Page 57 - Read Online

Page 2 of 21 Wen et al. J. Mater. Inf. 2025, 5, 30 https://dx.doi.org/10.20517/jmi.2024.102

HTMs is proposed, aiming to accelerate the discovery and application of high-performance SM-HTMs. A custom-
developed molecular splicing algorithm (MSA) generated a sample space of 200,000 intermediate molecules,
culminating in the creation of a comprehensive database of over 7,000 potential SM-HTM candidates. In total, six
promising HTM candidates were identified through MSA, density functional theory calculations and high-
throughput screening. Furthermore, three machine learning algorithms, namely random forest, gradient boosting
decision tree, and extreme gradient boosting (XGBoost), were employed to construct predictive models for key
material properties, including hole reorganization energy, solvation free energy, maximum absorption wavelength,
and hydrophobicity. Among these, the XGBoost-based model demonstrated the best overall performance. The
MSA methodology combining comprehensive SM-HTM database and performance prediction models, as
introduced in this study, offers a powerful and universal toolkit for the design and optimization of next-generation
SM-HTMs, thereby paving the way for future advancements of PSCs.
Keywords: Perovskite solar cell, small-molecule hole transport materials, molecular splicing, density functional
theory, high-throughput computational screening, machine learning

INTRODUCTION
With the growing urgency of addressing energy demands, climate change, and environmental challenges,
the development of solar cells, which play a pivotal role in converting solar energy into electricity, has
become increasingly significant . Among various solar cell technologies, perovskite solar cells (PSCs) have
[1-3]
emerged as a groundbreaking innovation. Due to their high power conversion efficiency (PCE), low cost,
tunable optical bandgap, excellent carrier transport properties, broad spectral response, and straightforward
fabrication processes, PSCs have rapidly become a focal point of research in the field of solar energy .
[4-7]
[8]
Notably, their PCE has reached an impressive value of 26.7% .
Hole transport materials (HTMs) are the essential component in PSCs for facilitating the transport of
photo-generated holes to the electrodes, and the optimization of their performance can significantly
improve the PCE of solar cells [9-11] . These materials can be broadly categorized into inorganic, polymeric and
small-molecule HTMs (SM-HTMs) [9,12] . Inorganic HTMs, while promising, are limited by poor film-
forming properties and a narrow range of material options, hindering their development. Polymer HTMs
face limitations in their application scope due to the relatively intricate synthesis and purification processes,
along with the challenges associated with accurately characterizing their molecular weight. On the other
hand, SM-HTMs offer abundant availability and flexible structural tunability, enabling the design of target
molecules with high hole mobility through various structural combinations [9,13] . Currently, spiro-OMeTAD
is the most representative SM-HTM used in PSCs. It consists of two rigid π-conjugated systems connected
by an orthogonal molecular conformation, offering excellent thermal stability and strong film-forming
capabilities [14,15] . However, due to weak intermolecular interactions, spiro-OMeTAD films exhibit low hole
mobility. To enhance device performance, dopants such as lithium salts and tBP are often introduced to
improve conductivity and hole mobility. Unfortunately, the use of dopants can lead to device
instability [16,17] . Moreover, the commercialization of spiro-OMeTAD is hindered by its complex synthesis,
low yield, difficult purification, and high production cost . Consequently, there is growing interest in the
[9]
development of new SM-HTMs to overcome these limitations.

The function of the SM-HTM is to extract holes from the perovskite absorber layer and efficiently transport
them to the electrode, thereby enhancing the performance of the solar cells. An ideal SM-HTM should meet
the following criteria [9,18] : (i) its highest occupied molecular orbital (HOMO) energy level should match that
of the perovskite layer, facilitating exciton separation at the interface, enabling easy hole injection into the
hole transport layer (HTL), and preventing electron migration into the HTL; (ii) it should exhibit high hole