Efficient Hole-transporting Materials Research Articles

Annealing- and doping-free hole transport material for p-i-n perovskite solar cells with efficiency achieving over 21%

• A small molecule hole transport materials (HTM, DFBT-PMTP) has been designed. • DFBT-PMTP canbe applied without doping and annealing in perovskites solar cells. • The efficiency of the corresponding p-i-n device reaches to as high as 21.23%. Hole transport materials (HTMs) play an important role in perovskites solar cells (Pero-SCs) which can greatly affect the stability and power conversion efficiency (PCE). While most of the efficient HTMs need doping (which results in poor stability) or the treatment of thermal annealing (which increases the complexity of the device fabrication process). In this work, we synthesized a small molecule HTM DFBT-PMTP based on 3,3′-difluoro-2,2′-bithiophene and applied it as HTM without the need for doping and thermal annealing in p-i-n Pero-SCs with the structure of ITO/HTM/MAPbI 3−x Cl x /C 60 /BCP/Ag. The PCE of the corresponding devices achieves a high value of 21.23% (19.8% for the PTAA-based reference ones), with an open-circuit voltage of 1.17 V and a high fill factor of 82.28%, which is to date among the highest values for the p-i-n MAPbI 3−x Cl x –based Pero-SCs. Interestingly, the PCE of the devices declined to 18.65 and 15.5%, while the annealing temperature of DFBT-PMTP increased to 100 and 150 °C, respectively. And the reasons for this abnormal phenomenon have been investigated including the intrisinc properties, the energy levels of MAPbI 3−x Cl x on top and the decices performance of the DFBT-PMTP films without annealing and annealed at 100 and 150 °C, respectively. The results revealed that the precise molecular structure design for the HTMs can make huge changes in the corresponding photovoltaic properties and fabrication process due to the effects on the top perovskites.

Rational design of phenothiazine-based hole transport material with fluorene-containing asymmetric peripheral donor group for perovskite solar cells

Developing low-cost and highly efficient hole transport materials (HTMs) is extremely important to promote the industrial application of perovskite solar cells (PSCs). Phenothiazine (PTZ) is validated as promising building block to construct low-cost and highly efficient HTM for PSCs. In this work, by using PTZ as core unit, we reported two HTMs, termed PTZT-MPF and PTZT-FF, through substituting the peripheral methoxybenzene group with fluorene. The asymmetric fluorene substituted donor provides the corresponding HTM PTZT-MPF with higher hole mobility, suitable frontier molecular orbital energy levels and smoother film morphology. As a result, the PTZT-MPF based PSCs exhibits a promising PCE of 20.15% with an enhanced open-circuit (Voc) of 1.13 V, even higher than well-known Spiro-OMeTAD. Meanwhile, with the fluorene substitution, the hydrophobicity of HTM is promoted with improved, and guarantee improved long-term stability for PTZT-MPF based PSCs.

Amplifying the photovoltaic properties of azaBODIPY core based small molecules by terminal acceptors modification for high performance organic solar cells: A DFT approach

Molecular engineering of azaBODIPY based molecule was done to construct four novel small photoactive materials (SM1-SM4) by varying terminal thiophene spaced acceptors for theoretical investigation of photovoltaic properties. In the present study, DFT and TD-DFT methods using B3LYP functionalwith a basis set of 6-31G were employed to systematically evaluate wavelength, determine parameters (absorption values, first excitonic energy, LHE), charge transfer elements (fill factor FF, open circuit voltage Voc), reactivity descriptors (IP and EA) and electronic processes (FMO, DOS, TDM) of modelled molecules (SM1-SM4). All studied molecules showed intense optical absorption in the visible region (710–751 nm), strong light harvesting capability, clearly assigned HOMO-LUMO placement, and lessened exciton binding energies due to highly electron withdrawing acceptor moieties. Greater dipole moment in solution phase than in the gas phase as compared to gaseous medium indicated that these small molecules possessed good solubility in Benzonitrile solvent.The internal reorganization energy values of novel molecules suggested their reliability as efficient hole transport materials. Additionally, results interpreted through analysis of values of Voc (with PTB7-Th), and FF, anticipated an escalation of power conversion efficiency. Among all considered molecules, SM2 and SM3 emerged as promising photovoltaic materials owing to their significant optical properties, reduced band gaps, rapid exciton dissociation, and high estimated PCE. Conclusively, these supportive outcomes favor the structural amendment of small molecules through effective terminal acceptor entities which could enhance the efficiency of photovoltaic materials.

