There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long-term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for high-performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping-induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant-free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low-cost, effective, and scalable materials for devices with long-term stability. It summarizes recent literature reports on non-doped, alternative, and more stable HTMs used in PSCs as essential components for high-efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant-free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant-free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

Development of Dopant-Free Organic Hole Transporting Materials for Perovskite Solar Cells

Since 2009, perovskite solar cells (PSCs) have witnessed dramatic developments with the record power conversion efficiency (PCE) exceeding 25% within a single decade. One bottleneck for the commercialization of PSCs is the lack of stability due to the commonly used hole transport materials (HTMs) with dopants. These hygroscopic dopants not only deteriorate the long-term stability due to moisture ingress and ion diffusion, but also increase the complexity and total cost. Therefore, the development of dopant-free HTMs is of great significance. In this work, the recent progress in dopant-free HTMs, including organic small molecules, polymers, organometallic compounds, and inorganic materials, has been systematically reviewed and summarized. To bring current dopant-free HTMs closer to the ideal ones, detailed solutions for making dopant-free HTMs more efficient, stable, and cost-effective are analyzed and discussed. In the end, we try to answer the key scientific questions such as what makes HTMs dopant-free, how to obtain the maximum PCE for a new dopant-free HTM, and what is the most efficient way to find ideal dopant-free HTMs based on the literature survey and our own understanding. With continuous efforts in developing dopant-free HTMs, PSC commercialization can be realized in the near future.

Toward ideal hole transport materials: a review on recent progress in dopant-free hole transport materials for fabricating efficient and stable perovskite solar cells

D-A-π-A-D-type Dopant-free Hole Transport Material for Low-Cost, Efficient, and Stable Perovskite Solar Cells

A Realistic Methodology for 30% Efficient Perovskite Solar Cells

The power conversion efficiency (PCE) of Cs2AgBiBr6-based perovskite solar cells (PSCs) is still low owing to the inherent defects of Cs2AgBiBr6 films. Herein, we demonstrate a carboxy-chlorophyll derivative (C-Chl)-sensitized mesoporous TiO2 (m-TiO2) film as an electron transport layer (ETL) to enhance and extend the absorption spectrum of Cs2AgBiBr6-based PSCs. The C-Chl-based device achieves a significantly improved PCE, exceeding 3% for the first time, with an increase of 27% in short-circuit current density. Optoelectronic investigations confirm that the introduction of C-Chl reduces the defects, accelerates the electron extraction, and suppresses charge recombination at the interface of ETL/perovskite. Moreover, the unencapsulated PSCs display restrained hysteresis and great stability under ambient conditions.

Chlorophyll Derivative-Sensitized TiO2 Electron Transport Layer for Record Efficiency of Cs2AgBiBr6 Double Perovskite Solar Cells.

Rational Design of Dopant-Free Coplanar D-π-D Hole-Transporting Materials for High-Performance Perovskite Solar Cells with Fill Factor Exceeding 80%

Delicately designed dopant-free hole-transporting materials (HTMs) with ordered structure have become one of the major strategies to achieve high-performance perovskite solar cells (PSCs). In this work, we report two donor-π linker-donor (D-π-D) HTMs, N01 and N02, which consist of facilely synthesized 4,8-di(n-hexyloxy)-benzo[1,2-b:4,5-b']dithiophene as a π linker, with 10-bromohexyl-10H-phenoxazine and 10-hexyl-10H-phenoxazine as donors, respectively. The N01 molecules form a two-dimensional conjugated network governed by C-H⋅⋅⋅O and C-H⋅⋅⋅Br interaction between phenoxazine donors, and synchronously construct a three-dimension lamellar structure with the aid of interlaminar π-π interaction. Consequently, N01 as a dopant-free small-molecule HTM exhibits a higher intrinsic hole mobility and more favorable interfacial properties for hole transport, hole extraction and perovskite growth, enabling an inverted PSC to achieve a very impressive power conversion efficiency of 21.85 %.

Synergistical Dipole–Dipole Interaction Induced Self-Assembly of Phenoxazine-Based Hole-Transporting Materials for Efficient and Stable Inverted Perovskite Solar Cells

ND-π-D molecular layer electronically bridges the NiOx hole transport layer and the perovskite layer towards high performance photovoltaics

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Perovskite Cells: Rational Design of Dopant‐Free Coplanar D‐π‐D Hole‐Transporting Materials for High‐Performance Perovskite Solar Cells with Fill Factor Exceeding 80% (Adv. Energy Mater. 39/2019)

Rational introduction of flexible alkyl chains into rigid conjugated molecules is a facile but effective strategy to develop advanced organic semiconductor materials for considerable enhancement of photovoltaic performance. Herein, seven hole‐transporting materials (HTMs) via attaching ethyl, hexyl, or ethylhexyl to benzodithiophene π‐linker and bromoethyl, bromobutyl, or bromohexyl to phenoxazine donor (D), respectively, named B2P2, B6P2, B8P2, B6P4, B2P6, B6P6 (N01), and B8P6, are reported. Joint differential scanning calorimetry measurements and thin‐film absorption spectra of representative HTMs, B8P6, B6P4, and B2P2 reveal that alkyl chain modulation on coplanar D–π–D HTMs enables synchronous optimization of their solution processability, molecular packing, and thermal phase transition. Consequently, benefiting from the favorable self‐assembly and surface characteristics of hole‐transporting layers and further induced superior upper perovskite‐growth, B8P6‐ and B6P4 ‐based inverted perovskite solar cells exhibit decent power conversion efficiencies of 20.67% and 20.13%, respectively, prior to that of amorphous B2P2 (19.04%). Analysis on steady‐state/transient photoluminescence spectra and light intensity‐dependent short‐circuit photocurrent and open‐circuit voltage (Voc) demonstrates that more ordered assemblies of HTMs obtained via alkyl chain engineering can facilitate efficient interfacial charge transport/extraction and inhibit detrimental charge recombination, accounting for an enhanced Voc and fill factor.

Modulating the Self‐Assembly of Coplanar D–π–D Hole‐Transporting Materials via Alkyl Chain Engineering for Efficient and Stable Inverted Perovskite Solar Cells

Ruixi Luo

Papers

Rational Design of Dopant-Free Coplanar D-π-D Hole-Transporting Materials for High-Performance Perovskite Solar Cells with Fill Factor Exceeding 80%

Synergistical Dipole–Dipole Interaction Induced Self-Assembly of Phenoxazine-Based Hole-Transporting Materials for Efficient and Stable Inverted Perovskite Solar Cells

ND-π-D molecular layer electronically bridges the NiOx hole transport layer and the perovskite layer towards high performance photovoltaics

Perovskite Cells: Rational Design of Dopant‐Free Coplanar D‐π‐D Hole‐Transporting Materials for High‐Performance Perovskite Solar Cells with Fill Factor Exceeding 80% (Adv. Energy Mater. 39/2019)

Modulating the Self‐Assembly of Coplanar D–π–D Hole‐Transporting Materials via Alkyl Chain Engineering for Efficient and Stable Inverted Perovskite Solar Cells