Improved Efficiency of Perovskite Solar Cells with Low‐Temperature‐Processed Carbon by Introduction of a Doping‐Free Polymeric Hole Conductor

Abstract

Perovskite solar cells (PSCs) have increased tremendously in power conversion efficiencies (PCEs) during the short period of their existence, from 3.8% in 2009 to 25.5% currently. [1,2] PSCs are based on lead halide perovskite semiconductors, which have attracted much attention due to their outstanding optoelectronic properties, such as high absorption coefficient with a direct bandgap, which is tunable by modification of their composition, [3] long carrier diffusion length, [4] high carrier mobility, [5] and their versatile fabrication methods, including simple solution processes. [6,7] PSCs can rival with commercial monocrystalline silicon solar cells. Despite the high efficiency of PSCs, the use of hazardous solvents in preparation and the presence of the toxic element lead increase environmental concerns. To minimized adverse effects, green solvent processing [8,9] and encapsulation techniques to avoid lead leakage [10] should be applied. However, long-term stability issues still remain, limiting so far the practical commercialization of PSC technology. [11] In the conventional nÀiÀp device structure of PSCs, the metal back contact, Au (gold) or Ag (silver), give limited stability due to chemical migration of the metals and reaction with the perovskite. [12][13][14] Furthermore, these metals are expensive and need a high-vacuum deposition technique, which makes it costly and energy intensive. As an alternative, a variety of carbon materials, such as carbon black, graphite, graphene, and carbon nanotubes, are suited as contact materials in PSCs. [15,16] These carbon materials possess a work function of about 5.0 eV and allow for PSC fabrication at low temperature (<120 C). [17] In addition, carbon electrodes (CEs) offer advantages such as low cost and vacuumfree preparation, as well as giving superior stability, by acting as a moisture barrier and by suppressing ion migration. [18][19][20] There are two types of C-PSCs. Type I C-PSCs have a mesoscopic n-i-p architecture and are typically prepared by screen printing porous layers of TiO 2 , insulator (ZrO 2 , Al 2 O 3 ), and carbon, which are heated at a high temperature (≈450 C) and subsequently infiltrated with a perovskite precursor solution. Initial reports from 2013 yielded devices with a PCEs of 6.6%, [21] which improved to 17.0-17.5% in recent years. [22,23] In type II C-PSCs, the CE is deposited on the top of the perovskite layer.

Enhanced Thermal Stability of Low‐Temperature Processed Carbon‐Based Perovskite Solar Cells by a Combined Antisolvent/Polymer Deposition Method

“…We investigated the hysteresis in J – V curves by analyzing the hysteresis indices (HI) calculated by the formula [ 37 ] where – and + were the PCE from reverse and forward scans, respectively. Similar average hysteresis index (HI) values for 0.164 ± 0.016 (pristine), 0.166 ± 0.01 (P3HT), and 0.171 ± 0.009 (PTAA) can be observed (see Table 1).…”

“…We investigated the hysteresis in J-V curves by analyzing the hysteresis indices (HI) calculated by the formula [37] HI ¼…”

“…Thereafter, the clean substrates were quickly transferred to the N 2 ‐filled glove box. The perovskite precursor solution was prepared by dissolving 239 mg of MAI and 692 mg of PbI 2 in a mixed solvent of 0.107 mL DMSO and 0.953 mL N , N ‐dimethylformamide (DMF) [ 37 ] and it was stirred at 60 °C overnight under the dark at Ar glove box. The resultant mixture was filtered through a 0.45 μm PTFE filter.…”

“…[36] In our recent work, we inserted undoped P3HT between carbon and perovskite (MAPbI 3 ), which led to devices with a PCE of 15.7% that maintained 70% of its initial performance for 900 h at 82 °C in air. [37] In other recent work, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was added into the perovskite of a device with undoped P3HT/carbon, leading to an enhanced PCE of 15.1% due to improved energy level matching. [38] To date, the highest efficiency of 24.6% was achieved when gallium (iii) acetylacetonate (Ga(acac) 3 ) was incorporated into the P3HT/Au device, showing the superior stability in air for 2000 h. [39] Jin et al reported that ZnO-based C-PSC device reached efficiency up to 16.05% when P3HT was inserted between perovskite and carbon leading to suitable energy alignment and retained its initial efficiency over 90% for 1200 h in air.…”

“…[ 36 ] In our recent work, we inserted undoped P3HT between carbon and perovskite (MAPbI 3 ), which led to devices with a PCE of 15.7% that maintained 70% of its initial performance for 900 h at 82 °C in air. [ 37 ] In other recent work, 2,3,5,6‐tetrafluoro‐7,7,8,8‐ tetracyanoquinodimethane (F4TCNQ) was added into the perovskite of a device with undoped P3HT/carbon, leading to an enhanced PCE of 15.1% due to improved energy level matching. [ 38 ]…”