Efficiency improvement for perovskite-inspired Cs3Sb2I9 solar cells using P3HT as the hole transport material

In this work, we introduce P3HT (poly(3-hexylthiophene-2,5-diyl)) as an efficient hole transport material (HTM) for lead-free Cs3Sb2I9 perovskite-inspired solar cells.

Photovoltaic properties of hole transport materials for organic solar cell (OSC) applications: physiochemical insight and in silico designing

Hole transport materials (HTMs) play a dominant role in enhancing the photovoltaic and optoelectronic properties of solar cells. These materials efficiently transport the hole, which significantly boosts the power conversion efficiencies of solar cells. In order to obtain better photovoltaic materials with efficient optoelectronic characteristics, we theoretically designed five new hole transport materials (Y3D1–Y3D5) after end-capped donor modifications of the recently synthesized highly efficient hole transport material Y3N (R). The relationships among photovoltaic, photophysical, optoelectronic and structural properties of these newly designed molecular models were studied at 6-31G(d,p) basis set and MPW1PW91 functional levels. Time‐Dependent Density Functional Theory (TDDFT) and density functional theory (DFT) proved to be excellent approaches for the studied systems. Geometrical parameters, molecular orbitals (MOs), open-circuit voltage (Voc), energy of binding and density of states were calculated. Low reorganization energy (RE) was noted; compared with the parent molecule (Reference/R), the designed molecular models possess high mobility. Molecular electrostatic potential (MEP) also supports our conclusion. Last but not least, the Y3D3:PC61BM complex was also studied to comprehend the role of charge distribution. These analyses showed that our modelled molecules are more efficient than the Y3N molecule. Thus, recommendations are made for experimentalists to develop extremely efficient solar cells in the near future.

Theoretical modelling of novel indandione-based donor molecules for organic solar cell applications

The development of efficient hole transport materials (HTMs) are highly attractive for the photovoltaic community to further improve the power conversion efficiency (PCE) and stability of organic solar cells. Therefore, in the present study, we have designed six new (H3T1-H3T6) small-molecule based HTMs for bulk-heterojunction (BHJ)-OSC. The photophysical characteristics and optoelectronic characterizations are systematically investigated. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) are employed to study the arrangement of frontier molecular orbitals (FMOs), optical characteristics, open-circuit voltages, density of states, transition density matrix, and reorganization energies of holes and electrons. The designed materials (H3T1-H3T6) have shown a better absorption phenomenon that ranges maximum up to ∼563.00 nm and offering optical band gap at ∼2.10 eV. Additionally, all the designed materials have displayed a well-matched HOMO along with higher LUMO energy levels regarding PC61BM molecule. The calculations of H3T-2/PC61BM provide a deep insight about the charge shifting at the donor-acceptor (D-A) interface. The results of these theoretical characterizations have shown that an efficient molecular designing and strategy are prerequisite to get a desirable photovoltaic precursor best fitted for BHJ-OSCs. Thus, these modelled materials (H3T1-H3T6) are being suggested for synthesis and future development of efficient organic solar cell devices.

Low-cost and efficient hole transport materials based on 9-phenyl-9H-carbazole branch for perovskite solar cells

Hole transport materials (HTMs) play a pivotal role in the hole extraction and transportation processes in perovskite solar cells (PSCs). Although a great number of HTMs based on various planar conjugated cores have been explored, the development of branches is rather limited. In this work, branches of 9-phenyl-9H-carbazole derivatives have been incorporated in different N-alkylated dithieno[3,2-b:2′,3′-d]pyrrol (DTP) core to construct HTMs (TM-9 to TM-14). Among them, TM-13 with 9-(4-methoxyphenyl)-9H-carbazole branch exhibites excellent performance in hole extraction and transportation ability. Consequently, the PSCs based on TM-13 have achieved the power conversion efficiency (PCE) of 20.44%, which is comparable to the PCE of 20.62% with the widely used Spiro-OMeTAD HTM. More importantly, the synthetic cost of TM-13 (62 $/g) is much lower than that of Spiro-OMeTAD (400 $/g), suggesting its promising commercialization for low-cost and efficient HTMs for PSCs.

Dopant-Free Hole Transporting Material Based on Nonconjugated Adamantane for High-Performance Perovskite Solar Cells.

