Efficient and Stable Colloidal Quantum Dot Solar Cells with a Green‐Solvent Hole‐Transport Layer

Abstract

of 4 based on the National Fire Protection Association (NFPA). In addition, the layer-by-layer deposition of the CQD-EDT HTL prevents scalable fabrication of CQD-SCs. [4] As an alternative, polymeric p-type materials have been recently explored as a HTL in CQD-SCs because of their single-step solution processability, nontoxic nature, and optoelectronic properties tunable through molecular design. [5-13] A power conversion efficiency (PCE) of 13.1% [14] was recently reported for CQD-SCs using a polymer HTL, requiring further development to compete with CQD-SCs based on the conventional CQD-EDT HTL. [15] The organic HTL is generally processed with a halogenated solvent, such as chlorobenzene (CB) (Figure 1a; MSDS hazard symbols: GHS07, GHS09). Halogen atoms are easily accumulated into the human body and ecosystems; thus, the substitution of toxic solvents is considered a priority for large-area fabrication and commercialization of CQD-SCs. [16] One candidate is 2-methylanisole (2-MA), a non-toxic food additive. [17,18] 2-MA shows higher polarity than CB, which suggests reengineering the polymer HTL for higher polarity. According to the like-dissolves-like principle, [19] this can be realized by breaking the symmetry of polymers. [20,21] A further potential benefit to this approach is the achievement of low crystallinity polymeric HTLs, an approach recently demonstrated to achieve high conductivity (by reducing grain boundaries) and thermal stability (by reducing lateral grain growth upon thermal stress). [22,23] Here, we design and synthesize random polymeric HTLs (asy-ranPBTBBDT) and compare them with control HTLs (asy-PBTBDT) having ethylhexyl branched alkyl chains (Figure 1b). The asy-ranPBTBBDT can be dissolved in various solvents owing to its extended branching points and random copolymerization. As a result, we demonstrate processing from 2-MA and report progress in both efficiency and thermal stability. The asy-ranPBTBDT-based CQD-SCs increase PCE to 13.2% while asy-BTBDT reference devices exhibit PCE 11.4% (Table S1, Supporting Information). The asy-ranPBTBDT CQD-SCs also improve operating stability, a finding we assign to the arresting of the process of lateral grain growth.

Open‐Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

“…PTB7-Th and PC 71 BM blended HTL shows better band alignment with PbS CQD film, lower current leakage, and lower interface recombination. With n-i-p structure, the CQDSC with the blend HTL can achieve a V oc of 0.65 V, which is 41 mV higher than that with PTB7-Th HTL (improved by 6.7%), and the V oc loss reaches ≈0.45 V. Moreover, recently reported organic HTLs, including asy-ran PBTBDT, [78] TIPS-TPD, [80] and PD2FCT-29DPP [21] all enable lead chalcogenide CQDSCs to achieve high PCEs of above 13%, which is comparable with that of EDT-PbS HTLs. However, the CQDSCs with organic HTLs still have a large V oc loss of ≈0.45 V, indicating that more research efforts should be devoted to the V oc loss of lead chalcogenide CQDSCs.…”

“…Organic HTLs can be solution-processed and have adjustable compositions, energy levels, and corresponding optical and electrical properties. [78,204] We summarize the key developments of organic HTLs in lead chalcogenide CQDSC in Figure 16. In 2016, Neo et al [204] applied poly(3-hexylthiophene) (P3HT) as HTLs in CQDSCs.…”

“…The PCE and voltage loss evolution of CQDSCs with organic HTLs over the years. The organic materials include P3HT, [204] PTB7, [77] PBDTTT-E-T:IEICO, [6] PBDB-TF, [205] PTB7-Th:PC 71 BM, [206] PBDTTPD-HT, [79] TIPS-TPD, [80] asy-ranPBTBDT, [78] and PD2FCT-29DPP. [21] organic HTLs show significantly higher stability.…”

Quantum Dots for Photovoltaics: A Tale of Two Materials

“…[184] Park and coworkers developed a new random polymeric HTL, asy-ran PBTBDT, and fabricated PbS QD solar cells showing a PCE of 13.2%. [185] In addition, there are a few other noteworthy device engineering strategies. Sargent and coworkers developed PbS QD solar cells with a bulk heterojunction device structure based on a homogeneous n-type and p-type QDs mixed film.…”

“…[ 184 ] Park and coworkers developed a new random polymeric HTL, asy‐ran PBTBDT, and fabricated PbS QD solar cells showing a PCE of 13.2%. [ 185 ]…”