broadly impacts the perovskite photovoltaic scientific and industrial research community. We also envision our new self-assembled HTM to be a promising interconnecting layer for the tandem solar cell, where conformal coating on the textured surface is essential for high performance tandem cell.

The impact of bottom HTM layer on the device stability.
Degradation under humid condition can be affected by various factors: the perovskite material degradation, moisture absorbed by interface layer that expedite the degradation. [4,5] Therefore, both bottom layer and top layer can affect the perovskite device lifetime under humid condition.
In addition, as shown from the all other stability test results and the high performance of the PSCs, the effect of the perovskite crystallinity and crystal orientation is much more pronounced than that of perovskite grain size. This is implying that the larger grain size is one of the contributing factors, however, it does not explain all of the improvements found in P3HT-COOH (SA) films. Moreover, the P3HT-COOH (SA) device exhibits much better crystallinity and high-quality perovskites layer formation in this study which suppressed the degradation process and demonstrated a much robust and stable performance among P3HT-COOH (SP) and PEDOT:PSS (SP) devices under the same testing conditions of moisture, heat, and photostability.

Ion migration.
It has been reported that the diffusion of metal ions in ITO contact (indium and tin) into the perovskite layer can significantly degrade the PCE. [6] The ion migration process is normally assisted by vacancy or defect. When using PEDOT:PSS as HTM, the hydrophilic and acidic -SO3H groups on PSS may induce a faster water absorption and the ITO electrode corrosion which accelerated the ion migration, [5] thus resulting in rapid degradation of PCE. In contrast, the carboxylic acid anchoring groups not only can self-assembly on ITO surface as HTL but also can passivate the ITO surface and suppressing the ion migration.      Table S1 in the Supporting Information. Figure S6. UV-vis absorbance spectra of P3HT-COOH with various film thickness. The relatively thick film exhibits an absorption peak at ~565 nm with a vibronic absorption shoulder at ~603 nm. As the film thickness decreases, the shoulder band becomes ambiguous and the absorption signatures are broadened and blue-shifted towards higher energies, implying the formation of a less aggregated structure and a more coil-like chain conformation that downshifts the HOMO energy level. The optimal thicknesses of SP and SA P3HT-COOH layers, estimated through the Beer-Lambert-law approach, are around 1.3 and 1.0 nm, respectively. The value of 1.0 nm is close to the length of hexyl carboxylic acid side chain, suggesting the P3HT-COOH (SA) forms a monolayer on top of ITO using the carboxylate as anchoring group. This finding demonstrates that SA route is an effective and simple approach to lower the Ip of conjugated homopolymers without incorporating electron-deficient units into polymer backbone through complicated synthetic steps.