A new dopant-free hole transporting material (HTM) 4′,4‴,4‴'',4‴''''-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)-[1,1′-biphenyl]-4-amine) (Ad-Ph-OMeTAD) (named FDY for short), which consists of a nonconjugated 3D bulky caged adamantane (Ad) as the core, triphenyl amines as side arms, and phenyl units as a linking bridge, is synthesized and applied in an inverted planar perovskite solar cell (PSC). As a result, the champion device with FDY as HTM yields an impressive power of conversion efficiency (PCE) of 18.69%, with JSC = 22.42 mA cm−2, VOC = 1.05 V, and FF = 79.31% under standard AM 1.5G illumination, which is ca. 20% higher than that of the device based on PEDOT:PSS (only 15.41%). Notably, the stability of PSC based on FDY is much better than that of devices based on PEDOT:PSS, and the corresponding devices retain over 90% of their initial PCEs after storing for 60 days in a nitrogen glove box without any encapsulation. Even when stored in an open air condition with 50–60% relative humidity for 188 h, the retained PCE is still over 81% of its initial one. All these results demonstrate that the new design strategy by combing the bulky and nonconjugated (aliphatic) adamantane unit as the core and triphenyl amines as side arms can efficiently develop highly efficient HTMs for PSCs, which is different from the traditional way based on conjugated backbones, and it may open a new way for scientists to design small-molecule HTMs for PSCs.

Side-chain tailoring of benzodithiophene derivatives as hole-transporting materials for stable perovskite solar cells

The high cost of Spiro-OMeTAD and the poor stability of devices remain a critical challenge for commercialization of perovskite solar cells (PSCs). Hence, the development of efficient hole-transporting materials (HTMs) seems imperative. Herein, three alkyls-modified HTMs named BTPA-C, BTPA-3C and BTPA-8C were designed and synthesized. Through tailoring their alkyl side-chain length, the structure-property relationships such as photophysical, electrochemical properties and the photovoltaic performance of corresponding PSCs were systematically investigated. Results demonstrate that the HTMs exhibit similar energy level, absorption and emission spectra. However, through tailoring their alkyl side-chain length, the film-forming and hole-transporting ability of HTMs can be greatly affected. Compared to BTPA-C or BTPA-8C, BTPA-3C with a moderate side chain length was endowed with the highest PCE of 18.60%, which was even comparable to that of PSCs based on Spiro-OMeTAD. Meanwhile, compared to the latter, PSCs based on BTPA-3C exhibited significantly improved stability at a relative humidity of 45 ± 5%, maintaining more than 80% of the initial efficiency after aging for 30 days.

Hole-Transporting Materials for Perovskite Solar Cells Employing an Anthradithiophene Core.

A decade after the report of the first efficient perovskite-based solar cell, development of novel hole-transporting materials (HTMs) is still one of the main topics in this research field. Two of the main advance vectors of this topic lie in obtaining materials with enhanced hole-extracting capability and in easing their synthetic cost. The use of anthra[1,9-bc:5,10-b′c′]dithiophene (ADT) as a flat π-conjugated frame for bearing arylamine electroactive moieties allows obtaining two novel highly efficient HTMs from very cheap precursors. The solar cells fabricated making use of the mixed composition (FAPbI3)0.85(MAPbBr3)0.15 perovskite and the novel ADT-based HTMs show power conversion efficiencies up to 17.6% under 1 sun illumination compared to the 18.1% observed when using the benchmark compound 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Detailed density functional theory calculations allow rationalization of the observed opto-electrochemical properties and predict a flat molecular structure with a low reorganization energy that supports the high conductivity measured for the best-performing HTM.

Design of small molecular hole-transporting materials for stable and high-performance perovskite solar cells

Perovskite solar cells represent one of the most exciting developments in photovoltaics in the past decade, with the power conversion efficiencies of over 25% being achieved to date. In high-performance perovskite solar cells, hole-transporting materials are generally employed to extract and transport holes from perovskite. Among them, small molecular hole transporting materials have attracted intense interest due to their tunable energy levels, structural variety, and simple synthesis. The commonly used hole-transporting material is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl amino)-9,9′-spirobifluorene (spiro-OMeTAD). Considering the high synthetic cost of spiro-OMeTAD and the device stability issue associated with the use of dopants, much research has been focused on the development of alternative high-performance hole-transporting materials. Herein, this review summarizes the recent developments in highly efficient small molecular hole-transporting materials with a power conversion efficiency close to or over 20%. On the basis of their structural features, three categories of small molecules are identified and discussed as highly efficient hole-transporting materials: spiro molecules with new terminal groups or a new spiro skeleton, star-shaped small molecular hole-transporting materials with three or four branches, and linear hole-transporting materials with a D-A, D-π-D, D-A-D, or D-A-π-A-D structure. The relationships of the optoelectronic properties of these hole-transporting materials and the device performance are discussed, with a comparison to those of model compounds in some cases. Finally, an outlook is addressed on the future development of hole-transporting materials for high-performance perovskite solar cells. We hope that this review can provide important guidance for the design and synthesis of new hole-transporting materials and finally help to promote the commercialization of perovskite solar cells.

Blending isomers of fluorine-substituted sulfonyldibenzene as hole transport materials to achieve high efficiency beyond 21% in perovskite solar cells

In this article, two fluorine-substituted sulfonyldibenzene compounds are synthesized and characterized (denoted as 3, 6-SFY and 2, 7-SFY), and they are mixed as hole transport material (HTM) in conventional perovskite solar cells (PSCs). 3, 6-SFY and 2, 7-SFY show good compatibility when they are blended due to their highly similar conjugated structures. The mixed HTM film exhibits suitable energy level and uniform surface morphology. Moreover, improved hole mobility and hole extraction ability was detected in the mixed isomers-based HTM compared to the single-component HTM. As a result, conventional perovskite solar cells using the mixed isomers as HTM deliver a maximum power conversion efficiency of 21.41%, significantly higher than that obtained in 3,6-SFY (19.02%) and 2,7-SFY-based devices (20.29%). These results demonstrate that mixing two isomers as HTM is a promising way to improve device performance. As far as we know, this is the first example of blending two isomers in conventional PSCs, and the results in this work also provide a new sight in developing highly efficient hole transport materials.

Molecular designing of triphenylamine-based hole-transporting materials for perovskite solar cells

To maximize the performance of perovskite solar cells (PSCs), controlling the hole mobility is of fundamental importance. In this regard, developing efficient hole transport materials (HTMs)is highly desirable in next-generation solar cells. Herein, several dopant-free HTMs, with the donor–acceptor-donor (D-A-D) architecture, are designed based on benzophenone and di (pyridine-3-yl) methanone as the acceptors and triphenylamine (TPA) as the donor. The correlation between electronic structure and hole transport property of the TPA-based HTMs were systematically investigated by tuning the connection between donor and acceptor (single vs. double bond) and also the substitution position of the methoxy groups (ortho, meta, and para). Our results from density functional theory calculations indicate that introducing the vinyl group as the connecting π-bridge can effectively increase the hole mobilities of the designed HTMs. Among the investigated compounds, the combination of benzophenone core acceptor with para-methoxy TPA terminals via the vinyl bridge, called p-TEBET, shows the most promising features and achieves excellent hole mobility (1.58 × 10-1 cm2 V−1 s−1) which is larger than that of spiro-OMeTAD as the most commonly used HTM. The results of the present computational study can be further employed in the process of synthesizing new HTMs with promising features in terms of proper energy alignment, stability, and hole mobility.

Heteroatom effect on linear-shaped dopant-free hole transporting materials for perovskite solar cells

A series of linear-shaped D-π-D and D-A-D hole transporting materials (HTMs) with different heteroatom-containing bridges, such as furan, thiophene, selenophene, 2,1,3-benzoxadiazole, 2,1,3-benzothiadiazole and 2,1,3-benzoselenadiazole, are designed, prepared and implemented in perovskite solar cells (PSCs). The effect of the variation heteroatom bridge on the photophysical, electrochemical, and the photovoltaic properties in devices are systematically investigated. D-π-D HTMs exhibit gradually bathochromic spectrum responding along with increasing atomic numbers (from O, S to Se). However, D-A-D molecules exhibit no regular variation trend of photophysical and electrochemical properties. When used in PSCs, the photoelectric conversion efficiency of the devices gradually decreased along with increasing atomic numbers whether D-π-D or D-A-D HTMs. Unexpectedly, Se-containing molecules show the best hole extraction ability according to photoluminescence study. The relatively low conductivity of Se-based HTMs, resulted from gradually increased dihedral angles between the bridge and the adjacent triphenylamine side arms, may mainly limited the device performance. The results would give some insights for designing novel efficient HTMs for PSCs.

C[tbnd]N-based carbazole-arylamine hole transporting materials for perovskite solar cells: Substitution position matters

Hole transporting materials (HTMs) containing passivating groups for perovskite materials have attracted much attention for efficient and stable perovskite solar cells (PSCs). Among them, CN-based molecules have been proved as efficient HTMs. Herein, a series of novel CN functionalized carbazole-arylamine derivatives with variable CN substitution positions (para, meta, and ortho) on benzene-carbazole skeleton (on the adjacent benzene of carbazole) were synthesized (p-HTM, m-HTM and o-HTM). The experimental results exhibit that the substitution positions of the CN unit on HTMs have minor difference on the HOMO energy level and hydrophobicity. m-HTM has a relatively lower glass transition temperature compared with that of p-HTM and o-HTM. The functional theory calculations show that the CN located on meta position exposed very well, and the exposure direction is also the same with the methoxy. Upon applying these molecules as HTMs in PSCs, their device performance is found to sensitively depend on the substitution position of the CN unit on the molecule skeleton. The devices using m-HTM and o-HTM exhibit better performance than that of p-HTM. Moreover, m-HTM-based devices exhibit better light-soaking performance and long-term stability, which could be resulted from better interaction with the perovskite according to DFT results. Moreover, we further prepared a HTM with two CN units on the symmetrical meta position of molecular skeleton (2m-HTM). Interestingly, 2m-HTM-based devices exhibit relatively inferior performance compared with that of the m-HTM, which could be resulted from weak negative electrical character of CN unit on 2m-HTM. The results give some new insights for designing ideal HTM for efficient and stable PSCs.

Learning from hole-transporting polymers in regular perovskite solar cells to construct efficient conjugated polyelectrolytes for inverted devices

• A polyelectrolyte was designed with the same main-chain of a reported polymer HTM. • HTM based on it showed an efficiency of 19.92% in an inverted solar cell. • More thiophene unit gave a superior hole-extraction and defect-passivation ability. Owing to its high wettability and the resulting compatibility with the future industrial device fabrication process, the conjugated polyelectrolyte (CPE) has becoming a promising hole-transporting material (HTM) in the inverted perovskite solar cell (iPSC); however, only a few highly efficient CPEs have been reported probably due to the limited pool of molecular designing strategies. Here we construct a CPE named DTB(Na) with the same main-chain of a polymer DTB(EH) that was previously employed as a dopant-free HTM in a highly efficient regular type PSC (rPSC) and with simple water/alcohol soluble side-chains. Compared with its analog bearing the only variation of less thiophene units in the main-chain, DTB(Na) shows stronger capacities of hole-extraction and defect-passivation, which should be ascribed to the more intensive exposure of thiophene functional unit to the perovskite layer. As a result, the DTB(Na) iPSC device presents enhanced values on all the photovoltaic parameters and long-term stability, and a power conversion efficiency of 19.92% is realized. Our results demonstrate the feasibility for efficient polymeric HTMs in rPSCs and iPSCs to share the same main-chains, thus enriching the molecular designing strategies and probably expanding the library of efficient HTMs for iPSCs.

Nanostructured bilayer CuSCN@CuI thin films as efficient inorganic hole transport material for inverted perovskite solar cells

In this work, the development of bilayer CuSCN@CuI inorganic hole transport material by a simple electrochemical approach is demonstrated. The thickness and the morphology of the bilayer thin films are controlled by electrochemical potential and deposition time. Uniformly distributed triangle-shaped CuI nanosheets formation is observed at 2 min deposition time. Inverted perovskite solar cells are fabricated using electrochemically grown CuSCN@CuI bilayer and tested its photovoltaic performance. The maximum short circuit current density of 18.24 mA/cm2 and open-circuit voltage of 1080 mV is achieved for uniformly distributed triangle-shaped CuI nanosheets grown at 2 min deposition time. The power conversion efficiency (PCE) of 15.58% is achieved with 1400 h of stability. The moderate thickness (~180–230 nm) of bilayer CuSCN@CuI nanostructures showed better charge transport and photovoltaic performance. The favourable band alignment of the designed CuSCN@CuI/perovskite/PC61BM/Carbon delivers stable open-circuit voltage than the earlier reports. The optimized bilayer CuSCN@CuI nanostructure with carbon back contact showed improved device stability.

Novel D-A-D type small-molecular hole transport materials for stable inverted perovskite solar cells

Hole transport materials (HTMs), as a critical role in the hole extraction and transportation processes, highly influence the efficiency and stability of perovskite solar cells (PSCs). Despite that several efficient dopant-free HTMs have been reported, there is still no clear structure-property relationship that could give instructions for the rational molecular design of efficient HTMs. Thus, in this work, a series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been carefully designed and synthesized. By varing the electron acceptor unit from benzene to pyridine, pyrazine and diazine, their packing structure in single crystals, optical and electronic properties have shown a great difference. While as dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13%. Moreover, the unencapsulated device based on TM-2 retains about 80% of its initial efficiency after 500 h storage in ambient environment, showing the superior stability. • A series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been designed and synthesized, showing a great difference in their single crystal packing structure, optical and electronic properties. • As the dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13% along with a superior stability.

2.2]Paracyclophane-based hole-transporting materials for perovskite solar cells

To develope an alternative efficient hole-transporting materials (HTMs) to 2,2′,7,7′,-tertrakis(N,N-p-dimethoxyphenylamino)-9,9′-spirobifuorene (spiro-OMeTAD) for high performance perovskite solar cells (PSCs), we demonstrate a series of donor-π-donor HTMs (WS-1, WS-2, and WS-4 HTMs) with [2.2]paracyclophane ([2.2]PCP) as the core structure and triphenylamine as four arms at pseudo-para and pseudo-ortho orientations. Compared with the well-known HTM of spiro-OMeTAD, WS-HTMs has a simpler synthetic route and short synthesis steps (3–4 steps). Due to the improved hole mobility and good charge transfer efficiency of pseudo-para-[2.2]PCP HTMs (WS-1 and WS-2), the out-of-plane carrier transport is enhanced and the PSC base on WS-1/WS-2 HTMs achieve higher Jsc and ff values than the device based on pseudo-ortho-[2.2]PCP HTM (WS-4). A SnO2 electron transport layer (ETL) with WS-1 HTM shows the best power conversion efficiency of 19.13% in a PSC, which is higher than that of spiro-OMeTAD (17.71%) under the same conditions. The WS-HTMs also provided better stability and moisture resistance in PSCs, which prolongs the lifetime in the ambient environment than in case of spiro-OMeTAD.

Side-Chain Engineering of Diketopyrrolopyrrole-Based Hole-Transport Materials to Realize High-Efficiency Perovskite Solar Cells.

The design and synthesis of a stable and efficient hole-transport material (HTM) for perovskite solar cells (PSCs) are one of the most demanding research areas. At present, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-MeOTAD) is a commonly used HTM in the fabrication of high-efficiency PSCs; however, its complicated synthesis, addition of a dopant in order to realize the best efficiency, and high cost are major challenges for the further development of PSCs. Herein, various diketopyrrolopyrrole-based small molecules were synthesized with the same backbone but distinct alkyl side-chain substituents (i.e., 2-ethylhexyl-, n-hexyl-, ((methoxyethoxy)ethoxy)ethyl-, and (2-((2-methoxyethoxy)ethoxy)ethyl)acetamide, designated as D-1, D-2, D-3, and D-4, respectively) as HTMs. The variation in the alkyl chain has shown obvious effects on the optical and electrochemical properties as well as on the molecular packing and film-forming ability. Consequently, the power conversion efficiency (PCE) of the PSC under one sun illumination (100 mW cm-2) is shown to increase in the order of D-1 (8.32%) < D-2 (11.12%) < D-3 (12.05%) < D-4 (17.64%). Various characterization techniques reveal that the superior performance of D-4 can be ascribed to the well-aligned highest occupied molecular orbital energy level with the counter electrode, the more compact π-π stacking with a higher coherence length, and the excellent hole mobility of 1.09 × 10-3 cm2 V-1 s-1, thus providing excellent energetics for effective charge transport with minimal charge-carrier recombination. Furthermore, the addition of the dopant Li-TFSI in D-4 is shown to deliver a remarkable PCE of 20.19%, along with a short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) of 22.94 mA cm-2, 1.14 V, and 73.87%, respectively, and superior stability compared to that of other HTMs. These results demonstrate the effectiveness of side-chain engineering for tailoring the properties of HTMs, thus offering new design tactics to fabricate for the synthesis of highly efficient and stable HTMs for PSCs.